SURFACE ENGINEERING
FOR CORROSION AND WEAR RESISTANCE
Edited by
J.R. Davis
Davis & Associates
tf
10M Communications
The Mate rials
Information Society
Materials Park, OR 44073-0002
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is a
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10M Book No. B751
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Library of Congress Cataloging-in-Publication
Data
Surface engineering for corrosion and wear resistance / edited by J .R. Davis
p. cm. Includes index.
1. Corrosion and anti-corrosives. 2. Mechanical wear.
3. Surfaces (Technology) 1. Davis, J.R. (Joseph R.)
TA462.S789 2001 620.1' 1233-dc21
00-048537
ISBN: 0-87170-700-4
ASM International®
Materials Park, OH 44073-0002
www.asminternationaLorg
Printed in the United States of America
Contents
Preface • • • • • • • . • • . • • • • • • • • • • • • • • • • • • • • • • • . • • • • • . . •vi i
CHAPTER 1: Introduction to Surface Engineering for Corrosion
and Wear Resistance ....•.•.•.•••.•.••..••..•••..•
Surface Engineering to Combat Corrosion and Wear
CHAPTER 2:
Principles of Corrosion ••••••••••••.••••.•.
Electrochemical Corrosion Basics
Corrosive Conditions
Forms of Corrosion
Uniform Corrosion
Galvanic Corrosion
Pitting
Crevice Corrosion
Erosion-Corrosion
Cavitation
Fretting Corrosion
Intergranular Corrosion
Exfoliation
Dealloying Corrosion
Stress-Corrosion Cracking
Corrosion Fatigue
Hydrogen Damage
Coatings and Corrosion Prevention
Corrosion Testing
Field Tests
Simulated Service Tests
Salt Spray Tests
Humidity Cabinet Tests
Electrochemical Tests
iii
1
3
11
11
13
15
15
16
19
21
22
23
24
25
26
26
27
29
30
31
35
36
36
38
39
39
CHAPTER 3:
Principles of Friction and Wear
Friction
Wear
Classification of Wear
Abrasive Wear
Solid Particle Erosion
Liquid Erosion
Slurry Erosion
Adhesive Wear
Galling
Fretting
Rolling-Contact Wear
Lubrication
Modes of Lubrication
Lubricants
Wear Testing
Test Methods
43
43
54
54
56
61
68
69
72
75
76
77
77
78
78
81
81
CHAPTER 4: Surface Engineering to Change the Surface
Metallurgy
Selective Surface Hardening
Flame Hardening
Induction Hardening
High-Energy Beam Hardening
Laser Melting
Shot Peening
87
87
87
88
90
91
93
CHAPTER 5: Surface Engineering to Change the Surface
Chemistry
95
Phosphate Chemical Conversion Coatings
95
Types of Phosphate Coatings
96
Applications
98
Chromate Chemical Conversion Coatings
100
Aluminum Anodizing
102
Chromic Anodizing
102
Sulfuric Anodizing ..................................103
Hardcoat Anodizing
· 104
Sealing of Anodized Coatings
105
Corrosion Resistance of Anodized Aluminum
106
Oxidation Treatments
108
Diffusion Heat Treatment Coatings
110
Carburizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Nitriding .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113
Carbonitriding and Ferritic Nitrocarburizing
115
iv
Pack-Cementation
Ion Implantation
Laser Alloying
Diffusion Coatings
116
120
122
CHAPTER 6: Surface Engineering to Add a Surface Layer or
Coating ..•..........•........................
Organic Coatings
Paints
Ceramic Coatings and Linings
Glass Linings
Porcelain Enamels
Concrete and Cementatious Coatings and Linings
High-Performance Ceramic Coatings and Linings
Hot Dip Coatings
Batch and Continuous Processing
Coating Microstructure
Galvanized Coatings
Galvanneal Coatings
Zinc-Aluminum Coatings
Aluminum Coatings
Teme Coatings
Electrochemical Deposition
Aqueous Solution Electroplating
Continuous Electrodeposition
Fused-Salt Electroplating
Precious Metal Plating
Electroless Plating
Composite Coatings
Weld-Overlay Coatings
Thermal Spray Coatings
Cladding
Corrosion Control through Cladding
Chemical Vapor Deposition
Physical Vapor Deposition Processes
Thermoreactive Deposition/Diffusion Process
CHAPTER 7:
Process Comparisons
Process Availability
Corrosion Resistance
Wear Resistance
Cost of Surface Treatments
Distortion or Size Change Tendencies
Coating Thickness Attainable
v
.••..•••••.••••••..•
125
127
128
132
132
133
134
136
138
138
.138
139
142
142
143
144
145
145
147
148
149
150
151
153
160
166
166
168
172
176
183
184
185
186
190
191
192
CHAPTER 8: Practical Design Guidelines for Surface
Engineering
Surface-Engineering Solutions for Specific Problems
Structural Parts in Corrosive Environments
Base Material
Neutral Environments
Specific Corrosive Environments
Parts in Static Contact with Vibration (Fretting)
Base Material
Contact Conditions
Fretting Fatigue
Oxidative Wear
Parts in Static Contact with a Product
Base Material
Specific Applications
Parts in Sliding or Rolling Contact with Another Surface
Base Material
General Contact Conditions
Surface-Engineering Options
Specific Contact Conditions
Parts in Low-Load Sliding Contact with an Abrasive Product
Base Material
Specific Applications
Parts in High-Load Sliding or Erosion with an Abrasive
Product
Base Material
Surface-Engineering Options
Parts in Contact with Another Engineering Component in
the Presence of an Abrasive and Corrosion Product or
Environment
Base Material
Surface-Engineering Options
Preprocessing and Postprocessing Heat Treatment
Coating Thickness, Case Depth, and Component Distortion
Considerations
Surface Roughness and Finishing
General Design Principles Related to Surface Engineering
Design Guidelines for Surface Preparation Processes
Design Guidelines for Organic Coating Processes
Design Guidelines for Inorganic Coating Processes
Other Important Considerations for the Design Engineer
Glossary .....••....•.....•............................
Index ..•....•........................................
vi
195
196
197
197
197
197
199
199
199
200
200
200
200
201
201
202
202
203
205
206
206
207
208
208
208
208
209
209
209
210
213
213
218
219
222
226
231
257
Preface
Corrosion, wear, or the combined effects of these destructive failure
modes cost industrial economies hundreds of billions of dollars each year.
One of the more effective means of mitigating damage due to corrosion
and wear is to treat, or "engineer," the surface so that it can perform functions that are distinct from those functions required from the bulk of the
material. For example, a gear must be tough and fatigue resistant yet have
a surface that resists wear. For applications requiring only a moderate degree of impact strength, fatigue resistance, and wear resistance, a higher
carbon through-hardening steel may be sufficient. For more severe conditions, however, a surface hardened steel may have to be used. What are the
options? Should the gear be flame or induction hardened, carburized or nitrided, or would high-energy processes such as laser- or electron-beam
hardening be more appropriate? As a second example, consider the use of
steels for various outdoor structural applications. Steel is popular because
it is inexpensive, strong, and easily fabricated. Unfortunately steel is
highly susceptible to severe corrosion in many environments and must be
coated to achieve a satisfactory service life. Once again there are a variety
of options. Should the component be painted, hot dip galvanized or aluminized, electroplated, thermally sprayed, or clad with a more corrosion
resistant material? For large steel components, such as bridge members,
size, weight, and handling problems may limit the type of surface treatment considered. Finally, take into consideration parts that require wearresistant, thin-film coatings. Can more conventional chromium or hard
nickel electroplating be used, or will harder coatings deposited by vapor
deposition techniques or ion implantation be required? Will processing
time or temperature be a factor in coating selection?
From the above discussion, it is apparent that engineers are faced with
a bewildering number of choices when selecting the appropriate surface
engineering treatment for a specific corrosion and/or wear application.
But where does one start? Where can a design engineer find practical
guidelines to aid in the selection process? The answers to these questions
vii
lie within Surface Engineering for Corrosion and Wear Resistance. In addition to devoting an entire chapter to process comparisons (see Chapter
7), this book contains dozens of useful tables and figures that compare
surface treatment thickness and hardness ranges; abrasion and corrosion
resistance; processing time, temperature, and pressure; costs; distortion
tendencies; and other surface treatment characteristics that must be considered when choosing the right coating for the job.
The starting point for this publication was an excellent overview published by the Institute of Materials (10M) entitled "Surface Engineering to
Combat Wear and Corrosion: A Design Guide," which was written by
Keith Stevens (A.T. Poeton Ltd.). Assisting 10M in the project was AEA
Technology plc, and their National Centre of Tribology located in Risley,
United Kingdom. The 10M booklet presents a concise methodology for
understanding corrosion and wear problems and the many factors that
must be considered before selecting a surface treatment. Material from the
10M design guide can be found primarily in Chapter 7, "Process Comparisons," and Chapter 8, "Practical Design Guidelines for Surface Engineering." Special thanks are due to Stephen Harmer, the editor of the 10M
"Design Guide" series, who also reviewed several key chapters, and Bill
Jackson, Head of Publishing for 10M, who worked out the copublishing
agreement with Scott Henry, Assistant Director of Reference Publications
for ASM International.
Other key contributions for this book originated from Volumes 4, Heat
Treating, 5, Surface Engineering, 13, Corrosion, 18, Friction, Lubrication, and Wear Technology, and 20, Materials Selection and Design, of the
ASM Handbook series and from the Metals Handbook Desk Edition, Second Edition. Of particular note are articles authored by Arnold R. Marder
(Lehigh University) and Eric W. Brooman (Concurrent Technologies Corporation) originally published in Volume 20 of the ASM Handbook. These
are acknowledged at the conclusions of Chapters 4, 5, 6, and 8. Tabular
data comparing various surface engineering processes were also adapted
from the ASM Materials Engineering Institute course "Surface Engineering Processes for Wear and Corrosion" developed by Ralph B. Alexander
(R.B. Alexander & Associates).
Joseph R. Davis
Davis & Associates
Chagrin Falls, Ohio
viii
CHAPTER
1
Introduction to Surface
Engineering for
Corrosion and Wear
Resistance
SURFACE ENGINEERING is a multidisciplinary activity intended to
tailor the properties of the surfaces of engineering components so that
their function and serviceability can be improved. The ASM Handbook defines surface engineering as "treatment of the surface and near-surface
regions of a material to allow the surface to perform functions that are
distinct from those functions demanded from the bulk of the material"
(Ref 1). The desired properties or characteristics of surface-engineered
components include:
•
•
•
•
•
•
•
Improved corrosion resistance through barrier or sacrificial protection
Improved oxidation and/or sulfidation resistance
Improved wear resistance
Reduced frictional energy losses
Improved mechanical properties, for example, enhanced fatigue or
toughness
Improved electronic or electrical properties
Improved thermal insulation
Improved aesthetic appearance
As indicated in Table 1, these properties can be enhanced metallurgically,
mechanically, chemically, or by adding a coating.
The bulk of the material or substrate cannot be considered totally independent of the surface treatment. Most surface processes are not limited
to the immediate region of the surface, but can involve the substrate by
2 / Surface Engineering for Corrosion and Wear Resistance
Table 1
Surface engineering options and property benefits
Primary property benefits
Surface treatment/coating type
Changing the surface metallurgy
Localized surface hardening (flame, induction,
and electron-beam hardening)
Laser melting
laser,
Shot peening
Improved wear resistance
through the development
of a hard martensitic
surface
Improved wear resistance through grain refinement and the formation of fine
dispersions of precipitates
Improved fatigue strength due to compressive stresses induced on the exposed
surface, also relieves tensile stresses that contribute to stress-corrosion cracking
Changing the surface chemistry
Phosphate
chemical conversion
coatings
Chromate chemical conversion coatings
Black oxide chemical conversion coatings
Anodizing (electrochemical conversion coating)
Steam treating
Carburizing
Nitriding
Carbonitriding
Ferritic nitrocarburizing
Diffusion (pack cementation)
Diffusion (pack cementation)
Diffusion (pack cementation)
Boronizing (boriding)
Ion implantation
Laser alloying
chromizing
aluminizing
siliconizing
Used primarily on steels for enhanced corrosion resistance, increased plating or paint
adhesion, and for lubricity (e.g., to increase the formability of sheet metals)
Enhanced bare or painted corrosion resistance, improved adhesion of paint or
other organic finishes, and provides the metallic surface with a decorative finish
Used for decorative applications, e.g., the "bluing" on steel gun barrels
Used primarily for aluminum for increased corrosion resistance, improved decorative
appearance, increased abrasion resistance (hard anodizing), improved paint adhesion, and improved adhesive bonding (higher bond strength and durability)
Used on ferrous powder metallurgy parts to increase wear resistance and transverse
rupture strength
Used primarily for steels for increased resistance to wear, bending fatigue, and
rolling-contact fatigue
Used primarily for steels for improved wear resistance, increased fatigue resistance,
and improved corrosion resistance (except stainless steels)
Used primarily for steels for improved wear resistance
Improved antiscuffing characteristics of ferrous alloys
Improved molten-salt hot corrosion
Improved oxidation resistance, sulfidation resistance, and carburization resistance
Improved oxidation resistance
Improved wear resistance, oxidative wear, and surface fatigue
Improved friction and wear resistance for a variety of substrates
Improved wear resistance
Adding a surface layer or coating
Organic coatings (paints and polymeric or elastomeric
coatings and linings)
Ceramic coatings (glass linings, cement linings,
and porcelain enamels)
Slip/sinter ceramic coatings
Hot-dip galvanizing (zinc coatings)
Hot-dip aluminizing
Hot-dip lead-tin alloy-coatings (teme coatings)
Tin plate (continuous electrodeposition)
Zinc-nickel alloy plate (continuous electrodeposition)
Electroplating
Electroless
plating
Mechanical plating
Weld overlays
Thermal spraying
Cladding (roll bonding, explosive bonding, hot
isostatic pressing, etc.)
Laser cladding
Carbide (salt bath) diffusion
Chemical vapor deposition (CVD)
Physical vapor deposition
(PVD)
Improved corrosion resistance,
wear resistance,
and aesthetic appearance
Improved corrosion resistance
Improved wear resistance and heat resistance
Improved corrosion resistance via sacrificial protection of steel substrate
Improved corrosion and oxidation resistance of steel substrate
Improved corrosion resistance of steel substrate
Improved corrosion resistance of steel substrate
Improved corrosion resistance of steel substrate
Depending on the metal or metals being electrodeposited, improved corrosion
resistance (e.g., nickel-chromium multilayer coatings, and cadmium and zinc sacrificial coatings), wear resistance (e.g., hard chromium coatings), electrical properties (e.g., copper and silver), and aesthetic appearance (e.g., bright nickel or decorative chromium plating)
Improved corrosion resistance (nickel-phosphorus)
and wear resistance
(nickel-phosphorus
and nickel-boron)
Improved corrosion resistance
Improved wear resistance (hardfacing alloys) and corrosion resistance (stainless steel
or nickel-base overlays) and dimensional restoration (buildup alloys)
Primarily used for improved wear resistance (many coating systems including ceramics
and cermets), but also used for improved corrosion resistance (aluminum, zinc, and
their alloys) and oxidation resistance (e.g., MCrAIY), thermal barrier protection
(partially stabilized zirconia), electrically conductive coatings (e.g., copper and silver), and dimensional restoration
Improved corrosion resistance
Improved wear resistance
Used primarily for steels for improved wear resistance in tooling applications
Improved wear (e.g., tools and dies), erosion, and corrosion resistance; also used for
epitaxial growth of semiconductors
Improved wear (e.g., tools and dies) and corrosion resistance, improved optical and
electronic properties, and decorative applications
Introduction to Surface Engineering for Corrosion and Wear Resistance / 3
exposure to either a thermal cycle or a mechanical stress. For example,
diffusion heat treatment coatings (e.g., carburizing/nitriding) often have
high-temperature thermal cycles that may subject the substrate to temperatures that cause phase transformations and thus property changes, or
shot-peening treatments that deliberately strain the substrate surface to induce improved fatigue properties. It is the purpose of this book, and in
particular Chapters 4 to 6, to review information on surface treatments
that improve service performance so that metallurgists, chemists" mechanical engineers, and design engineers may consider surface-engineered components as an alternative to more costly materials.
Surface Engineering to Combat Corrosion and Wear
The Economic Effects of Corrosion and Wear. The progressive deterioration, due to corrosion and wear, of metallic surfaces in use in major industrial plants ultimately leads to loss of plant efficiency and at worst a shutdown. Corrosion and wear damage to materials, both directly and indirectly,
costs the United States hundreds of billions of dollars annually. For example, corrosion of metals costs the U.S. economy almost $300 billion per year
at current prices. This amounts to about 4.2% of the gross national product.
However, about 40% of the total cost could be avoided by proper corrosion prevention methods. Table 2 provides a breakdown of the cost of
metallic corrosion in the United States. Similar studies on wear failures
have shown that the wear of materials costs the U.S. economy about $20
billion per year (in 1978 dollars) compared to about $80 billion annually
(see Table 2) for corrosion during the same period. Table 3 illustrates the
extent of wear failures by various operations within specific industrial
segments. Highway vehicles alone use annually 14,600 X 1012 Btu/ton of
energy represented in lost weight of steel and 18.6% of this energy could
be saved through effective wear-control measures.
Table 2
Cost of metallic corrosion in the United States
Billions of U.S. dollars
Industry
All industries
Total
Avoidable
Motor vehicles
Total
Avoidable
Aircraft
Total
Avoidable
Other industries
Total
Avoidable
Source: Ref 2
1975
1995
82.0
33.0
296.0
104.0
31.4
23.1
94.0
65.0
3.0
0.6
13.0
3.0
47.6
9.3
189.0
36.0
4 / Surface Engineering for Corrosion and Wear Resistance
Table 3 Industrial operations with significant annual
wear economic consequences
Industry
Utilities (28% total U.S.
consumption)
Transportation (26% total
U.S. consumption)
Mining
Agriculture
Primary metals
Operation
Loss mass(a), 1012 Btu
Seals
Accessories
Bearings
Reliability
Total
Brakes
Valve trains
Piston ring assemblies
Transmission
Bearings
Gears
Total
Ore processing
Surface mining
Shaft mining
Drilling
Total
Tillage
Planting
Total
Hot rolling
Cold rolling
Total
185
120
55
145
505
(b)
(b)
(b)
(b)
~)
(b)
(b)
22.80
13.26
10.70
5.58
52.34
16.85
2.47
19.32
14.30
0.14
14.44
(a) Assumes 19.2 X 106 Btu per ton of energy represented in lost weight of steel.
(b) Lost mass not estimated. Source: Ref 3
Corrosive Wear. Complicating matters is the fact that the combined effects of wear and corrosion can result in total material losses that are much
greater than the additive effects of each process taken alone, which indicates a synergism between the two processes. Although corrosion can
often occur in the absence of mechanical wear, the opposite is rarely true.
Corrosion accompanies the wear process to some extent in all environments, except in vacuum and inert atmospheres. Corrosion and wear often
combine to cause aggressive damage in a number of industries, such as
mining, mineral processing, chemical processing, pulp and paper production, and energy production. Corrosion and wear processes involve many
mechanisms, the combined actions of which lead to the mutual reinforcement of their effectiveness. As listed in Table 4, 17 synergistic relationships among abrasion, impact, and corrosion that could significantly increase material degradation in wet and aqueous environments have been
identified.
The combined effects of corrosion and wear can also lead to galvanic
corrosion in some applications, such as crushing and grinding (comminution) of mineral ores. Wear debris and corrosion products that are formed
during comminution affect product quality and can adversely affect subsequent benefication by altering the chemical and electrochemical properties of the mineral system (Ref 5-8). Electrochemical interactions between minerals and grinding media can occur, causing galvanic coupling
that leads to increased corrosion wear. More detailed information on galvanic corrosion can be found in Chapter 2.
Introduction to Surface Engineering for Corrosion and Wear Resistance / 5
Methods to Control Corrosion. Owing to its many favorable characteristics, steel is well suited and widely used for a broad range of engineering applications and is referenced here to demonstrate the various corrosion-control steps that can be considered. Steel has a variety of excellent
mechanical properties, such as strength, toughness, ductility, and dent resistance. Steel also offers good manufacturability, including formability,
weldability, and paintability. Other positive factors include its availability,
ferromagnetic properties, recyclability, and cost. Because steel is susceptible to corrosion in the presence of moisture, and to oxidation at elevated
temperatures, successful use of these favorable characteristics generally
requires some form of protection.
Methods of corrosion protection employed to protect steel include:
•
•
•
•
Altering the metal by alloying, that is, using a more highly alloyed and
expensive stainless steel rather than a plain carbon or low-alloy steel
Changing the environment by desiccation or the use of inhibitors
Controlling the electrochemical potential by the application of cathodic or anodic currents, that is, cathodic and anodic protection
Applying organic, metallic, or inorganic (glasses and ceramics) coatings
Application of corrosion-resistant coatings is one of the most widely used
means of protecting steel. As shown in Table 1, there are a wide variety
of coatings to choose from, and proper selection is based on the component size and accessibility, the corrosive environment, the anticipated
Table 4
Synergistic relationships between wear and corrosion mechanisms
Abrasion
Removes protective oxidized metal and polarized coatings to expose unoxidized metal, in addition to removing metal particles.
Forms microscopic grooves and dents for concentration cell corrosion.
Increases microscopic surface area exposed to corrosion.
Removes strain-hardened surface layers.
Cracks brittle metal constituents forming sites for impact hydraulic splitting.
Plastic deformation by high-stress metal-mineral contact causes strain hardening and susceptibility to chemical attack.
Corrosion
Produces pits that induce microcracking.
Microcracks at pits invite hydraulic splitting during impact.
Roughens surface, reducing energy needed to abrade away metal.
May produce hydrogen with subsequent absorption and cracking in steel.
Selectively attacks grain boundaries and less noble phases of multiphase microstructures,
weakening
adjacent metal.
Impact
Plastic deformation makes some constituents more susceptible to corrosion.
Cracks brittle constituents, tears apart ductile constituents to form sites for crevice corrosion, hydraulic splitting.
Supplies kinetic energy to drive abrasion mechanism.
Pressurizes mill water to cause splitting, cavitation, and jet erosion of metal and protective oxidized material.
Pressurizes mill water and gases to produce unknown temperatures, phase changes, and decomposition or reaction products from ore and water
constituents.
Heats ball metal, ore, fluids to increase corrosive effects.
Source: Ref 4
6 / Surface Engineering for Corrosion and Wear Resistance
Nitrocarburizing
vapor d Position
1
vapor deposition
l
Resin or laquer bonding
Ion implantation
I
10-5
10-4
10-3
10-2
0.1
10
Thickness, mm
Fig. 1
Approximate
thickness of various surface engineering treatments
temperatures, component distortion, the coating thickness attainable
(Fig. 1), and costs. Many of these selection criteria are addressed in Chapters 6 to 8 in this book.
Painting is probably the most widely used engineering coating used to
protect steel from corrosion. There are a wide variety of coating formulations that have been developed for outdoor exposure, marine atmospheres,
water immersion, chemical fumes, extreme sunlight, high humidity, and
moderately high temperatures (less than about 200°C, or 400 OF).
The most widely used corrosion-resistant metallic coatings are hotdipped zinc, zinc-aluminum, and aluminum coatings. These coatings exhibit excellent resistance to atmospheric corrosion and are widely used in
the construction, automobile, utility, and appliance industries.
Other important coating processes for steels include electroplating,
electroless plating, thermal spraying, pack cementation aluminizing (for
high-temperature oxidation resistance), and cladding (including weld
cladding and roll-bonded claddings). Applications and corrosion performance of these coatings are described in Chapter 6 in this book.
Methods to Control Wear. As is described in Chapter 3 in this book,
there are many types of wear, but there are only four main types of wear
systems (tribosystems) that produce wear and six basic wear control steps
(Ref 9). The four basic tribosystems are:
Introduction to Surface Engineering for Corrosion and Wear Resistance / 7
•
•
•
•
Relatively smooth solids sliding on other smooth solids
Hard, sharp substances sliding on softer surfaces
Fatigue of surfaces by repeated stressing (usually compressive)
Fluids with or without suspended solids in motion with respect to a
solid surface
As shown in Fig. 2, the wear that occurs in these tribosystems can be addressed by coatings or by modifications to the substrate metallurgy or
chemistry.
The six traditional techniques applied to materials to deal with wear
produced in the preceding tribosystems include:
•
•
•
•
Separate conforming surfaces with a lubricating film (see Chapter 3 in
this book).
Make the wearing surface hard through the use of hardfacing, diffusion heat treatments, hard chromium plating, or more recently developed vapor deposition techniques or high-energy processes (e.g., ion
implantation).
Make the wearing surface resistant to fracture. Many wear processes
involve fracture of material from a surface; thus toughness and fracture resistance play a significant role in wear-resistant surfaces. The
use of very hard materials such as ceramics, cemented carbides, and
hard chromium can lead to fracture problems that nullify the benefits
of the hard surface.
Make the eroding surface resistant to corrosion. Examples include the
use of cobalt-base hardfacing alloys to resist liquid erosion, cavitation,
and slurry erosion; aluminum bronze hardfacing alloys to prevent cavitation damage on marine propellers or to repair props that have
Wear-causing
effect/1
Coatings to reduce wear
• Polymers/elastomers
• Electrochemical
(plating, etc.)
• Chemical (CVD,
electro less plating)
• Thermal spraying
• Fusion welding
• Thin films (PVD,
sputtering, ion plating)
iD~
I
e
~~
----,
I
I
I
I
• Wear tiles
• Cladding (cast, explosion,
hot rolling)
Substrate treatments to
reduce wear
• Through hardening
• Surface hardening (flame,
induction, EB, laser)
• Diffusion of a hardening
species (carburizing,
nitriding, etc.)
o Laser/EB alloying
• Ion implantation
• Work hardening
• Lubricants
Tribosystem
Fig. 2
Surface
wear
Surface engineering processes used to prevent wear. CVD, chemical vapor deposition; PVD,
physical vapor deposition; EB, electron beam
8 / Surface Engineering for Corrosion and Wear Resistance
o
e
suffered cavitation damage; nickel-base hardfacing alloys to resist
chemical attack; and epoxy-filled rebuilding cements used to resist
slurry erosion in pumps.
Choose material couples that are resistant to interaction in sliding
(metal-to-metal wear resistance). Hardfacing alloys such as cobaltbase and nickel-chromium-boron
alloys have been used for many
years for applications involving metal-to-metal wear. Other surfaceengineering options include through-hardened tool steels, diffusion
(case )-hardened surfaces, selective surface-hardened alloy steels, and
some platings.
Make the wearing surface fatigue resistant. Rolling-element bearings, gears, cams, and similar power-transmission
devices often
wear by a mechanism of surface fatigue. Repeated point or line contact stresses can lead to subsurface cracks that eventually grow to
produce surface pits and eventual failure of the device. Prevention is
possible through the use of through-hardened steels, heavy casehardened steels, and flame-, induction-, electron beam-, or laserhardened steels.
More details on these surface-engineering techniques can be found in
Chapters 5 through 8 in this book.
Material/Process Selection (Ref 10). Faced with the wide range of
possibilities indicated in Table 1 and the discussions on "Methods to Control Corrosion" and "Methods to Control Wear," selection of surface engiPredict working
environment from
consideration of design _
Analyze service failures
to assist selection of
better materials
Identify material
requirements for
~
structure and surface
Proceed with one-piece
construction (see note .....below)
Yes ~Consider
one-piece ~
construction
No
Select substrate material
to suit strength, heat, and
corrosion needs
:-----l
~
Note: One-piece construction is often least
expensive for small parts as some surfacing alloys
are available as castings machined to finished size
or as powder metallurgical parts.
Select surfacing material
to suit requirements
I
t
Select from surfacing processes
suitable for chosen material and job,
Reconsider
(must satisfy needs for coating density,
materials
thickness, dilution, etc.)
~
Decide if chosen process
suits substrate material ~
and design (adhesion,
HAZ, access, distortion,
etc.)
~s
~No
+
t
\
Identify quality assurance --~
and control needs
Fig. 3
Yes ~
t
I
\....
••. None
Reconsider process
and/or material
t
Decide manufacturing
details, procedures,
health and safety
requirements, etc.
-------I~
Finalize choice of
materials and process
Checklist for surface engineering material/process selection. HAZ, heat-affected zone
Introduction to Surface Engineering for Corrosion and Wear Resistance / 9
neering material and process may seem difficult, but it is normally
straightforward. Often there are constraints placed on the choice because
of availability (e.g., laser melting and/or alloying are not widely used, and
these processes can only be obtained by a special arrangement with laser
job shops). In many cases there is a precedent, but when considering a
new problem it helps to follow a checklist of the type shown in Fig. 3.
The sequence of decisions to be made covers several fundamental
points. The first is the need to be clear about service conditions, based on
experience or plant data. This is the key to material selection. The second
decision is the choice of application process for the material. This involves
the question of compatibility with the coating material; that is, not all materials can be applied by all processes. A further question of compatibility
arises between both material and process with the substrate, for example,
whether distortion from high-temperature processes be tolerated. All these
issues are covered in subsequent chapters in this book (see, in particular,
Chapters 7 and 8).
References
1. C.M. Cotell and I.A. Sprague, Preface, Surface Engineering, Vol 5,
ASM Handbook, ASM International, 1994, p v
2. Economic Effects of Metallic Corrosion in the United States, Battelle
Columbus Laboratories and the National Institute of Standards and
Technology, 1978 and Battelle updates in 1995
3. "Tribological Sinks in Six Major Industries," Report Number PNL5535, Sept 1985, Pacific Northwest Laboratory, Richland, WA, operated for the U.S. Department of Energy by Battelle Memorial Institute
(NTIS No. DE86000841)
4. D.J. Dunn. Metal Removal Mechanisms Comprising Wear in Mineral
Processing, Wear of Materials, K.C. Ludema, Ed., American Society
of Mechanical Engineers, 1985, p 501-508
5. R.L. Pozzo and I. Iwasaki, Pyrite-Pyrrhotite Grinding Media Interactions and Their Effects on Media Wear and Flotation, J. Electrochem.
Soc., Vol 136 (No.6), 1989, p 1734-1740
6. R.L. Pozzo and I. Iwasaki, Effect of Pyrite and Pyrrhotite on the Corrosive Wear of Grinding Media, Miner. Me tall. Process., Aug 1987, p
166-171
7. K.A Natarajan, S.C. Riemer, and I. Iwasaki, Influence of Pyrrhotite on
the Corrosive Wear of Grinding Balls in Magnetite Ore Grinding, Int.
J. Miner. Process., Vol 13 1984, p 73-81
8. R.L. Pozzo and I. Iwasaki, An Electro-chemical Study of PyrrhotiteGrinding Media Interaction Under Abrasive Conditions, Corrosion,
Vol 43 (No.3), 1987, p 159-169
10 / Surface Engineering for Corrosion and Wear Resistance
9. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, Inc., 1988, p 6-10
10. Engineering Coatings-Design
and Application, 2nd ed., S. Grainger
and J. Blunt, Ed., Woodhead Publishing Ltd., 1999, P 7
CHAPTER
2
Principles of Corrosion
CORROSION of metal is a chemical or electrochemical process in
which surface atoms of a solid metal react with a substance in contact with
the exposed surface. Usually the corroding medium is a liquid substance,
but gases and even solids can also act as corroding media. In some instances, the corrodent is a bulk fluid; in others, it is a film, droplets, or a
substance adsorbed on or absorbed in another substance.
All structural metals corrode to some extent in natural environments
(e.g., the atmosphere, soil, or waters). Bronze, brass, most stainless steels,
zinc, and pure aluminum corrode so slowly in service conditions that long
service life is expected without protective coatings. Corrosion of structural grades of cast iron and steel, the 400 series stainless steels, and some
aluminum alloys, however, proceeds rapidly unless the metal is protected
against corrosion. As described in Chapter 1, corrosion of metals is of particular concern because annual losses in the United States attributed to
corrosion amount to hundreds of billions of dollars.
Although emphasis in this Chapter has been placed on irons and steels,
the electrochemical corrosion basics and the forms of corrosion described
are applicable to all metallic materials. For more detailed information on
the corrosion resistance of various metals and their alloys, the reader
should consult the selected references listed at the conclusion of this
Chapter, as well as Corrosion, Vol 13, of the ASM Handbook or Corrosion: Understanding the Basics, published by ASM International in 2000.
Electrochemical Corrosion Basics
Electrochemical corrosion in metals in a natural environment, whether
atmosphere, in water, or underground, is caused by a flow of electricity
from one metal to another, or from one part of a metal surface to another
part of the same surface where conditions permit the flow of electricity.
12 / Surface Engineering for Corrosion and Wear Resistance
Metallic conductor between
the anode and the cathode
Oxygen or other
depolarizer in
electrolyte
Oxidation reaction
occurs at anode
Electrolyte, water containing
conductive salts
Reduction reaction
occurs at cathode
Current flow through
the elsctrolvte
Fig. 1
Simple electrochemical
cell showing the components
necessary for corrosion
For the flow of energy to take place, either a moist conductor or an
electrolyte must be present. An electrolyte is an electricity-conducting solution containing ions, which are atomic particles or radicals bearing an
electrical charge. Charged ions are present in solutions of acids, alkalis,
and salts. The presence of an electrolyte is necessary for corrosion to
occur. Water, especially salt water, is an excellent electrolyte.
Electricity passes from a negative area to a positive area through the
electrolyte. For corrosion to occur in metals, one must have (a) an electrolyte, (b) an area or region on a metallic surface with a negative charge,
(c) a second area with a positive charge, and (d) an electrically conductive
path between (b) and (c). These components are arranged to form a closed
electrical circuit. In the simplest case, the anode would be one metal, such
as iron, the cathode another, perhaps copper, and the electrolyte might or
might not have the same composition at both anode and cathode. The
anode and cathode could be of the same metal under conditions described
later in this article.
The cell shown in Fig. 1 illustrates the corrosion process in its simplest
form. This cell includes the following essential components: (a) a metal
anode, (b) a metal cathode, (c) a metallic conductor between the anode and
the cathode, and (d) an electrolyte in contact with the anode and the cathode. If the cell were constructed and allowed to function, an electrical current would flow through the metallic conductor and the electrolyte, and if
the conductor were replaced by a voltmeter, a potential difference between
the anode and the cathode could be measured. The anode would corrode.
Chemically, this is an oxidation reaction. The formation of hydrated red
iron rust by electrochemical reactions may be expressed as follows:
Principles of Corrosion / 13
4Fe ~ 4Fe2+
4Fe
+ 8 e:
+ 302 + H20 ~ 2 Fe203"H20
+ 202 + 4H20 ~ 4Fe (OH)2
4Fe (OH)2 + 02 ~ 2Fe203·H20 + 2H20
(Eq 1)
4Fe
(Eq 2)
During metallic corrosion, the rate of oxidation equals the rate of reduction. Thus, a nondestructive chemical reaction, reduction, would proceed simultaneously at the cathode. In most cases, hydrogen gas is produced on the cathode. When the gas layer insulates the cathode from the
electrolyte, current flow stops, and the cell is polarized. However, oxygen
or some other depolarizing agent is usually present to react with the hydrogen, which reduces this effect and allows the cell to continue to function.
Contact between dissimilar metallic conductors or differences in the
concentration of the solution cause the difference in potential that results
in electrical current. Any lack of homogeneity on the metal surface or its
environment may initiate attack by causing a difference in potential, and
this results in localized corrosion. The metal undergoing electrochemical
corrosion need not be immersed in a liquid but may be in contact with
moist soil or may have moist areas on the metal surface.
Corrosive Conditions
If oxygen and water are both present, corrosion will normally occur on
iron and steel. Rapid corrosion may take place in water.fhe rate of corrosion being accelerated by several factors such as: (a) the velocity or the
acidity of the water, (b) the motion of the metal, (c) an increase in temperature or aeration, and (d) the presence of certain bacteria. Corrosion
can be retarded by protective layers or films consisting of corrosion products or adsorbed oxygen. High alkalinity of the water also retards the rate
of corrosion on steel surfaces. Water and oxygen remain the essential factors, however, and the amount of corrosion is generally controlled by one
or the other. For example, corrosion of steel does not occur in dry air and
is negligible when the relative humidity of the air is below 30% at normal
or lower temperatures. This is the basis for prevention of corrosion by dehumidification.
Water can readily dissolve a small amount of oxygen from the atmosphere, thus becoming highly corrosive. When the free oxygen dissolved in
water is removed, the water becomes practically noncorrosive unless it
becomes acidic or anaerobic bacteria incite corrosion. If oxygen-free
water is maintained at a neutral pH or at slight alkalinity, it is practically
14 / Surface Engineering for Corrosion and Wear Resistance
noncorrosive to structural steel. Steam boilers and water supply systems
are effectively protected by deaerating the water. Additional information
on corrosion in water can be found in Ref 1.
Soils. Dispersed metallic particles or bacteria pockets can provide a natural electrical pathway for buried metal. If an electrolyte is present and the
soil has a negative charge in relation to the metal, an electrical path from
the metal to the soil will occur, resulting in corrosion. Differences in soil
conditions, such as moisture content and resistivity, are commonly responsible for creating anodic and cathodic areas (Fig. 2). Where a difference exists in the concentration of oxygen in the water or in moist soils in
contact with metal at different areas, cathodes develop at points of relatively high-oxygen concentrations and anodes at points of low concentration. Further information on corrosion in soils is available in Ref 2.
Chemicals. In an acid environment, even without the presence of oxygen, the metal at the anode is attacked at a rapid rate. At the cathode,
atomic hydrogen is released continuously, to become hydrogen gas. Corrosion by an acid can result in the formation of a salt, which slows the reaction because the salt formation on the surface is then attacked.
Corrosion by direct chemical attack is the single most destructive force
against steel surfaces. Substances having chlorine or other halogens in their
composition are particularly aggressive. Galvanized roofing has been
known to corrode completely within six months of construction, the building being downwind of an aluminum ingot plant where fluorides were always present in the atmosphere. Consequently, galvanized steel should not
have been specified. Selection of materials and evaluation of service conditions are extremely important in combating corrosion. The response of
various materials to chemical environments is addressed in Ref 3 and 4.
Atmospheric corrosion differs from the corrosion action that occurs in
water or underground, because sufficient oxygen is always present. In at-
~"r-!/:.~~\:r.·:,:,~::0..:.:.~
-,::l f4.~~1 .-, I ~
° ° °t
Oxygen diffusing into earth
from ground surface
Cathodic area (steel :;'
2 2 2
at top of pipe)
- :~11>! .. 0.
:u.~'~
'(~';i.~~
. " ••,:\
Electrolyte
.f-current
'%;~
1 (soil with
ground water high in
oxygen content)
flow
•.
~".:"
I
.
Electrolyte 2 (soil with
ground water deficient
in oxygen content)
Fe2+ (rust)
Fig. 2
A metal pipe buried in moist soil forming a corrosion cell. A difference
in oxygen content at different levels in the electrolyte will produce a
difference of potential. Anodic and cathodic areas will develop, and a corrosion
cell, called a concentration cell, will form.
Principles of Corrosion / 15
mospheric corrosion, the formation of insoluble films and the presence of
moisture and deposits from the atmosphere control the rate of corrosion.
Contaminants such as sulfur compounds and salt particles can accelerate
the corrosion rate. Nevertheless, atmospheric corrosion occurs primarily
through electrochemical means and is not directly caused by chemical attack. The anodic and cathodic areas are usually quite small and close together so that corrosion appears uniform, rather than in the form of severe
pitting, which can occur in water or soil. A more detailed discussion on atmospheric corrosion can be found in Ref 5.
Forms of Corrosion
The differing forms of corrosion can be divided into the following eight
categories based on the appearance of the corrosion damage or the mechanism of attack:
•
•
•
•
•
•
•
Uniform or general corrosion
Galvanic corrosion
Pitting corrosion
Crevice corrosion, including corrosion under tubercles or deposits, filiform corrosion, and poultice corrosion
Erosion-corrosion, including cavitation erosion and fretting corrosion
Intergranular corrosion, including sensitization and exfoliation
Dealloying
Environmentally assisted cracking, including stress-corrosion cracking (SCC), corrosion fatigue, and hydrogen damage (including hydrogen embrittlement, hydrogen-induced blistering, high-temperature hydrogen attack, and hydride formation)
Figure 3 illustrates schematically some of the most common forms of corrosion. More detailed information pertaining to recognition and prevention of these forms of corrosion can be found in Ref 6 and 7.
Uniform Corrosion
General Description. Uniform or general corrosion, as the name implies, results in a fairly uniform penetration (or thinning) over the entire
exposed metal surface. The general attack results from local corrosion-cell
action; that is, multiple anodes and cathodes are operating on the metal
surface at any given time. The location of the anodic and cathodic areas
continues to move about on the surface, resulting in uniform corrosion.
Uniform corrosion often results from atmospheric exposure (especially
polluted industrial environments); exposure in fresh, brackish, and salt
waters; or exposure in soils and chemicals.
16 / Surface Engineering for Corrosion and Wear Resistance
More noble
metal
No corrosion
Uniform
Galvanic
Flowing
corrodent
Erosion
Cyclic
movement
~
Fretting
.I
Metal or
nonmetal
Crevice
Tensile stress
Pitting
Exfoliation
Fig. 3
Dealloying
Intergranular
Stress-corrosion
cracking
Corrosion
fatigue
Schematics of the common forms of corrosion
Metals Affected. All metals are affected by uniform corrosion, although materials that form passive films, such as stainless steels or nickelchromium alloys, are normally subjected to localized forms of attack. The
rusting of steel, the green patina formation on copper, and the tarnishing
of silver are typical examples of uniform corrosion. In some metals, such
as steel, uniform corrosion produces a somewhat rough surface by removing a substantial amount of metal, which either dissolves in the environment or reacts with it to produce a loosely adherent, porous coating of
corrosion products. In such reactions as in the tarnishing of silver in air,
the oxidation of aluminum in air, or attack on lead in sulfate-containing
environments, thin, tightly adherent protective films are produced, and the
metal surface remains smooth.
Prevention. Uniform corrosion can be prevented or reduced by proper
materials selection, the use of coatings or inhibitors, or cathodic protection. These corrosion prevention methods can be used individually or in
combination.
Galvanic Corrosion
General Description. The potential available to promote the electrochemical corrosion reaction between dissimilar metals is suggested by the
galvanic series, which lists a number of common metals and alloys
arranged according to their tendency to corrode when in galvanic contact
(Table 1). Metals close to one another on the table generally do not have
a strong effect on each other, but the farther apart any two metals are separated, the stronger the corroding effect on the one higher in the list. It is
possible for certain metals to reverse their positions in some environments, but the order given in Table 1 is maintained in natural waters and
the atmosphere. The galvanic series should not be confused with the sim-
Principles of Corrosion / 17
Table 1
Galvanic series in seawater at 25 0(77
OF)
Corroded end (anodic, or least noble)
Magnesium
Magnesium alloys
Zinc
Galvanized steel or galvanized wrought iron
Aluminum alloys 5052,3004,3003,
1100,6053, in this order
Cadmium
Aluminum alloys 2117,2017,2024,
in this order
Low-carbon steel
Wrought iron
Cast iron
Ni-Resist (high-nickel cast iron)
Type 410 stainless steel (active)
50-50 lead-tin solder
Type 304 stainless steel (active)
Type 316 stainless steel (active)
Lead
Tin
Copper alloy C28000 (Muntz metal, 60% Cu)
Copper alloy C67500 (manganese bronze A)
Copper alloys C46400, C46500, C46600, C46700 (naval brass)
Nickel 200 (active)
Inconel alloy 600 (active)
Hastelloy alloy B
Chlorimet 2
Copper alloy C27000 (yellow brass, 65% Cu)
Copper alloys C44300, C44400, C44500 (admiralty brass)
Copper alloys C60800, C61400 (aluminum bronze)
Copper alloy C23000 (red brass, 85% Cu)
Copper C11000 (ETP copper)
Copper alloys C65100, C65500 (silicon bronze)
Copper alloy C71500 (copper nickel, 30% Ni)
Copper alloy C92300, cast (leaded tin bronze G)
Copper alloy C92200, cast (leaded tin bronze M)
Nickel 200 (passive)
Inconel alloy 600 (passive)
Monel alloy 400
Type 410 stainless steel (passive)
Type 304 stainless steel (passive)
Type 316 stainless steel (passive)
Incoloy alloy 825
Inconel alloy 625
Hastelloy alloy C
Chlorimet 3
Silver
Titanium
Graphite
Gold
Platinum
Protected end (cathodic, or most noble)
ilar electromotive force series, which shows exact potentials based on
highly standardized conditions that rarely exist in nature.
The three-layer iron oxide scale formed on steel during rolling varies
with the operation performed and the rolling temperature. The dissimilarity of the metal and the scale can cause corrosion to occur, with the steel
acting as the anode in this instance. Unfortunately, mill scale is cathodic
to steel, and an electric current can easily be produced between the steel
and the mill scale. This electrochemical action will corrode the steel without affecting the mill scale (Fig. 4).
A galvanic couple may be the cause of premature failure in metal components of water-related structures or may be advantageously exploited.
18 / Surface Engineering for Corrosion and Wear Resistance
Electrolyte
(water)
Cathode
(broken mill
scale)
Fig. 4
Mill scale forming a corrosion cellon
steel
Galvanizing iron sheet is an example of useful application of galvanic action or cathodic protection. Iron is the cathode and is protected against corrosion at the expense of the sacrificial zinc anode. Alternatively, a zinc or
magnesium anode may be located in the electrolyte close to the structure
and may be connected electrically to the iron or steel. This method is referred to as cathodic protection of the structure. Iron or steel can become the
anode when in contact with copper, brass, or bronze; however, iron or steel
corrode rapidly while protecting the latter metals. Also, weld metal may be
anodic to the basis metal, creating a corrosion cell when immersed (Fig. 5).
While the galvanic series (Table 1) represents the potential available to
promote a corrosive reaction, the actual corrosion is difficult to predict.
Electrolytes may be poor conductors, or long distances may introduce
large resistance into the corrosion cell circuit. More frequently, scale formation forms a partially insulating layer over the anode. A cathode having a layer of adsorbed gas bubbles, as a consequence of the corrosion cell
reaction, is polarized. The effect of such conditions is to reduce the theoretical consumption of metal by corrosion. The area relationship between
the anode and cathode may also strongly affect the corrosion rate; a high
ratio of cathode area to anode area produces more rapid corrosion. In the
reverse case, the cathode polarizes, and the corrosion rate soon drops to a
negligible level.
The passivity of stainless steels is attributed to either the presence of a
corrosion-resistant oxide film or an oxygen-caused polarizing effect,
Electrolyte
Fig. 5
(water)
Weld metal forming a corrosion cell on steel. Weld metal may be anodic to steel, creating a corrosion cell when immersed.
Principles of Corrosion / 19
durable only as long as there is sufficient oxygen to maintain the effect,
over the surfaces. In most natural environments, stainless steels will remain in a passive state and thus tend to be cathodic to ordinary iron and
steel. Change to an active state usually occurs only where chloride concentrations are high, as in seawater or reducing solutions. Oxygen starvation also produces a change to an active state. This occurs where the oxygen supply is limited, as in crevices and beneath contamination on
partially fouled surfaces.
Prevention. Galvanic corrosion can be prevented or reduced by proper
materials selection (i.e., select combinations of metals as close together as
possible in the galvanic series), insulating dissimilar metals, applying a
barrier coating to both the anodic (less noble) and cathodic (noble) metal,
applying a sacrificial coating (aluminum, zinc, or cadmium) to the cathodic part, applying nonmetallic films (e.g., anodizing aluminum alloys),
and by providing cathodic protection.
Pitting
General Description. Pitting is a type of localized cell corrosion. It is
predominantly responsible for the functional failure of iron and steel
water-related installations. Pitting may result in the perforation of water
pipe, rendering it unserviceable, even though less than 5% of the total
metal has been lost through rusting. Where confinement of water is not a
factor, pitting causes structural failure from localized weakening while
considerable sound metal still remains.
Pitting develops when the anodic or corroding area is small in relation
to the cathodic or protected area. For example, pitting can occur where
large areas of the surface are covered by mill scale, applied coatings, or
deposits of various kinds and where breaks exist in the continuity of the
protective coating. Pitting may also develop on bare, clean metal surfaces
because of irregularities in the physical or chemical structure of the metal.
Localized, dissimilar soil conditions at the surface of steel can also create
conditions that promote pitting.
Electrical contact between dissimilar materials or concentration cells
(areas of the same metal where oxygen or conductive salt concentrations
in water differ) accelerates the rate of pitting. In closed-vessel structures,
these couples cause a difference of potential that results in an electric current flowing through the water or across the moist steel from the metallic anode to a nearby cathode. The cathode may be copper, brass, mill
scale, or any portion of a metal surface that is cathodic to the more active
metal areas. In practice, mill scale is cathodic to steel and is found to be
a common cause of pitting. The difference of potential generated between
steel and mill scale often amounts to 0.2 to 0.3 V. This couple is nearly
as powerful a generator of corrosion currents as is the copper-steel couple. However, when the anodic area is relatively large compared with the
20 / Surface Engineering for Corrosion and Wear Resistance
cathodic area, the damage is spread out and usually negligible, but when
the anode is relatively small, the metal loss is concentrated and may be
very serious.
On surfaces having some mill scale, the total metal loss is nearly constant as the anode is decreased, but the degree of penetration increases.
Figure 4 shows how a pit forms where a break occurs in mill scale. When
contact between dissimilar materials is unavoidable and the surface is
painted, it is preferred to paint both materials. If only one surface is
painted, it should be the cathode. If only the anode is coated, any weak
points such as pinholes or holidays in the coating will probably result in
intense pitting.
As a pit, perhaps at a break in mill scale, becomes deeper, an oxygen
concentration cell is started by depletion of oxygen in the pit. The rate of
penetration by such pits is accelerated proportionately as the bottom of the
pit becomes more anodic. Fabrication operations may crack mill scale and
result in accelerated corrosion.
Metals Affected. Pitting occurs in most commonly used metals and alloys. Iron buried in the soil corrodes with the formation of shallow pits,
but carbon steels in contact with hydrochloric acid or stainless steels immersed in seawater characteristically corrode with the formation of deep
pits. Aluminum tends to pit in waters containing chloride ions (for example, at stagnant areas), and aluminum brasses are subject to pitting in polluted waters.
Despite their good resistance to general corrosion, stainless steels are
more susceptible to pitting than many other metals. High-alloy stainless
steels containing chromium, nickel, and molybdenum are also more resistant to pitting but are not immune under all service conditions.
Pitting failures of corrosion-resistant alloys, such as Hastelloy C,
Hastelloy G, and Incoloy 825, are relatively uncommon in solutions that
do not contain halides, although any mechanism that permits the establishment of an electrolytic cell in which a small anode is in contact with a
large cathodic area offers the opportunity for pitting attack.
Prevention. Typical approaches to alleviating or minimizing pitting
corrosion include the following:
•
•
o
•
Use defect-free barrier coatings
Reduce the aggressiveness of the environment, for example, chloride
ion concentrations, temperature, acidity, and oxidizing agents
Upgrade the materials of construction, for example, use molybdenumcontaining (4 to 6% Mo) stainless steels, molybdenum + tungsten
nickel-base alloys, overalloy welds, and use corrosion-resistant alloy
linings
Modify the design of the system, for example, avoid crevices and the
formation of deposits, circulate/stir to eliminate stagnant solutions,
and ensure proper drainage
Principles of Corrosion / 21
Crevice Corrosion
General Description. Crevice corrosion is a form of localized attack
that occurs at narrow openings or spaces (gaps) between metal-to-metal or
nonmetal-to-metal components. This type of attack results from a concentration cell formed between the electrolyte within the crevice, which is
oxygen starved, and the electrolyte outside the crevice, where oxygen is
more plentiful. The material within the crevice acts as the anode, and the
exterior material becomes the cathode.
Crevices may be produced by design or accident. Crevices caused by
design occur at gaskets, flanges, rubber O-rings, washers, bolt holes,
rolled tube ends, threaded joints, riveted seams, overlapping screen wires,
lap joints, beneath coatings (filiform corrosion) or insulation (poultice
corrosion), and anywhere close-fitting surfaces are present. Figure 6
shows crevice corrosion in a riveted assembly caused by concentration
cells. Occluded regions are also formed under tubercles (tuberculation),
deposits (deposit corrosion), and below accumulations or biological materials (biologically influenced corrosion). Similarly, unintentional
crevices such as cracks, seams, and other metallurgical defects could serve
as sites for corrosion.
Metals Affected. Resistance to crevice corrosion can vary from one
alloy-environment system to another. Although crevice corrosion affects
both active and passive metals, the attack is often more severe for passive
alloys, particularly those in the stainless steel group. Breakdown of the
passive film within a restricted geometry leads to rapid metal loss and
penetration of the metal in that area.
Metal ion concentration cell
Low metal ion concentration
High metal ion concentration
High oxygen concentration
Oxygen concentration cell
Low oxygen concentration
Fig. 6
Corrosion caused at crevices by concentration cells. Both types of concentration cells shown sometimes occur simultaneously as in a reentry
angle in a riveted seam.
22 / Surface Engineering for Corrosion and Wear Resistance
Prevention. Crevice corrosion can be prevented or reduced
proved design to avoid crevices, regular cleaning to remove
selecting a more corrosion-resistant material, and by coating
or cast iron components with epoxy or other field-applied
applied organic coatings.
through imdeposits, by
carbon steel
or factory-
Erosion-Corrosion
General Description. Erosion-corrosion is the acceleration or increase
in the rate of deterioration or attack on a metal because of mechanical
wear or abrasive contributions in combination with corrosion. The combination of wear or abrasion and corrosion results in more severe attack than
would be realized with either mechanical or chemical corrosive action
alone. Metal is removed from the surface as dissolved ions, as particles of
solid corrosion products, or as elemental metal. The spectrum of erosioncorrosion ranges from primarily erosive attack, such as sandblasting, filing, or grinding of a metal surface, to primarily corrosion failures, where
the contribution of mechanical action is quite small.
All types of corrosive media generally can cause erosion-corrosion, including gases, aqueous solutions, organic systems, and liquid metals. For
example, hot gases may oxidize a metal then at high velocity blow off an
otherwise protective scale. Solids in suspension in liquids (slurries) are
particularly destructive from the standpoint of erosion-corrosion.
Erosion-corrosion is characterized in appearance by grooves, waves,
rounded holes, and/or horseshoe-shaped grooves. Analysis of these marks
can help determine the direction of flow. Affected areas are usually free of
deposits and corrosion products, although corrosion products can sometimes be found if erosion-corrosion occurs intermittently and/or the liquid
flow rate is relatively low.
Metals Affected. Most metals are susceptible to erosion-corrosion
under specific conditions. Metals that depend on a relatively thick protective coating of corrosion product for corrosion resistance are frequently
subject to erosion-corrosion. This is due to the poor adhesion of these
coatings relative to the thin films formed by the classical passive metals,
such as stainless steels and titanium. Both stainless steels and titanium are
relatively immune to erosion-corrosion in many environments. Metals that
4------
Fig. 7
Water flow
Schematic of erosion-corrosion
of a condenser tube
Principles of Corrosion / 23
are soft and readily damaged or worn mechanically, such as copper and
lead, are quite susceptible to erosion-corrosion. Even the noble or precious metals, such silver, gold, and platinum, are subject to erosion-corrosion. Figure 7 shows a schematic of erosion-corrosion of a condenser
tube wall. The direction of flow and the resulting attack where the protective film on the tube has broken down are indicated.
Prevention. Erosion-corrosion can be prevented or reduced through improved design (e.g., increase pipe diameter and/or streamline bends to reduce impingement effects), by altering the environment (e.g., deaeration and
the addition of inhibitors), and by applying hard, tough protective coatings.
Cavitation
General Description. Cavitation is a form of erosion-corrosion that is
caused by the formation and collapse of vapor bubbles in a liquid against
a metal surface. Cavitation occurs in hydraulic turbines, on pump impellers, on ship propellers, and on many surfaces in contact with high-velocity liquids subject to changes in pressure. The appearance of cavitation
is similar to pitting except that surfaces in the pits are usually much
rougher. The affected region is free of deposits and accumulated corrosion
products if cavitation has been recent.
Figure 8 is a simplified representation of the cavitation process. Figure
8(a) shows a vessel containing a liquid. The vessel is closed by an airtight
plunger. When the plunger is withdrawn (Fig. 8b), a partial vacuum is
created above the liquid, causing vapor bubbles to form and grow within
Partial
vacuum
Pressurized
(a) Rest
Quiescent liquid
at standard
temperature
and pressure
(b) Expansion
Liquid boiling
at room
temperature
{]
(e) Compression
.
... ::.,
..;~t::
Collapse of
vapor bubbles
.
(d)
Approaching
microjet
torpedo
Fig 8
Destruction of
metal oxide
on impact
Repair of
metal oxide at
expense of metal
Schematic representation of cavitation showing a cross section through a vessel and plunger enclosing a fluid.
•
(a) Plunger stationary, liquid at standard temperature and pressure. (b) Plunger withdrawn, liquid boils at room
temperature. (c) Plunger advanced, bubbles collapse. (d) Disintegration of protective corrosion product by impacting microjet "torpedo." Source: Ref 8
24 / Surface Engineering for Corrosion and Wear Resistance
the liquid. In essence, the liquid boils without a temperature increase. If
the plunger is then driven toward the surface of the liquid (Fig. 8c), the
pressure in the liquid increases, and the bubbles condense and collapse
(implode). In a cavitating liquid, these three steps occur in a matter of milliseconds. As shown in Fig. 8(d), implosion of a vapor bubble creates a
microscopic "torpedo" of water that is ejected from the collapsing bubble
at velocities that may range from 100 to 500 mls (330 to 1650 ft/s). When
the torpedo impacts the metal surface, it dislodges protective surface films
andlor locally deforms the metal itself. Thus, fresh surfaces are exposed
to corrosion and the reformation of protective films, which is followed by
more cavitation, and so on. Damage occurs when the cycle is allowed to
repeat over and over again.
Prevention. Cavitation can be controlled or minimized by improving design to minimize hydrodynamic pressure differences, employing stronger
(harder) and more corrosion-resistant materials, specifying a smooth finish
on all critical metal surfaces, and coating with resilient materials such as
rubber and some plastics.
Fretting Corrosion
General Description. Fretting corrosion is a combined wear and corrosion process in which material is removed from contacting surfaces
when motion between the surfaces is restricted to very small amplitude
oscillations (often, the relative movement is barely discernible). Usually,
the condition exists in machine components that are considered fixed and
not expected to wear. Pressed-on wheels can often fret at the shaftlwheel
hole interface.
Oxidation is the most common element in the fretting process. In oxidizing systems, fine metal particles removed by adhesive wear are oxidized and trapped between the fretting surfaces (Fig. 9). The oxides act
like an abrasive (such as lapping rouge) and increase the rate of material
removal. This type of fretting in ferrous alloys is easily recognized by the
red material oozing from between the contacting surfaces.
Fretting corrosion takes the form of local surface dislocations and deep
pits. These occur in regions where slight relative movements have occurred between mating, highly loaded surfaces.
Fig. 9
Schematic
of the fretting process
Principles. of Corrosion / 25
Prevention. Fretting corrosion can be controlled by lubricating (e.g.,
low-viscosity oils) the faying surfaces, restricting the degree of movement,
shot peening (rough surfaces are less prone to fretting damage), surface
hardening (e.g., carburizing and nitriding), anodizing of aluminum alloys,
phosphate conversion coating of steels, and by applying protective coatings by electrodeposition (e.g., gold or silver plating), plasma spraying, or
vapor deposition (Ref 9).
Intergranular
Corrosion
General Description. Intergranular corrosion is defined as the selective dissolution of grain boundaries, or closely adjacent regions, without
appreciable attack of the grains themselves. This dissolution is caused by
potential differences between the grain-boundary region and any precipitates, intermetallic phases, or impurities that form at the grain boundaries.
The actual mechanism differs with each alloy system. Although a wide variety of alloy systems are susceptible to intergranular corrosion under very
specific conditions, the majority of case histories reported in the literature
have involved austenitic stainless steels and aluminum alloys and, to a
lesser degree, some ferritic stainless steels and nickel-base alloys.
Precipitates that form as a result of the exposure of metals at elevated
temperatures (for example, during production, fabrication, and welding)
often nucleate and grow preferentially at grain boundaries. If these precipitates are rich in alloying elements that are essential for corrosion resistance, the regions adjacent to the grain boundary are depleted of these
elements. The metal is thus sensitized and is susceptible to intergranular
attack in a corrosive environment. For example, in austenitic stainless
steels such as AISI type 304, the cause of intergranular attack is the precipitation of chromium-rich carbides ((Cr,Fe)23C6) at grain boundaries.
These chromium-rich precipitates are surrounded by metal that is depleted
in chromium; therefore, they are more rapidly attacked at these zones than
on undepleted metal surfaces.
Impurities that segregate at grain boundaries may promote galvanic action in a corrosive environment by serving as anodic or cathodic sites.
Therefore, this would affect the rate of the dissolution of the alloy matrix
in the vicinity of the grain boundary. An example of this is found in aluminum alloys that contain intermetallic compounds, such as Mg5AIg and
CuAI2, at the grain boundaries. During exposures to chloride solutions,
the galvanic couples formed between these precipitates and the alloy matrix can lead to severe intergranular attack. Susceptibility to intergranular
attack depends on the corrosive solution and on the extent of intergranular precipitation, which is a function of alloy composition, fabrication, and
heat treatment parameters.
Prevention. Susceptibility to intergranular corrosion in austenitic stainless steels can be avoided by controlling their carbon contents or by
26 / Surface Engineering for Corrosion and Wear Resistance
adding elements (titanium and niobium) whose carbides are more stable
than those of chromium. For most austenitic stainless steels, restricting
their carbon contents to 0.03% or less will prevent sensitization during
welding and most heat treatment.
Intergranular corrosion in aluminum alloys is controlled by material selection (e.g., the high-strength 2xxx and 7xxx alloys are the most susceptible) and by proper selection of thermal (tempering) treatments that can
effect the amount, size, and distribution of second-phase intermetallic precipitates. Resistance to intergranular corrosion is obtained by the use of
heat treatments that cause precipitation to be more general throughout the
grain structure (Ref 10).
Exfoliation
Exfoliation is a form of macroscopic intergranular corrosion that primarily affects aluminum alloys in industrial or marine environments. Corrosion proceeds laterally from initiation sites on
the surface and generally proceeds intergranularly along planes parallel to
the surface. The corrosion products that form in the grain boundaries force
metal away from the underlying base material, resulting in a layered or
flakelike appearance (see, for example, the schematic shown in Fig. 3).
Prevention. Resistance to exfoliation corrosion is attained through
proper alloy and temper selection. The most susceptible alloys are the
high-strength heat-treatable 2xxx and 7xxx alloys. Exfoliation corrosion in
these alloys is usually confined to relatively thin sections of highly
worked products. Guidelines for selecting proper heat treatment for these
alloys can be found in Ref 10.
General Description.
Dealloying Corrosion
General Description. Dealloying, also referred to as selective leaching
or parting corrosion, is a corrosion process in which the more active metal
is selectively removed from an alloy, leaving behind a porous weak deposit of the more noble metal. Specific categories of de alloying often
carry the name of the dissolved element. For example, the preferential
leaching of zinc from brass is called dezincification. If aluminum is removed, the process is called dealuminification, and so forth. In the case of
gray iron, dealloying is called graphitic corrosion.
In the de alloying process, typically one of two mechanisms occurs:
alloy dissolution and replating of the cathodic element or selective dissolution of an anodic alloy constituent. In either case, the metal is left
spongy and porous and loses much of its strength, hardness, and ductility.
Table 2 lists some of the alloy-environment combinations for which dealloying has been reported. By far the two most common forms of dealloying are dezincification and graphitic corrosion.
Copper-zinc alloys containing more than 15% zinc are susceptible to
dezincification. In the dezincification of brass, selective removal of zinc
Principles of Corrosion / 27
leaves a relatively porous and weak layer of copper and copper oxide.
Corrosion of a similar nature continues beneath the primary corrosion
layer, resulting in gradual replacement of sound brass by weak, porous
copper.
Graphitic corrosion is observed in gray cast irons in relatively mild environments in which selective leaching of iron leaves a graphite network.
Selective leaching of the iron takes place because the graphite is cathodic
to iron, and the gray iron structure establishes an excellent galvanic cell.
Prevention. Dezincification can be prevented by alloy substitution.
Brasses with copper contents of 85% or more resist dezincification. Some
alloying elements also inhibit dezincification (e.g., brasses containing 1%
tin). Where dezincification is a problem, red brass, commercial bronze, inhibited admiralty metal, and inhibited brass can be successfully used.
Attack by graphitic corrosion is reduced by alloy substitution (e.g., use
of a ductile or alloyed iron rather than gray iron), altering the environment
(raise the water pH to neutral or slightly alkaline levels), the use of inhibitors, and avoiding stagnant water conditions.
Stress-Corrosion Cracking
General Description. Stress-corrosion cracking (SeC) is a cracking
phenomenon that occurs in susceptible alloys and is caused by the conjoint action of a surface tensile stress and the presence of a specific corrosive environment. For
to occur on an engineering structure, three
conditions must be met simultaneously, namely, a specific crack-promoting environment must be present, the metallurgy of the material must be
susceptible to SCC, and the tensile stresses must be above some threshold
value. Stresses required to cause SCC are small, usually below the macroscopic yield stress. The stresses can be externally applied, but residual
stresses often cause
failures. This cracking phenomenon is of particular importance to users of potentially susceptible structural alloys because SCC occurs under service conditions that can result, often with no
warning, in catastrophic failure. Failed specimens exhibit highly branched
sec
see
Table 2 Combinations of alloys and environments subject to dealloying and elements preferentially
removed
Alloy
Brasses
Gray iron
Aluminum bronzes
Silicon bronzes
Tin bronzes
Copper-gold single crystals
Monels
Gold alloys with copper or silver
Tungsten carbide-cobalt
High-nickel alloys
Medium- and high-carbon steels
Iron-chromium alloys
Nickel-molybdenum
alloys
Environment
Element removed
Many waters, especially under stagnant conditions
Soils, many waters
Hydrofluoric acid, acids containing chloride ions
High-temperature
steam and acidic species
Hot brine or steam
Ferric chloride
Hydrofluoric and other acids
Sulfide solutions, human saliva
Deionized water
Molten salts
Oxidizing atmospheres, hydrogen at high temperatures
High-temperature
oxidizing atmospheres
Oxygen at high temperature
Zinc (dezincification)
Iron (graphitic corrosion)
Aluminum (dealuminification)
Silicon (desiliconification)
Tin (destannification)
Copper
Copper in some acids, and nickel in others
Copper, silver
Cobalt
Chromium, iron, molybdenum, and tungsten
Carbon (decarburization)
Chromium, which forms a protective film
Molybdenum
Table 3
Some environment-alloy combinations known to result in stress-corrosion cracking (SCC)
Alloy system
Aluminum
alloys
Environment
Amines, aqueous
Ammonia, anhydrous
Ammonia, aqueous
Bromine
Carbonates, aqueous
Carbon monoxide, carbon
dioxide, water mixture
Chlorides, aqueous
Chlorides, concentrated,
boiling
Chlorides, dry, hot
Chlorinated solvents
Cyanides, aqueous,
acidified
Fluorides, aqueous
Hydrochloric acid
Hydrofluoric acid
Hydroxides, aqueous
Hydroxides, concentrated,
hot
Methanol plus halides
Nitrates, aqueous
Nitric acid, concentrated
Nitric acid, fuming
Nitrites, aqueous
Nitrogen tetroxide
Poly thionic acids
Steam
Sulfides plus chlorides,
aqueous
Sulfurous acid
Water, high-purity, hot
Carbon
steels
Copper
alloys
X
X
X
Nickel
alloys
Austenitic
Stainless Steels
Martensitic
Duplex
Titanium
alloys
Zirconium
alloys
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X, known to result in see
Stress-corrosion cracking control
I
Mechanical
I
Avoid stress
concentrators
I
Relieve fabrication
stresses
I
Introduce surface
compressive
stresses
I
Reduce operating
stresses
I
Nondestructive
testing implications
for design
I
I
Metallurgical
I
Change alloy
composition
I
Change alloy
structure
I
UsemetaJlic
or conversion
coating
I
Environmental
I
Modify
environment
I
Apply anodic or
cathodic protection
I
Add inhibitor
I
Use organic
coating
I
Modify
temperature
Fig. 10
Methods used to control
Ref 11
sec. Source:
Principles of Corrosion / 29
cracks (see Fig. 3) that propagate intergranularly and/or transgranularly,
depending on the metal-environment combination.
Table 3 lists some of the alloy-environment combinations that result in
SCC. This table, as well as others published in the literature, should be
used only as a guide for screening candidate materials prior to further indepth investigation, testing, and evaluation.
Prevention. Figure 10 summarizes the various approaches to controlling SCC. Surface engineering treatments like shot peening, metallic coatings, and organic coatings play a key role in controlling SCC.
Corrosion Fatigue
General Description. Corrosion fatigue is a term that is used to describe the phenomenon of cracking, including both initiation and propagation, in materials under the combined actions of a fluctuating or cyclic
stress and a corrosive environment. Corrosion fatigue depends strongly on
the interactions among the mechanical (loading), metallurgical, and environmental variables listed in Table 4.
Corrosion fatigue produces fine-to-broad cracks with little or no branching (see Fig. 3); thus, they differ from SCC, which often exhibits considerable branching. They are typically filled with dense corrosion product.
The cracks may occur singly but commonly appear as families or parallel
cracks. They are frequently associated with pits, grooves, or some other
form of stress concentrator. Transgranular fracture paths are more common than intergranular fractures.
Table 4 Mechanical, metallurgical, and environmental variables that
influence corrosion fatigue behavior
Variable
Mechanical
Metallurgical
Environmental
Type
Maximum stress or stress-intensity factor, (J"max or Kmax
Cyclic stress or stress-intensity range, d(J' or dK
Stress ratio, R
Cyclic loading frequency
Cyclic load waveform (constant-amplitude
loading)
Load interactions in variable-amplitude
loading
State of stress
Residual stress
Crack size and shape, and their relation to component size and geometry
Alloy composition
Distribution of alloying elements and impurities
Microstructure and crystal structure
Heat treatment
Mechanical working
Preferred orientation of grains and grain boundaries (texture)
Mechanical properties (strength, fracture toughness, etc.)
Temperature
Types of environments: gaseous, liquid, liquid metal, etc.
Partial pressure of damaging species in gaseous environments
Concentration of damaging species in aqueous or other liquid environments
Electrical potential
pH
Viscosity of the environment
Coatings, inhibitors, etc.
30 / Surface Engineering for Corrosion and Wear Resistance
Prevention. All metals and alloys are susceptible to corrosion fatigue.
Even some alloys that are immune to SCC, for example, ferritic stainless steels, are subject to failure by corrosion fatigue. Both temporary
and permanent solutions for corrosion involve reducing or eliminating
cyclic stresses, selecting a material or heat treatment with higher corrosion fatigue strengths, reducing or eliminating corrosion, or a combination of these procedures. These objectives are accomplished by changes
in material, design, or environment and by the application of surface
treatments. Shot peening, nitriding of steels, and organic coatings can
successfully impede corrosion fatigue. Noble metal coatings (e.g.,
nickel) can be effective, but only if they remain unbroken and are of sufficient density and thickness. The relatively low corrosion-fatigue
strength of carbon steel is reduced still further when local breaks in a
coating occur.
Hydrogen Damage
General Description. The term hydrogen damage has been used to
designate a number of processes in metals by which the load-carrying capacity of the metal is reduced due to the presence of hydrogen, often in
combination with residual or applied tensile stresses. Although it occurs
most frequently in carbon and low-alloy steels, many metals and alloys
are susceptible to hydrogen damage. Hydrogen damage in one form or another can severely restrict the use of certain materials.
Because hydrogen is one of the most abundant elements and is readily
available during the production, processing, and service of metals, hydrogen damage can develop in a wide variety of environments and circumstances. The interaction between hydrogen and metals can result in the
formation of solid solutions of hydrogen in metals, molecular hydrogen,
gaseous products that are formed by reactions between hydrogen and elements constituting the alloy, and hydrides. Depending on the type of hydrogen/metal interaction, hydrogen damage of metal manifests itself in
one of several ways.
Specific types of hydrogen damage, some of which occur only in specific alloys under specific conditions include:
o
Hydrogen embrittlement: Occurs most often in high-strength steels,
primarily quenched-and-tempered and precipitation-hardened steels,
with tensile strengths greater than about 1034 MPa (150 ksi). Hydrogen sulfide is the chief embrittling environment.
Hydrogen-induced blistering: Also commonly referred to as hydrogen-induced cracking (HIC), it occurs in lower-strength (unhardened)
steels, typically with tensile strengths less than about 550 MPa (80
ksi). Line pipe steels used in sour gas environments are susceptible to
HIC.
Principles of Corrosion / 31
•
•
•
Cracking from precipitation of internal hydrogen: Examples include
shatter cracks, flakes, and fish eyes found in steel forgings, weldments, and castings. During cooling from the melt, hydrogen diffuses
and precipitates in voids and discontinuities.
Hydrogen attack: A high-pressure, high-temperature form of hydrogen damage. Commonly experienced in steels used in petrochemical
plant equipment that often handles hydrogen and hydrogen-hydrocarbon streams at pressures as high as 21 MPa (3 ksi) and temperatures
up to 540°C (1000 OF)
Hydride formation: Occurs when excess hydrogen is picked up during
melting or welding of titanium, tantalum, zirconium, uranium, and
thorium. Hydride particles cause significant loss in strength and large
losses in ductility and toughness.
Prevention. The primary factors controlling hydrogen damage are material, stress, and environment. Hydrogen damage can often be prevented
by using more resistant material, changing the manufacturing processes,
modifying the design to lower stresses, or changing the environment. Inhibitors and post-processing bake-out treatments can also be used. Baking
of electroplated high-strength steel parts reduces the possibility of hydrogen embrittlement (see Chapter 8 for additional information).
Coatings and Corrosion Prevention
As described in the previous section, surface treatments, and in particular protective coatings, are widely used to control corrosion in its varying
forms. The problems of corrosion should be approached in the design
stage, and the selection of a protective coating is important. Paint systems
and lining materials exist that slow the corrosion rate of carbon steel surfaces. High-performance organic coatings such as epoxy, polyesters,
polyurethanes, vinyl, or chlorinated rubber help to satisfy the need for corrosion prevention. Special primers are used to provide passivation, galvanic protection, corrosion inhibition, or mechanical or electrical barriers
to corrosive action.
Corrosion Inhibitors. A water-soluble corrosion inhibitor reduces galvanic action by making the metal passive or by providing an insulating
film on the anode, the cathode, or both. A very small amount of chromate,
polyphosphate, or silicate added to water creates a water-soluble inhibitor.
A slightly soluble inhibitor incorporated into the prime coat of paint may
also have a considerable protective influence. Inhibitive pigments in paint
primers are successful inhibitors except when they dissolve sufficiently to
leave holes in the paint film. Most paint primers contain a partially soluble inhibitive pigment such as zinc chromate, which reacts with the steel
32 / Surface Engineering for Corrosion and Wear Resistance
substrate to form the iron salt. The presence of these salts slows corrosion
of steel. Chromates, phosphates, molybdates, borates, silicates, and
plumbates are commonly used for this purpose. Some pigments add alkalinity, slowing chemical attack on steel. Alkaline pigments, such as metaborates, cement, lime, or red lead, are effective, provided that the environment is not too aggressive. In addition, many new pigments have been
introduced to the paint industry such as zinc phosphosilicate and zinc
flake.
Barrier coatings are used to prevent the electrolyte from reaching the
component surface. Examples of barrier coatings include painted steel
structures, steels lined with thick acid-proof brick, steels lined with rubberlike materials, or steels electroplated with a noble (see Table 1) metal
(e.g., chromium, copper, or nickel). Protection is effective until the coating is penetrated, either by a pit, pore, crack, or by damage or wear. The
substrate will then corrode preferentially to the coating (since it is anodic
to the coating material), and corrosion products will lift off the coating
and allow further attack (Fig. 11).
Generally, electroplated coatings that are completely free of pores and
other discontinuities are not commercially feasible. Pits eventually form
at coating flaws, and the coating is penetrated. The resulting corrosion cell
is shown in Fig. 12. The substrate exposed at the bottom of the resulting
pit corrodes rapidly. A crater forms in the substrate, and because of the
(a)
(b)
Fig 11
Illustration of the mechanism of corrosion for painted steel. (a) A void
•
in the paint results in rusting of the steel, which undercuts the paint
coating and results in further coating degradation. (b) Photograph showing blistering and/or peeling (undercutting) of paint where exposed steel is rusting.
Principles of Corrosion / 33
Steel sUbstratJ
(anode)
Fig. 12
Crater formation in a steel substrate beneath a void in a noble metal
coating, for example, passive chromium or copper. Corrosion proceeds under the noble metal, the edges of which collapse into the corrosion pit.
)
Substrate (M3)
Fig. 13
Corrosion pit formation in a
noble metal coating. The top
coating underlayer (M2), which is in turn
Fig. 12, the coating tends to collapse into
substrate beneath a void in a duplex
coating layer (M1) is cathodic to the
cathodic to the substrate (M3). As in
the pit.
large area ratio between the more noble coating and the anodic crater, the
crater becomes anodic, and high corrosion current density results. Electrons flow from the substrate to the coating as the steel dissolves. Hydrogen ions (H+) in the moisture accept the electron and, with dissolved oxygen, form water at the noble metal surface near the void. Use of an
intermediate coating that is less noble than a surface coating but more
noble than the base metal can result in the mode of corrosion shown in
Fig. 13. This would be typical of a costume jewelry item with a brass substrate, an intermediate nickel coating, and a tarnish-resistant gold top coat.
It is also exemplified by nickel-chromium coating systems.
Sacrificial coatings, which corrode preferentially to the substrate, include zinc, aluminum, cadmium, and zinc-rich paints. Initially these sacrificial coatings will corrode, but their corrosion products are protective
and the coating acts as a barrier layer. If the coating is damaged or defective, it remains protective as it is the coating that suffers attack and not the
substrate. Figure 14 shows the sacrificial (galvanic) protection offered by
a zinc coating to a steel substrate.
Cathodic protection involves the reversal of electric current flow
within the corrosion cell. Cathodic protection can reduce or eliminate
corrosion by connecting a more active metal to a metal that must be
34 / Surface Engineering for Corrosion and Wear Resistance
Water drop
Fig. 14
Principles and mechanism of galvanic protection of a substrate by a
coating. Galvanic protection of a steel substrate at a void in a zinc
coating. Corrosion of the substrate is light and occurs at some distance from the
zinc.
protected. The use of cathodic protection to reduce or eliminate corrosion
is a successful technique of long-standing use in marine structures,
pipelines, bridge decks, sheet piling, and equipment and tankage of all
types, particularly below water or underground. Typically, zinc or magnesium anodes are used to protect steel in marine environments, and the anodes are replaced after they are consumed.
Cathodic protection uses an impressed direct current (de) supplied by
any low output voltage source and a relatively inert anode. As is the case
in all forms of cathodic activity, anelectrolyte is needed for current flow.
Cathodic protection and the use of protective coatings are most often employed jointly, especially in marine applications and on board ships where
impressed current inputs do not usually exceed 1 V. Beyond 1 V, many
coating systems tend to disbond. Current source for cathodic protection in
soils is usually 1.5 to 2 V.
Choice of anodes for buried steel pipe depends on soil conditions. Magnesium is most commonly used for galvanic anodes; however, zinc can
also be used. Galvanic anodes are seldom used when the resistivity of the
soil is over 30
m (3000
em); impressed current is normally used
for these conditions. Graphite, high-silicon cast iron, scrap iron, aluminum, and platinum are used as anodes with impressed current. The
availability of low-cost power is often the deciding factor in choosing between galvanic or impressed current cathodic protection. Figure 15 illustrates both types of galvanic protection systems.
Protective coatings are normally used in conjunction with cathodic protection and should not be disregarded where cathodic protection is contemplated in new construction. Because the cathodic protection current
must protect only the bare or poorly insulated areas of the surface, coatings that are highly insulating, very durable, and free of discontinuities
lower the current requirements and system costs. A good coating also enables a single-impressed current installation to protect many miles of piping. Coal-tar enamel, epoxy powder coatings, and vinyl resin are exam-
n.
n.
Principles of Corrosion / 35
ac line
Insulated copper wire
(b)
(a)
Fig. 15
Cathodic protection for underground
rent anode. ac, alternating current
pipe. (a) Sacrificial or galvanic anode. (b) Impressed-cur-
ples of coatings that are most suitable for use with cathodic protection.
Certain other coatings may be incompatible, such as phenolic coatings,
which may deteriorate rapidly in the alkaline environment created by the
cathodic protection currents. Although cement mortar initially conducts
the electrical current freely, polarization, the formation of an insulating
film on the surface as a result of the protective current, is believed to reduce the current requirement moderately.
Cathodic protection is used increasingly to protect buried or submerged
metal structures in the oil, gas, and waterworks industries and can be used
in specialized applications, such as for the interiors of water storage tanks.
Pipelines are routinely designed to ensure the electrical continuity necessary for effective functioning of the cathodic protection system. Thus,
electrical connections or bonds are required between pipe sections in lines
using mechanically coupled joints, and insulating couplings may be employed at intervals to isolate some parts of the line electrically from other
parts. Leads may be attached during construction to facilitate the cathodic
protection installation when needed.
Corrosion Testing
Many tests exist for establishing the reliability of protective coatings on
metal substrates. Existing tests and standards are under continuous development, and new tests are being designed. Organizations active in the development and standardization of corrosion tests for coatings include
ASTM, NACE International, the Society of Automotive Engineers (SAE),
the National Coil Coaters Association (NCCA), the International
Standards Organization (ISO), international systems (e.g., DIN), and
commercial (e.g., automotive, architectural, electronics), proprietary, and
36 / Surface Engineering for Corrosion and Wear Resistance
military organizations. This section provides a brief review of the most
widely used test methods including:
•
•
•
Field tests
Simulated service tests
Laboratory (accelerated) tests (e.g., salt spray tests, humidity tests,
and electrochemical tests)
Table 5 lists selected tests used for determining the effectiveness of protective coatings in corrosive environments.
More detailed information on testing of coated specimens can be
found in several excellent sources. Gaynes (Ref 13) and Munger (Ref
14) give descriptions and the framework for effective use of tests and
standards. Gaynes provides detailed descriptions including photographs, cross-listing ASTM to federal tests and a broader perspective
encompassing the federal standard, miscellaneous tests, and some
caveats of traditional testing. Munger offers practical material directed
toward large structures and provides a listing based on ASTM standards. Altmayer (Ref 15) compiled a table of 13 applicable corrosion
tests for 30 metallic, inorganic, and organic coating/substrate combinations. Other useful sources of information can be found in review articles by Simpson and Townsend (Ref 16) and Granata (Ref 12), which
describe tests for metallic coatings and nonmetallic coatings, respectively.
Field Tests
The most reliable performance data are obtained by field tests/surveys.
One example would be to monitor and test the corrosion of autobody panels that sit in junkyards. Another example of in-service testing would be
to monitor the behavior of the materials in a fleet of captive vehicles. This
enables better control and recording of the exposure and driving conditions. The use of fleet vehicles also makes it possible to test coupons representing a larger database of materials.
