CHAPTER 3
FORMS OF CORROSION
Chapter Outlines
3.1
3.2
3.3
3.4
Galvanic or Two-Metal Corrosion
Crevice Corrosion
Pitting Corrosion
Intergranular Corrosion
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GALVANIC CORROSION
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3.1 Galvanic or Two-Metal Corrosion
 also called ' dissimilar metal corrosion‘.
 Takes place when two metals are in physical contact with each other and are
immersed in a conducting fluid.
 corrosion damage induced when two dissimilar materials are coupled in a
corrosive electrolyte.
 Examples:
1. Plate and screw of different electrical potentials due to differences in
processing
2. Multiple component implant using different metals for each component
3. Copper and steel tubing are joined in a domestic water heater, the
steel will corrode in the vicinity of the junction
 The following fundamental requirements have to be met for galvanic
corrosion:
1. Dissimilar metals (or other conductors, such a graphite).
2. Electrical contact between the dissimilar conducting materials (can be
direct contact or a secondary connection such as a common grounding
path).
3. Electrolyte (the corrosive medium) in contact with the dissimilar
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conducting materials.
The relative nobility of a material can be predicted by measuring
its corrosion potential. The well known galvanic series lists the
relative nobility of certain materials in sea water. A small
anode/cathode area ratio is highly undesirable. In this case, the
galvanic current is concentrated onto a small anodic area. Rapid
thickness loss of the dissolving anode tends to occur under these
conditions. Galvanic corrosion problems should be solved by
designing to avoid these problems in the first place.
Fig. Galvanic corrosion between
stainless steel screw and Aluminium.
Fig. Galvanic corrosion between
Steel and Brass.
Noble, cathodic end
Platinum
Gold
Graphite
Titanium
Silver
Hastelloy C
18-8 austenitic stainless steels (passive condition)
Iron-chromium alloys (passive condition)
Inconel (passive)
Nickel
Monel
Cupronickel alloys
Bronzes
Copper
Brasses
Inconel (active)
Nickel (active)
Tin
Lead
18-8 Austenitic stainless steels (active)
13% Chromium stainless steel (active)
Cast iron
Mild steel and iron
Cadmium
Aluminum alloys
Zinc
Magnesium and magnesium alloys
Active, anodic end
Fig. Anodic- cathodic behavior of steel with zinc
and tin outside layers exposed to the
Atmosphere.
(a) zinc is anodic to steel and corrodes
(b) steel is anodic to tin and corrodes (the tin
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layer was perforated before the corrosion
began)
The following have been described as "main factors"
influencing galvanic corrosion rates in Skanaluminium's online publication "Alubook - Lexical knowledge about
aluminium".
• Potential Difference between materials
• Cathode Efficiency
• Surface areas of connected materials (area ratio)
• Electrical resistance of the connection between the
materials and of the electrolyte.
Note that the area ratio of the anode: cathode
is an important variable affecting the dissolution
current density (and hence corrosion rate)
pertaining to the anode. The area ratio is also
important when considering the relative amount
of current "available" from the cathodic
reaction.
Fig. Brass on Weathering Steel - rust forms in discrete
crystallites that are fine, red and diffusely reflecting, like
hematite. The massive re-crystallized layer is a shiny blue,
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approaching the blue-black of secular hematite.
CREVICE CORROSION
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Rivets : a metal pin for passing through holes in two or more plates or pieces to hold
them together, usually made with a head at one end, the other end being hammered into
a head after insertion.
Depletion : to decrease seriously or exhaust the abundance or supply of
3.2 Crevice Corrosion
 Crevice corrosion is a localized form of corrosion usually associated with a stagnant
solution on the micro-environmental level.
 Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those
formed under gaskets, washers, insulation material, fastener heads, surface deposits,
disbonded coatings, threads, lap joints and clamps.
 Occurs under gaskets, rivets and bolts, between valve disks and seats.
 Well-known examples of such geometries including flanges, gaskets, disbonded
linings/coatings, fasteners, lap joints and surface deposits.
Crevice corrosion is initiated by changes in local chemistry
within the crevice:
•
•
•
•
Depletion of inhibitor in the crevice
Depletion of oxygen in the crevice
A shift to acid conditions in the crevice
Build-up of aggressive ion species (e.g. chloride)
in the crevice
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Mechanism- Chronology of Crevice Corrosion
Stage 1
At time zero, the oxygen
content in the water
occupying a crevice is equal
to the level of soluble
oxygen and is the same
everywhere.
