corrosion of metals intergranular corrosion
TRANSCRIPT
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Corrosion of Metals
Michael Pfeifer, PhD., P.E.Industrial Metallurgists, LLC
Northbrook, IL 60062847.528.3467
www.imetllc.comwww.materialscoursesonline.com
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Module 7: Intergranular Corrosion
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Module learning objectives
1. Explain the mechanisms for intergranular corrosion
2. List three sets of alloys that are susceptible to intergranular corrosion
3. Explain the microstructure features and processing that leads to intergranular corrosion.
4. Describe different approaches for controlling intergranular corrosion in different alloys.
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Intergranular corrosionLocalized attack at grain boundaries with relatively little corrosion of grains
Two forms of intergranular corrosion
Grains fall outMetal disintegrates
Metal loses strength
Corrosion products push out grainsMetal appears to be flaking
Called exfoliation
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Two metallurgical causes behind intergranular corrosion
1. Segregation of impurities to grain boundaries
• Difference in composition can lead to galvanic corrosion
• If grain boundary is anode, then it will be attacked
• If region next to the grain boundary is anode, then it will be attacked
2. Formation of precipitates on the grain boundaries
• Precipitates are different metallurgical phase than grains
• Decrease corrosion resistance of areas adjacent to grain boundaries or develop microscopic galvanic cells.
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Segregation of elements to grain boundariesOccurs when metal heated during heat treatment or cooling after being cast
Increase concentration on grain boundaries
Decrease concentration within grains
May promote galvanic corrosion
Interstitials often impurities such as sulfur or phosphorous
Best controlled by minimizing amount of interstitial impurities in an alloy
Interstitial atom
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Precipitates at grain boundaries
In many alloys second phase particles form along grain boundaries
Precipitates a different metallurgical phase than grains
Two possible consequences
1. Decrease corrosion resistance of adjacent area due to depletion
2. Develop microscopic galvanic cells in area of grain boundary
Discrete particles Continuous film
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Sensitization of Austenitic (3xx series) stainless steelDepletion of one or more elements from areas adjacent to grain boundaries
• Due to formation of precipitates on grain boundaries
• Increases susceptibility to corrosion
In austenitic stainless steels sensitization involves depletion of chromium
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Corroded grainboundaries
304 stainless steel
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Chromium carbide precipitates form on grain boundaries
Required conditions
• High chromium content
• > 0.02% carbon
• Exposure between 425 and 870 C
Sensitization best known in steels with 18% chromium and 8% nickel
• 304 stainless steel contains up to 0.08% C
• Plenty of carbon available to form chromium carbide precipitates
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Chromium carbide precipitates in 304 stainless steel
0.0025 inch0.064 mm
0.0007 inch0.018 mm
Courtesy of Aston Metallurgical Services
Courtesy of Aston Metallurgical Services
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Sensitization process
Occurs when steel heated between 425 and 870 C (797 and 1598 F)
1. Carbon and chromium diffuse to grain boundaries
2. React to form chromium carbide (Cr4C or Cr23C6) precipiates
Chromium diffusion rate between 425 and 870 C is low
Chromium atomCarbon atom
Depleted region
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GrainGrain
Chromium gives stainless steel its corrosion-resistant properties
Need more than 12% chromium to make stainless steel corrosion resistant
Depleted regions have less than 12% chromium
• Areas near grain boundaries susceptible to attack
Carbide
18
12
%Chromium
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Problem 1
What is expected to occur as the time to cool a 304 stainless steel from 800 C to 400 C increases?
Depleted region decreases in width
Depleted region increases in width
No change in width of depleted region
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 1
What is expected to occur as the time to cool a 304 stainless steel from 800 C to 400 C increases?
Depleted region decreases in width
Depleted region increases in width
No change in width of depleted region
OK
INCORRECT
There will be more diffusion for longer time at elevated temperatures
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 1
What is expected to occur as the time to cool a 304 stainless steel from 800 C to 400 C increases?
Depleted region decreases in width
Depleted region increases in width
No change in width of depleted region
OK
CORRECT
Exposure to longer time allows for more carbon and chromium atoms to diffuse to grain boundaries.
Results in larger chromium carbides and wider depleted region
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 1
What is expected to occur as the time to cool a 304 stainless steel from 800 C to 400 C increases?
Depleted region decreases in width
Depleted region increases in width
No change in width of depleted region
OK
INCORRECT
There will be more diffusion for longer time at elevated temperatures.
