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Oxidation and Corrosion in High-Temperature Systems
G. H. MeierDepartment of Materials Science & Engineering
University of Pittsburgh
ICMR Summer Program on Advanced Thermostructural Materials
August 14, 2006
Introduction
The rate of reaction between a reactive material and a gas is determined by the nature of the reaction product.
Alloy or Ceramic
Product
Gas
ThermodynamicsReactive Potentials in Gas µO2, µC, µS2
Stable Product(s)
Vaporization
DiffusionD’s in Reaction Product
D’s in Alloy, Ceramic
ThermomechanicalStabilityPBR (VProd/VAlloy)
X > 2EΓ/σ2
Film PropertiesASR = ρx
x
N. Birks, G. H. Meier, and F. S. Pettit, Introduction to High Temperature Oxidation of Metals, 2nd Edition, Cambridge University Press, Cambridge, 2006.
Oxidation and Corrosion of Metals and Alloys
a. Review of thermodynamic principlesb. Review of diffusion principles: point defects in
metals and oxides c. Principles of oxidation and corrosion
• Parabolic and transition from parabolic• External vs. internal oxidation (Selective oxidation)• Oxide adherence• Environmental influences in real systems (Evaporation,
H2O content, dual-atmosphere, pressure, breakaway corrosion, corrosion in multi component gases)
d. Protective CoatingsCoating FabricationCoating Durability
Thermodynamic Principles''' TSHG −=
At constant temperature and pressure:ΔG′< 0 spontaneous reaction expectedΔG′= 0 equilibriumΔG′> 0 thermodynamically impossible process
aA + bB = cC +dD
⎟⎟⎠
⎞⎜⎜⎝
⎛+°Δ=Δ b
BaA
dD
cC
aaaaRTGG ln'
BADC badcG μμμμ −−+=′Δ
ioii aRT ln+= μμ
°≅≡i
io
i
ii p
pffa
oB
oA
oD
oC
o GbGaGdGcG Δ−Δ−Δ+Δ=Δ
KRTaaaaRTG
eqbB
aA
dD
cC lnln −=⎟⎟
⎠
⎞⎜⎜⎝
⎛−=°Δ “Law of Mass Action”
At equilibrium:
Note: Free energy of formation data can be accessed either directly or as “Log KP. The latter allows direct calculation of the equilibrium constant for the overall reaction.
Calculation of Activities in a Gaseous Environment
Problem A gas consisting of 60 vol % H2 and 40 vol% CO2 is let into a reaction chamber and heated to 1200K at a total pressure of 1 atm. Calculate the oxygen partial pressure and carbon activity in this gas when it comes to equilibrium.
Solution The common species which can form in such a gas mixture and their log KP values at 1200K are listed in the following table.
Species log KPCO2 17.243CO 9.479H2O 7.899H2 0.00
H2(g) + CO2(g) = H2O(g) + CO(g)
135.0loglogloglog 22 =−+= COP
COP
OHPR KKKK
22
2365.1COH
COOHR pp
ppK ==
tottot
ii P
nnp =
22
2365.1COH
COOHR nn
nnK ==
Species Initial Comp. (moles) Final Comp. (moles)H2 0.6 0.6-λCO2 0.4 0.4-λCO 0 λH2O 0 λ
)4.0)(6.0(365.1
λλλλ
−−==RK λ = 0.2575 moles.
Species Final Comp. (moles) Partial Pressures (atm)H2 0.3425 0.3425CO2 0.1425 0.1425CO 0.2575 0.2575H2O 0.2575 0.2575
Calculation of Activities
Oxygen Partial PressureH2(g) + 1/2O2(g) = H2O(g)
2/12/17
222
2
3425.02575.010924.7
OOH
OHP ppp
pxK ===
atmxpO18100.9
2
−=
Carbon Activity2CO(g) = CO2(g) + C(s)
715.1log2loglog 2 −=−= COP
COPR KKK
22 )2575.0(1425.0193.0 2 C
CO
CCOR
ap
apK ===
aC = 0.009
)()()( 221 sNiOgOsNi =+
Will this gas with oxidize Ni or Cr?
