National Energy Technology Laboratory

Office of Fossil Energy

Evaluation of a Surface Treatment on the Performance of Stainless Steels for SOFC

Interconnect Applications

D.E. Alman, G.R. Holcomb, T.A. Adler,R.W. Wilson, and P.D. Jablonski

NETL, Albany, ORwww.netl.doe.gov

International Conference on Metallurgical Coatings and Thin FilmsSan Deigo, CA April 23-27,2007

ICMCTF-2007, Apr. 27, 2007

ACKNOWLEDGEMENTS

• Christopher Johnson at NETL-Morgantown• W.K. Collins, D.Smith, R.Chinn, A. Hunt, P.

Danielson, M. Arnold, M Hayes, D. Davis and NETL-Albany

ICMCTF-2007, Apr. 27, 2007

OUTLINE

• WHY RARE EARTH SURFACE TREATMENTS• OXIDATION BEHAVIOR OF SURFACE

TREATED CROFER 22APU• WHY SURFACE TREATMENT WORKS IN

IMPROVING OXIDATION RESISTANCE• SOFC BUTTON CELL WITH TREATED

ANNODE CURRENT COLLECTORS• IMPROVING ELECTRICAL PERFORMANCE

− “Half-cell” ASR-measurements• SUMMARY AND CONCLUSION

ICMCTF-2007, Apr. 27, 2007

Low CostPhysical Compatibility

? SOFC Degradationi. Cr poisoning due to evaporation

from alloyii. Formation of non-conductive

internal oxides at alloy/oxide scale interface

iii. Excessive oxide scale growth

CHROMIA FORMING FERRITIC STEELS AS METALLIC INTERCONNECTS

Anode

Cathode

Interconnector

ICMCTF-2007, Apr. 27, 2007

REACTIVE ELEMENT EFFECT • Characteristics

Reduction in the oxidation rate• Change in scale growth mechanisms

• cation transport anion transport• Modification of scale microstructure

• Large columnar grains small grains

Stabilize Cr2O3 scales at lower Cr levels• Lower Cr levels

Improvement in scale adhesion• resistance to spallation

Alloy Fe Cr Mn Si Ti Al La

Crofer22APU Bal 22.0 0.5 -- 0.08 -- 0.06 La

ZMG232 Bal 22.0 minor: Mn, Ni, Zr, La

ICMCTF-2007, Apr. 27, 2007

IMPROVING OXIDATION RESISTANCE WITH RARE EARTHS

• Melt addition+Elements added during ingot production (single

manufacturing step)−Difficulty in melting (react with crucibles)−Surface concentration limited by solubility and

diffusivity• Surface treatments

+Rare Earth concentrated where needed (at surface and have most benefit)

−$“Extra” manufacturing step.? Long term effectiveness (as with any coating or

surface treatment)

ICMCTF-2007, Apr. 27, 2007

NETL DEVELOPED SURFACE TREATMENT

• Investigate rare earth surface treatment for improving oxidation resistance of ferritic steels for SOFC interconnect applications.

• Two different surface treatments investigated−Developed at NETL

• Similar to pack cementation: coated with a powder mixture containing CeO2 and halide activator followed by heating in a controlled atmosphere (900oC-12 hrs), after which residual “pack” coating is washed off the surface.

• Patent application filed with USPTO in September, 2005.• Applied to over 50 alloys.

−Described in a paper by P.Y. Hou and J. Stringer (H/S)• J. Electrochem Soc., Vol 134, No. 7, July 1987, pp. 1836-1849• Coupons heated to 200oC were coated with a cerium-nitrate slurry

(10w/o nitrate adjusted with HNO3 to pH=2), followed heating in air at 400oC to decompose to CeO2

ICMCTF-2007, Apr. 27, 2007

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

CROFER 22APU+Ce SURFACE TREATMENT

Crofer

Crofer +Ce (NETL)

Crofer +Ce (H/S)

~2-3x reductionin weight gain

ICMCTF-2007, Apr. 27, 2007

CROFER 22APU+Ce SURFACE TREATMENT800oC-4000hrs-Air+3%H2O

“smaller” internal oxidation zone

Crofer + Ce (NETL)

Crofer + Ce (H/S)

Crofer

ICMCTF-2007, Apr. 27, 2007

CROFER 22APU+Ce SURFACE TREATMENT

Early batch of Crofer w/ high Al and Si contents (610 ppm Al, 530 ppm Si by GDMS)

SiO2

ICMCTF-2007, Apr. 27, 2007

CROFER 22APU+Ce SURFACE TREATMENT

Early batch of Crofer w/ high Al and Si contents (610 ppm Al, 530 ppm Si by GDMS)