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 2
What is expected to occur as the carbon content decreases in a 304 stainless steel that is cooled from 1050 C to room temperature?
Degree of sensitization decreases
Degree of sensitization increases
Degree of sensitization the same as at higher carbon content
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Problem 2
What is expected to occur as the carbon content decreases in a 304 stainless steel that is cooled from 1050 C to room temperature?
Degree of sensitization decreases
Degree of sensitization increases
Degree of sensitization the same as at higher carbon content
OK
CORRECT
As carbon content decreases there is less carbon to form precipitates
Precipitates will be smaller and fewer in number compared to a steel with higher carbon content
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Problem 2
What is expected to occur as the carbon content decreases in a 304 stainless steel that is cooled from 1050 C to room temperature?
Degree of sensitization decreases
Degree of sensitization increases
Degree of sensitization the same as at higher carbon content
OK
INCORRECT
With less carbon available, will it be as easy for chromium carbide precipitates to form compared to an alloy with higher carbon content?
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Problem 2
What is expected to occur as the carbon content decreases in a 304 stainless steel that is cooled from 1050 C to room temperature?
Degree of sensitization decreases
Degree of sensitization increases
Degree of sensitization the same as at higher carbon content
OK
INCORRECT
With less carbon available, will it be as easy for chromium carbide precipitates to form compared to an alloy with higher carbon content?
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Welding is a common cause of sensitization
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Top view
Side view
Heated to carbide formation temperatures
Cooling rate sufficiently high to avoid carbide precipitation
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Problem 3
What is expected to occur as the welding time to join two pieces of 304 stainless steel component increases?
Decrease width of steel that is sensitized
Increase width of steel that is sensitized
No change in width of steel that is sensitized
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 3
What is expected to occur as the welding time to join two pieces of 304 stainless steel component increases?
Decrease width of steel that is sensitized
Increase width of steel that is sensitized
No change in width of steel that is sensitized
OK
INCORRECT
As welding time increases, heat put into areas away from weld increases.
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 3
What is expected to occur as the welding time to join two pieces of 304 stainless steel component increases?
Decrease width of steel that is sensitized
Increase width of steel that is sensitized
No change in width of steel that is sensitized
OK
CORRECT
As welding time increases, heat put into areas away from weld increases.
Increased portion heated into temperature range for carbide formation.
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 3
What is expected to occur as the welding time to join two pieces of 304 stainless steel component increases?
Decrease width of steel that is sensitized
Increase width of steel that is sensitized
No change in width of steel that is sensitized
OK
INCORRECT
As welding time increases, heat put into areas away from weld increases.
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Sensitization during annealing
Sometimes necessary to anneal stainless steel to soften for cold working
For example, deep drawing may require more than one forming step• Steel work hardens to the point that it cannot be shaped in one step
Steel annealed above 1000 C to restore ductility
Carbides will form if steel not cooled quickly enough between 870 and 425 C
Step 1 Step 2
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Problem 4
What are possible methods for reducing susceptibility to sensitization of 304 stainless steel that must be annealed?
Increase carbon content or fast cool from annealing temperature
Increase carbon content or slow cool from annealing temperature
Decrease carbon content or fast cool from annealing temperature
Decrease carbon content or slow cool from annealing temperature
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 4
What are possible methods for reducing susceptibility to sensitization of 304 stainless steel that must be annealed?
Increase carbon content or fast cool from annealing temperature
Increase carbon content or slow cool from annealing temperature
Decrease carbon content or fast cool from annealing temperature
Decrease carbon content or slow cool from annealing temperature
OK
INCORRECT
Increasing carbon content results in more carbon available to form chromium carbide precipitates
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 4
What are possible methods for reducing susceptibility to sensitization of 304 stainless steel that must be annealed?
Increase carbon content or fast cool from annealing temperature
Increase carbon content or slow cool from annealing temperature
Decrease carbon content or fast cool from annealing temperature
Decrease carbon content or slow cool from annealing temperature
OK
INCORRECT
Increasing carbon content results in more carbon available to form chromium carbide precipitates.
Slow cool from annealing temperature gives more time for chromium carbide precipitates to form.
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 4
What are possible methods for reducing susceptibility to sensitization of 304 stainless steel that must be annealed?
Increase carbon content or fast cool from annealing temperature
Increase carbon content or slow cool from annealing temperature
Decrease carbon content or fast cool from annealing temperature
Decrease carbon content or slow cool from annealing temperature
OK
CORRECT
Decreasing carbon content in less carbon available to form chromium carbide precipitates.
