Intergranular Corrosion
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Intergranular Corrosion
INTERGRANULAR CORROSION
(INTERGRANULAR ATTACK.. IGA)
Metals are usually “polycrystalline” . . . an assemblage of single-crystal grains
separated by grain boundaries.
Grain boundary in a
polycrystalline metal (twodimensional representation).
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The atoms in the grain boundaries are in a distorted lattice (i.e.,
disordered).
The higher energies of grain boundary atoms make them slightly more
reactive than grains.
BUT: difference is NOT NOTICEABLE in general corrosion.
SOMETIMES . . . grain boundaries can become highly reactive:
–
by concentration of impurity atoms (e.g., Fe in Al has low
solubility in metal, segregates in grain boundaries which corrode
more rapidly than grains, and intergranular attack results);
–
by enrichment of an alloying element (e.g., Zn in brass);
–
by depletion of an alloying element (e.g., Cr in SS).
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IGA (Intergranular Attack) in Austenitic SS (Stainless Steel)
What is austenite?
The lower-left corner
receives prime attention
in heat-treating of steels.
(In calculations, 0.77 % is
commonly rounded to 0.8
%.)
Fe-Fe3C Phase Diagram.
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Nomenclature
•
•
•
•
•
cast iron / CS . . . . > 2%C / < 4% C;
 - iron ( - ferrite not to be confused with ferrite oxides).. is BCC
 - iron (ferrite) is also BCC;
iron carbide (cementite) is Fe3C, orthorhombic;
 - iron (austenite) is FCC.
austenite
• is non-magnetic;
• is unstable below 727C
decomposes on slow cooling to ferrite + pearlite if hypoeutectoid;
pearlite + eutectic if hyperentectoid
(N.B. pearlite is the lamellar mixture of ferrite and carbide that forms on
cooling austenite of eutectoid composition . . . 0.8% C).
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Pearlite.
This microstructure is a lamellar
mixture of ferrite (lighter matrix)
and carbide (darker).
Pearlite forms from austenite of
eutectoid composition. Therefore,
the amount and composition of
pearlite are the same as those of
eutectoid austenite.
Pearlite Formation. Carbon must
diffuse from the eutectoid austenite
(0.8 percent) to form carbide (6.7
percent). The ferrite that is formed
has negligible carbon.
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AUSTENITE decomposes on rapid cooling below 727C (i.e., quenching) to:
MARTENSITE - a metastable forced solution of C in ferrite that is very hard,
has BCT (body-centered-tetragonal) structure.
N.B. IN STAINLESS STEELS, THE THREE MAJOR CARBON STEEL PHASES (FERRITE,
AUSTENITE, MARTENSITE) CAN ALSO BE FORMED.
Also:
∙
∙
“ferritic-austenitic” (duplex)
“precipitation-hardened”.
Stability and mechanical/physical properties depend on combination of
alloying elements.
austenite stabilizers:
C, N, Mn, Ni, (q.v. Ni alloys);
ferrite stabilizers:
Si, Cr, Mo, Nb (“Columbium”- Cb), Ti.
Selection of a steel/alloy for a particular application depends on mechanical
or physical property considered to be most important.
