surface modifications for oxidation resistance · max temp 900oc cen -based max temp 400oc...

Surface Modifications for Oxidation Resistance

National Energy Technology Laboratory

8th Annual SECA Workshop – San Antonio, TX, August 7-9, 2007

David E. AlmanPaul D. Jablonski

Office of Research & Development


Accomplishments

• Surface treatment developed at NETL based on CeO2

− Applied to over 50 commercial and experimental alloys including; T430, T441, Crofer22APU.

− For comparative purposes applied other RE surface treatment that are described in the literature.

− RE Treatments are effective in enhancing oxidation resistance.• Initiated long term testing to determine effectiveness.

− ASR measurements and single-cell test indicate surface treatment can enhance SOFC performance.

− Modified NETL treatment to use La2O3

• Investigated influence of Si levels on behavior of interconnect alloys− Oxidation as a function of Si level in T430 (objective is to determine

critical Si-level).


Reactive Element (RE) Effect• Well known that the addition of small amounts of

RE (Ce, La, Y, etc) improves oxidation resistance• Characteristics

Reduction in the oxidation rate• Change in scale growth mechanisms

• cation transport anion transport• Modification of scale microstructure

• large columnar grains small equiaxial grains

Stabilize Cr2O3 scales at lower Cr levelsImprovement in scale adhesion

minor: Mn, Ni, Zr, La22.0BalZMG232

0.06 La--0.08--0.522.0BalCrofer22APU

LaAlTiSiMnCrFeAlloy


RE Surface Additions

• Surface treatments+ Rare Earth concentrated where needed (at surface)+ Applied to any alloy − ($) “Extra” manufacturing step.? Long term effectiveness (as with any coating or surface

treatment)

La-rich particles

-- Most commercial Most commercial ferriticferriticstainless steels do not stainless steels do not

contain RE ($)contain RE ($)• Melt addition+ Elements added during ingot

production (single manufacturing step)− Difficulty in melting (react with

crucibles)− Surface concentration limited by

solubility and diffusivity


Effect of RE on Oxidation

800oC-Air+3%H2O

Time (hrs)

0 500 1000 1500 2000

Spe

cific

Mas

s C

hang

e (g

/cm

2 )

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014

0.0016

F1 F2=F1+0.05 mischmetalF3=F1+0.1 mischmetalCrofer 22APU (0.1 La)

w/ NETL-Ce surface treatment

~50-60%reduction in massgain w/ surfacetreatment

F1=Fe-22Cr-0.5Mn-0.1Ti Mischmetal is a combination of Ce, La and other RE


Cerium Surface Treatments

• Developed a combination of pack cementation and superficially applied coating technique (NETL).

Coat surface with a slurry mixture: CeO2 and halide (NaCl) activator. Heat (900oC) in a controlled atmosphere (x10-3Torr)Residual “pack” coating is washed off the surface.

• Applied treatment described by Hou & Stringer (H/S).J. Electrochem. Soc., Vol 134, No. 7, July 1987, pp. 1836-1849.Coupons heated to 200oC were coated with a cerium-nitrateslurry (10w/o nitrate adjusted with HNO3 to pH=2), followed by heating in air at 400oC to decompose to CeO2.Surface also cleaned in water after treatment.


Surface After Treating (CeO2-NETL)

• The surface treatment pre-oxides the surface. Ce-rich oxide forms at the gas-substrate surface. A Cr-Mnoxide forms underneath the Ce-rich oxide.

Crofer+Ce (NETL)


Thermodynamics

CeO2 reacts with Crduring treatment to form CeCrO3

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.10.20.30.40.50.60.70.80.9

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CeO2

Mn Femole fractions /(CeO2+Fe+Mn)

Cr(s) + Fe(s) + Ce2O

3(s) + Mn(s2) + CeCrO

3(s)

MnO(s) + Fe(s) + Ce2O

3(s) + Mn(s2) + CeCrO

3(s)

MnO(s) + Fe(s) + Ce2O

3(s) + Ce

18O

31(s) + CeCrO

3(s)

Ce18

O31

(s) + Fe(s) + Ce6O

11(s) + Monoxide1 + CeCrO

3(s)

CeO2 - Fe - Mn - Cr800oC, mole Cr/(CeO2+Fe+Mn) = 0.2

5/4/2007N:\myfiles\Months\2007\April 2007\David CeO2 Coating\May 4\CeO2-Fe-Mn-20Cr.emf

Fe(s), CeCrO3, Ce6O11, Ce18O31, MnO


Surfaces After Treating CeN-based (H/S: 400oC)

CeO2-based (NETL:900oC)Ce-oxide

Ce-oxide

Fe-Cr


Surfaces After Treating (Prior to Oxidation)Crofer+Ce (NETL) Crofer+Ce (H/S)

CeO2-BasedMax Temp 900oC

CeN -BasedMax Temp 400oC

68Fe-20Cr-0Mn-1.2 Ce-8.7O*

29Fe-10Cr-0Mn-16Ce-43O*

76Fe-23Cr-1Mn

*not accurate due to edge effect (rounding of edge during sample preparation), however,

indicates Ce is present at the surface.