Simulated Service Tests
The most widely used simulated service test for static atmospheric testing is described in ASTM G 50, "Practice for Conducting Atmospheric
Corrosion Tests on Metals." It is used to test coated sheet steels for a variety of outdoor applications. Test materials, which are in the form of flat
test panels mounted in a test rack (Fig. 16), are subjected to the cyclic
effects of the weather, geographical influences, and bacteriological factors
that cannot be realistically duplicated in the laboratory. Test durations can
last from several months up to many years. Some zinc-coated steel specimens have undergone testing for more than 30 years.
Principles of Corrosion / 37
Table 5
Widely
used tests for determining
Test
resistance
of protective
coatings
Description and remarks
Salt spray (ASTM B 117)
100% relative humidity (ASTM D 2247)
Acetic acid-salt spray ASTM G 85, Al (formerly
ASTMB 287)
Sulfur dioxide-salt
spray (ASTM G 85, A 4)
Copper-accelerated
salt spray, or CASS
(ASTM B 368)
FACT (formerly ASTM B 538)
Accelerated
weathering
Lactic acid
Acidified synthetic seawater testing or SWAAT
(ASTM G 85, A3; formerly ASTM G 43)
Electrographic
Adhesion
the corrosion
and chemical porosity tests
(ASTM D 3359-90)
T-bend adhesion (ASTM D 4145)
Most widely specified test. Atomized 5% sodium chloride (NaCl), neutral pH, 35°C (95 OF)
(a), follow details of ASTM B 117, Appendix Xl. Emphasizes wet surfaces (nondrying),
high oxygen availability, neutral pH, and warm conditions. Control of comparative specimens should be run simultaneously. Corrosivity consistency should be checked as described in ASTM B 117, Appendix X3. Notes: May be the most widely misused test. Requires correlation to service tests for useful results. Do not assume correlation exists.
Widely used test. Condensing humidity, 100% RH, 38°C (100 OF). Emphasizes sensitivity to
water exposure
Widely used test. Atomized 5% NaCI, pH 3.2 using acetic acid, 35°C (95 OF). More severe
than ASTM B 117. The lower pH and the presence of acetate affect the solubility of corrosion products on and under the protective coatings.
Atomized 5% NaCI, collected solution pH = 2.5-3.2,35
°C (95 OF), S02 metered (60 min35 cm3/min per m3 cabinet volume) 4 times per day
Atomized 5% NaCI, pH 3.2 with acetic acid, 0.025% cupric chloride-dihydrate,
35°C (95°F).
Galvanic coupling due to copper salt reduction to copper metal. More severe than ASTM B
117
Testing anodized aluminum specimens. Electrolyte as in salt spray or CASS test. Specimen is
made the cathode to generate high pH at defects.
Exposure of coated specimens to effects of ultraviolet radiation experienced in outdoor sunlight
conditions, which may be combined with other exposures such as moisture and erosion.
Exposure cabinets use carbon arc (ASTM D 822), xenon lamp (ASTM G 26), or fluorescent lamp (ASTM G 53).
\~
On substrates of brass and copper alloys, determines coatings porosity and resistance to handling
(perspiration). Consists of immersion in 85% lactic acid solution, drying, and incubating
above acetic acid vapors for 20 h to reveal discoloration spots at failure points or delaminations
Atomized synthetic seawater (ASTM D 1141) with 10 mL glacial acetic acid per L of solution,
pH 2.8 to 3.0, 35°C (95 OF). More severe than ASTM B 117. The lower pH and the presence of acetate affect the solubility of corrosion products on and under the protective coatings.
Pores and active defects in nonmetallic coatings can be revealed by color indication or deposit
formation. On nickel substrates, dimethylglyoxime,
or steel, potassium ferricyanide (ferroxyl test) indicator can be applied to surface on filter paper while substrate is made the
anode. Alternatively, a substrate immersed in acidic copper sulfate can be made the cathode to form copper nodules at conductive coatings defects.
Knife and fingernail test consists of cutting through the coating with knife or awl and dislodging coating with thumbnail or fingernail (pass/fail). The ASTM D 3359 test consists of
"X" scribes or parallel cross-hatches followed by adhesive tape stripping of loosened coating.
Combined flexibility and adhesion test consists of clamping end of coated flat metal panel in
vise or similar tool bending (convex) through 90°, reclamping to bend through 180° to
give "OT' bend (where Tis panel thickness and the numeral (0, 1,2, ...) is the number of
panel thicknesses). Rebending over the 180° bend gives a 1T bend. Adhesive tape is
pressed down along edge of bend and any loose coating stripped off.
OT---=:J 1T CJ 2T51
Scab test
Exterior exposure (ASTM D 1014)
Service test data
3T I CJ I
Cyclic testing consisting of short salt exposure, short drying period, and long period of high
humidity. Undercutting from scribe is measured.
Method for conducting exterior exposure tests of paints on steel. Well-defined exposure setup,
not necessarily equivalent to service tests
Performance data of coatings systems under use conditions. Slowest evaluation method; provides tangible results
FACT, Ford anodized aluminum corrosion test. (a) Note that dissolved CO2 concentration at 0 °C (32 OF) is three times that of concentration at 35°C (95 OF) and can affect
corrosion. Source: Ref 12
38 / Surface Engineering for Corrosion and Wear Resistance
Fig. 16
Atmospheric
corrosion test rack
Salt Spray Tests
As indicated in Table 6, salt spray testing is the most popular form of
testing for protective coatings. These tests have been used for more than
90 years as accelerated tests in order to determine the degree of protection
afforded by both inorganic and organic coatings on a metallic base. Table
5 lists several widely used salt spray tests.
The neutral salt-spray (fog) test (ASTM B 117-Method
811.1 of
Federal Test Method 151b) is perhaps the most commonly used salt spray
test in existence for testing inorganic and organic coatings, especially
where such tests are used for material or product specifications. The duration of this test can range from 8 to 3000 h, depending on the product
type of coating. A 5% sodium chloride (NaCI) solution that does not contain more than 200 ppm total solids and with a pH range of 6.5 to 7.2 when
atomized is used. The temperature of the salt spray cabinet is controlled
to maintain 35 + 1.1 or -1.7 °C (95 + 2 or - 3 OF)within the exposure
zone of the closed cabinet.
The acetic acid-salt spray (fog) test (ASTM G 85, Annex AI; Former
Method B 287) is also used for testing inorganic and organic coatings but
is particularly applicable to the study or testing of decorative chromium
Table 6 Results of a survey to determine the most
widely used tests for protective coatings
Test
% respondents(a)
Salt spray
Immersion
Outdoor
Ultraviolet/condensation
Accelerated/weathering
Humidity/condensation
Cathodic disbondment
Adhesion
Atlas cell test (NACE TMOI74)
Other physical tests
Other chemical tests
Flexibility
(a) Multiple
52
24
22
20
14
10
7
7
4
4
3
2
tests used (total greater than 100%). Source: Ref 12
Principles of Corrosion / 39
plate (nickel-chromium or copper-nickel-chromium) plating and cadmium
plating on steel or zinc die castings and for the evaluation of the quality of
a product.
This test can be as brief as 16 h, although it normally ranges from 144
to 240 h or more. As in the neutral salt spray test, a 5% NaCI solution is
used, but the solution is adjusted to a pH range of 3.1 to 3.3 by the addition of acetic acid, and again, the temperature of the salt spray cabinet is
controlled to maintain 35 + 1.1 or -1.7 °C (95 + 2 or -3 OF) within the
exposure zone of the closed cabinet.
The copper-accelerated
acetic acid-salt spray (fog) test (CASS
test), which is covered in ASTM B 368, is primarily used for the rapid
testing of decorative copper-nickel-chromium
or nickel-chromium plating on steel and zinc die castings. It is also useful in the testing of anodized, chromated, or phosphated aluminum. The duration of this test
ranges from 6 to 720 h. A 5% NaCI solution is used, with 1 g of copper
II chloride (CuCI2-2H20) added to each 3.8 L of salt solution. The solution is then adjusted to a pH range of 3.1 to 3.3 by adding acetic acid.
The temperature of the CASS cabinet is controlled to maintain 49 + 1.1
or -1.7 °C (120 + 2 or -3 OF) within the exposure zone of the closed
cabinet.
Humidity Cabinet Tests
In a humidity cabinet the humidity is raised to a value chosen as
appropriate to the material under test. The temperature is generally cycled, so that the specimen is exposed to alternating humid air and condensation.
The apparatus is automated to ensure that conditions are controlled
within narrow limits. Other corrodent materials, such as sulfur dioxide,
may also be introduced. Examples of humidity cabinet tests include
ASTM D 2247 and ASTM G 85 listed in Table 5.
Electrochemical Tests
Corrosion of metallic substances is an electrochemical process. An alternate approach to field or other accelerated tests in understanding and
predicting metallic corrosion is the use of electrochemical parameters/
tests. Electrochemical tests often complement other test methods by providing kinetic and mechanistic data that would be otherwise difficult to
obtain. Electrochemical tests are typically grouped as direct current (de)
or alternating current (ac) methods based on the type of perturbation signal that is applied in making the measurements. A number of investigators
have used de and ac electrochemical methods to study the performance
and the quality of protective coatings, including passive films on metallic
substrates, and to evaluate the effectiveness of various surface pretreatments. Several are discussed below.
40 / Surface Engineering for Corrosion and Wear Resistance
Anodized Aluminum Corrosion Test. One such method isthe Ford
anodized aluminum corrosion test (FACT) listed in Table 5. This test involves the cathodic polarization of the anodized aluminum surface by
using a small cylindrical glass clamp-on cell and a special 5% NaCI solution containing cupric chloride (CuCI2) acidified with acetic acid. A
large voltage is applied across the cell by using a platinum auxiliary
electrode. The alkaline conditions created by the cathodic polarization
promote dissolution at small defects in the anodized aluminum. The
coating resistance is decreased, more current begins to flow, and the
voltage decreases. The cell voltage (auxiliary electrode to test specimen
voltage) is monitored for 3 min, and the parameter cell voltage multiplied by time is recorded.
A similar test, known as the cathodic breakdown test, involves cathodic
polarization to -1.6 V (versus saturated calomel electrode, SCE) for a period of 3 min in acidified NaCl. Again, the test was designed for anodized
aluminum alloys because the alkali created at the large applied currents will
promote the formation of corroded spots at defects in the anodized film.
The electrolytic corrosion test was designed for electrodeposits of
principally nickel and chromium on less noble metals, such as zinc or
steel. Special solutions are used, and the metal is polarized to +0.3 V versus the SCE. The metal is taken through cycles of 1 min anodically polarized and 2 min unpolarized. An indicator solution is then used to detect
the presence of pits that penetrate to the substrate. Each exposure cycle
simulates 1 year of exposure under atmospheric-corrosion conditions. The
ASTM standard B 627 describes the method in greater detail.
The paint adhesion on a scribed surface (PASS)test involves the cathodic polarization of a small portion of painted metal. The area exposed
contains a scribed line that exposes a line of underlying bare metal. The
sample is cathodically polarized for 15 min in 5% NaCl. At the end of this
period, the amount of delaminated coating is determined from an adhesive
tape pulling procedure.
The impedance test for anodized aluminum (ASTM B 457) is used
to study the seal performance of anodized aluminum. In this sense, the test
is similar to the FACT test, except that this method uses a 1 V root mean
square 1 kHz signal source from an impedance bridge to determine the
sealed anodized aluminum impedance. The test area is again defined with
a portable cell, and a platinum or stainless steel auxiliary electrode is typically used. The sample is immersed in 3.5% NaCl. The impedance is determined in ohms X 103. In contrast to the methods discussed previously,
this test is essentially nondestructive and does not accelerate the corrosion
process.
Electrochemical impedance spectroscopy (EIS) offers an advanced
method of evaluating the performance of metallic coatings (passive film
forming or otherwise) and organic barrier coatings. The method does not
accelerate the corrosion reaction and is nondestructive. The technique is
Principles of Corrosion / 41
quite sensitive to changes in the resistive-capacitive nature of coatings.
The technique has been used to evaluate phosphate coverage/stability on
galvanneal, painted cold-rolled steel, electrogalvanized steel, and electrogalvannealed steel (Ref 16).
It is also possible to monitor the corrosion rate with this technique. In
this respect, the electrochemical impedance technique offers several advantages over de electrochemical techniques in that the polarization resistance related to the corrosion rate can be separated from the high dc resistance of the dielectric coating. This is not possible with the dc methods.
References
1. Corrosion of Steels in Waters, ASM Specialty Handbook: Carbon and
Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 408-429
2. Corrosion of Steels in Soils, ASM Specialty Handbook: Carbon and
Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 430-438
3. Corrosion of Steels in Chemical Environments, ASM Specialty Handbook: Carbon and Alloys Steels, J.R. Davis, Ed., ASM International,
1996, p 439-451
4. Types of Corrosive Environments, Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000, p 193-236
5. Atmospheric Corrosion of Steels, ASM Specialty Handbook: Carbon
and Alloys Steels, J.R. Davis, Ed., ASM International, 1996,
p 393-407
6. Forms of Corrosion: Recognition and Prevention, Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000,
p 99-192
7. Corrosion Control by Proper Design, Corrosion: Understanding the
Basics, J.R. Davis, Ed., ASM International, 2000, p 301-362
8. H.M. Herro and R.D. Port, Cavitation Damage, The Nalco Guide to
Cooling Water System Failure Analysis, McGraw-Hill, Inc., 1993,
p 270-271
9. R.B. Waterhouse, Fretting Wear, Friction, Lubrication, and Wear
Technology, Vol 18, ASM Handbook, ASM International, 1992,
p 242-256
10. Intergranular and Exfoliation Corrosion, Corrosion of Aluminum and
AluminumAlloys, J.R. Davis, Ed., ASM International, 1999, p 63-74
11. R.N. Parkins, An Overview-Prevention
and Control of StressCorrosion Cracking, Mater. Perform., Vol 24, 1995, p 9-20
12. R.D. Granata, Nonmetallic Coatings, Corrosion Tests and Standards:
Application and Interpretation, R. Baboian, Ed., ASTM, 1995,
P 525-530
13. N.I. Gaynes, Testing of Organic Coatings, Noyes Data Corp., 1977
42 / Surface Engineering for Corrosion and Wear Resistance
14. C.G. Munger, Corrosion Prevention by Protective Coatings, National
Association of Corrosion Engineers, 1984, Chapter 12
15. F. Altmayer, "Choosing an Accelerated Corrosion Test," Met. Finish.,
61st Guidebook and Directory Issue, Vol 91 (No. lA), Jan 1993,
p 483
16. T.C. Simpson and H.E. Townsend, Metallic Coatings, Corrosion
Tests and Standards: Application and Interpretation, R. Baboian,
Ed., ASTM, 1995, p 513-524
Selected References
•
•
•
•
•
Corrosion, Vol 13, ASM Handbook, ASM International, 1987
Corrosion Basics-An Introduction, L.S. Van Delinder, Ed., NACE International, 1984
Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000
M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill, 1986
H.H. Uhlig and R.W. Revie, Corrosion and Corrosion Control, 3rd
ed., John Wiley & Sons, 1985
CHAPTER
3
Principles of Friction
and Wear
FRICTION, WEAR, AND LUBRICATION are complex, interwoven
subjects that may all affect the service life of a component or the efficient
operation of a machine. While all three are important factors, the major
emphasis in this Chapter will be on wear and the various methods used to
reduce or prevent it, including the application of surface engineering treatments. More detailed information on the science and technology of friction, wear, and lubrication-known
as tribology-can be found in Friction,
Lubrication, and Wear Technology, Volume 18 of the ASM Handbook.
Friction
Friction is the resistance to motion when two bodies in contact are
forced to move relative to each other. It is closely associated with any
wear mechanisms that may be operating and with any lubricant andlor
surface films that may be present, as well as the surface topographies. The
heat generated as a result of the dissipation of frictional interaction may
affect the performance of lubricants, may change the properties of the
contacting materials andlor their surface films, and, in some cases, may
change the properties of the product being processed. Any of these results
of frictional heating can cause severe safety problems because of the danger of mechanical failure of components due to structural weakening, severe wear (for example, seizure), or fire and explosion.
In moving machinery, friction is responsible for dissipation and loss of
much energy. It has been estimated, for example, that 10% of oil consumption in the United States is used simply to overcome friction. The energy lost to friction is an energy input that must continually be provided
in order to maintain the sliding motion. This energy is dissipated in the
44 / Surface Engineering for Corrosion and Wear Resistance
system, primarily as heat-which
may have to be removed by cooling to
avoid damage and may limit the conditions under which the machinery
can be operated. Some of the energy is dissipated in various deformation
processes, which result in wear of the sliding surfaces and their eventual
degradation to the point where replacement of whole components becomes necessary. Wear of sliding surfaces adds another, very large component to the economic importance of friction, because without sliding
friction, these surfaces would not wear.
The need to control friction is the driving force behind its study. In many
cases low friction is desired (bearings, gears, materials processing operations), and sometimes, high friction is the goal (brakes, clutches, screw
threads, road surfaces). In all of these cases, constant, reproducible, and
predictable friction values are necessary for the design of components and
machines that will function efficiently and reliably.
Important Terms and Concepts. It is useful to clearly separate the various terms and concepts associated with friction, such as "friction force,"
"friction coefficient," "frictional energy," and "frictional heating." These
terms are defined subsequently in the context of solid friction, which can
be defined as "the resistance to movement of one solid body over another."
The movement may be by sliding or by rolling.
The friction force is the tangential force that must be overcome in order
for one solid contacting body to slide over another. It acts in the plane of
the surfaces and is usually proportional to the force normal to the surfaces,
N, or:
F=
~N
(Eq 1)
The proportionality constant is generally designated f..L or f and is termed
the friction coefficient, which is the ratio between the friction force, F, and
the load, N:
~ = FIN
(Eq 2)
The friction coefficient typically ranges from 0.03 for a very welllubricated bearing, to 0.5 to 0.7 for dry sliding, and even ~5 for clean metal
surfaces in a vacuum. A u-value of 0.2 to 0.3 allows for comfortable walking; however, walking on ice is very difficult because the u-value for the
ice/shoe pair may be <0.05, and a slippery floor may have a u-value of
0.15. Nature has provided highly efficient lubrication to another component of walking, the knee joint which has a u-value of 0.02.
In most cases, a greater force is needed to set a resting body in motion
than to sustain the motion; in other words, the static coefficient offriction,
J.Ls' is usually somewhat greater than the dynamic or kinetic coefficient of
friction, J.Lk.
Principles of Friction and Wear / 45
A body of weight W on a flat surface will begin to move when the surface is tilted to a certain angle termed the friction angle, 8, as defined in
Fig. 1. The static friction coefficient is given by:
f.Ls =
tan
e
(Eq 3)
This represents a simple way to measure f-Ls' but force measurements are
more generally used to measure both the static and the dynamic, or kinetic, coefficients of friction. The results obtained from these measurements do, however, depend on the nature and cleanliness of the surfaces
and .also to some extent on the various characteristics of the measuring
system. This dependence underscores the basic fact that the friction coefficient is not a unique, clearly defined materials property, as may become
evident from the following discussion on "Basic Mechanisms of Friction."
To overcome friction, the tangential force must be applied over the entire sliding distance; the product of the two is friction work. The resulting
energy is lost to heat in the form oi frictional heating and to other general
increases in the entropy of the system, as represented, for example, in the
permanent deformation of the surface material. Thus, friction is clearly a
process of energy dissipation.
Basic Mechanisms of Friction. Surfaces are not completely flat at the
microscopic level. At high magnification, even the best polished surface
will show ridges and valleys, asperities, and depressions. When two surfaces are brought together, they touch intimately only at the tips of a few
asperities. At these points, the contact pressure may be close to the hardness of the softer material; plastic deformation takes place on a very local
scale. Cold welding may form strongly bonded junctions between the two
materials. When sliding begins, these junctions have to be broken by the
friction force, and this provides the adhesive component of the friction.
Some asperities may plow across the surface of the mating material, and
tan ()
=
F/ N
=
I.l. s
w
(a)
Fig. 1
(b)
Inclined plane used to determine coefficient of static friction, JLs• (a) Tilting flat surface
through smallest angle, S, needed to initiate movement of the body down the plane.
(b) Relation of the friction angle to the principal applied forces
46 I Surface Engineering for Corrosion and Wear Resistance
the resulting plastic deformation or elastic hysteresis contributes to the
friction force. Additional contributions may be due to wear by debris particles that become trapped between the sliding surfaces.
Because so many mechanisms are involved in generating the friction
force, it is clear that friction is not a unique materials property, but instead
depends to some extent on the measuring conditions, on the surface
roughness, on the presence or absence of oxides or adsorbed films, and so
on. In spite of this complexity, the values of f.1 obtained by different methods and by different laboratories tend to fall into ranges that are representative of the material pair in question under reasonably similar conditions;
that is, values obtained by different laboratories tend to fall within '""'-'20to
30% of each other if the testing conditions are generally similar. It is important, however, to understand that the values of f.1 listed in this Chapter
are intended only to provide rough guidelines and that more exact values,
if needed, must be obtained from direct measurements on the system in
question under its typical operating conditions.
The deformation at asperities and junctions is extremely localized, and
very high temperatures may therefore be generated over very short periods of time. At these local hot spots, rapid oxidation, plastic flow, or interdiffusion can take place, and these all affect the wear process. In some
cases, sparks may even form. The temperatures obtained depend on how
fast heat is generated (that is, on the operating conditions of load and velocity) and on how fast heat is removed (that is, on the thermal properties
of the sliding surfaces). These temperatures can be calculated with some
degree of certainty, as shown in Friction, Lubrication, and Wear Technology, Volume 18 of the ASM Handbook (see pages 39 to 44).
Friction Coefficients for Selected Materials. The friction coefficient
between solids sliding, or about to slide, over one another under the influence of a nonzero normal force is a function of several factors whose
relative contributions vary on a case-by-case basis:
•
•
•
•
•
•
•
•
•
Composition of the materials
Surface finish of each solid
Nature of the surrounding environment
Force holding the solids in contact (load)
Velocity of relative motion
Nature of the relative motion (for example, unidirectional, back and
forth, steady, variable, and so on)
Nature of the contact (conforming versus nonconforming surfaces)
Temperature of the interfacial region
Prior sliding history of the surfaces
Characteristics of the machine and fixtures in which the materials are
affixed
No single source has generated a comprehensive list of friction coefficients for materials under identical testing conditions; therefore, nearly all
Principles of Friction and Wear / 47
existing handbooks rely on compilations of data produced under a variety
of testing conditions. Readers should he aware of this shortcoming and
use the values only as very approximate guides, unless their applications
are exactly the same as those methods used in generating the data.
The five tables of friction coefficient values in this Chapter contain both
static and kinetic friction coefficients. They are arranged by material type
as follows:
•
•
•
•
•
Table
Table
Table
Table
Table
1: metals on metals
2: ceramics on various materials
3: polymers on various materials
4: coatings on various materials
5: miscellaneous materials
Table 1 Friction coefficient data for metals sliding on metals and corresponding references and test
conditions
Metals tested in air at room temperature
Material
Fixed specimen
Ag
Al
AI, alloy 6061- T6
Au
Brass,60Cu-40Zn
Cd
Co
Cr
Cu
Moving specimen
Ag
Au
Cu
Fe
Al
Ti
AI, alloy 6061- T6
Cu
Steel,1032
Ti-6AI-4V
Ag
Au
Steel, tool
Cd
Fe
Co
Cr
Co
Cr
Co
Cr
Cu
Fe
In
Mg
Mo
IS
IS
IS
IS
IS
IS
FOF
FOF
FOF
Zn
Steel, 4619
Co
Cr
Fe
FOF
IS
IS
POR
IS
IS
IS
IS
IS
IS
IS
IS
IS
IS
IS
IS
BOR
IS
IS
IS
Mg
Mo
Ti
W
Zn
In
Mg
Fe
Mo
IS
IS
IS
IS
IS
IS
IS
IS
IS
Ni
CU,OFHC
Fe
Test
geometry(a)
Friction coefficient
Static
0.50
0.53
0.48
0.49
0.57
0.54
0.42
0.28
0.35
0.34
0.53
0.49
Kinetic
1
1
1
1
1
1
0.34
0.23
0.25
0.29
0.24
2
2
2
2
1
1
3
1
0.79
0.52
0.56
0.41
0.41
0.46
0.44
0.46
0.55
0.50
0.49
0.56
1
1
1
1
1
1
1
1
1
1
1
0.82
0.41
0.48
0.51
Ref
4
1
1
1
0.51
0.46
0.49
0.47
0.55
1.46
0.69
0.46
0.44
(continued)
(a) Test geometry codes: BOR, flat block pressed against the cylindrical surface of a rotating ring; FOF, flat surface sliding on another flat surface; IS, sliding down an inclined
surface; POR, pin sliding against the cylindrical surface of a rotating ring; RSOF, reciprocating, spherically ended pin on a flat surface; SPOF, spherically ended pin on a flat
coupon
Table 1
(continued)
Material
Fixed specimen
Nb
Ni
Pb
Pt
Sn
Steel
Steel, 1020
Steel,1032
Steel, 52100
Steel, mild
Steel, M50 tool
Steel, stainless
Steel, stainless 304
Stellite
Ti
Ti-6AI-4V
W
Zn
Zr
Moving specimen
Test
geometry(a)
Nb
Cr
Ni
Pt
Ag
Au
Co
Cr
Fe
Pb
Steel
Ni
Pt
Fe
Sn
Cu
Pb
Steel,4619
AI, alloy 6061- T6
Cu
Steel, 1032
Ti-6AI-4V
Ni3Al, alloy IC-396M
Ni3Al, alloy IC-50
Steel, 1015 annealed
Steel, dual-phase DP-80
Steel, 02 tool
Steel, mild
Ni3A1, alloy Ie-50
Steel, tool
Cu
Steel, tool
Al
Steel, 17-4 stainless
Ti
Ti
Ti-6AI-4V
AI, alloy 6061- T6
Cu-AI (bronze)
Nitronic 60
Steel, 17-4 stainless
Steel, Type 440C stainless
Stellite 12
Stellite6
Ta
Ti-6Al-4V
Ti-6Al-4V
Cu
Fe
W
Cu
Fe
Zn
Zr
IS
IS
IS
IS
IS
IS
IS
IS
IS
IS
SPOF
IS
IS
IS
IS
SPOF
SPOF
BOR
FOF
FOF
FOF
FOF
RSOF
RSOF
BOR
BOR
BOR
BOR
RSOF
POR
FOF
POR
IS
POF
POF
FOF
POF
FOF
POF
POF
POF
POF
POF
POF
POF
FOF
POF
IS
IS
IS
IS
IS
IS
IS
Reference
Kinetic
0.46
0.59
0.50
0.64
0.73
0.61
0.55
0.53
0.54
0.90
0.80
0.64
0.55
0.55
0.74
0.47
0.32
0.31
0.36
0.23
0.54
0.48
0.47
0.55
0.43
0.41
0.36
0.38
0.36
0.44
0.35
0.45
0.53
0.36
0.36
0.41
0.47
0.51
0.56
0.55
0.75
0.63
0.80
1.40
0.54
0.38
0.25
0.23
0.32
1.08
0.70
0.74
0.55
0.49
0.62
0.68
0.53
0.21
0.60
0.48
0.40
0.36
0.38
0.27
0.31
0.31
0.37
0.29
0.36
0.53
0.30
0.31
Ref
1
1
1
1
1
1
1
1
1
1
5
1
1
1
1
5
5
4
2
2
2
2
6
6
7
7
7
3
6
3
2
3
1
8
8
1
8
2
8
8
8
8
8
8
8
2
8
1
1
1
1
1
1
1
Test condition
1. E. Rabinowicz,ASLE
Trans., Vol 14, 1971, p 198
2. "Friction Data Guide," General Magnaplate Corporation,
1988
3. J.E Archard, ASME Wear Control Handbook, M.B. Peterson and W.o.
Winer, Ed., American Society of Mechanical Engineers, 1980, p 38
4. A.W. Ruff, L.K. Ives, and W.A. Glaeser, Fundamentals of Friction
and Wear of Materials, ASM International, 1981, p 235
5. EP. Bowden and D. Tabor, The Friction and Lubrication of Solids,
Oxford Press, 1986, p 127
6. PJ. Blau and C.E. DeVore, Tribol., Int., Vol 23 (No.4), 1990, p 226
7. P.I. Blau, 1. Tribology, Vol 107, 1985, p 483
8. K.G. Budinski, Proceedings of Wear of Materials,
of Mechanical Engineers, 1991, p 289
Friction coefficient
Static
American
Society
Plate sliding on plate at 50% relative humidity
TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load, ground
specimens, 54% relative humidity, average of five tests
Pin-on-rotating ring, 3.9 N (0.40 kgf) load, 1.8 rnIs (350 ft/min) velocity
Flat block-on-rotating
35 mm (1 3/8 in.) diameter ring, ION (1.02 kgf)
load, 0.2 rnIs (40 ft/min) velocity
Sphere-on-flat, unspecified load and velocity
Reciprocating ball-on-flat, 10 Hz, 25 N (2.6 kgf) load, 10 mm stroke
Flat block-on-rotating
35 mm (1 3/8 in.) diameter ring, 133 N (13.6 kgf)
load, 5.0 crnls (2.0 in.ls) velocity
Modified ASTM G 98 galling test procedure
(a) Test geometry codes: BOR, flat block pressed against the cylindrical surface of a rotating ring; FOF, flat surface sliding on another flat surface; IS, sliding down an inclined
surface; POR, pin sliding against the cylindrical surface of a rotating ring; RSOF, reciprocating,
spherically ended pin on a flat surface; SPOF, spherically ended pin on a flat
coupon
Principles of Friction and Wear / 49
It should be emphasized that the data in the tables are for unlubricated
solids at room temperature and in ambient air. The references provided
with each table list both the sources of the data for the table and a brief description of the testing conditions used to generate these data, if such information was available in the reference. If accurate friction information
is required for a specific application, the use of carefully simulated
Table 2 Friction coefficient data for ceramics sliding on various materials and corresponding references
and test conditions
Specimens tested in air at room temperature
Material
Fixed
specimen
Ag
Al
Alumina
Boron carbide
Cr
Cu
Fe
Glass, tempered
Silicon carbide
Silicon nitride
Steel, M50 tool
Ti
Tungsten carbide
Moving specimen
Test
geometry(a)
Alumina
Zirconia
Alumina
Zirconia
Alumina
Alumina
Alumina
WRA(b)
WRZTA(c)
ZTA(d)
Boron carbide
Alumina
Zirconia
Alumina
Zirconia
Alumina
Zirconia
AI, alloy 6061- T6
Steel, 1032
Teflon(e)
Silicon carbide
Silicon nitride
Silicon nitride
Silicon nitride
Silicon carbide
Silicon carbide
Silicon carbide
Silicon nitride
Boron carbide
Silicon carbide
Silicon nitride
Tungsten carbide
Alumina
Zirconia
Tungsten carbide
RPOF
RPOF
RPOF
RPOF
SPOD
SPOD
SPOD
SPOD
SPOD
SPOD
POD
RPOF
RPOF
RPOF
RPOF
RPOF
RPOF
FOF
FOF
FOF
SPOD
SPOD
SPOD
SPOD
SPOD
SPOD
SPOD
SPOD
POD
POD
POD
POD
RPOF
RPOF
POD
Reference
1. K. Demizu, R. Wadabayashim, and H. Ishigaki, Tribol. Trans., Vol
33 (No.4), 1990, p 505
2. P.J. Blau, Oak Ridge National Laboratory
3. PJ. Blau, Oak Ridge National Laboratory
4. P.J. Blau, Oak Ridge National Laboratory
5. C.S. Yust, Tribology of Composite Materials, P.K. Rohatgi, P.J.
Blau, and C.S. Yust, Ed., ASM International, 1990, p 27
6. B. Bhushan and B.K. Gupta, table in Handbook of Tribology,
McGraw-Hill, 1991
7. "Friction Data Guide," General Magnaplate Corporation, 1988
Friction coefficient
Static
Kinetic
Ref
0.37
0.39
0.75
0.63
0.50
0.52
0.33
1
1
1
1
2
3
4
5
5
5
6
1
1
1
1
1
1
7
7
7
6
4
2
3
4
2
3
6
6
6
6
6
1
1
6
0.53
0.50
0.56
0.53
0.50
0.61
0.43
0.40
0.45
0.35
0.14
0.12
0.10
0.52
0.53
0.71
0.63
0.54
0.67
0.84
0.17
0.29
0.29
0.15
0.19
0.42
0.27
0.34
0.17
0.13
0.10
Test condition
1.5 mm (0.06 in.) radius pin reciprocating on a flat, 4 N (0.4 kgf)
load, 0.17 mmls (0.0067 in.ls) velocity, 50% relative humidity
1.0 N (0.10 kgf) load and 0.1 m/s (20 ft/min) velocity
10 N (1.0 kgf) load and 0.1 m/s (20 ft/min) velocity
9.5 mm (3/8 in.) diameter sphere-on-disk, 2 to 9 N (0.2 to 0.9 kgf)
load, 0.3 m/s (60 ft/min) velocity
20 N (2.0 kgf), 3 mmls (0.12 in.ls) velocity
TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load,
ground specimens, 54% relative humidity, average of five tests
(a) Test geometry codes: FOF, flat surface sliding on another flat surface; POD, pin-on-disk (pin-tip geometry not given); RPOF, reciprocating pin-on-flat; SPOD, spherically
ended pin-on-flat disk; SPOF, spherically ended pin on a flat coupon. (b) WRA, silicon carbide whisker-reinforced
alumina. (c) WRZTA, silicon carbide whisker-reinforced,
zirconia-toughened
alumina. (d) ZTA, zirconia-toughened
alumina. (e) Teflon, polytetrafluoroethylene
Table 3 Friction coefficient data for polymers sliding on various materials and corresponding references
and test conditions
Specimens tested in air at room temperature
Material(a)
Fixed specimen
Friction coefficient
Moving specimen
Test geometry(b)
Static
Kinetic
Ref
0.06
0.06
0.80
0.17
0.50
0.20
0.04
0.08
0.07
0.07
1
1
2
1
2
2
2
3
Polymers sliding on polymers
Acetal
Nylon 6/6
PMMA
Polyester PBT
Polystyrene
Polyethylene
Teflon
Acetal
Nylon 6/6
PMMA
Polyester PBT
Polystyrene
Polyethylene
Teflon
Teflon
TW
TW
NSp
TW
NSp
NSp
NSp
FOF
0.24
0.07
Dissimilar pairs with the polymer as the fixed specimen
Nylon 6 (cast)
(extruded)
Nylon 6/6
Nylon 6/6 (+ PTFE)
PA66
PA 66 (+ 15% PTFE)
PA 66 (PTFE/glass)
PEEK
PEEK (+ 15% PTFE)
PEEK (PTFE/glass)
PEl
PEl (+ 15% PTFE)
PEl (PTFE/glass)
PETP
PETP (+ 15% PTFE)
PETP (PTFE/glass)
Polyurethane( c)
Polyurethane( d)
POM
POM (+ 15% PTFE)
POM (PTFElglass)
PPS
PPS (+ 15% PTFE)
PPS (PTFE/glass)
Teflon
UHMWPE
Steel, mild
Steel, mild
Polycarbonate
Steel, mild
Steel,52100
Steel,52100
Steel,52100
Steel,52100
Steel,52100
Steel, 52100
Steel,52100
Steel,52100
Steel,52100
Steel,52100
Steel,52100
Steel, 52100
Steel, mild
Steel, mild
Steel,52100
Steel,52100
Steel,52100
Steel,52100
Steel, 52100
Steel, 52100
AI, alloy 6061- T6
Cr plate
Cu
Ni (0.001 P)
Steel, 1032
Ti-6AI-4V
TiN (Magnagold)
Steel, mild
TPOD
TPOD
TW
TPOD
BOR
BOR
BOR
BOR
BOR
BOR
BOR
BOR
BOR
BOR
BOR
BOR
TPOD
TPOD
BOR
BOR
BOR
BOR
BOR
BOR
FOF
FOF
FOF
FOF
FOF
FOF
FOF
TOPD
0.35
0.25
0.24
0.09
0.13
0.15
0.27
0.17
0.15
0.37
0.04
0.35
0.57
0.13
0.31
0.49
0.18
0.20
0.43
0.21
0.21
0.68
0.14
0.18
0.51
0.35
0.45
0.21
0.23
0.70
0.30
0.39
0.19
0.08
0.11
0.12
0.27
0.14
0.12
0.14
4
4
1
4
5
5
5
5
5
5
5
5
5
5
5
5
4
4
5
5
5
5
5
5
3
3
3
3
3
3
3
4
Dissimilar pairs with the polymer as the moving specimen
Steel, carbon
Steel, mild
Steel,52100
Steel, carbon
Steel,
Steel,
Steel,
Steel,
Steel,
Steel,
Steel,
Steel,
52100
mild
carbon
mild
carbon
mild
carbon
mild
ABS resin
ABS
ABS + 15% PTFE
Acetal
Acetal
HDPE
HDPE
LDPE
Lexan 101
Nylon (amorphous)
Nylon 6
Nylon 6
Nylon 6/6
Nylon 6/6
Nylon 6/10
Nylon 6/10
POF
TW
TW
TW
POD
POD
POF
POF
POD
TW
POF
TW
POF
TW
POF
TW
0.40
0.30
0.13
0.14
0.36
0.48
0.23
0.54
0.22
0.53
0.20
0.53
0.23
0.27
0.35
0.16
0.21
0.31
0.25
0.23
0.28
0.60
0.32
0.37
0.26
0.38
0.28
0.38
0.31
6
1
1
1
7
7
6
6
7
1
6
1
6
1
6
1
(continued)
(a) ABS, acrylonitrile butadiene styrene; HDPE, high-density polyethylene; LPDE, low-density polyethylene; Lexan, trademark of the General Electric Co. (polycarbonate);
nylon, one of a group of polyamide resins (see also PA); PA, polyamide; PBT, polybutylene terephthalate; PEEK, polyetheretherketone;
PEl, polyetherimide;
PETP, polyethylene terephthalate;
PMMA, polymethylmethacrylate;
POM, polyoxymethylene;
PPS, polyphenylene
sulphide; PTFE, polytetrafluoroethylene;
PVC, polyvinyl chloride;
UHMWPE, ultra high molecular weight polyethylene; Magnagold, product of General Magnaplate, Inc.; Teflon, trademark of E.1. Du Pont de Nemours & Co., Inc. (PTFE).
(b) Test geometry codes: BOR, flat block-on-rotating
ring; FOF, flat surface sliding on another flat surface; NSp, not specified; POD, pin-on-disk; POF, pin-on-flat; TPOD,
triple pin-on-disk; TW, thrust washer test. (c) Green polyurethane. (d) Cream-colored
polyurethane
Principles of Friction and Wear / 51
Table 3
(continued)
Material(a)
Fixed specimen
Friction coefficient
Moving specimen
Test geometry(b)
Nylon 6/12
PEEK (Victrex)
Phenol formaldehyde
PMMA
PMMA
Polycarbonate
Polyesther PBT
Polyethylene
Polyimide
Polyoxylmethylene
Polypropylene
Polypropylene
Polystyrene
Polystyrene
Polysulfone
PVC
PTFE
Teflon
Teflon
Teflon
Teflon
Teflon
Teflon
Teflon
Steel, carbon
Steel,52100
Steel, carbon
Steel, mild
Steel, carbon
Steel, mild
Steel, carbon
Steel, mild
Steel, carbon
AI, alloy 6061- T6
Cr plate
Glass, tempered
Ni (0.001 P)
Steel,1032
Ti-6AI-4V
TiN (Magnagold)
TW
TW
PDF
POD
PDF
TW
TW
TW
PDF
PDF
PDF
TW
PDF
TW
TW
PDF
PDF
FOF
FOF
FOF
FOF
FOF
FOF
FOF
Static
Kinetic
Ref
0.24
0.20
0.51
0.31
0.25
0.44
0.68
0.50
0.38
0.25
0.13
0.34
1
1
6
7
6
1
0.64
0.31
0.19
0.09
0.46
0.30
0.36
0.17
0.26
0.11
0.37
0.32
0.37
0.38
0.08
0.43
0.28
0.29
0.53
0.37
0.19
0.21
0.10
0.22
0.18
0.23
0.16
Reference
0.09
0.18
0.19
0.10
0.19
0.16
0.21
0.11
1
1
6
6
6
1
6
1
1
6
6
3
3
3
3
3
3
3
Test condition
1. "Lubricomp Internally-Lubricated
Reinforced Thermoplastics and
Fluoropolymer Composites," Bulletin 254-688, ICI Advanced Materials
2. EP. Bowden and D. Tabor, Appendix IV, The Friction and Lubrication of Solids, Oxford Press, 1986
3. "Friction Data Guide," General Magnaplate Corporation, 1988
4. J.M. Thorp, Tribol. Int., Vol 15 (No.2), 1982, p 69
5. J.W.M. Mens and A.W.J. de Gee, Wear, Vol 149, 1991, p 255
6. R.P. Steijn, Metall. Eng. Quart., Vol 7, 1967, p 9
7. N.P. Suh, Tribophysics,
Prentice-Hall,
1986, p 226
Thrust washer apparatus, 0.28 MPa (40 psi), 0.25 mls (50 ft/min),
after running-in for one full rotation
Unspecified
testing conditions
TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load,
ground specimens, 54% relative humidty, average of five tests
Three pin-on-rotating disk apparatus, 0.1 mls (20 ft/min)
Flat block-on-rotating
ring, 1.5 MPa (0.22 ksi) pressure, 150 N
(15 kgf) load, 0.1 mls (20 ft/min) velocity
12.7 mm (0.500 in.) diameter ball-on-flat, 9.8 N (1.0 kgf) load,
0.01 mmls (4 X 10-4 in.ls) velocity
Pin-on-disk, 4.4 N (0.45 kgf) load, 3.3 cmls (1.3 in.ls) velocity, 65%
relative humidity
(a) ABS, acrylonitrile butadiene styrene; HDPE, high-density polyethylene; LPDE, low-density polyethylene; Lexan, trademark of the General Electric Co. (polycarbonate);
nylon, one of a group of polyamide resins (see also PA); PA, polyamide; PBT, polybutylene terephthalate; PEEK, polyetheretherketone;
PEl, polyetherimide;
PETP, polyethylene terephthalate;
PMMA, polymethylmethacrylate;
POM, polyoxymethylene;
PPS, polyphenylene
sulphide; PTFE, polytetrafluoroethylene;
PVC, polyvinyl chloride;
UHMWPE, ultra high molecular weight polyethylene; Magnagold, product of General Magnaplate, Inc.; Teflon, trademark of E.I. Du Pont de Nemours & Co., Inc. (PTFE).
(b) Test geometry codes: BOR, flat block-on-rotating
ring; FOF, flat surface sliding on another flat surface; NSp, not specified; POD, pin-on-disk; POF, pin-on-flat; TPOD,
triple pin-on-disk; TW, thrust washer test. (c) Green polyurethane. (d) Cream-colored
polyurethane
Table 4
Friction
coefficient
data for coatings sliding on various materials
Specimens tested in air at room temperature
Material
Fixed specimen
AI, alloy 6061-T6
Au, electroplate
Friction coefficient
Moving specimen
Test geometry(a)
Cr plate
Ni (0.001 P) plate
TiN (Magnagold)( c)
60Pd-40Ag, plate
60Pd-40Au, plate
70Au-30Ag, plate
FOF
FOF
FOF
POF
POF
PDF
Static
Kinetic
Ref
0.27
0.33
0.25
0.22
0.25
0.22
2.40
0.30
3.00
1
I
1
2
2
2
(continued)
(a) Ams, Amsler circumferential,
rotating disk-on-disk machine; FOF, flat surface sliding on another flat surface; POD, pin-on-disk; POF, pin-on-flat; SPOD, spherically ended
pin-on-flat disk. (b) Teflon is a registered trademark of E.I. Du Pont de Nemours & Co., Inc. (polytetrafluoroethylene).
(c) Magnagold is a product of General Magnaplate, Inc.
Cd) CVD, chemical vapor deposition
Table 4
(continued)
Friction coefficient
Material
Fixed specimen
Au, electroplate
(continued)
Cr plate
Niobium carbide, coating
Ni (0.001 P) plate
Steel
Steel,1032
Steel, type 440C stainless
Steel, bearing
Steel, stainless
Teflon(b)
TiC on type 440C
stainless steel
TiN on type 440C stainless steel
TiN (Magnagold)( c)
Test geometry(a)
Moving specimen
80Pd-20Au, plate
99Au-1 Co, plate
Au plate
Au-0.6 Co, plate
Pd plate
AI, alloy 6061- T6
Ni (0.001 P) plate
Steel, 1032
Teflon(b)
Ti-6Al-4V
Niobium carbide, coating
AI, alloy 6061- T6
Cr plate
Ni (0.001 P) plate
Steel,1032
Steel, D2 tool
Teflon(b)
TiN (Magnagold)(c)
Cu film on steel
In film onAg
In film on steel
Pb film on Cu
Cr plate
Ni (0.001 P) plate
TiN (Magnagold)( c)
TiC on type 304 stainless
TiN on type 304 stainless
Chrome carbide
SiC (CVD)(d)
TiC (CVD)(d)
TiN (CVD)(d)
A1203, plasma-sprayed
Cr plate
Cr203, plasma-sprayed
Ti02, plasma-sprayed
WC-12 Co, plasma-sprayed
Cr plate
Ni (0.001 P) plate
TiN (Magnagold)(c)
Al
Ti
TiC on type 440C stainless
TiN on type 440C stainless
Al
Steel, type 304 stainless
Ti
TiC on type 440C stainless
TiN on type 440C stainless
AI, alloy 6061- T6
Steel, 1032
Teflon(b)
Ti-6A1-4V
TiN (Magnagold)( c)
steel
steel
0.20
0.19
0.20
0.21
0.38
0.19
0.26
0.41
0.32
0.35
0.43
0.22
0.33
0.30
0.10
0.08
0.18
0.25
0.37
0.31
0.12
0.50
0.09
0.15
0.15
0.50
0.65
0.22
0.25
0.27
0.29
0.50
0.05
0.65
0.30
0.38
0.16
0.26
0.25
Kinetic
Ref
1.80
2.40
2.80
0.40
2
2
2
2
2
1
1
1
1
1
3
1
1
0.60
0.19
0.17
0.17
0.19
0.33
0.13
0.23
0.36
0.28
0.31
0.33
0.19
0.26
1
1
1
1
1
4
4
4
4
0.21
0.30
0.28
0.17
0.75
0.79
0.23
0.25
0.49
0.13-0.30
0.30-0.38
0.14-0.15
0.10-0.15
0.11-0.13
0.08
0.12
0.12
0.85
0.80
0.20
0.20
0.40
0.41
0.76
0.06
0.45
0.26
0.31
0.11
0.23
0.21
1
1
1
5
5
6
6
6
6
7
7
7
7
7
1
1
1
5
5
5
5
5
5
5
5
5
1
1
1
1
1
Test condition
Reference
1. "Friction Data Guide," General Magnaplate
steel
steel
POF
POF
POF
POF
POF
FOF
FOF
FOF
FOF
FOF
FOF
FOF
FOF
FOF
FOF
FOF
FOF
FOF
SPOD
SPOD
SPOD
SPOD
FOF
FOF
FOF
POD
POD
POD
POD
POD
POD
Ams
Ams
Ams
Ams
Ams
FOF
FOF
FOF
POD
POD
POD
POD
POD
POD
POD
POD
POD
FOF
FOF
FOF
FOF
FOF
Static
Corporation,
1988
2. M. Antler and E.T. Ratcliff, Proceedings of the Holm Conference
on Electrical Contacts, 1982, p 19
3. M.J. Manjoine, Bearing and Seal Design in Nuclear Power Machinery, American Society of Mechanical Engineers, 1967
4. EP. Bowden and D. Tabor, The Friction and Lubrication of Solids,
Oxford Press, 1986, p 127
5. B. Bhushan and B.K. Gupta, Handbook of Tribology, McGrawHill, 1991, Table 14.16a
6. B. Bhushan and B.K. Gupta, Handbook ofTribology, McGrawHill, 1991, Table 14.65
7. B. Bhushan and B.K. Gupta, Handbook of Tribology, McGrawHill, 1991, Table 14.12
TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load,
ground specimens, 54% relative humidity, average of five tests
Sphere-on-reciprocating
flat, 0.49 N (0.050 kgf) load, 1.0 mmls
(0.039 in.ls) velocity
Flat plate-on-flat plate, 28 MPa (4.1 ksi) contact pressure, 0.25 mmls
(0.010 in.ls) velocity
Sphere-on-flat, low-speed sliding, 39.2 N (4 kgf) load
Pin-on-disk, 12 N (1.2 kgf) load, 14 to 16 cmls (0.55 to 0.63 in.ls)
velocity
Pin-on-disk, 5 N (0.5 kgf) load, 1.0 cmls (0.39 in.ls) velocity, 50%
relative humidity
Amsler disk machine, 400 rev/min, 250 N (26 kgf) load
(a) Ams, Amsler circumferential,
rotating disk-on-disk machine; FOF, flat surface sliding on another flat surface; POD, pin-on-disk; POF, pin-on-flat; SPOD, spherically ended
pin-on-flat disk. (b) Teflon is a registered trademark of E.I. Du Pont de Nemours & Co., Inc. (polytetrafluoroethylene).
(c) Magnagold is a product of General Magnaplate, Inc.
(d) CVD, chemical vapor deposition
Principles of Friction and Wear / 53
Table 5
Friction coefficient data for miscellaneous materials
Specimens tested in air at room temperature
Material
Fixed specimen
Brick
Cotton thread
Diamond
Explosives(b)
HMX(c)
PETN(d)
RDX(e)
Lead azide [Pb(N3)21
Silver azide (AgN3)
Glass, tempered
Glass, thin fiber
Glass, clean
Graphite, molded
Graphite (clean)
Graphite (outgassed)
Hickory wood, waxed
Ice
Leather
Metal
Mica (cleaved)
Mica (contaminated)
Nylon fibers
Paper, copier
Sapphire
Silk fibers
Steel (clean)
Wood (clean)
Friction coefficient
Moving specimen
Test geometry(a)
Static
Wood
Cotton thread
Diamond
UnSp
UnSp
UnSp
0.6
0.3
0.1
Glass
Glass
Glass
Glass
Glass
AI, alloy 6061-T6
Steel, 1032
Teflon(t)
Brass
Graphite
Porcelain
Steel, stainless
Teflon(t)
Glass (clean)
AI, alloy 2024
AI, alloy 2219
Graphite, extruded
Graphite, molded
Inconel X -7 50(g)
Steel, type 304 stainless
Steel, type 347 stainless
Graphite (clean)
Graphite (outgassed)
Snow
Bronze
Ebonite
Ice
Ice
Ice
Metal (clean)
Glass (clean)
Mica (cleaved)
Mica (contaminated)
Nylon fibers
Paper, copier
Sapphire
Silk fibers
Graphite
Metals
Wood (clean)
RPOF
RPOF
RPOF
RPOF
RPOF
FOF
FOF
FOF
StOD
StOD
StOD
StOD
StOD
UnSp
FOF
FOF
FOF
FOF
FOF
FOF
FOF
UnSp
UnSp
UnSp
UnSp
UnSp
UnSp
UnSp
FOF
UnSp
UnSp
UnSp
UnSp
UnSp
FOF
UnSp
UnSp
UnSp
UnSp
UnSp
Reference
0.9-1.0
0.16
0.22
0.20
0.18
0.16
0.18
0.19
0.10
0.5-0.8
Ref
0.55
0.40
0.35
0.28
0.40
2
2
2
2
2
3
3
0.14
0.12
0.10
0.16-0.26
0.15
0.36
0.31
0.10
5
0.17
0.14
5
5
5
5
5
5
1
1
0.14
0.02
0.02
0.05-0.15
>0.01
0.6
0.5-0.7
1.0
0.2-0.4
0.15-0.25
0.28
0.2
0.2-0.3
0.1
0.2-0.6
0.25-0.5
3
4
4
4
4
4
1
0.02
>0.01
6
6
6
6
6
3
1
1
1
1
1
0.26
3
1
1
1
1
1
Test condition
1. EP. Bowden and D. Tabor, Appendix IV, The Friction and Lubrication of Solids, Oxford Press, 1986
2. J.K.A. Amuzu, BJ. Briscoe, and M.M. Chaudhri, 1. Phys. D, Appl.
Phys., Vol 9, 1976, p 133
3. "Friction Data Guide," General Magnaplate
4. P.K. Gupta,1.
0.17
0.13
0.10
Kinetic
Corporation,
Am. Ceram. Soc., Vol 74 (No.7),
1988
1991, P 1692
5. M.J. Manjoine, Bearing and Seal Design in Nuclear Power Machinery, American Society of Mechanical Engineers, 1967
6. EP. Bowden and D. Tabor, The Friction and Lubrication of Solids,
Oxford Press, 1986
Method unspecified
Reciprocating, single-crystal flat sliding on smooth fired glass surfaces, range 5 to 20 gf (0.049 to 0.1962 N load), 0.20 mm1s
(0.008 in.ls) velocity
TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load,
ground specimens, 54% relative humidity, average of five tests
Strand lying on a rotating drum, 1.96 N (0.200 kgf) load, 8.5 mm1s
(0.33 in.ls) velocity
Flat plate-on-flat plate, 28 MPa (4.1 ksi) contact pressure, 0.25 mm1s
(0.010 in.ls) velocity
Unspecified method, 4.0 mls (790 ft/min) at 0 °C
(a) FOF, flat surface sliding on another flat surface; RPOF, reciprocating pin-on-flat; StOD, strand wrapped over a drum; UnSp, unspecified method. (b) Explosives reported
here were tested as reciprocating, single-crystal, flat-ended pin-on-moving
flat. (c) HMX, cyclotetramethylene
tetranitramine.
(d) PETN, pentaerithritol
tetranitrate. (e) RDX,
cyclotrimethylene
trinitramine. (0 Teflon is a registered trademark of E.I. Du Pont de Nemours & Co., Inc. (g) Inconel is a product of INCO, Inc.
54 / Surface Engineering for Corrosion and Wear Resistance
conditions or instrumentation of the actual machine should be conducted in
lieu of using tabulated values, because even a small change in contact conditions (for example, sliding speed or relative humidity for .some materials)
can result in a marked change in the measured or apparent friction coefficient. More detailed information on friction measurement techniques can be
found in the article "Laboratory Testing Methods for Solid Friction" in Friction, Lubrication, and Wear Technology, Volume 18 in the ASM Handbook.
Wear
In general, wear may be defined as damage to a solid surface .caused by
the removal or displacement of material by the mechanical action of a
contacting solid, liquid, or gas. Gradual deterioration is often implied, and
the effects are for the most part surface-related phenomena; but these restrictions should not be rigorously applied when analyzing wear problems
or failures. Neither should the assumption that wear is entirely mechanical be accepted, because chemical corrosion may combine with other
wear factors.
Classification of Wear
Wear has been classified in various ways. One of the simplest classifications of wear is based on the presence or absence of effective lubricantsnamely lubricated or nonlubricated wear. (The various types of lubricationllubricants are described later in this Chapter.) Another possibility is to
classify wear on the basis of the fundamental mechanism that is operating.
Unfortunately, this approach is complicated by the fact that more than one
mechanism may be operating at the same time and by the fact that those
developing wear classification schemes have come from different backgrounds and experiences with wear. As a result, different classification
schemes based on wear mechanisms have been developed and no one
scheme is universally accepted, although most of them have reasonably
similar features. Figures 2 and 3 show two wear classifications based on
the work of Blau (Ref 1) and Budinski (Ref 2). Blau places wear processes
into three categories based on the type of motion encountered, that is, sliding, impact, and rolling contact (Fig. 2). Budinski reduces wear .processes
into four categories, that is, abrasion, erosion, adhesion, and surface fatigue
(Fig. 3). Although both of the wear classifications schemes shown in Fig.
2 and 3 have merit, they also point out the difficulties inherent in classifying wear processes based on the commonality of mechanism.
A third approach to wear classification emphasizes the nature of the
contacting materials and the experimental conditions, using descriptive
terms that are widely understood and accepted. The following is an example of this type of classification:
Principles of Friction and Wear / 55
Metal against nonmetallic abrasive
High-stress gouging or grinding:
Wet, as in ball and rod mills
Dry, as in jaw-type or roll-type ore crushers
Low-stress scratching or sliding:
Wet, such as conveyor screws for wet sand
Dry, as against plows or earthmoving devices operating in dry soil
Impact of loose abrasive (erosion):
Wet, as against impellers in slurry pumps
Dry, as in sandblasting
Metal against metal
Sliding:
Lubricated, such as engine crosshead or shaft in a bearing
Nonlubricated, such as fasteners, nuts, and bolts
Rolling:
Lubricated, such as roller bearings and gears
Nonlubricated, such as wheels on tracks
Liquid or vapor impingement on metals
Wet steam,
Combustion
Water, such
Cavitations,
such as turbines
gases, such as gas turbines
as pump impellers
as in turbulent, high-velocity
flowing liquids
Frequently, these conditions are combined in service so an application that
was originally metal against metal may evolve into metal against nonmetal wear, such as the generation of oxide wear debris and the introduction of nonmetallic particles through imperfect seals. Other combinations
include rolling with sliding and lubricated-nonlubricated situations.
Abrasive
(cutting) wear
L 2-body
L Multibody
Adhesive
(3-body)
wear
Roiling/sliding
Sliding
Erosion
SOlidS
Fatigue wear
Liquids
L Delamination
Fretting wear
contact
Gases
Slurries
~
Electric sparks
Polishing wear
(chemo-mechanical
abrasion)
Rolling
~
Fig 2
Major categories of wear classified by the type of relative motion encountered (sliding, impact,
•
and rolling contact). Using this classification system, galling, scuffing, and scoring are not
strictly considered forms of wear because material is not necessarily removed (it may instead be displaced to one side). Rather, these latter phenomena are referred to as "surface damage." Source: Ref 1
56 / Surface
Engineering
for Corrosion
and Wear Resistance
Require hard,
sharp surfaces
imposed on
softer surfaces
Fig. 3
Require
repetitive
compressive
stresses
Require fluid
action
Major categories of wear based
surface fatigue. Source: Ref 2
Require
interaction
between
conforming
surfaces
on abrasion,
erosion,
adhesion,
and
Abrasive Wear
General Description. Abrasive wear is defined as wear due to hard
particles or hard protuberances forced against and moving along a solid
surface. This form of wear in metals is most frequently caused by nonmetallic materials, but metallic particles can also cause abrasion. Generally, a material is seriously abraded or scratched only by a particle harder
than itself. Figure 4 shows the damage caused on the surface of a soft copper substrate abraded by a hard ceramic particle.
The cost of abrasion is high and has been estimated as ranging from 1
to 4% of the gross national product of an industrialized nation. The effect
of abrasion is particularly evident in the industrial areas of agriculture,
mining, mineral processing, earth moving, and essentially wherever dirt,
rock, and minerals are handled. Examples include plows, ore loading/
moving buckets, crushers, and dump truck beds.
When two surfaces contact, wear occurs on both surfaces. Individuals and
industry tend to focus on the wearing surface that has the greatest potential
for their own economic loss, and consider the other surface to be the abrasive. For example, an individual walking up the stairs of a building would
Principles of Friction and Wear / 57
Fig. 4
Scanning electron micrograph showing surface damage by chip formation, plastic deformation, and pickup of fragments of a ceramic particle abrading a copper surface
be more likely to think that his shoes, rather than the stairs, were experiencing abrasive wear, whereas the maintenance staff would have the opposite opinion. In actuality, both surfaces are being subjected to abrasive wear.
The rate at which the surfaces abrade depends on the characteristics of
each surface, the presence of abrasives between the first and second surfaces, the speed of contact, and other environmental conditions. In short,
loss rates are not inherent to a material. With reference to the above example, changing the material of either the shoes or the steps could, and
often would, change the wear on the opposite counterface. The addition of
an abrasive, such as a layer of sand, on the steps would further change the
situation, in that the sand would be the second surface that contacts both
the shoes and the steps.
Abrasive Wear Categories. Abrasion is typically categorized according to types of contact, as well as contact environment. Types of contact
include two-body and three-body wear. The former occurs when an abrasive slides along a surface, and the latter, when an abrasive is caught between one surface and another. Two-body systems typically experience
from 10 to 1000 times as much loss as three-body systems for a given load
and path length of wear. Contact environments (Fig. 5) are classified as either open (free) or closed (constrained).
Tests have shown that for a given load and path length of wear, the wear
rate is about the same for both open and closed systems. However, measurements of the loss in closed systems will often appear higher than the
loss in open systems. This probably occurs because most closed systems
experience higher loads.
As shown in Fig. 3, abrasion is often further categorized as being lowstress abrasion, high-stress abrasion, gouging abrasion, and polishing
58 / Surface Engineering for Corrosion and Wear Resistance
Machining
(b)
(a)
Plow penetrating
sandy soil
Jaw crusher
(c)
(d)
Fig. 5
Types of contact during abrasive wear. (a) Open two-body. (b) Closed two-body.
(c) Open three-body. (d) Closed three-body
abrasion. As described in the following paragraphs, each of these forms of
abrasion is characterized by varying amounts of surface or subsurface
damage.
Low-stress abrasion (scratching) is defined as wear that occurs clue to
relatively light rubbing contact of abrasive particles with the metal. The
criteria established for low-stress abrasion is that the forces must be low
enough to prevent crushing of the abradant. Wear scars usually show
scratches, and the amount of subsurface deformation is minimal. Consequently, the surface does not work harden appreciably. Parts such as
screens, chute liners, blades, and belts that are exposed to sand slurries or
abrasive atmospheres could experience low-stress abrasion. Many machine components such as bushings, seals, and chains that operate in dust
will wear by low-stress abrasion. Figure 6(a) shows a surface that was
subjected to low-stress abrasion.
High-stress abrasion is wear under a level of stress that is high enough
to crush the abrasive. Considerably more strain hardening of the metal
surface occurs. The abrasion of ore grinding balls is an example of highstress abrasion in the mining industry. Other examples include abrasion
experienced by rolling-contact bearings, gears, cams, and pivots. Figure
6(b) illustrates this form of wear.
Principles of Friction and Wear / 59
(a)
(b)
(e)
(d)
Fig. 6
Schematics illustrating the four types of abrasive wear. (a) Low-stress abrasion where material
is removed by hard, sharp particles or other hard, sharp surfaces plowing material out in
furrows. (b) High-stress abrasion characterized by scratching, plastic deformation of surfaces, and pitting
from impressed particles. Damage is almost always more severe than low-stress abrasion. (c) Gouging
abrasion where material removal is caused by the action of repetitive compressive loading of hard materials such as rocks against a softer surface, usually a metal. (d) Polishing wear where material is removed from the surface by the action of rubbing from other solids under such conditions that material
is removed without visible scratching, fracture, or plastic deformation. The example shown is polishing
metal removal with a buffing wheel. Source: Ref 2
Gouging Abrasion. The term gouging abrasion is used to describe the
high-stress abrasion that results in sizable grooves or gouges on the worn
surface (Fig. 6c). It occurs on parts such as crusher liners, impact hammers in pulverizers, and dipper teeth handling large rocks. Strain hardening and plastic deformation are the dominant factors.
Polishing wear is an extremely mild form of wear for which the mechanism has not been clearly identified, but that may involve extremely finescale abrasion, plastic smearing of microasperities, andlor chemical corrosion (Fig. 6d). Surfaces that have been subjected to polishing wear are
usually smoothed and brightened, but this smoothing or brightening requires material removal. It can cause loss of serviceability in some instances, for example, worn and slippery stair treads.
Several mechanisms have been proposed to explain how material is removed from a surface during abrasion. These mechanisms include fracture, fatigue, and melting. Because of the complexity of abrasion, no one
mechanism completely accounts for all the loss. Figure 7 depicts some of
the processes that are possible when a single abrasive tip traverses a surface. They include plowing, wedge formation, cutting, microfatigue, and
micro cracking .
60 / Surface Engineering for Corrosion and Wear Resistance
Plowing
Microfatigue
Wedge
Microcracking
Cutting
Fig. 7
Five mechanisms of abrasive wear
Plowing is the process of displacing material from a groove to the sides.
This occurs under light loads and does not result in any real material loss.
Damage occurs to the near surface of the material in the form of a buildup of
dislocations through cold work. If later scratches occur on this cold-worked
surface, then the additional work could result in loss through microfatigue.
When the ratio of shear strength of the contact interface relative to the
shear strength of the bulk rises to a high enough level (from 0.5 to 1.0), it
has been found that a wedge can develop on the front of an abrasive tip.
In this case, the total amount of material displaced from the groove is
greater than the material displaced to the sides. This wedge formation is
still a fairly mild form of abrasive wear.
The most severe form of wear for ductile material is cutting. During the
cutting process, the abrasive tip removes a chip, much like a machine tool
does. This results in removed material, but very little displaced material
relative to the size of the groove. For a sharp abrasive particle, a critical
angle exists forwhich there is a transition from plowing to cutting. This
angle depends on the material being abraded. Examples of critical angles
range from 45° for copper to 85° for aluminum (Ref 3,4).
Brittle materials have an additional mode of abrasive wear, namely, microcracking or microfracture. This occurs when forces applied by the
abrasive grain exceed the fracture toughness of the material. This is often
the predominant mode of severe wear for ceramic materials, and is active
in materials such as white cast irons.
Principles of Friction and Wear / 61
Effects of Material Properties on Abrasive Wear. Although hardness
is the most important factor in the resistance to abrasion, other properties
such as elastic modulus, yield strength, fracture toughness, microstructure, and composition also play an important role. For example, the
abrasion resistance of ferrous metals is highly dependent on three metallurgical variables: microstructure, hardness, and carbon content. The inherently hard martensitic structure is preferable to the softer ferritic and
austenitic structures. This is especially significant in low-stress abrasion,
where little subsurface deformation occurs. When high-stress abrasion is
encountered, alloys with high work-hardened hardness values have improved wear resistance when compared with alloys with low work-hardened hardness values. Figure 8 compares the hardness of ferrous alloy
constituents with that of various minerals.
Prevention of abrasive wear is possible through proper material selection and the use of surface engineering treatments. A number of material
families have demonstrated good resistance to abrasive wear. They are
typically hard materials that resist scratching, and include ceramics, carbide materials, alloyed white cast irons containing hard chromium carbides (see Fig. 8), and hardened alloy steels.
Applicable surface treatments include:
•
•
•
•
•
•
•
Weld hardfacing coatings, for example, high-carbon iron-chromium
alloys, tool steels, nickel-chromium-boron alloys, cobalt-base alloys,
and austenitic manganese steels
Ceramic or cermet thermal spray materials deposited by plasma spraying, detonation gun (D-gun), or high-velocity oxyfuel processes
Hard chromium plating
Case hardening treatments
Selective hardening treatments, for example, flame hardening
Wear plates, for example, white cast iron or manganese steels
Hard coatings produced by vapor deposition, for example, TiN
Solid Particle Erosion
General Description. Solid particle erosion (SPE) is the loss of surface
material that results from repeated impacts of small, solid particles. In
some cases SPE is a useful phenomenon, as in sandblasting and highspeed abrasive waterjet cutting, but it is a serious problem in many engineering systems, including steam and jet turbines, pipelines and valves
carrying particulate matter, and fluidized bed combustion systems.
Solid particle erosion is to be expected whenever hard particles are entrained in a gas or liquid medium impinging on a solid at any significant
velocity (greater than 1 mis, or 3.3 ft/s). Manifestations of SPE in service
usually include thinning of components, a macroscopic scooping appearance following the gas/particle flow field, surface roughening (ranging
62 / Surface Engineering for Corrosion and Wear Resistance
from polishing to severe roughening, depending on particle size and velocity), lack of the directional grooving characteristic of abrasion, and, in
some but not all cases, the formation of ripple patterns on metals.
Solid particle erosion can occur in a gaseous or liquid medium containing solid particles. In both cases, particles can be accelerated or decelerated, and their directions of motion can be changed by the fluid. This is
more significant in liquid media, and slurry erosion is generally treated as
8000
7000
6000
5000
Vanadium carbide
(VC)
10
Diamond
9
Corundum (AI203)
8
Topaz
7
Quartz
6
Feldspar
5
Apatite
4
Fluorite
4000
Titanium carbid~
3000
__
""= = = =
(TiC)
Silicon carbide/""
(SiC)
»: :':
Tungsten
Mineral
Hardness
Mohs
Knoop
Microconstituent
car~-
2000
--
(WC)
Chromium carbide
[(M,CrhC31
/-
--
Cementite (Fe3C)
I
Martensite
Austenite
1000
900
800
700
600
500
-«
I
-<",
,
Pearlite
Ferrite ---
\
I
I
't,
-<
,,
400
300
200
Silica sand
Olivene
Taconite,
Glass
Leucite,
Ilmenite
Flint, garnet
Iron pyrite
M·agnetite
Hematite
Limonite
Siderite
Dolomite
3
Calcite
100
90
80
70
60
50
40
Bauxite
Biotite mica
Kaolin
2
Gypsum
30
Anthracite
Bituminous coal
Plastics
20
Talc
10
Fig. 8
Hardness of some carbides, minerals, and alloy microconstituents. In general, the
harder the material or constituent, the higher the abrasion resistance
Principles of Friction and Wear / 63
a different, though related, subject. In gaseous media, at least for particles
larger than about 50 urn, deflection of the particles by the gas stream can
often be ignored in erosion tests. However, it should be borne in mind that
in an engineering application, such effects can be quite important-as,
for
example, the spatial distribution and severity of erosion damage within
turbines and the effects of particle size and the change of direction and
speed when particles rebound from surfaces.
Erosion versus Abrasion. The distinction between erosion and abrasion should be clarified, because the term erosion has often been used in
connection with situations that might be better classed as abrasion. Solid
particle erosion refers to a series of particles striking and rebounding from
the surface, while abrasion results from the sliding of abrasive particles
across a surface under the action of an externally applied force. The clearest distinction is that, in erosion, the force exerted by the particles on the
material is due to their deceleration, while in abrasion it is externally applied and approximately constant. This serves as a good working definition of the difference between the two phenomena. A clear-cut distinction
between erosion and abrasion is difficult in some cases, particularly for
very dense particle distributions in liquid or gas media, in which a "pack"
of particles can develop and slide across the surface, which would be
classed as abrasion.
Variables Influencing Erosion. In practice, erosion rarely takes place
in inert atmosphere or vacuum, but at room temperature the effect of oxidation can generally be ignored. Erosion can be thought of as "pure," that
is, there are no synergistic interactions between erosion and corrosion
(erosion-corrosion phenomenon are described in Chapter 2). Variables affecting pure erosion can be broadly separated into three types: impingement variables describing the particle flow, particle variables, and material variables. The primary impingement variables are particle velocity (v),
angle of incidence (o.), and flux (particle concentration). u is defined here
and throughout virtually all erosion literature as the angle between the incident particle direction and the particle surface. Particle rotational speed
is an additional impingement variable; it is difficult to measure and has
not been studied in much detail. Particle variables include particle shape,
size, hardness, and friability (ease of fracture). Material variables include
all the material properties, such as hardness, work hardening behavior,
and microstructure.
In the erosion literature, materials are broadly classified as ductile or brittle, based on the dependence of their erosion rate on u. Ductile materials,
such as pure metals, have a maximum erosion rate, E, at low angles of incidence (typically 15 to 30°), while for brittle materials, such as ceramics,
the maximum is at or near 90°. These two classical extremes are illustrated
in Fig. 9. A variety of curves intermediate between these classical extremes
exist and in some cases the same material exhibits behavior that shifts from
one extreme to the other, depending on erosion conditions.
64 / Surface Engineering for Corrosion and Wear Resistance
The erosion rate, E, is commonly given in terms of mass or volume of
material removed per unit mass of erodent impacted, volume being preferred because it permits thickness loss comparisons between materials of
different density. Implicit is the assumption that the dimensions of the
eroded area and the particle concentration are unimportant, which is a
good approximation for dilute flows. Metals and ceramics differ in the dependence of E on ex, as mentioned above, and also in their response to velocity and particle size and shape. E generally shows a power-law velocity dependence:
E
=
kv"
(Eq 4)
where k is a constant and n is a velocity exponent that generally depends
on material and erosion conditions. The value of n usually falls in the
range of 2 to 2.5 for metals and 2.5 to 3 for ceramics, although observations exist outside these ranges. Material removal in ductile materials involves considerable plastic flow, while in brittle materials, fracture is of
primary importance, at least for higher angles of incidence. Theories predict that E should be inversely proportional to hardness for metals, while
for ceramics there should be a much weaker dependence on hardness, but
fracture toughness should be important. Most theories further predict no
effect of particle size for metals, although it is often observed that E increases strongly with particle size, at least up to about 100 urn, For ceramics, a particle size exponent is predicted and observed.
Hansen (Ref 5) compared the erosion rates of a large number of alloys,
ceramics, and cermets. Although limited to a particular set of conditions,
Hansen's study provides the practicing engineer with a good comparison
of erosion rates for a very broad range of materials under the same condi20------~--------------20
b
~
~
s
S
~c
o
"u;
~
o
6
~
AI1100-0
16
~-+-~---+------+----~
16
b
~
~
12
12 .8!
~
~
8
8
i
0
m
4H-----+-I"-----t-------i4
O~
~
~
«
~O
900
Angle of impingement, a
Fig. 9
Erosion of 1100-0 aluminum relative to AI203 when both are eroded
by 127 mm SiC particles impinging at a velocity of 152 m/s (499 ft/s)
Principles of Friction and Wear / 65
tions. Estimation of the effects of variables such as velocity on the erosion
rates of different classes of materials can be made from the remainder of
this article. It should be noted that the order of material rankings would
change with any change of variables such as velocity, particle type or size,
and angle of incidence. The erosion tests were performed using 27 urn
Al203 particles at normal incidence and 170 mls (560 ftls) at 20 and
700°C (68 and 1300 OF) in nitrogen. As in most other studies, the tests
were conducted with a gas-jet erosion apparatus in which particles are fed
from a hopper into a nozzle, where they mix with and are accelerated by
a flowing gas stream (ASTM G 76 as described later in this Chapter).
Hansen (Ref 5) normalized the erosion rates by defining the relative erosion factor (REF) as specimen volume loss divided by that of a standard
material, Stellite 6B (Table 6). Figures 10 and 11 show this data for
Table 6 Erosion test results for selected metals and ceramics evaluated at room temperature and at
elevated temperature
Test parameters: 90° impingement;
test duration; N2 atmosphere
27 mm AI203 particles; 5 glmin (0.18 ozlmin) particle flow; 170 rn/s (560 ftls) particle velocity;
3 min
Relative erosion factor (REF)(c)
Material
Manufacturing
method(a)
Composition (manufacturer) (b)
20°C
(70°F)
700°C
(1290 OF)
1.26
1.25
1.19
1.08
1.04
1.00
1.00
0.54
1.00
1.16
1.06
1.61
1.00
Metals
Ti-6AI-4V
Haynes 93
25Cr iron
Stellite 6K
Stellite 3
Stellite 6B
Type 304 stainless steel
Type 316 stainless steel
Haynes 188
Haynes 25
Type 430 stainless steel
HK-40
Inconel600
RA330
Incoloy 800H
Beta III Ti
Incoloy 800
RA333
Inconel671
Mild steel
Molybdenum
Thngsten
W
C
C
W
C
W
W
W
W
W
W
C
W
W
W
W
W
W
W
W
W
W
17Cr-16Mo-6.3Co-3C-bal
Fe (Stellite)
25Cr-2Ni-2Mn-0.5Si-3.5C-bal
Fe (OGC)
30Cr-4.5W-l.5Mo-l.7C-bal
Co (Stellite)
31Cr-12.5W-2.4C-bal
Co
30Cr-4.SW-l.SMo-l.2C-bal
Co (Stellite)
17Cr-9Ni-2Mn-lSi-bal
Fe
17Cr-12Ni-2Mn-lSi-2.SMo-bal
Fe
22Cr-14.5W-22Ni-0.lSC-bal
Co (Stellite)
22Cr-lSW-10Ni-l.SMn-0.15C-bal
Co (Stellite)
17Cr-lMn-lSi-0.lC-bal
Fe
26Cr-20Ni-OAC-bal Fe
76Ni-lS.SCr-8Fe
(HA)
19Cr-35Ni-1.5Mn-1.3Si-bal
Fe (RA)
32.5Ni-21Cr-0.07C-46Fe
(RA)
11.SMo-6Zr-4.5Sn-bal
Ti
32.5Ni-46Fe-21 Cr (HA)
2SCr-1.5Mn-1.3Si-3Co-3Mo-3W-18Fe-bal
Ni (RA)
SONi-48Cr-OATi (HA)
O.ISC-bal Fe
(GE)
0.99
0.97
0.96
0.93
0.93
0.92
0.91
0.91
0.90
0.83
0.80
0.77
0.76
0.S2
0.48
0.73
0.56
0.83
0.8S
0.62
0.78
0.61
0.79
0.S4
0.57
0.57
0.80
0.62
0.17
Ceramics
ZRBSC-M
Chromite
Refrax 20C
HD43S
Carbofrax D
HD430
Si3N4
Norbide
BT-9
HP
PS
PS
PS
HP
HP
PS
ZrB2-SiC-graphite
(N)
(UCAR)
SiC-Si3N4 bond (Carbor)
Recrystallized SiC (N)
SiC-ceramic bond (Carbor)
Recrystallized SiC (N)
(N)
B4C (N)
2MgO-25TiB2-3.5WC-bal
Al203 (OGC)
6.36
2.44
0.91
0.80
0.49
0040
0040
0.38
0.37
>S.OO
3.43
LIS
0.32
1.38
0.38
0.12
0.21
0.36
(continued)
(a) W, wrought; C, cast; HP, hot pressed; PS, pressed and sintered. (b) Manufacturer:
BW, Babcock and Wilcox; Carbor, Carborundum
Co.; GE, General Electric Co.; HA,
Huntington Alloy Products; N, Norton Co.; OGC, Oregon Graduate Center; RA, Rolled Alloys Corp.; Stellite, Stellite Div., Cabot Corp.; UCAR, Union Carbide Corp. (c) REF
= Volume loss material/volume
loss Stellite 6B
66 / Surface Engineering for Corrosion and Wear Resistance
Table 6
(continued)
Relative erosion factor (REF)(c)
Material
BT-12
BT-ll
ZRBSC-D
BT-24
BT-IO
Noroe 33
TiC-AI203
SiC
CBN
GEdiamond
Manufacturing
method(a)
PS
PS
HP
PS
PS
HP
PS
HP
Composition (manufacturer) (b)
1.5MgD-49TiB2-3.5WC-bal
1.7MgO-38TiB2-3.5WC-bal
ZrB2-SiC (N)
2MgO-30TiB2-3.5WC-bal
2MgO-30TiB2-3.5WC-bal
Si3N4-SiC (N)
(BW)
(N)
(GE)
(GE)
Al203 (OGC)
Al203 (OGC)
Al203 (OGC)
Al203 (DGC)
20°C
(70 OF)
700°C
(1290 OF)
0.35
0.33
0.32
0.32
0.30
0.20
0.19
0.12
0.16
0.26
0.07
0.20
0.25
0.42
0.30
0.02
o
o
o
o
(a) W, wrought; C, cast; HP, hot pressed; PS, pressed and sintered. (b) Manufacturer:
BW, Babcock and Wilcox; Carbor, Carborundum
Co.; GE, General Electric Co.; HA,
Huntington Alloy Products; N, Norton Co.; OGC, Oregon Graduate Center; RA, Rolled Alloys Corp.; Stellite, Stellite Div., Cabot Corp.; UCAR, Union Carbide Corp. (c) REF
Volume loss material/volume
loss Stellite 6B
=
metals and ceramics, respectively. Tungsten carbide-cobalt (WC-Co) cermets gave REFs from about 0.1 to 1.6, and REF was found to increase
with binder content. The REFs of most metals were similar at 20 and 700
°C (68 and 1300 OF), typically within about 20% of unity (Fig. 10). The
three lowest room-temperature REFs for metals were for tungsten (0.48),
molybdenum (0.52), and 1015 steel (0.76), and the highest was for Ti6AI-4V (1.26). The 700°C (1300 OF)erosion rate of the standard (Stellite
6B) was 20% higher than the room-temperature value, so that 700°C
(1300 OF)REF values greater than 0.8 represent increases of erosion rate
with temperature for a given material.
These results illustrate the unfortunate fact that alloy-strengthening
mechanisms such as solution or precipitation hardening that increase
hardness do not significantly improve erosion resistance. According to
Hansen (Ref 5), if service experience reveals an erosion problem for a
metallic component, substitution of another metallic alloy will generally
provide little improvement. Most ceramics tested had REF values in the
range 0.3 to 0.6, although a few were much higher, and a few were nearly
zero. It is important to note here (as discussed later) that for erodent particles of lower hardness than Al203 (used in Hansen's study), significant
improvements of erosion resistance can be obtained when the ratio of particle to target hardness, HplHt, is less than 1.
Prevention. Various design solutions have been developed in which
high erosion rates are avoided by reconfiguring the system-such
as the
blocked tee configuration, in which a tee joint with one end closed is used
in place of a gradual bend in a pipeline to prevent low-angle impingement.
A good example of the variety of engineering solutions to SPE is provided
by the case of power-generating steam turbines, in which exfoliation of
iron oxide scale formed on steel heater tubes generates large pieces of
scale that are fragmented into approximately 100 urn particles, causing
erosion of turbine blades, shrouds, valves, rivets, and other components.
Liquid droplet erosion is also present. Solid particle erosion solutions include minimizing of scale formation by using austenitic steels or chromiz-
Principles of Friction and Wear / 67
Ti-6AI-4V
Haynes 93
25Cr iron
Haynes Stellite 6 K
Haynes Stellite 3
Haynes Stellite 6B
Type 304 stainless steel
t·.::'·:;
Type 316 stainless steel
Haynes 188
Haynes 25
Fig. 10
Relative erosion factors for
selected commercially available metals at an impingement angle of
90°. Stellite 6B cobalt-base alloy was
used as the reference material. Source:
Ref 5
Type 430 stainless steel
HK-40
Inconel600
RA-330
Incoloy 800H
Beta][Ti
Incoloy 800
RA333
Inconel671
Mild steel
Molybdenum
_700°C
h:///::xJ 20°C
Tungsten
o
0.2
0.4
0.6
0.8
1.0
I
I
1.2
1.4
1.6
1.8
Relative erosion factor (REF)
specimen perforat~d
6.36
...:::,:.'.::::...:::'::i.::::,::!:.--t--
.:.::::'::.:::.::::';::.:::.::::: :.:::.:::.::'::::::'.:::.:::.:.:::.::::':::'::
....
.':.::" ::...::!::!::: ::.:::.::::!::!::::::.:::.:::::::!::
is:.•.ii t:.•::::i.:i ...::'.:'::i;.:ii::.·i:;:'.':i.::I.i;.::.:!:;:::!!:~;;:-::
.:':·::.·i:,::,:.·:;:,:,.•:;.:::,.-":':';.!;f::'.;::'i:•.::,.::,.:•;:.::'.::':'fi•.:...:,:,.:.:i:::-·:'::.::.·::i:.:• :ii .•::...:: ••::: i•...:•...::.i::;:.·.: ....t:.•.;.::i.;::!.:.::;: ...•
HD430
Si3N4
Norbide
BT-9
BT-12
BT-11
ZRBSC-D
BT-24
~
~ ••~
--:f:::'.::;·:::;::';
:'.:.::':::':'::::':
•••
..::'::,:
:'::'!::'.:::.::::':':.