37Cr-24Mn

39Cr-0.5Mn

SiO2

43Cr-1Mn

23Cr-21Mn

ICMCTF-2007, Apr. 27, 2007

REPEATABLE RESULTS

800oC-Air+3%H2O

Time (hrs)

0 1000 2000 3000 4000 5000

Spec

ific

Mas

s C

hang

e (g

/cm

2 )

0.0000

0.0005

0.0010

0.0015

#1, Nov 04#2, Nov 04#3, Nov 04#4, Mar 05#5, Nov 05#6, Nov 05

#1, Nov 04#2, Nov 04#3, Nov 04 #4, Mar 05 #5, Nov 05 #6, Nov 05

Crofer 22APU+Ce (NETL method)

Crofer 22APU

Crofer 22APU

Crofer 22APU+Ce

Crofer 22APU – 1mm thick (Crofer 20)

ICMCTF-2007, Apr. 27, 2007

THERMODYNAMIC PREDICTION

CeO2 reacts with Crduring treatment to form CeCrO3

ICMCTF-2007, Apr. 27, 2007

AS-TREATED SURFACE: PRIOR TO TESTING

18Cr-5.4Mn-8.8Ce-61O

24Cr-11Mn-1.1Ti-2.2Ce-64O26Cr26Cr--13Mn13Mn--0.5Ti0.5Ti--0Ce0Ce--62O62O

72Fe-23Cr-0.3Mn-0.1Ti-4.3O

x 75Fex 75Fe--23Cr23Cr--0.1Mn0.1Mn--0.1Ti0.1Ti--1O1O

composition in at%determined through WDX analysis

ICMCTF-2007, Apr. 27, 2007

SURFACE PRIOR TO OXIDATION

Diffraction Angle (2 Theta)20 25 30 35 40 45 50 55 60 65 70 75 80

Rel

ativ

e In

tens

ity

0

20

40

60

80

100

120CROFER 22APUFe=(Fe-Cr) alloyC=CeO2T=CeCrO3T2=(Cr,Mn)3O4

Fe

Fe

subjected to thermal portion of treatment w/o Ce modification

Ce surface modified

C TC C C TTTT2

ICMCTF-2007, Apr. 27, 2007

THERMAL PORTION OF SUFACE TREATMENTSurface modification process includes a thermal treatment in a controlled

atmosphere. Does this influence oxidation behavior by “pre-oxidizing” surface?

800oC-Dry Air

Time (hrs)0 500 1000 1500 2000

Spe

cifo

c M

ass

Cha

nge

(g/c

m2 )

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014Crofer 22APU+Ce Crofer 22APU+TCCrofer 22APU

as received

subjected to thermal portion of treatment w/o Ce modification

Ce surface modified

Thermal portion of process does not improve oxidation resistance. Presence of Rare earth (Ce) improves oxidation resistance.

ICMCTF-2007, Apr. 27, 2007

OXIDATION RESITANCE

• Nucleation −CeO2 CeCrO3 during treatment nominally

continuous surface oxide? Finer Cr2O3 grain size.

• Ce in scale changes mechanism of subsequent Cr2O3 (or (Cr,Mn)xOy) scale growth−oxygen controlled mechanism slower scale

growth, less internal oxidation.• Future work: TEM examination of scales.

ICMCTF-2007, Apr. 27, 2007

LABORATORY SCALE SOFC

• SOFC button cell−Nextech Materials (Nextcell-2.5D).−2.5 cm diameter Zirconia-based electrolyte supported cell.−1.27 cm diameter, 50 μm thick LSM cathode−Gd-doped CeO2 layer at the electrolyte-cathode interface.−1.27 cm diameter, 50 μm thick Ni-GDC anode.

• Fe-22Cr-0.5Mn steel current collector−Crofer, NETL-Alloy F5

• Results of experiments in press:−D.E. Alman, Johnson, Collins, Jablonski, J. Power

Sources, available on line at www.sciencedirect.com

ICMCTF-2007, Apr. 27, 2007

STAINLESS STEEL CATHODE CURRENT COLLECTOR

• Alloy Composition (wt%)

Alloy Fe Cr Mn Ti Al Si

F5 73.3 22.0 0.44 0.007 0.03 0.02

Crofer22APU 75.9 22.7 0.45 0.095 0.11 0.27

F5: Produced in house at NETL-Albany (VIM, forging and rolling).Crofer 22APU: procured from ThyessenKrupp.1 mm thick

ICMCTF-2007, Apr. 27, 2007

CATHODE CURRENT COLLECTORMachined Current Collectors were Ce Surface Treated

ICMCTF-2007, Apr. 27, 2007

SOFC ASSEMBLY

• 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

ICMCTF-2007, Apr. 27, 2007

SOFC TEST APPARATUS

Pt mesh(anode side)

