D C Dunand

J A Scott

CiteSeerX

Oxidation and Creep Behavior of Porous E-Brite for Solid-Oxide Fuel Cell 
Interconnects 
 
J. A. Scott and D. C. Dunand 
 
Department of Materials Science & Engineering, Northwestern University 
Evanston, Illinois 60208, USA 
 
 
The effects of isothermal oxidation on the creep response and 
porosity of E-Brite foams at 800ºC were investigated. Samples of 
43.2% and 51.4% porosity were first oxidized in static laboratory 
air up to 200 hours to determine oxidation kinetics. Select samples 
were removed after 10 and 100 hours of oxidation and then creep 
tested to characterize the impact of scale growth on mechanical 
response and changes in porosity. Significant creep strengthening 
was observed even when minimal oxide growth occurred. 
Although oxide growth led to reduced pore size and choked 
fenestrations, the reduction in open porosity after 200 hours of 
exposure remained less than 10%. 
 
 
Introduction 
 
Recent reductions in solid oxide fuel cell (SOFC) operating temperatures permit the 
use of metallic interconnects in place of ceramics but their application remains a 
challenge. Potential alloys are limited due to the harsh environmental conditions imposed 
by simultaneous exposure to an oxidizing and reducing atmosphere at elevated 
temperatures as high as 800ºC (1). Given the requirements of service, chromia-forming 
ferritic steels remain one of the most popular candidate families of alloys to examine for 
interconnect components due to their electrically conducting oxide, coefficient of thermal 
expansion match with the stack, and low cost. Several studies have investigated the 
oxidation behavior, thermal expansion, and electrical properties of dense ferritic steels in 
traditional SOFC operating conditions (1-3). Little work, however, has examined these 
properties in light of a new class of porous interconnects being developed for planar 
SOFCs.  
 
Porous interconnects are proposed to help reduce the weight and cost of the stack 
while simultaneously serving as flow channels for the fuel and oxidant (4-11). Most 
porous interconnect substrates are prepared using powder metallurgy techniques with 
porosities in range of 30% – 50% (9-14). The increased surface area of these porous 
structures, however, can lead to rapid oxidation (10).  Oxidation can significantly affect 
the performance of the interconnect and, to date, most work involving porous 
interconnects has focused on the resulting change in area specific resistance (ASR) (7, 
10). However, the thickness of oxide scale influences mechanical properties and leads to 
choking of pore fenestrations or connections, which can impede fluid flow. Accordingly, 
ferritic steel samples with porosities ca. 40% and 50% were prepared and tested to 
determine the effect of oxidation on mechanical response and the reduction in 
fenestration size at various growth intervals. 
ECS Transactions, 25 (2) 1361-1368 (2009)
10.1149/1.3205665 ©  The Electrochemical Society
1361 ) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-10-07 to IP 
Experimental Procedures 
 
E-Brite (Fe-26Cr-1Mo, wt.%) is a commercial ferritic steel alloy and was chosen as 
the subject of this study due to its coefficient of thermal expansion match with ceramic 
SOFC components, oxidation resistance, and low material cost (1). Samples were 
prepared by first combining elemental powders of Fe (APS 6-10 µm), Cr (APS < 10 µm), 
and Mo (APS 3-7 µm) acquired from Alfa Aesar (Ward Hill, MA) in the appropriate 
weight fraction. The resulting mixture was tumbled end over end at 30 RPM for one 
hour. NaCl was then crushed and sieved to a size of 53-106 µm. Subsequently, NaCl 
powder was added to the E-Brite powder mixture in the appropriate volume fraction and 
again mixed end over end at 30 RPM for one additional hour. The mixture was then 
uniaxially cold pressed at 350 MPa and vacuum sintered at 1250°C for 4 hours, 
whereupon the NaCl place holder evaporated and the metallic powders interdiffused and 
densified.  
 