Fig. 11
Relative erosion factors for
selected ceramics at an impingement angle of 90°. Ratings based
on using Stellite 6B cobalt-base alloy as
the reference material. Source: Ref 5
;:':':::::',:.::,:)
•••••
:':':::,::.f::::,:.
',::.:.':;:J
BT-10
Noroc 33
TiC-AI203
SiC
Cubic boron
nitride
Diamond
~
~
~
_700°C
L////d 20°C
~=o
0
0.2
0.4
0.6
0.8
1.0
1.2
I
1.4
Relative erosion factor (REF)
I
1.6
1.8
2.0
68 / Surface Engineering for Corrosion and Wear Resistance
ing treatments, particle removal with cyclones or screens, application of
plasma-sprayed or diffusion coatings to blades, and redesign of turbine
configurations.
Liquid Erosion
General Description. Erosion of a solid surface can take place in a liquid medium even without the presence of solid abrasive particles in that
medium. Cavitation, one mechanism of liquid erosion, involves the formation and subsequent collapse of bubbles within the liquid. The process
by which material is removed from a surface is called cavitation erosion,
and the resulting damage is termed cavitation damage. The collision at
high speed of liquid droplets with a solid surface results in a form of liquid erosion called: liquid impingement erosion.
Cavitation damage has been observed on ship propellers and hydrofoils;
on dams, spillways, gates, tunnels, and other hydraulic structures; and in
hydraulic pumps and turbines. High-speed flow of liquid in these devices
causes local hydrodynamic pressures to vary widely and rapidly. In mechanical devices, severe restrictions in fluid passages have produced cavitation damage downstream of orifices and in valves, seals, bearings, heatexchanger tubes. and venturis. Cavitation erosion has also damaged
water-cooled diesel-engine cylinder liners.
Liquid impingement erosion has been observed on many components
exposed to high-velocity steam containing moisture droplets, such as
blades in the low-pressure end of large steam turbines. Rain erosion, one
form of liquid-impingement erosion, frequently damages the aerodynamic
surfaces of aircraft and missiles when they fly through rainstorms at high
subsonic or supersonic speeds. Liquid impingement and cavitation erosion are of concern in nuclear power systems, which operate at lower
steam quality than conventional steam systems, and in systems using liquid metals as the working fluid, where the corrosiveness of the liquid
metal can promote rapid erosion of components.
Basic Mechanisms. Liquid erosion involves the progressive removal of
material from a surface by repeated impulse loading at microscopically
small areas. Liquid dynamics is of major importance in producing damage, although corrosion also plays a role in the damage process,
at least with certain fluid-material combinations. The process of liquid
erosion is not as well understood as most other wear processes. It is
difficult to define the hydrodynamic conditions that produce erosion and
the metallurgical processes by which particles are detached from the surface. Evidently, both cavitation and liquid impingement exert similar hydrodynamic forces on a solid surface. In any event, the appearance of
damaged surfaces (Fig. 12) and the relative resistance of materials to damage are similar for both liquid impingement and cavitation erosion. Additional information on the mechanism of material removal during cavitation can be found in Chapter 2, "Principles of Corrosion."
Principles of Friction and Wear / 69
Fig. 12
A cast steel feedwater-pump impeller severely damaged by cavitation. Note how
damage is confined to the outer edges of the impeller where vane speed was
maximum.
Prevention. Damage from liquid erosion can be prevented or minimized by reducing the intensity of cavitation or liquid impingement
through design, using erosion-resistant materials, for example, cobaltbase alloys .and tool steels including weld overlays of these materials, or,
under certain conditions, using elastomeric coatings.
Slurry Erosion
General Description. Slurry erosion is progressive loss of material
from a solid surface by the action of a mixture of solid particles in a liquid (slurry) in motion with respect to the solid surface. If the solid surface
is capable of corroding in the fluid portion of the slurry, the slurry erosion
will contain a corrosion component. Figure 13 shows an example of slurry
erosion.
A slurry by definition is a physical mixture of solid particles and a liquid (usually water) of such a consistency that it can be pumped. The particles must be in suspension in the liquid, and most pump able slurries contain at least 10% solids.
Apparent Abrasivity. Typical pumpable slurries possess inherent "apparent abrasivity," which must be determined by testing to enable cost predictions for pump replacement parts or other equipment used for slurries.
Apparent abrasivity, without inhibition, is the complex synergistic reaction of many factors (Fig. 14). This reaction, known as the MorrisonMiller effect (Ref 6), is such that the wear response of a given material in
70 / Surface Engineering for Corrosion and Wear Resistance
Fig. 13
Schematic of slurry erosion.
Resistance of protective
film of corrosion products
to abrasivity of slurry
Corrosive
liquid
Galvanic corrosion (if
two metals involved)
True abrasivity of solids
(particle hardness, size,
shape, and concentration)
Released corrosive
connate water from
ore particles
Soluble elements
in solids forming
corrosive solution
Fig. 14
Dissolved air
(oxygen or
environment)
Synergistic effects of seven factors in slurry abrasivity
a certain slurry does not indicate how that material would respond to another slurry. Similarly, the effect of a certain slurry on one material does
not indicate how it would affect another material.
Other modes of wear are also encountered when handling slurries. As
shown in Fig. 15, these include abrasion-corrosion (the most severe wear
mode), scouring wear, abrasive metal-to-metal wear (crushing and grinding), high-velocity erosion, low-velocity erosion, saltation wear (rapid
wear caused when particles are moved forward in a series of short intermittent bounces from a bottom surface), and cavitation.
Principles of Friction and Wear / 71
(8)
(b)
Velocity
profile
Pipe wall
(c)
(d)
(e)
Pipe wall
\
~~fO\;:::j J \ ~
Large tumbling
(f)
//1~
Collapsing vapor
bubbles
rocks
(g)
Fig. 15
Slurry erosion wear modes. (a) Abrasion-corrosion. (b) Scouring wear, with wear areas equal
(left) and unequal (center and right). (c) Crushing and grinding. (d) High-velocity erosion.
(e) Low-velocity erosion. (f) Saltation erosion. (g) Cavitation
Miller numbers are used to determine the abrasivity of slurries, based
on the rate of metal loss from a standard 27% chrome-iron wear block that
reciprocates through any slurry, on a rubber lap, with an imposed load of
22.2 N (5 lbf) placed on the wearing block. The higher the number, the
greater the aggressive effect of the slurry on part life. The additional effect of corrosion (usually present in liquid slurries, even those mixed with
distilled water) was slow to be recognized. This was because such
chrome-iron is rather corrosion resistant and the original test actually fulfilled its objective to reveal the "true abrasivity" of the dry particles. The
effects of both abrasion and corrosion must be recognized in the operation
of any slurry-handling system.
Table 7 lists typical Miller numbers for selected slurry materials. The
wide variation in Miller numbers for some materials is due to the inclusion of varying amounts of "tramp" materials that usually occur with the
basic mineral. Information about the factors that contribute to Miller number abrasivity can be found in the ASTM G 75 standard.
Prevention of slurry erosion is accomplished through design changes,
for example, lessening the severity of pipe bends or using replaceable
72 / Surface Engineering for Corrosion and Wear Resistance
Table 7
Typical Miller numbers for selected slurry materials
Material
Alundum (400 mesh)
Alundum (200 mesh)
Aragonite
Ash
Ash, fly
Bauxite
Calcium carbonate
Carbon
Carborundum (220 mesh)
Clay
Coal
Copper concentrate
Detergent
Dust, blast furnace
Gilsonite
Gypsum
Iron ore (or concentrate)
Kaolin
Lignite
Limestone
Limonite
Magnesium hydrate
Magnetite
Microsphorite
Mud, drilling
Nickel
Phosphate
Potash
Pyrite
Quartzite
Rutile
Salt brine
Sand and sand fill
Sea bottom
Shale
Serpentine
Sewage, digested
Sewage, raw
Sodium sulfate
Soda ash tailings
Sulfur
Tailings (all types)
Tar sand
Waste, nickel
Waste, coal
Miller number(s)
241
1058
7
127
83,14
9, 33, 50, 76, 134
14
14,16
1284
34,36
6, 10, 21, 28,47,57
19, 37,58,68,
111, 128
6,8
57
10
41
28, 37, 64, 79, 122, 157, 234
7,30
14
22,30,39,43,46
113
4
64, 71, 134
76
10
31
68, 74, 84, 134
1,2
194
99
10
11
51,68, 85, 116, 138, 149, 246
11
53,59
134
15
25
4
27
1
24,61,91,
159,217,480,644
70
53
22,28
wear backs on 90° elbows in high-velocity slurry pipelines and protective
coatings. These include hardfacing alloys (e.g., cobalt alloys), plasmasprayed ceramics and cermets, hard platings, ceramic and carbide wear
tiles, ceramic-filled repair cements, chromized steels, cast cylinder liners,
plastic-lined pipe, and basalt-lined pipe (Ref 2).
Adhesive Wear
General Description. Adhesive wear is defined as wear by transference of material from one surface to another during relative motion under
load due to a process of solid-state welding (Fig. 16); particles that are removed from one surface are either permanently or temporarily attached to
the other surface.
Adhesive wear may be between metallic materials, ceramics, or polymers, or combinations of these. It is dependent on adhesion between the
Principles of Friction and Wear / 73
material, and that, in tum, depends on surface films like oxides or lubricants, as well as the mutual affinity of one material for another.
If loads are light and the natural spontaneous oxidation of a metal can
keep up with the rate of its removal by wear, then that wear rate will be
relatively low (the oxide acting asa lubricant). This is called mild wear.
If loads are high and the protective oxide is continually disrupted to
allow intimate metal to metal contact and adhesion, then the wear rate will
be high. This is called severe wear.
Theory. Solid surfaces are almost never perfectly smooth but rather
consist of microscopic or macroscopic asperities of various shapes. When
two such surfaces are brought into contact under a load normal to the general planes of the surfaces, the asperities come into contact and elastically
or plastically deform until the real area of contact is sufficient to carry the
load. A bond may then occur between the two surfaces that is stronger
than the intrinsic strength of the weaker of the two materials in contact.
When relative motion between the two surfaces occurs, the weaker of the
two materials fails, and material is transferred to the contacting surface. In
subsequent interactions, this transferred material may be retransferred to
the original surface (probably at a different location) or may become
totally separated as a wear debris particle of an irregular morphology (Ref
7). Formulas that have been proposed (Ref 8, 9), to describe this phenomenon are of the form:
Vad
==
kSL
3H or V
==
kSL
H
CEq 5)
where V is the wear scar volume, S is the distance of sliding, L is the load,
H is the indentation yield strength (hardness) of the softer surface, and k
is a probability factor that a given area contact will fracture within the
weaker material rather than at the original interface.
Formulations similar to Eq 5 have been shown to describe adhesive
wear over fairly wide ranges of sliding distances, under a variety of conditions, over limited ranges of load, and over limited ranges of hardness
when the same classes of material were compared (Ref 10, 11). While initial theoretical considerations assumed bare metal-to-metal contact, later
Fig. 16
Schematic of adhesive wear
74 / Surface Engineering for Corrosion and Wear Resistance
work assumed that oxide films, adsorbed films, andlor lubricant effects
could be accounted for by changing k or by using more complex formulations (Ref 12). It has also been proposed that true metal-to-metal adhesive
wear occurs at some time after motion is initiated when surface films or
contaminants are worn away. Presumably, therefore, more than one adhesive wear mechanism could be operating at any given time, depending
upon the presence or absence of various surface films in local areas.
Changes in the apparent value of k or klH as a function of load may be the
result of penetration of such films at sufficiently high load or the generation of new films as a result of frictional heating.
The wear coefficient, k, has been determined experimentally for a large
number of materials couples under various test conditions and geometries.
The values found range from about 10-3 to 10-8 (Ref 7). For example,
representative values of k for the end of a cylinder sliding against the flat
surface of a ring at 1.8 m1s (6 ft/s) under a 400 g load are given for various combinations of cylinder and ring materials in Table 8. In many laboratory experiments, a stationary specimen with a small surface area rubs
against a moving specimen with a large area. This frequently leads to a
much higher wear rate on the smaller specimen than on the larger because
of the constant contact and associated heating of the smaller specimen.
This relative area effect may influence the wear mechanisms operating
and may not be representative of field use. For most practical applications,
volume loss, as predicted by Eq 5, must be converted to a linear value representing penetration or decrease in length, for example, increase in diameter of a journal bearing bushing, reduction in shaft diameter, or reduction in the length of brush in an electric motor.
Primary Material Parameters. Materials selection for adhesion resistance requires careful consideration of the operating environment of the
workpiece in addition to the total functional performance required of the
workpiece itself. Wear properties of the steels vary widely with processing and heat treatment. Polymers are selected for sliding contact applications because of inherent properties such as inertness to manychemicals,
relatively low galling tendency, and self-lubricating properties. Ceramics
Table 8 Wear coefficients for various combinations of materials under
conditions of dry sliding
Sliding combination
Cylinder material
Ring material
Low-carbon steel
60-40 brass
PTFE
Bakelite
Beryllium copper
Tool steel
Stellite
Tungsten carbide
Tungsten carbide
Low-carbon steel
Hardened steel
Hardened steel
Hardened steel
Hardened steel
Hardened steel
Hardened steel
Low-carbon steel
Tungsten carbide
Wear coefficients
Wear coefficient, k
7.0 X
6.0 X
2.5 X
7.5 X
3.7 X
1.3 X
5.5 X
4.0 X
1.0 X
10-3
10-4
10-5
10-6
10-5
10-4
10-5
10-6
10-6
Hardness of softer
member, 106 gfcm2
18.6
9.5
0.5
2.5
21.0
85.0
69.0
18.6
130.0
given are for the end of a cylinder sliding against the flat surface of a ring at 1.8 mJs (6 ftls) under a 400 g load.
Principles of Friction and Wear / 75
are used where extreme resistance to high-temperature oxidation or resistance to highly corrosive materials or gases is required.
Prevention. The following guidelines are recommendations to prevent
adhesive wear in metals, polymers, and ceramics:
•
•
Avoid sliding similar materials together, particularly metals.
If fatigue due to repeated high-contact pressure is not likely to be a
problem, then high hardness is a desired property. However, avoid
sliding hard metals against hard metals in lubricated systems to avoid
scuffing and to accommodate debris.
• Consider the effect of relative hardness of phases in materials. For example, a high-chromium cast iron may have a hardness of 400 HB,
which is moderate. However, that cast iron may contain Cr7C3, which
has a hardness of about four times that of 400 HB and will damage the
countersurface considerably. The same applies to polymers, which
seem rather soft relative to metals. However, wear-resisting polymers
often contain glass or some other hard filler that wears metal counterfaces rather severely. Hard phases in one body may fragment and become embedded in the counterface, which causes abrasion if the fragments extend above the surface.
• . Even if done inadequately, lubrication will reduce wear. Some lubrication can be applied by providing an atmosphere that is corrosive in
order to form surface films, many of which produce lower friction
than if that film were not present.
Galling
General Description. Galling can be considered a severe form of adhesive wear. With high loads and poor lubrication, surface damage can
occur on sliding metal components. The damage is characterized by localized macroscopic material transfer, that is, large fragments or surface
protrusions that are easily visible on either or both surfaces. This gross
damage is usually referred to as galling, and it can occur after just a few
cycles of movement between the mating surfaces. Severe galling can result in seizure of the metal surfaces.
The terms scuffing and scoring are also used to describe similar surface
damage under lubricated conditions. Scuffing is the preferred term when
the damage occurs at lubricated surfaces, such as the piston ring-cylinder
wall contact. Scoring typically describes damage that takes the form of
relatively long grooves.
Primary Material Parameters. Materials that have limited ductility
are less prone to galling, because under high loads surface asperities will
tend to fracture when interlocked. Small fragments of material may be
lost, but the resultant damage will be more similar to scoring than
to galling. For highly ductile materials, asperities tend to plastically
76/ Surface Engineering for Corrosion and Wear Resistance
deform, thereby increasing the contact area of mated surfaces; eventually, galling occurs.
Another key material behavior during plastic deformation is the ease
with which dislocations cross slip over more than one plane. In face centered cubic (fcc) dislocations easily cross slip. The rate of cross slip for a
given alloy or element is usually indicated by its stacking-fault energy.
Dislocation cross slip is hindered by the presence of stacking faults, and a
high stacking-fault energy indicates a low number of impeding stacking
faults and an increased tendency to cross slip and, hence, gall. Table 9 lists
the stacking-fault energies of four fcc elements. Nickel and aluminum
have poor galling resistance, whereas gold and copper have good galling
resistance.
Materials that have a hexagonal close-packed (hcp) structure with a high
cIa ratio have a low dislocation cross slip rate and are less prone to galling.
This explains why cobalt-base alloys and cadmium-plated alloys resist
galling while titanium alloys tend to gall.
Prevention of galling is accomplished through proper design, for example, parts should have sufficient clearance, because tightly fitted parts
are more prone to galling. Adequate lubrication and various hard surface
coatings also can help prevent galling. Control of surface roughness
is another important factor. Highly polished surfaces «0.25
I-1m, or
10 p.in.) or very rough finishes (> 1.5 I-1m,or 60 u.in.) increase the tendency for wear and galling. It is theorized that very smooth surfaces lack
the ability to store wear debris because of the absence of valleys between asperities, which means the asperities will have greater interaction. Also, lubricants will tend to wipe off the smoother surface. Too
rough a finish results in interlocking asperities, which promote severe
tearing and galling.
Fretting
General Description. Fretting is a wear phenomenon that occurs between two mating surfaces; initially, it is adhesive in nature, and vibration
or small-amplitude oscillation is an essential causative factor. Fretting is
frequently accompanied by corrosion. In general, fretting occurs between
two tight-fitting surfaces that are subjected to a cyclic, relative motion of
extremely small amplitude.
Fretting generally occurs at contacting surfaces that are intended to be
fixed in relation to each other but that actually undergo minute alternating
Table 9
Metals
Gold
Copper
Nickel
Aluminum
Source: Ref 13
Stacking-fault energies of some common metals
Stacking-fault
energy,
30
40
80
200
eV gs/cm2
Principles of Friction and Wear / 77
relative motion that is usually produced by vibration. The relative displacements between bodies are quite small (<50 to 150 J.Lm).Fretting further differs from ordinary wear in that the bulk of the debris produced is
retained at the site of fretting. In ferrous materials, the fretting process creates a mass of reddish oxide particles. Fretting also occurs in nonoxidizing materials, such as gold, platinum, and cupric oxide.
Common sites for fretting are in joints that are bolted, keyed, pinned,
press fitted, or riveted; in oscillating bearings, splines, couplings, clutches,
spindles, and seals; in press fits on shafts;. and in universal joints, baseplates, shackles, and orthopedic implants. One additional problem with
fretting is that it may initiate fatigue cracks, which, in highly stressed
components, often result in fatigue fracture.
Prevention. Fretting, because of the significant role of oxidation, is best
combatted by oxidation-resistant coatings, for example, electroless nickel
or softer self-lubricating coatings like silver or indium. Solid-film lubricants are also successfully employed. Additional information on prevention of fretting corrosion can be found in Chapter 2, "Principles of Corrosion."
Rolling-Contact Wear
General Description. The rolling of one body over another, as in a
rolling-element bearing, can result in repeated stressing of the subsurface
material, the nucleation of microcracks, and the eventual production of
pits and spalls. Because rolling-contact wear is generally produced by
repetitive mechanical stressing, it is often associated with, or even referred
to as, rolling-contact fatigue.
Analysis of bearings and gears indicates that some degree of slip occurs
in many rolling-contact situations, such as in the cam and roller assembly
in an automobile valve train and in the engagement of gear teeth. Thus, it
is common to observe sliding wear (e.g., scuffing or polished-looking
areas) on components that are ordinarily considered to be in "rolling contact."
Prevention of rolling-contact fatigue is accomplished through proper
design and load ratings, lubrication, and the use of through-hardened or
case-hardened premium quality (clean) alloy steels.
Lubrication
One important means of reducing wear (as well as friction) is lubrication. Lubrication not only reduces the power consumption needed
to overcome friction but also protects rolling and sliding contact surfaces from excessive wear. Even with lubrication, however, wear still
occurs.
78 / Surface Engineering for Corrosion and Wear Resistance
On lubricated surfaces, the wear process is usually mild and generates
fine debris of a particle size as small as 1 or 2 urn, Abrasive wear or
delamination wear predominates under lubricated conditions. Electron
microscope examination of worn surfaces from lubricated assemblies frequently reveals a multitude of fine scratches oriented in the direction of
relative motion. The fine debris generated by abrasion becomes suspended
in the oil or grease. In devices using circulating -oil lubrication, advantage
has been taken of the fact that wear debris can be analyzed by spectroscopy and that deterioration of the device by wear can be diagnosed
from these results. This technique is used to monitor the condition of vital
components in aircraft and locomotive engines.
Modes of Lubrication
There are several basic modes of lubrication. In all modes, contact surfaces are separated by a lubricating medium, which may be a solid, a
semisolid, or a pressurized liquid or gaseous film.
Hydrodynamic lubrication is a system in which the shape and relative
motion of the sliding surfaces cause the formation of a fluid film having
sufficient pressure to separate the surfaces. Hydrostatic lubrication is a
system in which the lubricant is supplied under sufficient external pressure to separate the opposing surfaces by a fluid film. Elastohydrodynamic lubrication is a system in which the friction and film thickness
between the two bodies in relative motion are determined by the elastic
properties of the bodies in combination with the viscous properties of
the lubricant at the prevailing pressure, temperature, and rate of shear.
Dry-film (solid-film) lubrication is a system in which a coating of solid
lubricant separates the opposing surfaces and the lubricant itself wears
away. Boundary lubrication and thin-film lubrication are two modes in
which friction and wear are affected by properties of the contacting surfaces as well as by the properties of the lubricant. In boundary lubrication, each surface is covered by a chemically bonded fluid or semisolid
film, which mayor may not separate opposing surfaces, and viscosity of
the lubricant is not a factor affecting friction and wear. In thin-film lubrication, the lubricant usually is not bonded to the surfaces and it does
not separate opposing surfaces. Lubricant viscosity affects friction and
wear.
Lubricants
Almost any surface film can act as a lubricant, preventing cold welding
of asperities on opposing surfaces or allowing opposing surfaces to slide
across one another at a lower frictional force than would prevail if the film
were not present. Lubricants may be either liquid or solid (in some cases,
gas films may act as lubricants). One of the functions of a lubricant is to
carry away heat generated by two surfaces sliding under contact pressure.
Principles of Friction and Wear / 79
Liquid lubricants can dissipate heat better than solid or semifluid lubricants, but in all types, the shear properties of the lubricant are critical to
its performance.
Properties. Liquid lubricants maintain separation or opposing surfaces
by pressure within the film, which opposes the contact force. This pressure may be generated within the film, usually as a result of the shape of
the opposing surfaces, or the liquid may be forced between the opposing
surfaces by pressure from an external source. Regardless of the means of
creating pressure within the film, the opposing surfaces slide on a film of
liquid. Friction and wear are directly influenced by the thickness and
shear properties (viscosity) of the liquid. Where appropriate, the use of a
high-viscosity lubricant usually results in a relatively thick film and a low
wear rate. However, high sliding speeds cannot be accommodated by a
viscous film, because excessive heat generated within the film causes it to
become less viscous and to decompose chemically.
Full-film (thick-film) lubrication, such as occurs under hydrostatic or
hydrodynamic conditions, effectively separates asperities on opposing
surfaces, whereas thin-film and boundary lubrication allow asperity contact. The differences among these three conditions of liquid lubrication are
illustrated schematically in Fig. 17.
Some special types of boundary lubricants, most notably the extreme
pressure (EP) lubricants, react with a metallic surface, often at high temperatures, to produce a monomolecular film on the surface. This very thin
film contaminates the mating surfaces and prevents metal-to-metal contact
or adhesion. Extreme-pressure lubricants often contain extremely reactive
constituents that re-form the film instantly if it is scraped off one of the
surfaces. Film formation of this type is, in effect, corrosion; when it is uncontrolled or when the film is repeatedly scraped off and re-formed, deterioration of the surface can result.
Solid-film lubricants must be adherent to be effective, or they allow
metal-to-metal contact or introduce unwanted particles that roll and slide
Full-film lubrication
Fig. 17
Thin-film
lubrication
Boundary
lubrication
Schematic showing the relation of surface roughness to film thickness. Shown are conditions
of full-film, thin-film, and boundary lubrication.
80 / Surface
Engineering
for Corrosion
and Wear Resistance
within the joint. When they can be kept within the joint, graphite and
molybdenum disulfide make good lubricants because they shear easily in
certain crystallographic directions. Hard, adherent oxide films, such as
Fe304 on steel or anodized Al203 on aluminum, withstand wear because
they resist penetration and do not bond with most mating surfaces.
Lubricating oils are relatively free-flowing organic substances that are
used to lower the coefficient of friction in mechanical devices. They are
available in a broad range of viscosities, and many are blended or contain
additives to make them suitable for specific uses. In general, lubricating
substances that are fluid at 20°C (70 OF) are termed oils; lubricating substances that are solid or semifluid at 20 °C (70 OF) are termed greases or
fats.
Oils are derived from petroleum (mineral oils) or from plants or animals
(fixed oils). Mineral oils are classified according to source (type of crude),
refining process (distillate or residual), and commercial use. The commercial mineral oil base products consist mainly of saturated hydrocarbons (even though naphthene-base crudes are predominantly unsaturated)
in the form of chain or ring molecules that are chemically inactive and do
not have polar heads. These commercial products mayor may not contain
waxes, volatile compounds, fixed oils, and special-purpose additives.
Fixed oils and fats differ from mineral oils in that they consist of an alcohol radical and a fatty-acid radical, can be reacted with an alkali (sodium
hydroxide or potassium hydroxide, for example) to form glycerin or soap,
cannot be distilled without decomposing, and contain 9 to 12.5% oxygen.
All fixed oils are insoluble in water and, except for castor oil, are insoluble in alcohol at room temperature.
Fixed oils are generally considered to have greater oiliness than mineral
oils. Oiliness is a term that describes the relative ability of any lubricant
to act as a boundary lubricant.
Lubricating grease, as defined by ASTM, is a solid to semifluid product consisting of a dispersion of a thickening agent in a liquid lubricant.
In more practical terms, most greases are stabilized mixtures of mineral
oil and metallic soap. The soap is usually a calcium, sodium, or lithium
compound and is present in the form of fibers whose size and configuration are characteristic of the metallic radical in the soap compound.
Solid lubricants, which are solids with lubricating properties, can be
maintained between two moving surfaces to reduce friction and wear. Numerous solid inorganic and organic compounds, as well as. certain metals
and composite materials, may be classified as solid lubricants. Molybdenum disulfide (MoS2), graphite, polytetrafluoroethylene
(PTFE) , and
graphite fluoride (CFx) are the solid lubricants most commonly used. Several hundred different compounds and mixtures have been described as
potential solid lubricants. Increasingly, solid lubricants are being vapor
deposited for use in harsh environments in which liquids would evaporate
or congeal.
Principles of Friction and Wear I 81
Wear Testing (Ref 1)
Because different types of wear occur in machinery, many different
types of wear tests have been developed to evaluate effects of wear on materials and surface treatments. Consequently, the selection of the right type
of wear test for each investigation is important in order to achieve useful
and meaningful engineering data. More than one type of wear can attack
the same part, such as sliding wear and impact wear in printing presses, or
erosive wear and abrasive wear on extrusion machine screws for plastics.
Sometimes wear can operate in the presence of corrosive or chemically
active environments, and synergistic chemomechanical effects are possible. Selection of appropriate wear test methods begins with an assessment
of the type of wear involved in each problem area.
Wear testing is performed for one or more of the following reasons: to
screen materials, surface treatments, or lubricants for a certain application; to help develop new, wear-resistant materials, surface treatments, or
lubricants; to establish the relationship between the manufacturing, processing, or finishing methods applied to a certain machine part and its
wear performance; or to better understand and model the fundamental nature of a certain type of wear. Surprising to some, the wear resistance of a
given material is not a basic material property, like elastic modulus or
yield strength. Rather, the wear behavior of a material depends on the conditions of its use. Therefore, the first step in wear testing is to recognize
how the results of the work will be used. Only then can the appropriate
test methodes), testing parameters, and a useful format for reporting the
results be selected.
Test Methods
Standardized Wear Testing Methods. A list of ASTM standardized
wear test methods, organized by type of wear or surface damage, is given
in Table 10. This is by no means a complete list of all available test methods; many nonstandardized tests developed by individual companies or research organizations are also frequently used. Oftentimes if a suitable
standard test method does not exist for a specific type of wear problem, an
organization may decide to develop its own internal wear-testing standards best suited to its purposes.
Variables to be Controlled in Wear Testing. Each wear mode is influenced by a different set of physical variables. Therefore, it is important
to recognize what factors must be controlled, or at least monitored, in the
design of wear testing procedures. Table 11 lists the major experimental
variables that are controlled in conducting wear tests of various types. Environmental and other factors, which should he considered in interpreting
the results of the wear tests, are also listed. Sometimes these secondary
factors must be controlled in order to simulate a given application, but
82 / Surface Engineering for Corrosion and Wear Resistance
Table 10 ASTMwear test methods grouped by wear type illustrated in Fig. 2
Form of wear
Abrasive wear, 2-body
Designation
G56
G 132
G 119
Abrasive wear, 3-body
G65
G 81
G 105
Erosive wear,
cavitating fluid
Erosive wear,
liquid droplets
Erosive wear, slurry
G32
Erosive wear,
solid particles
Fretting wear
G76
Sliding wear
D2266
G73
G75
D4170
D2670
D2882
D 2981
D 3702
D 3704
D4l72
D 5001
G77
G99
G 119
G 133
G 137
Surface damage, galling
G98
Surface damage, scoring
D 2782
Means of wear measurement
Title
Test Method for Abrasiveness of Ink-Impregnated Fabric
Printer Ribbon
Test Method for Pin Abrasion Testing
Guide for Determining Synergism between Wear
and Corrosion
Test Method for Measuring Abrasion Using the Dry
SandlRubber Wheel Apparatus
Practice for Jaw Crusher Gouging Abrasion Test
Test Method for Conducting Wet SandlRubber Wheel
Abrasion Tests
Test Method for Cavitation Erosion Using Vibratory
Apparatus
Practice for Liquid Impingement Erosion Testing
Test Method for Determination of Slurry Abrasivity
(Miller Number) and Slurry Abrasion Resistance
Response of Materials (SAR Number)
Test Method for Conducting Erosion Tests by Solid
Particle Impingement Using Gas Jets
Test Method for Fretting Wear Protection of Lubricating
Greases
Test Method for Wear Preventative Characteristics of
Lubricating Grease (Four-Ball Method)
Test Method for Measuring Wear Properties of Fluid
Lubricants (Falex Pin and Vee Block Method)
Test Method for Indicating Wear Characteristics of
Petroleum and Non-Petroleum Hydraulic Fluids in
a Constant Volume Vane Pump
Test Method for Wear Life of Solid Lubricants in
Oscillating Motion
Test Method for Wear Rate of Materials in Self-Lubricated
Rubbing Contact Using a Thrust Washer Testing
Machine
Test Method for Wear Preventative Properties of
Lubricating Greases Using the (Falex) Block on
Ring Test Machine in Oscillating Motion
Test Method for Wear Preventative Characteristics of
Lubricating Fluid (Four-Ball Method)
Test Method for Measurement of Lubricity of Aviation
Turbine Fuels by the Ball-on-Cylinder Lubricity
Evaluator (BOCLE)
Test Method for Ranking Resistance of Materials to
Sliding Wear Using Block on Ring Wear Test
Test Method for Wear Testing with a Pin-on-Disk
Apparatus
Guide for Determining Synergism between Wear and
Corrosion
Test Method for Linearly Reciprocating Ball-on-Flat
Sliding Wear
Test Method for Ranking Resistance of Plastic
Materials to Sliding Wear Using a Block-on-Ring
Configuration
Test Method for Galling Resistance of Materials
Test Method for Extreme-Pressure
Lubricating Fluids
Properties
of
Surface profiling or other method
Mass loss
Mass loss and corrosion-related
measurements
Mass loss
Mass loss ratio
Mass loss, normalized by
wheel dimensions
Mass loss
Mass loss
Mass loss
Mass loss
Mass loss ratio
Wear scar diameter
"Teeth wear" apparatus-specific
measurement of wear
Mass loss
Number of revolutions to failure,
as indicated by friction
Thickness change
Wear scar width
Wear scar diameter
Wear scar diameter
Wear scar width
Ball: wear scar diameter,
disk: profile
Mass loss and corrosion-related
measurements
Ball: wear scar diameter, flat:
profile
Mass loss
Visual inspection, critical load
for galling
"OK" value of for load just
below critical scoring
condition
usually it is sufficient just to measure and document them as an aid to interpreting the data.
Wear Testing Devices: Commercially Manufactured and Custom
Made. Table 12 exemplifies the types of simple testing geometries common to evaluating the various forms of wear. While simple geometries,
such as those in Table 12, represent one approach to testing, some wear
Principles of Friction and Wear / 83
Table 11
types
Category
Sliding
Parameters that are commonly controlled and reported when conducting wear tests of various
Subcategory
Abrasive wear, 2-body
Abrasive wear, 3-body
"Adhesive"
Fretting wear
Polishing wear
Impact
2-body
Impingement,
liquid and solid
Cavitation erosion
Rolling
Rolling contact fatigue
Rolling with slip
Supplementary
characterizations or variables(a)
Typical variables
Load (contact pressure), abrasive type, binder
type, backing body, whether repeated
contact or sliding against fresh abrasive
lubricant or coolant, surface speed,
temperature, duration of contact
Load (contact pressure), abrasive type,
concentration, hardness of counterbody(b),
coolant or lubricant, whether repeated
contact or continual motion against fresh
abrasive surface, surface speed, temperature,
duration of contact
Load (contact pressure or stress), relative velocity,
contact geometry, type of motion (unidirectional
or oscillating), duration, sliding distance, or
time of sliding, temperature
Load (contact stress), contact geometry, amplitude of
oscillation, frequency of oscillation, number of
cycles or time, choice of lubricant
Size of polishing medium, concentration of medium,
liquid used for suspension, normal pressure, type
of motion bodies (platten and specimen), time of
exposure, temperature, substrate (pad type)
Force of impact, speed of impact, geometry of contact,
angle of impact, repetition rate, duration/number
of impacts, temperature
Average impact velocity, particle stream shape (by
nozzle design), impingement angle of the stream
to the surface, duration of exposure, temperature
of the specimen and/or jet
Test geometry, frequency of moving body oscillation,
temperature of the fluid, fluid type, duration of
exposure
Load (elastic contact stress), rpm of roller(s), test
duration, temperature
Load (elastic contact stress), rpm of roller(s), % slip,
test duration, temperature
Method of surface preparation,
characterization
material
Method of surface preparation,
material characterization
Method of surface preparation, cleaning, surface finish of bodies, type
of material/lubricant,
method of
supplying the lubricant, relative
humidity
Surface finish, relative humidity,
debris characteristics
Particle composition and geometric
description, method of medium
introduction, initial surface finish
of specimen
Material characterization, environment
and relative humidity, surface
finish of bodies
Particle velocity or flux distribution,
density of particles, particle shape
description, particle size
distribution, particle composition
Material characterization
Lubricant/material
characterization,
surface finish of rollers
Lubricant/material
type, surface finish
of rollers
(a) These quantities are often used to characterize the testing conditions or materials even though they may not be directly controlled in an experiment. In certain cases, they
could be treated as variables themselves. (b) In certain types of 3-body abrasive wear tests, notably the dry sand-rubber wheel test, the hardness of the material that is pressing
the loose abrasive particles against the test specimen can have a significant effect on the results.
machines are either one-of-a-kind or highly specialized for simulating a
particular application. Commercially manufactured sliding and rollingcontact wear testing machines are available in a number of contact geometries. Abrasion and erosion testing machines are also commercially available. Some testers, called "universal" or "multimode" testers, are
configured to permit the user to change the contact geometry from, for example, block-on-ring to pin-on-disk, using accessory fixtures and drive
mechanisms. Several manufacturers or retail sellers of wear testing machines advertise on the Internet and can be found through key-word
searches.
As indicated earlier, wear and chemical attack can have synergistic effects. Special procedures have been developed to study these phenomena
(ASTM G 119). Specialized commercial testing machines have also been
developed to study such effects, like machines that simulate the movements
of surgical knee and hip replacement components in bodylike fluids.
84 / Surface Engineering for Corrosion and Wear Resistance
Table 12 Typical testing geometries for wear tests of various types
Category
Sliding
Subcategory
Abrasive wear, 2-body
Abrasive wear, 3-body
"Adhesive"
Testing geometry
Flat pin-on-rotating abrasive drum (spiral path), reciprocating
pin-on-abrasive flat, flat pin-on-moving abrasive belt, traversing pin-on-abrasive disk (spiral path), twin rotating abrasive wheels
Dry sand fed between a rotating rubber wheel and a flat coupon,
reciprocating flat pin-on-a plate in a slurry bath, block-onrotating ring in a slurry bath, ball mill, or tumbling wear test
Block-on-ring (flat or conformal face), pin-on-disk, double rubshoe on rotating disk, reciprocating pin-on-flat, flat-on-flat
(thrust washer), pin clamped between V-blocks, ball spinning on three flats (120 apart)
Oscillating pin-on-flat, pivoting ball-in-socket, clamped specimen
on the sides of a tensile coupon
Flat specimen-on-vibrating lap, flat specimens in an orbital
polishing or lapping machine
Repetitive "hammer"-on-flat
Liquid jet aimed at the specimen, gas jet with entrained
particles, spinning specimens through a gravity-fed stream
of particles, centrifugal particle "slinger" apparatus
Oscillatory "hom" suspended above the specimen in a fluid,
flowing fluid through a submerged nozzle aimed at the
specimen
Disk-on-disk rolling contact (equal circumferential speed), rod
spinning between three captive balls
Disk-on-disk rolling contact (unequal circumferential speed)
0
Fretting wear
Polishing wear
Impact
2-body
Impingement, liquid and solid
Cavitation erosion
Rolling
Rolling contact fatigue
Rolling with slip
As with mechanical testing in general, commercial wear testing machines are being computer automated. While automation has definite
advantages, it also drives up the price of these machines. Testers with
infrequent wear problems who do not want to make a significant capital investment in wear testing. may be faced with the decision as to
whether to construct their own machine, purchase a commercial machine, have a custom machine built, or obtain the services of a fee-testing laboratory.
Published surveys, conducted years ago by organizations such as the
American Society of Lubrication Engineers (now called the Society of
Tribologists and Lubrication Engineers) and the European Space
Agency, have revealed the existence of hundreds of different wear testing devices. Some of these devices have similar geometries and operational features; however, even relatively similar-looking machines can
produce different wear results due to subtle differences in construction
features (fixture stiffness, method of specimen mounting, mechanical
damping capacity, natural frequencies, heat flow, etc.). The number of
custom-designed wear testing devices probably exceeds the number of
commercially produced machines. Unfortunately, people who decide to
build their own wear testing machines may not be aware of certain subtleties in wear tester design and, thus, may ultimately generate questionable results or results that cannot be reproduced elsewhere. There is
still a great deal of applied research needed to better understand differences in wear results arising from different machine designs and measurement techniques.
Principles of Friction and Wear / 85
References
1. P.J. Blau, Wear Testing, Metals Handbook Desk Edition, 2nd ed., J.R.
Davis, Ed., ASM International, 1998, p 1342-1347
2. K.G. Budinski, Wear Modes, Surface Engineering for Wear Resistance, Prentice Hall, 1988, p 15-43
3. A.J. Sedriks and T.O. Mulhearn, The Effect of Work-Hardening on the
Mechanics of Cutting in Simulated Abrasive Processes, Wear, Vol 7,
1964, P 451
4. T. Sasaki and K. Okamura, The Cutting Mechanism of Abrasive
Grain, Bull. Jpn. Soc. Mech. Eng., Vol 12, 1960, p 547
5. J.S. Hansen, Relative Erosion Resistance of Several Metals, Erosion:
Prevention and Useful Applications,
STP 664, ASTM, 1979,
P 148-162
6. R.A. Corbett, "A Modified G-75 Abrasion Test for Corrosive Environments," Corrosion Testing Laboratories, Inc., Newark, DE
7. L.E. Samuels et aI., Sliding Wear Mechanisms, Fundamentals of Friction and Wear of Materials, D.A. Rigney, Ed., American Society for
Metals, 1981, p 13-42
8. F.P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Vol
1 and 2, Clarendon Press, 1950 and 1966
9. J.F. Archard, Contact and Rubbing of Flat Surfaces, J. Appl. Phys., Vol
24, 1953, p 981-988
10. M.M. Kruschov, Resistance of Metals to Wear by Abrasion; Related
to Hardness, Institute of Mechanical Engineers Conference: Lubrication arzd Wear, 1975, p 655-659
11. R.C. Tucker, Jr., Plasma and Detonation Gun Deposition Techniques,
Deposition Technologies for Films and Coatings, R.F. Bunshah et aI.,
Ed., Noyes Publications, 1982, p 454-489
12. C.N. Rowe, Some Aspects of the Heat of Absorption in the Function
of a Boundary Lubricant, Trans. ASLE, Vol 9, 1966
13. L.K. Ives, M.B. Peterson, and E.P. Whitenton, "Galling: Mechanism and Measurement," National Bureau of Standards Report,
p 33-40
Selected References
•
•
•
Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook,
ASM International, 1992
M.B. Peterson and W.O. Winer, Ed., Wear Control Handbook, American Society for Mechanical Engineers, 1980
D.A. Rigney, Ed., Fundamentals of Friction and Wear of Metals,
American Society for Metals, 1981
86 / Surface Engineering for Corrosion and Wear Resistance
•
•
J.A. Schey, Tribology in Metalworking: Friction, Lubrication, and
Wear, American Society for Metals, 1983
A.Z. Szeri, Tribology: Friction, Lubrication, and Wear, McGraw-Hill,
1980
CHAPTER
4
Surface Engi neeri ng to
Change the Surface
Metallurgy
THE SURFACE-ENGINEERING
TREATMENTS described in this
Chapter include selective surface hardening by flame, induction, or highenergy beam heating, laser melting and quenching, and shot peening. None
of these processes change the surface chemistry, but rather they improve
properties such as wear and fatigue by altering the surface metallurgy.
Selective Surface Hardening
Surface hardening in a general sense involves many processes that improve the wear resistance. of parts while utilizing the tough interior properties of the steel or cast iron component. In this section, surface hardening is limited to localized heat treating processes that produce a hard
quenched surface without introducing additional alloying species. This
approach consists of hardening the surface by flame, induction, laserbeam, or electron-beam heating. More detailed information on surface
hardening of steels can be found in Heat Treating, Volume 4 of the ASM
Handbook.
Flame Hardening
Flame hardening consists of austenitizing the surface of steel by heating
with an oxyacetylene or oxyhydrogen torch and immediately quenching
with water. After quenching, the microstructure of the surface layer consists of hard martensite over a lower-strength interior core of other steel
morphologies such as ferrite and pearlite. A prerequisite for proper flame
88 / Surface Engineering for Corrosion and Wear Resistance
hardening is that the steel must have adequate carbon and other alloy additions to produce the desired hardness, because there is no change in
composition. Flame-hardening equipment utilizes direct impingement of
a high-temperature flame or high-velocity combustion product gases to
austenitize the component surface and quickly cool the surface faster than
the critical cooling rate to produce martensite in the steel. This is necessary because the hardenability of the component is fixed by the original
composition of the steel. Thus, equipment design is critical to success of
the operation. Flame-heating equipment may be a single torch with a specially designed head or an elaborate apparatus that automatically indexes,
heats, and quenches parts. With improvements in gas-mixing equipment,
infrared temperature measurement and control, and burner rig design,
flame hardening has been accepted as a reliable heat treating process that
is adaptable to general or localized surface hardening for small or
medium-to-high production requirements.
The flame-hardening process is used for a wide variety of applications.
These include (1) parts that are so large that conventional furnace treatments
are impractical or uneconomical, (2) prevention of detrimental treatment of
the entire component when only small segments of the part require heat
treatment, and (3) use of less costly material to obtain the desired surface
properties where alloyed steels would be normally applied. Flame hardening is limited to hardenable steels (wrought or cast) and cast iron. Typical
hardnesses obtained for the flame-hardened grades depend on the quench
media (Table 1). The practical level of minimum surface hardness attainable
with water quenching for various carbon contents is shown in Fig. 1.
Induction Hardening
Induction hardening is a versatile heating method that involves placing
a steel part in the magnetic field generated by high-frequency alternating
65
,./
60
J
0
~
:c
en
(J)
"E
ro
50
:c
45
~
~
/
55
cti
c
~
J
Y
40
0.20
V
0.30
0.40
0.50
0.60
0.70
Carbon, %
fig. 1
Relationship of carbon content to minimum surface hardness attainable by flame or induction heating and water quenching. Practical
minimum carbon content can be determined from this curve. Source: Ref 1
Surface Engineering to Change the Surface Metallurgy / 89
Table 1 Response of steels and cast irons to flame hardening
Typical
Material
hardness,
HRC,
as affected
by quenchant
Air(a)
Oil(b)
Water(b)
50-60
55-62
52-58
58-62
58-62
45-55
50-55
52-57(c)
55-60
33-50
55-60
60-63
62-65
45-55
55-62
58-64
50-60
50-60
58-62
60-63
62-65
62-65
45-55
50-60
55-60
55-60
52-57(c)
55-60
58-62
61-63
50-55
52-56
58-62
53-57
56-60
52-56
55-60
52-60
52-57
55-63
55-62
60-64
63-65
63-65
55-60
55-60
62-65
60-63
62-65
60-63
62-64
55-60
58-62
62-64
55-60
58-62
58-62
62-65
58-62
63-65
64-66
62-65
41-44
42-47
49-56
55-59
41-44
42-47
49-56
55-59
52-56
49-48
48-52
35-43
52-56
56-59
Plain-carbon steels
1025-1035
1040-1050
1055-1075
1080-1095
1125-1137
1138-1144
1146-1151
Carburized grades of plain-carbon steels(d)
1010-1020
1108-1120
Alloy steels
1340-1345
3140-3145
3350
4063
4130-4135
4140-4145
4147-4150
4337-4340
4347
4640
52100
6150
8630-8640
8642-8660
52-56
58-62
53-57
56-60
52-56
55-60
48-53
55-63
Carburized grades of alloy steels(d)
3310
4615-4620
8615-8620
Martensitic stainless steels
410,416
414,431
420
440 (typical)
Cast irons (ASTM classes)
Class 30
Class 40
Class 45010
50007,53004,60003
Class 80002
Class 60-45-15
Class 80-60-03
52-56
43-48
48-52
35-45
55-60
56-61
35-45
55-60
(a) To obtain the hardness results indicated, those areas not directly heated must be kept relatively cool during the heating process. (b)
Thin sections are susceptible to cracking when quenched with oil or water. (c) Hardness is slightly lower for material heated by spinning or combination progressive-spinning
methods than it is for material heated by progressive or stationary methods. (d) Hardness values of carburized cases containing 0.90-1.10% C. Source: Ref 1
current passing through an inductor, usually a water-cooled copper coiL
The depth of hardening increases as the frequency of the alternating current decreases. Other variables important to the process include the coil
current, heating time, and the coil design (Ref 2). The specific ferrous alloys that are commonly used in induction surface hardening are the same
as those used in flame hardening (Table 1). The minimum carbon contents
to obtain specific surface hardness are also shown in Fig. 1. Electrical
90 / Surface Engineering for Corrosion and Wear Resistance
Table 2
Comparison of flame- and induction-hardening
Characteristic
Equipment
Applicable
material
Speed of heating
Depth of hardening
Processing
Part size
Tempering
Can be automated
Operator skills
Control of process
Operator comfort
Cost
Equipment
Per piece
processes
Flame
Induction
Oxyfuel torch, special head quench
system
Ferrous alloys, carbon steels, alloy
steels, cast irons
Few seconds to few minutes
1.2-6.2 mm (0.050-0.250 in.)
Power supply, inductor, quench system
Same
One part at a time
No limit
Required
Yes
Significant skill required
Attention required
Hot, eye protection required
1-10 s
0.4-1.5 mm (0.015-0.060 in.); 0.1 mm
(0.004 in.) for impulse
Same
Must fit in coil
Same
Yes
Little skill required after setup
Very precise
Can be done in suit
Low
Best for large work
High
Best for small work
Source: Ref 2
properties of the alloy are an important consideration when selecting induction treatment as a surface-hardening technique. In induction hardening, the electrical resistivity and magnetic properties of the alloy can produce significant differences in heating characteristics. Thus, different
steels require differing induction-heating parameters. Table 2 compares
the flame- and induction-hardening processes.
High-Energy Beam Hardening
Electron- and laser-beam methods use high-energy beams to heat treat
the surface of hardenable steel. The electron-beam (EB) heat treating
process uses a concentrated beam of high-velocity electrons as an energy
source to heat selected parts of the steel component. In laser heat treatment, a laser beam is used to harden localized areas of ferrous parts. These
processes are similar to flame and induction hardening, except that the
need for quenchants is eliminated as long as a sufficient size workpiece is
being used.
Electron-Beam Hardening. In EB hardening, the surface of the hardenable steel is heated rapidly to the austenitizing temperature, usually
with a defocused electron beam to prevent melting. The mass of the workpiece conducts the heat away from the treated surface at a rate that is rapid
enough to produce hardening. Materials for application of EB hardening
must contain sufficient carbon and alloy content to produce martensite.
With the rapid heating associated with this process, the carbon and alloy
content should be in a form that will quickly allow complete solid solution in the austenite at the temperatures produced by the electron beam. In
addition, the mass of the workpiece should be sufficient to allow proper
quenching; for example, the part thickness must be at least ten times the
depth of hardening, and hardened areas must be properly spaced to prevent tempering of previously hardened areas. The most suitable materials
for EB hardening are the same steels used in flame hardening (Ref 2):
Surface Engineering to Change the Surface Metallurgy / 91
•
•
•
•
1045 to 1080 carbon steels
Medium- to high-carbon alloy steels (4140, 4340, 8645, 52100, etc.)
Pearlitic matrix cast irons
WI, W2, 01, 02, L2, L6, Sl, S2 tool steels
There are two basic types of EB systems: stationary or movable. In the
movable process, the workpiece is fixed and the gun is moved to produce
heating for the hardening. Stationary guns require manipulation of the
workpiece under the beam. However, in both cases the area to be hardened
on the workpiece must be in a line of sight with the beam. Gun movement
or workpiece manipulation is accomplished by computer control to produce any desired pattern, and the beam can be oscillated or pulsed by standard controls. To produce an electron beam, a high vacuum of 10-5 torr
(10-3 Pa) is required in the region where the electrons are emitted and accelerated. This vacuum environment protects the emitter from oxidizing
and avoids scattering of the electrons while they are still traveling at a relatively low velocity. Electron-beam hardening in hard vacuum units requires that the part be placed in a chamber that is sufficiently large to manipulate the gun or the workpiece. Out-of-vacuum units usually involve
shrouding the workpiece; a partial vacuum (10-2 torr, or 13 Pa) is obtained
in the work area by mechanical pumps.
Laser-Beam Hardening. Lasers can be used to perform selective hardening with hardening depths and material constraints similar to those of
EB hardening. Laser transformation hardening produces thin surface
zones, which are heated and cooled rapidly, resulting in very fine martensitic microstructures, even in steels with relatively low hardenability. This
process produces typical case depths for steel ranging from 0.75 to 1.3
mm (0.030-0.050 in.) depending on the laser power range, and hardness
values as high as 60 HRC. Laser processing has advantages over EB hardening in that laser hardening does not require a vacuum, wider hardening
profiles are possible, and there can be greater accessibility to hard-to-get
areas with the flexibility of optical manipulation of light energy. A major
disadvantage of lasers is the need to use surface treatments to prevent reflectivity of the laser beam.
laser Melting (Ref 3)
Processing. Laser melting requires higher power densities than the levels used for laser transformation hardening. The workpiece is often made
absorptive either by using coatings similar to those used for laser heating
or by increasing surface roughness, for example, by sand blasting. Laser
melting can harden alloys that cannot be hardened by laser transformation
hardening. In ferritic malleable gray iron, melting enhances the diffusion
of carbon, and the ensuing rapid quench produces a hardened region.
92 / Surface Engineering for Corrosion and Wear Resistance
Fig. 2
Cross sections of laser-melted cast iron surfaces. (a) Gray iron. (b) Ductile
iron. Source: Ref 3
Metallurgical changes with laser melting are in the forms of grain refinement, solid solutions, and fine dispersions of precipitates. All of these
can contribute to the hardening and strengthening of the surface. Lasermelted surfaces of cast irons appear dendritic, as shown in Fig. 2(a) for
gray iron and in Fig. 2(b) for ductile iron. Below the melt zone is the heataffected zone, which appears in lighter contrast in Fig. 2.
In the solidified melt in cast irons, a ledeburite (mixture of austenite
and cementite) structure generally forms. Hardening is caused by
graphite dissolution for form. cementite and austenite transformation to
martensite.
Wear Behavior of Laser-Melted Surfaces. Figure 3 shows that significant improvements in erosion behavior of gray and ductile iron are possible with laser melting. Improved wear resistance can also be obtained
with laser-melted tool steels.
Fig. 3
Erosive wear behavior of as-received and laser-melted gray and ductile
irons. Source: Ref 3
Surface Engineering to Change the Surface Metallurgy I 93
100
90
80
-5
OJ
c
~
2
"v;
c:
70
s
'0
?fi.
~~
60
E
en
50
40
Number of cycles to failure
Fig. 4
Fatigue curves for peened and unpeened steel spring wires
Shot Peening
Shot peening is a method of cold working in which compressive stresses
are induced in the exposed surface layers of metallic parts by the impingement of a stream of shot, directed at the metal surface at high velocity under
controlled conditions. It differs from blast cleaning in primary purpose and
in the extent to which it is controlled to yield accurate and reproducible results. Although shot peening cleans the surface being peened, this function
is incidental. The major purpose of shot peening is to increase fatigue
strength (Fig. 4). The process has other useful applications, such as relieving tensile stresses that contribute to stress-corrosion cracking, forming and
straightening of metal parts, and testing the adhesion of silver-plate on
steel. More detailed information on shot peening can be found in Ref 4.
References
1. T. Ruglic, Flame Hardening, Heat Treating, Vol 4, ASM Handbook,
ASM International, 1991, p 260-267
2. K.G. Budinski, Surface Hardening by Flame and Induction, Chapter
5, Surface Engineering for Wear Resistance, Prentice Hall, 1988,
p 120-137
94/ Surface Engineering for Corrosion and Wear Resistance
3. K.P. Cooper, Laser Surface Processing, Friction, Lubrication, and
Wear Technology, Vol 18, ASM Handbook, ASM International, 1992,
p 861-872
4. T. Kostilnik, Shot Peening, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 126-135
CHAPTER
5
Surface Engineering to
Change the Surface
Chemistry
SURFACE TREATMENTS that change the surface chemistry of a
metal or alloy, but that do not involve intentional buildup or increase in
part dimension, include:
•
•
•
•
•
Chemical or electrochemical conversion treatments that produce complex phosphates, chromates, or oxides on the metal surface
Thermochemical diffusion heat treatments that involve the introduction of interstitial elements, such as carbon, nitrogen, or boron, into a
ferrous alloy surface at elevated temperatures
Pack cementation diffusion treatments that involve the introduction of
aluminum, chromium, or silicon into an alloy surface
Surface modification by ion implantation, which involves the introduction of ionized species (virtually any element) into the substrate
using a beam of high-velocity ions
Surface modification by a combination of laser-beam melting and alloying
Phosphate Chemical Conversion Coatings
Phosphate coating is the treatment of iron, steel, galvanized steel, or aluminum with a dilute solution of phosphoric acid and other chemicals
in which the surface of the metal, reacting chemically with the phosphoric acid media, is converted to an integral, mildly protective layer of
96 / Surface Engineering for Corrosion and Wear Resistance
insoluble crystalline phosphate. The weight and crystalline structure of
the coating and the extent of penetration of the coating into the base metal
can be controlled by (Ref 1):
•
•
o
o
•
Method of cleaning before treatment
Use of activating rinses containing titanium and other metals of compounds
Method of applying the solution
Temperature, concentration, and duration of treatment
Modification of the chemical composition of phosphating solution
The method of applying phosphate coatings is usually determined by
the size and shape of the article to be coated. Small items, such as nuts,
bolts, screws, and stampings, are coated in tumbling barrels immersed in
phosphating solution. Large fabricated articles, such as refrigerator cabinets, are spray coated with solution while on conveyors. Automobile bodies are sprayed with or immersed in phosphating solution. Steel sheet and
strip can be passed continuously through the phosphating solution or can
be sprayed.
Phosphate coatings range in thickness from less than 3 to 50 urn (0.12 mils). Coating weight (grams per square meter of coated area), rather
than coating thickness, has been adopted as the basis for expressing the
amount of coating deposited.
Types of Phosphate Coatings
Three principal types of phosphate coatings are in general use: zinc,
iron, and manganese.
Zinc phosphate coatings encompass a wide range of weights and crystal characteristics, ranging from heavy films with coarse crystals to ultrathin microcrystalline deposits. Zinc phosphate coatings vary from light to
dark gray in color. Coatings are darker as the carbon content of the underlying steel increases, as the ferrous content of the coating increases, as
heavy metal ions are incorporated into the phosphating solution, or as the
substrate metal is acid pickled prior to phosphating. Zinc phosphating solutions containing active oxidizers usually produce lighter-colored coatings than do solutions using milder accelerators.
Zinc phosphate coatings can be applied by spray, immersion, or a combination of the two. Coatings can be used for any of the following applications of phosphating: base for paint or oil; aid to cold forming, tube
drawing, and wire drawing; increasing wear resistance; or rustproofing.
Spray coatings on steel surfaces range in weight from 1.08 to 10.8 g/m2
(3.5 X 10-3 to 3.5 X 10-2 oz/fr'); immersion coatings, from 1.61 to
43.0 g/m2 (5.28 X 10-3 to 1.41 oz/ft'').
Iron phosphate coatings were the first to be used commercially. Early
iron phosphating solutions consisted of ferrous phosphate/phosphoric acid
Surface Engineering to Change the Surface Chemistry / 97
used at temperatures near boiling and produced dark gray coatings with
coarse crystals. The term iron phosphate coatings refers to coatings resulting from alkali-metal phosphate solutions operated at pH in the range
of 4.0 to 5.0, which produce exceedingly fine crystals. The solutions produce an amorphous coating consisting primarily of iron oxides and having an interference color range of iridescent blue to reddish-blue color.
A typical formulation for an iron phosphate bath is:
Component
Phosphate salts
Phosphoric acid
Molybdate accelerator
Detergents (anionic/nonionic)
Composition, %
12-15
3-4
0.25-0.50
8-10
Basically, then, iron phosphate formulations consist of primary phosphate
salts and accelerators dissolved in a phosphoric acid solution. It is the acid
that initiates the formation of a coating on a metal surface. When acid attacks the metal and begins to be consumed, solution pH at the metal surface rises slightly. This is what causes the primary phosphate salts to drop
out of solution and react with the metal surface, forming a crystalline coating.
Although iron phosphate coatings are applied to steel to provide a receptive surface for the bonding of fabric, wood, and other materials, their
chief application is as a base for subsequent films of paint. Processes that
produce iron phosphate coatings are also available for treatment of galvanized and aluminum surfaces. Iron phosphate coatings have excellent adherence and provide good resistance to flaking from impact or flexing
when painted. Corrosion resistance, either through film or scribe undercut, is usually less than that attained with zinc phosphate. However, a
good iron phosphate coating often outperforms a poor zinc phosphate
coating.
Spray application of iron phosphate coatings is most frequently used, although immersion application also is practical. The accepted range of
coating weights is 0.21 to 0.86 g/m2 (6.9 X 10-4 to 0.26 oz/ft''). Little
benefit is derived from exceeding this range, and coatings of less than 0.21
g/m2 (6.9 X 10-4 oz/ft2) are likely to be nonuniform or discontinuous.
Quality iron phosphate coatings are routinely deposited at temperatures
from 25 to 65°C (80-150 OF) by either spray or immersion methods.
Manganese phosphate coatings are applied to ferrous parts (bearings,
gears, and internal combustion engine parts, for example) for break-in and
to prevent galling. These coatings are usually dark gray. However, because
almost all manganese phosphate coatings are used as an oil base and the
oil intensifies the coloring, manganese phosphate coatings are usually
black in appearance. In some instances, a calcium-modified zinc phosphate coating can be substituted for manganese phosphate to impart
break-in and antigalling properties.
98 / Surface Engineering for Corrosion and Wear Resistance
Manganese phosphate coatings are applied only by immersion, requiring times ranging from 5 to 30 min. Coating weights normally vary from
5.4 to 32.3 g/m2 (1.8 X 10-2 to 9.83 oz/ft"), but can be greater if required.
The manganese phosphate coating usually preferred is tight and finegrain, rather than loose and coarse-grain. However, desired crystal size
varies with service requirements. In many instances, the crystal is refined
as the result of some pretreatment (certain types or cleaners and/or conditioning agents based on manganese phosphate) of the metal surface.
Manganese-iron phosphate coatings are usually formed from high-temperature baths from 90 to 95°C (190-200 OF).
Applications
On the basis of pounds of chemicals consumed or tons of steel treated,
the greatest use of phosphate coatings is as a base for paint. Phosphate
coatings are also used to provide:
•• A base for oil or other rust-preventive material
o
Lubricity and resistance to wear, galling, or scoring of parts moving in
contact, with or without oil
• A surface that facilitates cold forming
o
Temporary or short-time resistance to mild corrosion
o
A base for adhesives in plastic-metal laminations or rubber-to-metal
applications
Corrosion Protection. Conversion of a metal surface to an insoluble
phosphate coating provides a metal with a physical barrier against moisture. The degree of corrosion protection that phosphate coatings impart to
surfaces of ferrous metals depends on uniformity of coating coverage,
coating thickness, density, and crystal size, and the type of final seal employed. Coatings can be produced with a wide range of thicknesses, depending on the method of cleaning before treatment, composition of the
phosphating solution, temperature, and duration of treatment. In phosphating, no electric current is used, and formation of the coating depends
primarily on contact between the phosphating solution and the metal surface and on the temperature of the solution. Consequently, uniform coatings are produced on irregularly shaped articles, in recessed areas, and on
threaded and flat surfaces, because of the chemical nature of the coating
process.
The affinity of heavy phosphate coatings for oil or wax is used to increase the corrosion resistance of these coatings. Frequently, phosphatecoated articles are finished by a dip in nondrying or drying oils that contain corrosion inhibitors. The articles are then drained or centrifuged to
remove the excess oil.
Medium to heavy zinc phosphate coatings, and occasionally, heavy
manganese phosphate coatings are used for corrosion resistance when
Surface Engineering to Change the Surface Chemistry / 99
supplemented by an oil or wax coating. Zinc phosphate plus oil or wax is
usually used to treat cast, forged, and hot-rolled steel nuts, bolts, screws,
cartridge clips, and many similar items. Manganese phosphate plus oil or
wax is also used on cast iron and steel parts.
Phosphate Coating as an Aid in Forming Steel. The contact pressure
used in deep-drawing operations sets up a great amount of friction between the steel surface and the die. The phosphate coating of steel as a
metalforming lubricant, before it is drawn:
•
•
•
•
Reduces
Increases
Reduces
Increases
friction
speed of the drawing operations
consumption of power
the life of tools and dies
Wear Resistance. Phosphating is a widely used method of reducing
wear on machine elements. The ability of phosphate coating to reduce wear
depends on uniformity of the phosphate coating, penetration of the coating
in to metal, and affinity of the coating for oil. A phosphate coating permits
new parts to be broken in rapidly by permitting retention of an adequate
film of oil on surfaces at that critical time. In addition, the phosphate coating itself functions as a lubricant during the high stress of break-in.
Heavy manganese phosphate coatings (10.8 to 43.0 g/m2, or 3.5 X 10-2
to 0.14 oz/fr'), supplemented with proper lubrication, are used for wearresistance applications. Parts that are manganese phosphate coated for
wear resistance are listed in Table 1.
When two parts, manganese phosphated to reduce friction by providing
lubricity, are put into service in contact with each other, the manganese
coating is smeared between the parts. The coating acts as a buffer to prevent galling or, on heavily loaded gears, welding. The phosphate coating
need not stand up for an extended length of time, because it is in initial
movements that parts can be damaged and require lubricity. For example,
scoring of the mating surfaces of gears usually takes place in the first few
revolutions. During this time, the phosphate coating prevents close contact
of the faces. As the coating is broken down in operation, some of it is
packed into pits or small cavities formed in gear surfaces by the etching
action of the acid during phosphating.
Table 1 Parts immersion coated with manganese phosphate for wear resistance
Part(a)
Components for small
arms, threaded fasteners(b)
Bearing races
Valve tappets, camshafts
Piston rings
Gears(c)
Material
Coating time,
min
Supplementary
coatings
Cast iron or steel; forged steel
15-30
Oils, waxes
High-alloy steel forgings or bar stock
Low-alloy steel forgings or bar stock
Forged steel, cast iron
Forged steel, cast iron
7-15
7-15
15-30
15-30
Oils, colloidal graphite
Oils, colloidal graphite
Oils
Oils
(a) Coating weights range from to.8 to 43.0 glm2 (3.5 X to-2 to 0.14 ozlft2). (b) Coating may be applied by barrel tumbling.
ing weights range from 5.4 to 43.0 g/m2 (1.8 X to-2 to 0.14 ozlft2).
(c) Coat-
100 / Surface Engineering for Corrosion and Wear Resistance
Long after break-in, the material packed into the pits or coating that was
originally formed in the pits prevents direct contact of mating surfaces of
gear teeth. In addition, it acts as a minute reservoir for oil, providing continuing lubrication. As work hardening of the gear surfaces takes place,
the coating and the etched area may disappear completely, but by this time
scoring is unlikely to occur.
Chromate Chemical Conversion Coatings
Chromate conversion coatings are formed by a chemical or an electrochemical treatment of metals or metallic coatings in solutions containing
hexavalent chromium (Cr6+) and, usually, other components. The process
results in the formation of an amorphous protective coating composed of
the substrate, complex chromium compounds, and other components of
the processing bath.
Chromate conversion coatings are applied primarily to enhance bare or
painted corrosion resistance, to improve the adhesion of paint or other organic finishes, and to provide the metallic surface with a decorative finish.
Chromating processes are widely used to finish aluminum, zinc, steel,
magnesium, cadmium, copper, tin, nickel, silver, and other substrates.
Chromate conversion coatings are most frequently applied by immersion
or spraying, but other methods of application, such as brushing, roll coating, dip and squeegee, electrostatic spraying, or anodic deposition, are
used in special cases.
Processing Steps. Chromate coatings are applied by contacting the
processed surfaces with a sequence of processing solutions. The processing baths are arranged in a series of tanks, and the surfaces to processed
are transferred through the sequence of stages by using manual, semiautomatic, or automatic control. The chromate coatings are usually applied
to metal parts or to a continuous metal strip running at speeds to 5 m/s
(1000 ft/min).
The basic processing sequence consists of the following six steps:
cleaning, rinsing, conversion coating rinsing, posttreatment rinsing or decorative color rinsing, and drying. In many applications, this sequence is
expanded to accommodate pickling, deoxidizing, dyeing, brightening, and
other rinsing stages, or the sequence can be shortened when cleaning or
posttreatment rinsing is not necessary. More detailed information on key
processing steps can be found in Ref 2.
Corrosion Protection. Chromate conversion coatings provide excellent bare or painted corrosion protection to the metal. The level of protection depends on the substrate metal, the type of chromate coating used,
and the chromium coating weight. In unpainted applications, corrosion
protection for the different conversion coatings generally increases with
Surface Engineering to Change the Surface Chemistry 1101
coating weight, and the upper limit of the coating weight is determined by
the process limitations or by the color requirement. Table 2 lists typical
corrosion data measured by the ASTM B 117 method.
In painted applications, the conversion coating must improve corrosion
resistance and provide for good paint adhesion. The upper limit of the
coating weight for the painted surfaces is normally defined by the onset of
weaker paint adhesion or of corrosion problems related to paint delamination.
Opinions differ widely regarding the mechanism of corrosion protection
provided by the chromate coatings. The most widely advanced concepts
suggest that the chromate coatings provide a barrier insulation from the
environment and inhibit the cathodic corrosion reactions.
Hardness and Abrasion Resistance. The hardness of chromate coatings depends strongly on the temperature during chromating and drying.
Freshly made wet films are very soft and can be easily damaged by abrasion. After drying, the films develop good hardness, which allows for safe
handling. However, even the dry films are susceptible to severe scratching
or abrasion.
Health and Safety Considerations. The disposal of spent solutions and
rinse waters requires waste treatment. Hexavalent chromium must be reduced to Cr3+ before neutralizing and precipitation. Sodium pyrosulfite
(N~S205) is usually used as the reducing agent in smaller operations, while
for larger plants, sulfur dioxide (S02) is preferred for economic reasons.
Wastewater treatment sludges from chromating operations are considered hazardous waste. As a result, the use and disposal of chromium and
chromium compounds have received much regulatory attention because of
the toxicity of chromium and indications that it is a cancer-causing agent.
Due to worker health and safety concerns, alternatives to chromate conversion coatings are being sought. Unfortunately, there are currently no
drop-in substitutes to chromate conversion coatings that adequately match
their corrosion resistance, paint adhesion, and so forth.
Possible elimination of chromate conversion coatings due to regulatory
restrictions is particularly troublesome for the aircraft industry. In applying the coating to the entire aircraft aluminum structure, the subsequent
Table 2 Typical salt-spray data for chromate coatings
on zinc and aluminum
Substrate
Electroplated
zinc
Hot-dip zinc
Aluminum
alloy 3003
Type of chromate
coating
Time to
corrosion stain, h
Untreated
Clear
Iridescent
Olive drab
Electrolytic
Untreated
Clear passivate
Untreated
Clear
Yellow-brown
<4
24-48
100-200
100-400
1000
<4
24-100
<24
60-120
250-800
102 / Surface Engineering for Corrosion and Wear Resistance
rinse process can generate large quantities of chromium-containing
wastes. The challenges of adequately maintaining aging aircraft will help
drive the search for effective chromate substitutes.
Aluminum Anodizing
Aluminum anodizing is an electrochemical method of converting aluminum into aluminum oxide (AI203) at the surface of the item being
coated. It is accomplished by making the workpiece the anode while suspended in a suitable electrolytic cell. Although several metals can be anodized, including aluminum, titanium, and magnesium, only aluminum
anodizing has found widespread use in industry. A more detailed discussion on anodizing can be found in Ref 3.
Because a wide variety of coating properties can be produced through
variations in the process, anodizing is used in almost every industry in
which aluminum can be used. The broadest classification of types of anodize is according to the acid electrolyte used. Various acids have been
used to produce anodic coatings, but the most common ones in current use
are sulfuric (H2S04) and chromic (Cr03) acids. There are two types of
H2S04 anodizing. The first is a room-temperature H2S04 process termed
conventional anodizing, and the second is a low-temperature H2S04
process termed hardcoat anodizing. In addition to Cr03, conventional,
and hardcoat anodizing, a process known as sealing can be used to enhance certain characteristics.
The three common types of anodize described above are usually controlled and described through the use of military specification MIL-A8625 (Table 3). It has become standard in the industry to describe anodic
coatings with the type and class nomenclature outlined in this specification.
Chromic Anodizing
The Cr03 anodizing process produces a coating that is nominally 2 J.1m
(0.08 mil) thick. It is relatively soft and susceptible to damage through
abrasion or handling. The color of the class 1 coating ranges from clear to
Table 3 Classification of anodizing processes according to MI L-A-8625
Thickness
Type
II
III
Class
Description
Dye
Seal
f.lm
mils
1
2
1
2
1
2
Cr03 anodize
Cr03 anodize, dyed
H2S04 anodize
H2S04 anodize, dyed
Hardcoat anodize
Hardcoat anodize, dyed
No
Yes
No
Yes
No
Yes
Yes
Yes
Yes
Yes
No
Yes
1.3-2.5
1.3-2.5
7.5-15
7.5-15
46-56
46-56
0.05-0.1
0.05-0.1
0.3-0.6
0.3-0.6
1.8-2.2
1.8-2.2
Surface Engineering to Change the Surface Chemistry / 103
gray, depending on whether the coating is sealed and on the alloy coated.
The coating can be dyed to produce a class 2 coating; however, this is not
generally done, because the coating is thin and does not retain the dye
color well. About two-thirds of the coating thickness penetrates the base
metal; one-third of the coating builds above the original base metal dimension. Thus, for a coating thickness of 2 urn (0.08 mil) per side, the dimensional change of the workpiece would be 0.7 urn (0.028 mil) per side.
Although the industry has adopted the penetration/buildup terminology, the terms are somewhat misleading. Actually, when the aluminum is
converted to Al203 it takes up more space-approximately
133% of the
space previously occupied by the aluminum converted. The penetration/buildup terms are used only as a convenience in predicting dimensional change in a coated article. The corrosion resistance of this coating
is very good. The coating will pass in excess of 336 h in 5% salt (NaCI)
spray per ASTM B 117.
Advantages and Uses. Although Cr03 anodizing is the least used of
the three types of anodize, it has several advantages that make its use desirable. First, because Cr03 is much less aggressive toward aluminum
than H2S04, it should be used whenever part design is such that rinsing is
difficult. Difficult rinsing designs would include welded assemblies, riveted assemblies, and porous castings. Second, a typical Cr03 anodize
buildup is 0.7 urn (0.028 mil) per side with good repeatability. Therefore,
it is a very good coating to use when it is necessary to coat a precise dimension to size. Third, because Cr03 anodize produces the least reduction
in fatigue strength of the three coatings, it should be used where fatigue
strength is a critical factor. Fourth, the color of Cr03 anodize will change
with different alloy compositions and the heat treat conditions; this makes
it useful as a test of the homogeneity of structural components. Lastly,
when properly applied, Cr03 anodize can be used as a mask for subsequent hardcoat anodize operations.
Suitable Alloys. Most alloys can be successfully coated by the Cr03
process. Exceptions are high-silicon die-cast alloys and high-copper alloys. The rule for suitability is that any alloy containing more than 5% Cu,
7% Si, or total alloying elements of 7.5% should not be coated by this
process.
Relative Costs. Chromic anodize costs more than H2S04 but less than
hardcoat anodize.
Sulfuric Anodizing
The H2S04 process produces a coating that is normally 8 urn (0.31 mil)
in minimum thickness. Although harder than type 1 coatings, H2S04 anodize may still be damaged by moderate handling or abrasion. The color
of the class 1 coating is yellow-green because of the preferred sealing
method of immersion in sodium dichromate (Na2Cr207). Clear coatings
can also be produced by sealing in hot water. Clear coatings should be
104 / Surface Engineering for Corrosion and Wear Resistance
specified by the notation "class 1, clear." This coating can also be dyed to
produce a class 2 coating. This type of anodize produces the most pleasing colors of the three anodizing methods. Dyed H2S04 anodize coatings
have deep colors with good repeatability. Like Cr03 anodize, H2S04 anodize coatings penetrate the base metal for two-thirds of their thickness
and build above the original base metal dimension for one-third the total
thickness. As with all types of anodize, the corrosion resistance of H2S04
anodize is very good; it has an ASTM B 117 salt-spray resistance of at
least 336 h.
Advantages and Uses. Sulfuric anodize is the most widely used type
of anodize and has many desirable benefits. First, because it has a fairly
hard surface, it can be used in situations that require light to moderate
wear resistance. Applications include lubricated sliding assemblies and
items subject to handling wear, such as front panels. Second, because it is
the most aesthetically pleasing type of anodize, it should be used where
final appearance is important. It can be dyed almost any color and produces deep, rich shades that make the item appear to be made of a material bearing a color throughout, rather than an applied coating. Lastly, because corrosion resistance is good, it should be used whenever corrosion
resistance is needed and the specialized benefits of the other two anodize
types are not required.
Suitable Alloys. With the exception of high-silicon die-cast alloys, all
alloys can be successfully coated with H2S04 anodize. Clarity and depth
of color of the anodize increase with the purity of the alloy. Therefore, alloys should be chosen for maximum purity consistent with the physical
requirements needed in the item.
Relative Costs. Sulfuric anodize is the least costly and most widely
available type of anodize.
Hardcoat Anodizing
The hardcoat anodize process produces a coating that is normally 50 urn
(2 mils) thick, although other thicknesses can be specified. The coating is
extremely hard. It is described as file hard (equal to about 60-70 HRC).
The color of the class 1 coating ranges from gray to bronze to almost black,
depending on the alloy coated, the coating thickness, and the electrolyte
temperature. The coating can be dyed to produce a class 2 coating. Because
thick coatings are naturally very dark, only colors darker than natural are
possible. Generally, this limits the dying of hardcoat to black in common
processes. If a more extensive color choice is required, there are several
proprietary hardcoat processes available to accomplish this.
Hardcoat penetrates the base metal for one-half of its thickness and
builds above the original base metal dimension for one-half of its thickness. Thus, for a thickness of 50 p.m (2 mils) per side, the dimensional
change of the workpiece would be 25 urn (1 mil) per side. Commercially
available coating thickness tolerances are the greater of ±5 f.Lmor ± 10%
Surface Engineering to Change the Surface Chemistry / 105
of the total targeted thickness. The corrosion resistance of the unsealed
class 1 coating is very good and comparable to the other types of anodize.
When the hardcoat anodize is sealed, as in a class 2 coating, it becomes
the most corrosion-resistant type of anodize.
Advantages and Uses. Hardcoat anodize, because of its variety of desirable properties, has found widespread use in manufactured products.
First, because of it extreme hardness, it is used in situations in which wear
resistance is required. Applications include valve/piston assemblies, drive
belt pulleys, tool holders and fixtures, and many other items requiring
wear resistance.
Second, because of its excellent resistance to corrosion, hardcoat is used
on aluminum components in harsh environments. These include outside
exposure in salt air, marine components, automobile wash equipment,
components for the aircraft and aerospace industries, and food preparation
machines.
Third, because hardcoat is an excellent electrical resistor, it can be used
to insulate heat sinks for direct mounting of electrical or electronic equipment. Also, it is used in welding fixtures where some areas may need to
be insulated from work.
Fourth, because hardcoat is a naturally porous substance, it is used in
many areas in which the bonding or impregnation of other materials to aluminum is needed. This coating bonds very well with paints and adhesives.
Also, it can be impregnated with teflon (polytetrafluoroethylene, or PTFE)
and many dry film lubricants to impart lubricating properties to the coating.
Lastly, because of its desirable properties and also because it produces
a buildup of coating, it is widely accepted as a salvage coating to restore
worn or improperly machined parts to usable dimensions. Coating thicknesses in excess of 250 urn (10 mils) per side are possible on some alloys
with certain proprietary hardcoat processes.
Suitable Alloys. Although almost all alloys can be coated, the 6000-series aluminum alloys produce the best hardcoat properties. As with the
other anodize types, high-silicon die castings produce the lowest-quality
coatings. Also, because the hardcoat process is sensitive to copper, alloys
in the 2000 series should be avoided if possible. Alloys containing copper
can be hardcoated, but only a relatively few commercial sources have the
ability to coat these alloys with reliability.
Relative Costs. Hardcoat anodize is the most expensive type of anodize. It is generally twice the cost of H2S04 anodize and 50% more than
Cr03 anodize.
Sealing of Anodized Coatings
Because all of the anodic processes produce porous Al203 coatings,
it is often desirable to seal the coating to close these pores and to eliminate the path. between the aluminum and the environment. Sealing
involves immersing the coating in hot water; this hydrates the Al203
106 I Surface Engineering for Corrosion and Wear Resistance
and causes the coating to swell in order to close the pores. Conventional sealing is generally done at a minimum temperature of 95°C
(200 OF) for not less than 15 min. There are also several proprietary
nickel-base sealing agents available that area said to produce sealing at
low temperature through catalytic action. Chromic and sulfuric anodizes are almost always sealed. However, because sealing softens the
coating somewhat, hardcoat anodize is usually not sealed unless criteria other than hardness have the maximum importance in the finished
coating.
Corrosion Resistance of Anodized Aluminum
In general, corrosion resistance of anodic coatings is greatest in approximately neutral solutions, but such coatings are usually serviceable
and protective if the pH is between 4 and 8.5. More acidic and more alkaline solutions attack anodic coatings.
Under atmospheric weathering, the number of pits developed in the base
metal decreases exponentially with increasing coating thickness (Fig. 1).
The pits may form at minute discontinuities or voids in the coating, some
of which result from large second-phase particles in the microstructure.
The pit density was determined by dissolving the anodic coating in a
stripping solution that does not attack the metal substrate. After the 81/2year exposure, the pits were of pinpoint size and had penetrated less than
50 urn (2.0 mils). Specimens with coatings at least 22 lLm (0.9 mil) thick
were practically free of pitting.
Weathering of anodic coatings involves relatively uniform erosion of the
Original anodic coating thickness, mils
1 09
cu
o
0.25
0.5
0.75
1.0
1.25
r------r-r-~__r"---r-..,-,----.---r-r--,
lOB
Q)
E
Q)
ro::J
107
xr
t/)
cuCo
~
'0.
'0
Highly aggressive
/' environments
106
Cii
.0
E
::J
2
105
104
0
10
20
25
30
35
Original anodic coating thickness, urn
Fig. 1
Number of corrosion pits in anodized aluminum
coating thickness. Source: Ref 4
1100 as a function of
Surface Engineering to Change the Surface Chemistry / 107
coating by windbome solid particles, rainfall, and some chemical reaction
with pollutants. The available information indicates that such erosion occurs at a reasonably constant rate, which averaged 0.33 um/yr (0.013
mil/yr) for several alloys exposed to an industrial atmosphere for 18 years
(Fig. 2).
A three-year seacoast exposure of specimens of several alloys with 23
urn (0.9 mil) thick sulfuric acid coatings caused no visible pitting except
in several alloys of the 7xxx series and in a 2xxx alloy (Table 4). Alloys
that exhibited pitting were not protected any more effectively by 50 urn (2
mils) thick coatings. This confirms a general observation that optimal protection against atmospheric corrosion is achieved in the coating thickness
range of 18 to 30 urn (0.7-1.2 mils) and that thicker coatings provide little additional protection.