Log KP = 5.75
5106.5121
2
xp
KO
P ==
atmxpeqO
12102.32
−=
Ni cannot oxidize
)()()(2 32223 sOCrgOsCr =+
atmxpeqO
24102.12
−=
Cr will tend to oxidize
atmxpO18100.9
2
−=
Ellingham (Richardson) Diagram
Ellingham (Richardson) Diagram
atmxpO18100.9
2
−=
33.12
2 =OH
H
pp
Isothermal Stability (Ellingham) Diagram
Ni-S-O System, 1250KTernary – Type 1 Diagram
Generalized “Phase Diagrams”Gibbs Phase Rule: F = C – P +2
Combined 1st and 2nd Laws
332211 dndndnVPdSTdUd μμμ +++′−′=′
inVSi
i GnU
j
=⎟⎟⎠
⎞⎜⎜⎝
⎛∂
′∂≡
′′ ,,
μ
T, P, µi = potentials (φ)
S/, V/, ni = conjugate extensive variables (q)
Type 1: φi vs φj Invariant Equilibria = “point”
Type 2: φ vs qi/qj Invariant Equilibria = “line”
Type 3: qi/qk vs qj/qk Invariant Equilibria = “area”
Pelton, A. D. and Schmalzried, H., Met. Trans., 4, (1973) 1395.
Fe-O Isobaric DiagramBinary – Type 2 Diagram
Fe-Cr-O Isothermal Diagram Ternary – Type 2 Diagram
Fe-Cr-O Isothermal Section Ternary – Type 3 Diagram
Solution Thermodynamics
Positive Deviation from Ideality
Negative Deviation from Ideality
NB NB
Solution Thermodynamicsai = Ni Ideal Solutionai = γiNi Real Solution, γi = f(Ni)ai = γi(∞)Ni Dilute Solution, γi(∞) = constant
Sievert’s LawConsider Hydrogen Dissolving into Nickel
)()(221 inNiHgH =
2/12/122
)(
H
HH
H
H
pN
paK ∞
==γ
2/12/122)( HSH
HH pkpKN =
∞=γ
Oxide Evaporation
)(2)()( 3223
32 gCrOgOsOCr =+
43
2
21
32
21
3 OOcrCrO paKp =
Chromia Evaporation
CHROMIA
• At high T and high PO2 volatile oxides develop
Cr2O3 + 3/2 O2 → 2 CrO3(g)Cr2O3 + 2 H2O + 3/2 O2 → 2 CrO2(OH)2
Al – O 1473K Si – O 1250K
Vapor Species Diagrams for the Al-O and Si-O Systems
Diffusion Fundamentals
xC
DJ BB ∂
∂−= ~
2
2~xC
Dt
C BB
∂∂
∂∂
=
Fick’s First law
Fick’s Second Law
Steady State Diffusion of H2 Through Ni T=800°C
xCCD
dxdcDJ HH
HHH
12 −−=−=
)(1.0)/(1009.2)/(1088.6
3525
1
cmcmgxscmx
xC
DJ HHH
−−==
JH
x
[H]
CH(1)
CH(2)=0
1mm0
atmpH 1.02=
02≈Hp )/(1044.1 28 scmgxJ H ⋅= −
Fick’s Second Law
∂∂
∂∂
Nt
DNx
B B= ~ 2
2
Boundary Conditions: At time t: NB = NB(S) at x = 0
NB = NBo at x = ∞
Concentration Profile for a Solute B Being Lost by Evaporation
N A B erf xDtB = +
⎛⎝⎜
⎞⎠⎟1 1 2General Solution
( ) ⎟⎠
⎞⎜⎝
⎛−+=DtxerfNNNN S
BoB
SBB 2
Solution Dtx 5≥
Calculation of Local Flux
xN
VDx
CDJ B
mB
B ∂∂
−=∂∂
−= ~~
xN B
∂∂
• Intrinsic and Extrinsic Elemental Semiconductors
Intrinsic:
Extrinsic:
-ionized native defects and dopants
kTE
i
kTE
i
g
g
eKnp
eKpn
−°
−°
=
== 2
Point Defects in Semiconductors
Defects in Compounds (e. g. Oxides) Defect Notations
• The main symbol: The defect species, which may be an ion, indicated by the atomic symbol for the species, or a vacant lattice site, denoted by V.
• The subscript: Indicates the lattice or interstitial site, I, occupied by the defect.• The superscript: Indicates the difference in charge at the defect site relative to the charge at that site
in the perfect crystal. A dot is used for an extra positive charge, and a slash denotes an extra negative charge.