Foams with total porosities within 3% of the target value were achieved, each with a 
closed porosity of less than 2%. Closed porosity of specimens prior to oxidation was 
determined by helium pycnometry based on a known bulk density of 7.69 g/cm3 for E-
Brite. To obtain open porosity, Archimedes density measurements were performed after 
coating porous samples with a thin layer of vacuum grease to prevent infiltration of water 
into the open porosity. When reporting porosity, the total (open plus closed) porosity is 
used unless otherwise specified.  
 
Prior to oxidation, cylindrical compression samples were machined for mechanical 
testing. Specimens were then placed on an alumina boat and isothermally oxidized in 
static laboratory air (uncontrolled humidity) in a muffle furnace at 800ºC for up to 250 
hours. Samples were periodically returned to ambient temperature to allow measurement 
of weight gain from oxidation. The weight increase was normalized by surface area, 
which was determined as follows. Foams with 40% and 50% porosity were 
simultaneously oxidized for ~5 hours with an E-Brite plate with an easily determinable 
surface area.  Weight gain of each sample was measured individually and foam surface 
area was estimated under the assumption that it was proportional to the increase in 
weight. This allowed for calculation of the parabolic rate constants from the measured 
oxidation curves. 
 
Cylindrical samples for mechanical testing were prepared by wire electro-discharge 
machining the sintered compacts to dimensions of 6 mm (D) × 12 mm (H). A total of 
three specimens were machined from each compact to determine creep response of 
samples that were non-oxidized, preoxidized 10 hours, and preoxidized 100 hours. Creep 
was measured in compression using an ATS Series 3210 compressive creep frame with a 
lever arm arrangement. Displacement was measured by a LVDT that directly monitored 
deflection of the alumina pushrod to an accuracy of 6 µm. A testing temperature of 
800°C was chosen to simulate typical SOFC operating conditions and was achieved by 
means of a three-zone, resistively-heated furnace with a temperature stability of ±2°C. A 
retort setup was assembled inside the furnace using a quartz tube to enable flowing argon 
and ensure negligible oxidation throughout the course of the test. Constant-load creep 
tests were performed until accumulated strains were a maximum of 0.1 to ensure 
consistency of structure among samples tested. Up to four increasing loads were applied 
ECS Transactions, 25 (2) 1361-1368 (2009)
1362 ) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-10-07 to IP 
on a single sample. Corresponding secondary creep rates were determined by the slopes 
of strain-time plots acquired for each stress.  
 
Specimens were cross-sectioned, mounted in an epoxy resin, and polished using 
standard metallographic procedures. Polishing was finished with a 0.05 µm Al2O3 slurry. 
Optical microscopy and SEM were used to examine the microstructures of foams and 
oxide scale growth.  
 
Results 
 
Oxidation Behavior 
 
Figure 1 displays the kinetics of mass gain at 800ºC in static laboratory air up to 200 
hours for four E-Brite foams: two at 43.2% porosity and two at 51.4% porosity. When 
normalized by mass as in Figure 1(a), the 51.4 % porous foams exhibit higher mass gain 
than the 43.2% porous foams as expected due to the difference in surface area. However, 
if normalized by surface area as in Figure 1(b), no difference is observed within 
experimental error, indicating the mechanism of oxidation for the two foams are the same 
in the static air environment.  
 
Based on Wagner’s oxidation theory, when scale growth is dominated by diffusion, it 
can be predicted by the parabolic dependence: 
 
    ( ) tKm p ×=∆ 2 ,            [1]  
 
where ∆m is mass gain per unit area, Kp is a parabolic rate constant, and t is time. It has 
been shown in previous work that this may not be a suitable prediction for long-term 
behavior due to the variance of Kp over time (15). Such deviation from linearity is evident 
in Figure 2, which plots ∆m as a function of t. Thus, a more appropriate method of 
comparison is to determine the instantaneous value of Kp, which was accomplished by 
computing the slope of every pair of neighboring data collected. The resulting range of 
Kp values for the samples tested are seen in Table I.  
 