Stage 2
Because of the difficult access
caused by the crevice geometry,
oxygen consumed by normal
uniform corrosion is very soon
depleted in the crevice. The
corrosion reactions now specialize
in the crevice (anodic) and on the
open surface (cathodic).
Fig. Schematic illustration (initial stage) of the mechanism for
crevice corrosion between two riveted sheets.
Stage 3
The crevice development a few more accelerating factors fully develop:
1. The metal ions produced by the anodic corrosion reaction readily hydrolyze giving off protons
(acid) and forming corrosion products. The pH in a crevice can reach very acidic values,
sometimes equivalent to pure acids.
2. The acidification of the local environment can produce a serious increase in the corrosion rate of
most metals. See, for example, how the corrosion of steel is affected as a function of water pH.
3. The corrosion products seal even further the crevice environment.
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4. The accumulation of positive charge in the crevice becomes a strong attractor to negative ions in
the environment, such as chlorides and sulfates, that can be corrosive in their own right.
Fig. Crevice corrosion on aircraft
Fig. Pack rust is a form a localized corrosion typical of steel
components that develop a crevice into an open atmospheric
environment. This expression is often used in relation to bridge
inspection to describe built-up members of steel bridges which
are showing signs of rust packing between steel plates.
Zebra mussels- an example of marine environment
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Close-up picture showing the severity of corrosion
Underside of panel where severe corrosion was found
Crevice Corrosion Testing
ASTM G78
Standard Guide for Crevice Corrosion Testing
A good example of how crevice corrosion can be reproduced and accelerated in a laboratory environment is the
formation of occluded cells with multiple crevice assemblies (MCAs), as described in the ASTM G78 Standard Guide for
Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing
Aqueous Environments.
In this test, washers make a number of contact sites on either side of the specimens. The number of sites showing
attack in a given time can be related to the resistance of a material to initiation of localized corrosion, and the average
or maximum depth of attack can be related to the rate of propagation. The large number of sites in duplicate or
triplicate specimens is amenable to probabilistic evaluation.
The susceptibility to localized corrosion becomes quite visible once a specimen equipped with these Teflon washers has
been exposed to a corrosive environment for an extended period of time.
... after 30 days in 0.5 FeCl3 + 0.05 M10
NaCl
PITTING CORROSION
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3.3 Pitting Corrosion
 Pitting corrosion is a localized form of corrosive attack that produces holes or small
pits in a metal.
 the bulk of the surface remains unattacked.
 Pitting is often found in situations where resistance against general corrosion is
conferred by passive surface films.
 Localized pitting attack is found where these passive films have broken down.
 Pitting attack induced by microbial activity, such as sulfate reducing bacteria (SRB)
also deserves special mention.
 Pitting can be one of the most dangerous forms of corrosion because it is difficult to
anticipate and prevent, relatively difficult to detect, occurs very rapidly, and penetrates
a metal without causing it to lose a significant amount of weight.
Special case of crevice corrosion:
1.
2.
Initiated by inclusions, scratches, or handling damage instead of
deep cracks
The presence of static flow conditions and reduced oxygen
availability are less important than in crevice corrosion
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Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable
membrane of corrosion products. Pits can be either hemispherical or cup-shaped.
Pitting is initiated by:
1.
2.
3.
Localized chemical or mechanical damage to the protective oxide film; water chemistry factors which
can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend
to render a protective oxide film less stable) and high concentrations of chloride (as in seawater)
Localized damage to, or poor application of, a protective coating.
The presence of non-uniformities in the metal structure of the component, e.g. nonmetallic
inclusions.
A local cell that leads to the initiation of a pit can
be caused by an abnormal anodic site surrounded
by normal surface which acts as a cathode, or by
the presence of an abnormal cathodic site
surrounded by a normal surface in which a pit will
have disappeared due to corrosion.
Fig. Local Cathode on a corroded piece of material
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






Alloying can have a significant impact on the pitting resistance of stainless steels.
Conventional steel has a greater resistance to pitting than stainless steels, but is still susceptible,
especially when unprotected.
Aluminum in an environment containing chlorides and aluminum brass (Cu-20Zn-2Al) in contaminated
or polluted water are usually susceptible to pitting.