!E
E
~
rn
(J)
Q)
~u
:£
·E
25
1.0
rn
O.B
~uc
£
(J)
Q)
c
20
0)
Alloy type
• 1xxx
o 5x57
c
.~
0
o
15
c
·c
E
Q)
a::
Fig. 2
0.6
o 3003
6 6063, 6053
0)
.n; 10
0)
.s
0
Weathering
mosphere
8
12
co
0
o
0)
0.4
16
20
24
28
32
36
a::
Exposure time, years
data for anodically
c::
·c
.n;
EQ)
coated aluminum
in an industrial at-
Table 4 Results of three-year seacoast exposure testing of anodized aluminum alloys
Alloy and temper
Results
Sheet
1100
2024- T3, alclad
5456-H343
5086-H34
606l-T6
7039-T6
7075-T6
7075-F, alclad
7079-T6
No visible pitting
Edge pitting only
No visible pitting
No visible pitting
No visible pitting
No visible pitting
Edge pitting only
Edge pitting only
Edge pitting only
Extrusions
635l-T6
6061-T6
6063-T5
6070-T6
7039-T6
No visible pitting
No visible pitting
No visible pitting
No visible pitting
Scattered small pits
H2S04 anodic coatings 23 urn (0.9 mil) thick, sealed in boiling water on test panels
X 150 mm (4 X 6 in.) cut from sheet and extrusions
100
108 / Surface Engineering for Corrosion and Wear Resistance
Oxidation Treatments
Tool Steels. Oxidation is a well-established process used for high-speed
steel cutting tools. Increases in tool life of up to 100% are achieved,
mostly due to a decrease in friction, because of the hard oxide coating and
the ability of the porous oxide to entrap lubricant and draw it to the
tool/workpiece interface. Steam oxidation of a finished tool is accomplished either by exposing it to steam at a temperature of about 565°C
(1050 OF) or by treating it in liquid sodium hydroxide and sodium nitrate
salts at approximately 140 °C (285 OF) for 5 to 20 min. These treatments
result in a black oxidized layer that is less than 5 J.Lm(0.2 mil) thick and
will not peel, chip, or crack, even when the tool is bent or cut. Tool life
improvements due to steam oxidation are listed in Table 5.
Steam Treating of Powder Metallurgy (P/M) Steels. Many PIM parts
have traditionally been steam treated for improved wear resistance, corrosion resistance, and sealing capacity. In this process, PIM parts are heated
in a specific manner under a steam atmosphere at temperatures between
510 and 570°C (950 and 1060 OF) to form a layer of black iron oxide,
identified as magnetite, in the surface porosity. Magnetite has a hardness
equivalent to 50 HRC.
SpaUing or flaking of the surface oxide layer can occur if the process
temperature exceeds 570°C (1060 OF) and process times exceed 4 h. The
maximum thickness of the surface oxide layer should not exceed 7 J.Lm
(0.28 mil). Beyond this thickness, flaking can occur due to an increase in
surface tensile stress.
Sintered density is an important consideration when applying steam
treating for improved strength and hardness. Its ability to increase the
wear resistance of the substrate material depends on the available porosity for oxidation. As density is increased, the amount of oxide formed is
Table 5
Machining tool life improvements due to steam oxidation
Tool life
Tool
M2 broaches
M2 drills
M7 end mill tools
A6 hobs
M2 milling cutters
M2 saw blades
M2 taps
Source: Ref 5
Application
Cutting AISI 1010 latch
Drilling Bakelite plastic
insulating blocks
Phenolic terminal plates
Drilling AISI 4030 steel,
25 nun (1 in.) thick
Cutting 8740 steel forgings
Cutting teeth on AISI 3140
forged gear
Two slots in 1020 steel
Slotting 1020 steel bars
Cutting 75 nun (3 in.) rods,
austenitic steel
Cutting SAE 52100 steel
Before steam treating
After steam treating
20 h per grind
10 holes
70 h per grind
25 holes
1700 holes per grind
17 holes
8500 holes per grind
81 holes
30 pieces
62.2% increased
200 pieces
62.2% increased life
life
150 cuts per grind
2000 per grind
100% endurance at 0.52
mls (102 sfm)
1800 pieces
306 cuts per grind
7000 per grind
120% endurance at 0.57
mls (112 sfm)
3000 pieces
Surface Engineering to Change the Surface Chemistry I 109
Density, Ib/in.3
0.217
130
0.224
0.231
0.238
0.246
0.253
0.260
0.80% C, steam-treated
CD
a:
::I:
u)
CI)
Q)
90
e
"0
Co
..c:
E
e
(1]
70
Q.
Q.
<C
30L-------~------~--------~------~------~------~
6.4
6.6
6.2
6.0
6.8
7.0
7.2
Density, g/cm3
Fig. 3
Effect of steam treating on the hardness of sintered P/M carbon steels. Source: Ref 6
decreased, which minimizes the improvement in apparent hardness attributed to steam treating. This is shown in Fig. 3 for sintered steel.
The increase in density and apparent hardness produced by steam treating is illustrated in the micrograph of a sintered steel (Fig. 4). By filling
Fig. 4
Micrograph
of steam-treated structure of a P/M steel. Unetched. 200X
110 / Surface Engineering for Corrosion and Wear Resistance
the porosity with a hard second phase, the PIM steel offers a better support to the indentation hardness tester. Figure 5 illustrates that the transverse rupture strength is increased significantly by steam treatment for
low-carbon PIM steels, but only modestly for high-carbon (0.8% C) PIM
steels.
Diffusion Heat Treatment Coatings
The diffusion coatings described in this section involve heat treating
processes that cause carbon, nitrogen, or a combination of the two to diffuse
into the surface of a ferrous part to alter the surface chemistry/properties. As
listed in Table 6, these processes include carburizing, nitriding, and carbonitriding. Each of these depends on the concentration gradient of the diffusing
species, the diffusivity of the atomic species in the host material, and the time
and temperature at which the process takes place.
All carburizing and nitriding processes increase the surface carbon or nitrogen content of the alloy to allow the surface to respond to quench hardening. The heat treater usually relies on empirical data to determine how long
to expose the part to achieve the desired carbon or nitrogen diffusion. The
term used for the entire field of surface-hardening processes is case hardening, and the case indicates the depth of hardening below the surface. Although
Density, Ib/in.3
0.217
0.224
0.231
0.238
0.246
0.253
0.260
875~--------~~------~--------~----------~----------~----------~127
0.80% C,steam-treated
co
a..
Ow
~
~
~c,
c:
~en
~:::J
c.
2
Q)
en
-5
C)
91
625
~
t)
~
a.
:J
73
500
2
Q)
en
Q)
Qi
>
en
c
co
t!:
c
>
en
c:
co
375
54
250~------~------~--------~------~----------~----------~36
6.0
6.2
6.4
6.6
6.8
7.0
.=
7.2
Density, g/cm3
Fig. 5
Effect of steam treating on transverse rupture strength of sintered P/M carbon steels.
Source: Ref 6
Surface Engineering to Change the Surface Chemistry / 111
the depth of hardening decreases gradually because the diffused species
does not stop abruptly, the effective case depth is considered to be the
depth at which the hardness falls below 50 HRC. More detailed information on the case-hardening procedures described in this section can be
found in Heat Treating, Volume 4 of ASM Handbook.
Table 6
Typical characteristics of carburizing, nitriding, and carbonitriding diffusion treatments
Process
temperature
Name of
case
Process
°C(OF)
Case
hardness,
typical case depth
Hardness,
HRC
Typical base
metals
Process
characteristics
Carburizing
Pack
Diffused carbon
815-1090
(1500-2000)
125 f.Lm-I.5 mm
(5-60 mils)
50-63 (a)
Gas
Diffused carbon
815-980
(1500-1800)
75 f.Lm-1.5 mm
(3-60 mils)
50-63 (a)
Liquid
Diffused carbon
and possibly
nitrogen
815-980
(1500-1800)
50 f.Lm-1.5 mm
(2-60 mils)
50-65 (a)
Low-carbon steels,
low-carbon alloy
steels
Vacuum
Diffused carbon
815-1090
(1500-2000)
75 f.Lm-1.5 mm
(3-60 mils)
50-63 (a)
Low-carbon steels,
low-carbon alloy
steels
Gas
Diffused nitrogen,
nitrogen
compounds
480-590
(900-1100)
125 f.Lm-O.75 mm
(5-30 mils)
50-70
Alloy steels, nitriding
steels, stainless
steels
Salt
Diffused nitrogen,
nitrogen,
compounds
510-565
(950-1050)
2.5 f.Lm-O.75 mm
(0.1-30 mils)
50-70
Most ferrous metals
including cast
iron
Ion
Diffused nitrogen
nitrogen
compounds
340-565
(650-1050)
75 f.Lm-0.75 mm
(3-30 mils)
50-70
Alloy steels, nitriding
steels, stainless
steels
Gas
Diffused carbon
and nitrogen
760-870
(1400-1600)
75 f.Lm-O.75 mm
(3-30 mils)
50-65(a)
Low-carbon steels,
low-carbon alloy
steels, stainless
steels
Liquid
(cyaniding)
Diffused carbon
and nitrogen
760-870
(1400-1600)
2.5-125 urn
(0.1-5 mils)
50-65(a)
Low-carbon
steels
Ferritic
ni trocarburizing
Diffused carbon
and nitrogen
480-590
(900-1090)
2.5-25 urn
(0.1-1 mil)
40-60(a)
Low-carbon
steels
Low-carbon steels,
low-carbon alloy
steels
Low-carbon steels,
low-carbon alloy
steels
Low equipment costs,
difficult to control case
depth accurately
Good control of case
depth, suitable for
continuous operation,
good gas controls
required, can be
dangerous
Faster than pack and gas
processes, can pose salt
disposal problem, salt
baths require frequent
maintenance
Excellent process control,
bright parts, faster than
gas carburizing, high
equipment costs
Nitriding
Hardest cases from
nitriding steels,
quenching not required,
low distortion, process
is low, is usually a
batch process
Usually used for thin hard
cases <25 urn « 1 mil),
no white layer, most are
proprietary processes
Faster than gas nitriding,
no white layer, high
equipment costs, close
case control
Carhonitriding
(a) Requires
quench from austenitizing
temperature.
Source: Ref 7
Lower temperature than
carburizing (less
distortion), slightly
harder case than
carburizing, gas
control critical
Good for thin cases on
noncritical parts, batch
process, salt disposal
problems
Low-distortion process for
thin case on lowcarbon steel, most
processes are
proprietary
112 / Surface Engineering for Corrosion and Wear Resistance
Carburizing
Carburizing is the addition of carbon to the surface of low-carbon steels
at temperatures (generally between 850 and 950°C, or 1560 and 1740 OF)
at which austenite, with its high solubility for carbon, is the stable crystal
structure. Hardening of the component is accomplished by removing the
part and quenching or allowing the part to slowly cool and then reheating
to the austenitizing temperature to maintain the very hard surface property. On quenching, a good wear- and fatigue-resistant high-carbon
martensitic case is superimposed on a tough, low-carbon steel core. Carburized steels used in case hardening usually have base carbon contents of
about 0.2 wt%, with the carbon content of the carburized layer being fixed
between 0.8 and 1.0 wt% (Ref 8). Carburizing methods include gas carburizing, vacuum carburizing, plasma (ion) carburizing, salt-bath carburizing, and pack carburizing. These methods introduce carbon by use of an
atmosphere (atmospheric gas, plasma, and vacuum), liquids (salt bath), or
solid compounds (pack). The vast majority of carburized parts are
processed by gas carburizing, using natural gas, propane, or butane. Vacuum and plasma carburizing are useful because of the absence of oxygen
in the furnace atmosphere. Salt-bath and pack carburizing have little commercial importance, but are still done occasionally.
Gas carburizing can be run as a batch or a continuous process. Furnace
atmospheres consist of a carrier gas and an enriching gas. The carrier gas
is supplied at a high flow rate to ensure a positive furnace pressure, minimizing air entry into the furnace. The type of carrier gas affects the rate of
carburization. Carburization by methane is slower than by the decomposition of CO. The enriching gas provides the source of carbon and is supplied at a rate necessary to satisfy the carbon demand of the work load.
Most gas carburizing is done under conditions of controlled carbon potential by measurement of the CO and CO2 content. The objective of the
control is to maintain a constant carbon potential by matching the loss in
carbon to the workpiece with the supply of enriching gas. The carburization process is complex, and a comprehensive model of carburization requires algorithms that describe the various steps in the process, including
carbon diffusion, kinetics of the surface reaction, kinetics of the reaction
between the endogas and enriching gas, purging (for batch processes), and
the atmospheric control system. Possible models of each of these steps
have been outlined (Ref 9).
Vacuum carburizing is a nonequilibrium, boost-diffusion-type carburizing process in which austenitizing takes place in a rough vacuum, followed by carburization in a partial pressure of hydrocarbon gas, diffusion
in a rough vacuum, and then quenching in either oil or gas (Ref 10). Vacuum carburizing offers the advantages of excellent uniformity and reproducibility because of the improved process control with vacuum furnaces,
improved mechanical properties due to the lack of intergranular oxidation,
Surface Engineering to Change the Surface Chemistry / 113
and reduced cycle time. The disadvantages of vacuum carburizing are predominantly related to equipment costs and throughput.
Plasma (ion) carburizing is basically a vacuum process utilizing glowdischarge technology to introduce carbon-bearing ions to the steel surface
for subsequent diffusion (Ref 11). This process is effective in increasing
carburization rates because the process bypasses several dissociation steps
that produce active soluble carbon. For example, because of the ionizing
effect of the plasmas, active carbon for adsorption can be formed directly
from methane (CH4) gas. High temperatures can be used in plasma carburizing because the process takes place in an oxygen-free vacuum, thus
producing a greater carburized case depth than both atmospheric gas and
vacuum carburizing (Fig. 6).
Nitriding
Nitriding is a process similar to carburizing, in which nitrogen is diffused into the surface of a ferrous product to produce a hard case. Unlike
carburizing, nitrogen is introduced between 500 and 550°C (930 and
1020 OF), which is below the austenite formation temperature (Ac}) for
ferritic steels, and quenching is not required. As a result of not austenitizing and quenching to form martensite, nitriding results in minimum distortion and excellent control. The various nitriding processes (Table 6) include gas nitriding, liquid nitriding, and plasma (ion) nitriding.
All hardenable steels must be quenched and tempered prior to nitriding.
The nitriding process is used to obtain a high surface hardness, improve
wear resistance, increase fatigue resistance, and improve corrosion resistance (except for stainless steel). The case structure of a nitrided steel, conDepth below surface, in.
1.6
0
0.010
0.020
0.030
0.040
A Atmosphere carburized, 30 min
B Vacuum carburized, 30 min
C Plasma carburized, 30 min
1.4
1.2
'cF.
1.0
c-
0.8
~
0
.c
ro
0
0.6
0.4
0.2
0
0
0.25
0.5
0.75
1.0
Depth below surface, mm
Fig. 6
Carbon gradient profile of atmosphere, vacuum, and plasma carburizing of AISI 8620 steel at 980°C (1800 OF) saturation conditions for 30
min and followed by direct oil quenching. Source: Ref 12
114 / Surface Engineering for Corrosion and Wear Resistance
taining a diffusion zone with or without a compound zone (Fig. 7), depends on the type and concentration of alloying elements and the timetemperature exposure of a particular nitriding treatment (Ref 13). The diffusion zone is the original core microstructure with the addition of nitride
precipitates and nitrogen solid solution. The compound zone is the region
where "/' (Fe4N) and e (Fe2_3N) intermetallics are formed. Commercial
steels containing aluminum, chromium, vanadium, tungsten, and molybdenum are most suitable for nitriding because they readily form nitrides
that are stable at the nitriding temperatures (Ref 14). The following steels
can be nitrided for specific applications:
•
•
o
•
o
o
•
•
•
Aluminum-containing low-alloy steels: Nitralloys
Medium-carbon, chromium-containing low-alloy steels: 4100, 4300,
5100, 6100, 8600, 8700, and 9800 series
Low-carbon, chromium-containing low-alloy steels: 3300, 8600, and
9300 series
Hot-working die steels containing 5% Cr: Hll, H12, and H13
Air-hardenable tool steels: A2, A6, D2, D3, and S7
High-speed tool steels: M2 and M4
Nitronic stainless steels: 30, 40, 50, and 60
Ferritic and martensitic stainless steels: 400 series
Austenitic stainless steels: 200 and 300 series
Precipitation-hardened stainless steels: 13-8 PH, 15-5 PH, 17-4 PH,
17-7 PH, A-286, AM 350 (Ref 14), and AM 355
Gas nitriding
(Ref 14) is a case-hardening process that takes place in
the presence of ammonia gas. Either a single-stage or a double-stage
1
~
en
en
OJ
c:
"E
co
..c:
eo
~
Compound
zone
Case depth (il Y)
Fig. 7
Factors affecting the microhardness profile of a nitrided steel. The hardness of the compound zone is unaffected by alloy content, while the
hardness of the diffusion zone is determined by nitride-forming elements (AI, Cr,
Mo, Ti, V, Mn). ~ is influenced by the type and concentration of alloying elements; Ll Y increases with temperature and decreases with alloy concentration.
Source: Ref 13
Surface Engineering to Change the Surface Chemistry / 115
process can be used when nitriding with anhydrous ammonia. The singlestage process, in which a temperature of 495 to 525°C (925-975 OF) is
used, produces the brittle nitrogen-rich compound zone known as the
white nitride layer at the surface of the nitrided case. The double-stage
process, or Floe process, has the advantage of reducing the white nitrided
layer thickness. After the first stage, a second stage is added which either
by continuing at the first-stage temperature or increasing the temperature
to 550 to 565°C (1025-1050 OF).The use of the higher-temperature second stage lowers the case hardness and increases the case depth.
Liquid nitriding (nitriding in a molten salt bath) uses similar temperatures as in gas nitriding and a case-hardening medium of molten, nitrogenbearing, fused-salt bath containing either cyanides or cyanates (Ref 15).
Similar to salt-bath carburizing, liquid nitriding has the advantage of processing finished parts because dimensional stability can be maintained
due to the subcritical temperatures used in the process. Furthermore, at the
lower nitriding temperatures, liquid nitriding adds more nitrogen and less
carbon to ferrous materials than that obtained with high-temperature treatments because ferrite has a much greater solubility for nitrogen (0.4%
max) than carbon (0.02% max).
Plasma (ion) nitriding is a method of surface hardening using glow-discharge technology to introduce nascent (elemental) nitrogen to the surface
of a metal part for subsequent diffusion into the material (Ref 13). The
process is similar to plasma carburizing in that a plasma is formed in a vacuum using high-voltage electrical energy and the nitrogen ions are accelerated toward the workpiece. The ion bombardment heats the part, cleans the
surface, and provides active nitrogen. The process provides better control of
case chemistry, case uniformity, and lower part distortion than gas nitriding.
Properties of plasma nitrided ferrous alloys are listed in Table 7.
Carbonitriding and Ferritic Nitrocarburizing
Carbonitriding
introduces both carbon and nitrogen into the austenite
of the steel. The process is similar to carburizing in that the austenite
Table 7
Properties of ion nitrided ferrous metals
Nitriding
temperature
Type of
metal
Carbon steel
Gray cast iron
Alloy steel
Nitriding steel
Hot-work tool steel
Cold-work tool steel
High-speed tool steel
Stainless steel
Designation
1045
G2500
4140
9310
Nitralloy 135
H13
D2
M2
303
17-4 PH
°C
510-570
510-540
480-540
510-550
480-540
480-540
450-540
480-510
540-570
510-540
Source: R.B. Alexander and Associates, Inc., Huntington Woods, MI
OF
950-1050
950-1000
900-1000
950-1025
900-1000
900-1000
850-1000
900-950
1000-1050
950-1000
Surface
hardness,
HR15N
File hard
77-82
84-90
77-92
90-95
90-94
90-94
92-95
90-95
90-95
Typical
case depth
mm
0.30-0.76
0.10-0.20
0.30-0.76
0.30-0.76
0.25-0.76
0.10-0.33
0.13-0.25
0.03-0.10
0.05-0.13
0.10-0.20
Compound
zone thickness
mils
12-30
4-8
12-30
12-30
10-30
4-13
5-10
1-4
2-5
4-8
f.Lm
3.8-15
5.1-10
3.8-15
3.8-10
1.3-10
2.5-5.1
mils
0.15-0.6
0.2-0.4
0.15-0.6
0.15-0.4
0.05-0.4
0.1-0.2
116 / Surface Engineering for Corrosion and Wear Resistance
composition is enhanced and the high surface hardness is produced by
quenching to form martensite. This process is a modified form of gas carburizing in which ammonia is introduced into the gas-carburizing atmosphere (Ref 16). As in gas nitriding, elemental nitrogen forms at the workpiece surface and diffuses along with carbon into the steel. Typically,
carbonitriding takes place at a lower temperature and a shorter time than
gas carburizing, producing a shallower case. Steels with carbon contents
up to 0.2% are commonly carbonitrided; these include 1000, 1100, 1200,
1300, 1500, 4000, 4100, 4600, 5100, 6100, 8600, and 8700 series.
Ferritic nitrocarburizing
is a subcritical heat treatment process, carried out by either gaseous or plasma techniques, and involves the diffusion
of carbon and nitrogen into the ferritic phase. The process results in the
formation of a thin white layer or compound layer,with an underlying diffusion zone of dissolved nitrogen in iron, or alloy nitrides (Ref 17). The
white layer improves surface resistance to wear, and the diffusion zone increases the fatigue endurance limit, especially in carbon and low-alloy
steels. Alloy steels, cast irons, and some stainless steels can be treated.
The process is used to produce a thin, hard skin, usually less than 25 um
(1 mil) thick, on low-carbon steels in the form of sheet metal parts, powder metallurgy parts, small shaft sprockets, and so forth.
Pack-Cementation
Diffusion Coatings
The pack-cementation process originally involved pack carburizing, which
is the process of diffusing carbon into the sun ace of iron or low-carbon
steel by heating in a closed container filled with activated charcoal. The
simplest and oldest carburizing process involves filling a welded sheet
metal or plate box with granular charcoal that is activated with chemicals
such as barium carbonate to assist the formation of carbon monoxide (CO).
In the heated box, charcoal forms carbon dioxide (C02), which converts to
CO in an environment with an excess of carbon. The CO then forms atomic
carbon at the component .surface and diffuses into the part. The packcarburization process is of little commercial importance, although it is ...
still
done occasionally. It has given rise to other pack-diffusion processes including aluminizing, siliconizing, chromizing, and boronizing.
Basic Principles. Pack cementation is a batch vapor-phase process that
involves heating a closed/vented pack to an elevated temperature (e.g.,
1050 °C, or 1920 OF) for a given time (e.g., 16 h) during which a diffusional coating is produced (Ref 18). The traditional pack consists of four
components: the substrate or part to be coated, the master alloy (i.e., a
powder of the element or elements to be deposited on the surface of the
part), a halide salt activator, and relatively inert filler powder. The master
alloy, the filler, and halide activator are thoroughly mixed together, and the
part to be coated is buried in this mixture in a retort (Ref 19). When the
Surface Engineering to Change the Surface Chemistry / 117
mixture is heated, the activator reacts to produce an atmosphere of source
element(s) halides that diffuse into the pack and transfer the source element(s) to the substrate on which the coating is formed (Ref 20).
Aluminizing. An aluminizing pack-cementation process is commercially practiced for a range of alloys, including nickel- and cobalt-base superalloys, and carbon, low-alloy, and stainless steels. Simple aluminide
coatings resist high-temperature oxidation by the formation of an alumina
protective layer and can be used up to about 1150 °C (2100 OF), but the
coating can degrade by spallation of the oxide during thermal cycling. For
extended periods of time at temperatures in excess of 1000 °C (1830 OF),
interdiffusion of the coating will cause further degradation, and therefore
practical coating life is limited to operating temperatures of 870 to 980°C
(1600 to 1800 OF). Pack compositions, process temperatures, and process
times depend on the type of base material to be aluminized and fall into
the following classifications (Ref 21):
Class
Alloy
I
II
III
IV
Carbon and low-alloy steels
Ferritic and martensitic stainless steels
Austenitic stainless steels with 21-40% Ni and iron-base superalloys
Nickel- and cobalt-base superalloys
As a general rule, overall aluminum diffusion is slowed as the nickel,
chromium, and cobalt contents increase. Thus, higher temperatures and
longer processing times are required to produce greater aluminum diffusion thicknesses as the base material increases in alloy content.
Stainless steels are oxidation resistant as a result of the formation of a
thin chromium-rich oxide on the component surface. A similar reaction
occurs in aluminized steels in which a thin, slower-growing aluminumrich oxide forms. Unlike chromium oxide, Al203 does not exhibit volatility in the presence of oxygen above 927°C (1700 OF).Figure 8 compares
~
~:::s
~
1095
2000
815
1500
1000
540
0.
0.
E
E
500
260
-18
Fig. 8
E
Q)
Q)
~
~
~
~
Steel type
Oxidation of steels in air at the temperature
than 10 mg/crn-. Source: Ref 22
~
0
at which scaling is less
118 / Surface Engineering for Corrosion and Wear Resistance
an aluminized carbon steel with several alloys at a temperature in which
scaling remains less than 10 mg/cm/ for oxidation in air. In sulfidizing environments, pack-aluminized coatings have excellent resistance to corrosive attack. In contrast to stainless steels, the aluminum-rich surface (50%
AI) and diffusion zone (20% Al min) of the coating is far more resistant
than chromium to sulfidation corrosion. Figure 9 compares the corrosion
rates of bare and aluminized 9Cr-1Mo steel in a hydrogen sulfide (H2S)
environment.
As mentioned in the preceding text, pack aluminizing is commonly
carried out on nickel- and cobalt-base superalloys. Diffusion-coated superalloys develop an aluminide (NiAI or CoAl) outer layer with enhanced corrosion resistance. It is estimated that more than 90% of all
coated gas turbine engine hot section blades and vanes made from superalloys are coated by pack cementation and related processes. Detailed information on protective diffusion coatings for superalloys can
be found in Ref 24.
Siliconizing, the diffusion of silicon into steel, occurs similarly to aluminizing. There are pack and retort processes in which parts are subjected
to gas atmospheres that react with the heated part surface to produce nascent silicon that diffuses into the substrate to be coated. In a pure silicon
pack that is activated with NH4CI, SiCl4 and SiHCl3 gases form, which are
reduced by hydrogen gas to deposit elemental silicon on the surface of the
parts (Ref 20). Another process involves tumbling parts in a retort with
SiC. When a temperature of 1010 °C (1850 OF) is reached, silicon tetrachloride gas is introduced, which reacts with the part and the SiC particles
to produce a concentration gradient of silicon on the part surface as the silicon diffuses into the substrate. The process normally takes place on lowcarbon steels, and these steels develop case depths up to 1 mm (0.040 in.)
with a silicon content of 13 wt% (Ref 25). Case depths developed on these
siliconized steels have hardnesses of about 50 HRC and therefore can be
used for wear resistance. The presence of silicon on the surface allows for
3.5
I...
<:-
3.0
1:.:.:.:.:.:.:.:.:.:.:.:.:.
~
Bare steel
Aluminized steel
120
2.5
100
2.0
80
0
1.5
e
60
1.0
40
E
E
s
e
c:
'u;
0
u
I...
<:.!!l
'E
a)
E
e
0
'en
e
0
o
0.5
20
370°C (700 OF)
Fig. 9
140
480 °C (900 OF)
0
Relative corrosion rates of 9Cr-1 Mo alloy steel in 5 mol?
kPa (515 psi) for 300 h. Source: Ref 23
H2S at 3550
Surface Engineering to Change the Surface Chemistry / 119
the formation of a stable silicon dioxide (Si02) phase in oxidizing environments and excellent corrosion resistance.
Chromizing. Chromium can be applied in the same manner as aluminum and silicon to produce a chromium-rich coating, and many of the
same principles of aluminizing packs apply to chromizing packs. Parts are
packed in chromium powder with an inert filler such as aluminum oxide.
A halide salt activator is added that changes to the vapor phase at the processing temperature and serves as a carrier gas to bring chromium to the
surface of the part. Diffusion coatings can be formed on nickel-base superalloys by pack cementation using ammonium chloride as a chromiumalumina activator. These coatings usually contain 20 to 25 wt% Cr at the
outer surface and involve approximately equal rates of interdiffusion of
chromium and nickel. Significant depletion of aluminum and titanium
from the alloy surface occurs, thus producing a coating that is a solid solution of the chromium in the remaining nickel-base superalloy. The deposited coating is usually overlaid with a thin layer of ce-chromium, which
must be removed chemically (Ref 20).
In low-alloy steels, it has been shown that chromizing is much more
complex, leading to microstructures that may behave detrimentally in
some environments (Ref 26). In a chromized 2.25Cr-lMo alloy, the coating contains a thin outer layer (~5 urn, or 0.2 mil) of mostly chromium
(>80 wt%), which is essential for corrosion protection. Large columnar
ferritic grains, containing between 30 and 15 wt% Cr, are found beneath
the outer layer. The columnar grain boundaries, as. well as the boundary
between the outer chromium-rich layer and the columnar grains, are decorated with chromium carbides that were found to contribute to coating
degradation. A layer of Kirkendall voids (also decorated by carbides), iron
carbides at the coating/substrate interface, and a large decarburized zone
in the substrate are also produced by the process.
Evaluation of samples exposed to a fossil-fired boiler up to two years
revealed two degradation mechanisms: cracking and sulfidation corrosion.
Cracking of the outer coating layer allowed ingress of sulfur, resulting in
intergranular sulfidation corrosion attack. Once the outer protective
chromium layer has been breached, the columnar grain-boundary orientation promotes crack initiation and propagation along the carbides when
the tube is subjected to axial thermal loading. However, it should be noted
that chromized coatings have been used up to 10 years in some fossil-fired
boilers. Therefore, stress and environmental conditions are critical to the
successful use of these pack-cementation coatings, as long as the effect of
the processing thermal cycle on the coating and substrate morphology is
understood.
Boriding, or boronizing, is a thermochemical surface-hardening
process that can be applied to a wide variety of ferrous, nonferrous, and
cermet materials. The boronizing pack process is similar to pack carburizing with the parts to be coated being packed with a boron-containing
120 / Surface Engineering for Corrosion and Wear Resistance
compound such as boron powder or ferroboron. Activators such as chlorine and fluorine compounds are added to enhance the production of the
boron-rich gas at the part surface. Processing of high-speed tool steels that
were previously quenched hardened is accomplished at 540°C (1000 OF).
Boronizing at higher temperature up to 1090 °C (2000 OF) causes diffusion rates to increase, thus reducing the process time. The boron case does
not have to be quenched to obtain its high hardness, but tool steels
processed in the austenitizing temperature range need to be quenched
from the coating temperature to harden the substrate.
Boronizing is most often applied to tool steels or other substrates that are
already hardened by heat treatment. The thin (12-15 urn, or 0.48-0.6 mil)
boride compound surfaces provide even greater hardness, improving wear
service life. Distortion from the high processing temperatures is a major
problem for boronized coatings. Finished parts that are able to tolerate a
few thousandths of an inch (75 urn) distortion are better suited for this
process sequence because the thin coating cannot be finish ground (Ref 25).
Although pack boriding is the most widely used boriding process, it is
important to note that other thermochemical boriding techniques are also
used. These include paste boriding, liquid (salt-bath) boriding, gas boriding, plasma boriding, and fluidized-bed boriding. These alternative techniques are described in Ref 27.
Ion Implantation
Ion implantation involves the bombardment of a solid material with
medium- to high-energy ionized atoms and offers the ability to alloy virtually any elemental species into the near-surface region of any substrate.
The advantage of such a process is that it produces improved surface
properties without the limitations of dimensional changes or delamination
found in conventional coatings. During implantation, ions come to rest beneath the surface in less than 10 to 12 s, producing a very fast quench rate
and allowing the development of nonequilibrium surface alloys or compounds. In almost all cases the modified region is within the outermost
micrometer of the substrate, often only within the first few hundred
angstroms (i.e., microinches) of the surface. Details of the process and associated equipment are documented in Ref 28. Ion implantation is commercially applied to various steels, tungsten carbide/cobalt materials, and
alloys of titanium, nickel, cobalt, aluminum, and chromium, although applications are restricted to temperatures below 250°C (480 OF) for steels
and 450 °C (840 OF) for carbides. Advantages and limitations of the ion
implantation process are outlined in Table 8.
Applications. Table 9 lists some of the applications for the ion-implantation process. Ion-implantation surfaces produce exceptional results in
Surface Engineering to Change the Surface Chemistry / 121
Table 8 Advantages and limitations of ion implantation
Advantages
Limitations
Produces surface alloys independent of thermodynamic criteria
No delamination concerns
No significant dimensional changes
Ambient-temperature
processing possible
Enhance surface properties while retaining bulk properties
High degree of control and reproducibility
Limited thickness of treated material
High-vacuum process
Line-of-sight process
Alloy concentrations dependent on sputtering
Relatively costly process; intensive training required compared
surface treatment processes
Limited commercial treatment facilities available
reducing wear, friction, and corrosion (Ref 28, 29). Commercial applications involve tooling, bearings, and biomedical components. Nitrogen implantation, especially in alloy surfaces containing elements forming stable
nitrides, has found use in tools and dies such as cobalt-cemented tungsten
carbide wire-drawing inserts. Nitrogen implantation has been especially
successful in increasing the life (Up to 20 times) of tools and parts used in
the manufacture of injection-molded plastics. Ion implantation with nitrogen or titanium and carbon has provided increased tool life for stamping
and other forming tools. For example, the life of punches and dies for the
manufacturing of aluminum beverage cans has increased to 6 to 10 times
that of untreated tooling. Table 10 lists examples of extending tool life
with ion implantation. Titanium and cobalt-chromium alloy orthopedic
prostheses for hip and knee joints are among the most successful commercial applications for ion-implantation components for wear resistance.
Table 9
Research and development applications for ion implantation
Surface properties
modified
Ions species used
Substrates studied
N,C
Friction
Steels, WC, Ti, ColCr
alloys, TiN coatings,
electroplated Cr
Steels
Fatigue
Ti alloys, steels
N,C
Fracture toughness
Ceramics: A1203, TiN
Ar
Aqueous corrosion
catalysis
Steels, Ti alloys, Pt
Cr, Ta, Cr+P
;:::1017 ions/ern-
Oxidation
Superalloys
Y,Ce
Wear
10-20 at.%
;:::1017 ions/ern?
Ti plus C implants
;:::1017 ions/ern-
;:::1017 ions/ern-
;:::1015 ions/ernElectrical
conductivity
Polymers
Ar,F
1015_1017 ions/em-
Optical: refractive index
Source: Ref 28
Glasses, electrooptics
Li,Ar
1015_1017 ions/ern-
Comments
Ti, Co/Cr alloys largest
use commercially in
orthopedic devices
Dual implants give
amorphous surface
layer
Implantation effective for
surface initiated fatigue
Radiation damage critical;
ion-induced
compressive stress
helpful
Ion implant can mimic
"normal" alloys;
amorphous and unique
surface alloys possible
Low effective doses;
implanted species stay
at metal-oxide interface
Permits chain scissoning,
doping; conductivity
approaches disordered
metal levels
Chemical doping and
lattice disorder both
important
to other
122 / Surface Engineering for Corrosion and Wear Resistance
Table 10
Examples of extending tool life via ion implantation
Tool
Chasers
Material
Application
Tool steel
Cutting threads in 380
aluminum
Drawing 3.5 mm (0.140 in.)
hot-rolled steel
2 mm (0.080 in.) hot-rolled
steel (flywheels)
1 mm (0.042 in.)
cold-rolled steel
Forming electrical connectors
13 mm (1/2 in.)
hot-rolled steel
1.3 mm (0.050 in.) Hastelloy
Get engine part)
0.08 mm (0.003 in.) 301
stainless steel
Shaping vinyl siding
Gaging powdered metal part
Finish reaming gray cast iron
Cutting rubber (belts)
Injection-molding
thermoset (20% glass)
Tapping cold-rolled steel
nuts
Tapping 380 aluminum
Welding aluminum to steel
Compacting copper-base
composite wire
Draw die
D2
Flanging ring
D2
Forming dies
WI
Forming
Pierce punches
D2
M2
M2
Pilot pins
M2
Plastic forming
Plug gage
Reamers
Slitters
Sprue bushings
Carbide
Carbide
P20 (chromium plated)
Taps
M2 ( chromium plated)
Thread form taps
Ultrasonic electrodes
Wire compacting dies
Stainless steel
06
M2
D2
Inconel, M2
Results
5X life
22 X life before polishing
80 X life before polishing
Pickup reduced, improved
finish on part
At least 60% longer life
12X life
30% longer life
5x life
Wear rate reduced for dies
At least 2X life
2X life
Friction reduced 30%
At least 4.5 X life
8X life
2X life
2.5 X life (mechanical
At least 16x life
wear)
Source: Ref 30
Laser Alloying
Processing. A technique of localized alloy formation is laser surface
melting with the simultaneous, controlled addition of alloying elements.
These alloying elements diffuse rapidly into the melt pool, and the desired
depth of alloying can be obtained in a short period of time. By this means,
a desired alloy chemistry and microstructure can be generated on the sample surface; the degree of microstructural refinement will depend on the
solidification rate. The surface of a low-cost alloy, such as mild steel, can
be selectively alloyed to enhance properties, such as resistance to wear, in
such a way that only the locally modified surface possesses properties typical of tribological alloys. This results in substantial cost savings and reduces the dependence on strategic materials.
One method of alloying is to apply appropriate mixtures of powders
on the sample surface, either by spraying the powder mixture suspended
in alcohol to form a loosely packed coating, or by coating a slurry suspended in organic binders (Ref 31). The use of metal powders in laser
alloying is the least expensive, but, with appropriate process modifications, alloys in the form of rods, wires, ribbons, and sheets can also be
added.
Applications. Laser alloying has been primarily applied to improve
corrosion resistance. A very common technique is to alloy steels with
Surface Engineering to Change the Surface Chemistry / 123
chromium. An example of laser alloying to improve wear resistance is exhaust valves fabricated by Fiat Research Laboratory.
Acknowledgment
Portions of this chapter were adapted from A.R. Marder, Effects of Surface Treatments on Materials Performance, Materials Selection and Design, Volume 20, ASM Handbook, ASM International, 1997, p 470-490.
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124 / Surface Engineering for Corrosion and Wear Resistance
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D.G. Bhat, and M. Jeandin, Ed., TMS, 1991, P 471
27. A.K. Sinha, Boriding (Boronizing), Heat Treating, Vol 4, ASM Handbook, ASM International, 1991, p 437-447
28. J.K. Hirvoven and B.D. Sartwell, Ion Implantation, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 605-610
29. G.R. Fenske, Ion Implantation, Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook, ASM International, 1992, p 850-860
30. R.E. Hoisington, Extending Tool Life with Ion Implantation, G.E.
Technol., Jan 1986, p 9-12
31. K.P. Cooper, Laser Surface Processing, Friction, Lubrication, and
Wear Technology, Vol 18, ASM Handbook, ASM International, 1992,
p 861-872
CHAPTER
6
Surface Engineering
to Add a Surface Layer
or Coating
THE SURFACE-ENGINEERING METHODS described in this Chapter are those that involve an intentional buildup or addition of a new layer
on a metal substrate, that is, the application of a coating or lining. A wide
range of processes are used to deposit metal, ceramic, and organic (paints
or plastic and rubber linings) coatings or combinations of these materials
(composite coatings). As shown in Table 1, each has their own distinct
processing parameters (e.g., temperature, pressure, and time), advantages, and limitations. Coating materials or coating methods discussed
below include:
•
•
•
•
•
•
•
•
•
•
•
Organic coatings and linings
Ceramic coatings
Hot dip metallic coatings
Electroplating (metal or composite coatings)
Electroless plating (metal or composite coatings)
Weld overlays (metal or cermet coatings)
Thermal spraying (metal, plastic, ceramic, or composite coatings)
Cladding (thick metal coatings)
Chemical vapor deposition (metals, graphite, diamond, diamondlike
carbon, and ceramics)
Physical vapor deposition (metals, ceramics, or solid lubricants)
Thermoreactive deposition/diffusion process (carbides, nitrides, or
carbonitrides)
More detailed information on these coating processes can be found in
Volumes 5 (Surface Engineering), 13 (Corrosion), and 18 (Friction,
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Surface Engineering to Add a Surface Layer or Coating / 127
Lubrication, and Wear Technology) of the ASM Handbook as well as the
references cited throughout this Chapter.
Organic Coatings
Painting and the application of various organic (plastic and rubber) coatings and linings are among the most widely used surface-engineering
processes. Paints or linings that act as protective film to isolate the substrate from the environment exist in a number of different forms. Sheet linings, commonly of the vinyl or vinylidene chloride family, are one such
type of coating that can be either adhered to the surface to be protected or
suspended as a bag within a tank, for example, to provide protection. Hotapplied organisols, or plastisols, again usually of the vinyl family, can also
be applied to a surface, typically by dipping or flow coating, to provide a
protective film.
Powder coatings are being increasingly used to protect concrete-reinforcing rod, as pipeline coatings, and as coating materials in the original
equipment manufacturing markets. Fine powders produced from high-molecular-weight resins of the thermoplastic vinyl and fluorinated hydrocarbon families or from thermoset resins of the epoxy and polyester families
are applied to the surface to be protected by either electrostatic spray or
fluidized-bed deposition. The metal being protected is usually preheated
at the time of application, and after application it is reheated to an elevated
temperature (generally from 150-315 °C, or 300-600 OF). The specific
time/temperature baking schedule depends on the metal temperature at the
time of application and the type of powder being applied.
Alternatively, some coating systems are characterized by the application
method used. For example, for coil-coated metal sheet (commonly steel or
galvanized steel), very specialized high-speed roller application equipment is used to coat the sheet steel as it is unwound from a coil. The paint
used in the coil-coating process can be of virtually any generic type, although alkyds, polyesters, epoxies, and zinc-rich epoxy coatings are the
most prevalent.
Certain lining materials, such as hand lay-up fiberglass-reinforced plastics, are also used to protect steel from corrosion. Such coating systems
usually consist of an epoxy primer applied to a blast-cleaned steel surface,
followed by one or more polyester gel coats, with one or more layers of a
fiberglass veil or woven roving mat laid within the gel coats as reinforcement. The system is then sealed with a layer of the polyester gel coat (a
semiclear, 100% solids resin coat). Similarly, rubber linings are used to
protect against corrosion. There are various types of rubbers, but they can
generally be categorized as prevulcanized or postvulcanized (vulcanized
after application). Similarly, rubbers (or elastomers) can be formulated
with different hardnesses and chemical resistances. Commonly, a rubber
128 / Surface Engineering for Corrosion and Wear Resistance
lining is a composite of two or three different types of rubbers adhered to
each other and to the surface. The environmental resistance of some common rubber lining materials is summarized in Table 2.
Paints
Despite the importance of the coatings and linings discussed previously,
the most commonly used organic materials are the liquid-applied (usually
by brush, roller, or spray) coating and lining materials, that is, paints. Liquid-applied organic coatings have four basic components: a resin, a solvent, pigments, and other miscellaneous compounds. The resins, often
called binders, are identified by their generic type, as indicated in Table 3.
These are based on the organic compound structure that makes up the
resin. The solvent can be either water or an organic solvent. Increasing environmental regulations are limiting the use of organic solvent-base systems. The pigments are added for such functions as rust inhibition, decreased permeability, to provide color, or to increase resistance to
ultraviolet or weathering conditions. Typical pigments include zinc phosphate, zinc molybdate, zinc phosphorus silicate, zinc chromate, and strontium chromate. Miscellaneous compounds added include dryers, flowcontrol and gloss-control agents, and suspension agents. Although the
resin or organic binder of the coating material is most influential in determining the resistance and properties of the paint, the type and amount of
pigments, solvents, and additives will dramatically influence the application properties and protective capability of the coating system. Furthermore, hybridized systems can be formulated that are crosses between the
categories. For example, an acrylic monomer or prepolymer can be incorporated with virtually any other generic type of resin to produce a product
with properties that are a compromise between the acrylic and the original polymer. In many cases, this is advantageous, as in the mixing of
Table 2
Environmental
Common
name
Butadiene
rubber
Natura! rubber,
isoprene rubber
Chloroprene
rubber
Styrene-butadience
(nitrile) rubber
Acrylonitrile-butadiene
(nitrile) rubber
Isobutylene-isoprene
(butyl) rubber
Ethylene-propylene
(-diene) rubber
Silicone rubber
Fluoroelastomer
(a) 0, outstanding;
resistance of common rubber lining materials
ASTM
D 1418
designation
Ozone
Oxidation
Water
Alkalies
Aliphatic
Aromatic
Halogenated
Alcohol
BR
NR,IR
P
P
G
G
E
E
F-G
F-G
P
P
P
P
P
P
G
G
F-G
F-G
CR
VG
VG
G
E
G
F
P
G
F-E
Low-medium
SBR
P
G
E
F-G
P
P
P
G
F-G
Low
NBR
P
F-G
E
F-G
E
G
P
VG
E
Very low
IIR
E
E
E
E
F
F-G
P
VG
G-E
Very low
0
E
E
G-E
P-G
P
P-F
P-G
F-E
Medium
E
E
0
E
VG
P-F
F-G
P-G
E
P-G
0
F
G
F
VG
G-VG
F-E
EPM,EPDM
E, excellent;
VMQ
FKM
Resistance to(a):
VG, very good; G, good; F, fair; P, poor
E
Acids
Permeability
to gases
Low
Low
High
Low
Surface Engineering to Add a Surface Layer or Coating / 129
vinyls and acrylics or the heat curing of alkyds and acrylics. In other
cases, such as with an epoxy, acrylic modification can be a detriment.
Table 3 lists the properties and applications of the principal coating resins.
Additional information on coating resins can be found in Ref 1 to 3.
Basic Function. Paint coatings may be applied for appearance, to meet
functional requirements, or to meet combined function and appearance
Table 3 Properties and applications of coating resins
Forms
available
Resin
Drying
method
Favorable
characteristics
Unfavorable
characteristics
Cost
Uses
Acrylic
Solvent,
waterborne,
powder
Air dry, bake
Water white, outdoor
durability, chemical,
heat resistance
Poor-fair adhesion,
tendency to be
brittle
Moderate,
Alkyd
Solvent,
waterborne
Air dry, bake
High gloss, flexibility,
good durability,
versatility
Low, moderate
Chlorinated
rubber
Solvent
Air dry
Water, alkali, acid
resistance
Poor alkali
resistance,
generally not
hard, tendency
to yellow,
depending on
resin
Abrasion resistance,
hardness, gloss,
sensitivity to
solvents
Epoxy
Solvent,
waterborne,
powder
Air dry, bake
Excellent adhesion,
chemical resistance,
flexibility, abrasion
resistance, hardness
Moderate,
Fluorocarbon
Solvent,
powder
Bake
Highest exterior
durability, chemical
resistance
Solvent
Air dry, bake
Extremely fast drying,
good hardness,
abrasion resistance
Phenolic
Solvent,
waterborne
Air dry, bake
Hardness, adhesion,
resistance to
chemicals, corrosion
Rapid chalking on
exterior
exposure, poor
resistance to
oxidizing acids,
yellows in clears
Adhesion,
recoatability,
high baking
temperatures
Low solids content,
fair to good
exterior
durability, low
flash point
solvents
Darkens, can only
be used in darkcolored coatings
Polyester
Solvent,
waterborne,
powder
Solvent,
waterborne,
powder
Air dry, bake
Solvent,
waterborne
Air dry, bake
Vinyl
Solvent,
powder
Air dry, bake
Fair adhesion, may
hydrolize under
certain conditions
Some types yellow
and chalk readily
on exterior
exposure
Tendency toward
brittleness.
Unmodified
types require
high baking
temperatures
Generally low
solids, low flash
points
High
Silicone
High gloss, hardness,
chemical resistance,
high film build
Chemical resistance,
abrasion resistance,
hardness, exterior
durability
High heat resistance,
exterior durability,
gloss and color
retention
Nitrocellulose(
a)
Polyurethane
(a) Must be modified
with other resins
Air dry, bake
Chemical resistance,
flexibility, fast air
dry, formability,
resistance to acid,
alkali, abrasion
high
Moderate
high
Automotive topcoats,
appliances, coil
coatings, aluminum
siding, general
industrial use
Trade sales enamels,
trim paints, exterior
enamels, general
metal finishing
Maintenance coatings,
ship bottom paints,
swimming pool
paints, chemical
process equipment
Maintenance paints,
automotive primers,
appliances, metal
products
High
Coil coatings, siding
Low, moderate
Furniture finishes,
touch-up lacquers,
general- purpose
product finishes,
aerosol lacquers
High
Can linings, tank
linings,
maintenance paint
on metals
Wood finishes, coil
coatings, specialty
bake coats
Aircraft finishes,
maintenance paints,
metal and plastic
coatings
Any finish for high
heat resistance,
exterior metal
coatings
Moderate,
High
Moderate
high
Can and tank linings,
maintenance paints,
metal decorating
paints
130 / Surface Engineering for Corrosion and Wear Resistance
needs. If the basic purpose is appearance, the gloss, color, and retention of
these properties in service are emphasized. In some applications, functional requirements are of equal importance to appearance. On office furniture, for example, paint films must provide attractive appearance and resist marring and abrasion. On automobiles, paint films must be attractive
in appearance, easily applied, and readily repaired, but be resistant to
abrasion, marring, and impact as well as capable of protecting the underlying metal from corrosion. In other applications, such as corrosion protection of tanks or chemical equipment, the functional requirements of the
paint film are of prime concern. Corrosion resistance is the most important functional requirement.
Corrosion of steel and cast iron occurs in all common environments.
The rate and extent of corrosion vary from mild attack in dry, clean environments to highly accelerated attack in marine or industrial areas where
corrosive fumes are present in air. Table 4 lists paints selected for service
in a wide range of corrosive conditions. The rate of the base metal corrosion where paints are used should not exceed approximately 1.3 mm/yr
Table 4 Organic coatings selected for corrosion resistance in various
environments
Coatings
Applications
Outdoor exposure
Oil paints
Alkyds
Amino resin-modified alkyds
Nitrocellulose lacquers
Acrylics
Buildings, vehicles, bridges; maintenance
Trim paints, metal finishes, product finishes
Automotive, metal awnings, aluminum siding
Product finishes, aerosol lacquers
Automotive finishes
Marine atmosphere
Alkyds, chlorinated
vinyl-alkyds
Urethanes
rubber, phenolics,
epoxies, vinyls,
Superstructures
and shore installations
Clear marine varnishes
Water immersion
Phenolics
Vinyls
Chlorinated
Urethanes
Epoxies
Ship bottoms
Ship bottoms, locks
Ship bottoms, swimming pools
Clear marine varnishes
Ship hulls, marine structures
rubber
Chemical fumes
Epoxies, chlorinated
rubber, vinyls, urethanes
Chemical-processing
equipment
Extreme sunlight
Vinyls
Acrylics
Silicone alkyds
Metal awnings
Automotive finishes
Petroleum-industry
processing
equipment
High humidity
Amino resin-modified alkyds
Epoxies
Catalyzed epoxies, chlorinated
rubber, phenolics
Refrigerators, washing machines
Air conditioners
Maintenance; chemical and paper plants
High temperature
Epoxies
Modified silicones
Silicones
Inorganic zinc-rich
Motors, piping, 120°C (250
Stove parts, roasters, 205°C
Stove parts, roasters, 290°C
pigmented paints 650°C
Structural steel, chimneys to
"F) max
(400 OF) max
(550 OF) max; aluminum(1200 OF) max
370 °C (700 OF)
Surface Engineering to Add a Surface Layer or Coating / 131
(50 mils/yr). For corrosion Oratesabove this, both in atmospheric and immersion service, or where catastrophic failure is of concern, paints should
generally not be used. For more severe corrosion applications, alternative
corrosion prevention measures such as the use of more corrosion-resistant
alloys, sheet or rubber coatings and linings, fiberglass layups, and metallic coatings and claddings should be considered.
In service, paint films are frequently required to resist exposure to
highly deleterious materials. For example, decorative finishes, such as
those on home laundry equipment, must resist detergents, and paint films
on equipment powered by gasoline engines must withstand attack from
gasoline.
Paint films also may be required to resist acids and alkalis, solvents,
staining, heat, impact, marring, and abrasion. Some coatings must be able
to withstand flexing without cracking or flaking. Table 5 lists paints that
have proven successful in withstanding mechanical and chemical action.
Surface Preparation. The importance of proper surface preparation to
the durability of any coating system cannot be overemphasized. Without
proper surface preparation, the finest paint, applied with the greatest of
skill, will fall short of its maximum performance or may even fail miserably. A coating can perform its function only so long as it remains intact
and firmly bonded to the substrate.
An adequately prepared surface not only provides a good anchor for the
coating, but also ensures a surface free of corrosion products and contaminants that might shorten the life of the film by spreading along the coating/substrate interface and destroying adhesion or by actually breaking
through the coating.
Before being painted, metals usually are exposed to one or more fabricating processes, such as rolling, stamping, forming, forging, machining,
and heat treating. In these processes, the metal surfaces pick up various
contaminants that can either interfere with the adhesion of the paint film
or allow corrosion to progress beneath the paint film and cause it to fail
prematurely.
The principle surface contaminants that adversely affect the performance of paint films include oils, greases, dirt, rust, mill scale, water, and
Table 5 Paints selected for resistance to mechanical
or chemical action
Action
Abrasion
Impact
Marring
Flexing
Acids
Solvents
Detergents
Staining
Gasoline
Alkalis
Heat
Paint
Vinyls; plastisols; polyurethanes
Epoxies; vinyls; polyurethanes
Thermosetting acrylics; vinyls
Epoxies; vinyls
Chlorinated rubber; vinyls; epoxies
Epoxies; phenolics
Thermosetting acrylics; epoxies
Thermosetting acrylics
Alkyds; epoxies
Phenolics
Alkyd-amines; silicone resins
132 / Surface Engineering for Corrosion and Wear Resistance
salts such as chlorides and sulfides. These contaminants must be removed
from the surface before paint is applied.
Selection of cleaning process is governed by the soil or contaminant to
be removed, the degree of cleanness required, thetype of paint to be applied, and the size, shape, material, and end use of the part. In addition, the
speed with which the process runs will affect the cleaning characteristics.
Methods of cleaning metal surface can be classified as:
•
•
Mechanical cleaning, including power brushing, grinding, and abrasive blasting
Chemical cleaning, including emulsion cleaning, solvent cleaning,
vapor degreasing, alkaline cleaning, acid cleaning, pickling, and
steam cleaning
To meet rigid requirements for surface cleanness, mechanical and chemical cleaning methods can be used in conjunction. For example, before
structural steel intended for an application involving exposure to corrosive
chemical environments is painted, oil, grease, rust, mill scale, and any
other surface contaminants must be completely removed. Chemical paints
strippers or solvent cleaners are often used prior to mechanical cleaning to
remove oil, grease, or old paint. More detailed information on various surface cleaning methods can be found in Surface Engineering, Volume 5 of
the ASM Handbook.
Ceramic Coatings and Linings
The ceramic coatings and linings discussed in this section include glass
linings, porcelain enamels, cement linings, and high-performance ceramic
coatings applied by various processing methods. Additional information
on ceramic and cermet (ceramic-metal) coatings applied by vapor deposition processes or thermal spraying can be found later in this chapter.
Glass Linings
Glass, particularly low-expansion borosilicate glass (e.g., Pyrex), finds
numerous application as a material of construction in the chemical, food,
pharmaceutical, petrochemical, and electronics industries. The principal
attributes of interest in these applications are chemical durability, thermal
stability, resistance to thermal shock, ease of cleaning, transparency and
economy-requirements
identical to those in the laboratory but now
scaled up to reflect process capacity. Modular glass components may be
used to make systems such as distillation columns, boilers, scrubbers, and
so on. Glass piping systems, glass-lined steel reactor vessels, and glass
heat exchangers are some examples of applications in which corrosion or
Surface Engineering to Add a Surface Layer or Coating / 133
product purity may dictate the use of glass. Increasingly, glass reaction
vessels are also being used in the treatment of hazardous (and, again, corrosive) wastes.
Glass-lined steel vessels are used as reaction vessels in applications in
which the additional mechanical integrity of the outer steel shell is a
safety imperative. An acid recovery or concentration unit is a good example. Thus, a sulfuric acid concentrator would consist of a glass-lined steel
container with a horizontal tantalum boiler. Waste sulfuric acid streams
can be concentrated to 90 wt% H2S04, The recovered concentrate may
still contain trace amounts of organics. Vapor that arises during the
process is condensed in yet another glass heat exchanger and then goes in
to a glass-phase separator where the organic and the water are separated
for reuse. The glass linings or vessel walls are completely inert to sulfuric
acid in all concentrations, so that the purity of the product is dependent
only on the quality of the feed and makeup fluids. There are two additional points:
•
•
Abrasion resistance is not an issue with glass linings but can be a concern when alternate fluorocarbon linings are considered.
Glass lining can be damaged through internal mechanical impact or
abuse; in such cases, the repair is performed with tantalum patches or
a similar material.
Modular glass components are employed in solvent recovery applications where the use of glass ensures product purity.
Porcelain Enamels
Porcelain enamels are glass coatings applied primarily to products or
parts made of sheet steel, cast iron, or aluminum to improve appearance
and to protect the metal surface. Porcelain enamels are distinguished from
other ceramic coatings on metallic substrates by their predominantly vitreous nature and the types of applications for which they are used. These
coatings are differentiated from paint by their inorganic composition and
coating properties. They are fused to the metallic substrate at temperature
above 425°C (800 OF) during the firing process. Detailed information on
the types of porcelain enamels and methods to apply them can be found
in Ref 4.
The most common applications for porcelain enamels are major appliances, water heater tanks, sanitary ware, and cookware. In addition, porcelain enamels are used in a wide variety of coating applications, including
chemical processing vessels, agricultural storage tanks, piping, pump
components, and barbecue grills. They also are used for coatings on architectural panels, signage, specially executed murals, and substrates for
microcircuitry. Porcelain enamels are selected for products or components
where there is a need for one or more special service requirements that
134 / Surface Engineering for Corrosion and Wear Resistance
porcelain enamel can provide. These include chemical resistance, corrosion protection, weather resistance, abrasion resistance, specific mechanical or electrical properties, appearance or color needs, cleanability, heat
resistance, or thermal shock capability.
Corrosion Resistance. Porcelain enamel is widely used because of its
resistance to household chemicals and foods. Mild alkaline or acid environments are generally involved in household applications. Table 6 presents examples of corrosive environments for which porcelain enamels are
widely used for long periods of service. Special enamel compositions are
available to resist most acids, except for hydrofluoric or concentrated
phosphorics, to temperatures of 230°C (450 OF). These compositions also
resist alkali concentration to pH 12 at 93°C (200 OF).
Wear Resistance. The hardness of porcelain enamels ranges from 3.5
to 6.0 on the Mohs scale. Porcelain enamels show a high degree of abrasion resistance. Under abrasive test conditions where plate glass retains
50% specular gloss, porcelain enamel compositions retain from 35 to 85%
specular gloss. Subsurface abrasion resistance varies with processing
variables that affect the bubble structure of the enamel, that is, gas bubbles
frozen in during cooling of the enamel. A decrease in abrasion resistance
occurs with an increase in the number or size of gas bubbles. Enamel compositions are available that contain crystalline particles (from mill additions or devitrification heat treatment) that increase abrasion resistance as
much as 50%.
Concrete and Cementatious Coatings and Linings
Cementatious linings have become one of the most widely used construction materials in designing protective linings for industrial installation in which high temperatures, aggressive corrosive media, and complicated substrate geometry exist, such as floors, trenches, sumps, ducts,
chimneys, and other air pollution control equipment. They are used in various industries, including power, steel and metalworking, chemical, pulp
and paper, refinery, waste treatment, and mining.
Table 6 Applications in which porcelain enamels are used for resistance to
corrosive environments
Corrosive environment
Temperature
Application
°C
OF
pH
Bathtubs
Chemical ware
$49
:::;100
:::;100
175-230
:::;120
:::;212
:::;212
350-450
5-9
12
1-2
1-2
Home laundry equipment
Range exteriors
Range oven lines, conventional
Range burner grates
Kitchen sinks
Water heaters
$71
21-66
66-315
66-590
:::;71
$71
:::;160
70-150
70-600
70-1100
:::;160
:::;160
11
2-10
2-10
2-10
2-10
5-8
Corrosive medium
Water; cleansers
Alkaline solutions
All acids except hydrofluoric
Concentrated sulfuric acid,
nitric acid, and hydrochloric
Water; detergents; bleach
Food acids; cleansers
Food acids; cleansers
Food acids; cleansers
Food acids; water; cleansers
Water
acid
Surface Engineering to Add a Surface Layer or Coating / 135
Inorganic monolithic linings have proved themselves in these industries
because of their chemical resistance to both high and low concentrations
of strong acids and solvents, thermal insulation that protects the substrates
from extremely high temperatures, temperature resistance to 870°C
(1600 OF), good compressive and flexural strength for environments in
which stress and strain are factors, and abrasion resistance. Monolithic
linings can be applied by cast or gunite (shotcreting) methods over old
and new steel or concrete, as well as by brick and mortar masonry.
Certain disadvantages were encountered during the development of the
acid-resistant silicate cements. The silicate cements were not resistant to
alkalies, hydrofluoric acid (HF) , and fluoride salts. The sodium silicate
(Na2Si03) cements formed a growth salt when exposed to H2S04 that put
undue internal stresses on the structure of the material.
Inorganic monolithic linings also have a certain amount of permeability
compared to organic surfacing materials. Over time, acid can penetrate the
lining and eventually reach the surface of the substrate. The problem is now
being combated by using a dual-lining system, which includes a chemically
resistant elastomeric membrane applied to the surface of the substrate.
Dual linings. In recent years, the trend toward using the superior technology of dual linings has emerged and is being recommended where corrosion problems occur. Figure 1 shows the design of a typical membrane/monolithic system in the chemical industry. Condensation occurs
not only on the face of the acid-resistant lining, but it can also penetrate
and condense on the substrate to be protected. Although acid-proof monolithic linings offer the proper chemical resistance, they are inherently inelastic, or brittle. In time, monolithic linings may tend to crack and absorb
Chemical- and
Permanently flexible
elastomeric expansion
joint
Fig. 1
Flexible chemical-
or
concrete
substrate
Schematic of a chemical-resistant dual-lining system that provides
double protection to the substrate in the form of a flexible membrane
and a rigid surface layer. The flexible, corrosion-resistant membrane is applied in
direct contact with steel or concrete substrates. It is then covered by the monolithic cement lining, which provides protection over a broad pH range as well as
against high temperatures.
136 / Surface Engineering for Corrosion and Wear Resistance
acids in acid gas condensate; therefore, it is advantageous to have a
backup membrane. In many applications, the coefficient of thermal expansion of the monolithic lining may not match that of the substrate.
Therefore, a flexible membrane will help accommodate stresses resulting
from these differences in thermal expansion, as well as other mechanically
induced stresses.
High-Performance Ceramic Coatings and Linings
High-performance ceramic coatings include high-temperature glasses;
high-temperature coatings based on oxides, carbides, nitrides, and silicides; and cermets. Ceramic coatings are applied to metals to protect them
from oxidation and corrosion at room temperature and at elevated temperature. Special coatings have been developed for specific uses, including wear resistance, chemical resistance, high reflectivity, electrical resistance, and prevention of hydrogen diffusion. Ceramic-coated metals are
used for furnace components, heat treating equipment, chemical processing equipment, heat exchangers, rocket motor nozzles, exhaust manifolds,
jet engine parts, and nuclear power plant components.
These coating materials are deposited by firing (sintering) a slurry or
slip that has been applied to a metal surface by spraying or dipping, by
various thermal spray techniques discussed later in this chapter, by the
pack cementation process described in Chapter 5, fluidized-bed processing, trowel coating, and electrophoresis. The method of applying the coating is restricted by the type of coating, the type of metal to be coated, and
the size and configuration of the work. More detailed information on ceramic coating methods can be found in Ref 5.
Silicate Glasses. Coatings prepared from glass powders, with or without additions of refractory compounds, have the greatest industrial usage
of all ceramic coatings. Glass coatings are used for such applications as
aircraft combustion chambers, turbines and exhaust manifolds, and heat
exchangers. Variations in composition of the silicate-based glass are virtually unlimited. The spray-sinter process is the most commonly used
method to deposit these coatings. Under certain conditions, electrostatic
spraying also can be used.
Oxides. Coatings based on oxide material provide underlying metal,
except refractory metals, with protection against oxidation at elevated
temperature and with a high degree of thermal insulation. Alumina
(AI203), zirconia (Zr02), and chromium oxide (Cr203) are the oxides
most commonly used as coatings. Alumina coatings are hard and have excellent resistance to abrasion and good resistance to corrosion. Zirconia is
widely used as a thermal barrier because of its low thermal conductivity.
Chromium oxide coatings are used in a variety of wear-resistant applications (see Chapter 8 for details). Oxide coatings are usually applied by
flame spraying or plasma spraying, although chromium oxide can be applied by spraying/dipping plus sintering.
Surface Engineering to Add a Surface Layer or Coating / 137
Carbides as ceramic coatings are principally used for wear and seal applications, in which the high hardness of carbides is an advantage. These
applications include jet engine seals, metalforming tools, tools and dies
for ceramic and plastic processing, components used in the chemical and
general processing industries, and machine elements. Commonly employed carbides include titanium carbide (TiC), silicon carbide (SiC),
tungsten carbide (WC), chromium carbide (Cr3C2), and boron carbide
(B4C). These ceramics are applied by thermal spraying or vapor deposition techniques.
Nitrides are used for wear-resistant applications, most notably surface
engineering of cemented carbide cutting tools and tool steels. Titanium nitride (TiN) is the most frequently employed coating, but titanium carbonitride (TiCN), hafnium carbide (HfC) , titanium aluminum nitride
(TiAIN), titanium zirconium nitride (TiZrN), and chromium nitride (CrN)
have also been used commercially. These coatings are applied by vapor
deposition techniques (Fig. 2).
Silicides are the most important coating material for protecting refractory metals against oxidation. Silicide-based coatings protect by means of
a thin coating of silica that forms on the coating surface when heated in
an oxygen-containing atmosphere. Examples of silicide coatings include
(a)
(b)
(e)
Fig. 2
Nitride ceramic coatings deposited on cemented carbide substrates by physical vapor deposition. (a) TiN. (b) TieN. (c) TiAIN
138 / Surface Engineering for Corrosion and Wear Resistance
molybdenum silicide (MoSi2), niobium silicide (NbSi2), and tantalum silicide (TaSi2). To improve the stability and adherence of the silica coating,
other elements, such as chromium, boron, or aluminum are added to the
coating formula. Silicide coatings are usually applied by some variation of
the vapor-deposition process.
Hot Dip Coatings
Hot dip coatings are predominantly used to improve the aqueous corrosion of steel.
Batch and Continuous Processing
Processing of hot dip coatings involves either batch or continuous processing. The continuous process is more advantageous for sheet steels,
whereas the batch process is normally used for individual parts. Details of
the processing techniques are outlined in Ref 6. In the batch galvanizing
process, the two types of conventional practices are the wet process and the
dry process (Ref 7). The wet process involves a flux blanket on the top of
the molten zinc bath to remove impurities from the surface of the steel and
also to keep that portion of the surface of the zinc bath, through which the
steel is immersed, free from oxides. In the dry process the steel is usually
cleaned, treated with an aqueous solution, dried, and then dipped in the
molten zinc bath. The molten zinc bath is maintained at temperatures between 445 and 455°C (830 and 850 OF) and times in the range of 3 to 6 min.
The time of immersion is used to control the thickness of the coating, which
consists of iron-zinc alloy phases at the interface along with a top coat of
pure zinc. Good cooling control is necessary because the zinc can continue
to react with the substrate to produce further alloying and detrimentally affect the properties of the coating such as the spangle finish (or grain size).
In continuous hot dip processing, welded coils of steel are coated at
speeds of 200 mlmin. The flux or Cook-Norteman line is similar to the
batch process in that the sheet is cleaned and fluxed in line prior to immersion. The hot-processed continuous line is more complex in that the
steel sheet is first cleaned at temperature in a reducing environment, annealed above the recrystallization temperature of about 700°C (1290 OF)
and then immersed in the molten bath. As the strip exits the bath, the thickness of the molten metal film is controlled by gas wiping dies that remove
excess coating metal. After coating, the sheet is either cooled by forced air
or subjected to an in-line heat treatment, called galvannealing, before being
rewound into coil or sheared into cut lengths at the exit of the line.
Coating Microstructure
In general, the coating microstructure consists of the substrate, the interfacial alloy layer, and the overlay cast structure. Depending on the type
Surface Engineering to Add a Surface Layer or Coating / 139
of coating, the microstructure and composition of these constituents
changes. As expected, the substrate plays a major role in the type of coating obtained, and substrate composition can affect growth kinetics of the
phases formed. For example, if the substrate contains silicon then the
well-known Sandelin Effect can influence the iron-zinc phase reaction
and consequently the thickness of the coating (Ref 8). Similarly, alloy additions to the steel to improve sheet formability, for example, interstitialfree (IF) steels with titanium, titanium/niobium, and phosphorus, can influence the microstructure of the iron-zinc phases in galvanized and
galvannealed steel (Ref 9). Substrate grain size has also been shown to
greatly affect the nucleation of the iron-zinc phases. In aluminum-containing baths, the structure formed first is an inhibition layer that is dependent on bath composition and prevents further alloying for a certain
short time before the inhibition layer becomes unstable (Ref 10).
When the zinc galvanizing bath contains only a trace of aluminum, zinc
attack of the substrate is uniform and the phases that form are governed
by the iron-zinc binary phase diagram. In zinc baths containing aluminum,
the stability of the inhibition layer governs the amount of iron-zinc phases
formed. Once the inhibition layer is no longer stable, outbursts or rapid
growth of iron-zinc phases occur during hot dipping (Ref 11). During the
thermal cycle of the galvannealed process, the inhibition layer dissolves
and iron-zinc phase layer growth occurs in a controlled manner until the
entire coating is made up of iron-zinc phases (Ref 12). Both galvanized
and galvannealed alloy phase growth are determined by Fe-AI-Zn ternary
diffusion, and the overlay cast microstructure greatly depends on aluminum content of the bath. The pure-zinc and low-aluminum coatings
form an overlay of pure zinc (1") phase. Zn-5 wt%Al (Galfan) solidifies as
eutectic microstructure, and the Zn-55 wt%Al (Galvalume and Zincalume) solidifies as aluminum dendrites with zinc-rich interdendritic regions. The aluminum coatings (type I and type II) either form overlays of
aluminum-silicon or aluminum alloy, respectively.
Galvanized Coatings
Surface Finishes. Galvanized coatings are commonly characterized by
surface spangles. In cross section, an Fe2AIs(Zn) inhibition layer develops
first, preventing any iron-zinc intermetallic phase formation. The overlay
layer is made up of dendrites of pure zinc (1") phase and appears as a polycrystalline structure. The three surface finishes commonly produced are:
•
•
Regular spangle, where the coating solidifies from the dipping temperature by air cooling
Minimum spangle, where the coating is quenched using water, steam,
chemical solutions, or by zinc powder spraying
Extra-smooth temper roll finish carried out as an additional operation
with regular and minimum spangle material
140 / Surface Engineering for Corrosion and Wear Resistance
Alloying Effects. Aluminum is probably the most important alloying element added to the hot dip galvanizing bath, with different levels required
to produce different properties in the bath (Ref 13). Aluminum levels of
0.005 to 0.02 wt% are added to brighten the initial coating surface. The effect is related to the formation of a continuous Al203 layer on the coating
surface that inhibits further oxidation by acting as a protective barrier
layer. This effect is also responsible for the reduced atmospheric oxidation
of the zinc bath. In addition, aluminum in the range of 0.1 to 0.3 wt% is
added to the zinc bath to suppress the growth of brittle iron-zinc intermetallic phases at the steel coating interface by forming the Fe2AIs (Zn)
inhibition layer. The end of this incubation period is marked by the disruption of the initial layer, followed by rapid attack of the substrate steel.
An increase in the incubation period depends on increased aluminum bath
composition using a low bath temperature, having low bath iron content,
agitation, and the presence of solute additions in the steel. Thus, during
commercial production, the immersion time is kept below the incubation
period in order to obtain a highly ductile product.
Corrosion Protection. Zinc coatings add corrosion resistance to steel in
several ways. As a barrier layer, a continuous zinc coating separates the steel
from the corrosive environment. By galvanic protection, zinc acts as a sacrificial anode to protect the underlying steel at voids, scratches, and cut
edges of the sheet. The sacrificial properties of zinc can be seen in a galvanic series where the potential of zinc is less noble than steel in most environments at ambient temperatures. In addition, after dissolution of the
zinc metal, zinc hydroxide can precipitate at the cathodic areas of the exposed steel, forming a secondary barrier layer. Zinc corrodes at a slower rate
than the steel substrate, although.the corrosion rate of zinc varies depending
on the atmosphere to which it is exposed (Ref 14), as shown in Fig. 3.
Fabricability. During forming, especially stretch-forming operations,
increased friction of the zinc can result in less total stretch before fracture.
In severe forming operations, galling and coating pickoff can also occur.
Furthermore, coating particulate buildup on die surfaces can lead to poor
surface appearance of formed parts. Proper lubrication is essential in the
design of any forming process, especially when forming zinc-coated parts.
Weldability of zinc coatings is also an important property of the coating.
Spot weldability properties are particularly important because most galvanized product is joined in this manner. Zinc coatings reduce the life of
welding electrodes because the copper electrode alloys with zinc. This effect leads to higher resistance, localized heating, and increased pitting and
erosion of the electrode tip. As a result, manufacturing costs increase because lower tip life reduces productivity due to frequent downtime in the
welding operation to redress tips.
Paintability. Although zinc coatings are often used in the as-coated
state, some applications call for a painted surface, and therefore paintability is an important design property of the coating. It has been shown that
Surface Engineering to Add a Surface Layer or Coating / 141
Zinc coverage, g/m2 (Oz/ft2)
76
153
229
305
382
460
532
610
688
763
837
916
(0.25) (0.50) (0.75) (1.00) (1.25) (1.50) (1.75) (2.00) (2.25) (2.50) (2.75) (3.00)
80
70
60
>~
50
40
(J)
o
.~
(J)
(f)
30
20
o ..-::;;...........j'-------L_
11
(0.4)
21
(0.8)
32
(1.3)
43
(1.7)
54
(2.1)
65
(2.6)
75
(3.0)
86
(3.4)
97
(3.8)
108
(4.2)
118
(4.7)
129
(5.1)
Thickness of zinc, urn (mils)
Fig. 3
Service life (time to 5% rusting of steel surface) versus thickness of zinc
for selected atmospheres. Shaded area is thickness range based on
minimum thicknesses for all grades, classes, and so forth, encompassed by ASTM
A 123 and A 153. Source: Ref 15
large-spangle material is difficult to paint; therefore, most -painted products are either minimum spangle or temper rolled. It is usually necessary
to pretreat a hot dip galvanized coating with a zinc phosphate or complex
oxide thin coating before prepainting. In the automobile industry, following the pretreatment most automobile bodies are primed with an electrophoretic paint (e-coat), and, as a result, resistance to e-coat cratering is
an important property. At high e-coat voltages, sparking as a result of exceeding the dielectric properties of the deposited paint film causes localized heat generation, film disruption, and premature curing of the paint.
After paint curing, these sparked areas form pinpoint craters that result
in a paint surface with a detrimental appearance. Therefore, resistance to
e-coat cratering, expressed in cratering threshold voltage, is an essential
paintability property (Table 7).
Table 7 Effects of hot dip coatings on threshold voltages
for cratering of cathodic electrophoretic primer
Type of surface
Uncoated bare steel
Zinc
Zinc-iron
Zn-55Al
Aluminum
Source: Ref 16
Cratering threshold, V
>400
275
225
375
>400
142 / Surface Engineering for Corrosion and Wear Resistance
Galvanneal Coatings
Galvanneal coatings are essentially diffusion coatings that expose the
zinc galvanized steel to an annealing temperature around 500°C (930 OF)
to produce a fully alloyed coating containing iron-zinc intermetallic
phases. This is accomplished by inserting heating and cooling capacity
above the liquid zinc pot in order for the galvannealing process to be continuous. Good process control requires that the effects of heating rate,
hold temperature and time, and cooling rate on the iron-zinc reaction kinetics be well understood. Galvanneal coatings have been classified as
(Ref 12):
•
•
•
Type 0: Underalloyed coating containing predominantly ~-phase
Type 1: Optimal alloyed coating with less than a 1 urn interfacial
'Y-Iayer and a top layer containing 8-phase interspersed with a small
amount of ~-phase
Type 2: Overalloyed coating with a 'Y-phase more than 1 urn and an
overlay of 8-phase containing basal plane cracks
Fabricability. Formability is an important property in galvanneal coatings because iron-zinc intermetallic phases are considered brittle. As a
result, powdering and flaking of the coating can occur during the forming
operation, resulting in reduced corrosion resistance and impaired
paintability. The type 1 coating was found to have the best formability
properties (Ref 17), but as in most forming operations lubrication to improve metal flow is essential. Spot weldability of galvanneal coatings are
improved over galvanized coatings because it is more difficult to these
iron-zinc phases to alloy with the copper electrode. Paintability is also better than that of galvanized coatings because of the microscopically rough
surface formed as a result of the iron-zinc alloy phases throughout the
coating. However, galvanneal coatings are more prone to cratering during
e-coating (Table 7). Conversely, corrosion resistance can be slightly reduced because of the increased iron .in the coating from the iron-zinc
phases; the galvanic potential is not as great as it is for pure zinc.
Zinc-Aluminum
Coatings
Zn-5AI alloy coating (Galfan) is near the eutectic point in the aluminum-zinc equilibrium phase diagram. Two compositions have been reported based on additions to the eutectic composition: small (up to about
0.5%) mischmetal additions containing lanthanum and cerium and additions of 0.5% Mg. These additions are made to improve the wettability
and suppress bare spot formation as well as to produce a typical "minimized spangle" structure. The microstructure of Galfan is characterized
by a two-phase structure, a zinc-rich proeutectoid 'rl-phase surrounded
by eutectic phase consisting of lamellae of a-aluminum and zinc-rich
ri-phase. However, the microstructure can be varied depending on the
Surface Engineering to Add a Surface Layer or Coating / 143
cooling rate. In the range of normal bath temperatures, 420 to 440°C
(790-825 OF)there is no visible intermetallic layer or at least an extremely
thin layer «0.5 J-Lm)at the interface between the steel substrate and the
overlay coating. Thus, Galfan coatings have excellent formability and cutedge corrosion protection.
Zn-55AI alloy coating (marketed under the tradename Galvalume)
contains about 1.5% Si added for the purpose of preventing an exothermic
reaction at the coating overlay/substrate steel interface. As a result, the
coating contains «-aluminum dendrites, zinc-rich interdendritic regions,
and a fine dispersion of silicon particles, along with a prominent Fe-AIZn intermetallic alloy layer at the interface between the steel substrate and
the overlay coating. The surface of the coating contains characteristic
spangles that consist of aluminum dendrites with a clearly measurable
dendrite arm spacing. Cooling rate after dipping can significantly refine
the microstructure of the coating, increasing the number of silicon particles and constraining the growth of aluminum dendrites.
Initially, the atmospheric corrosion of the Zn-55AI coating takes place
in the zinc-rich interdendritic regions, enabling the coating to exhibit galvanic protection. As the coating continues to corrode, the zinc corrosion
products become trapped in the interdendritic regions and act as a further
barrier to corrosion. Eventually, the aluminum dendrites, which also acted
as a barrier layer, add to the corrosion protection, as does the Fe-AI-Zn intermetallic alloy layer. This results in a parabolic type of corrosion as evidenced in Fig. 4. Although its galvanic protection is less than that provided by galvanized coatings, Zn-55AI is generally adequate to protect
against rust staining at scratches and cut edges of the steel sheet.
Aluminum Coatings
Aluminum coatings are produced as type 1 coating, a thin (20-25 J-Lm)
aluminum-silicon alloy coating, and type 2, a thicker (30-50 J-Lm)pure
20
0.7
[
en
15
0.6
(J)
..Q
0.5
c
0
·en
g
10
0
0
Q)
Q)
(J)
..Q
c
0.4
0
·en
0.3
0
e
0
Q)
C>
~
~
sa
·E
en
0.2
5
C>
~
«>
Q)
0.1
2
4
6
8
10
12
14
Exposure time, yr
Fig. 4
Corrosion losses of hot dip coatings in the industrial environment
Bethlehem, PA. Source: Ref 18
of
144 / Surface Engineering for Corrosion and Wear Resistance
aluminum coating. Silicon is present in type 1 coatings in the range of 5
to 11 wt% to prevent formation of a thick iron-aluminum intermetallic
layer at the coating/steel substrate interface. Instead, a thin Fe-AI-Si intermetallic layer is formed, allowing for good formability and coating adherence. These coatings are intended primarily for applications requiring
improved appearance, good formability, and resistance to high temperatures, as in automobile exhaust components. The type 2 coating has a microstructure containing a pure aluminum overlay and a thick iron-aluminum intermetallic alloy layer. Thus, the formability and adhesion of this
coating is limited by the poor ductility of the alloy layer. Nevertheless, the
coating is used for outdoor construction applications (e.g., roofs, culverts,
etc.) that require resistance to atmospheric corrosion (Table 8). The aluminum outerlayer offers excellent corrosion resistance because of the
good barrier properties provided by the increased thickness of the coating
(Fig. 4).
Terne Coatings
Lead-tin alloy hot dip coatings are widely known as terne coatings.
Generally, 3 to 15% Sn is added tothe bath in order to facilitate wetting
of the steel substrate. Terne-coated steel has a long history of use in automotive fuel tanks and tubing because of its excellent weldability, solderability, and formability. Moreover, the low corrosion rate of terne allows
it to function primarily as a barrier coating without the evolution of corrosion products that might otherwise clog fuel and hydraulic systems.
Due to concerns about the effects of lead in the environment, work is
now under way to find suitable replacement materials. Zinc alloy coatings
(hot dip zinc-iron or electroplated zinc-nickel) with an organic topcoat are
among the most likely candidates.
Nickel-terne-coated
steel includes an electrolytic flash coating of
nickel (1-1.5 g/m") underneath a conventional lead-tin coating for enhanced corrosion resistance. Applications are similar to the conventional
lead-tin alloy coatings described previously.
Table 8 Coating thickness losses for galvanized steel and type 2 aluminized steel
in atmospheric exposure
Middletown,
Years
exposed
1
2
4
6
10
15
(a) G90 galvanized
Kure Beach,
OH
Type 2
G90(a)
NC
Type 2
G90
fl.m
mils
fl.m
mils
fl.m
mils
fl.m
mils
2.6
5.2
9.3
14.5
24.4
0.1
0.2
0.37
0.57
0.5
0.7
1.2
0.02
0.028
0.047
0.12
0.11
0.21
7.0
8.6
12.7
16.2
23.5
0.28
0.34
0.5
0.64
0.93
1.02
1.4
2.4
0.04
0.09
0.15
0.18
0.24
0.26
0.96
3.1
2.9
5.3
steel has a coating weight of 0.90 ozlft2 (270
26.0
glm2). Source: Ref 19
3.8
4.5
6.0
6.7
Surface Engineering to Add a Surface Layer or Coating / 145
Electrochemical Deposition
Electrochemical methods are well-established processes for applying
metal coatings for improved surface properties of materials. Electrodeposition or electroplating is defined as the deposition of a coating by electrolysis, that is, depositing a substance on an electrode immersed in an
electrolyte by passing electric current through the electrolyte. The process
can take place in an aqueous electrolyte near ambient temperatures (called
aqueous solution electroplating) or in a fused metal salt at high temperatures (called metalliding or fused-salt electroplating). Modifications of the
electroplating process include occlusion or composite deposition plating.