 
Figure 1. Isothermal oxidation kinetics for 43.2% and 51.4% porous E-Brite foams at 
800ºC in static laboratory air, with mass gain normalized by (a) sample weight and (b) 
sample surface area. 
ECS Transactions, 25 (2) 1361-1368 (2009)
1363 ) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-10-07 to IP 
 
Figure 2. Isothermal oxidation kinetics for E-Brite foams at 800ºC in static laboratory air 
with the square of weight gain normalized by sample surface area. 
 
TABLE I. Comparison of mean parabolic constant values for 43.2 % and 51.4 % porous E-Brite and previously 
reported literature values.  
Material  Mean Kp (×10-13 g2 cm-4 s-1) 
43.2 % Porous E-Brite 0.29 - 0.94 
51.4 % Porous E-Brite 0.42 - 0.55 
Dense E-Brite (1) 0.353 
Dense E-Brite (16) 0.88 
 
Figure 3 reveals the pyramid-like morphology of the Cr2O3 scale consistent with 
previous work on E-Brite oxidation in air (1, 17). As the scale grows it begins to decrease 
pore size and choke fenestrations, thus reducing open porosity as reported in Table II. 
The decrease in open porosity is visible in the series of optical micrographs shown in 
Figure 4 for 0, 10, 100, and 200 hours of oxidation in static laboratory air at 800ºC.  
 
 
Figure 3. SEM of Cr2O3 scale morphology on E-Brite surface after 100 hours of exposure 
at 800ºC in static laboratory air.  
ECS Transactions, 25 (2) 1361-1368 (2009)
1364 ) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-10-07 to IP 
 
Figure 4. Optical micrographs of polished cross sections for 51.4% porous E-Brite foam 
after 0, 10, 100, and 200 hours of exposure at 800°C in laboratory air. Oxide (dark gray) 
is visible at the edge of the E-Brite matrix (white) after just 10 hours of exposure. Some 
areas of larger scale thickness are highlighted with arrows.  
 
Table II. Porosity values for samples containing 43.2 % and 51.4 % total porosity at various lengths of exposure to 
static laboratory air at 800 ºC. 
Exposure Time (Hours) Sample Open Porosity (%) Sample Open Porosity (%) 
0 50.6 (51.4 Total) 42.0 (43.2  Total) 
10 46.0 38.4  
100 44.0 36.0  
200 40.9 32.5 
 
Creep Properties 
A typical compressive strain-time plot is shown in Figure 5 for a 43.2 % porous E-
Brite foam preoxidized for 10 hours prior to creep testing. All creep curves exhibited 
similar behavior consisting of an initial stage of primary creep followed by a secondary 
stage where the average strain rate was constant.  Samples were removed prior to tertiary 
stage creep (i.e., before densification).  
All creep data obeyed typical power-law behavior: 
  ⎟
⎠
⎞
⎜
⎝
⎛ −=
RT
QA n expσε                                                   [2] 
where A is the Dorn constant, n the stress exponent, Q the activation energy, R the gas 
constant, and T is temperature. Figure 6 is a double logarithmic plot of minimum strain 
rate, ε , as a function of applied stress, σ, at a typical SOFC operating temperature of 
800 °C for 43.2 % and 51.4 % porous E-Brite preoxidized for 0, 10, and 100 hours. Best-
ECS Transactions, 25 (2) 1361-1368 (2009)
1365 ) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-10-07 to IP 
fit apparent stress exponents are all in the range of 2.8 – 4.6, expected with dislocation-
dominated, power-law creep.  
 
Figure 5. Typical compressive strain-time plot of a 43.2% porous E-Brite foam, 
preoxidized for 10 hours. Two regions of behavior are visible: primary creep followed by 
secondary, steady-state creep. 
 
