Oxidation Studies on Crofer 22 APU Alloy Under Simulated
SOFC Operating Conditions
Wenhua Huang, Srikanth Gopalan, Uday B. Pal and Soumendra N. Basu
ECS Trans. 2007, Volume 7, Issue 1, Pages 2379-2384.
doi: 10.1149/1.2729359
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ECS Transactions, 7 (1) 2379-2384 (2007)
10.1149/1.2729359, © The Electrochemical Society
Oxidation Studies on Crofer 22 APU Alloy
Under Simulated SOFC Operating Conditions
Wenhua Huanga, Srikanth Gopalana, Uday B. Pala, Soumendra N. Basua
a
Department of Manufacturing Engineering
Boston University, Boston, MA, 02215, USA
The oxidation of Crofer 22 APU was studied under simulated
SOFC interconnection operating conditions. Crofer 22 APU
samples were exposed to cathodic conditions, i.e. air on one side,
and anodic conditions, i.e. fuel (hydrogen) on the other side, with
an externally imposed current flow simulating the passage of
current. Preliminary results indicate that the composition of the
oxide scale formed on the fuel side of the alloy with the externally
imposed current flow was significantly different from that of the
oxide scale formed when there was no current flow. However, the
results indicate that the current flow has no significant effect on the
composition of the oxide scale on the air side.
Introduction
Solid oxide fuel cells (SOFCs) have gained significant interest due to their high
energy conversion efficiency, low pollution emission, and high fuel flexibility. Recent
SOFC development reduces the operation temperature to 650-850oC, which makes it
possible to consider oxidation resistant alloys as replacement materials for the traditional
ceramic interconnect materials used in high-temperature (~1000oC) SOFC stacks (1-8).
Such alloys contain Cr and/or Al as the alloying elements, and form a protective oxide
scale by preferential oxidation of Cr (Cr2O3) or Al (Al2O3). Al2O3-forming alloys are less
interesting for SOFC interconnect applications due to the low electrical conductivity of
the oxide scale. Studies have focused on the Cr2O3-forming alloys, especially the Cr-
containing ferritic stainless steels due to their electrically conducting oxide scale,
appropriate thermal expansion behavior, and low cost (7-8).
Most oxidation studies of candidate interconnect materials have been carried out in
either an air or a fuel environment (9-13). Most recently, the oxidation of alloys exposed
to a dual-atmosphere environment have also been studied since it simulates environment
in real fuel cells (14-16). Yang et al. (15-16) observed that the oxidation behavior of a
ferritic stainless steel under an air/hydrogen dual atmosphere exposure condition is
significantly different from the behavior observed when the steels are exposed to air only.
The scales formed in air on the alloys exposed to a dual atmosphere contained iron-rich
spinel or Fe2O3 nodules, which were not present in the alloys exposed to air on both sides.
Nakagawa et al. (17) observed a similar phenomenon upon exposure of a ferritic stainless
steel to a dual steam air environment, in which an outer layer of Fe2O3 was formed.
More recently, Horita (18) studied the oxide scale formation and stability of Fe-Cr
alloy interconnects under dual atmospheres and current flow conditions. No significant
effects of dual atmosphere and current flow were reported on the formed oxide scale
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ECS Transactions, 7 (1) 2379-2384 (2007)
phases of both the anode and cathode on Fe-Cr alloys. In this paper, the oxidation of
Crofer 22 APU was studied under simulated SOFC interconnection operating conditions.
Crofer 22 APU samples were exposed to cathodic conditions, i.e. air on one side, and
anodic conditions, i.e. fuel (hydrogen) on the other side, with an externally imposed
current flow simulating the passage of current.
Experimental
Materials
Crofer 22 APU is an Fe-Cr based ferritic stainless steel. The chemical composition of
the alloy is shown in Table I. The dimensions of the sample used in this study were 50
mm in diameter and 0.5 mm in thickness. The sample was cleaned ultrasonically in
acetone for 5 minutes before evaluation.
Table I. Chemical composition of Crofer 22 APU.
Fe Cr Mn Si C Ti P S La
wt% Bal 22.8 0.45 n/a 0.005 0.08 0.016 0.002 0.06
Oxidation Experiment in Simulated SOFC Operation Conditions
The oxidation of Crofer 22 APU was conducted in simulated SOFC operation
conditions. The experimental setup for oxidation under dual gas atmospheres is illustrated
in Fig.1. The samples were positioned in the isothermal zone of the furnace. On the fuel
side, the alloy sample was sealed by a gold O-ring to insure gas tightness. A gas mixture
of 95% Ar/5% H2 was passed through a water vapor saturator at 0oC. The oxygen partial
pressure in the mixed gas from equilibrium calculations is around 8.6x10-21 atm. The
mixed gas was introduced to one side of the reaction tube, and dry air to the other side.