Oxide Layer


800oC-Air+3%H2O

Time (hrs)0 1000 2000 3000 4000

Spec

ific

Mas

s C

hang

e (m

g/cm

2 )

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

Influence of Surface Treatment on Oxidation

Crofer

Crofer +Ce (NETL)

Crofer +Ce (H/S)

~2-3x reductionin weight gain


Total Oxidation NETL Ce-Surface Treatment

Crofer 22APU:800oC-Air+3%H2O

Time (hours)

0 500 1000 1500 2000 2500

Spe

cific

Mas

s C

hang

e (m

g/cm

2 )

0.00

0.20

0.40

0.60

0.80

1.00

Crofer 22APU Crofer 22APU+Ce Crofer 22APU+Ce total weight gain (treatment+oxidation)



Diffraction Angle (2 Theta)15 20 25 30 35 40 45 50 55 60 65 70

Rel

ativ

e In

tens

ity

0

500

1000

1500

2000

2500

S1E R

C

S1

S2E

S1

ES1

S2

E

S1Fe

R

CE

S1

S1

E

R S2

E C

S1

E

E

Crofer

Crofer+Ce (NETL)

Crofer + Ce (H/S)

S1=(Mn,Cr,Fe)3O4

E=Cr2O3

R=TiO2

S2=(Mn,Cr,Fe)2O3

Fe=Fe-Cr alloyC=CeO2

4000 hrs-800oC-Air+3%H2O


Oxide Scale Formation

Diffraction Angle (2Theta)25 30 35 40 45 50

Rel

ativ

e In

tens

ity

0

10

20

30

40

50

60Crofer+Ce (NETL) 4000 h:800oC-Air+3%H2O

FeCS1

S1

ES1

MO

E

MOE

C

S1

ER

S1=(Mn,Cr,Fe)3O4

E=Cr2O3

MO=(Mn,Cr,Fe)2O3

O1=CrCeO3

R=TiO2

C=CeO2

Fe=Fe-Cr alloy

Crofer+Ce (NETL) – 0 hrsO1 O1



Detailed scale microstructures can be found in D.E. Alman and P.D. Jablonski, “Effect of Minor Elements and a Cerium Surface Treatment on the Oxidation Behavior of an Fe-22Cr-0.5Mn (Crofer 22APU) Ferritic Stainless Steel, International

Journal of Hydrogen Energy, accepted for publication (2006), currently available on line at www.sciencedirect.com.

thinner oxide scales with surface treatments800oC-2000h-Air+3%H2O

Crofer+CeCrofer+Ce (NETL)(NETL) Crofer+CeCrofer+Ce (H/S)(H/S)CroferCrofer

2µm

36Cr-0.5Mn (Cr2O3)

20Cr-20Mn

26Cr-13Mn

SiO2

Ce-rich oxide

17Cr-16Mn

25Cr-12Mn

36Cr-0.5Mn

Al-rich oxide

Ce-rich oxide

38Cr-1Mn

25Cr-13Mn

Crofer+CeCrofer+Ce (H/S)(H/S)


Influence of Surface Treatment on Oxidation800oC-4000hrs-Air+3%H2O

Minimize internal oxidation

Crofer + Ce(NETL)

Crofer + Ce (H/S)Crofer


Surface Treatments• Slows scale growth

− Scale microstructures were similar with NETL and H/S treatment methods.

• Minimizes internal oxidation.− Indicates slower oxygen diffusion through the scale.

• Ce at surface modifies initial stages of transient oxidation alters the subsequent growth of the scale enhanced

oxidation resistance.− formation of CeCrO3 – type oxide during transient oxidation.− Pre-oxidation during NETL treatment

• (initial oxidation of H/S?)• Why?

− Scale microstructure is changed• (high diffusivity columnar to low diffusivity equiaxed)

− Ce in oxide changes diffusion through oxide.− NETL-ORD IAES project at CMU to investigate influence of

RE on transient oxidation.


coating

J. Rakowski, 2006

2 to 3 timesreductionwith Ce-surface treatment

Ce-surface treated at NETL Tested at Allegheny

T430=Fe-17Cr-0.5Mn


Long Term Exposure

800oC-Air+3%H2O

Time (hrs)0 1000 2000 3000 4000 5000 6000Sp

ecifi

c M

ass

Cha

nge

(mg/

cm2 )

0.00

0.50

1.00

1.50

2.00

2.50Crofer 430-A 430-B 430-C 430-D

0.13

<0.01

<0.01

<0.01

<0.01

Al

0.120.0920.4522.42BalCrofer

<0.010.0800.5217.11Bal430-D

<0.01<0.010.4917.13Bal430-C

<0.01<0.010.4717.03Bal430-B

<0.01<0.010.4416.85Bal430-A

SiTiMnCrFe

bare alloys

430 alloys produced at NETLOxidation test in progress


Long Term Exposure

800oC-Air+3%H2O

Time (hrs)0 1000 2000 3000 4000 5000 6000Sp

ecifi

c M

ass

Cha

nge

(mg/

cm2 )