Important Defects:Vacancies (cations and anions)InterstitialsForeign atomsElectrons and holesCharged defectsVarious associated complexes
VNa/
Conservation Rules• Conservation of mass: Atoms are neither created nor destroyed within a system, but must
be conserved• Conservation of charge: The bulk of an ideal crystal is electrically neutral. Charged
defects must be created in combinations that are electrically neutral. Matter can be added to or removed from a crystal only in electrically neutral combinations.
• Conservation of structure (lattice site ratios): The creation of lattice defects must not violate the inherent ratio of cation sites to anion sites in the structure. Thus cation and anion sites can be created or destroyed only in ratios that correspond to the stoichiometry of the compound (i.e., in electrically neutral combinations).
• Conservation of electronic states: The total number of electronic states in a system derives directly from the electronic states of the component atoms, and must be conserved.
Law of Mass Action is ApplicableaA + bB = cC + dD
[ ] [ ][ ] [ ]
bTG
ba
dc
eBADC
°Δ−
=
Replace activities with concentrations
Stoichiometry and Defect ChemistryIntrinsic Ionic Disorder is such that the stoichiometry of the compound is maintained.
Example: Schottky Disorder for the compound MX
•+′= XM VVNull
[ ][ ]•′= XMS VVKExtrinsic Ionic Disorder usually involves the addition of a dopant, but there are also cases where such disorder can be established by reactions of the compound with the gas and stoichiometry is not maintained.
NixONi
NiO VOAlOAl ′′++⎯⎯→⎯ • 3232
•+′′+⎯⎯→⎯ hVOO NixO
NiO 221
2
Dopant
Gas Reaction
Construction of a Kroger-Vink DiagramPure MXNo Dopant - Schottky Defects
Equilibration with Atmosphere
Electrical Neutrality
[ ][ ]i
sXM
KnpKVV
==′ •
[ ]2
1
'
'22
1
2
)(
X
MR
MxX
P
pVK
hVXgX
=
++= •
[ ] [ ]•+=′+ XM VpVn The Kröger-Vink diagram for MX.KS>Ki
Growth of Oxides on Pure Metals
Focus on Diffusion-Controlled Growth
1. Simple Parabolic Oxidation Model
2. Effect of Oxygen Partial Pressure
3. Bulk vs. Grain Boundary Diffusion
Pure Ni exposed for 1 hour @ 1100°C
air air + water vapor
x
Cations
Electrons
Anions
Cation Vacancies
Metal GasOxideMO
MOO2eM 2212 =++ −+
−− =+ 222
1 O2eOor
−− +=+ 2eMOOM 2
−+ += 2eMM 2
or
1Ma 2Op
M O2
oMO2 2MO;O2M GΔ=+Overall reaction:
p'O2
( )M/MO
= exp 2 GΔ MOO
RT a"M
exp ΔGMOO
RT=p"
O2( )
1½
Simplified Treatment of Diffusion-Controlled Oxidation
xCC
Djj MM
MM
VVVVM
′−″=−=+2
xCC
Ddtdx
Vj MM
M
VVV
oxM
′−″==+
12
xk
dtdx ′
=
)( ′−″=′MMM VVoxV CCVDk
tkx ′= 22
2OGas; p
Ni/NiOV )(Ni
C2Ni OV lowfor pC
2Ni OV highfor pC
Effect of oxygen partial pressure(p-type oxide)
Effect of oxygen partial pressure
NiXO VhOO ′′++= ⋅222
1
2/122OVh KpCC
Ni=″
NiVh CC ′′= 2
6/12
. OV pconstCNi=′′
