4/16/2013
LECTURE #22
Corrosion in Materials
Learning Objectives:
• What is corrosion?
• What is the difference between an oxidation and reduction reaction?
• What are the most common reduction reactions associated with corrosion?
• What are different forms of corrosion?
• What are means to prevent corrosion?
Relevant Reading for this Lecture...
• Chapter 16, Pages 689-720
Corrosion in Everyday Life
Corrosion can also be attractive!
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Impact of Corrosion
• Materials will degrade in time as a result of their reaction to the environment.
• Eliminating corrosion entirely is i
impractical (if not impossible)
ti l (if t i
ibl )
– Mitigation is essential • Corrosion affects:
Planes, Trains, and Automobiles (& watercraft)
Appliances (Water heater)
Infrastructure (pipelines for water, oil, gas)
g ,
Bridges, steel reinforced concrete construction of all kinds
Art (statues, metal used for appearance) • Direct Costs
In the USA, an estimated ~$100+ billion a year. •
Indirect Costs
– Plant down time
– Loss of product
– Loss of efficiency
Loss of efficiency
– Contamination
– Overdesign
• In catastrophic failure, lives can be lost. Corrosion – Involves electrochemical Corrosion –
reactions = Redox reactions
• Corrosion is the destructive attack of a metal involving the transfer
(loss) of electrons from the metal to another chemical species that
gains (consumes) them. i (
) th
• Corrosion is a “Redox” process, reduction/oxidation
• Corrosion is an electro‐chemical process
• A reduction cannot occur without a corresponding oxidation!
• An oxidation cannot occur without a corresponding reduction!
• Loss of e
Loss of e‐ = oxidation
oxidation and occurs at sites called anodes and occurs at sites called anodes
‐
• Gain of e = reduction and occurs at sites called cathodes
Anodic = oxidation = loss of e‐ Cathodic = reduction = gain of e‐
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Corrosion ‐‐ Electrochemical Reactions
Corrosion Anodic = oxidation = loss of e‐ Cathodic = reduction = gain of e‐
Anodic (oxidation)
d ( d
)
Cathodic
h d (reduction)
( d
)
For corroding metals anodic reactions The most common* cathodic reactions are
all have the form, the metal dissolves hydrogen evolution & oxygen reduction:
as an ion (usually in H2O):
• 2H+ + 2e‐ = H2 hydrogen evolution, typically requires
• Fe = Fe2+ + 2e‐
an acidic electrolyte (source of H+)
half‐cell • Ni Ni = Ni
Ni2+ + 2e
2e‐
reactions
• O2 + 4H+ + 4e‐ = 2H2O • Al = Al3+ + 3e‐
oxygen reduction, requires O2 or air
“ionization”
* other species can be reduced IF they
are present, for example
Cu2+ + 2e‐ = Cu “If acid solution, we reduce it”
EMF Series ‐‐ Reduction Potentials
EMF Series for “standard” concentrations
Each metal has its own unique potential called the standard reduction potential
reduction potential.
“noble”
“active”
Each value is measured vs. a reference half‐cell, which is assigned a value of 0. Here the reference half‐cell is the standard hydrogen electrode (SHE) (Pt electrode in 1 atm H2, in a 1 mol/liter solution of H+)
See Figure 16.4
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Compare with Fig 16.2
An electrochemical cell can be produced by combining two different metals from the EMF Series. The more “anodic” metal corrodes. See Table 16.1.
= 0.780 V
Calculated on next slide
Chemical change is accompanied by electric Chemical
change is accompanied by electric
current in the wire.
Chemical energy is converted to electrical energy. half‐cell reactions
Anode:
Fe → Fe2+ + 2 e‐
Cl ‐
Cu‐Cl
electrolyte solution
Cl ‐
Here Cu2+
is present
Cathode:
C 2+ + 2 e
Cu
2 ‐ → Cu
→C
Cl ‐
1 mol/L of Fe+2 in solution
Cl ‐
Does the membrane have to be porous? Why? Yes, current is ‘moving charge.’ Charge (e‐) move through the wire, charge (ions) must be transported in the electrolyte.
Solutions must be charge neutral!
Voltmeter reads voltage difference.
Add the half‐cell reactions to get the overall (net) cell reaction:
Fe → Fe2+ + 2 e‐
Cu2+ + 2 e‐ → Cu
Cu2+ + Fe → Cu + Fe2+ net cell reaction
“noble”
Use eqn. 16.18 to calculate V
V  V
o
o
reduction
V
o
oxidation
V = 0.340 – (– 0.440) = 0.780 V
What if you switch the values?
X
V = – 0.440 – 0.340 = – 0.780 V
“active”
active
V must be > 0, if V < 0, the net cell reaction goes in the reverse direction
Metals toward bottom are more “active”
& tend to corrode (oxidize) more easily. 4
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In the previous example Fe was corroded, but corrosion can happen even if only one metal is involved. Oxidation: Zn = Zn2+ + 2e‐
Anodic site
Anodic and cathodic reactions can take place widely separated on the same surface
Fig. 16.1
Reduction: 2H+ + 2e‐ = H2
Cathodic site
SHOW VIDEO OF Zn IN ACID
Forms of Corrosion
Wet corrosion often occurs selectively instead of uniformly
Localized Crevice corrosion – results from a difference in dissolved O2 between the area inside the crevice and the area outside the crevice.