Fast cool from annealing temperature reduces time for chromium carbide precipitates to form.
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 4
What are possible methods for reducing susceptibility to sensitization of 304 stainless steel that must be annealed?
Increase carbon content or fast cool from annealing temperature
Increase carbon content or slow cool from annealing temperature
Decrease carbon content or fast cool from annealing temperature
Decrease carbon content or slow cool from annealing temperature
OK
INCORRECT
Slow cool from annealing temperature gives more time for chromium carbide precipitates to form.
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Preventing sensitization of Austenitic stainless steel
1. High-temperature solution anneal heat treatment
2. Addition of elements that are strong carbide formers (called stabilizers)
3. Reduce carbon content
Approaches especially important if control of thermal treatment is difficult
• Some components or structures require stress relief heat treatment at temperatures ideal for sensitization
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Solution anneal heat treatment to dissolve precipitates
1. Heat the alloy above 1035 C (1900 F)
• Dissolve the chromium carbides
• Puts chromium back into solid solution
• Restores the chromium-depleted zone
2. Cool rapidly (quench) below 425 C (797 F)
• Prevent formation of chromium carbide precipitates
Chromium atomCarbon atom
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Addition of strong carbide formers
Stabilizers
• Niobium (type 347 stainless steel)
• Titanium (type 321 stainless steel)
These elements combine with carbon
• Form niobium carbide or titanium carbide particles
• Leaves no carbon left for chromium to react
• Chromium carbides cannot form
• Chromium depletion does not occur
Permits fabrication of large vessels, repair welding, and other operations without subsequent heat treatment
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Stabilized stainless steel as received from a steel mill
• Contains titanium or niobium carbides
• Essentially no chromium carbides
Must follow certain procedures if heat treatment is required
If steel is solution annealed and cooled too fast
• Titanium or niobium remain in solid solution and titanium or niobium carbide particles do not form
• When heated into sensitization temperature range, steel behaves like a steel without titanium or niobium, and sensitization results
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Decrease carbon content
As carbon content decreases• Number and size of chromium carbide particles decreases
• Elevated temperature exposure time to form particles increases
Graph general guidelines. Should be verified before applied.
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Te
mp
era
ture
( C
)
Time to Sensitization
Te
mp
era
ture
( F
)
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Problem 5
How quickly should a 304 stainless steel alloy with 0.030% carbon be cooled below 600 C to avoid sensitization?
Within 100 hours
(Reprinted with permission of ASM International®. All rights reserved.)
Te
mp
era
ture
( C
)
Time to Sensitization
Te
mp
era
ture
( F
)
Within 30 hours Less than 1 hour Within 8 hours
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Problem 5
How quickly should a 304 stainless steel alloy with 0.030% carbon be cooled below 600 C to avoid sensitization?
Within 100 hours
(Reprinted with permission of ASM International®. All rights reserved.)
Te
mp
era
ture
( C
)
Time to Sensitization
Te
mp
era
ture
( F
)
Within 30 hours Less than 1 hour Within 8 hours
OK
INCORRECT
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 5
How quickly should a 304 stainless steel alloy with 0.030% carbon be cooled below 600 C to avoid sensitization?
Within 100 hours
(Reprinted with permission of ASM International®. All rights reserved.)
Te
mp
era
ture
( C
)
Time to Sensitization
Te
mp
era
ture
( F
)
Within 30 hours Less than 1 hour Within 8 hours
OK
INCORRECT
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 5
How quickly should a 304 stainless steel alloy with 0.030% carbon be cooled below 600 C to avoid sensitization?
Within 100 hours
(Reprinted with permission of ASM International®. All rights reserved.)
Te
mp
era
ture
( C
)
Time to Sensitization
Te
mp
era
ture
( F
)
Within 30 hours Less than 1 hour Within 8 hours
OK
INCORRECT
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 5
How quickly should a 304 stainless steel alloy with 0.030% carbon be cooled below 600 C to avoid sensitization?
Within 100 hours
(Reprinted with permission of ASM International®. All rights reserved.)