nOV pconstC
M
/12
.=
⎥⎦
⎤⎢⎣
⎡⎟⎠⎞⎜
⎝⎛ ′−⎟
⎠⎞⎜
⎝⎛ ″∝′
n
O
n
O ppk/1/1
22
ElectroneutralityCondition
General for p-type Scales
n
Opk/1
2⎟⎠⎞⎜
⎝⎛ ″∝′
(atm)log2Op
-2 -1 0 1 2
-8
-7
-6
Effect of oxygen partial pressure
Oxidation of Co
43OCo
Cross-section of Co oxidized in Air at 750oC
2OGas; p
Zn/ZnOZn )(i
C
2i OZn lowfor pC
2i OZn highfor pC
Effect of oxygen partial pressure(n-type oxide)
Effect of oxygen partial pressure
2212 OeZnZnO i +′+= ⋅⋅
2/121 2OeZn
pCCKi
′⋅⋅=′
eZnCC
i′=⋅⋅2
2/131 2
4 OZnpCK
i⋅⋅=′
6/16/13/1
1 22.4/ −− =⎟
⎠⎞⎜
⎝⎛ ′=⋅⋅ OOZn
pconstpKCi
xCC
Ddtdx
Vj ii
i
MMM
oxM
′′−′==+
12
)(iii MMoxM CCVDk ′′−′=′
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛
″−⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛
′′=′
6/16/1
22
11.OO
ZnOpp
constk ( )2Opfk ≠′
Pure Ni exposed for 1 hour @ 1100°C
air air + water vapor
Ni in high-angleboundaries
Ni in lattice
O in lattice
Ni in low-angle boundaries
Temperature (°C)1600 1200 1000 800 700 600 500
)(K101 14 −×
5 6 7 8 9 10 11 12 13
-18
-16
-14
-12
-10
-8
Transport Paths - NiO
Single Crystal Cr2O3
Grain #1 Grain #2 Grain #3
Polycrystalline Cr
O2(g) Polycrystalline Cr2O3
Oxidation of Chromium
Caplan, D. and Sproule, G. I., Oxid. Metals, 9 (1975) 459
Temperature (K)
FeO
CoO
NiO
Cr O2 3
SiO2α-Al O2 3
)(K101 14 −×
7.0 7.2 7.4 7.6 7.8 8.0 8.2
-6
-8
-10
-12
-14
1400 1350 1300 1250
Relative growth rates of several oxides
Selective Oxidation
The approach to developing high temperature corrosion resistance in alloys is to have the reactants, namely the alloy and the gas, form a reaction product that separates the reactants and that allows slow transport of the reactants through it.
Most environments encountered in practice contain some oxygen, hence protective barriers are usually α-Al2O3, Cr2O3 and SiO2.
Alloy
Product
Gas
Ellingham (Richardson) Diagram
Internal Oxidation
Cu-1 wt%Ti Alloy
T = 700 - 900oC
Mount
Internal Oxidation Kinetics
m
SO
OO XVND
dtdmJ
)(
== (moles cm-2s-1)
mN X
VBo
m
=( )ν
(moles cm-2s-1)
dm N dXBo( )ν
dt V dtm
=
DNXV
NV
dXdtO
OS
m
Bo
m
( ) ( )
=ν
XdXN D
NdtO
SO
Bo=
( )
( )ν
XN D
NtO
SO
Bo=
⎡
⎣⎢
⎤
⎦⎥
21 2( )
( )
/
ν
Ni -1 wt%Cr - Dry AirT = 1000oC, t = 24h
43μmNiO
Ni - 1wt% Al - Dry AirT = 1000oC, t = 24h
NiO
IOZ
Apparent Oxygen Permeability in Ni
• J-W Park and C. J. Altstetter, Met. Trans. A, 18A, 43 (1987)At 1000oC: = 4.2 x 10-12 cm2/sO
SO DN )(
( )sx
XNtNX
DNo
Bo
BAppO
SO 4
2)()(2
.)(
1064.875.0
2=⎥
⎦
⎤⎢⎣
⎡=
ν
Alloy NB Atmosphere X (μm) (NODO)App (cm2/s)
Ni-1Cr 0.0113 Dry Air 42.9 1.8 x 10-12
Ni-1Al 0.0215 Dry Air 89.1 1.5 x 10-11
Effect of Internal Oxide Morphology on Apparent Oxygen Permeability
F. H. Stott, G. C. Wood: Materials Science and Technology, 4, 1072 (1988).
Cross-sections of Ni-8Cr-6Al oxidized @ 1100°C
External Alumina
α-Al2O3
NiO
Al2O3
NiO
Al2O3g.b.