Ceramic Flange

Ceramic Flange

Fe-22Cr-0.5Mn(cathode side)

oxidant

fuel

ICMCTF-2007, Apr. 27, 2007

LABORATORY SCALE SOFC TESTING: OPERATING CONDITIONS

• Heated to 800oC (2hrs) N2 on anode side: air+3%H2O on cathode side

• 800oC (2hrs)10%H2/90%N2 mixture on anode side

• Cell OperationFuel: 97% H2/3%H2O at 400 cm3/minOxidant: air+3%H2O at 1000 cm3/minConstant voltage: 0.7VPeriodic voltage sweeps: 1.1V to 0.V

• Cathode Current Collector Surface ConditionUntreated condition: polished (1 μm diamond)Ce-treated condition: cleaned with scotch-brite® pad in water

ICMCTF-2007, Apr. 27, 2007

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

CELL PERFORMANCE0.7V/800oC; Fuel: H2+3%H2O; Oxidant: Air +3% H2O

LSM Cathode/Fe-22Cr-0.5Mn Interconnect

Voltage sweep

ICMCTF-2007, Apr. 27, 2007

CELL PERFORMANCE

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

F5+Ce

Crofer+Ce

0.7V/800oC; Fuel: H2+3%H2O; Oxidant: Air +3% H2OLSM Cathode/Fe-22Cr-0.5Mn Interconnect

initial oxide on surface

Cr poisoning

ICMCTF-2007, Apr. 27, 2007

PRE-OXIDIZED CROFER 22APU CURRENT COLLECTORS: PNNL

• S.P. Simner, Anderson, Xia, Yang, Pederson, Stevenson, J. Electrochemical Soc., vol 154 (4), pp. A740-A745, 2005

ICMCTF-2007, Apr. 27, 2007

ANALYSIS OF Cr IN CATHODE

Interconnect

ElectrolyteCathode

Crofer(78 h)

F5+Ce(117 h)

Crofer+Ce(38 h)

(1) 0.00 0.00 0.00

(2) 0.02 0.02 0.06

(3) 0.48 0.15 0.08

(4) 0.08 0.08 0.11

(5) 0.09 0.09 0.08

Cr composition by WDX(weight percent)

(1)Ce-rich (2) (3) La-rich

(4)(5)

Under Channel

ICMCTF-2007, Apr. 27, 2007

ANALYSIS OF Cr IN CATHODE

Interconnect

ElectrolyteCathode

Crofer(78 h)

F5+Ce(117 h)

Crofer+Ce(38 h)

(1) 0.00 0.00 0.00

(2) 0.12 0.09 0.00

(3) 0.56 0.24 0.08

(4) 0.09 0.14 0.09

(5) 0.13 0.17 0.10

Cr composition by WDX(weight percent)

(1)Ce-rich (2) (3) La-rich

(4)(5)

Adjacent Metal

ICMCTF-2007, Apr. 27, 2007

Haynes 230

800oC Air+3%H2O

Time (hrs)0 1000 2000 3000 4000

Spe

cific

Mas

s C

hang

e (m

g/cm

2 )

-0.20

-0.10

0.00

0.10

Haynes 230+Ce (#3)Haynes 230+Ce (#2)

Haynes 230 (#1)Haynes 230+Ce (#1)

Haynes 230 (#2)

ICMCTF-2007, Apr. 27, 2007

Cr-DepletionH230+Ce Ln 2

H230+Ce Ln 1

10 µm

20 µm

20 µm

H230

Chromium content determined by WDX analysis

Haynes 230: 800oC - Air+3%H2O - 4000h

Distance From Oxide Scale (μm)0 5 10 15 20 25

Chr

omiu

m C

onte

nt (w

t%)

10

12

14

16

18

20

22

24

Haynes 230 Haynes 230 +Ce (Ln 1)Haynes 230 +Ce (Ln 2)

ICMCTF-2007, Apr. 27, 2007

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

ICMCTF-2007, Apr. 27, 2007

ASR

Time at 800oC, (hrs)

0 200 400 600 800

ASR

, (mΩ

-cm

2 )

0

10

20

30

40

50

Low Initial ResistivityLow Initial Resistivity - PreoxidizedHigh Initial Resistivity

ICMCTF-2007, Apr. 27, 2007

SUMMARY• Ce-surface treatment Fe-22Cr-0.5Mn steels

Improves oxidation resistance thinner scales, less internal oxidationPre-oxidizes surfaceModifies scales w/ RE to get RE benefit upon subsequent oxidation.

• Ce-surface treated interconnects in SOFCAct in a similar manner as pre-oxidized steelsDelays Cr poisoningLonger duration testing required

• Working on improving process to improve initial contact resistance.