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COMMON STANDARD WROUGHT AUSTENITIC SS
AISI type
UNS
Cr
Ni
Mo
C
Si
Mn
Other
304
S30400
18-20
8-10.5
-
0.08
1.0
2.0
-
304L
S30403
18-20
8-12
-
0.03
1.0
2.0
-
304N
S30451
18-20
8-10.5
-
0.08
1.0
2.0
0.10-0.16N
316
S31600
16-18
10-14
2.0-3.0
0.08
1.0
2.0
-
316L
S31603
16-18
10-14
2.0-3.0
0.03
1.0
2.0
-
316N
S31651
16-18
10-14
2.0-3.0
0.08
1.0
2.0
0.10-0.16N
347
S34700
17-19
9-13
-
0.08
1.0
2.0
(10xC)(Cb+Ta)
COMMON STANDARD WROUGHT FERRITIC SS
AISI type
UNS
Cr
C
Mn
Si
P
S
Other
405
S40500
11.5-14.5
0.08
1.0
1.0
0.04
0.03
0.1-0.3Al
430
S43000
16-18
0.12
1.0
1.0
0.04
0.03
-
COMMON STANDARD WROUGHT MARTENSITIC SS
AISI type
UNS
Cr
Ni
Mo
C
Other
403
S40300
11.5-13.0
-
-
0.15
-
410
S41000
11.5-13.0
-
-
0.15
-
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Sensitization:
Cr is added to steels to make them “stainless”. The Cr-rich oxide film (based
on Cr2O3) is thin, adherent and very protective.
BUT if heated into range 510-790C, the steels “sensitize” and become prone
to IGA.
Sensitization involves the precipitation of Cr carbide (Cr23C6) at the grain
boundaries; at the high temperature its solubility is virtually zero.
The C diffuses readily, and the disorder in the boundaries provides
nucleation sites.
This depletes the boundaries of Cr.
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Diagrammatic representation
of a grain boundary in sensitized
type 304 stainless steel.
Cross section of area shown above.
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Electron photomicrograph of carbides isolated from sensitized type 304 stainless steel.
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Sensitization by welding, or “Weld Decay”
During welding, the weld “bead” and the metal on either side pass through
the temperature range for sensitization.
Temperature AND time are crucial for carbide precipitation: sensitized areas
are on either side of the bead.
Tablecloth analogy of heat flow
and temperatures during
welding. The rise and fall of each
stripe represents the rise and fall
of temperature in a welded
plate.
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Actual measurements made with thermocouples at points ABCD. Fontana says
metal at and between points B and C within sensitizing range for some time.
Discuss
Temperatures during electric-arc welding of type 304 stainless steel..
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N.B. Sensitized SS can be used in many environments which are not too
aggressive or where selective corrosion not a problem (domestic,
architecture)
Minimizing IGA of SS
(1) Heat Treatment “Quench-Annealing”
or ... “Solution-Annealing”
or .... “Solution-Quenching”
Involves heating to above Cr carbide precipitation temperature to dissolve
carbides, then water-quenching to cool through sensitization range rapidly.
Most austenitic SS supplied in solution-quenching condition; if welded
during fabrication, must be quench-annealed to avoid weld decay during
subsequent exposure to corrosive environments. Solution-quenching of large
components can be a problem.
Discuss: Why not heat-treat just the weld region?
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(2) Alloy “Stabilization”
Elements that are strong carbide formers are added:
Nb (or Nb+Ta)
type 347 SS
Ti
type 327 SS
Important to ensure that Nb (for example) carbide has precipitated, so that Cr
Carbide cannot precipitate and reduce corrosion resistance at grain
boundaries
(REMEMBER - it is the Cr that provides the corrosion resistance, not the
stabilizer).
Melting point, F
2250
1450
C
Cr + Nb carbides
dissolve
Cr carbide dissolves
Nb carbide precipitates
1230
790
Cr carbide precipitates
510
950
No reactions
70
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Schematic chart showing solution
and precipitation reactions in types
304 and 347 SS.
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Stabilized SS from supplier usually heat-treated by quenching from ~1070C.
- Nb carbide has precipitated,
- Cr left in solution, hence no C available for any reactions with Cr at
lower temperatures.
HOWEVER, care is needed during welding etc.
If welding involves a rapid cooling of metal from temperatures just at or
below the melting point (as can occur in thin sheets), BOTH Nb and Cr
remain in solution.
This metal can now be sensitized if it is heated to the Cr carbide precipitation
range (510 - 790C, as might occur during a stress-relief).
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“Knife-Line-Attack” (KLA) may now occur in narrow band next to weld if exposed to
corrosive environment.
Knife-line attack on
type 347 stainless
steel.