Excellent reviews of electroplating can be found in Ref 20 and 21. Wearand corrosion-resistance data for selected electrochemical coating methods are summarized in Table 9.
Aqueous Solution Electroplating
Aqueous solution electroplating provides decorative and protective finishes for use at ambient temperatures and in a variety of environments.
Table 9 Characteristics of wear- and corrosion-resistant
components
Copper
Characteristic
Maximum
working
temperature, °C
Nontoxicity
Covering
complex
shapes\
Thickness
range, urn
Wear
Hardness, HV
Electroplated
nickel
Electroless
nickel
electrochemical finishes for engineering
Electroless
nickel
+ chromium
Chromium
Electroplated
nickel
+ chromium
50
650
550
650
550
650
Excellent
Medium
Very good
Medium
Very good
Excellent
Very good
Very poor
Very good
Very good
Very good
Medium
12.5-500
12.5-500
12.5-500
12.5-500
12.5-500 + 25-50
12.5-500 + 25-50
450-500
(900-1000
850-950
850-950
850-950
Excellent
Excellent
Excellent
60-150
200-300
Low friction,
antistick
Poor
Poor
after heat
treatment)
Medium
Resistance
to impact
Medium
Very good
Medium
Medium
Medium
Very good
Corrosion
resistance
Very poor
Very good
Medium
Poor
Very good
Very good
Typical
applications
Buildup;
lubricant
in forming;
heat sink;
selective
case
hardening
Buildup
under or
instead of
chromium
in corrosive
conditions;
printing
surfaces
PVC
molding
tools and
dies;
moving
parts in
process
glass and
rubber
molds
Molds, tools,
valves,
rams,
pistons
shafts,
gauges,
dies, saw
blades
High-temperature,
anti seize
bolting; ball
valves; shafts
Marine crane
rams and
hydraulics,
mine roof
supports, print
rolls
Source: Ref 22
Comment
Short times at higher
temperatures
possible
Grinding needed
over 200 um
Indication of
abrasive-wear
resistance
Indication of
adhesive-wear
resistance
Thin coats and soft
substrates prone
to damage
Nickel at least 50 urn
for corrosive
environments
146 / Surface Engineering for Corrosion and Wear Resistance
A main drawback in electroplating is the inability of achieving uniform
deposition, which is related to the throwing power of the electrolyte.
Throwing power is substrate-shape dependent and also depends on the
anode/cathode configuration and the current density, as well as on the
composition and conductivity of the electrolyte. A second difficulty is
that not all metallic elements can be deposited. Another problem with
electroplating is evolution of hydrogen at the electrodes when the
cathode efficiency is less than 100%. If a ferrous substrate is to
be plated, absorbed atomic hydrogen can cause embrittlement of the
part. Unless the parts are heat treated to remove the absorbed hydrogen,
they will be brittle and unusable for any application involving elastic
strain.
Substrate preparation for plating is critical to ensure good adhesion
and surface quality. Maximum adhesion depends on both the elimination of surface contaminants in order to induce a metallurgical bond and
the generation of a completely active surface to initiate plating on all
areas. The cleaning steps for steel substrates usually involve precleaning, intermediate alkaline cleaning, electrocleaning, acid treatments,
and anodic desmutting (Ref 23). Electrodeposited metals can have a
very good bond to the substrate, but that bond will never be the same as
a fusion bond, and poor bonds can go undetected unless techniques are
used to test the actual bond strength. The electrodeposited coating usually ends up with a surface texture that is the same as the starting substrate surface texture, unless an intermediate leveling coating (such as
copper) is used.
Nevertheless, the electroplating process produces a coating with distinct
advantages (Ref 24):
•
•
•
•
•
•
•
•
The workpiece will not undergo distortion or metallurgical changes
because the operating temperature of the bath does not exceed 100 °C
(212 OF).
Plating conditions can be adjusted to modify hardness, internal stress,
and metallurgical characteristics of the coating.
Coatings are dense and adherent to the substrate.
The thickness of the coating is proportional to the current density and
length of time of the deposition.
Although deposition rate seldom exceeds 75 urn/h (3 mils/h), it can be
accelerated by forced circulation of the electrolyte and can be as high
as 150 or 200 urn/h (6 or 8 mils/h) for some metals in high-speed
plating.
There is no technical limit to the thickness of electrodeposits.
Application of coatings is not confined to the line of sight. Although
throwing power may be limited, the freedom of anode design and location is helpful.
Areas not requiring deposition can be masked.
Surface Engineering to Add a Surface Layer or Coating / 147
•
•
Only the tank size of the bath limits the dimensions of the part, although large parts such as gun barrels can be the tank itself; as another
alternative, brush plating can be used.
The process is suitable for automation and has economic advantages
over other coating processes.
In general, aqueous electroplating has minimal effect on substrate properties (apart from hydrogen embrittlement). Coated substrates can also be
heat treated to promote interdiffusion, although this may result in concentration of elements at grain boundaries, causing embrittlement. Specific
elemental electrodeposition processes and properties are reviewed in Ref
25; some examples are given here.
Nickel plating is widely used for a corrosion- and wear-resistant finish.
Typical applications, with a thin top coat of electrodeposited chromium,
are decorative trim for automotive and consumer products and office furniture. Nickel deposits are also used for nondecorative purposes for improved wear resistance, for example, on pistons, cylinder walls, ball studs,
and so forth.
Chromium electroplating is also used as decorative and hard coatings.
Colored and tarnish-resistant chromium decorative coatings are produced
over a base deposit of copper and/or nickel for applications such as those
noted above for nickel. Hard chromium coatings are used for hydraulic
pistons and cylinders, piston rings, aircraft engine parts, and plastic
molds, where resistance to wear, heat abrasion, and/or corrosion are required.
Cadmium and zinc electroplating provides galvanic corrosion protection when coated on steel. Deposit thickness can vary between 5 and
25 J.1m (0.2 and 1 mil), and typical applications for both coatings are
found in Table 10. Cadmium is preferred for the protection of steel in marine environments, whereas zinc is preferred in industrial environments.
Cadmium is also preferred for fastening hardware and connectors because
its coefficient of friction is less than zinc. Cadmium is toxic and should
not be used in parts that will have contact with food. Precautions for minimizing hydrogen embrittlement should be taken because cadmium plating is more susceptible to such embrittlement than any other plated metal.
Continuous Electrodeposition
Electrogalvanizing. The development of continuous electrogalvanizing
lines has produced a thin, formable coating that is ideal for deep drawing
or painting. Automotive body panels are typically new applications for
electrogalvanized zinc, zinc-nickel, and zinc-iron alloys. Processing details applicable to electrogalvanizing can be found in Ref 27.
Tinplate is another continuous electrolytic plating process that has been
used for the past 200 years to make containers for the long-term storage
of food (Ref 28). The typical tinplate product consists of five layers: an
148 / Surface Engineering for Corrosion and Wear Resistance
Table 10 Recommended minimum thicknesses and typical applications for zinc and cadmium coatings
electrodeposited on iron and steel
Time to
Coating
white
thickness(a)
Service
conditions
Electrodeposited
Chromate
f.lm
mils
finish
minimum wear
5
0.2
None
Clear
Iridescent
Olive drab
None
Clear
Iridescent
Olive drab
None
Clear
Iridescent
Olive drab
None
Moderate (mostly dry, indoor
atmosphere; occasional condensation,
wear, and abrasion)
8
0.3
Severe (exposure to condensation;
infrequent wetting by rain and
cleaners)
13
0.5
Very severe (exposure to bold
atmospheric conditions; frequent
exposure to moisture, cleaners, and
saline solutions; likely damage by
abrasion or wear)
25
in
Typical
h
applications
Screws, nuts and bolts, wire goods, fasteners
12-24
24-72
72-100
Tools, zipper pulls, shelves, machine parts
12-24
24-72
72-100
12-24
24-72
72-100
Tubular furniture, window screens, window
fittings, builders' hardware, military
hardware, appliance parts, bicycle parts
Plumbing fixtures, pole line hardware
cadmium
Mild (see above)
Moderate
salt spray,
zinc
Mild (indoor atmosphere;
and abrasion)
Electrodeposited
corrosion
(see above)
Severe (see above)
Very severe (see above)(b)
5
8
13
25
0.2
0.3
0.5
None
Clear
Iridescent
Olive drab
None
Clear
Iridescent '
Olive drab
None
Clear
Iridescent
Olive drab
None
Clear
Iridescent
Olive drab
(a) Thickness specified is after chromate coating, if used. (b) There are some applications
hot dipped or sprayed coatings. Source: Ref 26
12-24
24-72
' 72-100
12-24
24-72
72-100
12-24
24-72
72-100
Springs, lock washers, fasteners, tools,
electronic and electrical components
Television and radio chassis, threaded parts,
screws, bolts, radio parts, instruments
Appliance parts, military hardware,
parts for tropical service
electronic
24
24-72
72-100
for cadmium
coatings
in this environment;
however, these are normally
satisfied by
innermost layer of steel sheet, a tin-iron intermetallic compound layer, a
free-tin layer, a thin passivation layer based on chromium oxide, and a top
layer of oil film for lubrication. The corrosion characteristics of tinplate
are documented in the literature (Ref 29).
Fused-Salt Electroplating
Fused-salt electroplating, which is commonly referred to as "metalliding," is a process for surface modification and surface hardening by
electrodeposition from fused-salt electrolytes. Two unique aspects of this
electrodeposition process are; (1) elements that cannot be plated by
conventional processes may plate by fused-salt electrodeposition and
(2) if the deposition rate is controlled to match the diffusion rate of the
Surface Engineering to Add a Surface Layer or Coating I 149
deposition species in the substrate at the fused-salt temperatures 400 to
900°C (750--1650 OF), the substrate will develop a diffusion coating.
In electroplating (Ref 30), the molten-salt medium in which the anode and
cathode are immersed, consists of a soluble form of the metal to be plated
dissolved in a molten-salt solvent, such as an alkali metal halide, that does
not participate in the plating process. Coating thickness is determined by the
electrical charge, and a sharp interface between the coating and substrate is
maintained. At the higher-temperature plating range of the bath, some coating/substrate interdiffusion can occur. In metalliding (Ref 31), the element
to be diffused is made the anode of the molten-salt electrochemical cell
(usually fluoride) and the substrate is the cathode. A more electrochemically
active anode diffuses into the cathode when the electrodes are connected.
Because the process is diffusion controlled, a sharp coating/substrate interface does not exist; instead a diffusion gradient in the substrate occurs.
Although fused-salt electroplating has only found limited application
for refractory metals and ceramic coatings, some success has been obtained with the platinum-group metals. The process conditions for this
technology are too stringent and economically unfeasible. Nevertheless,
the process requirements for the electrodes, the melt, and cell operation
have been outlined (Ref 30). On the other hand, metalliding is a unique
electrodeposition process for applying elements that are difficult to electrodeposit on substrates that usually cannot be plated. It is an important
process for improving the surface hardness and corrosion resistance of
metals .without producing significant dimensional changes. Small-scale,
small-size, specialized or strategically important components can be considered for technological development for coating by metalliding if the
cost justification can be made, in view of the fact that a one-step diffusion
process can be achieved more easily.
Precious Metal Plating
Silver, gold, and the platinum metal groups are electroplated by either
aqueous solution electroplating or fused-salt electrodeposition, Both silver and gold are used for decorative purposes as well as industrial uses;
the aqueous plating process is reviewed in Ref 32 and 33. Decorative applications of both elements still predominate, but silver has been successfully substituted for gold in some functional uses in electronics. Silver is
used on metallic leadframes, the device that supports the majority of silicon chips. New silicon-to-silver bonding techniques have been used to replace the more expensive gold. However, in electrical contact applications, where long-term surface integrity is important, silver has not been
able to replace gold because of its tendency to oxidize or sulfidize on the
surface, increasing the contact resistance of the component.
Both aqueous and fused-salt electrolytes have been used for plating the
platinum group elements. Platinum has been used as a diffusion-barrier
layer in aluminiding nickel-base alloys and MCrAIY coatings. Platinum
150 / Surface Engineering for Corrosion and Wear Resistance
from the aqueous electrolyte is highly stressed unlike the fused-salt deposit.
Substances particularly considered for platinum-group metal coating are the
refractory group alloys of molybdenum, tungsten, tantalum, niobium, and
vanadium, which tend to form volatile oxides at high temperatures thus reducing their usefulness as corrosion-resistant materials. However, cost is
still a major factor, and these metals can be used for diffusion-barrier layers
only if components are small and strategically important.
Electroless Plating
Electroless Nickel Plating. Electroless plating baths have been developed for copper, nickel, silver, gold, and a number of other metals, but the
systems with the most importance for corrosion and wear applications are
the nickel-phosphorus and nickel-boron systems. Electroless nickel plating
is used to deposit nickel without the use of an electric current; thus it is
sometimes called autocatalytic plating. In this process, the part is immersed in an aqueous solution containing metal salts, a reducing agent, and
other chemicals that control the pH and reaction rates. The part acts as a
catalyst for the reduction of the nickel ions by the reducing agent. The reducing agent causes the metal ion reduction and the nickel coating on the
part continues to act as a catalyst as the plating process continues, unlike
in electroplating where the ions pick up electrons from the cathode. When
the process takes place using a hypophosphite-reducing agent, the finished
nickel coating is not pure nickel, but contains phosphorus inclusions. Phosphorus content can be as high as 13% (Ref 34). When the process takes
place using a borohydride compound reducing agent, the finished product
is a nickel-boron alloy. The boron content can be as high as 5% (Ref 35).
As applied, nickel-phosphorus coatings are uniform, hard, relatively
brittle, lubricious, easily solderable, and highly corrosion resistant (Ref
35). Wear resistance equivalent to hard chromium coatings can be obtained when the coating is heat treated at low temperatures to produce a
very hard precipitation-hardened structure. As applied, most of these coatings are amorphous metal glasses that when heated first form nickel phosphite (Ni3P) particles; at temperatures above 320°C (610 OF), the deposit
crystallizes. Internal stresses are primarily a function of coating composition, and coating thickness uniformity can be easily controlled. Adhesion
to most metals is excellent, and frictional properties are also excellent and
similar to chromium.
Nickel-boron coatings have excellent resistance. to wear and abrasion,
but because they are not completely amorphous they have reduced resistance to corrosive environments. Furthermore, they are much more costly
than nickel-phosphorus coatings.
As deposited, the microhardness of electroless nickel-phosphorus coating is about 500 to 600 HVN (48-50 HRC), equivalent to many hardened
steels. After precipitation hardening, hardness values as high as 1100
HVN are reported, which is equivalent to commercial hard-chromium
Surface Engineering to Add a Surface Layer or Coating / 151
coatings. Because of their high hardness, electroless nickel coatings have
excellent wear and abrasion resistance in both the as-deposited and hardened condition (Table 11).
Electroless nickel coatings can be easily soldered and are used in electronic applications to facilitate soldering of light metals such as aluminum.
Electroless nickel is often used as a barrier coating; to be effective, the deposit must be free of pores and defects. In the as-deposited amorphous
state, the coating corrosion resistance is excellent (Table 12), and in many
environments is superior to that of pure nickel or chromium alloys. However, after heat treatment the corrosion resistance can deteriorate.
Composite Coatings
Composite deposition plating is a further extension of aqueous solution
electroplating or electroless coatings in that particles or fibers are suspended in the electrolyte, then occluded in the deposit. Oxides, carbides,
silicides, refractory powder, metallic powder, and organic powder can be
introduced into the electrolyte. The most widely used electrodeposited
composites are cermet coatings, with A1203, Zr02, titania (Ti02), and SiC
added to increase strength, hardness, and wear resistance (Table 13). The
amount of ceramic particles incorporated in the coating depends on the
current density and the bath loading; that is, the amount of particulate in
the suspension. It has been shown that coatings up to 40 vol% Al203 were
produced at 0.5 A/dm2 and 5.3 vol% bath loading. The amount of Al203
incorporated into the coating was seen to decrease with increasing current
density and decreasing bath loading (Ref 36). Hardness values ranged
from approximately 250 to 580 HVN, depending on the amount of Al203
incorporated.
Metallic particles such as chromium can be introduced into a metal plating electrolyte (for example, nickel and cobalt), and the deposited composite can be subsequently heat treated toform high-temperature oxidation-resistant alloys. MCrAIY composites have been made by depositing
10 J.LmCrAIY powder in a cobalt or nickel matrix. Heat treatment bonds
Table 11 Comparison of the Taber: abrasive wear
resistance of electroless nickel coatings
Heat treatment
Coating
Watts nickel
Electroless Ni-P(b)
Electroless
Ni-B(c)
Hard chromium
for 1 h
°C
OF
None
None
300
500
650
None
400
None
None
None
570
930
1200
None
750
None
Taber
wear
index, mg/lOOO cycles(a)
25
17
10
6
4
9
3
2
(a) CS-lO abraser wheels, 1000 g load, determined as average weight loss per 1000 cycles
for total test of 6000 cycles. (b) Hypophosphite-reduced
electroless nickel containing approximately 9% P. (c) Borohydride-reduced
electroless nickel containing approximately
5%B
152 / Surface Engineeringfor Corrosion and Wear Resistance
Table 12
Corrosion of electroless nickel coatings in various environments
Corrosion rate
Electroless nickelphosphorus(a)
Temperature
Environment
°C
OF
Acetic acid, glacial
Acetone
Aluminum sulfate, 27%
Ammonia, 25%
Ammonia nitrate, 20%
Ammonium sulfate, saturated
Benzene
Brine, 3.5% salt, CO2 saturated
Brine, 3.5% salt, H2S saturated
Calcium chloride, 42%
Carbon tetrachloride
Citric acid, saturated
Cupric chloride, 5%
Ethylene glycol
Ferric chloride, 1%
Formic acid, 88%
Hydrochloric acid, 5%
Hydrochloric acid, 2%
Lactic acid, 85%
Lead acetate, 36%
Nitric acid, 1%
Oxalic acid, 10%
Phenol,90%
Phosphoric acid, 85%
Potassium hydroxide, 50%
Sodium carbonate, saturated
Sodium hydroxide, 45%
Sodium hydroxide, 50%
Sodium sulfate, 10%
Sulfuric acid, 65%
Water, acid mine, 3.3 pH
Water, distilled, N2 de aerated
Water, distilled, 02 saturated
Water, sea (3.5% salt)
20
20
20
20
20
20
20
95
95
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
95
20
20
20
100
95
95
68
68
68
68
68
68
68
205
205
68
68
68
68
68
68
68
68
68
68
68
68
68
68
68
68
68
68
205
68
68
68
212
205
205
f.l.lnlyr
0.8
0.08
5
16
15
3
Nil
5
Nil
0.2
Nil
7
25
0.6
200
13
24
27
1
0.2
25
3
0.2
3
Nil
1
Nil
0.2
0.8
9
7
Nil
Nil
Nil
millyr
p.m1yr
millyr
0.03
0.003
0.2
0.6
0.6
0.1
Nil
0.2
Nil
0.008
Nil
0.3
84
Nil
Nil
40
( c)
3.5
Nil
( c)
0.14
Nil
Nil
42
Nil
1.7
0.2
0.008
90
3.5
Nil
( c)
Nil
Nil
Nil
Nil
( c)
Nil
Nil
Nil
11
0.4
Nil
Nil
Nil
Nil
1
0.02
8
0.5
0.9
1.1
0.04
0.008
2
0.1
0.008
0.1
Nil
0.04
Nil
0.008
0.03
0.4
0.3
Nil
Nil
Nil
(a) Hypophosphite-reduced
electroless nickel containing approximately
10.5% P. (b) Borohydride-reduced
ing approximately
5% B. ( c) Very rapid. Specimen dissolved during test. Source: Ref 35
Table 13
Mechanical properties of electrodeposited
Hardness, HV
Cermet
Ni
Ni-2.02AI203
Ni-3.33Ti02
Ni-6.80Cr203
Ni-3.6(Ti02 + CrSi2)
Ni-24Co
Ni-23.4Co-3Ti02
Ni-22.5Co-6.07Cr203
Ni-23.7Co-3.8CrSi2
Ni-23.7Co-3.4(Ti02
+ CrSi2)
(a) Annealed.
As plated
Annealed
187
275.4
354
409
283
280
383
462
302
359
118
247
254
295
209
150
219
285
207
211
Electroless
nickel-boron(b)
electroless
3.3
1.6
nickel contain-
cermets
Yield strength(a),
MPa
93.0
67(a),68.4(b)
222.4
284.0
198.5
120.6
206.6
264
196.4
196.5
Elongation(a),
%
6.26
3.75
1.50
2.60
2.3
2.0
1.2
1.6
(b) As plated. Source: Ref 22
the coating to the substrate and interdiffuses the cobalt and nickel with the
CrAlY particles. The CoCrAlY coatings produced have been shown to be
superior to some plasma sprayed CoCrAlY and pack aluminized coatings
(Ref 22). Aluminum particles have been codeposited in a nickel matrix
Surface Engineering to Add a Surface Layer or Coating '·153
103
o Ni (800°C)
o Ni-AI (800°C)
•
Ni (900°C)
~ Ni-AI (900°C)
E
::t.
•
Ni (1000 °C)
•
Ni-AI (1000 °C)
102
cti
C/)
Q)
~c:0
;S
Q)
"C
'x
0
10
1~
1
~
~
10
102
~
Time, h
Fig. 5
Oxide depth versus time plotted on log-log scale for pure nickel and nickel-alloy
coatings exposed in air at 800, 900, and 1000 °C. Source: Ref 37
and subsequently heat treated to produce a nickel-aluminum intermetallic
coating with exceptional oxidation resistance (Ref 37) (Fig. 5).
Polytetrafluoroethylene (PTFE), diamond, and SiC particles can also be
incorporated into a nickel electroless plating for improved properties. Diamond and silicon carbide are used to enhance abrasion resistance; the
surface hardness of these composites is reported to be 1300 HVN. Polytetrafluoroethylene is added to the electroless nickel bath to provide a
composite coating with enhanced lubrication. Almost any particulate material can be deposited in a metallic matrix, provided the particles are sufficiently small to remain suspended in the bath and that the particles do
not react chemically with the bath during electrodeposition.
Weld-Overlay
Welding is a solidification method for applying coatings with corrosion,
wear, and erosion resistance. Weld-overlay coatings, sometimes referred
to as hardfacing, offer unique advantages over other coating systems in
that the overlay/substrate weld provides a metallurgical bond that is not
susceptible to spallation and can easily be applied free of porosity or other
defects. Welded deposits of surface alloys can be applied in thicknesses
greater than most other techniques, typically in the range of 3 to 10 mm.
Coatings
154 / Surface Engineering for Corrosion and Wear Resistance
Most welding processes are used for application of surface coatings and
on-site deposition can be more easily carried out, particularly for repair
purposes. Weld overlays are very versatile because a large number of commercially available alloys can be selected to provide protection from a
wide range of environmental degradation mechanisms.
During weld-overlay surfacing, the coating material is raised to its melting point and then solidified on the surface of the substrate, which means
that metals and alloys used for this purpose must have melting points similar to or less than the substrate material. The effectiveness of the weldoverlay coating depends mainly on the welding process and the overlay
alloy composition. The welding process must be selected and optimized
to apply protective overlays at high deposition rates and thermal efficiency, with good control over the overlay/substrate dilution and coating
thickness. The overlay alloy composition must be selected to provide the
required properties to prevent coating degradation, and the alloy composition must be readily weldable.
Applicable Welding Processes. A number of welding processes are
available for applying protective weld overlays, and many welding parameters must be considered when attempting to optimize a particular process
for a given application. The process principles and their characteristics for
some processes are summarized for comparison purposes in Table 14 and
are described in Ref 38 and 39. The processes can be grouped as torch
processes, arc welding processes, and high-energy-beam techniques. The
torch process, oxyacetylene welding (OAW) , is the oldest and simplest
hardfacing process and involves simply eating the substrate with the flame
and then melting the filler rod to get the hardfacing to melt. High-energybeam techniques use laser beam welding (LBW) or electron beam welding
(EBW) to alloy the surface by adding alloy powders to the weld pool.
In arc welding, the heat is generated by an arc between an electrode and
the workpiece. Arc welding processes can be grouped into nonconsumable
electrode processes and consumable electrode processes. Nonconsumable
electrode processes, gas tungsten arc welding (GTAW) and plasma arc
welding (PAW), both involve a tungsten electrode and the introduction of
the filler metal (in the form of rod or wire in GTAW and powder in PAW).
The arc melts the filler metal to form a molten pool that is protected from
the atmosphere by an inert gas shield. In plasma arc welding, an additional
inert gas flows through a constricted electric arc in the welding torch to
form the plasma. In general, for consumable electrode processes, the arc is
maintained between the consumable electrode and the workpiece. In
shielded metal arc welding (SMAW), the electrode consists of a core wire
surrounded by a flux covering, that upon melting forms a liquid slag and
gas to protect the molten metal pool. In flux core arc welding (FCAW), the
flux is contained in the core of the metallic tubular electrode, whereas in
gas metal arc welding (GMAW) the consumable wire electrode and substrate metal is protected from the atmosphere by a gas fed axially with the
Surface Engineering to Add a Surface Layer or Coating / 155
Table 14 Weld surfacing processes
Process
Approximate
deposit thickness
(min),mm
Deposition rate,
kglh
Dilution single
layer, %
1.5
:51
1-5
0.1
0.2-1
Oxyacetylene
(OAW)
Powder weld (PW)
Shielded metal arc
(SMAW)
Gas tungsten arc
(GTAW)
Plasma transferred
arc (PAW)
Gas metal arc
(GMAW)
Flux-cored arc
(FCAW)
Submerged arc
(SAW)
Wire
Strip
3
1-4
15-30
1.5
:52
5-10
2
:510
2-10
2
3-6
10-30
2
3-6
15-30
3
10-30
15-30
4
10-40
10-25
4
15-35
5-20
Bulk
Electroslag
(ESW)
Typical uses
Small area deposits
on light sections
Small area deposits
on light sections
Multilayers on
heavier sections
High-quality lowdilution work
High-quality lowestdilution work
Faster than SMAW,
no stub-end loss;
positional work
possible
Similar to GMAW.
Mainly for ironbase alloys for
high abrasion
resistance
Heavy section work;
higher-quality
deposits than
FCAW
Corrosion-resistant
cladding of large
areas
Similar to SAW wire
but other alloys
possible
High-quality
deposits at higher
deposit rates than
SAW. Limited
alloy range
Source: Ref 39
wire through the welding gun nozzle. In submerged arc welding (SAW),
the arc, which is submerged beneath a covering of flux dispenses from a
hopper, melts the electrode, the surface of the workpiece, and some of the
flux that protects the molten pool from oxidation. Electroslag welding
(ESW) uses equipment similar to SAW for strip cladding.
Processing Parameters. There are a large number of processing parameters that must be considered when attempting to optimize welding
processes for surface application:
All processes
Consumable
Voltage across the arc
Current through the arc
Current polarity
Current pulsing parameters
Travel speed of heat source
Shielding gas type (except SAW)
Filler metal feed rate
Electrode diameter
Electrode extension ("stick-out"
Nonconsumable
processes
length)
processes
GTAW electrode tip angle (vertex angle)
PAW plasma gas flow rate
156 / Surface Engineering for Corrosion and Wear Resistance
However, the important factors considered in terms of arc welding overlay parameter optimization and process performance include arc efficiency, melting efficiency, deposition rate, dilution, and coating thickness (Ref 40). Arc efficiency is only a function of the arc welding
process; melting efficiency increases with increasing arc power and
travel speed, and the maximum deposition rate is directly related to both
the arc and melting efficiency. During the deposition of the weld-overlay coating, the base metal and the filler metal are melted and mixed in
the liquid state to form a fusion bond. Depending on the weld-overlay
coating thickness, if a large portion of the substrate is melted' and allowed to mix appreciably with the weld overlay, dilution can cause the
overall alloy content of the coating to be significantly reduced. The level
of mixing is quantified as the dilution ratio and is one of the most important parameters in a surface application because the original filler
metal mechanical and corrosion properties can be altered. The extent to
which dilution occurs depends on the surfacing and substrate materials
used, the welding process chosen, and the parameters employed. Table
14 indicates the range for dilution expected for the various processes
employed. Figure 6 is a surfacing diagram that relates dilution for various arc welding processes according to filler metal feed rate and melting power (a function of arc and melting efficiency) and can be used to
280
Calculated
dilutions
240
200
----
Inoperable
Ch
(i)
E
E
l!f
160
~
"0
OJ
~
120
"'ffi
1i)
E
~
80
u::
40
100%
0
o
1000
2000
3000
4000
5000
6000
Melting power, W
Fig. 6
Effect of processing parameters on dilution
process. Source: Ref 41
with experimental
data plotted for SAW
Surface Engineering to Add a Surface Layer or Coating / 157
facilitate process selection and parameter optimization in weld-overlay
applications (Ref 41).
During welding, the base metal is subjected to peak temperatures that
are at least as high as the melting temperature of the substrate. The properties of the weld and the adjacent heat-affected zone (HAZ) strongly
depend on the thermal history as dictated by the heat input. Preheating
the part may be a necessary step in reducing the residual stress and distortion associated with welding. Preheat and maintenance of a specific
minimum temperature during the welding cycle can also reduce the
cooling rate to prevent the formation of a detrimental transformation region in the HAZ of ferrous alloys. Interpass temperature is another important factor that needs to be controlled in order to prevent increased
dilution and HAZ grain growth at high temperatures. Postweld heat
treatment can take many forms, depending on whether the weld-overlay
coating needs to be stress-relief annealed or must be heat treated for
specified properties.
Wear and Corrosion Resistance. Excellent reviews of hardfacing
metallurgy and the application of weld-overlay consumables are found in
Ref 39 and 42. For overlay coatings, components are designed to provide
resistance to various forms of wear, erosion, and corrosion over a large
temperature range. Thus, properties such as hardness, microstructure, and corrosion resistance are more important for the coating than tensile strength and elongation, which are usually provided by the substrate
material.
Generally, coatings selected for wear resistance require high hardness as
a characteristic, thus the term "hardfacing." It is believed that most hardfacing alloys develop their wear resistance by virtue of wear-resistance
carbides (Ref 43). Almost all hardfacing alloys can be separated into two
major groups based on chemical compositions of the primary solidified
hard phases:
•
•
Carbide hardening alloys, including cobalt-base/carbide (WC-Co)
and some iron-base superalloys
Intermetallic hardening alloys, for example, nickel-base superalloys,
austenitic stainless steels, and iron-aluminides
However, although increased hardness generally increases wear resistance, different microstructures containing the same carbide type can also
have significant effect on wear resistance (Fig. 7).
Erosion resistance of materials is very dependent on the erosion conditions, the effects of which are dominated by a number of variables including particle size, shape, composition, and velocity; angle of incidence; and
temperature. Unlike wear properties, the erosion rate of weld-overlay
coatings generally increases with increasing hardness (Fig. 8). However,
the erosion resistance of weld-overlay alloys depends on whether the
158 / Surface Engineering for Corrosion and Wear Resistance
1,8
~
'e:J
1,6 1-----+----+----4-----+-----1---:
e
jg
:eco
2f
1.4 1-----+----+-----+-
c:
j9
en
'Ci)
~
ro
~
Ferrite: Low-carbon steels, annealed or normalized
Pearlite: Medium- to high-carbon steels,
annealed or normalized
Bainite: Austempered medium- to high-carbon steels
Martensite: Quenched and tempered medium- to
high-carbon steels and carburized steels
~----f-
1,2
100
300
200
400
500
Hardness,
Fig. 7
600
700
800
HB
Effect of structure and hardness on abrasion resistance. Source: adapted from Ref 44
24
I
22
I
I
o Plastically deformed
•
Not deformed plastically
f-
---
I
Armacor-M
20
c:
18
'E
ME
E
16
M
0
.2f
~
14
---
c:
0
'Ci)
e
OJ
OJ
12 >-
H
t II
aSry-
E
:J
~
22
o
Stellite-6
iv
•
10
T
I
Type 316L
0
8
I
,rr-a,uminide
0
0
IN-625
I
Ultimet
I
1
B-60
TS-2
•
High-chromium
I
iron
•Type 1420
6
4
o
100
200
300
400
Average Vickers microhardness
Fig. 8
500
at 400°C
Volume steady-state erosion rates of weld-overlay
as a function of average microhardness at 400°C
erodent). Source: Ref 45
600
700
800
(500 g load)
coatings at 400°C (750 "F)
(90° impact angle; alumina
Surface Engineering to Add a Surface Layer or Coating 1159
coating can be classified as a brittle or ductile material (Ref 45). Those
materials that can be deformed plastically (ductile) produce a large plastic zone beneath the eroded surface, and the increased plastic zone size
can be directly correlated to an improved steady-state erosion resistance.
For those materials that cannot deform plastically (brittle), an increase
in coating hardness sometimes may lead to a decrease in volumetric erosion rate. Thus, materials that can dissipate particle impact energy
through plastic deformation (plastic zone) exhibit low. erosion rates.
However, for materials that do not deform plastically (no plastic zone)
and do not undergo plastic deformation, the ability to resist brittle fracture (i.e., cracking) becomes a major factor that can control the erosion
resistance.
The corrosion resistance of weld-overlay coatings follows the corrosion-resistant properties of the bulk materials and is also dependent on
the corrosive environment. Weld-overlay coatings provide resistance to
oxidation and sulfidation. Dilution, as discussed previously, can be expected to modify the behavior. of the coating alloy from the properties
quoted for the undiluted bulk materials. In weld-overlay coatings such
as austenitic steels, dilution can affect corrosion resistance because of a
reduction in the effective chromium content or an increase in carbon
content through carbon pickup from the substrate steel. Iron aluminides
appear to be potentially important weld-overlay coatings for sulfidation
environments. Figure 9 shows isothermal weight gain studies for a number of weld-overlay coatings exposed to H2S-H2-H20-Ar gas mixtures
at 800°C (1470 OF) (Ref 46). This work showed that compositions containing at least 30% Al and ;:::2% Cr had excellent sulfidation resistance,
and, at increased chromium levels, corrosion rates increased but were
still superior to other alloy classes such as stainless steels.
10
C\I
E
~
11AI-7Cr-7Ni
8
0)
E
<Ii
0)
c
co
.c::
0
:E:
0)
~
6
4
2
36AI-2Cr
0
0
50
100
150
200
Time, h
Fig. 9
Weight change versus time for specimens cut from iron-aluminide
weld overlays and isothermally exposed to H2S-H2-H20
at 800°C
(1470 OF). The elemental concentrations shown are in at. 0/0; the balance is iron.
Source: Ref 46
160 / Surface Engineering for Corrosion and Wear Resistance
Thermal Spray Coatings
Thermal spraying is a generic term for a group of processes that apply
a consumable in the form of a spray of finely divided molten or semimolten droplets' to produce a coating. A number of extensive reviews of
the topic can be found in Ref 47 to 51. The characteristics that distinguish
thermal spray processes from weld-overlay coatings are indicated as follows (Ref 51):
•
•
•
•
•
Substrate adhesion, or bond strength, is dependent on the materials
and their properties and generally is characterized as a mechanical
bond between the coating and the substrate, unlike the metallurgical
bond found in weld-overlay coatings.
Spray deposits can be applied in thinner layers than welded coating,
but thick deposits are also possible.
Provided there is a stable phase, almost all material compositions can
be deposited, including metals, cermets, ceramics, and plastics.
Thermal spray processes are usually used on cold substrates, preventing distortion, dilution, or metallurgical degradation of the substrate.
Thermal spray processes are line-of-sight limited, but the spray plume
often can be manipulated for complete coverage of the substrate.
Tables 15 and 16 compare thermal spraying, welding, and electroplating.
Thermal spray processes can be classified into two categories, arc
processes and gas combustion processes, depending on the means of
achieving the heat for melting of the consumable material during the
spraying operation.
In the lower-energy electric arc (wire arc) spray. process, heating and
melting occur when two electrically opposed charged wires, comprising
the spray material, are fed together to produce a controlled arc at the intersection. The molten material on the wire tips is atomized and propelled
onto the substrate by a stream of gas (usually air) from a high-pressure gas
jet. The highest spray rates are obtained with this process, allowing for
cost-effective spraying of aluminum and zinc for the marine industry. In
the higher-energy plasma arc spray process, injected gas is heated in an
electric arc and converted into a high-temperature plasma that propels the
coating powder onto the substrate at very high velocities. This process can
take place in air with air plasma spraying (APS), or in a vacuum with vacuum plasma spray (VPS) or low-pressure plasma spraying (LPPS).
For gas combustion processes, the lower-energy flame spray process
uses oxyfuel combustible gas as a heat source to melt the coating material,
which may be in the form of rod, wire, or powder. In the higher-energy,
high-velocity oxyfuel combustion spray (HVOF) technique, internal combustion of oxygen and fuel gas occurs to produce a high-velocity plume
Surface Engineering to Add a Surface Layer or Coating I 161
Table 15
Applications of thermal spraying, welding, and electroplating
Electroplating
Application
Thermal spraying
Welding
Base metal
Identification of alloy
by generic type is required.
Almost any alloy can be
sprayed.
Must be known before
coating material
selection can be made
Practical for buildup from
0.25 mm (0.010 in.) to
2.5 mm (0.100 in.) and
sometimes greater
Not useful
Excellent results-no
distortion
Precise identification of alloy is
required. In steels, composition
must be known. Some alloys are
difficult or impossible to weld.
Need not be known. It is usually
sufficient to match the properties
of the base metal.
Practical, but. comparatively costly,
particularly on alloys requiring
postweld heat treatment
Operating environment of
finished component
Restoring dimensions
Restoring strength
Precision-dimensioned
Good to excellent results
Distortion is a serious problem. It is
frequently difficult to predict
whether or not distortion will be
within acceptable limits.
Weld stresses may create problems.
Identification of alloy by
generic type is
required. Almost any
alloy can be plated.
Usually not required in
machinery applications
Practical up to 0.64 mm
(0.025 in.) by tank
plating and 0.13 mm
(0.005 in.) by "brush" plating
Not useful
Excellent results-no
distortion
Rotating or oscillating
machinery
Usually excellent. No
induced stresses in base
metal
Limited fatigue
Excellent results if stress
risers are not introduced
by machining and if
shot peening is done
prior to spraying
Usually excellent
Not recommended
Fair
Usually excellent if
hydrogen
embrittlement is
prevented. No induced
stresses in the base
metal
Excellent results if
preceded by shot
peening and followed
by hydrogen removal
when required
Usually excellent
Excellent
Impractical
Good
Excellent
Excellent but expensive
Excellent
Antiwear or antifriction
surface
Galvanic anticorrosion
coating
Corrosion-resistant
coating
Source: R.B. Alexander
Table 16
& Associates,
Huntington
Woods, MI
Process requirements in thermal spraying, welding, and electroplating
Welding
Application
Thermal spraying
Preheating
Always used to remove
moisture, otherwise
temperature is held
as low as possible.
Usual preheat range
is 95-150 °C
(200-300 OF)
Work frequently done on a
lathe with the spray
gun mounted on tool
post and the lathe used
to machine to plan size
None required, except for
one process variation
that fuses the coating
after application
Auxiliary
equipment
operation
Postheating
Restoring to plan
dimensions
Source: R.B. Alexander
Special machining and
grinding techniques
used. Finish machining
is sometimes
unnecessary after light
plasma spray antiwear
coatings.
& Associates,
Huntington
Electroplating
Treatment varies from
chilling to heating up
to 425°C (800 oF)
depending on
composition of base
metal.
Heat treatment sometimes
required for stress relief
after grinding on alloy
steels
None
Work frequently done on a
lathe with handheld
brush plating wand. No
auxiliary equipment
used for a tank plating
None required except for
hydrogen bake-out heat
treatment after
chromium
electroplating on high
strength alloys
Frequently used for
dimensional stability,
stress relieving, and
tempering depending on
composition of base
metal, geometry, and
end use of the part
Conventional machining
and grinding
Woods, MI
Special grinding required
on chrome plate
162 / Surface Engineering for Corrosion and Wear Resistance
capable of accelerating powders at supersonic speeds and lower temperatures than the plasma processes. Continuous combustion occurs in most
commercial processes, whereas the proprietary detonation gun (D-gun)
process uses a spark discharge to propel powder in a repeated operating
cycle to produce a continuous deposit.
In the lower-energy processes, electric arc (wire arc) spray and flame
spray processes, adhesion to the substrate is predominantly mechanical
and is dependent on the workpiece being perfectly clean and suitable
rough. Some porosity is always present in these coatings, which may present problems in both corrosion and erosion. The higher-energy processes-APS,
VPS, LPPS, and HVOF processes-were
developed to reduce porosity and improve adhesion to the substrate. In addition, these
processes are capable of spraying materials with higher melting points,
thus widening the range of applications to include high-temperature coatings and thermal and mechanical shock-resistant coatings. With these
higher-energy processes, bond strengths are higher because of the possible breakup of any oxide films present on the particles or the workpiece
surface, allowing for some diffusion bonding to take place (Ref 51). Typical design features of the various thermal spray processes are listed in
Table 17.
Properties of Thermal Spray Coatings. The variations in oxide content and porosity, as well as the chemical composition of the coating,
greatly affect the properties of the deposit and, in the case of corrosion,
the underlying substrate. The splat morphology and, more importantly, the
splat/splat and splat/substrate interface are critical to properties such as
bond strength, wear, erosion, and corrosion. The mechanical properties of
thermal spray coatings are not well documented except for their hardness
and bond strength. Table 18 contains typical mechanical property data for
a large range of plasma sprayed materials; however, the sensitivity of the
properties of the coatings to specific deposition parameters makes universal cataloging of properties by process and composition "virtually meaningless" (Ref 48).
Table 17 Typical design characteristics of thermal spray processes
Process
Flame
Arc wire
High-velocity
oxyfuel
(JetKote)
Detonation gun
Air plasma
spray
Vacuum plasma
spray
NA, not applicable.
Gas
temperature,
°C
3,000
NA
Particle,
velocity, Adhesion,
Oxide
rnIs
MPa
content, %
40
100
8
12
3,000
4,000
12,000
800
>70
800
>70
200-400 4 to >70
12,000
400-600
Source: Ref 51
> 70
Porosity, Spray rate,
%
kglh
Relative Typical deposit
cost, low = 1 thickness, mm
10-15
10-20
10-15
10
2-6
12
1
2
0.1-15
0.1 to >50
1-5
1-5
1-3
1-2
1-2
1-5
2-4
0.5
4-9
3
NA
4
0.1 to >2
0.05-0.3
0.1-1
ppm
<0.5
4-9
5
0.1-1
Table 18
Typical mechanical
properties of plasma sprayed coatings
Bond tensile
strength(a)
Material
MPa
ksi
Rockwell
macro/micro
hardness
g/cm3
Ib/ft3
45158 HRH
651142 HRB
7011450 HR15N
6511448 HRA
84/ ... HR15T
81/ ... HR15T
6111344 HRC
6511585 HRA
78/ ... HR15N
50/500 HRA
2.48
7.20
9.90
8.96
7.95
7.48
7.06
14.15
4.17
16.90
155
449
618
559
496
467
441
883
260
1055
Density
Pure metals
Aluminum
Copper
Molybdenum (fine)
Molybdenum (coarse)
Nickel (fine)
Nickel (coarse)
Niobium
Tantalum
Titanium
Tungsten
8.3
21.4
57.2
55.2
23.4
33.1
54.5
46.9
41.4
40.0
1.2
3.1
8.3
8.0
3.4
4.8
7.9
6.8
6.0
5.8
Alloy metals
304 stainless
316 stainless
431 stainless
80Ni-20Cr (fine)
80Ni-20Cr (coarse)
40Ni-60Cu
35Ni -5In-60Cu
10AI-90Cu (fine)
10AI-90Cu (coarse)
Hastelloy 31 (fine)
Hastelloy 31 (coarse)
5AI-95Ni
20AI-80Ni
6AI-19Cr-75Ni
12Si-88AI
5AI-5Mo-90Ni
Hastelloy X
Hastelloy C
420 stainless
0.9C stainless
Cast iron
Ti-6A1-4V
Monel
0.2C steel
17.6
23.4
31.0
31.0
29.0
24.1
24.1
28.3
22.1
41.4
23.4
68.3
47.6
49.6
16.5
37.9
42.7
42.1
22.1
33.8
35.9
33.1
44.8
22.1
3.2
88/ ... HR15T
70/ ... HR30T
351... HRC
901... HR15
90/ ... HR15T
72/ ... HRB
83/ ... HR15T
88/ ... HR15T
81/ ... HR15T
79/ ... HR15T
79/ ... RH15T
80/490 HRB
80/510 HRB
90/250 HRB
78/60 HR15T
80/200 HRB
89/ ... HR15T
901... HR15T
70/ ... HR15N
351... HRC
28/ ... HRC
351... HRC
35/ ... HR15N
951... HRB
4.9
4.7
4.65
6.2
4.0
7.0
80/500
86/500
72/660
92/250
80/250
801200
2.55
3.4
4.5
4.5
4.2
3.5
3.5
4.1
3.2
6.0
3.4
9.9
6.9
7.2
2.4
5.5
6.2
6.1
3.2
4.9
5.2
4.8
6.5
7.22
6.80
6.25
7.48
7.19
7.89
7.94
6.73
6.30
7.65
7.83
7.51
6.92
7.51
2.49
7.43
7.65
8.25
7.10
7.05
7.00
4.30
8.50
6.90
451
425
390
467
449
493
496
418
393
478
489
469
432
469
155
464
478
515
443
440
437
268
531
431
Metal composites
95Ni-5AI
80Ni-20AI
65Ni-35Ti
75Ni-19Cr-6AI
75Ni-9Cr-7 AI-5Mo-5Fe
90Ni-5AI-5Mo
Carbide
powders
HR15T
HR15T
HR15N
HR15T
HRB
HRB
7.39
7.02
6.62
7.71
6.90
7.40
461
438
413
481
431
462
13.75
12.41
14.55
11.10
6.41
6.23
858
775
908
693
400
389
5.80
362
5.30
4.80
5.00
4.10
331
300
312
256
3.50
3.50
4.0
3.30
3.30
4.20
218
218
250
187
187
262
and blends
88WC-12Co (cast fine)
88WC-12Co (cast coarse)
88WC-12Co (sintered)
83WC-17Co
75Cr3-C2-25NiCr (fine)
75Cr3-C2-25NiCr (coarse)
75Cr3-C2-25NiCr (composite)
85Cr 3-C2-15NiCr
Ceramic
33.8
32.4
32.1
42.7
27.6
48.3
44.8
44.8
55.2
68.9
41.4
34.5
6.5
6.5
8.0
10.0
6.0
5.0
88/ ... HR15N
811... HR15N
851... HR15N
85/950 HR15N
84/950 HR15N
8011850 HR15N
...11850 HR15N
8011850 HR15N
oxides
Zr02 (calcinated)
Chromium oxide
80Zr02 -20yttria
Ti02
Al203 (white)
87 A1203-13Ti02
60A1203-40Ti02
50A1203-50Ti02
Al20rgray (fine)
Al203-gray (coarse)
Magnesium zirconate
(a) Over a grit-blasted
surface roughened
44.8
44.8
15.2
6.5
6.5
2.2
44.8
15.5
27.6
6.5
2.25
4.0
6.9
1.0
17.2
2.5
to 2.5- 4.11J.ffi (100-160
701 ...
90/ ...
80/ ...
87/ ...
HR15N
HR15N
HR15N
HR15N
901... HR15N
90/850 HR15N
851... HR15N
871193 HR15N
851... HR15N
751 ... HR15N
uin.) AA (arithmetic
average).
Source: Ref 50
164 / Surface Engineering for Corrosion and Wear Resistance
Table 19 Abrasive wear data for selected thermal
spray coatings
Material
Wear rate, mm3/1000 rev
Type
Carballoy
WC-Co
883
1.2
Sintered
Detonation gun
Plasma spray
SuperD-gun
High-velocity oxyfuel
0.8
16.0
0.7
0.9
ASTM G 65 dry sand/rubber wheel test. 50nO mesh Ottawa silica. 200 rpm. 13.6 kg (30
lb) load. 3000-revolution
test duration. Source: Ref 48
One of the most extensive uses for thermal spray coatings is in wear applications. Generally, the wear resistance of coatings increases with their
density and cohesive strength, so that HVOF coatings provide the best
wear resistance in contrast to plasma spray coatings (Table 19). Carbide
cermets were found to be good for both wear and erosion environments,
and the optimal amount of hard phase (oxide and carbide) has been determined for erosion resistance. A comparison between the presprayed powder and the actual coating microstructure showed that the retention of the
FeCrAlY matrix is much better than the chromium carbide particles,
which can form oxides during HVOF thermal spraying (Ref 52). Erosion
tests show that both carbides and oxides increase the erosion rate of the
coating (Fig. 10) and that low amounts of hard constituents are preferable
for erosion resistance. Table 20 summarizes the various thermal spray
coatings used to prevent various forms of wear.
Thermal spray zinc, aluminum, and zinc-aluminum alloys are used for
sacrificial galvanic protection for corrosion resistance on bridges, ships,
and other large structures (Table 21). Other corrosion-resistant applications for thermal spray coatings include oxidation and sulfidation
resistance in power boilers and other high-temperature uses. In a comparison of several different thermal spray techniques, it was found that hightemperature corrosion-resistant coatings must have compositions that promote the formation of protective oxides at splat boundaries, be dense
enough so that protective oxides can form within and fill voids, and be
0.3
c:
~
E
0.25
0
0)
m
JB
~
c:
0
'en
e
0.2
0.15
w
0.1
0
20
40
60
80
100
Measured constituent content, %
Fig. 10
Steady-state erosion rates versus constituent composition.
Source: Ref 52
Surface Engineering to Add a Surface Layer or Coating / 165
thick enough to postpone the diffusion of corrosive species to the substrate
material along the fast diffusion paths of the coating (Ref 53). It has been
found that corrosion attack of the substrate generally decreases as the free
path to the substrate (that is, the diffusion path the corrosive species takes
to the substrate) increases. Thus, as in erosion and wear, the splat/splat and
splat/substrate boundaries are critical to properties of the thermal spray
coatings.
Table 20
Thermal spray coatings used for wear-resistant applications
Type of wear
Adhesive wear
Abrasive wear
Surface fatigue wear
Fretting: Intended motion
applications
Fretting: Small amplitude oscillatory
displacement applications:
Low temperature
«540°C,
or 1000 oF)
High temperature
(>540 °C,
or 1000 OF)
Erosion
Cavitation
Coating material
Soft bearing coatings:
Aluminum bronze
Coating process(a)
Applications
Tobin bronze
Babbitt
Tin
Hard bearing coatings:
MolNi-Cr-B-Si blend
Molybdenum
High-carbon steel
Alumina/titania
Tungsten carbide
Co-Mo-Cr-Si
Fe-Mo-C
Aluminum oxide
Chromium oxide
Thngsten carbide
Chromium carbide
Ni-Cr-B-SiCIWC (fused)
Ni-Cr-B-SiC (fused)
Ni-Cr-B-SiC (unfused)
OFW, EAW, OFP,
PA,HVOF
OFW,EAW
OFW, EAW, OFP
OFW, EAW, OFP
Babbitt bearings, hydraulic press
sleeves, thrust-bearing shoes,
piston guides, compressor
crosshead slippers
PA
OFW, EAW, PA
OFW,EAW
OFP,PA
OFP, PA, HVOF
PA,HVOF
PA
PA
PA
PA,HVOF
PA,HVOF
OFP,HVOF
OFP,HVOF
HVOF
Bumper crankshafts for punch
press, sugar cane grinding roll
journals, antigalling sleeves,
rudder bearings, impeller shafts,
pinion gear journals, piston ring
(internal combustion); fuel
pump rotors
Slush-pump piston rods, polish rod
liners, and sucker rod couplings
(oil industry); concrete mixer
screw conveyors; grinding
hammers (tobacco industry);
core mandrels (dry-cell
batteries); buffing and
polishing fixtures; fuel-rod mandrels
Molybdenum
MolNi-Cr-B-SiC
Co-Mo-Cr-Si
OFW,PA
PA
PA,HVOF
Servomotor shafts, lathe and grinder
dead centers, cam followers,
rocker arms, piston rings
(internal combustion), cylinder
liners
Aluminum
Cu-Ni-In
Cu-Ni
Co-Cr-Ni-W
Chromium carbide
OFW, EAW, PA,
HVOF
PA,HVOF
PA,HVOF
PA,HVOF
PA,HVOF
Chromium carbide
Tungsten carbide
WClNi-Cr-B-Si-C(fused)
WClNi-Cr-B-SiC
(unfused)
Chromium oxide
Ni-Cr-B-SiC-AI-Mo
Ni-AlINi-Cr-B-SiC
Type 316 stainless steel
Ni-Cr-B-SiC (fused)
Ni-Cr-B-SiC (unfused)
Aluminum bronze
Cu-Ni
PA,HVOF
PA,HVOF
OFP,HVOF
OFP,HVOF
PA
PA
PA
PA
OFP,HVOF
HVOF
PA,HVOF
PA,HVOF
Aircraft flap tracks (air-frame
component); expansion joints and
mid-span supports Get engine
components)
Compressor air seals, compressor
stators, fan duct segments and
stiffeners (all jet engine components)
Exhaust fans, hydroelectric valves,
cyclone dust collectors, dump
valve plugs and seats, exhaust
valve seats
bronze
Wear rings (hydraulic turbines),
water turbine buckets, water
turbine nozzles, diesel engine
cylinder liners, pumps
(a) OFW, oxyfuel wire spray; EAW, electric arc wire spray; OFP, oxyfuel powder spray; PA, plasma arc spray; HVOF, high-velocity
oxyfuel powder spray
166 / Surface Engineering for Corrosion and Wear Resistance
Table 21 Typical corrosion-resistant
thermal spraying
applications for
Application
Thermal spray materials
Pipe sections for saltwater pumps
Oil platform components
Bridges
Bridge fabrication shops
Chemical and water storage tanks
Power transmission poles
Piping in power plant cooling towers
Cooling water pump diffuser
Concrete bridge structures
Grandstands
Ski lifts
Decorative hand rails
Alloy 625 (Ni-22Cr-9Mo-5Fe-4Nb)
or Al
Al and Al alloys
Zn, Zn-AI, or Al
Zn, Zn-AI, or Al
Alloy 625, AI, Zn, and a high-CrINi alloy
85% Zn-15% Al
Al
Al
Zn or Zn-AI
Zn-Al
Zn
Zn
Source: Ref 3
Cladding
Clad metals are bonded metal-to-metallaminar
composite systems that
can be fabricated by a number of processes. The principal cladding techniques include hot-roll bonding, cold-roll bonding, explosive bonding,
and weld cladding (including laser cladding), although centrifugal casting, adhesive bonding, extrusion, and hot isostatic pressing have also been
used to produce clad metals. Clad metals can be provided in plate, sheet,
tube, rod, and wire forms. Most engineering metals and alloys can be clad.
Cladding combinations that have been commercially produced on a large
scale are shown in Fig. 11. The cladding of steel with stainless steel, copper, nickel alloys, titanium, and tantalum has become increasingly popular in the chemical processing industries. Applications include pressure
vessels, reactors, heat exchangers, and storage tanks.
Clad metals provide a means of designing into a composite material
specific properties that cannot be obtained in a single material. The early
use of clad metals in the jewelry industry combined the aesthetics of precious metals with the low-cost strength of base metals. These materials
systems are currently being used for electrical and electronics applications, such as contacts and connectors with selectively clad (inlay) precious metals for low contact resistance and high reliability.
Corrosion Control through Cladding
Clad metal systems designed for corrosion control can be categorized as
follows:
•
•
•
•
Noble metal clad systems
Corrosion barrier systems
Sacrificial metal systems
Transition metal systems
Surface Engineering to Add a Surface Layer or Coating / 167
CD
s
E
::J
c:
0
'cuc..
co
0
-e
«
:J
c,
CD
.::.:.
o
Z
u
u
0
0
0
Nickel
•
0
0
Copper
0
0
•
0
0
0
0
0
Aluminum
Carbon steel
Ferritic
stainless steel
0
0
0
0
0
0
Invar
0
0
0
0
Titanium
0
0
0
0
High-volume
Q)
(I)
e
01
co
«
'(0
Q)
~
•
•
ti~
::J c:
ti
Q)
N
c
:ee
(I)
en
co
m
0
•
0
0
0
•
0
0
• •
• • • • • • •
• • •
0
Austenitic
stainless steel
CD
Q)
'en
• •
• •
0
Martensltlc
stainless steel
u
:~ ti
c (I)
::::J
c:
'E
Fig. 11
E
(I)
0
• •
• • •
o
Commercial
•
Requires development
commercially
0
available clad metals
Proper design is essential for providing maximum corrosion resistance
with clad metals.
Noble metal clad systems are materials having a relatively inexpensive base metal covered with a corrosion-resistant metal. A typical example would be a carbon steel clad with a stainless steel or nickel-base alloy.
Another group of commonly used noble metal clad metals uses aluminum
as a substrate. For example, in stainless-steel-clad aluminum truck
bumpers, the stainless steel provides corrosion resistance, and the aluminum provides a high strength-to-weight ratio.
Corrosion-Barrier Systems. The combination of two or more metals to
form a corrosion-barrier system is most widely used where perforation
caused by corrosion must be avoided. This is shown schematically in Fig.
12. Low-carbon steel and stainless steel are susceptible to localized corrosion in chloride-containing environments and can perforate rapidly.
When steel is clad with a stainless steel layer, the corrosion-barrier mechanism prevents perforation. Localized corrosion of the stainless steel is
prevented; the stainless steel is protected galvanically by the sacrificial
corrosion of the carbon steel in the metal laminate. Therefore, only a thin
pore- free layer is required.
Sacrificial metals, such as magnesium, zinc, and aluminum, are in the
active region of the galvanic series and are extensively used for corrosion
protection. The single largest application for cold-roll-bonded materials
is stainless-steel-clad aluminum for automotive trim. The stainless steel
168 / Surface Engineering for Corrosion and Wear Resistance
Low-carbon
steel
(a)
Low-carbon
steel
Stainless
steel
(b)
Fig. 12
lIustrations of the corrosion-barrier principle.
(b) Carbon steel clad with stainless steel
(a) Solid carbon steel.
exterior surface provides corrosion resistance, high luster, and abrasion
and dent resistance, and the aluminum on the inside provides sacrificial
protection for the painted auto body steel and for the stainless steel.
The largest application for hot-roIl-bonded materials-alclad
aluminum-also falls into this category. In this case, a more active aluminum
alloy is bonded to a more noble aluminum alloy. In service, the outer clad
layer of aluminum corrodes sacrificially and protects the more noble aluminum substrate.
Clad transition metal systems provide an interface between two incompatible metals. They not only reduce galvanic corrosion where dissimilar metals are joined, but they also allow welding techniques to be
used when direct joining is not possible. Clad metals provide an ideal solution to the materials problem of dual environments. For example, in the
application of small battery cans and caps, copper-clad, stainless steelclad nickel (Cu/SSINi) is used where the external nickel layer provides atmospheric corrosion resistance and low contact resistance. The copper
layer on the inside provides the electrode contact surface as well as compatible cell chemistry. The. stainless steel layer provides strength and resistance to perforation corrosion.
Chemical Vapor Deposition
Chemical vapor deposition (CVD) involves the formation of a coating
by the reaction of the coating substance with the substrate. The coating
species can come from a gas or gases or from contact with a solid as in the
pack-cementation diffusion process described in Chapter 5. The process is
more precisely defined as the deposition of a solid on a heated surface by
a chemical reaction from the vapor or gas phase (Ref 54). In general, three
processing steps are involved in any CVD reaction: (1) the production of
a volatile carrier compound, (2) the transport of the gas to the deposition
Surface Engineering to Add a Surface Layer or Coating / 169
site without decomposition, and (3) the chemical reaction necessary to
produce the coating on the substrate. The numerous chemical reactions
used in CVD include thermal decomposition (pyrolysis), reduction, hydrolysis, disproportionation, oxidation, carburization, and nitridation.
These reactions take place singly or in combination and are controlled by
thermodynamics, kinetics, mass transport, chemistry of the reaction, and
processing parameters of temperature, pressure, and chemical activity.
Chemical vapor deposition processes can be classified as either open reactor systems, including thermal CVD and plasma CVD, or as a closedreactor system, as in pack cementation. In thermal CVD, reactions usually
take place above 900°C (1650 OF), whereas plasma CD usually operates
at temperatures between 300 and 700°C (570 and 1290 OF) (Table 22).
Using the lower-reaction-temperature plasma CVD enables coatings to be
produced on substrates with low melting points or that otherwise would
undergo solid-state transformations over the range of deposition temperatures. Furthermore, the low deposition temperature of plasma CVD coatings limits the stresses due to the large mismatches in thermal expansion
that can lead to cracking and delamination of the coating.
Materials that cannot ordinarily be deposited by electrodeposition ·for
example, the refractory metals tungsten, molybdenum, rhenium, niobium,
tantalum, zirconium, hafnium, and so forth·are deposited using CVD
processes. Typical products produced are crucibles, rocket nozzles, and
other high-temperature components; linings for chemical vessels; and
coatings for electronic components. These refractory metals are deposited
at temperatures far below their melting points or sintering temperatures,
and coatings can be produced with a preferred grain size and grain orientation. For example, tungsten that is deposited by the hydrogen reduction
of the halide and deposition at a lower temperature (500°C, or 930 OF)
gives a finer grain size with higher strength (83 MPa, or 12 ksi) than deposition at a higher temperature (700°C, or 1290 OF) (Ref 54).
Wear-, erosion-, and corrosion-resistance applications extensively
utilize CVD coatings, as do applications that require low friction characteristics. Table 23 lists the properties of typical CVD coating materials for
these applications. Some materials, such as titanium diboride, titanium
Table 22 Typical deposition temperatures for thermal
and plasma CVD
Deposition temperature
PlasmaCVD
Thermal CVD
Material
Silicon nitride
Silicon dioxide
Titanium carbide
Titanium nitride
Tungsten carbide
Source: Ref 54
°C
900
800-1100
900-1100
900-1100
1000
OF
1650
1470-2010
1650-2010
1650-2010
1830
°C
300
300
500
500
325-525
OF
570
570
930
930
615-975
170 / Surface Engineering for Corrosion and Wear Resistance
Table 23
Selected wear and corrosion properties of CVD coating materials
Thermal
conductivity,
Hardness
106
Material
GPa
Titanium carbide
31.4
4.5
17
7.6
Titanium nitride
20.6
3.0
33
9.5
24.5-29.4
22.1
3.5-4.3
3.2
20-30
11
8
10
Silicon carbide
27.4
4.0
125
3.9
Titanium diboride
33.0
4.7
25
6.6
Alumina
18.8
2.7
34
8.3
29-49
4.2-7.1
200
14.2
180
Titanium carbonitride
Chromium carbide
Diamondlike
Diamond
carbon
98
psi
Coefficient of thermal
expansion at 2S °C
W/m-K
(77 OF) 1O-6/K
2.9
Remarks
High wear and
abrasion
resistance, low
friction
High lubricity;
stable and inert
Stable lubricant
Resists oxidation
to 900°C
(1650 oF)
High conductivity,
shock resistant
High hardness,
high wear
resistance
Oxidation
resistant, very
stable
Very hard,
high thermal
conductivity
Extreme hardness
and high
thermal
conductivity
Source: Ref S4
carbide, and silicon carbide, provide extremely low wear rates. Table 24
lists specific production applications for the wear-, erosion-, and corrosion-protection provided by CVD coatings.
The cutting-tool industry relies heavily on coatings. The technology
associated with CVD has made some of its most important gains in this
area. Major applications are represented by titanium carbide coatings on
the majority of cemented (cobalt-bonded) tungsten carbide tools and both
titanium nitride and carbonitride coatings on high-speed tool steel and cemented carbide tools.
The materials identified in Table 23 can be used as multilayer structures
that utilize the strongest characteristics of each layer of material. Nearly
all coatings are multilayer systems that combine titanium nitride for lubricity and galling resistance; alumina for chemical inertness and thermal
insulation; and titanium carbide, as well as titanium carbonitride, for abrasion resistance. Selecting the optimal combination of materials depends
on the type of machining operation, the material to be machined, and other
factors. Criteria for such a selection are summarized in Table 25.
Titanium nitride, the most common coating material, is generally combined with a very thin undercoating of titanium carbide or titanium carbonitride to promote adhesion. Alumina coatings are preferred in highspeed machining applications in which oxidation resistance and
high-temperature stability are the critical factors. Like titanium nitride,
alumina is deposited on an intermediate titanium carbide layer.
Surface Engineering to Add a Surface Layer or Coating / 171
Table 24 Wear-, erosion-, and corrosion-resistance
applications of CVD
Metalforming (noncutting)
Tube and wire-drawing dies (TiN)
Stamping, chamfering, and coining tools (TiC)
Drawing punches and dies (TiN)
Deep-drawing dies (TiC)
Sequential drawing dies (Cr7C3)
Coating on dressing sticks for grinding wheels (B4C)
Ceramic and plastic processing
Molding tools and dies for glass-filled plastics [Ti(CN)]
Extrusion dies for ceramic molding (TiC)
Kneading components for plastic mixing (TiC)
Chemical- and general-processing industries
Pump and valve parts for corrosive liquids (SiC) and abrasive liquids (TiB2)
Valve liners (SiC)
Positive-orifice chokes (SiC, TiB2)
Packing sleeves, feed screws (TiC)
Thermowells (SiC, A1203)
Abrasive-slurry transport (WC)
Sandblasting nozzles (TiC, B4C, TiB2)
Textile-processing rolls and shafts (A1203, TiC, WC)
Paper-processing rolls and shafts (TiC)
Valves for coal-liquefaction components (TiB2)
Cathode coating for aluminum production (TiB2)
Oxidation-resistant
coatings for carbon-carbon composites (SiC)
Machine elements
Gear components (TiN)
Coating on stainless-steel spray-gun nozzles (TiC)
Components for abrasive processing (TiC)
Coating on ball bearings (TiC)
Turbine blades (SiC, TiC)
Nuclear
Coating
Coating
Coatings
Nuclear
for neutron flux control in nuclear reactors (B 4C)
for shielding against neutron radiation (B 4C)
for fusion reactor applications (SiC)
waste container coatings (SiC)
Instruments
Radiation sensor (SiC)
Thermionic cathodes (W- Th)
Target coatings for x-ray cathodes (W-Re)
Note: TiN, titanium nitride; TiC, titanium carbide; B4C, boron carbide; Ti(CN), titanium
carbonitride; SiC, silicon carbide; TiB2' titanium diboride; A1203, alumina; WC, tungsten
carbide
Table 25 Criteria for selecting coating materials for
cutting tools
Property
Best materials(a)
Oxidation and corrosion resistance; high-temperature
stability
Crater-wear resistance
Hardness and edge retention
Abrasion resistance and flank wear
Low coefficient of friction and high lubricity
Fine grain size
(a) For each property, best material
is identified
first. Source: Ref 55
A1203, TiN, TiC
A1203, TiN, TiC
TiC, TiN, Al203
A1203, TiC, TiN
TiN, A1203, TiC
TiN, TiC, Al203
172 / Surface Engineering for Corrosion and Wear Resistance
Diamond films grown by CVD exhibit outstanding properties approaching natural diamond, such as high electrical resistivity, high optical
transparency, extreme hardness, high refractive index, and chemical inertness. Different film-deposition techniques and system configurations result in films with different characteristics. Diamond films can be grown
using processing variables of different concentrations of methane in
methane-hydrogen gas mixtures and flow rates (Ref 56). The CVD of diamond requires the presence of atomic hydrogen, which selectively removes graphite and activates and stabilizes the diamond structure. The
basic reaction involves the decomposition of methane, which can be activated by microwave plasma, thermal means (hot filament), plasma arc, or
laser.
Physical Vapor Deposition Processes
Physical vapor deposition (PVD) processes involve the formation of a
coating on a substrate by physical deposition of atoms, ions, or molecules
of the coating species (Ref 57). There are three main techniques for applying PVD coatings: thermal evaporation, sputtering, and ion plating.
Thermal evaporation involves heating of the material until it forms a vapor
that condenses on a substrate to form a coating. Sputtering involves the
electrical generation of a plasma between the coating species and the substrate. Ion plating is essentially a combination of these two processes. A
comparison of the process characteristics of PVD, CVD, and ion implantation is provided in Table 26. Reviews of these processes can be found in
the literature (for example, Ref 57, 59, and 60).
Originally PVD was used to deposit single metal elements by transport
of a vapor in a vacuum without involving a chemical reaction. Today, PVD
technology has evolved so that a wide array of inorganic materials (including metals, alloys, compounds, or their mixtures) and organic compounds can be deposited. The PVD process occurs in a vacuum chamber
and involves a vapor source and the substrate on which deposition occurs.
Different techniques arise because of variations in atmospheres, vapor
source heating method, and electrical voltage of the substrate, all of which
contribute to the structure, properties, and deposition rate of the coating
(Ref 60). The steps in deposition occur as follows:
1. Synthesis of the material deposited (transition from a condensed state,
solid or liquid, to the vapor phase, or, for deposition of compounds, reaction between the components of the compound, some of which may
be introduced into the chamber as a gas or vapor)
2. Vapor transport from the source to the substrate
3. Condensation of the vapors followed by film nucleation and growth
Surface Engineering to Add a Surface Layer or Coating '.173
Table 26
Comparison of PVD, CVD, and ion implantation process characteristics
Process
Processing temperature,
°C
Throwing
power
Coating applications and
special features
Coating materials
Vacuum evaporation
RT -700, usually
<200
Line of sight
Ion implantation
200-400, best <250
forN
Line of sight
Ion plating, ARE
RT-O.7 Tm of coating.
Best at elevated
temperatures
Moderate to
good
Ion plating: AI, other metals
(few alloys) ARE: TiN
and other compounds
Sputtering
RT-O.7 Tm of metal
coatings. Best
>200 for
nonmetals
Line of sight
Metals, alloys, glasses, oxides.
TiN and other
compounds(a)
CVD
300-2000, usually
600-1200
Very good
Metals, especially refractory
TiN and other
compounds( a),
pyrolytic BN
Chiefly metal, especially AI
(a few simple alloys/a few
simple compounds)
Usually N (B, C)
Electronic, optical, decorative,
simple masking
Wear resistance for tools, dies,
etc. Effect much deeper than
original implantation depth.
Precise area treatment,
excellent process control
Electronic, optical, decorative.
Corrosion and wear
resistance. Dry lubricants.
Thicker engineering coatings
Electronic, optical, wear
resistance. Architectural
(decorative). Generally thin
coatings. Excellent process
control
Thin, wear-resistant films on
metal and carbide dies,
tools, etc. Free-standing
bodies of refractory
metals and pyrolytic C
orBN
RT, room temperature; ARE, activated reactive evaporation; Tm' absolute melting temperature. (a) Compounds: oxides, nitrides, carbides, silicides, and borides of AI, B, Cr, Hf,
Mo, Nb, Ni, Re, Si, Ta, Ti, V, W, Zr. Source: Ref 58
The PVD process produces coatings for a range of applications including electronics, optics, decoration, and corrosion and wear prevention.
Only engineering uses, that is, corrosion- and wear-resistant coatings, are
discussed below. The coatings used for wear applications are usually hard
compounds, and, from the designers point of view, thin-film wear coatings
can be used for the same type of applications as chromium electroplate.
Physical vapor deposition coatings have hardnesses greater than any metal
and are used in systems that cannot tolerate even microscopic wear losses.
Most processes are operated on a batch basis, and the component size is
limited by the size of the vacuum chamber. Provided that the substrate can
be manipulated to face the coating source, the size and shape of objects
are limited by the capital and operating expenditures involved rather than
by the fundamental characteristics of the process. Furthermore, cleanliness of the substrate is critical and far exceeds surface preparation requirements for other coatings.
Thermal evaporation is the oldest and probably the most widely used
PVD technique. It accounts for the major proportion of both the equipment in use and the area coated. Thermal evaporation occurs in a hard vacuum of 0.1 to 10 mPa, at which pressures the mean free path of a gas atom,
that is, the average distance the atom travels before colliding with another
atom, is greater than the chamber dimensions. An atom evaporating from
a source travels in a, straight line; thus the process is line-of-sight limited,
and coating around comers or reentrant angles is not possible without substrate manipulation. A description of the process and the equipment is
found in Ref 61.
174 / Surface Engineering for Corrosion and Wear Resistance
T
0.25 mm
/1
Dense inner
layer (Zr02)
Fig. 13
Oxidationresistant
bond
coat
Gas turbine
blade superalloy
Cross section illustrating the strain-tolerant columnar Zr02 microstructure of EB/PVD zirconia thermal barrier coatings. Source: Ref 62
Aluminum and chromium coatings for automotive trim are probably the
largest application of this process. Hard coatings of chromium and Al203
are sometimes deposited on steel or tungsten carbide tools, but this application has been replaced by sputter coatings. Aerospace applications use
aluminum and nickel-chromium for corrosion protection and aluminum
and silver as solid lubricants. Electron beam/physical vapor deposition
(EBIPVD) is widely used in the gas turbine industry for applying
MCrAIY metallic coatings on turbine blades and vanes for oxidation and
corrosion protection (Ref 62). Zirconia thermal barrier coatings (TBCs)
can also be deposited using this technique. Bond coats for EBIPVD TBCs
are normally MCrAIY type coatings, similarly processed. The major advantage of EBIPVD TBCs is the columnar outer structure of the Zr02
(Fig. 13), which reduces the stress buildup within the body of the coating.
These TBCs have superior degradation resistance that has been confirmed
in gas turbine flight tests. For example, EBIPVD zirconia was found to be
better than plasma sprayed zirconia or metallic MCrAIY after 4200 h tests
of first-stage blades.
Sputter coating is a vacuum process that involves the use of ions from
a gas-generated plasma to dislodge coating atoms or molecules from a target made of the material that will become the coating. The plasma is established between the target and the substrate by the application of a direct-current potential or an alternating potential (radio frequency). An
inert gas is introduced into the chamber to form the glow discharge plasma
between the electrodes. The materials that can be sputter coated are pure
metals, alloys, inorganic compounds, and some polymeric materials. A
major restriction to be considered for the substrate material is the temperature of the process, which can range from 260 to 540°C (500-1000 OF).
Details of the process and equipment have been reviewed (Ref 63). Sputtering is often used for depositing compounds and materials that are difficult to coat by thermal evaporation techniques. Engineering applications
of sputter coatings include:
Surface Engineering to Add a Surface Layer or Coating / 175
•
•
•
Corrosion and oxidation resistance, for
MCrAIY, and polymers
Lubrication, for example, silver, indium,
cides, and tellurides
Wear resistance, for example, TiN, other
num, carbides, borides, and diamondlike
example, nickel-chromium,
MoS2, PTFE, selenides, silinitrides, tungsten, molybdecarbon
Titanium nitride coatings are generally used for wear resistance, and Fig.
14 shows that TiN coatings increase the abrasion resistance of a hardened
steel (Ref 57).
Ion plating is a vacuum coating process in which a portion of the
coating species impinges on the substrate in ionic form (Ref 57). The
process is a hybrid of the thermal evaporation process and sputtering
with the evaporation rate being maintained at a higher rate than the
atoms that can be sputtered from the substrate. Some evaporant atoms
pass through the plasma in atomic form, while some atoms collide with
electrons from the substrate and become ions. They impinge on the substrate in ionic form, pick up electrons, and return to the atomic state,
forming the coating. A detailed description of the process and equipment is provided in Ref 65.
A variant process is reactive ion plating in which the metallic constituent (titanium) of the compound (TiN) is evaporated into the reactive
gas mixture of argon and nitrogen that is enhanced by the glow discharge,
depositing a golden-colored TiN coating on the substrate. Films of TiN are
applied to a wide range of tools such as bits, punches, dies, taps, and
so forth, to improve tool life by three to ten times (Ref 62). Figure 14
shows the improvement in abrasion resistance of reactive ion plating over
other PVD sputtering processes. The results show that the higher-energy
reactive ion plating process had more than an order of magnitude
52100 steel
(60 HRC)
RF-diode
sputtered TiN
dc-magnetron
sputtered TiN
Cathodic arc
sputtered TiN
Reactive ion
plated TiN
o
20
40
60
80
100
Relative low-stress abrasion resistance of
TiN coatings (ASTM G 56 test apparatus)
Fig. 14
Effect of coating technique on the relative abrasion resistance of TiN on hardened
steel appl ied by various processes. Source: Ref 64
176 / Surface Engineering for Corrosion and Wear Resistance
improvement over the radio-frequency diode sputtered coatings (Ref 57).
These results indicate that application technique should be made a coating
selection factor.
Thermoreactive Deposition/Diffusion
Process
The thermoreactive deposition/diffusion process (TRD) is a method of
coating steels with a hard, wear-resistant layer of carbides, nitrides, or
carbonitrides. In the TRD process, the carbon and nitrogen in the steel
substrate diffuse into a deposited layer with a carbide-forming or nitrideforming element such as vanadium, niobium, tantalum, chromium,
molybdenum, or tungsten. The diffused carbon or nitrogen reacts with the
carbide- and nitride-forming elements in the deposited coating so as to
form a dense and metallurgically bonded carbide or nitride coating at the
substrate surface.
The TRD process is unlike conventional case-hardening methods, where
the specific elements (carbon and nitrogen) in a treating agent diffuse into
the substrate for hardening. Unlike. conventional diffusion methods, the
TRD method also results in an intentional buildup of a coating at the substrate surface. These TRD coatings, which have thicknesses of about 5 to
15 f..Lm(0.2-0.6 mil), have applications similar to those of coatings produced by CVD or PVD. In comparison, the thickness of typical CVD
coatings (usually less than 25 urn, or 1 mil) has about the same range as
TRD coatings.
Process Characteristics. The hard alloy carbide, nitride, and carbonitridecoatings in the TRD method can be applied to steels by means of
salt-bath processing (Ref 66-69) or fluidized beds (Ref 70). The carbide
coating by salt-bath immersion was first developed in Japan and used industrially approximately 30 years ago under the name of the Toyota Diffusion (TD) coating process (Ref 66, 67). The TD method uses molten
borax with additions of carbide-forming elements such as vanadium, niobium, titanium, or chromium, which combine with carbon from the substrate steel to produce alloy carbide layers. Because the growth of the layers is dependent on carbon diffusion, the process requires a relatively high
temperature, from 800 to 1250 °C (1470-2280 OF), to maintain adequate
coating rates. Carbide coating thicknesses of 4 to 7 urn are produced in 10
min to 8 h, depending on bath temperature and type -of steel. The coated
steels may be cooled and reheated for hardening, or the bath temperature
may be selected to correspond to the steel austenitizing temperature, permitting the steel to be quenched directly after coating.
Applications and Properties. The most common application area for
TRD-processed steels is that of forming tools subjected to high wear and
galling problems. Table 27 lists typical applications.
Surface Engineering to Add a Surface Layer or Coating / 177
Table 27
Applications of TRD-processed tooling
Application
Tool
Sheet metal working
Pipe and tube manufacturing
Pipe and tube working
Wire manufacturing
Wire working
Cold forging and warm forging
Hot forging
Casting (aluminum,
Rubber forming
zinc)
Plastic forming
Glass forming
Powder compacting
Cutting and grinding
Draw die, bending die, pierce punch, form roll, embossing punch, coining
punch, shave punch, seam roll, shear blade, stripper guide pin and bushing,
pilot pin, and so on
Draw die, squeeze roll, breakdown roll, idler roll, guide roll, and so on
Bending die, pressure die, mandrel, expand punch, swaging die, shear blade,
feed guide, and so on
Draw die, straightening roll, descaling roll, feed roll, guide roll, cutting blade
Bending die, guide plate, guide roll, feed roll, shear blade
Extrusion punch and die, draw die, upsetting punch and die, coining punch and
die, rolling die, quill cutter, and so on
Press-forging die, rolling die, upsetting die, rotary swaging die, closed-forging
die, and so on
Gravity-casting core pin, die-casting core pin, core, sleeve, and so on
Form die, extrusion die, extrusion screw, torpedo, cylinder sleeve, piston,
nozzle, and so on
Form die, injection screw, sleeve, plunger, cylinder, nozzle, gate, and so on
Form die, plunger, blast nozzle, machine parts, and so on
Form die, core rod, extrusion die, screw, and so on
Cutting tool, cutting knife, drill, tap, gage pin, tool holder, guide plate, and
so on
I
TRD process {
Cr-C
I
NbC
I
VC
I
I
I
~
CVD method
I
TiC
TiC
r
PVD method
Fe2B
~
Boriding
Fe-8
1-----1
Cr
H
Plating
I
Ferritic nitrocarburizing
H
Sulfurizing
Ni-P
I
I
r-----t
Fe-S
r-----t
Spark hardening
WC, TiC
J----I
Hardened tool steel
I
Cemented carbide
o
Fig. 15
Fe-N
1000
I
2000
Microhardness, HV (50 g)
3000
Surface hardness of carbide layers by TRO process in relation to other surface-hardening processes. Source: Ref 71
4000
178 / Surface Engineering for Corrosion and Wear Resistance
Punch
1500
Wear
Lubricant: none
6
l1000
•
u)
CIJ
.E
:0
0 No scuffing
•
6
•
(1)
~
•
500
6
•
•
•
•••
0
0
1:J
sco
0
0
c
::::>
"C
Q)
E
1:J
Q)
"C
"C
(1)
Q)
"C
Q)
N
:2 C ~••••·c
co 0..2
0 c.
Z co
'3
.§ E::::I (J)
+
'E
z
e
.s::
o
Fig. 16
Slight scuffing
Severe scuffing
Spalling
Q)
0
0
1:J
~c
Q)
0
u
0 0 u
> .0
.s > >
:0 ~ ~
"C
0
z
z ~
I-
i=
~~~
E i=
Q)
u
Coated
Comparative cross-sectional area of wear, scuffing, and spalling on a
die radius in a sheet steel-bending test. Source: Ref 71
The most important properties associated with TRD coatings are high
hardness and wear resistance. Figures 15 and 16 compare the surface
hardness and wear properties of TRD coatings with various other surfacehardening processes.
Acknowledgment
Portions of this chapter were adapted from A.R. Marder, Effects of Surface Treatments on Materials Performance, Materials Selection and Design, Vol 20, ASM Handbook, ASM International, 1997, p 470-490.