Titanium is strongly resistant to pitting corrosion.
Proper material selection is very effective in preventing the occurrence of pitting corrosion. Another
option for protecting against pitting is to mitigate aggressive environments and environmental
components (e.g. chloride ions, low pH, etc.).
Inhibitors may sometimes stop pitting corrosion completely.
Further efforts during design of the system can aid in preventing pitting corrosion, for example, by
eliminating stagnant solutions or by the inclusion of cathodic protection.
FIG The pitting of a 304 stainless
steel plate by an acid-chloride solution.
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Sewer Explosion due to Pitting Corrosion
Fig. Corrosion Pits are the primary source
of leaks in water handling systems
Pitting:
corrosion of a metal surface, confined to a
point or small area, that takes the form of
cavities.
An example of corrosion damages with shared responsibilities was
the sewer explosion that killed 215 people in Guadalajara, Mexico, in
April 1992. Besides the fatalities, the series of blasts damaged 1,600
buildings and injured 1,500 people.
*Sewer: an artificial conduit, usually underground, for carrying off waste
water and refuse, as in a town or city.
Pitting factor:
ratio of the depth of the deepest pit
resulting from corrosion divided by the
average penetration as calculated from
weight loss.
Pitting resistance equivalent number
(PREN):
an empirical relationship to predict the
pitting resistance of austenitic and duplex
stainless steels. It is expressed as PREN =
Cr + 3.3 (Mo + 0.5 W) + 16N
This example of a pitted
surface was produced
by exposing a specimen
of aluminum A92519
to 3.5% NaCl during
seven days. The width
of the picture is
approximately 1 mm.
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Corrosion Pit Shapes
SIDEWAY PITS
THROUGH PITS
Subsurface
Narrow, deep
Shallow, wide
Undercutting
Elliptical
Horizontal grain attack
Vertical Grain Attack
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Table 1 Pitting corrosion incidents of aircraft and helicopters
Aircraft
Location of
Failure
Cause
Bell
Helicopter
Fuselage,
longeron
Fatigue, corrosion
and pitting present
DC-6
Engine, master
connecting rod
Piper PA-23
Incident
Severity
Place
Year
From
Serious
AR, USA
1997
NTSB
Corrosion pitting
Fatal
AK, USA
1996
NTSB
Engine,
cylinder
Corrosion pitting
Fatal
AL, USA
1996
NTSB
Boeing 75
Rudder Control
Corrosion pitting
Substantial
damage to plane
WI, USA
1996
NTSB
Embraer 120
Propeller Blade
Corrosion pitting
Fatal and
serious, loss of
plane
GA, USA
1995
NTSB
Gulfstream
GA-681
Hydraulic Line
Corrosion pitting
Loss of plane, no
injuries
AZ, USA
1994
NTSB
L-1011
Engine,
compressor
assembly disk
Corrosion pitting
Loss of plane, no
injuries
AK, USA
1994
NTSB
Embraer 120
Propeller Blade
Corrosion pitting
Damage to plane,
no injuries
Canada
1994
NTSB
Embraer 120
Propeller Blade
Corrosion pitting
Damage to plane,
no injuries
Brazil
1994
NTSB
F/A-18
Trailing-edge
Flap (TEF)
Outboard
Hinge Lug
Corrosion pitting,
fatigue
Loss of TEF
Australi
a
1993
AMRL
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INTERGRANULAR
CORROSION
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3.4 Intergranular Corrosion
 Intergranular corrosion refers to preferential (localized) corrosion along grain
boundaries.
 or immediately adjacent to grain boundaries, while the bulk of the grains remain largely
unaffected.
 This form of corrosion is usually associated with chemical segregation effects (impurities
have a tendency to be enriched at grain boundaries) or specific phases precipitated on
the grain boundaries.
 This selective dissolution may lead to the dislodgement of grains.
 Intergranular corrosion in sensitized stainless steels and exfoliation in aluminum
alloys represent industrially significant examples of this form of damage.


Also known as “knife- line attack”
classic example is the sensitization
19 of
stainless steels or weld decay
Fig. Severe problem in the welding of stainless steels,
when it is termed weld decay.
FIG. Intergranular corrosion of a failed aircraft
component made of 7075-T6 aluminum (picture
width = 500 mm)
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