Should have been heat-treated between 790 & 1230 C (Nb carbide precipitates, Cr
dissolves).
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(3) Use “Low-Carbon” (< 0.03%) Alloy.
At concentrations < 0.03%, not enough C can precipitate as Cr carbide to
sensitize. Get L-Grade or ELC alloys e.g., “type 304L”.
Elimination of weld decay
by type 304ELC.
weld bead at back
N.B. Must take care to avoid C contamination during casting, welding, etc.
Other Alloys and IGA
Alloy with precipitated phases may also show IGA:
• Duralumin(um) Al-Cu can precipitate CuAl2 and deplete Cu locally;
• Die-cast Zn alloys containing Al... IGA in steam, marine environments;
• Minor IGA effects in many Al alloys.
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Selective leaching
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Selective Leaching
SELECTIVE LEACHING
(“Dealloying”, “Parting”)
Corrosion in which one constituent of an alloy is
preferentially removed, leaving behind an altered
(weakened) residual structure.
Can occur in several systems.
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Combinations of alloys and environments subject to dealloying and elements
preferentially removed
Alloy
Environment
Element removed
Brasses
Many waters, especially under stagnant
conditions
Zn (dezincification)
Grey iron
Aluminium bronzes
Soils, many waters
HCl, acids containing Chloride
Fe (graphitic corrosion)
Al (dealuminification)
Silicon bronzes
High-temperature steam and acidic species
Si (desiliconification)
Tin bronzes
Copper-nickels
Hot brine or steam
High heat flux and low water velocity
(in refinery condenser tubes)
Sn (destannification)
Ni (denickelification)
Copper-gold single crystals
Monels
Ferric chloride
Hydrofluoric and other acids
Cu
Cu in some acids, and Ni in others
Gold alloys with copper or
silver
High-nickel alloys
Sulfide solutions, human saliva
Molten salts
Cu, Ag, Cr, Fe, Mo and T
Medium- and high-carbon
steels
Oxidizing atmospheres, hydrogen at high
temperatures
C (decarburization)
Iron-chromium alloys
High-temperature oxidizing atmospheres
Cr, which forms a protective film
Nickel-molybdenum alloys
Oxygen at high temperature
Mo
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Dezincification
All Cu-Zn alloys (Brasses) containing > 15% Zn are susceptible . . .
e.g. common yellow brass . . . 30 Zn 70 Cu, dezincifies to red copperrich structure. Dezincification can be uniform...
- potable water inside
Uniform dezincification of
brass pipe.
-
or plug-type.... (boiler water inside, combustion gases outside)
Plug-type
dezincification.
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Section of one of the plugs shown before
Overall dimensions of original material tend to be retained . . . residual is
spongy and porous . . . often brittle.
Can go unnoticed, especially if covered with dirt/deposit, etc.
Uniform dezincification...
- usually found in high brasses (high[Zn]), acid environments;
Plug-type dezincification...
- usually found in low brasses, alkaline, neutral or slightly acid environments.
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Mechanism
(1) Zn atoms leave lattice sites . . .
“are leached into the environment selectively’’
Discuss . . . w.r.t. last picture.
(2) Generally accepted . . .
- brass dissolves;
- Zn stays in solution;
- Cu re-deposits.
Discuss . . . w.r.t. last picture.
N.B. possibility for local anode-cathode couples .. Cu deposits accelerate
attack.
N.B. dezincified areas generally 90-95% Cu; some Cu2O/CuO present if O2 in
the environment.
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Prevention
-
Make environment less aggressive (e.g., reduce O2 content);
- Cathodically protect;
- Use a better alloy (common cure - above not usually feasible)...
- “red” brass (<15% Zn) almost immune
- Admiralty Brass. . .
70 Cu, 29 Zn, 1 Sn;
- arsenical Admiralty. . . 70 Cu, 29 Zn, 1 Sn, 0.04 As
(Sn and Sn-As in deposited films hinder redeposition of Cu);
- For very corrosive environments likely to provoke dezincification, or for
critical components, use . . .