 
Figure 6. Secondary steady-state compressive creep rate plotted as a function of applied 
stress for porous E-Brite tested at 800°C under flowing Ar. Values in parentheses 
indicate hours of preoxidation prior to creep testing. 
ECS Transactions, 25 (2) 1361-1368 (2009)
1366 ) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-10-07 to IP 
Discussion 
 
The range of values for parabolic oxidation rates of porous E-Brite correlates well 
with previously reported values for the bulk material. Among chromia-forming ferritic 
steel alloys, this rate is low compared to potential interconnect alloy alternatives such as 
430 or 446 (1). Cracking and spallation nonetheless remain an issue with chromia 
forming alloys, particularly during prolonged exposure and when subject to thermal 
cycling (18). Consequently, coatings containing reactive elements are recommended to 
control growth, improve adherence, and maintain consistent behavior over the lifetime of 
the fuel cell (16,19).  
 
Though scale growth continues well beyond 200 hours of exposure, further decreases 
in porosity induced by pore choking diminish beyond this time, consistent with the mass 
gain trend in Figure 1. This is evidenced by Table II and implies that a larger initial 
porosity can be used to maintain a desired threshold for percolation of the foam. Due to 
the oxide morphology, seen in Figure 3, it is not expected that significant additional 
closed porosity is likely. The majority of reduced porosity appears to stem from the 
reduction in pore size. Given the composite nature of the material, it was not possible to 
analyze the reduction in closed porosity using pycnometry. Usable open porosity was 
nonetheless determined and indicates that after 200 hours of exposure, a ~10% change in 
absolute porosity occurs. 
 
Significant creep strengthening is observed in the case of oxidized samples. With just 
10 hours of oxidation, strain rats are ca. 100 times lower. This behavior is in accordance 
with the anticipated reduction in porosity and growth of the oxide skeleton that serves to 
strengthen the E-Brite foams.  
 
It is important to note that the results of this study are most directly applicable to 
behavior on the anode side of the SOFC where the E-Brite is exposed to an oxidizing 
environment. A better understanding of the mechanical response of porous E-Brite and 
composite effects due to oxidation in a reducing environment is needed to fully 
characterize behavior in the SOFC stack. Previous work identifies anomalous behavior in 
dual-exposure conditions (1,17), which should also apply to porous materials, though 
additional work is needed to further examine this effect.   
 
 
Conclusions 
 
The oxidation kinetics at 800ºC of E-Brite foams with porosities of 43.2% and 51.4% 
are similar to those of the bulk material. Although the increased surface area of higher 
porosities leads to larger mass gains, the correspondence of parabolic rate constants with 
previously reported values indicates the difference in porosity does not affect oxide scale 
growth. Upon oxidation at 800ºC, however, mechanical properties of E-Brite foams are 
significantly influenced by the presence of oxide scale. After just 10 hours of exposure in 
static laboratory air, a difference in strain rate of nearly 100 times is observed. Though 
this creep strengthening effect diminishes with increasing exposure, its rapid occurrence 
alters the mechanical properties of the interconnect and should be acknowledged in the 
initial design of the stack. Preoxidation is one method that may help ensure more 
consistent performance of the fuel cell stack throughout its lifetime. Furthermore, if the 
oxide scale is dense enough and is able to resist spallation, the continuous Cr2O3 network 
may lead to changes in the coefficient of thermal expansion of the interconnect. This 
ECS Transactions, 25 (2) 1361-1368 (2009)
1367 ) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-10-07 to IP 
could be addressed with a method such as preoxidation. As oxide scale grows in E-Brite 
foams, pore fenestrations are choked, resulting in reduced porosity. This can be 
problematic in the case where foams are intended for use as fluid flow channels. Results 
indicate that larger initial porosities can be used to ensure desired percolation in the 
interconnect after oxide growth.  
 
