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