- cupronickels
70-90 Cu, 30-10 Ni.
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“Graphitization” (misnomer . . . graphitization is the breakdown of pearlite to
ferrite + C at high temperature)
Grey cast iron is the cheapest engineering metal . . . 2-4% C, 1-3% Si.
Hard, brittle, easily cast; carbon present as microscopic flakes of matrix
graphite within microstructure.
Microstructure of grey cast
iron.
100 m
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In some environments (notably mild, aqueous soils affecting buried pipe) the
Fe leaches out slowly and leaves graphite matrix behind . . appears graphitic . .
.soft . . . can be cut with a knife. Pores usually filled with rust. Original
dimensions are retained.
A 200-mm (8-in.) diameter grey-iron pipe that
failed because of graphitic corrosion. The pipe
was part of a subterranean fire control system.
The external surface of the pipe was covered
with soil; the internal surface was covered with
water. Severe graphitic corrosion occurred
along the bottom external surface where the
pipe rested on the soil.
The small-diameter piece in the foreground is a
grey-iron pump impeller on which the impeller
vanes have disintegrated because of graphitic
corrosion.
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(a) External surface of a grey-iron pipe
exhibiting severe graphitic corrosion.
(b) Close-up of the graphitically-corroded
region shown in (a).
(c) Micrograph of symmetrical envelopes of
graphitically-corroded iron surrounding flakes
of graphite.
20 m
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Selective Dissolution in Liquid Metals
In liquid metal coolants (LMFBR with Na or Na-K coolant), austenitic alloys can
lose Ni and Cr and revert to the ferrite phase...
Corrosion of Inconel* alloy 706 exposed to liquid sodium for 8,000 hours at 700C
(1290F); hot leg circulating system. A porous surface layer has formed with a
composition of  95% Fe, 2% Cr and < 1% Ni. The majority of the weight loss
encountered can be accounted for by this surface degradation. Total damage depth:
45 m. (a) Light micrograph. (b) SEM of the surface of the porous layer.
* Alloy 706 ... 39-44% Ni, 14.5-17.5% Cr, 0.06% C.
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Also in fusion-reactor environments (Li as coolant)....
Light micrograph of cross-section.
SEM of surface showing porous layer.
Corrosion of type 316 stainless steel exposed to thermally convective lithium for
7488 hours at the maximum loop temperature of 600C.
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Usually, the transport and deposition of leached elements is of more concern
than the actual corrosion.
(a)
(b)
SEM micrographs of chromium mass transfer deposits found at the 460C (860C)
position in the cold leg of a lithium/type-316-stainless-steel thermal convection loop
after 1700 hours. Mass transfer deposits are often a more serious result of corrosion
than wall thinning. (a) Cross section of specimen on which chromium was deposited.
(b) Top view of surface.
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100 m
Iron crystals found in a plugged region of a failed pump channel of
a lithium processing test loop.
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Selective Leaching in Molten Salts
Molten salts are ionic conductors (like aqueous solutions) and can promote
anodic-cathodic electrolytic cells . . . they can be aggressive to metals.
ALSO . . . some molten salts (notably fluorides) are “Fluxes” and dissolve surface
deposits that would otherwise be protective: dealloying of Cr from Ni-base
alloys and stainless steels can occur in the surface layers exposed to molten
fluorides; the vacancies in the metal lattice then coalesce to form subsurface
voids which agglomerate and grow with increasing time and temperature.
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(a)
(b)
(a) microstructure of type 304L SS exposed to LiF-BeF2-ZrF4-ThF4- UF4 (70-23-5-1-1
mole % respectively) for 5700 hours at 688C.
(b) microstructure of type 304L SS exposed to LiF-BeF2-ZrF4-ThF4- UF4 (70-23-5-1-1
mole % respectively) for 5724 hours at 685C.
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