Te
mp
era
ture
( C
)
Time to Sensitization
Te
mp
era
ture
( F
)
Within 30 hours Less than 1 hour Within 8 hours
OK
CORRECT
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Testing for sensitization
ASTM A262 Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels
• 6 different tests
• Tests involve exposing samples to acid solutions followed by different methods of evaluation
Tests bears little relationship to intended service environment
• Detect metallurgical conditions that can lead to intergranular corrosion
Use for process development, supplier evaluation, and problem solving
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Final remarks for sensitization
A sensitized stainless steel will not necessarily exhibit intergranular corrosion in all environments
• In environments where IG corrosion does not occur, the sensitized boundary and grains exhibit passive behavior
Ferritic stainless steels can be sensitized
• Information available in references in Module 1
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Intergranular corrosion of aluminum alloys
2 mechanisms
Exfoliation
Galvanic corrosionDue to formation of precipitates
on grain boundaries
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Galvanic corrosion
Galvanic cells form due to differences in composition at grain boundaries
Different forms of galvanic cells
Likelihood and severity of corrosion depends on • Alloy composition
• Degree of grain boundary precipitation • Corrosiveness of environment
Discrete particles Continuous film
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Al2Cu precipitates in Al-5Cu alloy
0.010 mm(0.0004 inches)
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Aluminum alloys of concernPrecipitation hardenable alloys
• 2xxx alloys (Cu is major alloying element)• 7xxx alloys (Zn and Mg are alloying elements; many with ≤ 3% Cu)
• Certain 2xx, 3xx, and 7xx cast alloys with Zn, Mg, and/or Cu
5xxx alloys• Contain magnesium as the major alloying element
• Strengthened by work hardening, such as by cold-rolling• Alloys with >3% magnesium can form Mg2Al3 precipitates along grain
boundaries when exposed to moderately elevated temperatures or after long time periods (many years) at room temperature.
Although, 6xxx alloys are precipitation hardenable, they are less of a concern
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Galvanic cell 1: Copper depletion at grain boundariesOccurs in alloys that contain copper and form Al2Cu precipitates
• 2xxx alloys
• Some 7xxx alloys
• Some cast alloys
Depletion occurs when alloy cooled too slow after solution anneal treatment
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1) Solution heat treat
2) Fast cool
Microstructure
25 C
Aluminum
Copper
Precipitation hardening
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Aluminum alloy used in solution annealed condition or aged
Microstructure after aging
Form precipitates at aging temperature
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1) Solution heat treat
2) Slow cool
25 CPrecipitates present
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Depleted of copper atoms
Copper atomAluminum atom
If cooling rate not fast enough
• Form Al2Cu or Al2Cu(Zn) precipitates on grain boundaries
Easier for precipitates to form on grain boundaries than within grains
• Unless cooling rate is extremely slow, precipitates do not form in grains
Grain boundary
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Copper depleted regions (anodes)
GrainGrain
Copper in solid solution (Cathodes)
Galvanic cell forms when exposed to a corrosive environment
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% Copper in grain phase
Grain boundary
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012Grain boundary
% Copper in grain phase
For underaged alloys some copper will still remain in solution within the
grain phase.
Possibility for galvanic cells between depleted layers and rest of grain
Important that intentionally underaged components not be exposed to
corrosive environments
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Problem 6
What are two approaches for preventing intergranular corrosion in aluminum-copper alloys that can be precipitation strengthened?
Fast cool alloy after solution anneal or underage
Slow cool after solution anneal or age to maximum strength
Fast cool alloy after solution anneal or age to maximum strength
Slow cool alloy after solution anneal or underage
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Problem 6
What are two approaches for preventing intergranular corrosion in aluminum-copper alloys that can be precipitation strengthened?
Fast cool alloy after solution anneal or underage
Slow cool after solution anneal or age to maximum strength
Fast cool alloy after solution anneal or age to maximum strength
Slow cool alloy after solution anneal or underage
OK
INCORRECT
Fast cooling after solution anneal will prevent intergranular corrosion
Underaging will result in a difference of copper in solid solution between depleted areas and areas away from grain boundaries
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Problem 6
What are two approaches for preventing intergranular corrosion in aluminum-copper alloys that can be precipitation strengthened?
Fast cool alloy after solution anneal or underage
Slow cool after solution anneal or age to maximum strength
Fast cool alloy after solution anneal or age to maximum strength
Slow cool alloy after solution anneal or underage
OK
INCORRECT
Slow cooling after solution anneal will result in a difference of copper in solid solution between depleted areas and areas away from grain boundaries
However, aging to maximum strength, or even overaging, will result in an equalization of the copper concentration in solid solution
Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012
Problem 6
What are two approaches for preventing intergranular corrosion in aluminum-copper alloys that can be precipitation strengthened?