Localized Intergranular corrosion – occurs when grain boundaries are more susceptible to corrosion compared to the bulk grain
Localized Pitting corrosion – preferential attack that occurs at breaks
in the natural oxide layer on passive metals Localized Galvanic corrosion –corrosion of a more active metal in the Galvanic Series when connected to a more noble metal in the Galvanic Series.
Localized Stress corrosion cracking – accelerated corrosion, localized
at cracks in tensile loaded components
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
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Crevice Corrosion
Results from a difference in dissolved O2 (i.e., O2 pressure) between the area inside the crevice and the area outside the crevice.
Anodic site: O2 starved region
Cathodic sites
All of the exterior surface
O2 + 4H+ + 4e‐ = 2H2O Anodic site: O2 starved region
Fe → Fe2+ + 2 e‐
Over time, the micro‐environment inside the crevice becomes more aggressive and corrosion rate increases. Why?
1. concentration of Fe+2 increases
due to Fe = Fe+2 + 2e2. Cl- diffuses into the crevice to
maintain charge neutrality (thus
increasing corrosion rate)
3. concentration of Fe+2 reaches a
maximum and forms H+, acid!
Fe+2 + 2H2O = Fe(OH)2 + 2H+
pH increases! Cl- increases! Leads to increased corrosion rates!
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Ions in Solution and pH
Corrosion by acids and alkalis is an electro‐chemical reaction
A chemical compound that dissociates
A chemical compound that dissociates
in water increases either the hydrogen
ion concentration or hydroxyl ion concentration The pH level of an environment can
The
pH level of an environment can
initiate corrosion by stimulating
a reaction in which a metal dissociates
into a metal ion and releases electrons
Figure 17.6
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Galvanic Corrosion
The EMF Series is useful, but engineers seldom design with pure metals in “standard” concentration solutions. “noble”
noble
For such conditions the Galvanic Series is useful, AND it includes alloys!
Metals /alloys toward bottom are more “active” & tend to corrode (oxidize) more easily when
(oxidize) more easily when connected to a metal higher in the series. “active”
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Galvanic Corrosion
A steel bolt in a brass plate creates a galvanic cell.
Anodic half cell reaction:
Fe → Fe2+ + 2 e‐
i th ½ cell reaction (oxidation)
is the ½ ‐
ll
ti ( id ti )
active
noble
Cathodic half cell reaction:
Cathodic
half cell reaction:
The reduction is NOT Zn2+ + 2e‐ → Zn (insufficient Zn2+ in solution)
The actual reduction is: ½ O2 + H2O + 2e‐ → 2 OH‐
The O2 is dissolved in the electrolyte (for example, seawater).
Galvanic Corrosion
If you were required to connect brass & steel which option would be best?
Brass
Steel
Large cathode area causes current to be “concentrated” on the g
small anode area. If galvanic couples are necessary, use a large area anode next to a small area cathode!
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Intergranular Corrosion
Stainless steel
“Stainless” requires > 12% Cr
Pitting Corrosion
Defect in passive oxide coating – allows anode and cathode to form and corrosion to occur
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Stress Corrosion Cracking (SCC)
Tensile (pulls it apart)
Susceptible environment and material
SCC ‐ Corrosion accelerated fracture (fatigue) Stress Corrosion Cracking (Fatigue)
Remember this?
max
m

S
min
time
Jet that island jumped in Hawaii (near salt water 24‐7))
Corrosion contributed to the (accelerated) fatigue – went faster than thought only with mechanical  Fewer cycles, lower stress
Pulling crack apart….
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Passivity
• Passive Metals: Many metals form thin, protective oxide films.
• Self‐generated passive coatings –a protective film f
forms spontaneously (Alumina
l (Al i –on Aluminum)
Al i
)
• By adding Cr, Al, and similar passive coating forming elements to steel (termed stainless steel) better corrosion resistance properties occur because these elements produce a protective oxide scale.
Alumina (Al2O3)
Aluminum
Oxide Scales
When most metals are exposed to air, an ultra‐thin surface film of oxide forms – this inhibits reaction with oxygen or any other reducing agent, e.g. H+
The oxide film separates the metal from the oxygen – to react
farther either oxygen atoms must diffuse inward through the
film to reach the metal or metal atoms must diffuse outward
atoms must diffuse outward through the film to reach the oxygen
The oxidation reaction M + O = MO occurs in two steps:
1) The metal forms an ion and releases electrons M = M2+ + 2e
2) Electrons are absorbed by oxygen to give an oxygen ion O + 2e = O2‐
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Fighting Corrosion: Four Strategies
Painting • Judicious design, meaning informed material choice and choice of geometry and configuration
t
d
fi
ti
• Protective coatings, passive metals (oxide films), paint, polymer coatings
• Corrosion inhibitors, chemicals added to the corrosive medium that retard the rate of the corrosion reaction
• Monitoring, with protective maintenance or regular replacement
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Design: Geometry and Configuration
Do’s and Don’ts
•
•
•
•
•
•
•
Allow for uniform attack
Avoid fluid trapping
Suppress galvanic attack (small cathode/large anode)
Avoid crevices
Consider cathodic protection
Consider cathodic protection
Beware of stress corrosion and corrosion‐fatigue
Design for inspection and maintenance
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
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Design changes to prevent trapped fluids reduce the rates of corrosion
Change design to g
g
minimize
galvanic attack
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Change design to avoid crevice corrosion
Crevices create oxygen starved regions – resulting in corrosion in that area.