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Surface Engineering to Add a Surface Layer or Coating / 179
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18. H.E. Townsend and J.C. Zoccola, Mater. Perform. A, Vol 10, 1979,
P 13-20
19. H.F. Graff, Aluminized Steel, Encyclopedia of Materials Science and
Engineering, Pergamon Press, 1986, p 138-141
20. F. Lowenheim, Modern Electroplating, 3rd ed., John Wiley, 1974
21. J.W. Dini, Electrodeposition: The Materials Science of Coatings and
Substrate, Noyes, 1993
22. M.G. Hocking, V. Vasatasree, and P.S. Sidky, Metallic and Ceramic
Coatings: Production, High Temperature Properties and Applications,
John Wiley & Sons, 1989, p 206
23. J.R. Davis, Surface Engineering of Carbon and Alloy Steels, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994,
p 701-740
180 / Surface Engineering for Corrosion and Wear Resistance
24. Engineering Coatings: Design and Application, S. Grainger, Ed.,
Abington Publishing, 1989, p 101
25. M.E. Browning, Section Ed., Plating and Electroplating, Surface
Engineering, Vol 5, ASM Handbook, ASM International, 1994,
p 165-332
26. Quality Metal Finishing Guide, Metal Finishing Suppliers' Association
27. S.G.Fountoulakis,
Continuous Electrodeposited Coatings for Steel
Strip, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 349-359
28. A.J. Killmeyer, Tin Plating,Suiface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 239-241
29~ D.J. Maykuth and W.B. Hampshire, Corrosion of Tin and Tin Alloys,
Corrosion, Vol 13, ASM Handbook, ASM International, 1987, p
770-783
30. K.H. Stern.. Electrodeposition of Refractory Metals from Molten
Salts, Metallurgical and Ceramic Protective Coatings, K. Stem, Ed.,
Chapman and Hall, London, 1996, p 9
31. K.H. Stem, Metalliding, Metallurgical and Ceramic Protective Coatings, K. Stem, Ed., Chapman and Hall, London, 1996, p 38
32. A. Blair, Silver Plating, Surface Engineering, Vol 5, ASM Handbook,
ASM International, 1994, p 245-246
33. A.M. Weisberg, Gold Plating, Surface Engineering, Vol 5, ASM
Handbook, ASM International, 1994, p 247-250
34. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 52
35. D.W. Baudrand, Electroless Nickel Plating, Surface Engineering, Vol
5,ASM Handbook, ASM International, 1994, p 290-310
36. K. Barnak, S.W. Banovic, C.M. Petronis, D.F. Susan, and A.R.
Marder, Structure of Electrodeposited Graded Composite Coatings of
Ni-AI-AI203, J.Microsc., Vol 185, part 2, Feb 1997, p 265
37. D.F. Susan, K. Barnak, andA.R. Marder, Diffusion and Oxidation Behavior of Electrodeposited Ni Al Particle Composite Coatings, Materials, Coatings and Processes for Improved Reliability of High Temperature Components, N.S. Cheruvu and K. Dannemann, Ed., to be
published 1997
38. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 209
39. Engineering Coatings, Design and Application, S. Grainger, Ed.,
Abington Publishing, .1989, p 33
40. J.N. DuPont and A.R. Marder, Thermal Efficiency of Arc Welding
Processes, Weld. I., Dec 1995, p 406-s
41. J.N. DuPont andA.R. Marder, Dilution in Single Pass Arc Welds, Metall. Mater. Trans., Vol 27B, 1996, p 481
Surface Engineering to Add a Surface Layer or Coating 1181
42. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988,p 242
43. M. Scholl, R. Devanathan, and P. Clayton, Abrasive and Dry Sliding
Wear Resistance of Iron-Molybdenum-Nickel-Silicon-Carbon
Weld
Hardfacing Alloys, Wear, Vol 135 (No.2), 1990, p 355
44. K.H. Zurn Gahr, How Microstructure Affects Abrasive Wear Resistance, Met. Prog., Sept 1971, p 46--49
45. B.F. Levin, J.N. DuPont, andA.R. Marder, Weld Overlay Coatings for
Erosion Control, Wear, Vol 181-183, 1995, p 810
46. P.F. Tortorelli, I.G. Wright, G.M. Goodwin, and M. Howell, HighTemperature OxidationlSulfidation
Resistance of Iron-Aluminide
Coatings, Elevated Temperature Coatings: Science and Technology II,
N.B. Dahorte and I.M. Hampikian, Ed., TMS, 1996, p 175
47. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 219
48. R.C. Tucker, Jr., Thermal Spray Coatings, Surface Engineering, Vol 5,
ASM Handbook, ASM International, 1994, p 497-509
49. H. Herman and S. Sampath, Thermal Spray Coatings, Metallurgical
and Ceramic Protective Coatings, K. Stem, Ed., Chapman and Hall,
1996, p 261
50. Thermal Spraying: Practice, Theory, and Application, American
Welding Society, 1985
51. Engineering Coatings: Design and Application, S. Grainger, Ed.,
Abington Publishing, 1989, p 77
52. K.I. Stein, B.S. Schorr, and A.R. Marder, Erosion of Thermal Spray
FeCrAIY-Cr3C2 Cermet Coatings, Elevated Temperature Coatings:
Science and Technology IL N.B. Dahorte and I.M. Hampikian, Ed.,
TMS, 1996, P 99
53. S.T. Bluni and A.R. Marder, Effects of Thermal Spray Coating Composition and Microstructure on Coating Response and Substrate Protection at High Temperatures, Corrosion, Vol 52, 1996, p 213
54. H.O. Pierson, Chemical Vapor Deposition of Semiconductor Materials, Surface Engineering, Vol 5, ASM Handbook, ASM International,
1994, p 510-516
55. D.G. Bhat and P.F. Woerner, Coatings for Cutting Tools, J. Met., Feb
1986, p 68
56. D.R. Chopra, A.R. Chourasia, M. Green, R.C. Hyer, K.K. Mishra, and
S.C. Sharma, Diamond and Amorphous Films, Surface Modification
Technologies Iv, T.S. Sudarshan, D.G. Bhat, and M. Jeandin, TMS,
1991, P 583
57. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 138
58. Engineering Coatings, Design and Application, S. Grainger, Ed.,
Abington Publishing, 1989, p 119
182 / Surface Engineering for Corrosion and Wear Resistance
59. D.M. Mattox, Section Ed., Vacuum and Controlled-Atmosphere Coating and Surface Modification Processes, Surface Engineering, Vol 5,
ASM Handbook, ASM International, 1994, p 495-626
60. M.G. Hocking, V. Vasatasree, and P.S. Sidky, Metallic & Ceramic
Coatings: Production, High Temperature Properties & Applications,
J. Wiley & Sons, 1989, p 49
61. D.M. Mattox, Vacuum Deposition. Reactive Evaporation and Gas
Evaporation, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 556-572
62. R.L. Jones, Thermal Barrier Coatings, Metallurgical and Ceramic
Protective Coatings, K. Stem, Ed., Chapman and Hall, 1996, p 194
63. S.L. Rhode, Sputter Deposition, Surface Engineering, Vol 5, ASM
Handbook, ASM International, 1994, p 573-581
64. E.J. Lee and R.G. Bayer, Tribological Characteristics of Titanium Nitride Thin Coatings, Met. Finish., July 11'985,p 39-42
65. D.M. Mattox, Ion Plating, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 582-592
66. T. Arai and N. Komatsu, Carbide Coating Process by Use of Salt Bath
and its Application to Metal Forming Dies, Proc. 18th International
Machine Tool Design and Research Conference, 14-16 Sept 1977,
p 225-231
67. T. Arai, Carbide Coating Process by Use of Molten Borax Bath in
Japan, J. Heat. Treat., Vol 18 {No.2), 1979, P 15-22
68. T. Arai, H. Fujita, Y. Sugimoto, and Y. Ohta, Vanadium Carbonitride
Coating by Immersing into Low Temperature Salt Bath, Heat Treatment and Surface Engineering, George Krauss, Ed., ASM International, 1988, p 49-53
69. I.E. Campbell, V.D. Barth, R.F. Hoeckelman, and B.W. Gonser, Salt
Bath Chromizing, 1. Electrochem. Soc., Vol 96 (No.4),
1949,
P 262-273
70. T. Arai, J. Endo, and H. Takeda, Chromizing and Boriding by Use of
a Fluidized Bed, Proc. International Congress Fifth Heat Treatment of
Materials Conference, 20-24 Oct 1986, P 1335-1341
71. T. Arai and S. Harper, Thermoreactive Deposition/Diffusion Process,
Heat Treating, Vol 4, ASM Handbook, ASM International, 1991,
p 448-453
CHAPTER
7
Process Comparisons
PROCESS COMPARISONS discussed in this Chapter include:
•
•
•
•
•
•
Process availability
Corrosion resistance
Wear resistance
Cost
Distortion or size change tendencies
Thickness attainable
In addition to the information presented below, tables and figures comparing surface-engineering process characteristics that appear in other
Chapters should also be referred to. These are summarized in Table 1.
Table 1 Additional process comparison data presented in other Chapters
Description
Source
Chapter 1
Fig. 1
Table 1
Compares the thickness of various engineering coatings
Categorizes the various surface-engineering
options and lists their property benefits
Chapter 3
Table 4
Gives friction coefficient
data for different coatings applied by various processes
Chapter 4
Table 2
Compares
flame- and induction-hardening
Compares
the typical characteristics
processes
Chapter 5
Table 6
of carburizing,
nitriding, carbonitriding,
and ferritic nitrocarburizing
Chapter 6
Fig. 14
Fig. 15
Fig. 16
Table 1
Compares the abrasion resistance of TiN coatings applied by various thin-film processes
Compares the surface hardness of hardened tool steel and a cemented carbide with that of the following surface-hardening
processes: TRD, CVD, PVD, boriding, chrome plating, electroless nickel-phosphorus
plating, ferritic nitrocarburizing, sulfurizing, and spark hardening
Compares the wear, scuffing, and spalling resistance of sheet-metal dies coated by the following surface-hardening
processes:
uncoated, nitrided, borided, nitrogen ion implanted, chrome plated, sulfurized, uncoated cemented carbide, TiC + TiN by
CVD, TiC by CVD, VC by TRD, and NbC by TRD
Compares the processing characteristics for electroplating, electroless plating, CVD, PVD, thermal diffusion, ion nitriding, TRD,
ion implantation, ion-beam assisted deposition, and thermal spraying
(continued)
TRD, thermoreactive
deposition/diffusion
process;
CVD, chemical
vapor deposition;
PVD, physical
vapor deposition
184 / Surface Engineering for Corrosion and Wear Resistance
Table 1
(continued)
Source
Description
Chapter
6 (continued)
Compares the wear and corrosion resistance of electroplated copper, electroplated nickel, electroless nickel, electroplated
chromium, and electroless nickel + chromium
Compares the Taber abrasion resistance of electroplated nickel, electroless nickel, and electroplated hard chromium
Compares the characteristics of various weld overlay coatings
Compares the applications of thermal spraying, welding, and electroplating
Compares the process requirements in thermal spraying, welding, and electroplating
Compares the design characteristics of flame, arc wire, high-velocity oxyfuel, detonation gun, air plasma, and vacuum plasma
thermal spray processes
Compares the abrasive wear resistance of tungsten carbide coatings applied by detonation gun, plasma, and high-velocity oxyfuel
thermal spray processes
Compares the deposition temperatures for thermal and plasma CVD
Compares the processing characteristics of PVD, CVD, and ion implantation processes
Table 9
Table
Table
Table
Table
Table
11
14
15
16
17
Table 19
Table 22
Table 26
Chapter
Table
Table
Table
Table
Table
Table
8
1
2
3
4
5
6
TRD, thermoreactive
Compares thickness ranges and hardness levels of a wide range of surface-engineering
Compares surface finish characteristics of various surface-engineering
processes
Compares size and weight limitations for different surface treatments
Summarizes design limitations for surface preparation/cleaning
processes
Summarizes design limitations for organic coating processes
Summarizes design limitations for inorganic (metal and ceramic) coating processes
deposition/diffusion
process;
CVD, chemical
vapor deposition;
PVD, physical
processes
vapor deposition
Process Availability
One of the key considerations in the materials selection process is material availability and delivery time. This is especially true if a
person/company has only a limited time for completing a part. Even without time constraints, materials engineers tend to use materials that are
readily available. Similarly, the choice of a surface-engineering process is
often based on process availability because poor logistics between the
customer and surface treatment supplier can result in added shipping time
and costs.
In general, the long-established surface-engineering processes are available from numerous job shops in varied locations. These would include localized surface-hardening treatments, diffusion heat treatments such as carburizing and nitriding, weld surfacing, thermal spraying, electroplating,
galvanizing, and painting. However, within these surface-treatment categories there may be a wide disparity in the availability of specific
processes. For example, most heat treating job shops offer flame and induction localized hardening, but few have facilities for electron beam or
laser localized surface hardening. The same can be said of diffusion heat
treatments. In a survey of 800 commercial heat treating shops in the United
States and Canada (Ref 1), 70% offered carburizing services, of which:
•
•
•
•
48% offered gas atmosphere carburizing
19% offered pack carburizing
12% offered salt-bath carburizing
5% offered carburizing in fluid beds
Process Comparisons / 185
•
•
2% offered vacuum carburizing
1% offered plasma (ion) carburizing
Thus, process availability might negate the selection of plasma carburizing over conventional methods, despite the reduced carburizing times and
more uniform case depths associated with plasma methods. A similar situation exists for gas nitriding and plasma (ion) nitriding.
The more specialized pack-cementation diffusion processes, such as
aluminizing, chromizing, siliconizing, and boronizing, are usually carried
out at companies that specialize in these processes. Some of these
processes are also performed by aerospace companies, for example, aluminizing of jet engine turbine components.
More recently developed coatings or surface modifications-such
as
chemical vapor deposition (CVD), physical vapor deposition (PVD) , ion
implantation, and laser melting, alloying, or cladding-are also performed
by companies that specialize in these processes/coatings. For example, most
cutting tool manufacturers offer CVD, PVD, or CVD + PVD processing.
The availability of facilities offering various surface-engineering options
can best be determined by contacting technical associations that offer information services for these surface treatments. Examples include:
•
•
•
•
•
•
•
•
•
•
American Electroplaters and Surface Finishers Society
American Galvanizers Association
American Welding Society
Association of Industrial Metallizers, Coaters, and Laminators
Federation of Societies for Coating Technology
National Paint and Coatings Association
Powder Coating Institute
Society of Vacuum Coaters
Steel Structures Painting Council
Thermal Spray Society or the International Thermal Spray Association
The Society for Protective Coatings
Descriptions of these organizations including their scope, addresses, telephone and fax numbers, web site or e-mail access, and so forth can be
found in the Encyclopedia of Associations, published by Gale Group Publishing and available at most local public libraries.
Corrosion Resistance
Corrosion-resistant protective coatings include various organic and inorganic coatings that provide barrier protection (e.g., a paint coating or
multilayer electroplate) or sacrificial protection (e.g., zinc and aluminum
186 / Surface Engineering for Corrosion and Wear Resistance
Table 2 Salt mist corrosion performance of various
steels and coatings
Surface
Coating life(a), h
Steels
Low-alloy steel or low-carbon steel
Induction-hardened
carbon steel
Stainless steel (316)
Carburized mild steel
Nitrided low-alloy steel
Nitrocarburizedloxidized
mild steel
Nitrided stainless steel
Phosphated mild steel
Steam-tempered alloy steel
2
2
<2000
2
20
400
2
500-1000
300-500
Coatings (on mild steel)
Hard chrome plate
Crack -free chrome
Electroless nickel (as-deposited)
Hardened electroless nickel
Electroless nickel + polymer
Electroless nickel-PTFE
Electroless nickel-SiC
Nickel electroplate
Nickel-ceramic electroplate
Cadmium plate
Zinc plate
Zinc-9Ni plate
Hot dip galvanized
Hot dip aluminized
PVDTiN
Plasma sprayed ceramic
High-velocity oxyfuel cermet
Spray and fused nickel-chromium
Slurry/sinter formed ceramic
Aluimnum alloy 6082
Anodized
Anodized + polymer in-fill
<10
30-50
100-1000
50--500
1500
<20
<20
<1000
500
<2000
1000
2000
1000
500
2
10
1000
2000
<2000
5
300
1800
PTFE, polytetrafluoroethylene.
(a) Time at which five or more individual corrosion spots
have formed on the upper facing of the test panel. Copyright ABA Technology pIc; used
with permission. Source: Ref 2
coatings). As described in Chapter 2, the corrosion resistance of these
coatings is often determined by accelerated laboratory tests. Table 2 lists
the results of the neutral salt-spray (fog) test described in ASTM B 117 on
substrate and coating materials. These data should be used with caution
because the corrosion response of a given coating changes from environment to environment. Coating suppliers should be consulted for final coating material selection. The type of coating process selected is dependent
on the design factors described in Chapter 8.
Wear Resistance
Hardness versus Wear Resistance. The wear processes that are usually
mitigated by the use of hard surfaces are low-stress abrasion, wear in systems involving relative sliding of conforming solids, fretting wear, galling,
and to some extent, solid-particle erosion (Ref 3). Unfortunately there are
many caveats to this statement, and substrate/coating selection should be
Process Comparisons / 187
r:7/J Boron
~
~
.,.,.,.,.,..,..,.,.,..,.,..,.,.,..,.,..,.,.,..,~~~.
~
Soft
steel
.--
PVDTiN
~
~
~
~
~
Boronizing
Chromium
coating
Silicon carbide
oxide ceramics
carbide cermets
of steels
Plasma sprayed (AI20s/Cr203IWC-Co
~
~
Aluminum
Cemented
carbide
CVD TiC coating
coatings)
electroplate
Hardened tool steels
~
Nitrided steel
~,.,.,.,.,.,.,.,.,.,.~ Fusion hardfacings
~
Laser/EB
~
~
~
~
o
Induction-hardened
Flame-hardened
~
~
harden
Carbonitride/carburize/cyanide
Electroless
steels
steels
nickel (before hardening)/nickel
electroplate
HSLA steels
Mn steels (before work hardening)
I
500
1000
2000
Hardness,
Fig. 1
3000
kg/mm2
Range of hardness levels for various materials and surface treatments. Source: Ref 3
carefully studied with proper tests carried out if necessary. Coating suppliers should also be consulted. Chapter 3 provides additional information on
wear processes and the means to prevent specific types of wear.
Figure 1 shows typical ranges in hardness for many of the surfaceengineering processes used to control wear. All of the treatments shown in
this figure have hardness values greater than ordinary constructional steel
or low-carbon steel. The surface-hardening processes that rely on martensitic transformations all have comparable hardness, and the diffusion
treatments that produce harder surfaces are nitriding, boronizing (boriding), and chromizing. The hardest metal coating is chromium plate, although hardened electroless nickel plate can attain values just under that
of chromium. The surfaces that exceed the hardness of chromium are the
cermets or ceramics, or surfaces that are modified so that they are cermets
or ceramics. These include nitrides, carbides, borides, and similar compounds. The popular solid ceramics used for wear applications-aluminum oxide, silicon carbide, and silicon nitride-generally
have hardnesses in the range of 2000 to 3000 kg/mm-, As shown in Fig. 1, when
materials such as aluminum oxide are applied by plasma spraying or other
thermal spray process, they have hardnesses that are less than the same
material in solid pressed-and-sintered form. This is because the sprayed
4000
188 / Surface Engineering for Corrosion and Wear Resistance
Table 3
Comparison of thermal spray methods
As described in the text, coating porosity affects coating hardness.
Flame or exit
plasma
temperature
Method
Combustion
powder
Combustion
wire
Arc wire
Plasma
High-energy
plasma
Vacuum
plasma
D-gun
HVOF
D-gun, detonation
Maximum
spray rate
°C
of
Atmosphere
around
particles
lb/h
%
11
400
2,200
4,000
CO, CO2, H20
30
100
7
15
6-15
71
2,500
2,800
5,000
N2, CO, CO2, H2O
180
600
9
20
6-15
5,500
5,500
11,000
10,000
10,000
20,000
N2,02
N2, Ar, H2, 02
N2• Ar, H2, 02
240
240
240-1,200
800
800
800-4,000
16
5
23
35
10
50
2-8
<2
<1
11,000
20,000
Ar,He
240-610
800-2,000
11
24
<0.5
3,100
3,100
5,600
5,600
3,000
2,000-5,000
1
14
2
30
<1
<0.5
Gas flow
m3/h
71
4.2
17-28
8.5
11
28-57
ft3/h
2,500
150
600-1,000
300
400
1,000-2,000
gun; HVOF, high-velocity
oxyfuel.
Particle impact velocity
mls
N2, CO, CO2, H2O
N2, CO, CO2, H2O
910
610-1,500
ftls
kglh
Coating
porosity,
Source: Ref 4
materials contain porosity and oxides that are not contained in the sintered
solid form. Table 3 shows the coating porosity that can be expected from
variations in the thermal spray process. The other hard surface for tools,
cemented carbide, has a hardness of about 2000 kg/mm-, about twice as
hard as the hardest metal. Recently developed diamond and diamondlike
carbon coatings deposited by CVD processing have hardness levels in excess of 5000 kg/mm-,
Table 4 Low-stress abrasive wear rankings for various
materials
See text for details.
Low wear rate
100
200
300
400
500
800
1,000
1,500
4,000
5,000
8,000
10,000
12,000
15,000
50,000
HVOFWC-Co
CVD cre (high-carbon low-alloy tool steel)
CVD CrN (high-chromium tool steel)
Carbide diffusion process
PVD CrN, 30 JJ.m thick
Hard chrome plate
Sprayed and IDP chromium
Plasma sprayed alumina-titania
Electroless nickel-ceramic
Boronized 316 stainless steel
Plasma sprayed chromium oxide
Spray and fused nickel-chromium-chromium
carbide
Carburized steel
Induction-hardened
0.4% C steel
Slurry/sinter formed ceramic
Nitrided 316 stainless steel
Hardened electroless nickel
As-plated electroless nickel
0.4% C steel, normalized
316 stainless steel
PVD CrN (2 JJ.m thick)
Anodized aluminum alloy
Aluminum alloy
High wear rate
CVD, chemical vapor deposition; PVD, plasma vapor deposition; HIP, hot isostatically
pressed. Copyright ABA Technology pIc; used with permission. Source: Ref 2
Process Comparisons / 189
TableS
Erosive wear rankings for various materials
Test conditions: 1000 ppm of silica sand in water with an impact velocity of 25 m/s (80 ftls)
Low wear rate
100
200
300
700
800
1000
1500
2000
High-chromium iron weld overlay
Spray and fused nickel-chromium-chromium
Boronized 316 stainless steel
High-energy sprayed WC-Co
Hard chrome plate
Nitrided 316 stainless steel
Electroless nickel
Slurry/sinter formed ceramic
PVDTiN
.
316 stainless steel
carbide
High wear rate
PVD, plasma vapor deposition.
Source: Ref 2
Copyright
ABA Technology
plc; used with permission.
Test Results. Table 4 shows results of the ASTM G 65 dry-sand/rubberwheel test on various coatings. The low-stress abrasion resistance performance is indexed to that of the best quality tungsten carbide-cobalt (WC-Co)
coating, denoted a value of 100, and is related to volume loss per revolution
of the wheel under a fixed load, at constant speed and abrasive throughput.
Table 5 shows the resistance of various coatings to erosive wear. The results are indexed to that of a high-chromium cast iron hardfacing alloy,
again denoted by a value of 100.
Table 6 shows typical adhesive dry rubbing wear values for surface
treatments and coatings. These were determined from a pin-on-plate
Table 6
Wear rate,
m3/N • m
10-17
10-16
10-15
10-14
10-13
10-12
10-11
10-10
10-9
10-8
Adhesive wear rates of various materials
Material
Lubricated through-hardened
steel
HVOFWC-Co
Plasma sprayed chrome oxide
PVD TiN (not at high loads)
CVD CrN or alumina
Hard chrome plate
Nitrided tool steel
Nitrided stainless steel (not at higher loads)
Slurry/sinter formed ceramic (not higher loads)
Carburized steel
Nitrided low-alloy steel
Unlubricated through-hardened
steel
Glass- filled PTPE
Anodized aluminum
Hardened electroless nickel
Electroless nickel, as plated
Normalized, unlubricated steel
Austenitic stainless steel
Copper plate
Electrolytic nickel plate
Aluminum alloy
Unfilled PTFE coating
Cadmium and zinc plates
Unfilled PFA or PEP polymer coatings
Silver plate
HVOF, high-velocity oxyfuel: PVD, plasma vapor deposition; CVD, chemical vapor deposition; PTFE, polytetrafluoroethylene;
PFA, perfluoro alkoxy alkaline; PEP, fluorinated
ethylene propylene. Copyright ABA Technology pIc; used with permission. Source: Ref 2
190 / Surface Engineering for Corrosion and Wear Resistance
sliding test using a polished hardened steel pin rubbing against the treated
surface at a load of 10 N/m2 (---100 gf/ft2).
Cost of Surface Treatments
Cost must be weighed against the performance required for the surfacetreatment system. A low-cost surface treatment that fails to perform its
function is a wasted expense. Unfortunately, it is nearly impossible to give
absolute comparative costs for different surface-engineering options.
Often, a range of prices will be offered for a particular job from different,
equally competent candidate suppliers. Probably the most important factor that relates to costs of producing a corrosion- or wear-resistant surface
on a part is part quantity. Treating many parts usually allows economies in
treatment and finishing.
Another consideration when assessing surface treatment costs is part
size. There are some critical sizes for each surface-treatment process
above which the cost of obtaining the treatment may be high. A number
of surface treatments require that the part fit into the work zone of a vacuum chamber. The cost of vacuum equipment goes up exponentially with
chamber volume.
SAW
FCAW
GMAW
SMAW
FLSP
PSP
EB and laser
2
3
4
5
6
7
8
9
10
Relative cost
Fig. 2
Relative costs (based on pounds of alloy deposited) for various weld
overlay and thermal spray processes. SAW, submerged
arc welding;
FCAW, flux-cored arc welding; GMAW, gas metal arc welding; SMAW, shielded
metal arc welding; OAW, oxyacetylene
gas welding; FLSp, flame spraying; PSp,
plasma spraying; EB, electron beam. Source: Ref 3
Process Comparisons / 191
Thermochemical:
Carburizing
rn
Nitriding
~
Nitrocarburizing
Electrochemical:
Chromium
II
III
Cobalt + Cr 3C2
Electroless:
Nickel
Plasma sprayed:
WC-Co
AI203
Combustion gun sprayed:
13% Cr wire
Ni-Cr-8 and fuse
Ni-Cr-8 + WC and fuse
Surface weld:
Iron-base
Cobalt-base
Vapor deposited:
CVDTiC
PVDTiN
Cost
Fig. 3
Approximate
--..
relative costs of various surface treatments
Other factors to be considered are:
•
•
•
•
•
•
•
•
The time required for a given surface treatment
Fixturing, masking, and inspection costs
Final finishing costs
Material costs
Energy costs
Labor costs
Environmentally related costs, for example, disposal of spent plating
solutions
Expected service life of the coating
Because of these various factors, it is difficult to compare costs with a
high degree of accuracy. Figures 2 and 3 provide some general guidelines
for cost comparisons.
Distortion or Size Change Tendencies
Figure 4 shows the surface temperatures that are encountered in various
surface-engineering processes. As indicated in the figure, the processes
are categorized into two groups: one group produces negligible part dis-
192 / Surface Engineering for Corrosion and Wear Resistance
Negligible part distortion
on ferrous metal
Likely distortion
Ferritic nitrocarburizing
w$)/////00'mA
Brazing of WC wear tiles
~ Wear plates
f?0Wff$ff4f///Jj
~A
I
Repair cements
I
I
I
l0W§#~d/0t$;0W4I
I
L
""11'~""'%-'y/,:""~""'~""'%-'~""(/,:""'~""'%-'~""'/J
Ion implantation
I
I
I
Sleeving
EB ~nd laser
melting
Sputter coating
w~"
Thermalevaporalion
coatings
.~dJCVDcoatings
I
~
Plating
W~§§/M
IH
I
I
I
f//WZ///pJ// J/////////I
I
Thermal
Carburizing
hardening
and pack cementation
Nitriding
I
~
Selective
10W)0?'"ff4
Quench hardening
alloy and tool steels
spray coatings
I
ORT
400
800
1200
1600
Temperature,
20
200
400
600
800
2000
of
1000
Temperature,
2400
1200
1400
2800
1600
3200
1800
°C
Fig. 4
Maximum surface temperatures that can be anticipated for various surface-engineering
processes. The dashed vertical line at 540°C (1000 OF) represents the temperature limit for
distortion for ferrous metals. Obviously, a temperature of 540°C (1000 OF) would melt a number of
nonferrous metals, and it would cause distortion on metals such as aluminum or magnesium. However, this process temperature information can be used to compare the heating that will be required
for a particular pr~cess. Source: Ref 3
tortion, and the other group contains processes that have varying potential
for causing distortion. Obviously if a part could benefit from a surface
treatment, but distortion cannot be tolerated, processes that require minimal heating should be considered.
Coating Thickness Attainable
Figure 5 shows the typical thickness/penetration capabilities of various
coating and surface treatments. As indicated in the figure, some surfaceengineering treatments penetrate into the surface and there is no intentional buildup on the surface. These are the surface-engineering processes
described in Chapters 4 and 5. Other surface treatments coat or intentionally build up the surface. This is a selection factor. Can a part tolerate a
buildup on the surface? If not, the selection process is narrowed to the
Process Comparisons 1193
0.1
~
Fig. 5
0.010
0.001
Depth of penetration of
surface treatment, in.
0.0001
o
t
Part
surface
0.0001
0.001
0.010
0.10
Surface coating
thickness, in.
-----.
Typical coating thickness/depth of penetration for various coating and surface-hardening
1.0
processes. Source: Ref 3
treatments that penetrate into the surface. Other factors affecting the thickness of a given surface treatment include dimensional requirements, the
service conditions, the anticipated/allowable corrosion or wear depth, and
anticipated loads on the surface. Questions or concerns related to coating
thickness should be discussed with the contractor. Available specifications
should also be reviewed. Additional information regarding the thicknesses
associated with various surface-engineering processes can be found in
Chapters 4 to 6 and 8.
References
1. W.L. Kovacs, Commercial and Economic Trends in Ion Nitriding/
Carburizing, in Ion Nitriding and Ion Carburizing, ASM International,
1990, p 5-12
2. K. Stevens, "Surface Engineering to Combat Wear and Corrosion: A
Design Guide," The Institute of Materials, London, United Kingdom,
1997
3. K. Budinski, Selecting a Wear-Resistant Surface, Chapter 12, in Surface Engineeringfor Wear Resistance, Prentice-Hall, 1988, p 303-345
4. M.L. Thorpe, Thermal Spray: Industry in Transition, Adv. Mater.
Process., Vol 143 (No.5), 1993, p 50-56
CHAPTER
8
Practical Design
Guidelines for Surface
Engi neeri ng
THE DESIGN ENGINEER is faced with a wide range of options when
selecting a surface treatment for a given problem or application. Some of
the important factors described in this Chapter that must be considered before selecting a surface treatment include (Ref 1):
•
•
•
•
•
•
•
•
•
The function of the component. Is it rolling, sliding, in static contact,
and so forth?
The base material. Is it a low-carbon steel, medium-carbon steel, lowalloy steel, a nonferrous alloy, and so forth?
The fabrication method. Is it cast, welded, machined, and so forth?
Temperature restrictions, that is, the temperature that must not be exceeded when carrying out a surface-engineering treatment. Will distortion of the component result?
The interactions to which the component will be subjected, for example, sliding, rolling contact, static corrosion, including special requirements for strength or fatigue resistance
The operating environment. Is it corrosive or abrasive in nature? Is it
saline, oxidizing, caustic, and so forth?
The temperature of the environment. What is the maximum temperature the component will likely see in service?
The material from which any component or product in rubbing contact with the part is made, that is the counterface material and its hardness. Does the counterface material contain a hard abrasive filler?
The predominant mode of degradation. Is it corrosion, wear, fatigue,
and so forth?
196 / Surface Engineering for Corrosion and Wear Resistance
•
•
•
•
•
•
•
•
•
•
•
The essential requirements for successful performance, for example,
low-stress abrasion, high-stress abrasion, nonsticking, and so forth
The contact load (maximum value) and likely contact area. Is it over
a large area or concentrated?
The contact conditions, for example, impact, cyclic loading, static
loading, or sliding
The required surface hardness of the component
The requirements for surface roughness (or smoothness)
Constraints on any final or finishing operations. Are there any critical
dimensions or tolerances that must be met after processing?
The required surface coverage and thickness of any treatment
The geometry of the component. Are holes, sharp edges, enclosures,
reentrants, and so forth, present?
The overall size and weight of the component
Are there special requirements that must be met, ·for example, Department of Defense (DOD), Food and Drug Administration (FDA)?
Appearance, for example, color or texture
Surface-Engineering Solutions for Specific Problems (Ref 1)
This section provides surface-engineering
conditions:
•
•
•
•
•
•
•
solutions for seven operating
Structural parts, for example, pipes, pump and valve bodies, casings,
housings, supports, rigs, tanks, and so forth, subjected. to corrosive
conditions in various environments
A part in static contact with another engineering component with
small relative motions or vibrations
A part in static contact with a product that is being cast, molded,
cured, and so forth
A part in rolling contact with another part, for example, shafts, journals, pistons, rings, gears, seals, and tools for metal pressing, forming,
drawing, and cutting
A part under light mechanical load but which handles, rubs, or slides
against an abrasive product, for example, paper, filled plastics, textile
yarns, leather, friction materials for clutches and brakes, pharmaceuticals, and some foodstuffs like wheat and soy
A part under high mechanical load, with or without impact, that handles or slides against abrasive or erosive materials, for example, coal
chutes and conveyors, crushers, digging equipment, and so forth
A part in rolling or sliding contact with another part in the presence of
corrosive or abrasive materials, for example, pumps, valves, mechanical seals, and slurry handling
Practical Design Guidelines for Surface Engineering / 197
Emphasis is placed on the base material, operating conditions, and applicable surface treatments. As can be seen, alternative coating processes!
materials may be recommended for a given material/operating condition
combination. Final selection may be based on some of the application and
performance requirements listed above and further examined in subsequent sections of this Chapter.
Structural
Parts in Corrosive Environments
If the part is structural with no sliding or rubbing contacts, then the main
concern will be corrosion. Also, if there is cyclic loading-that
is, fatigue-corrosion
can considerably accelerate mechanical failures. Environmentally assisted cracking due to corrosion fatigue, stress-corrosion
cracking (SCC), or hydrogen damage is discussed in Chapter 2.
Base Material
Base materials for structural parts are commonly an engineering steel,
cast iron, stainless steel (most probably a ferritic or martensitic type), or
an aluminum alloy.
Neutral Environments
If there is no concern about corrosion, but there is a requirement for improved strength or fatigue resistance, the following surface treatments
should be considered:
•
•
Shot peening
Improving the surface quality and finish by grinding, lapping, or polishing
Specific Corrosive Environments
If there is concern about corrosion,
the temperature are important. Also,
metallic component of a dissimilar
corrosion, which accelerates failure,
then both the corrosive medium and
if the part is in contact with another
material, then galvanically assisted
is very possible.
For outdoor, normal atmospheric corrosion, consider:
•
•
•
•
•
Hot dip galvanizing, which can provide prolonged protection even in
polluted environments
Thermally sprayed zinc or aluminum
Electrolytic zinc
Painting or powder coatings with appropriate surface preparation and
priming
Heavy electrolytic nickel provided there are no defects in the coating
(Ref 1)
198 / Surface Engineering for Corrosion and Wear Resistance
•
•
•
•
Electroless nickel-phosphorus coating
Aluminum ion plating
Phosphating for moderate protection
Anodizing, preferably sealed, for aluminum alloys
For more hostile environments, including marine and aerospace
where galvanic corrosion will be a major concern, consider:
•
•
•
•
•
Hot dip galvanizing, which will provide moderate protection
Thermally sprayed zinc or aluminum for moderate protection
Electrolytic zinc or zinc-nickel alloy (10-14% Ni) coating followed
by chromate passivation and an organic topcoat
Painting, with appropriate preparation and priming, perhaps zinc or
aluminum loaded
Cadmium plate, preferably chromate passivated for maximum protection
High-Temperature
Oxidation and Corrosion. The substrates for
high-temperature corrosion applications are often superalloys, stainless
steels, or titanium alloys. Protective coatings to be considered include:
•
•
•
Diffusion chromizing for oxidation resistance up to 750 to 800°C
(1380-1470 OF)
Diffusion aluminizing for protection against oxidation, carburizing,
and sulfur and vanadium corrosion in chemical plants and gas turbines; can be effective above 800°C (1470 OF)
Slurry/sinter formed ceramics (chromium oxide based) at temperatures up to 600 °C (1110 OF)
Thermally sprayed coatings, for example, MCrAIY corrosion protection layers and ceramic-based thermal barriers
For caustic environments, consider:
•
•
•
•
•
Slurry/sinter-formed ceramics (chromium oxide based)
Thermally sprayed ceramics, for example, chromium oxide, alumina,
preferably sealed
Cadmium plate, preferably chromate passivated for moderate protection
Electroless nickel
Heavy electrolytic nickel plating
For acidic environments, consider:
•
•
Slurry/sinter-formed ceramics (chromium oxide based)
Thermally sprayed ceramics, for example, chromium oxide, alumina,
preferably sealed
Practical Design Guidelines for Surface Engineering / 199
For stress-assisted corrosive conditions, consider shot peening followed by corrosion protection appropriate to environmental conditions,
but only those processes applied at near-ambient temperature.
Parts in Static Contact with Vibration (Fretting) (Ref 1)
If the part is in contact with another engineering component, but with no
relative movement, then the main concern will be with corrosion. If the
mating part is a dissimilar metal, then galvanic corrosion will be a significant risk (see Chapter 2).
If the contact also involves vibration or impact motion then fretting,
fretting corrosion, or even fretting fatigue must be considered.
This is the case with splines and couplings where motion is transmitted
from one part to another via a loaded contact and in parts fastened or fitted together where there is a source of external vibration, for example,
heat exchangers and bearing housings. Fretting-type failures are also
found on chains, pulleys, and wire ropes.
Base Material
Fretting corrosion is most prevalent with steel parts where the oxidation
process produces an obvious, distinctive, red oxide abrasive dust. Stainless steels are not immune, particularly ferritic types. Fretting of aluminum alloys produces a white oxide debris that is also very abrasive.
Contact Conditions
Fretting and Fretting Corrosion. With light loads or low-cycle fatigue,
the effects of fatigue will usually be small, and the preferred solution is to
reduce the tendency to oxidation by applying an inert coating. Consider
the following treatments:
•
•
•
•
•
•
Hot dip galvanizing
Heavy electrolytic nickel or copper plating, which will provide a lowcorrosion surface but one with a tendency to gall and to wear quickly
Electroless nickel, which will provide good oxidation protection, with
extra wear resistance if hardened. Additional improvements can be obtained by adding a further solid-lubricant coating of molybdenum
disulfide (MoS2), ideally in an epoxy binder
Hard chrome plate for maximum wear protection
Silver or indium plating, which provides a soft, ductile interface with
good oxidation resistance
Anodizing for protection of aluminum alloys, preferably sealed with a
self-lubricating polymer such as polytetrafluoroethylene (PTFE)
200 / Surface Engineering for Corrosion and Wear Resistance
fretting fatigue
With high loads or prolonged operation, fretting may lead to crack initiation followed by fretting fatigue. Since electroplating can impair fatigue
resistance of the substrate, the best solutions are usually the intrinsically
hard and tough thermally sprayed coatings. These include:
•
•
•
Nickel-chromium for corrosion resistance and toughness in impact
fretting
Tungsten carbide-cobalt (WC-Co) for maximum wear resistance
Nickel-chromium-chromium
carbide for higher-temperature fretting
Oxidative Wear
If there is a small, slow-speed relative sliding between the parts, this
may also lead to a fretting-type wear condition, with the oxidized wear debris trapped in the contact. It is common on chain links and wire ropes and
sometimes occurs on pulleys.
The only viable solution for wire ropes is regular oil or grease soaking.
For parts under high loading, and that are traditionally made of highstrength engineering steels, there is usually no easy way to reduce the corrosive contribution to the wear process. The best approach is to increase
the surface hardness so that it can resist the abrasion by the oxide debris.
For example, consider:
•
•
•
Local surface hardening, for example, flame, induction, or laser for
medium-carbon steels
Case hardening, for example, carburizing, carbonitriding for lowcarbon steels
Nitriding or nitrocarburizing if the loads are not too high and the steel
has some alloying elements such as chromium or molybdenum
Parts in Static Contact with a Product (Ref 1)
This operating condition applies to molding, casting, and activities such
as baking and curing when a product is held against the component surface for an extended time. The issue is not usually one of wear; rather the
principal requirement is that the product and component ·will separate
without adhesion or damage to either surface. However, in many cases, the
product must first flow into the mold and, if pressure is applied during
processing, the product may also creep across the surface as it cures or
sets. All of these can cause wear.
Base Material
In many food applications the substrate will be stainless steel or an aluminum alloy. Dies and molds for plastic molding are most likely to be
Practical Design Guidelines for Surface Engineering / 201
made of alloy steel, aluminum, brass, or copper. Die casting of aluminum
or zinc products will generally use hot-work (H-series) tool steels; glass
molding uses tool steels, cast irons, and beryllium-copper alloys.
Specific Applications
For food baking and molding, consider:
•
•
•
Fluorinated polymer coatings. Fluorinated ethylene propylene (PEP)
provides the best release. Perfluoro alkoxy alkane (PFA) gives release
and wear resistance. PTFE provides best low friction ..Ensure that the
grade chosen is approved for food use.
Electroless nickel plus PTFE, which will provide both low friction and
good release properties, but not high wear resistance
Anodizing plus PTFE seal for nonsticking aluminum alloy parts
For plastic injection molding tools, consider:
•
•
•
•
Nitriding of alloy steel parts when the plastic is filled and abrasive
Hard chrome plate for steel, brass, or copper parts when abrasion is
expected
Ion implantation for improved wear resistance of alloy tool steels and
chrome-plated parts
Anodizing plus PTFE seal for nonstick and wear resistance with aluminum alloy parts
For die casting, consider:
•
•
Nitriding for H-series tool steels
Physical vapor deposition (PVD) coatings, for example, TiN, TiAIN
For glass molding, consider:
•
•
Diffusion chromizing on cast iron molds for hot erosion-corrosion resistance
Slurry/sinter formed ceramics (chromium oxide based) coatings
Hard chrome plate (crack-free form is best)
Parts in Sliding or Rolling Contact with Another Surface (Ref 1)
If the part is in sliding or rolling contact with another engineering component then, even if it is lubricated, there is the likelihood of adhesive
wear.
Adhesive wear can occur in many engineering situations, for example,
shafts, journals, pistons and rings, cams, bearings, pads gears, seals, slide-
202 I Surface Engineering for Corrosion and Wear Resistance
ways, and so forth, in metal cutting, drawing, and forming. In general, if
both mating parts are metallic, it will be the softer part that suffers the
greater wear and should be surface engineered. However, in cases where
replacement of a particular part is difficult, then it is that part that should
be protected, even at the expense of extra wear on the mating surface.
If the mating part is nonmetallic, wear could still occur on the counterfacing component. If the counterface is plastic, determine whether it has
any fillers that could cause abrasive wear. This would also be the case if
the counterface was a ceramic.
Base Material
Common base metals include cast iron, low-carbon steel, mediumcarbon steel, alloy steel (including tool and bearing steels), stainless steel
(austenitic, martensitic, or ferritic) , aluminum alloys, titanium alloys or
other nonferrous metals, for example, bronzes, copper, and brasses.
General Contact Conditions
Is the Part Lubricated? If it is, or could be, then wear, even without
surface engineering, might be reduced by a factor of 1000 compared to
running unlubricated. In lubricated systems under high load and at high
speed (e.g., cams and tappets, piston bores and rings) there is still the possibility of scuffing.
Is the Part Unlubricated yet There is a Need to Reduce Friction?
For dry sliding it is important to specify the exact requirements. A low
friction coefficient (see Chapter 3) can be defined as 0.1 or less and is generally achieved with polymers such as PTFE, but these have high wear
rates. If, without surface engineering, the friction would be unacceptably
high, for example, galling between two soft steel parts, then most surface
treatments will reduce friction as well as reduce the wear.
Is the Specific Loading High? Loads above 100 MPa (14.5 ksi) are
considered high, in which case the hardness and thickness, or case depth,
of the surface treatment is the critical factor. Both the substrate and the
coating must be able to withstand that load. It is important to remember
that rolling parts are often under high specific loading.
What are the Requirements for Reducing Wear? In general, the
higher the hardness of the surface layer, the lower will be the wear. It is
vital to understand the consequences of the wear; for example, it may be
that an increase in clearance between two parts must be avoided in service. If the wear is concentrated in a small area, then even a low wear rate
will lead to a rapid increase in the clearance, and a high surface hardness
is needed in that area. If the wear is spread out over a wider area the corresponding increase in clearance will be smaller, and a simpler, less expensive solution may be adopted.
Is There an Element of Corrosion? For instance, if moisture or salt
water is present there is a major risk of combined wear and corrosion that
Practical Design Guidelines for Surface Engineering / 203
can rapidly increase- surface material loss. Select coatings with good corrosion resistance rather than high hardness. Corrosion is generally the
more damaging feature.
Surface-Engineering Options
For mild steel or cast iron parts, consider:
•
•
•
•
•
•
•
Case hardening, that is, carburizing or carbonitriding for high hardness and load-carrying capacity. With case-hardening processes, however, distortion problems must be considered.
Nitriding or nitrocarburizing. This gives a thin compounded layer.
Electroless nickel and associated composites for corrosive wear. Heat
treating at 400°C (750 OF)will provide additional hardness and wear
resistance. Ceramic-filled electro less nickel gives greater wear resistance. PTFE-filled electroless nickel will give low friction, but high
wear if the load is high.
Hard chrome plate for excellent wear protection, for example, on auto
body dies. Select the thickness according to the load. Good for corrosive wear if protected with an underlayer of electrolytic nickel.
Thermally sprayed metals or alloys. Use for wear and corrosion.
Thermally sprayed ceramics or cermets, for example, WC-Co, alumina, chromium oxide, and so forth, for maximum wear resistance. If
corrosion is likely, a corrosion barrier is required under the hard coating.
Hot dip galvanizing on steel parts. Use if corrosive wear is likely.
For medium-carbon steel parts, consider the surface treatments listed
above for mild steels, plus local surface hardening, for example, flame, induction, or laser for maximum loading, and for large rolling components
(e.g., large cylindrical roller bearings and tracks).
For low-alloy steel parts (steels containing chromium, vanadium,
and/or molybdenum), consider the surface treatments listed above for both
mild and medium-carbon steels, plus nitriding or nitrocarburizing to give
a diffused case. Follow with oxidation and oiling treatment for corrosive
conditions.
High-Alloy and Tool Steel Parts. Tool steels or high-speed steels (including AISI 440C, and the A, D, and M series steels) can be heat treated
to high hardnesses and are wear resistant in their own right. For additional
wear resistance and to reduce pickup, particularly in metalworking operations, consider:
•
•
Nitriding or nitrocarburizing. Follow with oxidation and oiling treatment for reduced pickup. Used on warm forming tools.
Hard chrome plate for excellent wear prevention, but postprocessing
heat treatment may be necessary to prevent hydrogen embrittlement.
Good for deep-drawing tools
204 / Surface Engineering for Corrosion and Wear Resistance
•
•
•
PVD coatings, for example, TiN, CrN, and diamondlike carbon will
give low friction and low wear under moderate loads. Used on cutting
and cold-forming tools
Chemical vapor deposition (CVD) coatings, for example, TiN,
TiCITiN, A1203, and TiC for higher loads than with PVD. Used on
carbide inserts and other cutting tools, cold- and hot-forming tools
Carbide diffusion, also called Toyota diffusion process (see Chapter
6), for high-carbon and precarburized steels. Uses a salt bath to produce a vanadium carbide (VC) layer
For austenitic (300 series) stainless steel parts, consider:
•
•
•
•
•
Electroless nickel and composites used as-deposited or heat treated to
provide some wear resistance. Ceramic-filled electroless nickel will
give greater wear resistance. PTFE-filled electroless nickel will give
low friction but high wear if the load is high.
Hard chrome plate for excellent wear resistance. Choose the thickness
according to the load.
Thermally sprayed metals or alloys for wear and corrosion
Thermally sprayed ceramics or cermets, for .example, WC-Co,
chromium oxide, alumina, and so forth, for maximum wear resistance
Nitriding or nitrocarburizing can produce a very high surface hardness. However, all corrosion resistance will be lost.
For aluminum or titanium alloy parts, consider:
•
•
•
•
•
•
Electroless nickel and composites for corrosive wear. Hardening at
400°C (750 OF) will provide additional wear resistance. Ceramicfilled electroless nickel will give greater wear resistance. PTFE- filled
electroless nickel will give low friction, but high wear if the load is
high.
Hard chrome plate for excellent wear resistance. Choose the thickness
on the basis of the load.
Thermally sprayed metals or alloys for wear and corrosion
Thermally sprayed ceramics or cermets, for example, we-co,
alumina, chromium oxide, and so forth, for maximum wear resistance
Anodizing for wear protection of aluminum alloys; can be sealed with
PTFE for reduced friction. Anodizing of titanium produces only a very
thin decorative layer.
Nitriding or nitrocarburizing for wear protection of titanium alloys
(requires high-temperature processing)
Bronze, Brass, and Copper Parts. Some copper and copper-base alloy
substrates have relatively low load-carrying capacity. The principal options to reduce wear are:
Practical Design Guidelines for Surface Engineering / 205
•
•
•
•
Electroless nickel and composites for corrosive wear conditions Heat
treating at 400°C (750 OF) will provide additional wear resistance.
Ceramic- filled electroless nickel will give additional wear resistance.
PTFE-filled electroless nickel will give low friction, but high wear if
the load is high.
Hard chrome plate for excellent wear resistance. Choose the thickness
according to the load.
Thermally sprayed metals or alloys for wear and corrosion
Thermally. sprayed ceramics or cermets, for example, WC-Co,
chromium oxide, alumina, and so forth, coatings for maximum wear
resistance
e
.
Specific Contact Conditions
Rolling and Rolling/Sliding Contact. In rolling-element bearings and
similar rolling components, the base material is usually a temper-sensitive
steel. Concern about the effect of surface engineering on tolerances and
possible distortion leave few options. In some circumstances, the following surface-engineering options may be considered:
•
•
•
PVD coatings, for example, TiN, CrN, MoS2, and so forth, but must
be processed at below the tempering temperature of the steel
Oxide treatments, for example, caustic treatment of needle rollers
Hard chrome plate, using the thin, dense variety (restricted to approximately 5 f.Lm thick)
For gears, where the motion is combined rolling and sliding, the main options are:
•
•
•
Case hardening, that is, carburizing or carbonitriding, of low-carbon
steels to give high hardness and wear resistance
Local surface hardening of medium-carbon steels to give maximum
load capacity
Nitriding of alloy steels for lower loads
Scuffing Conditions. Cams and tappets, cylinders and pistons, even
when lubricated, can be prone to scuffing. Options include:
•
•
•
•
Hard chrome plate for moderate-speed cylinder bores
Electrolytic nickel/ceramic composite for cylinder bores in highrevving engines
Nitrocarburizing for tappets and cams made from nitriding steels
Diamondlike carbon for high-revving cams and tappets on a polished
hard substrate
Reducing Friction in Dry Sliding. For any base material, certain polymer systems can be considered:
206 / Surface Engineering for Corrosion and Wear Resistance
•
•
•
•
•
PTFE in a binder, wet sprayed and cured
PFA wet sprayed and melt flowed at 400°C (750 OF)
MoS2 wet sprayed in a phenolic binder
Electroless nickel plus PTFE heat treated to give improved wear performance
Diamondlike carbon, but only on a hard, polished substrate
Sliding against Nonmetallic, Abrasive Counterfaces. This operating
condition might be the case for some plain or journal bearings or for mechanical seals. If the counterface is a polymer, it will probably contain an
abrasive filler. If the counterface is a ceramic or cermet, then its surface
roughness will greatly influence its abrasiveness.
For all of these conditions, high hardness must be the basis on which a
surface-engineering treatment is chosen. Examples include:
•
•
Hard chrome plate, which is the best of the electroplates. Electroless
nickel, even hardened is not recommended.
Sprayed ceramic or cermet, for example, chromium oxide. A ceramic
versus ceramic combination is possible.
Parts in Low-Load Sliding Contact with an Abrasive Product (Ref 1)
Many products are abrasive, either as a result of their basic structure and
composition or through the action of added fillers or pigments. In lowload situations (as defined by the product areas discussed later in this section), the choice of surface treatment can be made primarily on the basis
of surface hardness, since even very thin coatings are able to support the
contact loads.
The industrial areas covered in this section include textiles, printing,
plastics, packaging, food, pharmaceuticals, leather goods paints, inks, ceramic powders, and wood processing. It is assumed that the part in question is in direct contact with the product (e.g., a textile guide, a print roller,
a wood-cutting tool, a food chute, etc.) and not with another engineering
component.
The applications also cover seals, where a nonmetallic, for example, a
filled polymer or elastomer, part is in sliding contact with a shaft or a
thrust pad.
Base Material
The substrate will usually be mild steel, low-alloy steel, austenitic stainless steel, or an aluminum alloy. Tool steels will be used for knives or
other cutting or trimming tools.
Practical Design Guidelines for Surface Engineering / 207
Specific Applications
Chipboard, Wood, or Composite Products and Ceramic Powder
Handling. The content of wood products is always uncertain, with metal
and mineral contaminants being common. The only safe solutions are
thermally sprayed or welded ceramics or cermets, for example, WC-Co,
alumina, chromium oxide, and so forth, for maximum wear and damage
resistance.
Synthetic Textiles (Nylon, Polyester), Glossy Newsprint, GlassFilled Plastics (Including Seals), and Pigmented Plastics Other than
Black (i.e., Specifically White, Green, and Red). These are all abrasive
as they contain inorganic pigments or fillers. A surface hardness of at least
1000 HV is needed to ensure acceptable part lives. Applicable coatings include:
•
•
•
•
Thermally sprayed or welded ceramics or cermets, for example, WCCo, alumina, chromium oxide, and so forth, for maximum wear resistance
Nitriding may be used on austenitic stainless steel substrates to
achieve maximum hardness, but it destroys corrosion resistance.
Hard chrome plate will provide good wear resistance in applications
where the contact is not concentrated on one area of the part. It can be
used on textile feed rollers but not on eye-guides.
PVD coatings, for example, ceramics such as TiN orCrN
Slurry/sinter-formed ceramics, that is, chromium-oxide-based composites loaded with ceramic particles
Black and White Newsprint, Natural Textiles (Cotton, Wool),
Cardboard and Packaging, Carbon-fiber-Reinforced Plastics, BlackPigmented Plastics, Paints and Inks, Food Products, Leather, and
Pharmaceutical Products. These are mildly abrasive and require a surface hardness more than 600 HV for effective protection. Effective coatings include:
•
•
•
•
•
•
•
Thermally sprayed or welded ceramics orcermets, for example, WCCo, alumina, chromium oxide, for example, for maximum wear resistance
Nitriding or nitrocarburizing on any alloy steel substrate
Hard chrome plate will provide good wear resistance in all applications
PVD coatings, for example, TiN or CrN
Slurry/sinter-formed ceramics, that is, chromium-oxide-based composites loaded with ceramic particles
Case hardening, that is, carburizing, carbonitriding for low-carbon
steels to give high hardness and wear resistance
Local hardening, that is, induction, laser, and so forth, for mediumcarbon steels
208 / Surface Engineering for Corrosion and Wear Resistance
•
Anodizing of aluminum alloys (provides only limited protection and
is best for dry food products. under the lightest loads)
Parts in High-Load Sliding or Erosion with an Abrasive Product (Ref 1)
When abrasion takes place under high loads, and where impact occurs
and erosion is prevalent, then hardness alone is not a reliable parameter on
which to select the appropriate surface treatment. Applications include
coal chutes, mining conveyors, diggers, crushers, millers, extruders, cutters, and compactors. Erosive conditions also exist in turbines, impellers,
and pipework.
The surface must not only be hard, it must also be tough and resilient
and able to withstand high specific loading without deforming into the
substrate.
Base Material
The substrate is most likely to be constructional steel or low-alloy tool
steels in plate form.
Surface-Engineering Options
Suitable surface treatments for high-stress abrasive conditions include:
•
•
•
•
Welded or spray and fused coatings, including nickel or cobalt-base
materials with high carbide content, deposited at least 2 mm (0.08 in.)
thick
Thermally sprayed and hot isostatically pressed (HIP) coatings, including nickel and iron-base materials, with a high content of the carbides of tungsten, chromium, and titanium, deposited 5 mm (0.2 in.)
or more thick
High-velocity oxyfuel (HVOF) thermally sprayed coatings including
nickel or cobalt-base cermets with low porosity and high bond integrity, at least 1 mm (0.04 in.) thick
Elastomer-based coatings for high-angle erosive situations where
there is no abrasive, cutting element
Parts in Contact with Another
Engineering Component in the Presence of an Abrasive and
Corrosion Product or Environment (Ref 1)
When a component has surfaces that roll or slide against others with
abrasive and/or corrosive product trapped between them, it creates the
very extreme condition of three-body high-stress abrasive wear (see
Practical Design Guidelines for Surface Engineering / 209
Fig. 5 and 6 in Chapter 3). It is particularly common in pumps, valves, and
mechanical seals that are working in abrasive slurries such as sand, water,
andhydrocarbons found in oil and gas extraction. The condition is typified by a crushing and grinding action between the surfaces, perhaps the
two sliding surfaces of a journal bearing, which breaks down the abrasive
particles and continuously creates new cutting edges. When combined
with corrosion this creates a very extreme wear situation.
Base Material
The substrate is most likely to be austenitic or ferritic stainless steel,
high-alloy steels, nickel-base alloys (e.g., Inconels), or cast grades of Stellite (Co-Cr-W-C) materials.
Surface-Engineering
Options
Surface treatments for three-body high-stress abrasive wear and corrosion applications are limited to those which provide a combination of
hardness, toughness, load-carrying capacity, and corrosion resistance.
They include:
•
•
•
•
•
Welded or spray and fused coatings, including nickel or cobalt-base
materials with high tungsten, chromium, or titanium carbide content
Thermally sprayed and HIP coatings, including nickel and iron-base
materials with high tungsten, chromium, or titanium carbide content
HVOF coatings, including nickel or cobalt-base cermets with low
porosity and high bond integrity
Diffusion chromizing for combined corrosion and wear resistance, but
only on substrates with a sufficient carbon content to produce a surface layer of chromium carbide
Boronizing for high wear resistance of carbon and alloy steels, but
without appreciable corrosion resistance. Stellites, sintered cemented
carbides, and some sprayed coatings can be boronized to reduce wear
of their binder phases.
Hard chrome plate, provided the substrate is corrosion resistant and
the situation is not too aggressive; crevice corrosion can undermine
the plating
Preprocessing and Postprocessing Heat Treatment (Ref 1)
Heat treatments performed before or after surface processing are carried
out to:
•
•
•
Relieve residual stresses
Restore mechanical properties of the metal core
Reduce the risk of hydrogen embrittlement
210 / Surface Engineering for Corrosion and Wear Resistance
Stress Relieving. Residual stresses are left in components after manufacture, either from machining, casting, cold-forming, or forging operations. These residual stresses may cause distortion during subsequent
coating or surface treatments, and it is advisable to carry out a stressrelieving step prior to the final surface treatment. The following times and
temperatures are recommended for steels of varying strength levels:
Tensile strength
MPa
ksi
Stress-relief treatment
Up to 1100
1100-1650
1650-1800
Over 1800
Up to 160
160-240
240-260
Over 260
None required
1 h at 190-280 °C (375-535 OF)
18 h at 190°C (375 OF)
24 h at 190°C (375 OF)
Restoring Core Strength. If the surface hardening. or coating process
involves high temperature, the core strength or hardness of a steel component can be compromised. In the case of carburizing, particularly if carried out in a sealed quench furnace, the part will be quenched and tempered as part of the process, and the core properties (where the carbon
content will be lower than the case) will generally be restored. For hightemperature processes like boronizing or chromizing, there will need to be
a subsequent heat treatment step to reharden the core for most applications. After a CVD or carbide diffusion process on high-alloy tool steel,
there will usually need to be a vacuum heat treatment step to restore core
properties. These coatings are thin and depend on adequate support from
the substrate to perform properly.
Heat Treatment to Avoid Hydrogen Embrittlement. With most of the
electroplating processes, and in particular cadmium and hard chromium plating, and some of the chemical processes including electroless nickel, there is
a risk of hydrogen embrittlement of high-strength steel components. It is essential to carry out a postprocessing heat treatment immediately after plating.
The recommended treatments for steels of varying strength levels are:
Tensile strength
MPa
ksi
Heat treatment
Up to 1100
1100-1650
1650-1800
Over 1800
Up to 160
160-240
240-260
Over 260
None required
190-230 °C (375-450 OF) for 2 h
190-230 °C (375-450 oF) for 6 h
190-230 °C (375-450 OF) for 8 h
Coating Thickness, Case Depth, and
Component Distortion Considerations (Ref 1)
The thickness of a surface coating or case depth is governed by both
the process characteristics and the cost. For example, in theory it would
Practical Design Guidelines for Surface Engineering 1211
be possible to build up a PVD coating thickness of 100 urn, but it would
take so long as to be both impractical and too expensive. From the user's
point of view, it is important to know the thickness of the surface layer
so that its load-carrying capacity and potential service life can be assessed and any likely changes in dimensions of the component predicted. Assuming that the final dimensions of the part are critical, below
are some guidelines to help the designer predict coating thickness and
potential distortion problems. In addition, Table 1 lists the thickness
ranges and hardness values for a wide range of coating/surface-hardening processes.
Weld overlays will produce significant distortion of the part and a surface growth at least equal to the layer thickness. They will need to be surface ground.
High-temperature diffusion processes such as carburizing can produce component distortion. The only option is to allow for postgrinding
Table 1 Thickness ranges and hardness levels associated with various surfaceengineering processes
Treatment
Local surface hardened
Carburized (case hardened)
Nitrided or nitrocarburized
Boronized
Chromized
Aluminized (diffusion)
Phosphated
Chromated
Oxidized
Ion implanted
PVDTiN
PVDCrN
Diamondlike carbon
CVD chromium nitride
CVD chromium carbide
CVD alumina
Chromium plate
Nickel plate
Copper plate
Cadmium plate
Zinc plate
Electroless nickel
Electroless nickel/ceramic
Hot dip galvanizing
Electrogalvanized
steel strip
Hot dip aluminized steel strip
Thermally sprayed chromium
oxide
Thermally sprayed alumina
Thermally sprayed tungsten
carbide/cobalt
Thermally sprayed and spray
and fused chromium
carbide/nickel-chromium
Slurry/sinter-formed
ceramics
PVD, physical
vapor deposition;
Substrate
Thickness or case depth
Medium-carbon steel
Low-carbon steel
Low-carbon steel
Tool steel
Stainless steel
Mild steel
Low-alloy steel
Stainless steel (316)
Stainless steel (316)
Stainless steel (316)
Low-carbon steel
Various
Steel
Steel
Various
Various
Various
Stainless steel (316)
High-carbon steel
Steel
Various
Various
Various
Various
Various
Various
Various
Steel
Low-carbon steel
Low-carbon steel
Various
1-10mm
1-3mm
5-10 urn
50--200 urn
20-50 urn
10-20 f.Lm
20-30 urn
30-40!Lm
20-50 urn
20-50 urn
4-7 urn
1 or 2 urn
3-5 urn
0.1-1 urn
1-5 urn
2-20 urn
lor 2!Lm
10-15 urn
10-15 urn
5-10 urn
5-250 urn
lfl um to 1 mm
10-250 um
5-10 J.Lm
5-10 urn
5-50 urn
5-50 J.Lm
20-250 J.Lm
5-10 urn
5-10 urn
20-100 urn
CVD, chemical
Hardness, HV
700-900
700--900
400-600
800--1000
1000-1200
500-700
800-1000
1000-1200
300-400
400-500
-200
Not accurately known
250-350
Not accurately known
2000-3000
1800-2500
1500-2000
1100-1300
1500-2000
1500-2000
800--1000
250-650
70-90
-50
-50
500-1000
<1300
70-250
-70
-70
1200-1600
Various
Various
20-100
20-100
urn
urn
1500-1800
1100-1600
Various
Up to 1 mm
1000-1100
20-100
1000-1200
Steel
vapor deposition.
Source: Ref 1
urn
212 I Surface Engineering for Corrosion and Wear Resistance
to size. Case depths in the range 200 to 2000 urn allow for such finishing. The same principles apply to local surface hardening such as induction or flame hardening, provided the depth of hardening is sufficient.
CVD and carbide diffusion processes will produce some distortion
and growth of 50 to 100% of the coating thickness. There is insufficient
coating thickness (up to 20 urn, usually less) to allow for postcuring
grinding. Normally such processes are applied to tooling where trials and
experience prove that dimensional accuracy can be consistently maintained at acceptable levels.
Nitriding and nitrocarburizing produce only minimal distortion,
with a small surface growth (a few microns). Such processes often produce a thin "compound" layer (10 urn) on the surface of the main case
(200-500 urn), and this is usually removed by a finish-grinding operation.
Thermal spray processes impart little general heat to the part and,
therefore little distortion. The surface growth will equal the coating thickness and, if the finish is important, they will need grinding. Spray and
fused deposits, or coatings that are HIP after spraying, will grow and distort from the effects of high temperature. They will require grinding to improve surface finish.
Slurry-based ceramic coatings are sintered at high temperature and
experience surface growth equal to the coating thickness (typically
10-100 urn) that may cause some distortion. These coatings are normally
left unfinished.
Coatings for corrosion protection, for example, zinc or cadmium
plating, phosphating, and chromating will produce surface growth in the
range of 2 to 20 urn. They cannot be finished after processing. Electroless
nickel will produce growth equal to the .coating thickness (typically
10-100 urn) with no distortion unless the substrate is sensitive to the heat
treatment temperature of 400 DC (750 OF).
Hot dipped galvanized coating thickness ranges from 10 to 250 urn
and is controlled by the steel chemistry, section thickness, and immersion
time.
Electrolytic Coatings. Hard chrome and heavy nickel and copper plating can vary in thickness from just a few microns to 250 urn, ·Surface
growth will equal the coating thickness and, except for the thinnest layers,
they will need finish grinding.
PVD coatings are usually less than 10 urn thick and will produce minimal distortion. They are not finished after processing.
Paints and polymer coatings are usually around 10 to 30 urn thick.
They are left as-coated.
Anodizing produces no distortion. The surface growth is half that of the
coating thickness, the coating growing 50% in and 50% out of the original aluminum alloy surface.
Practical Design Guidelines for Surface Engineering / 213
Table 2
Surface finish characteristics of various surface-engineering
Process
processes
As-treated surface finish
Normal finishing operation
Overlays
Thermal spray
Local hardening
Case hardening
Nitriding and nitrocarburizing
Very rough
Very rough
May be rough and distorted
May be rough and distorted
Slightly roughened
Galvanizing
Phosphating
Oxidizing
Electroplating
Slight roughening
Slight roughening
Slight roughening
May roughen
Electroless nickel
PVD
CVD
Ion implantation
Shot peening
Paints and polymers
Replicates surface finish
Replicates surface finish
Some roughening
Replicates surface finish
Deliberate alteration
Some roughening possible
Grind
Grind
Grind
Grind
May be ground, but often used
as-treated
Not finished
Not finished
Not finished
Chromium and copper usually
ground, cadmium and zinc
used as-plated
Not finished
Not finished
Not usually finished
Never finished
Not finished
Not finished
PVD, physical
vapor deposition;
CVD, chemical
vapor deposition.
Source: Ref 1
Surface Roughness and Finishing (Ref 1)
The surface finish of the surface-engineered component will depend on
the process itself and, in some instances, on the finish before it was
processed. As described in the previous section, some parts will have to be
ground after treatment because of distortion or growth, or to develop an
acceptable finish. Others will be left untreated, regardless of their surface
roughness. In some cases, the primary objective will be to preserve original surface texture without the need for postfinishing operations. Table 2
gives some general surface-finish guidelines relevant to the various surface-engineering treatments.
General Design Principles Related to Surface Engineering (Ref 2)
There are a number of general design principles that apply to a variety
of surface-engineering processes, while others are specific to individual
treatments/techniques. These general principles are discussed in this section, and the following three sections discuss design aspects relating to:
(1) surface preparation techniques, including cleaning, (2) organic coating
processes, and (3) inorganic (metal and ceramic) coating processes.
Fabrication Processes. Some methods of fabrication such as the forging,
extrusion, molding, and casting of metals and ceramics can lead to surface
defects that must be removed by subsequent surface-finishing techniques,
such as grinding, lapping, and polishing or electropolishing, or hidden by
techniques such as applying a leveling copper deposit before a decorative
plated finish. Defects include laps, tears, cracks, pores, shrinkage cavities,
214 / Surface Engineering for Corrosion and Wear Resistance
• Functional
requirements
• Design constraints
- Material(s)
-Size
-Weight
• Tolerances
- Pretreatment
- Posttreatment
• Ease of rework
(stripping)
Fig. 1
• Part orientation
• Movement (agitation)
• Masking
• Shielding
• Manual operation
• Automatic operation
• Material(s)
.
• Size
• Weight
• Tolerances
• Ease of use
• Cleaning (stripping)
Interrelation between the component,
tions. Source: Ref 2
•
•
•
•
•
•
•
•
•
•
•
Process selected
Design restrictions
Modifications
Ease of control
Batch operation
Continuous operation
Single/mixed parts
Accuracy (calibration)
Reliability
Reproducibility
Maintainability
fixturing, and equipment limita-
gating and venting residues, ejection marks, and, parting lines. Careful design of the casting or molding operation-including
the dies, gates, vents,
and overflows-will minimize finishing problems by ensuring such defects
are avoided, occur on nonsignificant surfaces, or are hidden by specially incorporated design features, such as steps or ridges at parting lines.
Whatever the type of material being cast or molded, dimensional and
warpage allowances must be made in the design of the tooling (i.e., dies)
to accommodate shrinkage and distortion during solidification and cooling. Otherwise, parts may be undersized or require excessive machining
to obtain the specified dimensional tolerances.
Control of fastening or joining processes also can influence surface finishing. For example, two flat surfaces riveted together produce cavities
that can entrap processing solutions, impair coating, and lead to corrosion
(Ref 3). Spot or tack welding is no better in this regard. However, a continuous weld-with a smooth bead and no weld spatter-will prevent this
problem and make surface finishing easier. Also, the elimination of sharp
edges and comers will prolong the life of grinding, polishing, and buffing
belts and wheels.
Component Size and Weight and Handling Problems. The size, dimensions, and weight of a part to be surface engineered have a direct influence on part handling and fixturing and the size and type of equipment
that is used (Fig. 1). Put simply, there are two main issues in relation to
size and weight of components:
•
Are they too big for the process to accommodate, either in respect to
the pretreatment surface preparation/cleaning
facilities or plating
tanks, vacuum chambers, and the like?
Practical Design Guidelines for Surface Engineering / 215
•
Are they so small, or too numerous, to make the holding, manipulating, or cleaning for the chosen process impractical or too expensive?
Objects weighing in excess of 20 kg (about 50 lb) will probably need
hoists or overhead moving cranes to manipulate them through the cleaning lines and treatment chambers. Some heavy objects that are to be
treated in front-loading furnaces can often be handled by a fork lift. Table
3 provides some likely limits on size and weight for various surfaceengineering processes.
Aesthetics and Function. Another general consideration is that not all
surfaces may require the same high standard of surface finish. While surfaces exposed to view must be aesthetically pleasing, and surfaces subjected to more aggressive conditions of exposure or use require durable
coatings, hidden (internal) surfaces or less-exposed surfaces may not need
such a high-quality finish. Specifications for surface finishes for a part depend not only on the design and end-use application, but also must take
into account that the requirements may differ for different areas or surfaces on that part. A design should take this into consideration, as well as
the fact that different types of equipment or equipment operation settings
may be necessary for those areas and surfaces.
Functional requirements of a part also influence the selection of surfacepreparation processes. For example, grinding processes can introduce
stresses that could have a negative impact on fatigue properties. Choosing
an alternative process, such as chemical milling, or mitigating the stresses
by shot peening can alleviate the problem.
Design Features. Shape and features such as recesses, holes, threads,
keyways, slots, fins, and louvers can present problems to the finisher, and
the severity of the problem can depend on the finishing technique. For example, when holes are included in thin sections that require a finishing
Table 3
Size and weight limitations for various surface-engineering
processes
Process
Largest dimension restraint
Weight restraint
Small parts
Overlays
None, assuming access
Not less than about 100 nun
Thermal spray
Local hardening
Around2m
None, assuming access
Shot peening
Case hardening
Nitriding and
nitrocarburizing
Galvanizing
Phosphating
Oxidizing
Electroplating
Electroless nickel
PVD,CVD
None, assuming access
Around 3 m
Around 3 m
None, particularly if on-site
work is possible
Several tonnes
None, particularly if done
on-site
Often done on-site
About 1 tonne
About 1 tonne
Around 30 m
Around 5 m
Around 1 m
Around 3 m
Around 1 m
Around 1-3 m, usually
smaller
Around 1 m
None, assuming access
10-15 tonnes
Several tonnes
About 1 tonne
About 5 tonnes
About 0.5 tonne
About 0.5 tonne, usually
lighter
About 0.5 tonne
None assuming access
Ion implantation
Paints and polymers
PVD, physical
vapor deposition;
CVD, chemical
vapor deposition.
Source: Ref 1
Down to 10 nun
Not less than about 100 nun
Not less than about 100 nun
Not less than about 10 nun
Not less than about 5 nun
20 nun or M8 fastener
Not less than about 1 m
Down to 1 nun
Down to 10 nun
Down to 5 nun
Down to 10 nun
Down to 10 nun
Down to about 10 nun
216 / Surface Engineering for Corrosion and Wear Resistance
operation such as grinding, if too much pressure is applied edges and
comers might be chipped. If only a light pressure is used to avoid this possibility, then the desired finish might not be obtained. Another example is
when paint is applied by conventional solvent spraying or when a part is
electroplated, bowl-shaped recesses, blind holes, and similar features can
trap the paint or plating solution, leading to areas that sag or do not cure
properly (in the case of paint), or carry over trapped chemicals to subsequent processing steps (in electroplating). The latter can cause problems
such as rinse-water contamination and increased waste-treatment costs.
Also, solutions that are trapped can lead to blistering or delamination of
the plated coating, especially if there is a posttreatment step that requires
the part to be heated (such as for electroless nickel, cadmium, and hard
chromium deposition).
For parts that will be sprayed, especially with paint, .another problem
with deep recesses, closely spaced, large fins or partitions, and the like is
the entrapment of air. The back pressure of entrapped air causes incomplete coverage at the bottom of the recesses. One way to avoid this problem, if a change of design is not possible, is to use an "airless" spraying
technique (Ref 4). During electrostatic powder coating there is the problem associated with "Faraday cage" effect, in which the charged components of the powder- coating system are attracted by the high fields at the
edges and comers of parts, causing excessive coverage there and incomplete coverage in other areas (Ref 5), as shown in Fig. 2. Rounding corners and edges, tapering the sides and decreasing the depth of recesses,
minimizing the use of louvers or fins, or changing their dimensions are
ways to avoid the Faraday cage effect. In electroplating, a similar phenomenon exists whereby the depositing metal or alloy ions are attracted to
the high-current-density areas at edges and comers, and thicker coatings
are obtained in those locations. Rounding such edges, changing dimensions to allow for the excessive buildup, or using shields and current "robbers" or "thieves" will help the finisher to obtain the desired coating
thickness distribution. Reference 6 provides some examples of the use of
such devices.
Coating
Fig. 2
Faraday cage effect in powder coating. Adapted from Ref 5
Practical Design Guidelines for Surface Engineering / 217
Accessible
outside diameter
Beam or
spray
Accessible
inside diameter
Accessible
Beam or
spray
Beam or
spray
outside diameter
~
Inaccessible
inside diameter
Fig. 3
Some examples of line-of-sight
limitations
in spraying or ion-beam coating processes
In conventional paint spraying and many vacuum-deposition techniques, such as ion plating, ion implantation, PVD, and sputtering, attention has to be paid to the limitations imposed by the "line-of-sight" deposition process. Certain features, such as ridges, flanges, and fins, can
shadow or mask areas behind them leading to incomplete or nonuniform
coverage, as shown in Fig. 3 and 4. Similarly, if the aspect ratio of holes
and recesses is too high (i.e., the depth is much greater than the diameter
of the opening), it is not possible with line-of-sight limited techniques to
penetrate to the bottom surfaces and coat them (Fig. 4). Decreasing the
Beam or spray
Cross section of part
Fig. 4
Design features that cause shadowing
line-of-sight limitations
or poor coverage because of
218 / Surface Engineering for Corrosion and Wear Resistance
aspect ratio, providing rounded edges, and tapering the sides of ridges and
fins or holes will help to facilitate finishing, as will lowering the height of
features such as fins. Of course, rotating or translating a part in the spray
plume also will help to obtain complete and more uniform coverage, but
this approach usually requires longer times and more sophisticated finishing equipment and fixturing; hence, it often leads to higher costs. The
same can be said for using multiple line-of-sight sources to obtain better
coverage.
Finally, as a general rule of thumb, parts of the same size, weight, design, and material should always be finished at the same time so that the
finishing process(es) can be optimized for those parts. Batches of mixed
parts should be avoided unless they share some common features, such as
shape and substrate material.
Design Guidelines for Surface Preparation Processes
Surface preparation, including cleaning, is the essential first step in all
successful surface-engineering practice. To facilitate surface preparation
prior to subsequent coating operations, there are a number of design features that must be considered. Abrupt changes in surface contours should
be avoided, and features such as fine grooves, recesses, surface patterning,
blind holes, and reentrant areas should be avoided because they will be inaccessible to polishing media or would trap polishing media, making subsequent cleaning more difficult. Such features also would entrap cleaning
chemicals, making rinsing more difficult, or could possibly entrap air,
preventing cleaning of these areas.
Sharp comers and edges or protrusions can cause excessive wear of polishing wheels and belts and lead to uneven polishing because the high
areas are polished at the expense of the surrounding lower areas. As mentioned earlier, rounding edges and corners is a good design precept, while
minimizing the height of protuberances is beneficial, as is decreasing the
aspect ratio of holes, grooves, and recesses.
Large expanses of flat surfaces may be a problem if these are significant
surfaces, especially if these surfaces must be polished to a reflective, mirrorlike finish. Imperfections are exaggerated. Minimizing the area of such
surfaces and providing a slightly rounded contour will help to attain the
desired finish and help with visual appearance.
Simpler designs lend themselves to automatic finishing processes, while
more complex designs may require manual surface-preparation techniques. If parts are to be mass finished (e.g., by tumbling or vibratory finishing) significant flat areas should be avoided. Otherwise, parts may stick
together, and these occluded surfaces will not be finished. Designs that
prevent access by the deburring or polishing media (such as small recesses
Practical Design Guidelines for Surface Engineering / 219
and holes) or that entrap the media (such as narrowly spaced ribs) should
be avoided as mentioned above.
When it is impractical or impossible to use mechanical polishing, chemical etching, chemical milling, or electropolishing can be used. The design
principles for the latter are similar to those for electroplating, which is discussed later. In electropolishing, the workpiece is the anode, which is the opposite of electroplating. Current-density distribution is extremely important,
as is the original surface of the pan being electropolished. In high-currentdensity areas on susceptible materials, the surface layers may be removed
and etching of the substrate can occur. Polishing occurs on a microscopic
scale, so macro features such as large grooves or scratch marks will not be
removed, but will receive a luster and become more noticeable. Similarly,
parting lines can be smoothed, but not removed; therefore, parting fines must
be minimized by good die design and careful molding operations.
Solvent cleaning is a fairly forgiving surface-finishing process, but part
design can influence its efficacy, as already alluded to. If agitation or other
cleaning aids are used, such as ultrasonic energy, care must be taken to prevent soft materials or thin and fragile features or cross sections from being
damaged. The energy released during cavitation, for example, in ultrasonic
cleaning is very large. If techniques such as plastic media blasting are used,
the blasting parameters should be tailored to the part material and design,
and the part should be designed to allow easy access by the media and easy
removal of the media once the desired finish (cleanliness) is obtained.
If a power spray washing technique is used, the part design should allow
for proper drainage to conserve chemicals and minimize carryover to the
next process step. Providing drainage holes may be necessary. These
should be either a natural feature of the design or located on nonsignificant surfaces. As the design of a part becomes more complex, rinsing requirements become more stringent, and several rinsing stages may be necessary. If an air knife is used afterward to remove excess water, the part
must be capable of withstanding the pressure or must be fixtured such that
the air pressure does not distort any delicate design features while holding
the part steady.
Table 4 provides a summary of the design limitations of some surfacepreparation and cleaning processes and indicates which design features to
avoid.
Design Guidelines for Organic Coating Processes (Ref 2)
Organic coatings are applied by a variety of techniques, such as dipping,
brushing, spraying, airless spraying, or electrostatic spraying. In addition,
some primers are deposited using electrophoretic techniques, while electropolymerization is being looked at for certain types of organic coatings.
220 / Surface Engineering for Corrosion and Wear Resistance
Table 4
Summary of design limitations for selected surface-preparation
Process
Blasting/deburring
Broachinglhoning
Brushing/bumishing
Chemical
processes
Design limitations
milling
Conversion
coating
Electrocleaning
Electropolishing
Etching
Grinding
Lapping/buffing
Pickling
Polishing
Solvent cleaning, immersion
Solvent cleaning, ultrasonic
Avoid recesses, holes, channels, and similar features (such as closely spaced ribs) that could trap blasting media
Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by the blasting media
Avoid intricate designs and surface features
Typically used for inside diameters of tubes and other cylindrical parts, or for grooves, large holes, and other
cavities
Surfaces must be accessible to tools and withstand the local pressure and heat buildup
Avoid very thin cross sections/wall thickness
Surfaces must be accessible to tools and withstand the local pressure and heat buildup
Avoid very thin cross sections/wall thickness that could deflect
Avoid sharp comers and edges
Avoid intricate designs and surface features
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory
rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air or evolved gases and prevent chemical action from occurring or cause uneven
attack
Mask areas not to be attacked
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory
rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air and prevent surface chemical reactions from occurring or cause staining
Mask areas not to be attacked
Allow for electrical contact to be made on nonsignificant surfaces
Avoid features that would trap process chemicals or prevent satisfactory rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air or evolved gases and would prevent cleaning from occurring or cause staining
Allow for electrical contact to be made on nonsignificant surfaces
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory
rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air or evolved gases and prevent polishing action from occurring or cause staining
Mask areas not to be attacked
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory
rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air and prevent etching action from occurring
Avoid sharp corners and edges
Avoid shallow intricate designs and surface features
Mask areas not to be attacked
Surfaces must be accessible to tools and withstand the local pressure and heat buildup
Avoid very thin cross sections/wall thickness
Avoid sharp corners, edges, and protuberances
Avoid intricate designs and surface features
Surfaces must be accessible to tools (preferably flat or simple, curved contours
Avoid very thin cross sections/wall thickness that cannot withstand the local pressure and heat buildup
Avoid sharp comers and edges
Avoid intricate designs and surface features that would trap the lappinglbuffing compounds
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory
rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air and prevent pickling action
Avoid flat surfaces on small parts that could stick together, exclude the acid, and prevent the pickling action
Surfaces must be accessible to tools and withstand the local pressure and heat buildup
Avoid very thin cross sections/wall thickness
Avoid sharp corners, edges, and protuberances
Avoid intricate designs and surface features that could trap the polishing compound
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory
rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air and prevent cleaning from occurring
Avoid flat or curved surfaces on small parts that could stick together during immersion and prevent cleaning of
those surfaces
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory
rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air and prevent cleaning from occurring
(continued)
Source: Ref 2
Practical Design Guidelines for Surface Engineering / 221
Table 4
(continued)
Process
Design limitations
Solvent cleaning,
ultrasonic (continued)
Stripping, chemical
Stripping, mechanical
Stripping, thermal
Avoid thin cross sections that could be damaged by the energy released during cavitation
Avoid features (e.g., small recesses, blind holes, cavities) that would trap smut and process chemicals or prevent
satisfactory rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air and prevent coating removal from occurring
Mask areas not to be attacked
Avoid recesses, holes, channels, and similar features that could trap blasting media
Avoid thin cross sections or intricate designs that could be damaged by the stripping media
Mask areas not to be attacked
Avoid thin cross sections or intricate designs that could be distorted by the thermal cycling
Try to provide uniform cross-sectional mass throughout the part to help provide a uniform temperature distribution
during heating
Source: Ref 2
Table 5 summarizes these techniques and the design limitations associated
with each.