Al2O3
Internal Alumina
1 h 1 min
A-BA+
BO
BO
NB
a.
b.NB
A-B
x(t)
Transition to External Scale Formation
Transition to External Scale Formation
2/1)(
*)(
2 ⎥⎦
⎤⎢⎣
⎡>
oxB
mOSO
oB VD
VDNgNν
π
Equating the molar flux of solute B to that required to create a critical volume fraction of oxide
Maintaining the Growth of an External ScaleEquating the molar flux of solute B to that being required for oxide growth
2/1)(
32 ⎟⎟⎠
⎞⎜⎜⎝
⎛=
B
pmoB D
kVNπ
ν
JDV
Nx
kM
tBB
m
B
x
p
O
= ⎛⎝⎜
⎞⎠⎟
= • •=
−∂∂ ν0
1 21 21 1
2
//
fAdXV
ADV
Nx
dtm
B
m
B=⎡
⎣⎢
⎤
⎦⎥
∂∂
PWA 1484 oxidized at 1100°C
Al Depletion
Alumina
Transient Oxide
1300
1200
1100
1000
10 20 30 40
Tem
pera
ture
(°C
)
at% Al
Ni Al3
Steady stateexternal Al O2 3
External Al O overtaken byNiO + NIAl O
2 3
2 4
External NiOInternal Al O2 3
Effect of Temperature and Al Content on the Formation and Continued Growth of External Alumina on Ni-Al Alloys
700°C Aluminum and chromium not selectively oxidized
Wet 3200 cycles Dry 3200 cycles
PWA 1484 @ 700°C
Temperature (K)
FeO
CoO
NiO
Cr O2 3
SiO2α-Al O2 3
)(K101 14 −×
7.0 7.2 7.4 7.6 7.8 8.0 8.2
-6
-8
-10
-12
-14
1400 1350 1300 1250
Relative Growth Rates of Several Oxides
0 4 8 12 20 2416time (h)
1
2
Ni-50CrNi-50Cr-0.004CeNi-50Cr-0.010CeNi-50Cr-0.030CeNi-50Cr-0.080Ce
Isothermal Oxidation – RE Effect
Ecer, and Meier, Oxid. Of Metals, 1979
Singh, Ecer, and Meier, Oxid. Of Metals, 1982
900oC
MnCr2O4
Cr2O3
Internal Al2O3
Si rich oxide
MnCr2O4
Internal Al2O3
Cr2O3
700oC
Metal Layer
Internal Al2O3
Cr2O3
700oC
Crofer AL453Internal Oxidation of Impurities
900oC
Cr2O3 Metal Layer
Internal Al2O3900oC
Cr2O3 Metal Layer
Internal Al2O3
Oxide Vaporization
Cr Cr2O3 gas
O2-
e-
Cr3+
x
CrO3
)(2)(23)( 3232 gCrOgOsOCr =+
Oxide Vaporization• Diffusion Process in Series with a Vaporization Process
• kp is parabolic growth constant, kv is vaporization constant. When oxide stops growing
vp kxk
dtdx
−=
0=dtdx
v
p
kk
x =0
• causes increased metal consumption• this effect increases with oxygen pressure, gas flow rate and temperature
x
time
x°
time
ΔM/A
(ΔM/A)°
Oxide Vaporization
Vapor species diagram for the Cr-O system at 1250°K
Vapor Species Diagram for the Mo-O SystemT = 1250K
Vapor Species Diagram for the W-O SystemT = 1250K
Oxide
GasAlloy
Isothermal
Cyclic
Δm/At
Cyclic Oxidation
NB(Crit)-OK
Internal Ox.
Effects of Alloy Depletion and Scale Spallation
Origins of Oxide Stress
• Growth Stresses – stresses arising from the nature of the oxide growth process
• Thermal Stresses – stresses arising form the thermal expansion mismatch between metal and oxide.