Protection of steel by y
cathodic protection –
zinc acts as a sacrificial anode
Cathodic protection, Zn is a sacrificial anode
Galvanizing
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Galvanic Corrosion :
Sacrificial Anode Coatings
• Passive coatings – separate the material from the corrosive environment and are inherently corrosion
corrosive environment and are inherently corrosion resistant
• Self‐generated passive coatings – rely on alloying in sufficient concentrations so that a protective film forms spontaneously (Aluminum – Alumina)
Alumina (Al2O3)
Aluminum
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
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Ways of Applying Coatings
(a) ‐ Paint spraying
(b) ‐ Hot dip galvanizing
(c) ‐ Electroplating
(d) ‐ Metal flame spraying
(e) ‐ Polymer powder spraying
(f) ‐ Enameling
E
li
Figure 17.17
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Corrosion Inhibitors
Corrosion inhibitors reduce the rate of attack, when dissolved or dispersed in a corrosive medium
or dispersed in a corrosive medium
Some inhibitors work by increasing the pH or by coating the part and suppressing either the anodic or the cathodic reactions
Choice of inhibitor depends on material and environment
Choice of inhibitor depends on material and
Won’t work on a ship, but would in a chemical process tank.
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
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Monitoring, Maintenance, and Replacement
Regular inspection allows early indications
of corrosion to be detected
of corrosion to be detected
Maintenance
painting, recoating, or repair can then be carried out to minimize its down‐
time and risk of failure
The design of the system should allow for inspection of all vulnerable surfaces and p
permit access for maintenance.
Replacement at prescribed, regular intervals often used for safety‐critical components
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Summary:
•
•
•
•
•
Corrosion is the destructive attack of a metal involving the transfer (loss) of electrons
from the metal to another chemical species that gains (consumes) them – Redox! Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. Oxidation = Loss of electrons & Reduction = Gain of electrons
Oxidation = Loss of electrons & Reduction = Gain of electrons Different forms of corrosion: – Intergranular corrosion
– Pitting Corrosion
– Crevice Corrosion
– Galvanic Attack
– Stress Corrosion Cracking
Ways to prevent corrosion:
Ways to prevent corrosion: – Good Design
– Coatings
– Inhibitors
– Monitoring 16
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Review for Midterm #3 (a guide, not necessarily all encompassing – review all your notes)
• Mechanical Behavior
 Identify the mechanical behavior regions/points on an stress‐strain plot  What is toughness? Hardness (how measured)? Impact toughness (how measured)?
What are modes of mechanical failure? What do those fracture surfaces look like?
 Be able to use elastic equations and plastic equations (Holloman’s equation). What is work hardening and what are the consequences of work hardening?
is work hardening and what are the consequences of work hardening?
 Be able to determine the resolved shear stress in a material
What is DBTT? What is fatigue (endurance limit)?
 How are ceramics tested? Why do they fail easier in tension rather than compression?
What is KIC (eq. will be given) – how is it used in designing materials? Why are flaws critical in the failure of ceramics?
• Electrical properties
 Know Ohms law. How is conductivity and resistivity related. Be able to apply these relationships through given equations (see assessments)
 What are the physical phenomena that distinguish conductors, semiconductors, and insulators?
 For metals, how is conductivity affected by imperfections, T, and deformation?
 For semiconductors, how is conductivity affected by impurities (doping) and T? What is n‐type and p‐type conduction? How does that occur in semiconductors?
How do semiconductor devices work (p‐n junction, LED, transitors)
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•
•
•
Thermal properties  How do materials respond to the application of heat?
 How do the thermal properties of ceramics, metals,
and polymers differ?
 How do we define and measure...
-- heat capacity? thermal expansion? thermal conductivity (what
mechanisms contributes to k) ? thermal shock resistance?
p
p p
Optical properties
 What phenomena occur when light is shone on a material?
 What determines the characteristic colors of materials?
 What is refraction (be able to apply Snell’s law and other relevant equations if
given)
 What is the difference(s) between spectral and diffuse reflection?
 Why are some materials transparent and others are translucent?
 Use of optics - How does a laser’s operate (luminescence)? Fiber optics?
Magnetic properties
 Recognize the connection between electrical current and magnetism and be
able to apply those equations if given
 How do we explain magnetic phenomena (magnetic dipole, unpaired e-)?
 How are magnetic materials classified? On a B-H loop, what is a soft/hard
magnet?
 Know some application examples that use magnetic materials and how they
work
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