Most of the techniques are line-of-sight limited, and the guidelines provided in the previous section, "Design Guidelines for Surface-Preparation
Techniques," will apply. Allowance for drainage is important for processes
that involve dripping or spraying. Avoiding sags and runs on large, flat,
vertical surfaces can be accomplished by applying good coating practices
and by minimizing such surfaces in the design of the part.
Table 5
Summary of design limitations for selected organic coating processes
Process
Electrocoating
Electropolymerization
Painting, brushing or dipping
Painting, solvent spraying
Powder coating
Sol-gel coating
Solution coating
Source: Ref 2
Design limitations
Allow for electrical contact to be made on nonsignificant surfaces
Avoid features that could trap air and prevent wetting by process solutions
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle
Allow for electrical contact to be made on nonsignificant surfaces
Avoid features that could trap air and prevent wetting by process solutions
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle
Surfaces must be accessible to application tools (preferably flat or simple, curved contours)
Avoid features that would trap excess paint
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air and prevent coating from occurring
Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle
Surfaces must be accessible to application tools (preferably flat or simple, curved contours)
Allow for fixturinglracking on nonsignificant surfaces
Avoid features that would trap excess paint
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air and prevent coating from occurring
Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle
Allow for fixturing/racking on nonsignificant surfaces
Allow for electrical contact to be made on nonsignificant surfaces
Avoid deep recesses and blind holes that cause the "Faraday cage" effect
Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle
Allow for fixturinglracking on nonsignificant surfaces
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals
Avoid thin cross sections or intricate designs that could be distorted by the thermal cycling
Try to provide uniform cross-sectional mass throughout the part to help provide a uniform temperature
distribution during heating cycle
Allow for fixturing/racking on nonsignificant surfaces
Avoid features (e.g. small recesses blind holes cavities) that would trap process chemicals
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover
Avoid features that could trap air and prevent coating from occurring
Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle
222 / Surface Engineering for Corrosion and Wear Resistance
A few organic coating techniques use electric or electrostatic fields. Designing the fixtures and electrical grounding, such that points of contact
are on nonsignificant surfaces, will improve the appearance of the coated
part and ..give the impression of a better quality product. With spraying
techniques, proper fixturing and racking of parts can improve the use of
coating material because less empty space exists during a run. However,
the parts should not be racked so closely together that they shield some
surfaces and prevent some areas from being coated.
Avoiding thin cross sections and good fixturing will help prevent distortion during the curing and baking steps used after paint or powder is applied.
Optimizing a design for surface finishing, such as painting, becomes very
important as coating thickness is reduced to 30 urn or less. Access to all
surfaces must be possible, and any features that would prevent this should
be avoided. This is because the dimensions of the solid components in the
coating formulation (e.g., powder particle) are similar to the dimensions of
the desired dry film thickness (Ref 7). For example, during the first part of
curing, when the particles liquefy, the surface tension of the film formed
will tend to pull it away from sharp comers or edges, resulting in poor coverage. If a design modification is not possible, the powder formulation
should be changed to include higher-viscosity resins, and no, or only small
amounts, of surfactants (Ref 7). Thin-film coatings are best applied to parts
with simple geometries, with flat or curved surfaces, and few sharp edges.
Earlier, the problem with the Faraday cage effect was mentioned. This
phenomenon is further complicated by back-ionization with traditional corona-charging systems (Ref 5). Not only does the design of a recess, hole,
or channel control the distribution of coating thickness, but the buildup of
back-ionization at the areas of high field intensity lowers the effective
charge of the powder particles, further reducing their ability to reach the
bottom surfaces. Some possible design modifications were mentioned earlier, but if these are not possible, changing to a turbocharging system will
help. Back-ionization is greatly reduced, and the absence of free ions between the gun and the part promotes better coverage of all surfaces (Ref 5).
Design Guidelines
for Inorganic Coating Processes (Ref 2)
Inorganic finishes-including
metal- and ceramic-based coatings-are
applied by a variety of techniques, such as electroplating, electroless plating, thermal spraying, hot dipping, and various vapor-deposition techniques. Other techniques, such as ion implantation and laser melting/alloying, modify surface properties. Table 6 summarizes design limitations
for these and other types of inorganic coating processes.
Electroplating is widely used in industry to apply inorganic coatings, especially metals and alloys. Like some organic finishing processes, satisfactory coatings are only obtained when a uniform current density can be
Practical Design Guidelines for Surface Engineering / 223
Table 6
Summary of design limitations for selected inorganic coating processes
Process
Design limitations
Anodizing
Cementation/diffusion
Cladding
Electroless
plating
Electrophoretic
plating
Electroplating
(plating, electrodeposition)
Hot dipping, galvanizing
Inorganic painting, slurry coating
Ion implantation
Ion plating
Allow for electrical contact to be made on nonsignificant surfaces
Avoid, if possible, sharp edges and comers, ridges, blind holes, etc. that would prevent uniform
density distribution
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize
carryover
Avoid features that could trap air and prevent electrochemical reactions from occurring
Avoid features that could trap evolved gases and cause staining
Mask areas not to be anodized
Surfaces must be thoroughly deburred and cleaned before cladding, so design principles for these
processes also apply
Avoid thin cross sections or intricate designs that could become distorted during thermal cycling
Mask areas not to be coated
Only for relatively simple shapes, especially with flat surfaces
Surfaces must be thoroughly cleaned before cladding, so design principles for cleaning also
apply
Allow for fixturinglracking on nonsignificant surfaces
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or
prevent satisfactory rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize
carryover
Avoid features that could trap air and prevent chemical reactions from occurring or cause
staining
Mask areas not to be coated
Allow for electrical contact to be made on nonsignificant surfaces
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or
prevent satisfactory rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize
carryover
Avoid features that could trap air and prevent surface chemical reactions from occurring or cause
staining
Mask areas not to be coated
Allow for electrical contact to be made on nonsignificant surfaces
Avoid, if possible, sharp edges and comers, ridges, blind holes, etc., that would prevent uniform
current density distribution; or use current robbers and/or shields
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or
prevent satisfactory rinsing
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize
carryover
Avoid features that could trap air and prevent deposition from occurring
Avoid features that could trap evolved gases and cause staining
Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by internal stress
in the coating
Mask areas not to be coated
Allow for fixturinglracking on nonsignificant surfaces for discrete, small parts
Best for relatively simple shapes (e.g., tubing) and flat surfaces
Allow for excess coating material to drain quickly
Allow for doctor blades or air knives to be used to obtain uniform coating thickness
Avoid thin cross sections that could become distorted during thermal cycling
Surfaces must be accessible (preferably flat or simple, curved contours)
Allow for fixturing/racking on nonsignificant surfaces
Avoid features that would trap excess paint
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize
carryover
Avoid features that could trap air and prevent coating from occurring
Avoid thin cross sections or intricate designs that could become distorted during drying/fusing
cycle
Mask areas not to be coated
Allow for electrical contact to be made on nonsignificant surfaces or use a conductive screen
Avoid features that would shield the surface from the beam (line-of-sight limited) unless multiple
beams are used or part is rotated/translated
in beam
Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the beam to reach
the bottom surfaces
Mask areas not to be coated
Allow for electrical contact to be made on nonsignificant surfaces or use a conductive screen
Avoid features that would shield the surface from the beam (line-of-sight limited) unless
multiple beams are used or part is rotated/translated
in beam
(continued)
CVD, chemical
vapor deposition;
PVD, physical
vapor deposition.
Source: Ref 2
224 I Surface Engineering for Corrosion and Wear· Resistance
Table 6
(continued)
Process
Design limitations
Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the beam to reach
the bottom surfaces
Mask areas not to be coated
Allow for fixturing/racking on nonsignificant surfaces
Avoid features that would shield the surface from the laser beam (line-of-sight limited) unless
multiple beams are used or part is rotated/translated
in beam
Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the beam to reach
the bottom surfaces
Avoid thin cross sections or intricate designs that could be damaged by local heating during
glazing
Mask areas not to be treated
Allow for fixturing/racking on nonsignificant surfaces on large parts
Avoid features that could trap air and prevent activation by the process chemicals from occurring
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize
carryover of activating solutions
Avoid recesses, holes, channels, and similar features that could trap peening media
Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by the peening
action
Avoid sharp edges and comers that could be damaged by the peening media
Avoid intricate designs and small surface features that cannot be reached by the peening media
Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize
carryover
Mask areas not to be coated
Allow for fixturing/racking on nonsignificant surfaces
Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or
prevent satisfactory rinsing
Avoid features that could trap air and prevent surface chemical reactions from occurring or cause
staining
Mask areas not to be attacked
Allow for fixturing/racking on nonsignificant surfaces
Design should allow for surface roughening to promote adhesion, so blasting design precepts
also apply
Avoid features that would shield the surface from the spray (line-of-sight limited) unless multiple
sprays are used or part is rotated/translated
in spray plume
Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the spray to reach
the bottom surfaces
Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by the local heating
and kinetic energy
Mask areas not to be coated
Allow for fixturing/racking on nonsignificant surfaces
Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by heating, if
needed prior to coating deposition
Vacuum processes are line-of-sight limited, so similar design precepts to those for ion plating
will apply
Mask areas not to be coated
Ion plating (continued)
Laser melting/alloying
Mechanical
(peen) plating
Passivation
Thermal spraying
Vapor deposition
CVD, chemical
(CVD, PVD)
vapor deposition;
PVD, physical
vapor deposition.
Source: Ref 2
established on all surfaces to be finished. Phenomena like the Faraday cage
effect occur when design features prevent the establishment of a uniform
current density distribution. As mentioned earlier, techniques relating to
fixturing and racking can alleviate some of the problems. General design
approaches are discussed in Ref 6 and 8 and summarized in Table 7.
With the plating of fasteners, some special considerations apply, particularly in respect to threads (Ref 8). As might be expected, electroplated
metals build up faster on apexes of the threads, and coverage can be minimal at the bottom of the grooves. ANSI Specification B 1.1 states that
compared to flat surfaces, plating thickness builds up six times faster on
the major diameter than the minor diameter and that this results in a fourfold buildup on the pitch diameter, as illustrated in Fig. 5. This is known
as the "Rule of Four and Six."
Practical Design Guidelines for Surface Engineering / 225
Table 7 Influence of substrate design features on electroplateability
Poor design
Feature
Convex
surfaces
Influence on electroplateability
Better design
Ideal shape. Easy to plate
uniformly.
especially
where
edges are rounded
Flat surfaces
Sharply
angled
edges
Flanges
tI!;<;;;;;;;;;U;
Use a 0.4 mm (0.015 in.) crown to
minimize undulations
caused by
uneven buffing.
Undesirable.
Reduced coating
thickness
at center areas requires
increased
plating time to obtain a
minimum thickness of durable
electroplate.
All edges should be
rounded.
Edges that will contact
painted surfaces should have a
minimum radius of 0.8 mm W.03
in.).
Large flanges with sharp inside
angles should be avoided to
minimize plating costs. Use a
generous
radius on inside angles
and taper the abutment.
Slots
Narrow. closely spaced slots and
holes reduce electroplateability
and cannot be properly plated
unless corners are rounded.
Blind holes
Must usually be exempted
from
minimum thickness
requirements.
Where necessary.
limit depth to
50%;· of width. A void diameters
of less than 6 mm (0.24 in. L
Sharply
angled
indentations
Increase plating time and costs for
a specified minimum thickness.
and reduce the durability of the
plated part.
Flat-bottom
grooves
Inside and outside angles
rounded generously.
V-shaped
grooves
Deep V -shaped grooves cannot be
satisfactorily
plated and should
be avoided. Shallow. rounded
grooves are better.
Fins
Increase plating time and costs for
a specified minimum thickness
and reduce the durability of the
plated part.
Ribs
Narrow ribs with sharp angles
usually reduce electroplateability:
wide ribs with rounded edges
pose no problem. Taper each rib
from its center to both sides and
round off edges. Increase spacing
if possible.
be
Concave
recesses
Electroplateability
dimensions.
Deep scoops
Increase plating time and costs for
a specified minimum thickness.
Spearlike
Buildup on jut will rob corners
from their share of electroplate.
Crown the base and round off all
corners.
Rings
juts
depends
should
€«««««@
ctt$$$$~
I
I
I
cSS$S'Q
on
Electroplateability
depends on
dimensions.
Round off corners
and crown from center line.
sloping towards both sides.
Note: Distribution of electroplate on design shapes is intentionally exaggerated by solid black outline. Cross-hatched areas indicate part before plating.
226 / Surface Engineering for Corrosion and Wear Resistance
Buildup on pitch
diameter
= 4T
Fig. 5
~t
Rule of Four and Six as applied to coating external threads. Source:
Ref 8
Similarly, plating inside holes can be difficult. The general rule of thumb
is that if the hole diameter is x, the plating will occur to a depth of 2.x. However, for blind holes, plating will only occur to a depth of x. Agitation, solution flow, maximizing the throwing power of the plating bath, and other aids
can improve the situation somewhat, but the best approach is to eliminate or
minimize holes with high aspect ratios during the product-design stage.
In plasma-coating processes, the part design will have considerable
influence over the operating parameters of the coating-deposition
equipment. Complex shapes, blind holes, fins, slots, and similar features
will dictate that a high vacuum pressure, low part temperature, and light
plasma density be used (Ref 9). The converse will be true for simple
geometries. In plasma processing, consideration also must be given to
heating of the part by the plasma itself. Some design features with thin
cross sections and low mass, such as fins, louvers, and bosses will heat up
faster than the bulk material in the part. For parts that have been heat
treated, or otherwise finished to provide desirable mechanical properties,
overheating could destroy those properties or at least change the values
detrimentally. Reference 9 provides some examples of process and equipment modifications to avoid such problems during plasma nitriding. Reference 10 discusses the effect of part geometry on the growth of the nitride layer during ion nitridingand
how coating uniformity can be
improved for groovedsurfaces, As in electroplating, decreasing the aspect
ratio (depth of groove to width of groove) has a positive effect.
Other Important
Considerations
for the Design Engineer
Drawing up a Specification (Ref 1). The greatest source of dispute or
rejection after surface engineering is lack of communication and understanding, not a failure of the process itself. It is critically important that the
contractor is provided with all the needed information to ensure that the
component can, and will be, treated exactly as the customer expects. By establishing the right partnership and communication channels with all members of the purchase/supply chain, satisfactory results can be anticipated.
Practical Design Guidelines for Surface Engineering / 227
When a customer is seeking a quotation or evaluating a possible surface-engineering process, it is vital that the contractor has the information
listed below. This information should be viewed as the basis of a specification, to be agreed upon by both parties, with no margin for error or misunderstandings. Important information to be conveyed includes:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
A current issue engineering drawing of the part
Any applicable standards, for example, ASTM, ANSI, internal company standards, or international standards
Indication of part weight particularly if it is a large item
Indication of part number and quantity to be treated, in total and in
each batch
Packaging and handling requirements. Indication if parts must be returned in original packing
The base material, including composition or formal material specification. If there is more than one material in the part, make this point
very clearly. If it is a high-strength material being used in a highly
stressed situation (e.g., fatigue), then clearly state this.
Its heat treatment history during fabrication
What coating or treatment is required
Which areas on the part are to be treated. Mark them on the drawing.
Clearly show any areas that must not be treated and agree on masking
principles
Required thickness or case depth. Indication of a tolerance band allowance
If the part must be coated or plated to a final dimension, provide an
overall tolerance band. Remember that there will be two tolerances on
the part dimensions, the manufacturing tolerances and those of the
coating process. These will combine to a wider tolerance on the final
dimensions. Manufacturing sizes may have to be altered to accommodate the coating thickness.
Where, on its surface, can the part be supported or jigged (the area on
which it is supported will be obscured and not treated); anywhere
where it must not be jigged
Specify any precoating and postcoating heat treatment (e.g., stress relieving or deembrittlement) that will be the responsibility of the contractor. Inform the contractor of any heat treatment done elsewhere
prior to their receiving the work. Remember the more people in the
chain the greater the opportunities for them to deny responsibility for
problems.
Required. surface finish if it is controllable. Agree on any postfinishing procedures (e.g., a subcontracted grinding operation) if it is to be
part of the treatment service.
The inspection required, with a clear statement and understanding of
what is acceptable and what must result in rejection of the finished part
228 / Surface Engineering for Corrosion and Wear Resistance
•
•
•
Agree and establish responsibility for rework, if allowed,
scrapped work.
Any approvals required, for example, DoD or FDA
Date dispatched, date received by contractor, date required,
method of delivery and dispatch
and
and
Environmental Regulation of Surface Engineering. Environmental
protection regulations are often related directly or indirectly to surfaceengineering processes. This is particularly applicable for solvent cleaning
procedures, cadmium and chromium electroplating, chromate conversion
coatings, and organic coatings containing high amounts of volatile organic
compounds (solvents). The chemicals used for such processes may pose
serious health and environmental hazards. For information about specific
regulatory requirements, permitting conditions, and enforcement issues,
the design engineer is advised to seek assistance from federal, state, and
local regulatory agencies; consulting engineering firms; and law offices.
Another valuable source of information can be found in the Section "Environmental Protection Issues" in Surface Engineering, Volume 5 of the
ASM Handbook. Articles contained in this Section describe various environmental statutes affecting selection of surface-engineering processes
and review specific processes that can be used to replace cadmium coatings, chromium coatings, and chromate conversion coatings, as well as alternatives to vapor degreasing and wipe solvent cleaners.
Acknowledgment
This chapter was compiled and adapted from two primary sources:
•
•
K. Stevens, "Surface Engineering to Combat Wear and Corrosion: A
Design Guide," The Institute of Materials, London, United Kingdom,
1997. With permission
E.W. Brooman, Design for Surface Engineering, Materials Selection
and Design, Vol 20, ASM Handbook, ASM International, 1997, p
820-827
References
1. K. Stevens, "Surface Engineering to Combat Wear and Corrosion: A
Design Guide," The Institute of Materials, London, United Kingdom,
1997
2. E.W. Brooman, Design for Surface Engineering, Materials Selection
and Design, Vol 20, ASM Handbook, ASM International, 1997, p
820-827
Practical Design Guidelines for Surface Engineering / 229
3. M. Henthorne, Corrosion Causes and Control, Chemical Engineering
Series, McGraw-Hill, 1972, Part 7
4. D.L. Stauffer, Ed., Finishing Systems Design and Implementation, Society of Manufacturing Engineers, 1993, Chapter 1
5. S. Guskov, Faraday Cage, Finish Quality, and Recoating: New Technology for More Effective Powder Coating, Powder Coat., 1996, p
82-91
6. W.H. Safranek and E.W. Brooman, Finishing and Electroplating Die
Cast and Wrought Zinc, Zinc Institute, 1973, Chapter 7
7. B. Fawer, Thin-Film Powder Coatings: Design and Application Issues,
Powder Coat., Vol 7 (No.7), 1996, p 56-63
8. L.W. Flott, Quality Control: Becoming a Better Customer, Met. Finish., Vol 94 (No.2), 1996, p 79-82
9. R. Gunn, Industrial Advances for Plasma Nitriding, Ion Nitriding and
Ion Carburizing, T. Spalvins and W.L. Kovacs, Ed., ASM International, 1990, p 157-163
10. M.J. Park et al., Effect of Geometry on Growth of Nitride Layer in Ion
Nitriding, Ion Nitriding and Ion Carburizing, T. Spalvins and W.L.
Kovacs, Ed., ASM International, 1990,.p 203-209
Glossary of Terms
A
abrasion. (1) A process in which hard particles or protuberances are
forced against and moved along a solid surface. (2) A roughening or
scratching of a surface due to abrasive wear. (3) The process of grinding or wearing away through the use of abrasives. See also high-stress
abrasion and low-stress abrasion.
abrasive. (1) A hard substance used for grinding, honing, lapping, superfinishing, polishing, pressure blasting, or barrel finishing. Abrasives
in common use are alumina, silicon carbide, boron carbide, diamond,
cubic boron nitride, garnet, and quartz. (2) Hard particles, such as
rocks, sand, or fragments of certain hard metals, that wear away a surface when they move across it under pressure.
abrasive blasting. A process for cleaning or finishing by means of an
abrasive directed at high velocity against the workpiece.
abrasive erosion. Erosive wear caused by the relative motion of solid particles that are entrained in a fluid, moving nearly parallel to a solid surface. See also erosion.
abrasive wear. The removal of material from a surface when hard particles slide or roll across the surface under pressure. The particles may
be loose or may be part of another surface in contact with the surface
being abraded. Compare with adhesive wear.
accelerated corrosion test. Method designed to approximate, in a short
time, the deteriorating effect under normal long-term service conditions.
active metal. A metal ready to corrode or being corroded.
adhesion. (1) In frictional contacts, the attractive force between adjacent
surfaces. In physical chemistry, adhesion denotes the attraction
between a solid surface and a second (liquid or solid) phase. This definition is based on the assumption of a reversible equilibrium. In mechanical technology, adhesion is generally irreversible. In railway
232 / Surface Engineering for Corrosion and Wear Resistance
engineering, adhesion often means friction. (2) Force of attraction between the molecules (or atoms) of two different phases. Contrast with
cohesion. (3) The state in which two surfaces are held together by interfacial forces, which may consist of valence forces, interlocking action, or both.
adhesive wear. (1) Wear by transference of material from one surface to
another during relative motion due to a process of solid-phase welding.
Particles that are removed from one surface are either permanently or
temporarily attached to the other surface. (2) Wear due to localized
bonding between contacting solid surfaces leading to material transfer
between the two surfaces or loss from either surface. Compare with
abrasive wear.
alclad. Composite wrought product composed of an aluminum alloy core
having one or both surfaces a metallurgically bonded aluminum or aluminum alloy coating that is anodic to the core and thus electrochemically protects the core against corrosion.
alkaline cleaner. A material blended from alkali hydroxides and such alkaline salts as borates, carbonates, phosphates, or silicates. The cleaning action may be enhanced by the addition of surface-active agents
and special solvents.
aluminizing. Forming of an aluminum or aluminum alloy coating on a
metal by hot dipping, hot spraying, or diffusion.
anodizing. Forming a conversion coating on a metal surface by anodic oxidation; most frequently applied to aluminum.
arc spraying (ASP). A thermal spraying process using an arc between
two consumable electrodes of surfacing materials as a heat source and
a compressed gas to atomize and propel the surfacing material to the
substrate.
arc welding. A group of welding processes that produce coalescence of
metals by heating them with an arc, with or without the application of
pressure, and with or without the use of filler metal.
atmospheric corrosion. The gradual degradation or alteration of a material by contact with substances present in the atmosphere, such as
oxygen, carbon dioxide, water vapor, and sulfur and chlorine compounds.
B
barrel cleaning. Mechanical or electrolytic cleaning of metal in rotating
equipment.
barrel finishing. Improving the surface finish of workpieces by processing them in rotating equipment along with abrasive particles that may
be suspended in a liquid. The barrel is normally loaded about 60% full
with a mixture of parts, media, compound, and water.
barrel plating. Plating articles in a rotating container, usually a perforated
cylinder that operates at least partially submerged in a solution.
Glossary of Terms / 233
black oxide. A black finish on a metal produced by immersing it in hot
oxidizing salts or salt solutions.
blasting or blast cleaning. A process for cleaning or finishing metal objects with an air blast or centrifugal wheel that throws abrasive particles
against the surface of the workpiece. Small, irregular particles of metal
are used as the abrasive in gritblasting; sand, in sandblasting; and steel,
in shotblasting.
bluing. Subjecting the scale-free surface of a ferrous alloy to the action of
air, steam, or other agents at a suitable temperature, thus forming a thin
blue film of oxide and improving the appearance and resistance to corrosion. This term is ordinarily applied to sheet, strip, or finished parts.
It is used also to denote the heating of springs after fabrication to improve their properties.
boriding. Thermochemical treatment involving the enrichment of the
surface layer of an object with borides. This surface-hardening
process is performed below the ACl temperature. Also referred to as
boronizing.
bright finish. A high-quality finish produced on ground and polished
rolls. Suitable for electroplating.
bright nitriding. Nitriding in a protective medium to prevent discoloration of the bright surface.
bright plate. An electrodeposit that is lustrous in the as-plated condition.
brinelling. (1) Indentation of the surface of a solid body by repeated local
impact or impacts, or static overload. Brinelling may occur especially
in a rolling-element bearing. (2) Damage to a solid bearing surface
characterized by one or more plastically formed indentations brought
about by overload. See also false brinelling.
brush plating. Plating with a concentrated solution or gel held in or fed
to an absorbing medium, pad, or brush carrying the anode (usually insoluble). The brush is moved back and forth over the area of the cathode to be plated.
buffing. Developing a lustrous surface by contacting the work with a rotating buffing wheel.
burnishing. Finish sizing and smooth finishing of surfaces (previously
machined or ground) by displacement, rather than removal, of minute
surface irregularities with smooth-point or line-contact fixed or rotating
tools.
butler finish. A semilustrous metal finish composed of fine, uniformly
distributed parallel lines, usually produced with a soft abrasive buffing
wheel; similar in appearance to the traditional hand-rubbed finish on
silver.
C
calorizing. Imparting resistance to oxidation to an iron or steel surface by
heating in aluminum powder at 800 to 1000 °C (1470-1830 OF).
234 I Surface Engineering for Corrosion and Wear Resistance
carbonitriding. A case-hardening process in which a suitable ferrous material is heated above the lower transformation temperature in a gaseous
atmosphere of such composition as to cause simultaneous absorption of
carbon and nitrogen by the surface and, by diffusion, create a concentration gradient. The heat treating process is completed by cooling at a
rate that produces the desired properties in the workpiece.
carburizing. Absorption and diffusion of carbon into solid ferrous alloys
by heating, to a temperature usually above Ac3, in contact with a suitable carbonaceous material. A form of case hardening that produces a
carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching.
case. In heat treating, that portion of a ferrous alloy, extending inward
from the surface, whose composition has been altered during case hardening. Typically considered to be the portion of an alloy (a) whose
composition has been measurably altered from the original composition, (b) that appears light when etched, or (c) that has a higher hardness value than the core. Contrast with core.
case hardening. A generic term covering several processes applicable to
steel that change the chemical composition of the surface layer by absorption of carbon, nitrogen, or a mixture of the two and, by diffusion, create a concentration gradient. The processes commonly used
are carburizing and quench hardening; cyaniding; nitriding; and carbonitriding. The use of the applicable specific process name is preferred.
CASS test. Abbreviation for copper-accelerated salt-spray test.
cavitation. The formation and collapse, within a liquid, of cavities or bubbles that contain vapor or gas or both. In general, cavitation originates
from a decrease in the static pressure in the liquid. It is distinguished in
this way from boiling, which originates from an increase in the liquid
temperature. There are certain situations where it may be difficult to
make a clear distinction between cavitation and boiling, and the more
general definition that is given here is therefore to be preferred. In order
to erode a solid surface by cavitation, it is necessary for the cavitation
bubbles to collapse on or close to that surface.
cavitation corrosion. A process involving conjoint corrosion and cavitation.
cavitation damage. The degradation of a solid body resulting from its exposure to cavitation. This may include loss of material, surface deformation, or changes in properties or appearance.
cavitation erosion. Progressive loss of original material from a solid surface due to continuing exposure to cavitation.
cementation. The introduction of one or more elements into the outer portion of a metal object by means of diffusion at high temperature.
Glossary of Terms / 235
checks. Numerous, very fine cracks in a coating or at the surface of a
metal part. Checks may appear during processing or during service and
are most often associated with thermal treatment or thermal cycling.
chemical conversion coating. A protective or decorative nonmetallic
coating produced in situ by chemical reaction of a metal with a chosen
environment. It is often used to prepare the surface prior to the application of an organic coating.
chemical deposition. The precipitation or plating-out of a metal from solutions of its salts through the introduction of another metal or reagent
to the solution.
chemical polishing. A process that produces a polished surface by the action of a chemical etching solution. The etching solution is compounded so that peaks in the topography of the surface are dissolved
preferentially.
chemical vapor deposition (CVD). A coating process, similar to gas carburizing and carbonitriding, whereby a reactant atmosphere gas is fed
into a processing chamber where it decomposes at the surface of the
workpiece, liberating one material for either absorption by, or accumulation on, the workpiece. A second material is liberated in gas form and
is removed from the processing chamber, along with excess atmosphere
gas.
chromate treatment. A treatment of metal in a solution of a hexavalent
chromium compound to produce a conversion coating consisting of
trivalent and hexavalent chromium compounds.
chromating. Performing a chromate treatment.
chromizing. A surface treatment at elevated temperature, generally carried out in pack, vapor, or salt baths, in which an alloy is formed by the
inward diffusion of chromium into the base metal.
cladding. (1) A layer of material, usually metallic, that is mechanically
or metallurgically bonded to a substrate. Cladding may be bonded to
the substrate by any of several processes, such as roll cladding and
explosive forming. (2) A relatively thick layer (1 mm, or 0.04 in.) of
material applied by surfacing for the purpose of improved corrosion
resistance or other properties. See also coating, surfacing, and hardfacing.
clad metal. A composite metal containing two or more layers that have
been bonded together. The bonding may have been accomplished by
corolling, coextrusion, welding, diffusion bonding, casting, heavy
chemical deposition, or heavy electroplating.
coating. A relatively thin layer « 1 mm, or 0.04 in.) of material applied
by surfacing for the purpose of corrosion prevention, resistance to hightemperature scaling, wear resistance, lubrication, or other purposes.
coefficient of friction. The dimensionless ratio of the friction force (F)
between two bodies to the normal force (N) pressing these bodies together: (J.L or f) = (F/N).
236 / Surface Engineering for Corrosion and Wear Resistance
cohesion. (1) The state in which the particles of a single substance are
held together by primary or secondary valence forces. As used in the
adhesive field, the state in which the particles of the adhesive (or adherend) are held together. (2) Force of attraction between the molecules
(or atoms) within a single phase. Contrast with adhesion.
color buffing. Producing a final high luster by buffing. Sometimes called
coloring.
coloring. Producing desired colors on metal by a chemical or electrochemical reaction. See also color buffing.
composite coating. A coating on a metal or nonmetal that consists of two
or more components, one of which is often particulate in form. Example: a cermet composite coating on a cemented carbide cutting tool.
Also known as multilayer coating.
composite plate. An electrodeposit consisting of layers of at least two different compositions.
contact corrosion. A term primarily used in Europe to describe galvanic
corrosion between dissimilar metals.
contact plating. A metal plating process wherein the plating current is
provided by galvanic action between the work metal and a second
metal, without the use of an external source of current. .
conversion coating. A coating consisting of a compound of the surface
metal, produced by chemical or electrochemical treatments of the
metal. Examples include chromate coatings on zinc, cadmium, magnesium, and aluminum, and oxide and phosphate coatings on steel. See
also chromate treatment and phosphating.
copper-accelerated
salt-spray (CASS) test. An accelerated corrosion
test for some electrodeposits and for anodic coatings on aluminum.
core. In a ferrous alloy prepared for case hardening, that portion of the
alloy that is not part of the case. Typically considered to be the portion
that (a) appears dark (with certain etchants) on an etched cross section,
(b) has an essentially unaltered chemical composition, or (c) has a hardness, after hardening, less than a specified value.
corrodkote test. An accelerated corrosion test for electrodeposits.
corrosion. The chemical or electrochemical reaction between a material,
usually a metal, and its environment that produces a deterioration of the
material and its properties.
corrosion-erosion.
See erosion-corrosion.
corrosion fatigue. The process in which a metal fractures prematurely
under conditions of simultaneous corrosion and repeated cyclic loading
at lower stress levels or fewer cycles than would be required in the absence of the corrosive environment.
corrosion inhibitor. See inhibitor.
corrosion product. Substance formed as a result of corrosion.
corrosion protection. Modification of a corrosion system so that corrosion damage is mitigated.
Glossary of Terms I 237
corrosion rate. Corrosion effect on a metal per unit of time. The type of
corrosion rate used depends on the technical system and on the type of
corrosion effect. Thus, corrosion rate may be expressed as an increase
in corrosion depth per unit of time (penetration rate, for example,
mils/yr) or the mass of metal turned into corrosion products per unit
area of surface per unit of time (weight loss, for example, g/m2/yr). The
corrosion effect may vary with time and may not be the same at all
points of the corroding surface. Therefore, reports of corrosion rates
should be accompanied by information on the type, time dependency,
and location of the corrosion effect.
corrosion resistance. The ability of a material to withstand contact with
ambient natural factors or those of a particular, artificially created
atmosphere, without degradation or change in properties. For metals, this could be pitting or rusting; for organic materials, it could be
crazing.
corrosion system. System consisting of one or more metals and all parts
of the environment that influence corrosion.
corrosive wear. Wear in which chemical or electrochemical reaction with
the environment is significant. See also oxidative wear.
coveringpower; (1) The ability of a solution to give satisfactory plating
at very low current densities, a condition that exists in recesses and pits.
This term suggests an ability to cover, but not necessarily to build up, a
uniform coating, whereas throwing power suggests the ability to obtain
a coating of uniform thickness on an irregularly shaped object. (2) The
degree to which a porcelain enamel coating obscures the underlying
surface.
crevice corrosion. Localized corrosion of a metal surface at, or immediately adjacent to, an area that is shielded from full exposure to the environment because of close proximity between the metal and the surface of another material.
cyaniding. A case-hardening process in which a ferrous material is heated
above the lower transformation temperature range in a molten salt containing cyanide to cause simultaneous absorption of carbon and nitrogen at the surface and, by diffusion, create a concentration gradient.
Quench hardening completes the process.
D
dealloying. The selective corrosion of one or more components of a solidsolution alloy. Also called parting or selective leaching. See also·dezincification and graphitic corrosion.
degreasing. The removal of grease and oils from a surface. Can be accomplished by immersion in liquid organic solvent, by solvent vapors
condensing on the parts being cleaned (vapor degreasing), or by spraying the parts with solvent.
238 / Surface Engineering for Corrosion and Wear Resistance
deposit corrosion. Corrosion occurring under or around a discontinuous
deposit on a metallic surface. Also called poultice corrosion.
detonation flame spraying. A thermal spraying process variation in
which the controlled explosion of a mixture of fuel gas, oxygen, and
powdered coating material is utilized to melt and propel the material to
the workpiece.
dezincification. Corrosion in which zinc is selectively leached from zinccontaining alloys leaving a relatively weak layer of copper and copper
oxide. Most commonly found in copper-zinc alloys containing less than
85% Cu after extended service in water containing dissolved oxygen.
See also dealloying and selective leaching.
dichromate treatment. A chromate conversion coating produced on
magnesium alloys in a boiling solution of sodium dichromate.
differential coating. A coated product having a specified coating on one
surface and a significantly lighter coating on the other surface (such as
a hot dip galvanized product or electrolytic tin plate).
diffusion coating. Any process whereby a base metal or alloy is either
(1) coated with another metal or alloy and heated to a sufficient temperature in a suitable environment or (2) exposed to a gaseous or liquid
medium containing the other metal or alloy, thus causing diffusion of
the coating or of the other metal or alloy into the base metal. with resultant changes in the composition and properties of its surface.
diphase cleaning. Removing soil by an emulsion that produces two
phases in the cleaning tank: a solvent phase and an aqueous phase.
Cleaning is effected by both solvent action and emulsification.
distortion. Any deviation from an original size, shape, or contour that occurs because of the application of stress or the release of residual stress.
droplet erosion. Erosive wear caused by the impingement of liquid
droplets on a solid surface. See also erosion.
E
electrochemical corrosion. Corrosion that is accompanied by a flow of
electrons between cathodic and anodic areas on metallic surfaces.
electrodeposition. (1) The deposition of a conductive material from a
plating solution by the application of electrical current. (2) The deposition of a substance on an electrode by passing electric current through
an electrolyte. Electroplating, electroforming, electrorefining, and electrotwinning result from electrodeposition.
electroforming. Making parts by electrodeposition on a removable form.
electro galvanizing. The electroplating of zinc upon iron or steel.
electroless plating. (1) A process in which metal ions in a dilute aqueous solution are plated out on a substrate by means of autocatalytic
chemical reduction. (2) The deposition of conductive material from
an autocatalytic plating solution without the application of electrical
current.
Glossary of Terms / 239
electrolytic cleaning. A process of removing soil, scale, or corrosion
products from a metal surface by subjecting it as an electrode to an
electric current in an electrolytic bath.
electrolytic deposition. Same as electrodeposition.
electrolytic pickling. Pickling in which electric current is used, the work
being one of the electrodes.
electron beam heat treating. A selective surface hardening process that
rapidly heats a surface by direct bombardment with an accelerated
stream of electrons.
electroplate. The application of a metallic coating on a surface by means
of electrolytic action.
electroplating. The electrodeposition of an adherent metallic coating on
an object serving as a cathode for the purpose of securing a surface with
properties or dimensions different from those of the substrate.
electropolishing. A technique commonly used to prepare metallographic
specimens, in which a high polish is produced making the specimen the
anode in an electrolytic cell, where preferential dissolution at high
points smooths the surface.
electrotinning. Electroplating tin on an object.
emulsion cleaner. A cleaner consisting of organic solvents dispersed in an
aqueous medium with the aid of an emulsifyingagent,
environmental cracking. Brittle fracture of a normally ductile material in
which the corrosive effect of the environment is a causative factor.
erosion. (1) Loss of material from a solid surface due to relative motion
in contact with a fluid that contains solid particles. Erosion in which the
relative motion of particles is nearly parallel to the solid surface is
called abrasive erosion. Erosion in which the relative motion of the
solid particles is nearly normal to the solid surface is called impingement erosion or impact erosion. (2) Progressive loss of original material from a solid surface due to mechanical interaction between that surface and a fluid, a multicomponent fluid, and impinging liquid, or solid
particles. (3) Loss of material from the surface of an electrical contact
due to an electrical discharge (arcing). See also cavitation erosion and
erosion-corrosion.
erosion-corrosion. A conjoint action involving corrosion and erosion in
the presence of a moving corrosive fluid, leading to the accelerated loss
of material.
erosivity. The characteristic of a collection of particles, liquid stream, or
a slurry that expresses its tendency to cause erosive wear when forced
against a solid surface under relative motion.
exfoliation. Corrosion that proceeds laterally from the sites of initiation
along planes parallel to the surface, generally at grain boundaries,
forming corrosion products that force metal away from the body of the
material, giving rise to a layered appearance. Most commonly associated with wrought aluminum alloys.
240 / Surface Engineering for Corrosion and Wear Resistance
extreme-pressure
lubricant. A lubricant that imparts increased loadcarrying capacity to rubbing surfaces under severe operating conditions. Extreme-pressure lubricants usually contain sulfur, halogens, or
phosphorus.
F
false brinelling. (1) Damage to a solid bearing surface characterized by
indentations not caused by plastic deformation resulting from overload,
but thought to be due to other causes such as fretting corrosion.
(2) Local spots appearing when the protective film on a metal is broken
continually by repeated impacts, usually in the presence of corrosive
agents. The appearance is generally similar to that produced by
brinelling, but corrosion products are usually visible. It may result from
fretting corrosion. This term should be avoided when a more precise
description is possible. False brinelling (race fretting) can be distinguished from true brinelling because in false brinelling, surface material is removed so that original finishing marks are removed. The borders of a false brinell mark are sharply defined, whereas a dent caused
by a rolling element does not have sharp edges and the finishing marks
are visible in the bottom of the dent.
fatigue. The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate tensile
strength of the material. Fatigue failure generally occurs at loads that
applied statically would produce little perceptible effect. Fatigue fractures are progressive, beginning as minute cracks that grow under the
action of the fluctuating stress.
filiform corrosion. Corrosion that occurs under some coatings in the form
of randomly distributed threadlike filaments.
finish. Surface condition, quality, or appearance of a metal.
finish grinding. The final grinding action on a workpiece, of which the
objectives are surface finish and dimensional accuracy.
fixture. A device designed to hold parts to be joined in proper relation to
each other.
flame hardening. A process for hardening the surfaces of hardenable ferrous alloys in which an intense flame is used to heat the surface layers
above the upper transformation temperature, whereupon the workpiece
is immediately quenched.
flame spraying. A thermal spraying process in which an oxyfuel gas
flame is the source of heat for melting the surfacing material. Compressed gas mayor may not be used for atomizing and propelling the
surfacing material to the substrate.
fluidized bed. A contained mass of a finely divided solid that behaves like
a fluid when brought into suspension in a moving gas or liquid.
flux-cored arc welding (FCAW). An arc welding process that joins metal
by heating them with an arc between a continuous tubular filler-metal
Glossary of Terms I 241
electrode and the work. Shielding is provided by a flux contained
within the consumable tubular electrode. Additional shielding mayor
may not be obtained from an externally supplied gas or gas mixture.
See also flux-cored elctrode.
flux-cored electrode. A composite filler metal electrode consisting of a
metal tube or other hollow configuration containing ingredients to provide such functions as shielding atmosphere, deoxidation, arc stabilization, and slag formation. Minor amounts of alloying materials may
be included in the core. External shielding mayor may not be used.
fretting. A type of wear that occurs between tight-fitting surfaces subjected to cyclic relative motion of extremely small amplitude. Usually,
fretting is accompanied by corrosion, especially of the very fine wear
debris. Also referred to as fretting corrosion and false brinelling (in
rolling-element bearings).
fretting corrosion. (1) The accelerated deterioration at the interface between contacting surfaces as the result of corrosion and slight oscillatory movement between the two surfaces. (2) A form of fretting in
which chemical reaction predominates. Fretting corrosion is often characterized by the removal of particles and subsequent formation of oxides, which are often abrasive and so increase the wear. Fretting corrosion can involve other chemical reaction products, which may not be
abrasive.
fretting fatigue. (1) Fatigue fracture that initiates at a surface area where
fretting has occurred. The progressive damage to a solid surface that
arises from fretting. Note: If particles of wear debris are produced, then
the term fretting wear may be applied.
fretting wear. Wear arising as a result oi fretting.
friction. The resisting force tangential to the common boundary between
two bodies when, under the action of an external force, one body moves
or tends to move relative to the surface of the other.
friction coefficient. See coefficient of friction.
fused-spray deposit. A self-fluxing spray deposit that is deposited by
conventional thermal spraying and subsequently fused using either a
heating torch or a furnace.
G
galling. (1) A condition whereby excessive friction between high spots results in localized welding with subsequent spalling and a further roughening of the rubbing surfaces of one or both of two mating parts. (2) A
severe form of scuffing associated with gross damage to the surfaces or
failure. Galling has been used in many ways in tribology; therefore,
each time it is encountered its meaning must be ascertained from the
specific context of the usage. See also scoring and scuffing.
galvanic corrosion. Corrosion associated with the current of a galvanic
cell consisting of two dissimilar conductors in an electrolyte or two
242 / Surface Engineering for Corrosion and Wear Resistance
similar conductors in dissimilar electrolytes. Where the two dissimilar
metals are in contact, the resulting reaction is referred to as couple action.
galvanic series. A list of metals and alloys arranged according to their relative corrosion potentials in a given environment.
galvanize. To coat a metal surface with zinc using any of various
processes.
galvanneal. To produce a zinc-iron alloy coating on iron or steel by keeping the coating molten after hot dip galvanizing until the zinc alloys interdiffuse completely with the basis metal.
gaseous corrosion. Corrosion with gas as the only corrosive agent and
without any aqueous phase on the surface of the metal. Also called dry
corrosion. See also hot corrosion, oxidation, and sulfidation.
gas metal arc welding (GMAW). An arc welding process that produces
coalescence of metals by heating them with an arc between a continuous filler metal electrode and the workpieces. Shielding is obtained entirely from an externally supplied gas.
gas tungsten arc welding (GTAW). An arc welding process that produces coalescence of metals by heating them with an arc between a
tungsten (nonconsumable) electrode and the work. Shielding is obtained from a gas or gas mixture. Pressure mayor may not be used, and
filler metal mayor may not be used.
general corrosion. (1) A form of deterioration that is distributed more or
less uniformly over a surface. (2) Corrosion dominated by uniform
thinning that proceeds without appreciable localized attack. See also
uniform corrosion.
gouging abrasion. A form of high-stress abrasion in which easily observable grooves or gouges are created on the surface. See also abrasion.
graphitic corrosion. Corrosion of gray iron in which the iron matrix is selectively leached away, leaving a porous mass of graphite behind; it occurs in relatively mild aqueous solutions and on buried pipe and fittings.
grinding. Removing material from a workpiece with a grinding wheel or
abrasive belt.
grit blasting. Abrasive blasting with small irregular pieces of steel, malleable cast iron, or hard nonmetallic materials.
H
hard chromium. Chromium electrodeposited
for engineering purposes
(such as to increase the wear resistance of sliding metal surfaces) rather
than as a decorative coating. It is usually applied directly to substrate
and is customarily thicker (> 1.2 I-Lm, or 0.05 mil) than a decorative deposit, but not necessarily harder.
hardfacing. The application of a hard, wear-resistant material to the surface of a component by welding, spraying, or allied welding processes
Glossary of Terms / 243
to reduce wear or loss of material by abrasion, impact, erosion, galling,
and cavitation. See also surfacing,
hardfacing alloys. Wear-resistant materials available as bare welding rod,
flux-coated rod, long-length solid wires, long-length tubular wires, or
powders that are deposited by hard/acing.
high-stress abrasion. A form of abrasion in which relatively large cutting
forces are imposed on the particles or protuberances causing the abrasion and that produces significant cutting and deformation of the wearing surface.
high-temperature hydrogen attack. A loss of strength and ductility of
steel by high-temperature reaction of absorbed hydrogen with carbides
in the steel resulting in decarburization and internal fissuring.
holidays. Discontinuities in a coating (such as porosity, cracks, gaps, and
similar flaws) that allow areas of substrate to be exposed to any corrosive environment that contacts the coated surface.
honing. A low-speed finishing process used chiefly to produce uniform
high dimensional accuracy and fine finish, most often on inside cylindrical surfaces. In honing, very thin layers of stock are removed by simultaneously rotating and reciprocating a bonded abrasive stone or
stick that is pressed against the surface being honed with lighter force
than is typical of grinding.
hot corrosion. An accelerated corrosion of metal surfaces that results
from the combined effect of oxidation and reactions with sulfur compounds and other contaminants, such as chlorides, to form a molten salt
on a metal surface that fluxes, destroys, or disrupts the normal protective oxide. See also gaseous corrosion.
hot dip. Covering a surface by dipping the surface to be coated into a
molten bath of the coating material. See also hot dip coating.
hot dip coating. A metallic coating obtained by dipping the substrate into
a molten metal.
hydrogen blistering. The formation of subsurface planar cavities, called
hydrogen blisters, in a metal resulting from excessive internal hydrogen
pressure. Growth of near-surface blisters in low-strength metals usually
results in surface bulges.
hydrogen damage. A general term for the embrittlement, cracking, blistering, and hydride formation that can occur when hydrogen is present
in some metals.
hydrogen embrittlement. A loss of ductility of a metal resulting from absorption of hydrogen.
hydrogen-induced cracking. Stepwise internal cracks that connect adjacent hydrogen blisters on different planes in the metal or to the metal
surface. Also called stepwise cracking.
hydrogen stress cracking. Cracking that results from the presence of hydrogen in a metal in combination with tensile stress. It occurs most frequently with high-strength alloys.
244 / Surface Engineering for Corrosion and Wear Resistance
immersion cleaning. Cleaning in which the work is immersed in a liquid
solution.
immersion coating. A coating produced in a solution by chemical or electrochemical action without the use of external current.
immersion plating. Depositing a metallic coating on a metal immersed in
a liquid solution, without the aid of an external electric current. Also
called dip plating.
impact wear. Wear of a solid surface resulting from repeated collisions
between that surface and another solid body. The term erosion is preferred in the case of multiple impacts and when the impacting body or
bodies are very small relative to the surface being impacted.
impingement. A process resulting in a continuing succession of impacts
between liquid or solid particles and a solid surface.
impingement attack. Corrosion associated with turbulent flow of liquid. May be accelerated by entrained gas bubbles. See also erosioncorrosion and impingement corrosion.
impingement corrosion. A form of erosion-corrosion generally associated with the local impingement of a high-velocity, flowing fluid
against a solid surface.
impingement erosion. Loss of material from a solid surface due to liquid
impingement. See also erosion.
induction hardening. A surface-hardening process in which only the surface layer of a suitable ferrous workpiece is heated by electromagnetic
induction to above the upper critical temperature and immediately
quenched.
inhibitor. A substance that retards some specific chemical reaction, for
example, corrosion. Pickling inhibitors retard the dissolution of metal
without hindering the removal of scale from steel.
intergranular corrosion. Corrosion occurring preferentially at grain
boundaries, usually with slight or negligible attack on the adjacent grains.
intergranular stress-corrosion cracking (IGSCC).Stress-corrosion
cracking in which the cracking occurs along grain.boundaries,
interrupted-current plating. Plating in which the flow of current is discontinued for periodic short intervals to decrease anode polarization
and elevate the critical current density. It is most commonly used in
cyanide copper plating.
ion carburizing. A method of surface hardening in which carbon ions are
diffused into a workpiece in a vacuum through the use of high-voltage
electrical energy. Synonymous with plasma carburizing or glowdischarge carburizing.
ion implantation. The process of modifying the physical or chemical
properties of the near surface of a solid (target) by embedding appropriate atoms into it from a beam of ionized particles.
Glossary of Terms / 245
ion nitriding. A method of surface hardening in which nitrogen ions are
diffused into a workpiece in a vacuum through the use of high-voltage
electrical energy. Synonymous with plasma nitriding or glow-discharge nitriding.
ion plating. A generic term applied to atomistic film-deposition processes
in which the substrate surface and/or the depositing film is subjected to
a flux of high-energy particles (usually gas ions) sufficient to cause
changes in the interfacial region or film properties.
K
knife-line attack. Intergranular corrosion of an alloy, usually stabilized
stainless steel, along a line adjoining or in contact with a weld after
heating into the sensitization temperature range.
L
lapping. A finishing operation using fine abrasive grits loaded into a lapping material such as cast iron. Lapping provides major refinements in
the workpiece including extreme accuracy of dimension, correction of
minor imperfections of shape, refinement of surface finish, and close fit
between mating surfaces.
laser alloying. See laser surface processing.
laser beam welding (LBW). A welding process that produces coalescence of materials with the heat obtained from the application of a concentrated coherent light beam impinging upon the joint.
laser hardening. A surface-hardening process that uses a laser to quickly
heat a surface. Heat conduction into the interior of the part will quickly
cool the surface, leaving a shallow martensitic layer.
laser melting. See laser surface processing.
laser surface processing. The use of lasers to modify the metallurgical
structure of a surface and to tailor the surface properties without adversely affecting the bulk properties. The surface modification can
take the following three forms. The first is transformation hardening
in which a surface is heated so that thermal diffusion and solid-state
transformations can take place. The second is surface melting, which
results in a refinement of the structure due to the rapid quenching
from the melt. The third is surface (laser) alloying, in which alloying
elements are added to the melt pool to change the composition of the
surface. The novel structures produced by laser surface melting and
alloying can exhibit improved electrochemical and tribological behavior.
liquid carburizing.
Surface hardening of steel by immersion into a
molten bath consisting of cyanides and other salts.
liquid honing. Producing a finely polished finish by directing an airejected chemical emulsion containing fine abrasives against the surface
to be finished.
246 / Surface Engineering for Corrosion and Wear Resistance
liquid nitriding. A method of surface hardening
in which molten
nitrogen-bearing,
fused-salt baths containing both cyanides and
cyanates are exposed to parts at subcritical temperatures.
liquid nitrocarburizing. A nitrocarburizing process (where both carbon
and nitrogen are absorbed into the surface) utilizing molten liquid salt
baths below the lower critical temperature.
localized corrosion. Corrosion at discrete sites, for example, crevice corrosion, pitting, and stress-corrosion cracking.
low-stress abrasion. A form of abrasion in which relatively low contact
pressures on the abrading particles or protuberances cause only fine
scratches and microscopic cutting chips to be produced.
lubricant. (1) Any substance interposed between two surfaces in relative
motion for the purpose of reducing the friction or wear between them.
(2) A material applied to dies, molds, plungers, or workpieces that promotes the flow of metal, reduces friction and wear, and aids in the release of the finished part.
lubrication. The reduction of frictional resistance and wear, or other
forms of surface deterioration, between two load-bearing surfaces by
the application of a lubricant.
luster finish. A bright, as-rolled finish, produced on ground metal rolls; it
is suitable for decorative painting or plating, but usually must undergo
additional surface preparation after forming.
M
matte finish. (1) A dull texture produced by rolling sheet or strip between
rolls that have been roughened by blasting. (2) A dull finish characteristic of some electrodeposits, such as cadmium or tin.
mechanical plating. Plating wherein fine metal powders are peened onto
the work by tumbling or other means. The process is used primarily to
provide ferrous parts with coatings of zinc, cadmium, tin, and alloys of
these metals in various combinations.
mechanical polishing. A process that yields a specularly reflecting surface entirely by the action of machining tools, which are usually the
points of abrasive particles suspended in a liquid among the fibers of a
polishing cloth.
metallizing. Forming a metallic coating by atomized spraying with
molten metal or by vacuum deposition. Also called spray metallizing.
metal spraying. Coating metal objects by spraying molten metal against
their surfaces. See also thermal spraying.
N
nitriding. Introducing nitrogen into the surface layer of a solid ferrous
alloy by holding at a suitable temperature (below ACl for ferritic steels)
in contact with a nitrogenous material, usually ammonia or molten
Glossary of Terms / 247
cyanide of appropriate composition. Quenching is not required to produce a hard case. See also bright nitriding and liquid nitriding.
nitrocarburizing.
Any of several processes in which both nitrogen and
carbon are absorbed into the surface layers of a ferrous material at temperatures below the lower critical temperature and, by diffusion, create
a concentration gradient. Nitrocarburizing is performed primarily to
provide an antiscuffing surface layer and to improve fatigue resistance.
Compare with carbonitriding.
o
oxidation. A corrosion reaction in which the corroded metal forms an
oxide; usually applied to reaction with a gas containing elemental
oxygen, such as air. Elevated temperatures increase the rate of oxidation.
oxidative wear. (1) A corrosive wear process in which chemical reaction
with oxygen or oxidizing environment predominates. (2) A type of
wear resulting from the sliding action between two metallic components that generates oxide films on the metal surfaces. These oxide
films prevent the formation of a metallic bond between the sliding surfaces, resulting in fine wear debris and low wear rates.
oxyacetylene welding. An oxyfuel gas welding process in which the fuel
gas is acetylene.
oxyfuel gas welding (OFW). Any of a group of processes used to fuse
metals together by heating them with gas flames resulting from combustion of a specific fuel gas such as acetylene, hydrogen, natural
gas, or propane. The process may be used with or without the application of pressure to the joint, and with or without adding any filler
metal.
p
pack carburizing. A method of surface hardening of steel in which parts
are packed in a steel box with a carburizing compound and heated to elevated temperatures. This process has been largely supplanted by gas
and liquid carburizing processes.
pack nitriding. A method of surface hardening of steel in which parts are
packed in a steel box with a nitriding compound and heated to elevated
temperatures.
passive. (1) A metal corroding under the control of a surface reaction
product. (2) The state of the metal surface characterized by low corrosion rates in a potential region that is strongly oxidizing for the metal.
phosphating. Forming an adherent phosphate coating on a metal by immersion in a suitable aqueous phosphate solution. Also called phosphatizing. See also conversion coating.
physical vapor deposition (PVD). A coating process whereby the deposition species. are transferred and deposited in the form of individual
248 / Surface Engineering for Corrosion and Wear Resistance
atoms or molecules. The most common PVD methods are sputtering
and evaporation. Sputtering, which is the principal PVD process,
involves the transport of a material from a source (target) to a substrate by means of the bombardment of the target by gas ions that
have been accelerated by a high voltage. Evaporation, which was the
first PVD process used, involves the transfer of material to form a
coating by physical means alone, essentially vaporization. Physical
vapor deposition coatings are used to improve the wear, friction, and
hardness properties of cutting tools and as corrosion-resistant coatings.
pitting. (1) Forming small sharp cavities in a surface by corrosion, wear,
or other mechanically assisted degradation. (2) Localized corrosion of
a metal surface, confined to a point or small area, that takes the form of
cavities.
plasma arc welding (PAW). An arc welding process that produces coalescence of metals by heating them with a constricted arc between an
electrode and the workpiece (transferred arc) or the electrode and the
constricting nozzle (nontransferred arc). Shielding is obtained from
hot, ionized gas issuing from an orifice surrounding the electrode and
may be supplemented by an auxiliary source of shielding gas, which
may be an inert gas or a mixture of gases. Pressure mayor may not be
used, and filler metal mayor may not be supplied.
plasma-assisted chemical vapor deposition. A·chemical vapor deposition process that uses low-pressure glow-discharge plasmas to promote
the chemical deposition reactions. Also called plasma-enhanced chemical vapor deposition.
plasma carburizing. Same as ion carburizing.
plasma nitriding. Same as ion nitriding,
plasma spraying. A thermal spraying process in which a nontransferred
arc of a plasma torch is utilized to create a gas plasma that acts as the
source of heat for melting and propelling the surfacing material to the
substrate.
polishing. (1) Smoothing metal surfaces, often to a high luster, by rubbing
the surface with a fine abrasive, usually contained in a cloth or other
soft lap. Results in microscopic flow of some surface metal together
with actual removal of a small amount of surface metal. (2) Removal of
material by the action of abrasive grains carried to the work by a flexible support, generally either a wheel or a coated abrasive belt. (3) A
mechanical, chemical, or electrolytic process or combination thereof
used to prepare a smooth, reflective surface suitable for microstructural
examination that is free of artifacts or damage introduced during prior
sectioning or grinding. See also electropolishing.
porcelain enamel. A substantially vitreous or glassy, inorganic coating
(borosilicate glass) bonded to metal by fusion at a temperature above
425°C (800 OF). Porcelain enamels are applied primarily to compo-
Glossary of Terms / 249
nents made of sheet iron or steel, cast iron, aluminum, or aluminumcoated steels.
poultice corrosion. A term used in the automotive industry to describe the
corrosion of vehicle body parts due to the collection of road salts and
debris on ledges and in pockets that are kept moist by weather and
washing. Also called deposit corrosion or attack.
powder flame spraying. A thermal spraying process variation in which
the material to be sprayed is in powder form,
precoated metal products. Mill products that have a metallic, organic, or
conversion coating applied to their surfaces before they are fabricated
into parts.
Q
quench hardening. In ferrous alloys, hardening by austenitizing and then
cooling at a rate such that a substantial amount of austenite transforms
to martensite.
quenching. Rapid cooling of metals (often steels) from a suitable elevated
temperature. This generally is accomplished by immersion in water, oil,
polymer solution, or salt, although forced air is sometimes used.
R
residual stress. (1) The stress existing in a body at rest, in equilibrium, at
uniform temperature, and not subjected to external forces. Often caused
by the forming or thermal processing curing process. (2) An internal
stress not depending on external forces resulting from such factors as
cold working, phase changes, or temperature gradients. (3) Stress present in a body that is free of external forces or thermal gradients.
(4) Stress remaining in a structure or member as a result of thermal or
mechanical treatment or both. Stress arises in fusion welding primarily
because the weld metal contracts on cooling from the solidus to room
temperature.
robber. An extra cathode or cathode extension that reduces the current
density on what would otherwise be a high-current-density area on
work being electroplated.
rolling-contact
fatigue. Repeated stressing of a solid surface due to
rolling contact between it and another solid surface or surfaces. Continued rolling-contact fatigue of bearing or gear surfaces may result in
rolling-contact damage in the form of subsurface fatigue cracks and/or
material pitting and spallation.
rouge finish. A highly reflective finish produced with rouge (finely divided, hydrated iron oxide) or other very fine abrasive, similar in appearance to the bright polish or mirror finish on sterling silver utensils.
rust. A visible corrosion product consisting of hydrated oxides of iron.
Applied only to ferrous alloys.
250 / Surface Engineering for Corrosion and Wear Resistance
S
salt fog test ..An accelerated corrosion test in which specimens are exposed to a fine mist of a solution usually containing sodium chloride,
but sometimes modified with other chemicals. Also known as salt spray
test.
satin finish. A diffusely reflecting surface finish on metals, lustrous but
not mirrorlike. One type is a butler finish.
scaling. Forming a thick layer of oxidation products on metals at high
temperature. Scaling should be distinguished from rusting, which involves the formation of hydrated oxides. See also rust.
scoring. (1) The formation of severe scratches in the direction of sliding.
(2) The act of producing a scratch or narrow groove in a surface by
causing a sharp instrument to move along that surface. (3) The marring
or scratching of any formed metal part by metal pickup on the punch or
die.
scouring. (1) A wet or dry cleaning process involving mechanical scrubbing. (2) A wet or dry mechanical finishing operation, using fine abrasive and low pressure, carried out by hand or with a cloth or wire wheel
to produce satin or butler-type finishes.
scuffing. (1) Localized damage caused by the occurrence of solid-phase
welding between sliding surfaces, without local surface melting. (2) A
mild degree of galling that results from the welding of asperities due to
frictional heat. The welded asperities break, causing surface degradation.
seal coat. Material applied to infiltrate the pores of a thermal spray deposit.
sealing. Closing pores in anodic coatings to render them less absorbent.
seizure. The stopping of relative motion as the result of interfacial friction. Seizure may be accompanied by gross surface welding. The term
is sometimes used to denote scuffing.
selective leaching. Corrosion in which one element is preferentially removed from an alloy, leaving a residue (often porous) of the elements
that are more resistant to the particular environment. Also called dealloying or parting. See also dezincification and graphitic corrosion.
sensitization. In austenitic stainless steels, the precipitation of chromium carbides, usually at grain boundaries, on exposure to temperatures of about 540 to 845°C (about 1000-1550 OF), leaving the grain
boundaries depleted of chromium and therefore susceptible to preferential attack by a corroding medium. Welding is the most common
cause of sensitization. Weld decay (sensitization) caused by carbide
precipitation in the weld heat-affected zone leads to intergranular corrosion.
shielded metal arc welding (SMAW). An arc welding process that produces coalescence of metals by heating them with an arc between a
Glossary of Terms /251
covered metal electrode and the workpieces. Shielding is obtained from
decomposition of the electrode covering. Pressure is not used, and filler
metal is obtained from the electrode. Also commonly referred to as
stick welding.
shotblasting. Blasting with metal shot; usually used to remove deposits or
mill scale more rapidly or more effectively than can be done by sandblasting.
shot peening. A method of cold working metals in which compressive
stresses are induced in the exposed surface layers of parts by the impingement of a stream of shot, directed at the metal surface at high velocity under controlled conditions.
siliconizing. Diffusing silicon into solid metal, usually low-carbon steels,
at an el~vated temperature in order to improve corrosion or wear resistance.
solid lubricant. Any solid used as a powder or thin film on a surface to
provide protection from damage during relative movement and to reduce friction and wear.
sour gas. A gaseous environment containing hydrogen sulfide and carbon dioxide in hydrocarbon reservoirs. Prolonged exposure to sour gas
can lead to hydrogen damage, sulfide-stress cracking, and/or stresscorrosion cracking in ferrous alloys.
spalling. (1) The spontaneous chipping, fragmentation, or separation of a
surface or surface coating. (2) A chipping or flaking of a surface due to
any kind of improper heat treatment or material dissociation.
sputtering. The bombardment of a solid surface with a flux of energetic
particles (ions) that results in the ejection of atomic species. The
ejected material may be used as a source for deposition. See also physical vapor deposition.
steam treatment. The treatment of a sintered ferrous part in steam at
temperatures between 510 and 595°C (950 and 1100 OF) in order to
produce a layer of black iron oxide (magnetite, or ferrous-ferric oxide,
FeO· Fe203) on the exposed surface for the purpose of increasing hardness and wear resistance.
stray-current corrosion. Corrosion resulting from direct-current flow
through paths other than the intended circuit. For example, by an extraneous current in the earth.
stress corrosion. Preferential attack of areas under stress in a corrosive
environment, where such an environment alone would not have caused
corrosion.
stress-corrosion cracking (SCC). A cracking process that requires the
simultaneous action of a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate an alloy without applied or residual stress. Stress-corrosion
cracking may occur in combination with hydrogen embrittlement.
252 / Surface Engineering for Corrosion and Wear Resistance
stress relieving. Heating to a suitable temperature, holding long enough
to reduce residual stresses, and then cooling slowly enough to minimize
the development of new residual stresses.
strike. (1) A thin electrodeposited film of metal to be overlaid with other
plated coatings. (2) A plating solution of high covering power and low
efficiency designed to electroplate a thin, adherent film of metal.
submerged arc welding (SAW). An arc welding process that produces
coalescence of metals by heating them with an arc or arcs between a
bare metal electrode or electrodes and the workpieces. The arc and
molten metal are shielded by a blanket of granular, fusible material on
the workpieces. Pressure is not used, and filler metal is obtained from
the electrode and sometimes from a supplemental source (welding rod,
flux, or metal granules).
substrate. The material, workpiece, or substance on which the coating is
'deposited.
sulfidation. The reaction of a metal or alloy with a sulfur-containing
species to produce a sulfur compound that forms on or beneath the surface on the metal or alloy.
sulfide stress cracking (SSe). Brittle fracture by cracking under the combined action of tensile stress and corrosion in the presence of water and
hydrogen sulfide.
superabrasives. Synthetically produced diamond and cubic boron nitride
(CBN) used in a wide variety of cutting and grinding applications.
superfinishing. A low-velocity abrading process very similar to honing;
however, unlike honing, superfinishing processes focus primarily on
the improvement of surface finish and much lesson correction of geometric errors (dimensional accuracy). Also known as microhoning.
surface damage. In tribology, damage to a solid surface resulting from mechanical contact with another substance, surface, or surfaces moving relatively to it and involving the displacement or removal of material. In certain contexts, wear is a form of surface damage in which material is
progressively removed. In another context, surface damage involves a deterioration of function of a solid surface even though there is no material
loss from that surface. Surface damage may therefore precede wear.
surface finish. (1) The geometric irregularities in the surface of a solid
material. Measurement of surface finish shall not include inherent
structural irregularities unless these are the characteristics being measured. (2) Condition of a surface as a result of a final treatment.
surface hardening. A generic term covering several processes applicable
to a suitable ferrous alloy that produces, by quench hardening only, a
surface layer that is harder or more wear resistant than the core. There
is no significant alteration of the chemical composition of the surface
layer. The processes commonly ·used are carbonitriding, carburizing,
induction hardening, flame hardening, nitriding, and nitrocarburizing.
Use of the applicable specific process name is preferred.
Glossary of Terms 1253
surface .modiflcation.
The alteration of surface composition or structure by the. use of energy or particle beams. Two types of surfacemodification methods commonly employed are ion implantation and
laser surface processing.
surface roughness. Fine irregularities in the surface texture of a material, usually including those resulting from the inherent action of the
production process. Surface roughness is usually reported as the
arithmetic roughness average, Ra, and is given in micrometers or microinches.
surfacing. The deposition of filler metal (material) on a base metal (substrate) to obtain desired properties or dimensions, as opposed to making ajoint.
T
tarnish. Surface discoloration of a metal caused by formation of a thin
film of corrosion product.
terne. An alloy of lead containing 3 to 15% Sn,used as a hot dip coating
for steel sheet or plate. The term long terne is. used to describe. ternecoated sheet, whereas short terne is used for terne-coated plate. Terne
coatings, which are smooth and dull in appearance (terne means dull or
tarnished in French), give the steel better corrosion resistance and enhance its ability to be formed, soldered, or painted.
thermal spraying. A group of coating or welding ·processes in which
finely divided metallic or nonmetallic materials are deposited in a
molten or semimolten condition to form a coating. The surfacing material may be in the form of powder, rod, or wire. See also arc spraying,
flame spraying, plasma spraying, and powder flame spraying.
thermal stresses. Stresses in a material resulting from nonuniform temperature distribution.
thermal wear. Removal of material due to softening, melting, or evaporation during sliding or rolling. Thermal shock and high-temperature
erosion may be included in the general description of thermal wear.
Wear by diffusion of separate atoms from one body to the other, at high
temperatures, is also sometimes denoted as thermal wear.
thief. A racking device or nonfunctional pattern area used in the electroplating process. to provide a more uniform current density on plated
parts. Thieves absorb the unevenly distributed current on irregularly
shaped parts, thereby ensuring that the parts will receive an electroplated coating of uniform thickness. See also robber.
throwing power. The ability of a plating solution to produce a uniform
metal distribution on an irregularly shaped cathode. Compare with covering power.
tolerance. The specified permissible deviation from a specified nominal
dimension, or the permissible variation in size or other quality characteristic of a part.
254 / Surface Engineering for Corrosion and Wear Resistance
transformation
temperature. The temperature at which a change in
phase occurs. This term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used for
irons and steels:
ACern
AC1
AC3
AC4
Aeern, Ael' Ae3, Ae4
Arern
Arl
In hypereutectoid
The temperature
The temperature
The temperature
The temperatures
In hypereutectoid
The temperature
is completed
The temperature
The temperature
The temperature
The temperature
cooling
The temperature
steel
at which austenite begins to form during heating
at which transformation of ferrite to austenite is completed during heating
at which austenite transforms to 8 ferrite during heating
of phase changes at equilibrium
steel
at which transformation of austenite to ferrite or to ferrite plus cementite
during cooling
at which austenite begins to transform to ferrite during cooling
at which 8 ferrite transforms to austenite during cooling
at which transformation of austenite to pearlite starts during cooling
at which transformation of austenite to martensite is completed during
at which transformation
of austenite to martensite
starts during cooling
Note: All these changes, except formation of martensite, occur at lower
temperatures during cooling than during heating and depend on the rate
of change of temperature.
trees. Visible projections of electrodeposited metal formed at sites of high
current density.
tribology. (1) The science and technology of interacting surfaces in relative motion and of the practices related thereto. (2) The science concerned with the design, friction, lubrication, and wear of contacting surfaces that move relative to each other (as in bearings, cams, or gears,
for example).
tuberculation. The formation of localized corrosion products scattered
over the surface in the form of knob like mounds called tubercles.
tumbling. Rotating workpieces, usually castings or forgings, in a barrel
partly filled with metal slugs or abrasives, to remove sand, scale, or
fins. It may be done dry, or with an aqueous solution added to the contents of the barrel. See also barrel finishing.
u
ultraprecision finishing. Machining processes used to alter surface characteristics such as finish, waviness, roundness, and so forth, with substantial removal of the work material. Examples include lapping and
polishing of optical lenses, computer chips, or magnetic heads, and
honing of cylinder liners.
ultrasonic cleaning. Immersion cleaning aided by ultrasonic waves that
cause microagitation.
underfilm corrosion. Corrosion that occurs under organic films in the
form of randomly distributed threadlike filaments or spots. In many
cases, this is identical to filiform corrosion.
uniform corrosion. (1) A type of corrosion attack (deterioration) uniformly distributed over a metal surface. (2) Corrosion that proceeds at
Glossary of Terms / 255
approximately the same rate over a metal surface. Also called general
corrosion.
v
vacuum carburizing. A high-temperature
gas carburizing process using
furnace pressures between 13 and 67 kPa (0.1 and 0.5 torr) during the
carburizing portion of the cycle. Steels undergoing this treatment are
austenitized in a rough vacuum, carburized in a partial pressure of hydrocarbon gas, diffused in a rough vacuum, and then quenched in either
oil or gas.
vacuum deposition. Deposition of a metal film onto a substrate in a vacuum by metal evaporation techniques.
vacuum nitrocarburizing. A subatmospheric nitrocarburizing process
using a basic atmosphere of 50% ammonial50% methane, containing
controlled oxygen additions of up to 2%.
vapor degreasing. Degreasing of work in the vapor over a boiling liquid
solvent, the vapor being considerably heavier than air. At least one constituent of the soil must be soluble in the solvent. Modifications of this
cleaning process include vapor-spray-vapor, warm liquid-vapor, boiling
liquid-warm liquid-vapor, and ultrasonic degreasing.
vapor deposition. See chemical vapor deposition, physical vapor deposition, and sputtering.
vapor plating. Deposition of a metal or compound on a heated surface
by reduction or decomposition of a volatile compound at a temperature below the melting points of the deposit and the base materiaL
The reduction is usually accomplished by a gaseous reducing agent
such as hydrogen. The decomposition process may involve thermal
dissociation or reaction with the base materiaL See also vacuum deposition.
w
wear. Damage to a solid surface, generally involving progressive loss of
material, due to a relative motion between that surface and a contacting
surface or substance. Compare with surface damage.
wear debris. Particles that become detached in a wear process.
weathering. Exposure of materials to the outdoor environment.
welding. (1) Joining two or more pieces of material by applying heat or
pressure, or both, with or without filler material, to produce a localized
union through fusion or recrystallization across the interface. The
thickness of the filler material is much greater than the capillary dimensions encountered in brazing. (2) May also be extended to include
brazing and soldering. (3) In tribology, adhesion between solid surfaces
in direct contact at any temperature.
256 / Surface Engineering for Corrosion and Wear Resistance
white layer. (1) Compound layer that forms in steels as a result of the nitriding process. (2) In tribology, a white-etching layer, typically associated with ferrous alloys, that is visible in metallographic cross sections
of bearing surfaces.
white rust. Zinc oxide; the powder product of corrosion of zinc or zinccoated surfaces.
wiped coat. A hot dipped galvanized coating from which virtually all free
zinc is removed by wiping prior to solidification, leaving only a thin
zinc- iron alloy layer.
wiping effect. Activation of a metal surface by mechanical rubbing or
wiping to enhance the formation of conversion coatings, such as phosphate coatings.
wire flame spraying. A thermal spraying process variation in which the
material to be sprayed is in wire or rod form. See also flame spraying.
Z
zincrometal. A steel coil-coated product consisting of a mixed-oxide underlayer containingzinc particles and a zinc-rich organic (epoxy) topcoat. It is weldable, formable, paintable, and compatible with commonly used adhesives. Zincrometal is used to protect outer body door
panels in automobiles from corrosion.
Index
A
Abrasion. See also Abrasive wear; High-stress
abrasion; Low-stress abrasion.
definition
Abrasion-corrosion
Abrasive, definition
Abrasive blasting, definition
Abrasive erosion. See also Erosion.
231
70, 71 (F)
231
231
definition . . . . . . . . . . . . . . .
231
70, 71 (F)
56-61(F,T)
categories by type of contact
57-59(F)
definition . . .
56, 231
electroless nickel coatings . . . . . . . . . 151 (T)
gouging
57-58, 59(F)
hardness vs. wear resistance process
comparisons . . . . . . . . . . . . . . . . . . 186
high-stress
57-58, 59(F), 61, 243
low-stress (scratching)
57-58, 59(F), 61
multibody
55(F), 57
polishing
57-58, 59(F)
porcelain enamels. . . . . . . . . . . . . . . . . . 134
prevention through surface
treatments
61, 62(F), 75, 202
rate of
57, 164(T)
rate rankings for various materials,
low-stress
188(T)
surface treatments for prevention .. 206-209
synergistic relationship with corrosion
mechanisms
5(T)
test methods
.
82(T)
test variables to be controlled
81-82,
83(T)
of thermal spray coatings. . . . . . . . . . 164(T)
thermal spray coatings applications .. 165(T)
under lubricated conditions
78
versus erosion
63
wear testing devices
82, 84(T)
weld-overlay coatings
157, 158(F)
Abrasive metal-to-metal wear
Abrasive wear
Accelerated corrosion (weathering)
test
definition . . . . . . . . . . . . . . . .
Acidified synthetic seawater testing or
SWAAT (ASTM G 85, A3; formerly
ASTM G 43), description
37(T)
Acidity, of water, and corrosion. . . . . . . . . . 13
Acid treatments
146
Acrylics
applications . . . . . . . . . . . . . . . . . . . . 130(T)
characteristics, cost, and
applications
129(T)
in hybridized systems
128
modification of
128-129
thermosetting, resistant to mechanical
and chemical action .. . . . . . . . . 131 (T)
Acrylonitrile-butadiene (nitrile) rubber,
environmental
resistance ratings
Active metal, definition
Additives, for paints
Adhesion. See also Adhesive wear.
definition
Acetic acid-salt spray (fog) test (ASTM G
85), description
37(T), 38-39
231-232
Adhesion test (ASTM D 3359-90),
description
37(T)
56(F), 72-75(F,T)
coefficient . .
74(T)
definition
72, 232
description
72-73(F)
fretting
76-77
galling
75-7 6(F)
lubricants and
73-74
material combinations affected
72
materials selection . . . . . . . . . . . .
74-75
rates of various materials, pin on plate
sliding test . . . . . . . . . . . . . 189-190(T)
surface treatments for prevention .. 201-206
test variables to be controlled
81-82,
83(T)
thermal spray coatings
applications . . . . . . . . . . . . . . . . 165 (T)
wear testing devices
82, 84(T)
Adhesive wear
Aesthetic appearance
1
Agriculture, industrial operations and annual
wear economic consequences
37(T)
231
128(T)
231
128
4(T)
Aircraft, costs of metallic corrosion in U.S.
(1975, 1995)
3(T)
160
abrasive wear rate . . . . . . . . . . . . . . . 164(T)
Air plasma spraying (APS)
258 / Surface Engineering for Corrosion and Wear Resistance
design characteristics. . . . . . . . . . . . . 162(T)
Alclad, definition
232
Alkaline cleaner
definition
232
intermediate
146
Alkalinity, of water, and corrosion. . . . . 13-14
Alkyd-amines, resistant to mechanical and
chemical action . . . . . . . . . . . . . . . 131 (T)
Alkyds
127
applications . . . . . . . . . . . . . . . . . . . . 130(T)
characteristics, cost, and
applications . . . . . . . . . . . . . . . . 129(T)
resistant to mechanical and chemical
action. . . . . . . . . . . . . . . . . . . . . 131 (T)
Alloying, process availability . . . . . . . . . . . 185
Alloy steels
flame hardening
89(T)
hardened, abrasive wear
61
Alpha-aluminum
142, 143
Alpha-chromium,
used in chromizing
119
Alumina
136
as chemical vapor deposition coating
material
170(T), 171(T), 211(T)
thermal sprayed, thickness ranges and
hardness levels
211 (T)
Alumina/chromium
oxide/tungsten carbidecobalt coatings, plasma sprayed,
hardness range. . . . . . . . . . . . . . . . 187 (F)
Alumina plasma sprayed coating, cost,
relative. . . . . . . . . . . . . . . . . . . . . . 191(F)
Aluminizing
applications
198
characteristics and
requirements
117-118(F)
definition
232
diffusion (pack cementation)
198, 201,
209
process availability . . . . . . . . . . . . . . . . . 185
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Aluminum
as anode material with impressed
current
34
dealuminification . . . . . . . . . . . . . . . . . . . 26
oxidation . . . . . . . . . . . . . . . . . . . . . . . . . 16
pitting corrosion. . . . . . . . . .
20
pure, corrosion rate
11
Aluminum alloys
corrosion rates
11
erosion
63, 64(F)
exfoliation . . . . . . . . . . . . . . . . . . . . . . . . 26
fretting corrosion . . . . . . . . . . .
25
galvanic corrosion
17(T)
intergranular corrosion . . . . .
25
stress-corrosion cracking
28(T)
Aluminum anodizing
102-107 (F,T)
applications . . . . . . . . . . . . . . . . . . . . . . 102
chromic anodizing
. 102-103
classification of types . . . . . . . . . . . . . . . 102
corrosion pits per m2 as function of
coating thickness . . . . . . . . . . . . 106(F)
corrosion resistance of
products. . . . . . . . . . . . . . 106-107 (F,T)
corrosion test
40
erosion . . . . . . . . . . . . . . . . . . 106-107 (F,T)
hardcoat anodizing . . . . . . . . . . . . . 104-105
military specification (MIL-A-8625) for
classification . . . . . . . . . . . . . . . 102(T)
sealing . . . . . . . . . . . . . . . . . . . . . . . . . . 102
sealing of coatings . . . . . . . . . . . . . 105-106
sulfuric anodizing, types of
102
Aluminum bronze hardfacing alloys, to
prevent cavitation damage
7
Aluminum bronzes, dealuminification .. 27(T)
Aluminum coatings,
applications
143-144(F,T)
Aluminum ion plating, applications .....
198
Aluminum oxide ceramics, hardness
range
187(F)
Amino resin-modified alkyds,
applications . . . . . . . . . . . . . . . . . . 130(T)
Amsler circumferential
rotating disk-on-disk
machine (Ams) test geometry
52(T)
Anaerobic bacteria
13
Anode
12(F), 14, 15, 34
Anodic desmutting
146
Anodic protection
5
Anodized aluminum corrosion test,
description . . . . . . . . . . . . . . . . . . .... 40
Anodizing
applications
198, 199, 201, 204, 208, 212
benefits . . . .
2(T)
definition
232
design limitations
223(T)
to prevent fretting corrosion
25
Aqueous solution
electroplating
. . . . . . . . . . . . 145-147 (T)
Arc spraying (ASP), definition
232
Arc welding, definition . . . . . . . . . . . . . . . 232
Arc wire spray process,
characteristics
188(T)
Atmospheric corrosion
of anodized aluminum
106-107(F,T)
definition
232
description
14-15
Atmospheric corrosion tests on metal,
practice for conducting (ASTM
G 50)
36-38(F)
Austenitic manganese steels, as coatings .. 61
Austenitic stainless steels
intergranular corrosion
25-26
stress-corrosion cracking . . . . . . . .
28(T)
to prevent erosion
66-67
Autocatalytic plating
l5D-151(T), l52(T)
Index / 259
B
design limitations
Back-ionization
222
Barrel cleaning, definition
232
Barrel finishing, definition
232
Barrel plating, definition
232
Batch galvanizing process, wet or dry
138
Batch processing, of hot dip coatings . . . . 138
Binders
128
Biologically influenced corrosion
21
Black oxide, definition
233
Black oxide chemical conversion coatings,
benefits
2(T)
Blast cleaning, definition
233
Blasting, definition
233
Blastingldeburring, design limitation .. 220(T)
Block-on-ring (BOR) test
geometry
47-48(T)
Bluing
2(T)
definition
Borates
Borides, as sputter coating material . . .