• Applied Loads
Stress Generation by Internal Oxidation
AL453
Internal Al2O3
Extruded Metal
lOx lM tOx tM l1ltOx
Thermal Stress
Origin of Thermal StressTM
metalthermal Δαε = TOx
Oxthermal Δαε =
M
MMmetalmech E
)1( νσε −= Ox
OxOxOxmech E
)1( νσε −=
Oxmechanical
Oxthermal
metalmechanical
metalthermal εεεε +=+
0t2t OxOxMM =+ σσ
TM ΔαMM
MOxOxEt
)1(t2 νσ − TOxΔαOx
OxOxE
)1( νσ −- = +
( ) ( )Ox
Ox
MM
MOxMOx
Ox
E1
Et1t2
T)(νν
Δαασ−
+−
−−=
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛+−
−−=
MM
OxOx
MOxOxOx
EtEt
211
T)(E
ν
Δαασ
Force Balance
ΔT =TL -TH
If νM ≈ νOx
Timoshenko, S. P., J. Opt. Soc. Amer.,11(1925) 233
( )νΔαα
σ−−−
=1
T)(E MOxOxOx
Thermal Stress if tOx << tM
Oxide Failure
alloy
Al2O3 xStored elastic energy of alumina ≈ f(sox, xox)
Fracture resistance of the alumina/alloy interface ≈f(morphology, composition)
Stresses in OxidesXRD Techniques
Stress from d(hkl) v. sin2ψ
oo hklshklsd
ddσψσε )(2sin)( 1
222
1
0
0 +=−
= ΨΨ
( )12sin 12
221 +⋅+⋅⋅⋅= oooo sddsd σψσψ
oo ds ⋅⋅= σ221slope
Growth Stress Measurements, FIM/Rocking
Growth Stress Measurement for Alumina Formed on a Pt-Modified Aluminide Bond Coat at 1100°C
226 Rocking, (NI,Pt) aluminide, ridges removed, 100+24h @ 1100°C
y = -0.001x + 1.052
1.0515
1.0516
1.0517
1.0518
1.0519
1.052
1.0521
0 0.1 0.2 0.3 0.4 0.5
sine squared psi
d-sp
acin
g
Growth Stress
= -0.3±0.1GPa
After 124 h exposure
Response to Stresses
shear cracking of the oxide
plastic deformation of the oxide and alloy
buckling of the oxide
Polishingmark
Aluminascale
Example of Buckling
Spallation of Alumina Scale by BucklingFeCrAl (TMP) oxidized at 1100°C for 120 hours.
Circular spalled area
Intact alumina scale
Circular spalled area
Intact alumina scaleCircular Buckles
FeCrAlTi Cyclically Oxidized for 288h at 1100°C
Deformation of Crofer During Cyclic Oxidation
0 100 200 300 400 500 600-0.5
0
0.5
1.5
1
time (h)
FeCrAlY
FeCrAl (low S)
FeCrAl (normal S)
Effect of Sulfur and Reactive Element
alumina scale
FeCrAlY (TMP)
Effect of Yttrium
Long Term Cyclic Oxidation Testing
Cyclic oxidation kinetics for several Ni-Cr-Al alloys exposed at 1100°C.
0 4 8 12 16 20 24time (h)
Ni-50CrNi-49Cr-0.01CeNi-49Cr-0.08Ce
Air, 1100°C
1
0
-1
-2
-3
-4
-5
-6
Reactive Element Effect for a Chromia Former
Cyclic Oxidation
Cyclic Oxidation Degradation and Breakaway• Alloys initially are Cr2O3-formers
982°C 30 day cycles
More adherent oxide
wt%
Ni-Cr-Al Cyclic Oxidation Degradation
Water Vapor Effects on High Temperature Oxidation
• The effects produced by water vapor on the high temperature oxidation of alloys depends on the alloy system.
• In the case of oxidation resistant alloys such as Al2O3- and Cr2O3-formers, water vapor affects the oxidation resistance adversely.
Effect of Water Vapor on Cracking and Spalling of α-Al2O3 Scales
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
0 100 200 300 400 500 600 700 800 900
number of 1-hr cycles
delta
M/A
(mg/
cm s
q)
• The presence of water vapor in the oxidizing gas mixture causes theα-Al2O3 scales to crack and spall more profusely than in dry gases
LowS N5 dry
LowS N5 0.5atmN5 dry
N5 0.1atm
N5 0.5atm
LowS N5 0.1atm
increased degradation
1100°C
René N5 oxidized for 802 cycles @ 1100°C in air with 0.5atm water vapor
original α-Al2O3 has not spalled
Ni(Cr,Al)2O4
Al2O3 Ta oxide
transient oxides
γ’
NiONi(Cr,Al)2O4 Ta-rich oxide
alumina Al rich mixed oxides
original α-Al2O3 has spalled
regular (3-5ppm) sulfur content
low (<1ppm) sulfur content
• Cracking and spalling becomes less pronounced for very adherent TGO scales; in other words, the cracking and spalling under water vapor conditions tests becomes less evident as the interfacial toughness between α-Al2O3 and substrate becomes greater.