Acknowledgments 
 
The authors acknowledge Dr. R Bhat (GE Global Research Center) for useful 
discussions. This research was funded by NASA through a subcontract from GE (Award 
NNC06CB31C). JAS also thanks Bell Labs, the National Science Foundation, and the 
Nanoscale Science and Engineering Center at Northwestern University for graduate 
research fellowships. 
 
References 
 
1. Z. Yang, K. S. Weil, D. M. Paxton and J. W. Stevenson, J. Electrochem. Soc., 150, 
A1188 (2003). 
2. J. W. Fergus, Mater. Sci. Eng., A, 397, 271 (2005). 
3. W. J. Quadakkers, J. Piron-Abellan, V. Shemet and L. Singheiser, Mater. High 
Temp., 20, 115 (2003). 
4. M. Lang, T. Franco, G. Schiller, N. Wagner, J. Appl. Electrochem., 32, 871 (2002). 
5. S. Takenoiri, N. Kadokawa and K. Koseki, J. Therm. Spray Technol., 9, 360 (2000). 
6. N. Brandon, D. Corcoran, D. Cummins, A. Duckett, K. El-Khoury, D. Haigh, R. 
Leah, G. Lewis, N. Maynard, T. McColm, R. Trezona, A. Selcuk and M. Schmidt, J. 
Mater. Eng. Perform., 13, 253 (2004). 
7. S. Molin, B. Kusz, M. Gazda and P. Jasinski, J. Power Sources, 181, 31 (2008). 
8. J. D. Carter, T. A. Cruse, J.-M. Bae, J. M. Ralph, J. M. Deborah, R. Kumar and M. 
Krumpelt, Mater. Res. Soc. Symp. Proc., 756 (2003). 
9. I. Villarreal, C. Jacobson, A. Leming, Y. Matus, S. Visco and L. De Jonghe, 
Electrochem. Solid-State Lett., 6, A178 (2003). 
10. I. Antepara, I. Villarreal, L. M. Rodríguez-Martínez, N. Lecanda, U. Castro and A. 
Laresgoiti, J. Power Sources, 151, 103 (2005). 
11. R. Vaßen, D. Hathiramani, J. Mertens, V. A. C. Haanappel and I. C. Vinke, Surf. 
Coat. Technol., 202, 499 (2007). 
12. Y. B. Matus, L. C. De Jonghe, C. P. Jacobson and S. J. Visco, Solid State Ionics, 176, 
443 (2005). 
13. P. J. Panteix, V. Baco-Carles, P. Tailhades, M. Rieu, P. Lenormand, F. Ansart and M. 
L. Fontaine, Solid State Sci., 11, 444 (2009). 
14. E. Arahuetes, A. Bautista, F. Velasco, M. E. Sotomayor, Rev. Metal., 44, 406 (2008). 
15. Y. Liu, J. Power Sources, 179, 286 (2008). 
16. K. Huang, P. Y. Hou and J. B. Goodenough, Mater. Res. Bull., 36, 81 (2001). 
17. Z. Yang, M. S. Walker, P. Singh, J. W. Stevenson and T. Norby, J. Electrochem. 
Soc., 151, B669 (2004). 
18. W. N. Liu, X. Sun, E. Stephens, M. A. Khaleel, J. Power Sources, 189, 1044 (2009). 
19. Z. Yang, G.-G. Xia, X.-H. Li and J. W. Stevenson, Int. J. Hydrogen Energy, 32, 3648 
(2007). 
ECS Transactions, 25 (2) 1361-1368 (2009)
1368 ) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-10-07 to IP 


Oxidation and Creep Behavior of Porous E-Brite for Solid-Oxide Fuel Cell Interconnects

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=CF7508A4BC5F67DD47FD15F6CF7CE8D9?doi=10.1.1.1035.2573&rep=rep1&type=pdf

Oxidation and Creep Behavior of Porous E-Brite for Solid-Oxide Fuel Cell Interconnects

Abstract

Similar works

Full text

Available Versions

CiteSeerX