Boriding. See also Boronizing.
characteristics
definition
and requirements
Borohydride compound reducing
agents
Boron carbide
233
32
. 175
. . . 119-120
233
150
as chemical vapor deposition coating
material . . . . . . . . . . . . . . . . . . . 171 (T)
as coating material . . . . . . . . . . . . . . . . . 137
hardness range. . . . . . . . . . . . . . . . . . 187 (F)
Boronizing (boriding)
applications
209, 210
benefits
2(T)
characteristics and requirements . . . 119-120
process availability . . . . . . . . . . . . . . . . . 185
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Boron powder
119-120
Brasses
11, 26-27(T)
Brazing, temperature range and
distortion . . . . . . . . . . . . . . .. . . . . 192(F)
Break-in
100
Bright finish, definition
233
Bright nitriding, definition
233
Bright plate, definition
233
Brinelling. See also False brinelling.
definition
233
Broachinglhoning, design limitations .. 220(T)
Bronzes
11, 27(T)
Brushing, design limitations
221(T)
Brushinglburnishing, design
limitations . . . . . . .
220(T)
Brush plating, definition
233
Buffing
definition
233
. . 220(T)
Burnishing
definition
design limitations
233
220(T)
Butadiene rubber, environmental resistance
ratings
128(T)
Butler finish, definition
233
c
Cadmium, galvanic corrosion
Cadmium electroplating,
applications
17(T)
147, 148(T)
Cadmium plating
applications
198, 212
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Calorizing, definition
233
Carbide coatings, applications . . . . . . . . . 137
Carbide diffusion process
applications
benefits
204, 210, 212
2(T)
Carbide hardening alloys
157
Carbide materials, abrasive wear . . . . . . . . 61
Carbides
25, 175
Carbon, content effect on intergranular
corrosion
25-26
Carbon alloy steels, electron-beam
hardening . . . . . . . . . . . . . . . . .
91
Carbonitride/carburize/cyanide
coatings, hardness range . . . . . . . . 187 (F)
Carbonitriding
applications
203, 205, 207
benefits . . . . . . . . . . . . . . . . . . . . . . . . . 2(T)
characteristics and
requirements .... 110-111(T), 115-116
coating thickness/penetration
depth . . . . . . . . . . . . . . . . . 192-193 (F)
definition . . . . . . . . . .
234
Carbon steels
corrosion fatigue
30
crevice corrosion . . . . . . . . . . . . . .
22
electron-beam hardening
91
flame hardening. . . . . . . . . . . . . . . . . . 89(T)
hydrogen damage
30
pitting corrosion. . . . . . . . . . . . . . . . .
20
stress-corrosion cracking
28(T)
Carburization resistance, surface engineering
treatments for improvement. . . . . . . . 2(T)
Carburizing
7(F)
applications
203, 205, 207, 210, 211
benefits . . . . . . . . . . . . . .
2(T)
characteristics and
requirements .. 110-111(T), 112-113(F)
coating thickness/penetration
depth
192-193(F)
cost, relative
191(F)
260 / Surface Engineering for Corrosion and Wear Resistance
Carburizing (continued)
definition
234
phase transformations. . . . . . . .
3
process availability in U.S. and Canadian
commercial heat treating
shops
184-185
temperature range and distortion . . . . 192(F)
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
to prevent fretting corrosion
25
Case, definition . . . . . . . . . . .
234
Case hardening .. 61, 110-111(T), 112-113(F)
applications
200, 203, 205, 207
definition
234
size and weight limitations
215(T)
surface finish characteristics
213(T)
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
CASS test. See also Copper-accelerated saltspray test.
definition
234
Cast irons
crevice corrosion
22
electron-beam hardening
91
flame hardening
88, 89(T)
galvanic corrosion . . . . . . . . . . . . . . . . 17(T)
graphitic corrosion
26, 27
laser melting
92(F)
manganese phosphate coatings for
parts. . . . .
99(T)
structural grades, corrosion rate . . . . . . . . 11
Cast steel, erosion
'.'
68, 69(F)
Cathode
12(F), 14, 15
Cathodic breakdown test,
description
40
5, 16, 18, 33-35(F)
7, 23-24(F)
definition . . . . . . . . . . . . . . . . . . . . . . . . 234
thermal spray coatings applications .. 165(T)
Cavitation corrosion, definition
234
Cavitation damage
7-8,68
definition
234
Cavitation erosion
15, 16(F), 68, 70, 71(F)
definition
234
test variables to be controlled .. 81-82, 83(T)
wear testing devices
'.' .. 82, 84(T)
Cement
32, 135, 192(F)
Cementation, definition
234
Cementation/diffusion, design
limitations
223(T)
Cemented carbide cermets, hardness
range . . . . . . . . . . . . . . . . . . . . . . . 187 (F)
Cemented carbides, fracture of
7
Cement linings, benefits. . . . . .
2(T)
Ceramic coatings
132-138(F,T)
applications . . . . . . . . . . . . .
207
benefits
2(T)
porcelain enamels
133-134(T)
Cathodic protection
Cavitation
Ceramic high-performance
coatings and linings
applications
......................•
Ceramic linings
Ceramics
136-138(F)
136
132-138(F,T)
abrasive wear
60, 61
adhesive wear
74-75
erosion test results
65(T)-66(T)
fracture of . . . . . . . . . . . . . . . . . .
7
relative erosion factors
65-66, 67 (F)
wear coefficients, adhesive wear
74(T)
Ceramic thermal spray materials
61
Cermet coatings
151-153(F,T)
applications
203, 204, 205, 206, 207
erosion test results
66, 67 (F)
Cermet thermal spray materials
61
Checks, definition
235
Chemical attack
Chemical conversion coating,
definition
Chemical deposition, definition
Chemical etching
Chemical milling
design limitations
Chemical polishing, definition
Chemicals, as corrosion contributor.
Chemical vapor deposition
8
235
235
219
219
220(T)
235
. . . . . . 14
(CVD)
7(F), 168-172(T)
advantages, limitations, and processing
parameters . . . . . . . . . . . . . . . . . 126(T)
applications
169-172(T), 204, 210, 212
benefits
2(T)
characteristics compared to PVD and
ion implantation
172, 173(T)
chemical reactions . . . . . . . . . . . . . . . . . 169
closed reactor systems
169
coating thickness/penetration
depth
192-193(F)
cost, relative
191(F)
definition
235
design limitations
224(T)
open reactor systems
169
plasma process. . . . . . . . . . . . . . . . . . 169(T)
process availability
185
processing steps . . .. . . . . . . . . . . . 168-169
size and weight limitations
215(T)
surface finish characteristics
213(T)
temperature range and distortion . . . . 192(F)
thermal process
169(T)
Chemistry, surface changes
2(T)
Chlorimet 2, galvanic corrosion
17(T)
Chlorimet 3, galvanic corrosion . . . . . .. 17 (T)
Chlorinated rubbers
31, 129(T),
130(T), 131(T)
Chlorine
14
Chloroprene rubber, environmental
resistance ratings
128(T)
Chromate
31-32
Index / 261
Chromate chemical conversion
coatings
Coatings. See also Chemical vapor deposition;
100-102(T)
2(T)
benefits
salt-spray test data on zinc and
aluminum
101(T)
Chromate treatment, definition
235
Chromating
211(T), 212, 235
Chromatizing, to prevent erosion
66-68
Chromic anodizing
102-103, 105-106
Chromium carbide
abrasive wear
61
as chemical vapor deposition coating
material
170(T), 171(T), 211(T)
as coating material . . . . . . . . . . . . . . . . . 137
Chromium carbide/nickel-chromium, thermally sprayed and spray and fused, thickness ranges and hardness levels ... 211 (T)
Chromium electroplating
147, 187(F)
Chromium nitride, as coating material. .. 137,
Chromium oxide, as coating material ....
zurn
136,
211(T)
Chromium plating
applications 201, 203, 204, 205, 206, 207, 209
characteristics
145(T)
cost, relative
191(F)
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Chromium-rich carbides
25
Chromizing
applications
198, 201, 209, 210
characteristics and requirements . . . . . . . 119
definition
235
process availability. . . . . . . . . . . . . . . . . 185
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Cladding. See also Coating; Hardfacing;
Surfacing
7(F), 166-168(F)
benefits
2(T)
corrosion control metal
systems
166-168(F)
definition
235
design limitations
~.223(T)
high-volume, commercially produced
metal combinations. . . . . . . 166, 167 (F)
process availability. . . . . . . . . . . . . . . . . 185
techniques employed . . . . . . . . . . . . . . . 166
Clad metal, definition
235
Clad transition metal systems
168
Cleaning. See also Barrel cleaning; Blast
cleaning; Electrolytic cleaning; Emulsion
cleaner.
alkaline
146, 232
electro . . . . . . . . . . . . . . . . . . . . 146, 220(T)
precleaning. . . . . . . . . . . . . . . . . . . . . . . 146
surface cleaning methods before painting 132
ultrasonic
219
Coal-tar enamel
34-35
Physical vapor deposition; Weld-overlay
coatings
1, 187(F)
aluminum
117, 143-144(F,T)
barrier
32-33(F)
benefits . . . . . . . . . . .
2(T)
black oxide chemical conversion
2(T)
borides . . . . . . . . . . . . . . . . . . . . . . . . . . 175
carbide
137
cementatious
134-136(F)
ceramic
2(T), 132-138(F,T), 207
cermet
66, 67(F), 151-153(F,T), 203,
204,205,206,207
chlorinated rubber
31
chromate chemical conversion
2(T),
100-102(T)
chromium carbide 137, 170-171(T), 211(T)
chromium nitride
137, 211(T)
chromium oxide
136, 211 (T)
cobalt-base alloys
61
composite
151-153(F,T)
concrete . . . . . . . . . . . . . . . . . . . 134-136(F)
corrosion tests used
37(T)
definition
235
diamond-like carbon (DLC)
126(T),
170(T), 175, 211(T)
diffusion heat treatment .. 3, 7, 110-116(F, T)
distortion or size change
tendencies. . . . . . . . . . . . . . 191-192(F)
elastomeric
2(T)
electro less nickel/nickel
electroplate. . . . . . . . . . . . . . . . . 187 (F)
electroless nickel-boron
151(T)
electroless nickel-phosphorus
151(T),
152(T),198
electrolytic nickel .. 197, 199, 203, 205, 212
electrolytic zinc
197, 198
electroplated
32, 221(T), 192-193(F)
epoxy
31, 127
flame-sprayed. . . . . . . . . . . 136, 192-193(F)
fluorinatedethylene
propylene (PEP)
201
fluorinated polymer
201
fog test data
38, 186(T)
galvanized
140, 141(T), 142
of gold
33(F)
hafnium carbides as
137
high-carbon iron-chromium alloys as . . . . 61
of high-performance ceramic ... 136-138(F)
high-velocity oxyfuel (HVOF) thermally
sprayed
·208, 209
hot dip
138-144(F,T)
indium
175
inorganic
222-226(F,T), 130(T)
iron phosphate . . . . . . . . . . . . . . . ... 96, 97
laser/electron-beam hardened. . . . . . . 187 (F)
manganese phosphate
97-98, 99(T)
material availability and delivery
time
184-185
262 / Surface Engineering for Corrosion and Wear Resistance
Coatings (continued)
MCrAIY metallic
174
of nickel
33
nickel-aluminum intermetallic .. 152-153(F)
nickel-boron
150-151(T), 152(T)
of nickel-chromium
33
nickel-chromium-boron
61
nickel-phosphorus .... 0 150-151(T), 152(T)
nickel-teme 0
00. 0. 00
0. 144
niobium silicide . 0.. 0.. 0
0
137-138
nitride . 00 .....
0
00
137(F), 175
noble metal 0.. 00. 0
0
32-33(F)
organic .. 0
0002(T), 127-132(T)
organic high-performance 0.... 0.... 00031
organizations active in test development
and standardization
0.. 0
35-36
oxide application .. 0
00.. 0
0. 00 136
pack-cementation diffusion .. 0. . 116-120(F)
perfluoro alkoxy alkane (PFA)
applications. . . . . . . . 0.. 000. 201, 206
phosphate chemical conversion. 00. 0.. 2(T),
95-100(T)
polyester
0
0. 0
0
0. 031
polyurethane
0
0 .. 031
porcelain enamels applications .. 133-134(T)
powder. 0.. 0. 0
127, 197, 221(T)
process availability . . . . . . . . . . . . . 184-185
process comparisons found in
Chapters 1 to 6 0
0. . . . 183-184(T)
resins as 0. 0. 00
128-132(T)
sacrificial
33, 34(F)
salt mist corrosion performance of various
steels and coatings . . . . . . . . . . . 186(T)
sealing of . . . . . . . . . . . . . . . . . . . . 105-106
silicate glass 0. . .
0. . . . . . . . . . . 136
silicide
137-138, 175
silicon carbide
0
0
0
137
sol-gel
221(T)
sputter
7(F), 175, 192-193(F)
sulfuric anodized . 0
0
104
tantalum silicide
137-138
teme. 0
144
thermal spray
160-166(F,T), 192(F),
197-198,203-213(T),215
thickness available . . . . . . . . . . . 192-193(F)
thicknesses of various surface engineering
treatments . . . . . . . . . . . . . . . .
6(F)
titanium carbide
137
titanium carbonitride
137(F)
titanium nitride
61, 137(F)
to prevent cavitation. . . . . . .
24
to prevent corrosion
31-35(F)
to prevent corrosion fatigue . . .
30
to prevent crevice corrosion . . . . . . . .
22
to prevent erosion-corrosion
23
to prevent fretting corrosion
25, 77
to prevent galling
76
to prevent galvanic corrosion . . . . . . . . . . 19
to prevent pitting corrosion
20
to prevent slurry erosion
71-72
to prevent stress-corrosion cracking 28(F), 29
to prevent uniform corrosion . . . . . . . . . . 16
to reduce wear. . .
7(F)
trowel
136
tungsten carbide
137
tungsten carbide-cobalt.
27(T), 191(F),
200, 211(T)
vinyl
31
Watts nickel.
151(T)
weld hardfacing . . . . . . . . . . . . . . . .
61
zinc
2(T)
zinc-aluminum
142-143(F)
zinc phosphate
96
zirconia thermal barrier
136, 174(F)
Cobalt + chromium carbide, cost,
relative
191 (F)
Cobalt-base alloys, as coatings
61
Cobalt-base hardfacing alloys
8
to resist erosion and cavitation
7
Coefficient of friction. See also Friction,
coefficient of.
definition . . . . . . . . . . . . . . .
235
Coefficient of static friction. See Static
coefficient of friction.
Cohesion, definition
236
Color buffing, definition . . . .
236
Coloring. See also Color buffing.
definition . . . . . . . . . . . . . . . . . . . . . . . . 236
Combustion gun spraying, cost,
relative
191 (F)
Combustion powder spray process,
characteristics
188(T)
Combustion wire spray process,
characteristics
. . . . . . . . . . . . . . . . 188(T)
Comminution . . . . . . . . . . . . . .
4
Composite coating
151-153(F,T)
applications. 0
203, 204, 205
definition . . . . . . . . . . . . . . . . . . . . .
236
Composite deposition plating
145,
151-153(F,T)
Composite plate, definition
236
Compound white layer, ferritic
nitrocarburizing . . . . . . . . . . . . . . . . . 116
Compound zone
113-114(F), 115
Concentration cell . . . . . . . . . . . . . . . . . 14(F)
Conductor
12(F)
Contact corrosion, definition
236
Contact plating, definition
236
Contact stresses . . . . . . . . . . . . . . . . . . . . . . 8
Continuous electrodeposition . . . . . . 147-148
Continuous hot dip processing
138
Conversion coating. See also Chromate
treatment; Phosphating.
definition . . . . . . . . . . .
design limitations
Cook-Norteman line
. . . . . . . . ... 236
220(T)
138
Index / 263
Copper
erosion-corrosion
23
uniform corrosion. . . . . . . . . . . . . . . . . . . 16
Copper-accelerated
acetic acid-salt spray
(fog) test (CASS test)(ASTM B 368).
See also CASS test.
39
definition
236
description
37(T)
Copper alloys
galvanic corrosion
17(T)
pitting corrosion
20
stress-corrosion cracking
28(T)
Copper-gold single crystals, dealloying
corrosion
27(T)
Copper plating
characteristics
145(T)
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Copper-zinc alloys, dezincification
26-27
Core, definition . . . . . . . . . . . . . . . . .
236
Corrodkote test, definition
236
Corrosion. See also Specific forms of
corrosion. . . . . . . . . . . . . . . . . 13-31 (F,T)
abrasion
70, 71(F)
atmospheric
14-15, 36-38(F),
106-107(F,T),232
by salt spray on steels and coatings ... 37(T),
39, 186(T), 236
conditions for
13-15(F)
corrodent forms . . . . . . . . . . . . . . . . . . . . 11
dealloying
26-27(T)
definition . . . . . . . . . . . . . . . . . . . . . 11, 236
economic effects . . . . . . . . . . . . . . . . . . 3(T)
forms of
15-31(F,T)
galvanic
4, 15-19(F,T), 198-199,
241-242
prevention
1, 2(T), 16, 25,
31-35(F), 100, 103
rates of materials compared . . . . . . . . 11, 13
structural parts, surface engineering
solutions for
196, 197-199
synergistic relationships with wear
mechanisms. . . . . . .
~
5(T)
tests
35--41(F,T), 231
Corrosion-barrier
systems
167, 168(F)
Corrosion cell. . . . . . . . . . . . . . . . . . . . . 18(F)
Corrosion-erosion.
See Erosion-corrosion.
Corrosion fatigue
15, 16(F), 29-30(T)
definition
236
description. . . . . . . . . . . . . . .
29(T)
mechanical, metallurgical, and environmental
variables influencing behavior
29(T)
prevention . . . .
30
Corrosion inhibitor. See Inhibitor.
Corrosion product, definition
236
Corrosion protection, definition
236
Corrosion rate . . . . . . . . . . . . . . . . . . . . . . 11
definition
237
Corrosion resistance
1, 130, 131
definition
237
surface engineering treatments for
improvement
2(T)
Corrosion system, definition
237
Corrosion testing
35--41(F,T)
field tests
36
organizations active in test development and
standardization for coatings
35-36
simulated service tests
36-38(F)
tests used for corrosion resistance of
protective coatings
37(T)
Corrosive wear. See also
Oxidative wear
4-5(T)
definition . . . . . . . . . . . . . . . . . . . . . . . . 237
Costs
factors affecting . . . . . . . . . . . . . . . 190-191
of surface treatments . . . . . . . . . 190-191 (F)
Covering power, definition . . . . .
237
Cracking
of chromized steels. . . . . . . . . . . . . . . . . 119
environmental.
15, 16(F), 239
Cratering threshold voltage . . . . . . . . 141 (T)
Crevice corrosion
15, 16(F), 21-22(F)
definition
237
Cross slip . . . . . . . . . . . . . . . . . . . .
76
Cutting
59, 60(F)
Cyaniding
192-193(F)
definition
237
D
Deaeration, of water, and corrosion. . . . . . . 14
Dealloying. See also Dezincification;
Graphitic corrosion
15, 16(F)
definition
237
Dealloying corrosion
26-27(T)
Dealuminification
26, 27 (T)
Decarburization
27 (T)
Degreasing, definition . . . . . . . . . . . . . . . . 237
Dehumidification,
to prevent corrosion .... 13
Delamination wear, under lubricated
conditions
78
Depolarizing agents . . . . . . . . . . . . . . . . . . 13
Deposit corrosion . . . . . .
21
definition
238
Desiccation . . . . . . . . . . . . . . . . . . . . .
5
Design
of flame hardening equipment
88
to minimize cavitation . . . .
24
to prevent crevice corrosion . . . . . . . . . . . 22
to prevent erosion-corrosion
23
to prevent hydrogen damage
31
to prevent slurry erosion . . . . . . . . . . . 71-72
of wear testers
.
82-84
Design guidelines for surface
engineering
195-228(F,T)
264 / Surface Engineering for Corrosion and Wear Resistance
Design limitations
environmental regulations of surface
engineering. . . . . . . . . .
228
inorganic coating processes
222-226(F,T)
interrelation between the component,
fixturing, and equipment
limitations
214(F)
organic coating processes
219-222(T)
principles related to surface
engineering
213-218(F,T)
specifications
226-228
substrate features influence on
electroplateability
225(T)
surface preparation processes
218-219,
220-221(T)
Desiliconi6cation
..
27 (T)
Destanni6cation
27 (T)
Detonation flame spraying, definition
238
Detonation gun (D-gun) process
abrasive wear rate . . . . . . . . . . . . . . . 164(T)
characteristics . . . . . . . . . . . . . . . . . . 188(T)
design characteristics. . . . . . . . . . . . . 162(T)
Dezinci6cation. See also Dealloying;
Selective leaching. . . . . . .
26, 27 (T)
definition
238
Diamond, as particle additive for nickel
electroless plating
153
Diamond coatings
advantages, limitations, and processing
parameters . . . . . . . . . . . . . . . . . 126(T)
chemical vapor deposition .....
170(T), 172
Diamondlike carbon (DLC) coatings
advantages, limitations, and processing
parameters . . . . . . . . . . . . . . . . . 126(T)
as chemical vapor deposition coating
material
170(T)
as sputter coating material . . . . . . . . . . . 175
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Dichromate treatment, definition
238
Differential coating, definition
238
Diffusion aluminizing
2(T), 198
Diffusion (case)-hardened
surfaces
8
Diffusion chromizing, applications. . . . . . 198,
201,209
Diffusion coating, definition
238
Diffusion heat treatment coatings
3, 7,
110-116(F,T)
Diffusion (pack cementation)
aluminizing
198,201,209
applications . . . . . . . . . . . . . . . . . . . . . . 198
benefits
2(T)
Diffusion (pack cementation) siliconizing,
benefits . . . . ~. . . . . . . . . . . . . . . . . . . . 2(T)
Diffusion treatments
7(F)
Diffusion zone
113-114(F), 116
Dilution ratio
155(T), 156(F), 159
Diphase cleaning, definition
238
Dipping, design limitations. . . . . . . . . . 221 (T)
Dislocation cross slip
76·
Distortion
191-192(F)
definition
238
design guidelines;
210-212(T)
Droplet erosion. See also Erosion.
definition
238
Dryers
128
Dry-sandlrubber-wheel
test (ASTM G 65),
data for various coatings .....
188(T), 189
Duplex stainless steels, stress-corrosion
cracking
28(T)
Dynamic coefficient of friction. . .
44
E
e-coat cratering, resistance to . . . . . . . . . . 141
Economic costs
industrial operations with significant annual
wear consequences
4(T)
metallic corrosion in the U.S. .
3(T)
Elastomeric coatings and linings,
benefits . . . . . . . . . . . . . . . . . . . . . . . 2(T)
Electrical properties. . . . . . . . . . . . . . . . . . . 1
Electric arc (wire arc) spray process, design
characteristics . . . . . . . . . . . . . . . . 162(T)
Electrochemical
cell. . . . . . . . . . . . . . . . 12(F)
Electrochemical
coatings (plating)
7(F)
Electrochemical
conversion coatings,
benefits
2(T)
Electrochemical
corrosion. . . . . . . . 11-13(F)
definition
238
Electrochemical
deposition
145-153(F,T)
Electrochemical
impedance spectroscopy
(EIS), description . . . . . . . . . . . . . . 40-41
Electrochemical
tests
39-41
Electrocleaning.
. . . . . . . . . . . . . . 146, 220(T)
Electrocoating,
design limitations
221 (T)
Electrodeposition
25, 223(T)
definition
145, 238
Electroforming,
definition
238
Electrogalvanized
steel strip, thickness
ranges and hardness levels . .
211 (T)
Electrogalvanizing
147
definition
238
Electrographic
and chemical porosity
tests, description
37(T)
Electroless nickel + chromium plating,
characteristics
145(T)
Electroless nickel (before hardening)/nickel
electroplate coatings, hardness
range . ... . . . . . . . . . . . . . . . . ... . . 187 (F)
Electroless nickel-boron coatings
abrasive wear resistance
151(T)
corrosion rate in various
environments
152(T)
Electroless nickel/ceramic plating, thickness
ranges and hardness levels
211 (T)
Index /265
Electroless nickel-phosphorus coatings
abrasive wear resistance
151(T)
applications . . . . . . . . . . . . . . . . . . . . . . 198
corrosion rate in various
environments
152(T)
Electroless nickel plating
150-151(T),
152(T), 210, 212
applications
198, 199, 201,
203,204,205,206
characteristics . . . . . . . . . . . . . . . . . . 145(T)
cost, relative
191(F)
size and weight limitations
215(T)
surface finish characteristics
213(T)
thickness ranges and hardness
levels
211 (T)
Electroless plating .. 7(F), 150-151(T), 152(T)
advantages, limitations, and processing
parameters . . . . . . . . . . . . . . . . . 126(T)
benefits
o' ••••••••••
2(T)
coating thickness/penetration
depth
192-193(F)
definition . . . . .
238
design limitations
223(T)
Electrolyte . . . . . . . . . . . . . . . . . . . . . . . 12(F)
definition
12
Electrolytic cleaning, definition
239
'0'
•••
o
••••
Electrolytic copper plating,
applications
199, 212
Electrolytic corrosion test (ASTM B 627),
description
40
Electrolytic deposition, definition
239
Electrolytic nickel coating, applications. . 197,
199,203,205,212
Electrolytic pickling, definition
Electrolytic zinc coatings, applications
239
. . 197,
198
Electron beam (EB)
7(F)
Electron beam-hardened steels
8
Electron-beam (EB) hardening 2(T), 90-91,
192-193(F)
Electron beam heat treating, definition .. 239
Electron beam/physical vapor deposition
(EBIPVD)
174(F)
Electron beam welding (EBW)
154, 190(F),
192(F)
Electronic properties . . . . . . . . . . . . . . . . . . 1
Electrophoresis, to apply ceramic
coatings
136
Electrophoretic plating
219, 223(T)
Electroplate, definition . . . . . . .
239
Electroplated nickel + chromium plating,
characteristics
. . . . . . . . . . . . . . . . 145(T)
Electroplated nickel plating,
characteristics
. . . . . . . . . . . . . . . . 145(T)
Electroplating
advantages, limitations, and processing
parameters . . . . . . . . . . . . . 126(T), 146
applications
6, 210
benefits
2(T)
coating thickness/penetration
depth
192-193(F)
comparison of applications of thermal
spraying and welding. . . . . . . . . 161 (T)
definition
145, 239
design limitations
223(T)
process requirements compared to those of
thermal spraying and welding .. 161 (T)
size and weight limitations
215(T)
surface finish characteristics
213(T)
Electropolishing
219, 220(T)
definition
239
Electropolymerization
219, 221 (T)
Electroslag welding (ESW), for
weld-overlay coatings
155(T)
Electrotinning, definition
239
Emulsion cleaner, definition . . . . . . . .
239
Encyclopedia of Associations
185
Environmental cracking, definition
239
Environmentally assisted
cracking . . . . . . . . . . . . . . . . . . 15, 16(F)
Epoxies ..... 31,127, 129(T), 130(T), 131(T)
Epoxy-tilled rebuilding cements
8
Epoxy powder
34-35
Erosion. See also Cavitation erosion;
Erosion-corrosion
56(F)
abrasive . . . . . . . . . . .
231
of aluminum alloys
63, 64(F)
anodized, aluminum
106-107 (F,T)
cavitation
7, 15, 16(F), 68, 70, 71(F),
81-84(T)
definition
239
description . . . . . . .
61-63
high-velocity
70, 71(F)
of high-velocity oxyfuel spray
coatings
o. 164(F)
of laser-melted gray and ductile
irons
92(F)
liquid. . . .
7, 68-69(F)
liquid impingement
68
low-erosion
70, 71(F)
manifestations in service. . . . .
61-62
mechanisms of material removal .. 68, 69(F)
Miller numbers
71, 72(T)
particle to target hardness ratio and .
resistance
66
prevention
7, 66-68, 69
rain
68
rate
63, 64-66(T), 164(F)
rate rankings for various materials. . . 189(T)
related to plastic deformation
159
saltation
70, 71(F)
slurry
62-63, 69-72(F,T)
solid particle erosion (SPE) .. 61-68(F, T), 186
surface treatments for prevention
208
thermal spray coatings applications .. 165(T)
variables influencing
'063, 64(F)
~o
266 / Surface Engineering for Corrosion and Wear Resistance
Erosion (continued)
versus abrasion. . . . . . . . . .
63
weld-overlay coatings
157-159(F)
Erosion-corrosion
15, 16(F), 22-23(F)
definition
239
Erosion tests (ASTM G 76)
65
Erosive wear, test methods . .
82(T)
Erosivity, definition
239
Etching, design limitations . . . . . . . . .. 220(T)
Ethylene-propylene
(-diene) rubber,
environmental resistance ratings
128(T)
European Space Agency
84
Exfoliation
15, 16(F), 26
definition
239
Explosive bonding, benefits . . .
2(T)
Exterior exposure test (ASTM D 1014) .. 37(T)
Extreme-pressure
lubricant, definition ... 240
F
FACT test (formerly ASTM B 538),
description
37(T)
False brinelling, definition . . . . . . .
240
Faraday cage effect .....
216(F), 221(T), 222,
224
Fatigue
1
definition . . . . . . . . . . . . . . . . . . .
240
Ferritic nitrocarburizing
2(T), 111 (T),
115-116, 192-193(F)
Ferritic stainless steels, corrosion of
25, 30
Ferroboron
119-120
Ferrous alloys, hardness range . . . . . 61, 62(F)
Fiberglass, veil or woven roving mat
reinforcing gel coats . . . . . . . . . . . . . . 127
Fiberglass layups, for corrosion
resistance . . . . . . . . . . . . . . . . . . . . . . 131
Fiberglass-reinforced
plastics, hand
lay-ups
127
Filiform corrosion .. . . . . . . . . . . . . . . . . . 15
definition
240
Finish, definition . . . . .
240
Finish grinding, definition. . . . . . . . . . . .. 240
Fish eyes . . . . . . . . . . . . .
31
Fixed oils
80
Fixture, definition
240
Flakes
31
Flaking, of PIM steels surface oxide
layer
108
Flame hardening
7(F), 8,61, 87-90(F,T)
applications
88, 200, 203
benefits . . . . . . . . . . . . . . . . . . . . . . . . . 2(T)
coating thickness/penetration
depth
192-193(F)
definition . . . . . . .
240
Flame spraying
cost for process . . . . . . . . . . . . . . . . . 190(F)
definition
240
design characteristics. . . . . . . . . . . . . 162(T)
to apply oxide coatings. . . . . . . . . . . . . . 136
Flame spraying (FLSP) coatings, coating
thickness/penetration depth ... 192-193(F)
Flat block-on-rotating
ring (BOR) test
geometry
50(T)
Flat surface sliding on flat surface (FOF)
test geometry
47-48(T), 49(T),
50-51(T), 51-52(T), 53(T)
Floe process
115
Flow-control agents
128
Fluidized bed, definition
240
Fluidized-bed boriding
120
Fluidized-bed processing, to apply ceramic
coatings . . . . . . . . . . . . . . . . . . . . . . . 136
Fluorides. . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Fluorinated ethylene propylene (FEP)
coatings, applications
201
Fluorinated hydrocarbon
127
Fluorinated polymer coatings,
applications
...
201
Fluorocarbons,
characteristics, cost, and
applications . . . . . . . . . . . . . . . . . . 129(T)
Fluoroelastomer,
environmental resistance
ratings
128(T)
Flux
63
Flux-cored arc welding (FCAW). See also
Flux-cored electrode.
cost for weld-overlay coatings
190(F)
definition
240-241
for weld-overlay coatings
154, 155(T)
Flux-cored arc welding/gas metal arc
welding hardfacing, coating
thickness/penetration depth. . . 192-193 (F)
Flux-cored electrode, definition
241
Flux line
138
Fog test. See Neutral salt-spray test.
Ford anodized aluminum corrosion test
(FACT) test, description
37(T), 40
Fretting
76-77
definition . . . . . . . . . . . . . . . . .
241
description . . . . .
76-77
prevention
.............
77
sites
77
test methods .. .
82(T)
test variables to be controlled .. 81-82, 83(T)
thermal spray coatings applications .. 165(T)
wear testing devices
82, 84(T)
Fretting corrosion
15, 16(F), 24-25(F),
199-200
definition
241
Fretting fatigue, definition . . . . . . .
241
Fretting wear
186
definition . . . . . . . . . . . . . .
241
testing devices . . . . . .
82, 84(T)
Friction
43-54(F,T)
angle
4S(F)
definition . . . . . . . . . . . . . . . . .
43, 241
Index / 267
friction coefficient
44, 46-54(T)
friction force .. .
44
heating
43, 45, 74
mechanisms, basic
45-46
solid friction, definition . . . . . . . . . . . . . . 44
work
45
Frictional energy . . . . . . . . . . . . . . . . . . . 1
Frictional heating
43,45
Friction angle
45(F)
Friction coefficient. See also Coefficient of
friction
44, 46-54(T)
ceramics sliding on metals and
ceramics. . . . . . . . . . . . . . . . . . .. 49(T)
coatings sliding on metals, ceramics,
and polymers
51-52(T)
definition
235
factors contributing to
46
metals sliding on metals
47-48(T)
miscellaneous materials, sliding test
geometry
53(T)
polymers sliding on metals and
polymers
50(T)
Friction force
44, 46
Friction work
45
Furnace fusing, temperature range and
distortion . . . . . . . . . . . . . . . . . . . . 192(F)
Fused-salt electroplating
145, 148-149
Fused-spray deposit, definition
241
Fusion hardfacings, hardness range
187 (F)
Fusion welding
temperature range and distortion
7(F)
. . . . 192(F)
G
Galfan
139, 142-143
Galling. See also Scoring; Scuffing ..... 55(F),
75-76(T)
and adhesive wear
74
definition . .
241
galvanized coatings
140
hardness vs. wear resistance process
comparisons . . . . . . . . . . . . . . . . . . 186
test methods
...
82(T)
Galling test (ASTM G 98)
48(T)
Galvalume
139, 143(F)
Galvanic corrosion
4, 15, 16-19(F,T)
definition
241-242
description
16-19(F,T)
galvanic series in seawater at 25°C
(77 OF)
17(T)
prevention . . . . . . . . . . . . . . . . . . . 17(T), 19
surface treatments for prevention .. 198, 199
Galvanic protection, by a coating
33- 34(F)
Galvanic protection systems
33-35(F)
Galvanic series, definition
242
Galvanize, definition
242
Galvanized coatings
139-141(F,T)
alloying effects. . . . . . . . . . . . . . . . . . . . 140
corrosion resistance . . . . . . . . . . 140, 141 (F)
fabric ability and weldability . . . . . . . . . . 140
paintability
140-141
Galvanized steel, corrosion of
14, 17(T)
Galvanized wrought iron, galvanic
corrosion . . . . . . . . . . . . . . . . . . . . . 17(T)
Galvanizing
design limitations
hot dip .... 2(T), 197-199,203,
size and weight limitations
surface finish characteristics
Galvanneal, definition
223(T)
211-212(T)
215(T)
213(T)
242
Galvanneal coatings
141 (T), 142
Galvannealing
138, 139
Gas boriding
120
Gas carbonitriding, characteristics and
requirements
111(T), 115-116
Gas carburizing, characteristics and
requirements
111(T), 112, 113(F)
Gaseous corrosion. See also Hot corrosion;
Oxidation; Sulfidation.
definition
242
Gas metal arc welding (GMAW)
cost for weld-overlay coatings
190(F)
definition
242
for weld-overlay coatings
154-155(T)
Gas nitriding, characteristics and
requirements
111(T), 113, 114-115
Gas tungsten arc welding (GTAW)
cost for weld-overlay coatings . .. . . . 190(F)
definition
242
for weld-overlay coatings
154, 155(T)
Gas tungsten arc welding hardfacing, coating
thickness/penetration depth. . . 192-193(F)
General corrosion. See also Uniform corrosion.
definition
242
Glass linings
132-133
benefits . . . . . . . . . . . . . . . . . . . .
2(T)
for steel vessels
133
Gloss-control agents. . . . . . . . . . . . . . . . . 128
Gold
corrosion of
17(T), 23
for precious metal plating. . . . . . . . . . . . 149
galvanic corrosion
17(T)
Gold alloys with copper or silver, de alloying
corrosion . . . . . . . . . . . . . . . . . . . . . 27 (T)
Gouging abrasion. See also Abrasion.
definition
242
Graphite
80
as anode material with impressed
current
34
galvanic corrosion . . . . . . . . . . . . . . . . 17(T)
Graphite fluoride
80
Graphitic corrosion
26, 27(T)
definition
242
Gray irons
dealloying corrosion
26,27
268 / Surface Engineering for Corrosion and Wear Resistance
Gray irons (continued)
graphitic corrosion
laser melting
Grinding
definition
design limitations
Grit blasting, definition
Gunite method
27 (T)
91
242
220(T)
242
135
H
Hafnium carbide, as coating material . . . . 137
Halogens
14
Hard anodizing
2(T)
Hard chromium
7
definition
242
Hard chromium coatings, abrasive wear
resistance. . . . . . . . . . . . . . . . . . . . 151 (T)
Hard chromium plating
7, 61, 199
Hardcoat anodizing
104-105
Hardfacing. See also Surfacing; Weld-overlay
coatings
7
definition
242-243
Hardfacing alloys
157
Hardness
and adhesive wear ranges
73
range of carbides, mineral, and alloy
microconstituents
61, 62(F)
versus abrasion resistance
62(F)
Hastelloy alloys, corrosion of
17(T), 20
Health and safety precautions, cadmium
electroplating . . . . . . . . . . . . . . . . . . . 147
Heat resistance, surface engineering
treatments for improvement
2(T)
Heat treatments, preprocessing and
postprocessing
209-210, 226
Heavy case-hardened
steels
8
Hexavalent chromium
100, 101
High-carbon iron-chromium
alloys, as
coatings
61
High-carbon steels, decarburization
27 (T)
High-energy beam hardening
90-91
High-energy plasma spray process,
characteristics . . . . . . . . . . . . . . . . 188(T)
High-molecular-weight
resins
127
High-nickel alloys, dealloying
corrosion
27 (T)
High-nickel cast iron, galvanic
corrosion
17(T)
High-silicon cast iron, as anode material
with impressed current
34
High-strength
low-alloy steels, hardness
range . . . . . . . . . . . . . . . . . . . . . . . 187 (F)
High-strength
steels, hydrogen
embrittlement
30, 31
High-stress abrasion
57-58, 59(F)
definition
243
High-temperature
hydrogen attack
15
definition . . . . . . . . . . . . . . . .
243
High-velocity oxyfuel combustion spray
(HVOF) technique
160-162(T),
164(T), 188(T)
High-velocity oxyfuel (HVOF) thermally
sprayed coatings, applications .. 208, 209
Holidays, definition
243
Honing . . . . . . . . . . . . . . . . . . . '.'
220(T)
definition
243
Hot corrosion. See also Gaseous corrosion.
definition
243
Hot dip. See also Hot dip coating.
definition
243
Hot dip aluminized steel strip, thickness
ranges and hardness levels
211 (T)
Hot-dip aluminizing, benefits .. . . .
2(T)
Hot dip coatings
138-144(F,T)
aluminum coatings
143-144(F, T)
definition
243
galvanized coatings
139-141(F,T)
galvanneal coatings
141(T), 142
microstructure of coating
138-139
terne coatings. . . . . . . . . . . . . . . . . . . . . 144
zinc-aluminum coatings
142-143(F)
Hot dip galvanizing
applications
197, 198, 199, 203, 212
benefits
2(T)
thickness ranges and hardness levels .. 211 (T)
Hot-dip lead-tin alloys coatings,
benefits
2(T)
Hot-dipped aluminum coatings,
applications
6
Hot-dipped zinc-aluminum
coatings,
applications
6
Hot-dipped zinc coatings, applications
6
Hot dipping, design limitations
223(T)
Hot isostatically pressed (HIP)
coatings
2(T), 208, 209, 212
Hot-processed continuous line
138
Humidity cabinet tests (ASTM D 2247,
ASTM G 85), description
39
Humidity test, 100% relative (ASTM D
2247), description
37(T)
Hydride formation
15, 31
Hydrogen attack
31
Hydrogen blistering, definition
243
Hydrogen damage
15, 30-31
definition
243
Hydrogen embrittlement
'.' .. 15,30
definition
243
Hydrogen-induced
blistering
15,30
Hydrogen-induced
cracking (HIC)
30
definition
243
Hydrogen stress cracking, definition
243
Hydrogen sulfide, causing hydrogen
embrittlement
30
Index /269
Hypophosphite-reducing agents
150
Immersion cleaning, definition
Immersion coating, definition
Immersion plating, definition
Impact, synergistic relationships with
244
244
244
corrosion mechanisms
5(T)
55(F)
244
81-82,
83(T)
Impact wear
definition
test variables to be controlled
Impedance test for anodized aluminum
(ASTM B 457), description
40
Impingement. See also Erosion.
definition . .
244
Impingement attack. See also Erosioncorrosion; Impingement corrosion.
definition
244
Impingement corrosion, definition
244
Impingement erosion, definition
244
Impingement erosion wear, test variables to
be controlled
81-82, 83(T)
Impingement impact wear, wear testing
devices. .
82, 84(T)
Incidence, angle of
63, 64
Inclined surface test geometry (IS) 47-48(T)
Incoloy alloys, corrosion of.
17(T), 20
Inconel alloys, galvanic corrosion. . . . . . 17(T)
Indium, as sputter coating material
175
Indium plating, applications . . . . . . . . . . . 199
0
Induction-hardened steels
Induction hardening
8
7(F), 88-90(F,T)
200, 203, 207
2(T)
88(F)
applications
benefits
carbon content effect
coating thickness/penetration
depth
192-193(F)
definition
244
Information services, technical associations
listing surface treatment providers ... 185
Inhibitors
5, 31-32
definition . . . . . . . . . . . . . . . . . .
244
in phosphate coatings
98
to prevent stress-corrosion cracking
28(F)
to prevent uniform corrosion . . . . . . . . . . 16
Inorganic coatings, design
limitations
222- 226(F, T)
Inorganic painting, design
limitations
223(T)
Intergranular sulfidation corrosion
attack, of chromized steels. . . . . . . . . 119
Intermediate alkaline cleaning
146
Intermetallic hardening alloys
157
Interpass temperature
157
Interrupted-current plating,
definition
limitations, and processing
parameters
Ion carburizing, definition
126(T)
~. 244
Ion implantation
7(F), 120-122(T)
advantages, limitations, and processing
parameters . . . . . . . . . . . . . . . . . 126(T)
alloys suitable for
120, 121(T), 122(T)
applications
~
120-121(T), 201
benefits
2(T)
characteristics compared to CVD and
PVD
172, 173(T)
coating thickness/penetration
depth
192-193(F)
definition
244
design limitations
223(T)
line-of-sight limitations
217(F)
process availability. . . . . . . . . . . . . . . . . 185
size and weight limitations
215(T)
surface finish characteristics
213(T)
temperature range and distortion . . . . 192(F)
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Ion nitriding (plasma nitriding). . . . . 111 (T),
113, 115(T), 126(T)
definition
245
Ion plating
7(F), 172, 173(T), 175-176(F)
coating thickness/penetration
depth
192-193(F)
definition
245
design limitations
223-224(T)
line-of-sight limitations
217(F)
Iron, corrosion of
13, 20
Iron aluminides, as weld-overlay
coatings
159(F)
Iron-chromium alloys, dealloying
corrosion
27 (T)
Iron oxide scale
66
Iron phosphate bath
97
Iron phosphate coatings
96-97
o ••••
Isobutylene-isoprene (butyl) rubber,
environmental
resistance ratings
applications
corrosion
definition . . . . . . . .
resistance ratings ...
0
130(T)
15, 16(F), 25-26
244
••••••••••
Intergranular stress-corrosion cracking
(IGSCC), definition
244
128(T)
Isoprene rubber, environmental
Inorganic zinc-rich coatings,
Intergranular
244
Interstitial-free (IF) steels
139
lon-beam-assisted deposition, advantages,
0
••
128(T)
J
Jet Kote spray process, design
characteristics
. . . . . . . . . . . . . . . . 162(T)
270 / Surface Engineering for Corrosion and Wear Resistance
K
definition
Kinetic coefficient of friction
Kirkendall voids, produced by
44
chromizing . . . . . . . . . . . . . . . . . . . . . 119
245
62 (F)
Knife-line attack, definition
Knoop hardness scale
L
Lactic acid test, description
37(T)
Lapping, definition . . . . . . . . . . . . . . . . . . 245
Lapping/buffing, design limitations ... 220(T)
Laser alloying. See also Laser surface
processing
2(T), 122-123
91
245
Liquid erosion
7, 68-69(F)
Liquid honing, definition . .
245
Liquid nitriding (salt nitriding) . . . . . 111 (T),
113, 115
246
Liquid nitrocarburizing, definition. . . . . . 246
Liquid (salt-bath) boriding
120
Local hardening
213(T), 215(T)
Localized corrosion . . . . . . . . . . . . . . . . . . 13
definition . . . . .
246
Local surface hardening, thickness
ranges and hardness levels . . . . . . . 211 (T)
Low-alloy steels, hydrogen damage
30
Low-carbon steel, galvanic corrosion. . . 17(T)
Low-expansion borosilicate glass
132
definition . . . . . . .
Laser-beam hardening, description
Laser beam welding (LBW)
Low-pressure plasma spraying
cost for weld-overlay coatings . . . . . . 190(F)
definition . . . . . . . . . . . . . . . . . . . . . . . . 245
for weld-overlay coatings
154
Laser cladding, benefits. . . . . .
2(T)
Low-stress abrasion, definition
246
Lubricants .... 7(F), 73-74, 78-80(F), 126(T)
Laser/electron beam alloying
7(F)
Laser/electron-beam hardened coatings,
hardness range . . . . . . . . . . . . . . . . 187 (F)
Laser/electron-beam hardfacing, coating
thickness/penetration
depth
Laser-hardened steels
Laser hardening
192-193(F)
8
7(F)
applications
200, 203, 207
benefits
2(T)
coating thickness/penetration
depth
192-193(F)
definition . . . . . . . . . . . . . .
245
Laser melting. See also Laser surface
processing ... 2(T), 91-92(F), 185, 192(F)
Laser melting/alloying, design
limitations . . . . . . . . . . . . . . . . . . . 224(T)
Laser surface processing, definition ..... 245
Lead, corrosion of. . . . . . . . . . . . 16, 17(T), 23
Lead-tin alloy hot dip coatings,
applications
144
(LPPS)
160, 162, 164(T)
definition . . . . . . . . . . . . . . . . . . . . . . . . 246
Lubricating films. . . . . . . . . . . . . . . .... 7(F)
Lubrication
77-80(F)
boundary
78
circulating-oil. . . . . . . . . . . . . . . . . . . . ·78
definition . . . . . . . . . . . . . .
246
diagnosis of wear by spectroscopy
78
dry-film (solid-film)
78
elastohydrodynamic
78
hydrodynamic
78
hydrostatic
78
lubricants
78-80(F)
modes
78
polytetrafluoroethylene
added to
electroless nickel baths
153
sputter coatings for. . . . . . . . . . . . . . . .. 175
thin-film
78
to prevent adhesive wear
75, 202
to prevent fretting corrosion . .
25
to prevent galling
76
Luster finish, definition. . . . . . . . . . . .
246
Lead-tin solder (50-50), galvanic
corrosion . . . . . . . . . . . . . . . . . . . . . 17(T)
92
32
Ledeburite
Lime
Line pipe steels, hydrogen-induced
blistering
30
31
cementatious . . . . . . . . . . . . . . . 134-136(F)
ceramic
132-138(F,T)
concrete
134-136(F)
dual
135-136(F)
glass
132-133
of high-performance ceramics .. 136-138(F)
inorganic monolithic. . . . . . . . . . . . . . . . 135
Linings
Liquid carbonitriding (cyaniding),
characteristics
and requirements.
. . 111 (T)
Liquid carburizing. . . . . . . . . . . . . . . . 111 (T)
M
Magnesium, galvanic corrosion
17(T)
Magnesium alloys, galvanic corrosion .. 17(T)
Magnesium anodes, for cathodic
protection.
..............
Manganese phosphate coatings
parts immersed for wear resistance
34
97-98
. . . 99(T)
Manganese steels
before work hardening, hardness
range . . . . . . . . . . . . . . . . . . . . . 187 (F)
as wear plates. . . . . . . . . . . . . . . . . . . . .. 61'
Martensitic stainless steels ..... 28(T), 89(T)
Material/process selection checklist
8(F)
Matte finish, definition . . . . .
246
MCrAlY coatings .... 149-150, 174, 175, 198
Index / 271
Mechanical plating
2(T)
definition
246
peen plating, design limitations
224(T)
Mechanical polishing, definition
246
Mechanical properties
1
Medium-carbon
steels,
decarburization . . . . . . . . . . . . . . . . 27 (T)
Metaborates
32
Metal ion concentration
cell
21 (F)
Metallic coatings and cladding, for corrosion
resistance . . . . . . . . . . . . . . . . . . . . . . 131
Metalliding
145, 148-149, 192(F)
Metallizing, definition
246
Metallurgy, surface changes
2(T)
Metals
adhesive wear prevention . . . . . . .
7S
erosion test results
65(T)
fretting . . . . .
76-77
galling
~
75
relative erosion factors
65-66, 67 (F)
stacking-fault energies
76(T)
wear coefficients, adhesive wear
74(T)
Metal spraying. See also Thermal spraying.
definition . . . . . . . . . . . . . . . . . . . .....
246
Microcracking
5(T), 59, 60(F)
Microfatigue
59, 60(F)
Miller numbers
71, 72(T)
Mill scale
17, 18(F), 19, 20
Mineral oils
80
Mining, industrial operations and annual
wear economic consequences
4(T)
Mohs hardness scale
62(F)
Molybdates
32
Molybdenum
erosion rate
66
as sputter coating material . . . . . . . . . . . 175
Molybdenum disulfide
80
as sputter coating material . . . . . . . . . . . 175
Molydenum silicide, as coating
material
137-138
Monels, corrosion of
17(T), 27(T)
Morrison- Miller effect
69-70
Motor vehicles, costs of metallic
corrosion in U.S. (1975, 1995)
3(T)
Multihody abrasive wear
55(F), 57
N
Natural rubber, environmental resistance
ratings . . . . . . . . . . . . . . . . . . . . . . 128(T)
Neutral salt-spray (fog) test (ASTM
B 117)
38, 186(T)
Nickel, galvanic corrosion
17(T)
Nickel alloys, stress-corrosion cracking .. 28(T)
Nickel-aluminum
intermetallic
coating
152-153(F)
Nickel-base alloys, intergranular
corrosion
25
Nickel-base hardfacing alloys . . . . . . . . . .. 8
Nickel-boron coatings ... 15D-151(T), 152(T)
Nickel-chromium,
as sputter coating
material . . . . . . . . . . . . . . . . . . . . . . . 175
Nickel-chromium
boron alloys
8, 61
Nickel-chromium-chromium
carbide
plating, applications. .
200
Nickel-chromium
plating, applications
200
Nickel-molybdenum
alloys, dealloying
corrosion
27(T)
Nickel-phosphorus
coatings
lSD-1S1(T),
152(T)
Nickel plating
147, 211(T)
Nickel-terne coatings
144
Niobium, content effect on intergranular
corrosion
25-26
Niobium silicide, as coating
material
137-138
Ni-Resist, galvanic corrosion
17(T)
Nitride coatings
137(F), 175
Nitriding. See also Bright nitriding; Liquid
nitriding
7(F), 226
applications
200, 201, 203, 204, 205,
207,212
benefits . . . . . . . . . . . . . . . . . . . . . . . . . 2(T)
characteristics and requirements
11D-111(T),
113-115(F,T)
coating thickness/penetration
depth
192-193(F)
cost, relative
191 (F)
definition
246-247
Floe process
115
phase transformations. . . . . . . . . .
3
process availability in commercial heat
treating shops . . . . . . . . . . . . . . . . . 185
size and weight limitations
215(T)
steels treated in various applications . 114(F)
surface finish characteristics
213(T)
temperature range and distortion . . . . 192(F)
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
to prevent corrosion fatigue . . . . . . . . ... 30
to prevent fretting corrosion . . . . . . . . . . . 25
white nitride layer
115
Nitrocarburizing
applications
200,203,204,
204, 207, 212
cost, relative
191(F)
definition . . . . . . . . . . . . . . . . . . . . . . .. 247
size and weight limitations
215(T)
surface finish characteristics
213(T)
thickness ranges and hardness levels . 211 (T)
Nitrocellulose lacquers
129(T), 130(T)
Noble metal clad systems
167
Noble metals, erosion-corrosion
23
o
Occlusion
145
272 /Surface Engineering for Corrosion and Wear Resistance
Oil paints, applications.
Organic coatings
. . . . . . . . . . . . 130(T)
2(T), 127-132(T)
design limitations
219-222(T)
paints. . . . . . . . . . . . . . . . . . . . . 128-132(T)
Organisols, hot-applied
127
0
0
0
Oxidation
definition ....
247
and fretting corrosion
24
rate of
13
reaction. . . . . . . . . . . . . . . . . .
. . . 12-13
resistance
1, 159
treatments . . . . . . . . . . . . . . . . 108-11 O(F,T)
weld-overlay coatings
159
0
••
0
•
0
••••••••••••.••
Oxidative wear
definition
surface treatments for prevention
Oxide coatings, applications
Oxide treatments, applications
247
200
136
203, 205
55
211(T), 213(T), 215(T)
Oxide wear debris
Oxidizing
Oxyacetylene welding (OAW)
cost for weld-overlay coatings . . . . . . 190(F)
definition
247
for weld-overlay coatings
154, 155(T),
190(F)
Oxyfuel acetylene welding hardfacing,
coating thickness/penetration
depth
192-193(F)
Oxyfuel gas welding (OFW),
definition
247
21 (F)
Oxygen concentration cell
p
Pack aluminizing, characteristics and
requirements
Packcarburizing
o. • • • • • • • • • 117-118(F)
11(T), 112
247
0
definition
0
•
•
•
0
•••.••
0
•
•••••••
Pack-cementation diffusion
coatings. . . . .. . . . . . . . . . . . 116-120(F)
aluminizing
117-118(F)
boriding, or boronizing
119-120
chromizing .... '.' . . . . . . . . . . . . . . . . . 119
principles of process. . . . . . . . . . . . 116-117
siliconizing . . . . . . . . . . . . . . . . . . . 118-119
Pack-cementation diffusion processes
coating thickness/penetration
depth
192-193(F)
process availability in commercial heat
treating shops ..
185
temperature range and distortion . ... . 192(F)
to apply ceramic coatings.
136
Pack nitriding, definition
247
0
0
0
0
0
•
0
••
0
••
•••••••
0
o
•••••
•
0
•
0
••••
00
•
Paint adhesion on a scribed surface (PASS)
test, description ..
40
Painting
6, 20, 31, 221 (T)
Paints ..
32(F), 128-132(T)
0
0
0
•••••••••••
applications
•
0
•••••••••••••
•••••••••
0
••
0
••••
0
197, 198, 212
benefits ..
.2(T)
corrosion resistance as functional
requirement.
130
electrophoretic (e-coat) ....
141
function ...
129-131 (T)
resistant to mechanical and chemical
action
131 (T)
size and weight limitations
215(T)
surface cleaning methods used before
application
132
surface contaminants
131-132
surface finish characteristics
.213(T)
surface preparation ..
131-132
Paint spraying, line-of-sight
limitations . . . . . . . . . . . . . . .....
217 (F)
Particle rotational speed
63
Particle velocity . . . . . . . .
63
Parting corrosion. See Dea1loying corrosion.
Passivation, design limitations
224(T)
Passive, definition
247
Paste boriding
120
0
0
0
••
0
0
0
0
0
0
0
0
•
0
•
•
0
0
••
0
0
•••
••
0
0
0
0
•
'.0
0
•
0
•••
••••
•••••••••
0
0
0
0
•
•
0
0
•
••
•
'0
0
•
•
0
•••
•
•
•
••
'.'
•
••••••••
•
'0
0
0
0
0
•
0
•••
•••
','
••••••••••
0
•
•
•••••
Perfluoro alkoxy alkane (PFA) coatings,
applications
0
•
0
•
•
201, 206
Phenolics
35, 129(T), 130(T), 131(T)
Phosphate chemical conversion coatings. See
also Phosphating.
application method
96
applications
98~ 1OO(T)
articles coated. . . . . . . . .
96
benefits
2(T)
for corrosion protection
98-99
iron phosphate coatings
96-97
manganese phosphate coatings
97-98
as metalforming lubricant in forming
steel
99
thickness range and coating. weight
96
types
96-98
wear resistance reduction on machine
elements
~
99-1 OO(T)
weight and crystalline structure
95-96
zinc phosphate coatings . . . . . . . . . . .
96
Phosphates
32
Phosphating. See also Conversion coating;
Phosphate chemical conversion coatings.
applications
198, 212
definition
247
size and weight limitations
215(T)
surface finish characteristics
213(T)
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Physical vapor deposition (PVD)
7(F),
172-176(F,T)
advantages, limitations, and processing
parameters . . . . . . . . . . . . . . . ... 126(T)
application methods . . . . . . . . . . . . . . . . 172
benefits
2(T)
characteristics compared to CVD and ion
implantation. . . . . . . . . . . . 172, 173(T)
Index / 273
coatings, applications ... 201, 204, 205, 207,
210-211,212
coating thickness/penetration
depth
192-193(F)
cost, relative. . . . . . . . . . . . . . . . . . . . 191 (F)
definition
247-248
design limitations
.
224(T)
line-of-sight limitations
217(F)
process availability . . . . . . . . . . . . . . . . . 185
processing steps . . . . . . . . . . . . . . . 172-173
size and weight limitations
215(T)
surface finish characteristics
213(T)
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Pickling, design limitations
220(T)
Pickoff of coatings, galvanized coatings .. 140
Pigments, added to resins
128
Pin-on-disk (POD) test geometry
49(T),
50-51(T),52(T)
Pin-on-flat (POF) test geometry ... 50-51(T),
51-52(T)
Pin-on-ring (POR) test geometry .. 47-48(T)
Pitting
~
15, 16(F), 19-20
of anodized aluminum
106-107 (F,T)
definition
248
Plasma arcwelding (PAW), definition
248
Plasma arc welding (PAW) hardfacing,
coating thickness/penetration
depth
192-193(F)
Plasma-assisted chemical vapor deposition,
definition . . . . . . . . . . . . .
Plasma boriding
Plasma (ion) carburizing. See also Ion
248
120
carburizing.
characteristics and requirements
112,
113(F), 115
Plasma (ion) nitriding. See also Ion nitriding.
characteristics and requirements . . . . 111 (T),
113, 115(T)
properties of treated metals
115(T)
Plasma spraying
characteristics
188(T)
coating thickness/penetration depth
(PSP)
192-193(F)
cost for process . . . . . . . . . . . . . . . . . 190(F)
definition . .
248
to apply oxide coatings. . . . . . . . . . . . . . 136
to prevent fretting corrosion
25
Plasma transferred arc welding (PAW), for
weld-overlay coatings
154, 155(T)
Plastic deformation, and erosion rate .... 159
Plastisols
127, 131(T)
Plating
design limitations
reactive ion
temperature range and distortion
Platinum
as anode material with impressed
8
223(T)
175-176(F)
. . . . 192(F)
current
34
corrosion of . . . . . . . . . . . . . . . . . . 17(T), 23
for precious metal plating . . . . . . . . 149-150
Plowing
59, 60(F)
definition
60
Plumbates
32
Polarization
13, 35
Polishing. See also Electropolishing
219
definition
248
design limitations
220(T)
test variables to be controlled .. 81-82, 83(T)
wear testing devices
82, 84(T)
Polyesters
31, 127, 129(T)
Polymer coatings and linings
7(F)
adhesive wear prevention
75
applications
212
benefits
2(T)
size and weight limitations
215(T)
as sputter coating material . . . . . . . . . . . 175
surface finish characteristics
213(T)
Polyphosphate
31
Polytetrafluoroethylene (PTFE)
80
aslubricant
199,201,202,
203,204,205,206
as particle additive for nickel electroless
plating
153
as sputter coating material . . . . . . . . . . . 175
Polyurethanes
31, 129(T), 131(T)
Porcelain enamels
2(T), 133-134(T)
definition
248-249
Post-processing bake-out treatments, to
prevent hydrogen damage
31
Poultice corrosion. . . . . . . . . . . . . . . . . . . . 15
definition
249
Powder coatings
127, 197, 221(T)
Powder flame spraying, definition
249
Powder metallurgy (p/M) steels .. 108-11 O(F)
Powder weld (PW), for weld-overlay
coatings
155(T)
Power-law velocity dependence
64
Power spray washing technique
219
Precious metal plating,
applications
149-150
23
Precious metals, erosion-corrosion
Precipitation-hardenedsteels, hydrogen
embrittlement
'
30
Precleaning
146
Precoated metal products, definition
249
Primary metals, industrial operations and
annual wear economic consequences
Pure zinc ('Y) phase
Pyrex, applications
4(T)
139
132
Q
Quenched-and-tempered
embrittlement
steels, hydrogen
.............
30
274 I Surface Engineering for Corrosion and Wear Resistance
Quench hardening, definition
Quench hardening alloy and tool steels,
temperature range and distortion
Quenching, definition
249
. . 192(F)
249
Reactive ion plating . . . . . . . . . . . 175-17 6(F)
Rebuilding cements, coating thickness/
penetration depth . . . . . . . . . . 192-193(F)
Reciprocating pin-on-flat test geometry
(RPOF)
49(T), 53(T)
Reciprocating, spherically ended pin
on a flat surface (RSOF) test
geometry
47 -48(T)
Red lead
32
Reducing agents
150
Reduction, rate of
13
Relative erosion factor (REF) .. 65-66, 67 (F)
Repair cements, temperature range and
distortion . . . . . . . . . . . . . . . . . . . . 192(F)
249
. . . 128-132(T)
216
definition . . . . . . . . . . . . . . . . . .
249
Roll bonding, benefits . . . . . . . . .
2(T)
Rolling-contact fatigue. . . . . . . . . . . .
77
definition
249
test variables to be controlled .. 81-82, 83(T)
wear testing devices. . . . . . . . . . . . 82, 84(T)
Rolling-contact wear
55(F), 77
surface treatments for prevention .. 201-206
Residual stress, definition
Resins, as coatings
Robber
Rolling with slip wear
test variables to be controlled .. 81-82, 83(T)
wear testing devices. . . . . . . . . . .. 82, 84(T)
Rouge finish, definition
249
Rubbers
127-128(T), 131
"Rule of Four and Six"
224, 226(F)
Rust
12-13, 19, 32(F)
definition . . . . . . . .
249
s
Sacrificial metals for cladding . . . . . 167-168
Salt-bath carburizing, characteristics and
requirements
characteristics
and requirements
definition
Scoring
R
Salt fog test, definition. . . . . . . . . .
Salt nitriding (liquid nitriding),
Satin finish, definition
Scab test, description
Scaling. See also Rust.
112, 113(F)
250
. . 111 (T),
113, 115
Salt particles. . . . . . . . . . . . . . . . . . . . . . . . 15
Salt-spray test (ASTM B 117) .... 38-39(T),
101(T)
coating life data. . . . . . . . . . . . . . . . . 186(T)
description
37(T)
sulfuric anodized coatings
104
Salt water, as electrolyte
12(F)
Sandelin Effect
139
definition
of gears
test methods
Scouring, definition
Scouring wear
Scrap iron, as anode material with
250
37(T)
250
55(F), 75
250
99
. .. 82(T)
250
70, 71(F)
impressed current
Scuffing
definition
surface treatments
of TRD processed
Seal coat, definition.
Sealing
definition . . . . . .
Seizure
definition . . . . . .
34
55(F), 75
250
for prevention . . . . .. 205
sheet steel
178(F)
..............
250
102
............
250
75
.....
250
Selective hardening
temperature range and distortion . . . . 192(F)
treatments. . . . . . . .
61
Selective leaching. See also Dealloying corrosion; Dezincification; Graphitic corrosion.
definition . . . . . . . .
250
Selective surface-hardened alloy steels
8
Selenides, as sputter coating material. . . . . 175
Sensitization
15, 26
definition . . . . . . . . . . . . . . . . . . . . . . . . 250
37(T)
31
127
Service test data test, description
Shatter cracks
Sheet linings
Shielded metal arc welding (SMAW)
cost for weld-overlay coatings . . . . . . 190(F)
definition
250-251
for weld-overlay coatings
154, 155(T)
Shotblasting, definition. . . .
251
Shotcreting method
135
Shot peening
93(F)
applications . . . . . . . . . . . . . . . . . . . . . . 199
benefits . . . . . . . . .
2(T)
definition . . . . . . . . . . . . . . . . . . . . .
251
design limitations
224(T)
fatigue curves for steel spring wires
93(F)
phase transformations. . . . . .
3
size and weight limitations
215(T)
surface finish characteristics
213(T)
to prevent corrosion fatigue
30
to prevent fretting corrosion . . .
25
to prevent stress-corrosion cracking
29
Silicate cements
135
Silicate glass coatings, applications. . . . . . 136
Silicates
31, 32
Silicide coatings
applications
137-138
as sputter material. . . . . . . . . . . . . . . . . . 175
Index 1275
Silicon bronzes, desiliconification
Silicon carbide
.....
27 (T)
chromium
as chemical vapor deposition coating
material.
170(T), 171(T)
as coating material . . . . . . . . . . . . . . . . . 137
hardness range
187(F)
as particle additive for nickel electroless
plating
153
Silicon dioxide, as chemical vapor deposition
coating material. . . . . . . . . . . . . . . 169(T)
Silicone alkyds, applications. . . . . . . . . 130(T)
Silicone resins, resistant to mechanical and
chemical action . . . . . . . . . . . . . . . 131 (T)
Silicone rubber, environmental resistance
ratings
128(F)
Silicones
applications . . . . . . . . . . . . . . . . . . . . 130(T)
characteristics, cost, and applications 129(T)
modified, applications . . . . . . . . . . . . 130(T)
Siliconizing . . . . . . . . . . . . . . . . . . . . 118-119
definition . . . . . . . . . . . .
251
process availability. . . . . . . . . . . . . . . . . 185
Silicon nitride, as chemical vapor
deposition coating material . . . . . . 169(T)
Silicon-to-silver bonding techniques . . . . 149
Silver
erosion-corrosion . . . . . . . . . . . . . . . . . . . 23
for precious metal plating. . . . . . . . . . . . 149
galvanic corrosion . . . . . . . . . . . . . . . . 17(T)
as sputter coating material . . . . . . . . . . . 175
tarnishing. . . . . . . . . . . . . . . . . . . . . . . . . 16
uniform corrosion. . . . . . . . . . . . . . . . . . . 16
Silver plating, applications . . . . . . . . . . . . 199
Sintered thermal spray process, abrasive
wear rate
164(T)
Sleeving, temperature range and
distortion . . . . . . . . . . . . . . . . . . . . 192(F)
Sliding wear
55(F)
hardness vs. wear resistance process
comparisons . . . . . . . . . . . . . . . . . . 186
surface treatments for prevention .. 201-206,
208
test methods
82(T)
Slip/sinter ceramic coatings, benefits
2(T)
Slurry
abrasivity
definition
Miller number values
Slurry coating, design limitations
Slurry erosion
Slurry/sinter formed ceramics
Sodium pyrosulfite, to reduce hexavalent
69-70(F)
69
71, 72(T)
223(T)
7, 8, 71-72
applications
198, 201, 207, 212
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
Society of Tribologists and Lubrication
Engineers
Sodium dichromate, for sealing of sulfuric
84
anodized alloys. . . . . . . . . . . . . . . . . . 103
101
Sodium silicate cements
135
Soil, as corrosion contributor. . . . . . . . . . 14(F)
Sol-gel coating, design limitations
221 (T)
Solid lubricant, definition. . . . . . . .
251
Solid lubricants application, advantages,
limitations, and processing
parameters
Solution coating, design limitations
126(T)
221 (T)
Solvent cleaning . . . . . . . . . . . . . . .
219
immersion, design limitations
220(T)
ultrasonic, design limitations . .
220(T)
Solvents
128
Solvent spraying, design limitations. . . 221 (T)
Sour gas, definition . . . . . . . . . . . . . . . . . . 251
Spalling
aluminized coatings . . . . . . . . . . . . . . . . 117
definition . . . . . . . . . . . . . . . . . . .
251
of P/M steels surface oxide layer
108
of TRD processed sheet steel
178(F)
Spangle material
139-140, 141
Sparking, of galvanized coatings
141
Specification, drawing up of one
226-227
Spectroscopy, to analyze wear debris in
lubricants
78
Spherically-ended pin-on-a-flat coupon
(SPOF) test geometry
47-48(T),
49(T), 52(T)
Splat boundaries
164-165
Spraying/dipping plus sintering, to apply
chromium oxide coatings . . . . . . . . . . 136
Spray-sinter process, to apply silicate glass
coatings
136
Sputter coating
coating thickness/penetration
depth
192-193(F)
temperature range and distortion . . . . 192(F)
Sputtering. See also Physical vapor
deposition. . . . . . . . . . .
7(F)
definition . . . . . . . . . . . . . . .
251
line-of-sight limitations
217(F)
Stacking-fault energy
76(T)
Stacking faults
76(T)
Stainless steels
corrosion fatigue
30
corrosion rate . . . . . . . . . . . . . . . . . . . . . . 11
crevice corrosion . . . . . . . . . .
21
erosion-corrosion
22-23
galvanic corrosion
17(T)
intergranular corrosion
25-26
nitriding
114(F)
passivity . . . . . . . . . . . . . . . . . . . . . . . 18-19
pitting corrosion. . . . . . . . . . . . .
20
stress-corrosion cracking
28(T)
Static coefficient of friction . . . . . . . . . 44, 45
Static friction coefficient. See Static
coefficient of friction.
276/ Surface Engineering for Corrosion and Wear Resistance
Steam treatment
benefits . . . . . . . . . . . . . . . . . . . . . . . .. 2(T)
definition
251
Steels
boronized, hardness. range
' . . . 187 (F)
carbonitriding treatments . . . . . .
. . . 116
corrosion inhibitors for
32
corrosion protection methods
'
5
corrosion rate. . . . . . . . . . . .
. . . . . . . . 13
erosion rate
66
flame-hardened, hardness range
187(F)
flame hardening
88, 89(T)
fretting corrosion
25
induction-hardened, hardness range .. 187(F)
manganese phosphate coatings. for
parts
99(T)
nitrided, hardness range
187(F)
nitriding treatment. . . . . . . . . . . . . . . 114(F)
salt-spray test data, coating life
186(T)
structural grades, corrosion rate
~.... 11
uniform corrosion. . . . . . . . . . . . . . . . . . . 16
Strand lying on a rotating drum (StOD)
test geometry
S3(T)
Stray-current corrosion, definition
251
Stress corrosion, definition
251
Stress-corrosion cracking (SCC) .. 15, 16(F),
definition
'
chemical, design limitations
mechanical, design limitations
thermal, design limitations
221 (T)
221 (T)
221 (T)
128
Strontium chromate
Structural parts in corrosive environments,
surface engineering solutions. . . . . . . 196,
197-199
Styrene-butadiene (nitrile) rubber,
resistance ratings ...
128(T)
Submerged arc welding (SAW)
cost for weld overlays . . .
definition
for weld-overlay coatings
. . .. . . 190(F)
252
155(T), 156(F)
Submerged arc welding hardfacing, coating
thickness/penetration depth
192-193(F)
Substrate, definition
252
Substrate treatments, to reduce wear
7(F)
Sulfidation
Surface engineering material/process
selection checklist
8(F), 9
Surface fatigue
8, 56(F)
Surface fatigue wear, thermal spray coatings
applications
Sulfide stress cracking (SSe),
definition
252
Sulfur compounds
'.' '.'
Sulfur dioxide, to reduce hexavalent
"
15
101
165(T)
Surface finish
aesthetics and functional
requirements. . . . . . .. . . . . . . . ... 215
characteristics of various surface-engineering
processes
213 (T)
definition
252
design features
215-218(F)
size and weight effect on surface-engineering
processes
215(T)
Surface hardening
~
7(F)
definition
to prevent fretting corrosion
Surface modification, definition
Surface roughness, definition
'.'
252
25
253
253
139
253
128
Surface spangles
Surfacing, definition
Suspension agents
T
Tantalum, hydride formation
Tantalum silicide, as coating
material
31
137-138
Tarnish
definition
corrosion, of chromized steels
119
definition
252
resistance. . . . . ... . . . . . . . . . . . . .. . 1, 2(T)
weld-overlay coatings
159
chromium
alloys suitable for
'.. . . . . . . 104
conventional anodizing. . . . . . . .. .. . . .102
hardcoat anodizing . . . . . . .. . .. . . . .. . 102
sealing of coatings . . . . . . . . . . . . . 105-106
Superabrasives, definition
252
Super D-gun process, abrasive wear
rate
164(T)
Superfinishing, definition
252
Surface damage
55(F)
definition
252
Surface engineering
'.. 2(T)
definition . . . . . . . . . . . . . . . . . . . . . . . . . . 1
processes
213(T), 215(T)
properties or characteristics of
components
1
27-29 (F,T)
251
252
252
Stress relieving, definition
Strike, definition
Stripping
environmental
Sulfur dioxide-salt spray test (ASTM G 85,
A4), description
37(T)
Sulfuric acid, in glass-lined steel vessels .. 133
Sulfuric anodizing . . . 102, 103-104, 105-106
16
~
253
T-bend adhesion test (ASTM D 4145),
description
37 (T)
Technical associations, information services
source for surface treatments
185
Tellurides, as sputter coating material . . . . 175
Terne, definition
253
Terne coatings
2(T), 144
Thermal diffusion, advantages, limitations,
and processing parameters . . . . . . . 126(T)
Index / 277
Thermal evaporation
172, 173-174(T),
192(F)
Thermal insulation
Thermal spray coatings
1
160-166(F, T)
abrasive wear. . . . . . . . . . . . . . . . . . . 164(T)
of alloy metals. . . . . . . . . . . . . . . . .. 163(T)
aluminum, applications. . . . . . . . . . 197, 198
applications
203, 204, 205,
207,208,209,212
applications recommended for wear
resistance
165(T)
of carbide powders and blends
163(T)
of ceramic oxides
163(T)
ceramics, applications
198, 203,
204,205,206
corrosion-resistant applications and
materials used . . . . . . . . . . . . . . 166(T)
mechanical properties
163(T)
of metal composites
163(T)
process categories
. . . . . . . . . 160-162(T)
properties . . . . . . . . . . . . . . . . 162-166(F, T)
of pure metals
163(T)
size and weight limitations
215(T)
surface finish characteristics
213(T)
temperature range and distortion . . . . 192(F)
thickness ranges and hardness
levels . . . . . . . . . . . . . . . . . . . . . 211 (T)
versus weld-overlay coatings . . . . . . . . . 160
zinc, applications . . . . . . . . . . . . . . 197, 198
Thermal spraying. See also Arc spraying;
Flame spraying; Plasma spraying;
Powder flame spraying
7 (F)
advantages, limitations, and processing
parameters . . . . . . . . . . . . . . . . . 126(T)
benefits . . . . . . . . . . . . . . . . .
2(T)
comparison of applications of welding and
electroplating. . . . . . . . . . . . .. . 161 (T)
definition
253
design limitations
224(T)
methods compared
188(T)
process requirements compared to
those of welding and
electroplating. . . . .
. . . . . . . 161 (T)
to apply carbide coatings
137
to apply ceramic coatings
136
Thermal stresses, definition
253
Thermal wear, definition
253
Thermoreactive deposition/diffusion
process (TRD)
176-178(F,T)
advantages, limitations, and processing
parameters . . . . . . . . . . . . . . . . . 126(T)
applications of tooling
176-178(T)
Thief. See also Robber. .
216
definition
253
Thin dense chromium coatings, advantages,
limitations, and processing
parameters. . . . . . . . . . . . . . . . . . . 126(T)
Thorium, hydride formation
31
Three-body abrasive wear (multibody
wear)
55(F), 57
Through hardening
7 (F), 8
Throwing power
definition
of electrolyte in electroplating
Thrust washer (TW) test
geometry
Tin, galvanic corrosion
Tin bronzes, destannification
Tinplate (continuous
electrodeposition)
Titanium
content effect on intergranular
corrosion
erosion-corrosion
galvanic corrosion
hydride formation. . . . . .
253
146
50-51(T)
17(T)
27 (T)
2(T), 147-148
~
25~26
22-23
17(T)
31
Titanium alloys
erosion rate
stress-corrosion
66
28(T)
cracking
Titanium aluminum nitride, as coating
material . . . . . . . . . . . . . . . . . . . .. 137 (F)
Titanium carbide
as chemical vapor deposition coating
material .. 169(T), 170(T), 171(T), 187(F)
as coating material
137
Titanium carbonitride
as chemical vapor deposition
material.
as coating material
Titanium diboride, as chemical
deposition coating material
coating
170(T), 171(T)
137(F)
vapor
170(T), 171 (T)
Titanium nitride
as chemical vapor deposition coating
material.
169(T), 170(T), 171(T)
as coating material
61, 137(F)
as physical vapor deposition coating
material
187(F), 211(T)
as reactive ion plating material .. 175-176(F)
as sputter coating material . . . . . . . . . 175 (F)
Titanium zirconium nitride, as. coating
material
'.' . . . . . . .
. 137
Tolerance, definition
253
Tool steels
as coatings . . . . . . . . . . . . . . . . .
61
electron-beam hardening
91
hardened, hardness range
187(F)
laser melting
92
nitriding. . . . . . . . . . . . . . . . . . . . . . . 114(F)
oxidation. . . . . . . . . . . . . . . . . . . . . . 108(T)
Toughness
Toyota Diffusion (TD) coating process
1
176
applications
204
Transformation temperature, definition .. 254
Transition metals, for cladding. . . . . . . . . 168
Transportation, industrial operations and
annual wear economic consequences
4(T)
278 / Surface Engineering for Corrosion and Wear Resistance
Trees, definition. . . . . . . . . . . . . . . . . . . .. 254
Tribology, definition
254
Tribosystems
6-7
Triple pin-on-disk (TPOD) test
geometry
50(T)
Trowel coating, to apply ceramic coatings 136
Thberculation . . . . . . . . . . . . . . . . . . . .
21
definition . . . . . . . . . . . . . . . . . . . .
254
Thmbling. See also Barrel finishing.
definition . . . . . . . . . . . . . . . . .
254
Thngsten
erosion' rate
66
as sputter coating material . . . . . . . . . . . 175
Thngsten carbide
as chemical vapor deposition coating
material.
169(T), 171(T)
as coating material . . . . . . . . . . . . . . . . . 137
Thngsten carbide-cobalt
coatings
applications . . . . .
200
de alloying corrosion . . . . . . . . . . . . .. 27 (T)
plasma sprayed, cost . . . . . . . . . . . . . 191 (F)
thermally sprayed, thickness ranges and
hardness levels. . . . . .
211 (T)
Thngsten rhenium (W -Re), as chemical vapor
deposition coating material . . . . . . 171 (T)
Thngsten thorium (W-Th), as chemical
vapor deposition coating
material . . . . . . . . . . . . . . . . . . . . . 171 (T)
Two-body abrasive wear
55(F), 57
u
Ultraprecision
finishing, definition
254
Ultrasonic cleaning
219
definition . . . . . . . . . . . . . . . . . . .
254
Underfilm corrosion, definition
254
Uniform corrosion
15-16(F)
definition
254-255
Uranium, hydride formation
31
Urethanes, applications
130(T)
Utilities, industrial operations and annual
wear economic consequences
4(T)
v
Vacuum carburizing
characteristics and requirements
.... 111 (T),
112-113(F)
definition
255
Vacuum deposition, definition
255
Vacuum nitrocarburizing,
definition
255
Vacuum plasma spraying (VPS)
160,
162(T), 164(T), 188(T)
Vapor degreasing, definition
255
Vapor deposition. See also
Chemical vapor deposition;
Physical vapor deposition;
Sputtering
25, 61, 137, 138
Vapor plating. See Vacuum deposition.
Vinyl
31, 127, 129(T),
130(T), 131(T)
Vinyl-alkyds, applications
130(T)
Vinylidene chloride
127
Vinyl resin
34-35
w
Water
alkalinity effect on corrosion rate . . . . . . . 13
as corrosion contributor . . . . . . . . . . . . . . 13
Watts nickel coatings, abrasive wear
resistance
151(T)
Wear. See also Abrasive wear;
Adhesive wear; Erosion;
Rolling-contact wear
54-77(F,T)
abrasive
56-61(F,T)
adhesive
72-75(F,T)
classification schemes
54-56(F)
definition
54, 255
economic effects
3
erosion . . . . . . . . . . . . . . . . . . . . 61-72(F, T)
galling . . . . . . .
75-7 6(T)
parts in static contact with a product,
surface treatments
for
196, 200-201
rolling -contact . . . . . . . . . . . . . . . . . .... 77
synergistic relationships with corrosion
mechanisms
5(T)
Wear coefficient
74(T)
Wear debris, definition
255
Wear plates
coating thickness/penetration
depth
192-193(F)
temperature range and distortion . . . . 192(F)
Wear resistance
1
process comparisons . . . . . . . . 186-190(F, T)
surface engineering treatments
for improvement
2(T)
Wear scar volume
73
Wear testing
81-84(T)
computer automation
84
devices
82-84(T)
purposes
81
standardized methods
81, 82(T)
variables to be controlled
81-82, 83(T)
Wear tiles
7(F)
Weathering, definition
255
Wedge formation
59, 60(F)
Index / 279
Welding. See also
Weld-overlay coatings
161 (T)
definition
255
Weld-overlay coatings
153-159(F,T)
abrasive wear resistance
157, 158(F)
applications
207, 208, 209, 211
benefits . . . . . . . . . . . . . . . . . . . . . .... 2(T)
cost, relative. . . . . . . . . . . . . . . . . . . . 191 (F)
erosion
157-159(F)
iron aluminides
159(F)
oxidation resistance
159
processing parameters for
optimization
155-157(F,T)
size and weight limitations
215(T)
sulfidation resistance
159
surface finish characteristics
213(T)
thickness range of treatment . . . . . .
6(F)
versus thermal spray coatings. . . . . . . . . 160
welding processes available
154-155(T)
White cast irons, wear of
60, 61
White layer, definition
256
White rust, definition
256
Wiped coat, definition
256
Wiping effect, definition . . . . . . . . . . . . . . 256
Wire flame spraying. See also Flame
spraying.
definition
256
Work hardening
7(F)
Wrought iron, galvanic corrosion
17(T)
z
Zinc
corrosion of . . . . . . . . . . . . . . . . . . 11, 17(T)
dezincification . . . . . . . . . . . . . . . . . . . . . 26
as sacrificial coating material
33, 34(F)
Zincalume
139
Zinc-aluminum
coatings
142-143(F)
Zinc anodes, for cathodic
protection. . . . . . . . . . . . . . . .
34
Zinc chromate
31-32,128
Zinc coatings, benefits . . . . . . . . . . . . ... 2(T)
Zinc electroplating, applications .. 147, 148(T)
Zinc flake
32
Zinc molybdate
128
Zinc-nickel alloy plate (continuous
electrodeposition),
benefits
2(T)
Zinc phosphate
96, 128
Zinc phosphorus silicate. . . . . . . . . . . . . . 128
Zinc phosphosilicate . .'
32
Zinc plating
211(T), 212
Zinc-rich epoxy coatings
127
Zinc-rich 'l)-phase
142
Zincrometal, definition . . . . . . . . . . . . . . . 256
Zirconia thermal barrier coatings
(TBCs)
136, 174(F)
Zirconium, hydride formation
31
Zirconium alloys, stress-corrosion
cracking
28(T)