Mechanism: Stress Corrosion Cracking
• Although cyclically oxidized low S superalloys do not spall as much as the regular S superalloys, water vapor has access to the oxide/substrate interface in both cases.• Low S superalloys have increased spallation resistance because of a higher Al2O3/substrate interfacial toughness.• A crack that forms propagates until it is large enough to result in failure. Water vapor has access to the oxide/substrate interface through cracks in the oxide and, combined with the stress, can cause spalling of the scale by interfacial stress corrosion cracking.
air + H2O
oxide
substrate crack propagation delamination
Cross-section of Ni-8Cr–6Al oxidized for 1 h at 1100ºC (a) in dry air, and (b) in air with water vapor (0.1 atm).
Water Vapor Adversely Affects the Selective Oxidation of Aluminum in Alloys
a
b
Cross-sections of Ni specimens exposed for 1hr@1100°C in dry air/air withwater vapor showing position of Pt markers in the scale (see arrows);
Pt mesh deposited in dots of 1mm diameterby sputtering
dry air air with water vapor (0.1atm)
Highlights of important results:
• the initial stages of oxidation are affected by water vapor- more extensive transient oxidation stage- enhanced internal oxidation- inhibition of selective oxidation of Al
• NiO grows more rapidly during the transient period in water vapor- enhanced inward transport of oxygen-containing species alongthe oxide grain boundaries promotes growth of NiO- smaller grain size oxide
Planar SOFC Configuration
Repeat Components
Electrolyte – YSZ Cathode – La/Sr-Manganite (LSM) Anode – Ni/YSZ Interconnect – Metallic
Fuel gas
Oxidant gas
Compliant Seal
Ambient
Current collector/ Bi-polar gas separator
SOFC Current Collector (Interconnect)Exposure Conditions
T = 600 – 800oC
RequirementsLow ASR
Surface StabilityAir ± H2O, Fuel + H2O, Dual Atmospheres
No Contamination of Other Components
Metallurgical Stability
Dual Atmosphere Effect – 304 Stainless Steel
Air – Air Exposure Air Side – Dual Atmosphere Exposure
ReducingOxidizing
Metal
Oxidizing Oxidizing
Uniform Cr2O3 Fe-rich Oxide Nodules
Silver Exposed Under Dual Atmosphere Conditions for 24 h at 800oC
Air Side
Ar-H2-H2O Side
O
H
Air/Air
Ni-200 - 600 Hours 800°C
400 Hours 800°C Ar-10%H20-4%H2/Air
Ar-4%H2
Air Side
Air Side
Silver Via, 800°C 100 Hours, exposed under dual atmospheric conditions. The upper surface was exposed to dry air while the lower surface was exposed to simulated anode gas of Ar-10%H2O-4%H2.
High Conductivity Pathway
Ag
Ni
Mixed Oxidant Corrosion
The phases formed from other oxidants generally grow faster than the respective oxides (with some notable exceptions).
Cr
Mo
Ni
Nb
Fe
Fe
Ni
Co
Co
Cr
Mn
Mn
OxidationSulfidation
6 8 10 12 14
10-6
10-4
10-8
10-10
10-12
10 /T (K )4 -1
Growth of Oxides and Sulfides
Fe O + FeSduplex
3 4
FeO + FeSduplex
Iron
Duplex Sulfide/Oxide Scale on Iron in Ar-1%SO2 at 900oC
20 μm
Reaction Path
Reaction Path
Sulfidation/Oxidation of an Alloy
Other Reactants in Gas Affecting Cyclic Oxidation of Alumina Formers
The second reactant causes alumina formation to stop at shorter times
Effects of Carbon on the High Temperature Corrosion of Fe-Base and Ni-Base Alloys
•Gases with low carbon activities can disrupt the selective oxidation process.•Gases with high carbon activities (aC>1) can result in coking and metal dusting. (T range for dusting 400-850oC)
Effect of CO2 on the Oxidation
of Fe-Cr Alloys
Cross-sections of Fe-15wt%Cr Oxidized at 900oCAlloy on the left was exposed for 4 h in CO2
Alloy on the right was exposed for 16.6 h in 0.2 atm O2
Isothermal Stability Diagram for the Cr-C-O System at 1250K
Carbon Activity in Gaseous Mixtures
)()()(2 2 gCOsCgCO +=
22
CO
COC
ppa
K =
2
2
CO
COC p
Kpa =
Consider a gas consisting initially of 99%CO and 1% CO2 at P = 1 atm entering a system at 1100K
K1100 = 0.0885 → ac = 8.7 i.e. Gas is Supersaturated
Metal Dusting of Alloy 800 Exposed for 48 h at 850oC in Ar-10%CH4