Fast cool alloy after solution anneal or underage
Slow cool after solution anneal or age to maximum strength
Fast cool alloy after solution anneal or age to maximum strength
Slow cool alloy after solution anneal or underage
OK
CORRECT
Fast cooling after solution anneal will prevent formation of grain boundary precipitates
Aging to maximum strength, or even overaging, will result in an equalization of the copper concentration in solid solution
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Problem 6
What are two approaches for preventing intergranular corrosion in aluminum-copper alloys that can be precipitation strengthened?
Fast cool alloy after solution anneal or underage
Slow cool after solution anneal or age to maximum strength
Fast cool alloy after solution anneal or age to maximum strength
Slow cool alloy after solution anneal or underage
OK
INCORRECT
Slow cooling after solution anneal and underaging will result in a difference of copper in solid solution between depleted areas and areas away from
grain boundaries
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% Copper in grain phase
Fast cooling after solution anneal
Aging to maximum strength or overaging
Grain boundary
Preventing intergranular corrosion in alloys that form Al-Cu precipitates
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Duration of precipitation heat treatment (hours)
Yie
ld s
tren
gth
(MP
a)
Yie
ld s
tren
gth
(ksi
)
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Galvanic cell 2: Precipitates more active than grain interior
Cold rolled 5xxx alloys with > 3% magnesium
Precipitation hardenable copper-free 7xxx alloys
GrainGrain
Anode
Cathode
Precipitates preferentially attacked
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In general, 5xxx alloys have excellent resistance to corrosion
In 5xxx alloys with >3% magnesium• Mg2Al3 precipitates can form along grain boundaries
• Form when alloy exposed to moderately elevated temperatures or after long periods of time (many years) at room temperature
• Mg2Al3 highly anodic with respect to aluminum grain phase• Precipitates corrode, weakening the grain boundaries
Time required for precipitates to form depends on magnesium content, alloy temper, exposure temperature, and initial processing
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7xxx alloys without copper
Anodic MgZn2 precipitates form on grain boundaries
• Improper heat treatment
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Exfoliation
Occurs predominantly in aluminum alloy components
• Highly elongated grains that are parallel to metal surface
• Present in extruded and heavily cold-worked components
Corrosion initiates on surface and proceeds along grain boundaries
• Corrosion products take up greater volume than parent metal
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Susceptibility to exfoliation depends on
• Alloy composition
• Heat treatment
• Severity of corrosive environment
Not accelerated by stress and does not lead to stress corrosion cracking
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Susceptible alloys
Certain extruded products in both marine and industrial environments
• 2xxx copper-magnesium alloys
• 7xxx zinc-copper-magnesium alloys
• Certain cold worked 5xxx alloys
Attack generally associated with
• Alloy fabrication method and extent of aging
• Impurities in alloy matrix
• Metallic compounds at surface and in grain boundaries
Alloys resistant to exfoliation
• 1100 (UNS A91100)
• 3003 (UNS A93003)
• 5052 (UNS A95052)
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7xxx alloys
• Exfoliation susceptibility typically high in alloys aged to maximum strength
• For alloys with copper, overaging improves exfoliation resistance, but with significant decrease in strength from maximum strength
5xxx aluminum alloys
• Special processing for some alloys to control where Mg2Al3 precipitates form within grains
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Intergranular corrosion of other metals and alloys
Other metals susceptible to intergranular corrosion
• Zinc alloys that contain aluminum
• Some nickel alloys
• Possible, but uncommon in some copper alloys
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Module review1. Mechanisms by which an alloy made susceptible to intergranular corrosion
a. Galvanic cells due to segregation of impurities to grain boundaries
b. Depletion of an element that provides corrosion resistance
• Sensitization
• Austenitic stainless steels
c. Galvanic cells due to depletion of element near grain boundaries
• Precipitation hardenable Al alloys that form Al-Cu precipitates
d. Galvanic cells between grain boundary precipitates and regions adjacent to grain boundaries
• Precipitation hardenable Al alloys that form Mg-Zn precipitates
• 5xxx alloys with more than 3% magnesium
2. Various microstructure features enable intergranular corrosion
• Specific processing conditions lead to development of these features
3. There are different approaches for controlling intergranular corrosion
• Specific approaches that can be used depend on specific alloy and mechanical requirements for component in which alloy will be used
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End of Module 7