0.00

0.50

1.00

1.50

2.00

2.50Crofer 430-A 430-B 430-C 430-D

0.13

<0.01

<0.01

<0.01

<0.01

Al

0.120.0920.4522.42BalCrofer

<0.010.0800.5217.11Bal430-D

<0.01<0.010.4917.13Bal430-C

<0.01<0.010.4717.03Bal430-B

<0.01<0.010.4416.85Bal430-A

SiTiMnCrFe

bare alloys

Ce-treated (NETL)

430 alloys produced at NETLOxidation test in progress


Influnence of Si Content: T430

Testing is in progress


Time at 800oC, hrs

0 100 200 300 400 500

AS

R (m

Ohm

s-cm

-2)

5

6

7

8

9

Electrical Performance

Crofer 22APU

Lower ASR for SOFC interconnectCrofer

Measurements made by G. Xia & Z.G.Yang, PNNL

Crofer 22APU+Ce

Samples pre-oxidized at 800oC for100 hours prior to testing



0

5

10

15

20

25

0 200 400 600 800 1000

Time, h

ASR

, mO

hm.c

m2 Ce-treated T-441 (by NETL)

Bare T-441

ASR Measurements performed by PNNL (Z.G. Yang)800oC, air; LSM cathode//LSM contact//interconnect


Laboratory Scale Testing• “Button” cell test frames in Morgantown• Fe-22Cr-0.5Mn steel current collector was

attached to the cathode with Pt paste (a Pt mesh placed between interconnect and cathode).

• Pt mesh attached to anode.• Ag current cables and voltage taps spot

welded to current collectors

electrolyte

cathode

anode

interconnect

Pt meshPt pasteAg current cables

Ag voltage taps


Time (hrs)0 20 40 60 80 100 120

Pow

er D

ensi

ty (W

/cm

2 )

0.10

0.15

0.20

0.25

0.30

Crofer (as polished surface)

F5+Ce

Laboratory Scale Cell Performance0.7V/800oC; Fuel: H2+3%H2O; Oxidant: Air +3% H2O

LSM Cathode/Fe-22Cr-0.5Mn Interconnect

initial oxide on surface

Cr poisoning

Results published: D.E. Alman, C.D. Johnson, W.K. Collins, and P.D.Jablonski, “The Effect of Cerium Surface Treated Ferritic Stainless Steel Current Collectors on the Performance of Solid Oxide Fuel Cells

(SOFC),” Journal of Power Sources, Vol 168, 2007, pp. 351-355.


Pre-Oxidized Current Collectors• S.P. Simner, Anderson, Xia, Yang, Pederson, Stevenson,

J. Electrochemical Soc., vol 154 (4), pp. A740-A745, 2005

Behavior of cell with Ce-treated interconnect (previous slide) similar to behavior of cells with pre-oxidized interconnects.


NETL Treatment with La2O3

composition is at%

CeN-based (H/S: 400oC)

CeO2-based (NETL:900oC)

La2O3-based (NETL:900oC)

Ce-oxide

Ce-oxide

Fe-Cr

La-oxide


800C-Moist Air

Time (hrs)0 200 400 600 800 1000 1200 1400

Spec

ific

Mas

s C

hang

e (m

g/cm

2 )

0.00

0.20

0.40

0.60

0.80

1.00

Crofer Crofer +Ce (NETL)Crofer+La (NETL)

Effect of Rare Earth


Initial Conductivity: ASR Experimental Setup

Furnace at 800°C

CurrentSource

200 mA/cm2

V

Alloy

Alloy

LSM Compact

LSM Paste

LSM Paste

Platinum Leads

Sample under load



Time at 800oC, (hrs)

0 200 400 600 800

ASR

, (mΩ

-cm

2 )

0

10

20

30

40

50

Crofer+La

Crofer + Ce

Crofer+Ce

Crofer+La


Summary

• RE surface treatments effective in improving oxidation resistance.−Applied to ferritic stainless steels for interconnect

application (Crofer and Type 430).−Both Ce-based methods (NETL and H/S) were

effective.−Thinner oxide scales with treatment.−Thinner internal oxidation zone.−La modification to the NETL method was effective. −ASR measurements indicate that slower scale

growth will enhance SOFC performance.


On-Going and Future Work

• Continue long term exposures on Ce- and La-treated samples− to determine if and when breaks-down occurs−accelerated tests −Longer term ASR (or cell tests)

• Continue investigation influence of Si content−Determine critical Si level (oxidation and ASR)−Low cost production of low Si steels using recycled

scrap (develop innovative slag additives to getter Si during metling).


Experimental Alloys and RE Surface Treated Materials Available For Evaluation by SECA

Participants

Contact: Paul D. Jablonski: 541.967.5982 orDavid E. Alman: 541.967.5885


Thermal Treatment (TC) Only is Ineffective

surface modifications for oxidation resistance · max temp 900oc cen -based max temp 400oc...

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