A r/H 2 In
Pt Wire (-)
Stainless Steel
Spring
Heating Element
Gold O-ring Seal
Samples
Pt - Mesh
Alumina Tube
Air In
Pt wire (+)
Air Out
Figure 1. Schematic illustration of experimental setup for the oxidation test of alloys.
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ECS Transactions, 7 (1) 2379-2384 (2007)
Pt-mesh was attached on the top of the alloy as cathode and anode current collectors with
the aid of platinum paste. A constant current density of 400 mA/cm2 was applied to the
alloy through two platinum wires connected to the platinum meshes. For comparison, one
sample was oxidized in the same dual gas atmosphere without applying any current flow.
The alloys were oxidized at 800oC for 100 hours. The reaction furnace was heated at a
rate of 12oC /min) to 800oC and held for 100 hours, and was cooled at a rate of 5oC /min
to room temperature.
Analysis of the Oxidized Alloys
The oxidized samples were first examined by X-ray diffraction (Bruker D8 X-Ray
Diffractometer). Then the samples were epoxy mounted, sectioned, polished and
examined by SEM (JEOL JXA-733).
Results and Discussion
Figure 2 shows X-ray diffraction (XRD) patterns of two samples of the oxide scale on
the fuel side of Crofer 22 APU. Both samples were tested in a dual gas atmosphere and
were exposed to the same thermal treatment, where one sample had a 400 mA/cm2
current applied, and the other sample had no current flow through it. XRD analysis of the
fuel side of the Crofer 22 APU that was exposed to dual gas atmospheres without current
flowing shows the formation of Cr2O3 and MnCr2O4. In contrast, the XRD pattern from
the fuel side of Crofer 22 APU that was exposed to dual gas atmospheres with 400
mA/cm2 current flow revealed that in addition to Cr2O3 and MnCr2O4, the oxide scale
also contained a substantial amount of Mn2O3 and FeCr2O3. Thermodynamic calculations
show that the equilibrium oxygen partial pressure for the reaction of 2Mn(s)+3/2 O2
(g)=Mn2O3 (s) is 8.65x10-23 atm, and the equilibrium oxygen partial pressure for the
reaction of Fe(s)+1/2 O2 (g)+Cr2O3=FeCr2O4 (s) is 5.9x10-23 atm. In this system, the
oxygen partial pressure of the anode gas is around 8.6 x10-21 atm, which indicates that the
Mn2O3 and FeCr2O4 are thermodynamically stable on the fuel side of the Crofer 22 APU.
Figure 2. XRD pattern of oxide scale formed on the fuel side of the Crofer 22 APU that
was exposed in dual gas atmospheres, one sample had an applied current density of 400
mA/cm2, the other had no current applied. The samples were oxidized at 800oC for 100 h.
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ECS Transactions, 7 (1) 2379-2384 (2007)
The intensity ratio of the strongest MnCr2O4 peak to the strongest peak of Cr2O3 on
the fuel side with current flow is smaller than that of the same sample without current
flow. This indicates that the current flow suppressed the growth of MnCr2O4 in the oxide
scale. Cox et al. (19) showed that Mn2+ diffuses faster than Cr3+ in a Cr2O3 lattice. This
faster diffusion of Mn2+ led to the rapid growth of coarse MnCr2O4 spinel crystals in a
fine, continuous chromia matrix. Wolter also showed that DMn>DFe>DCr in MnCr2O4 (20),
with the applied current, the diffusion of Mn2+ is accelerated to the top of MnCr2O4 spinel
to form Mn2O3 crystals and this will also limit the growth of MnCr2O4.
Figure 3 shows elemental distribution maps of Crofer 22 APU that was exposed in
dual gas atmospheres with an applied current density of 400 mA/cm2 after testing (fuel
side). The oxide scale is composed of some oxide layers on the alloys: Mn-rich and Cr-
rich layers are identified around the oxide scale/alloy interfaces.
Pt
Oxide
Alloy
Figure 3. Elemental distribution maps of Crofer 22 APU that was exposed in dual gas
atmospheres with an applied current density of 400 mA/cm2after testing (fuel side).
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ECS Transactions, 7 (1) 2379-2384 (2007)
Figure 4. XRD pattern of the oxide scale formed on the air side of the Crofer 22 APU that
was exposed in dual gas atmospheres where one sample had a current density of 400
mA/cm2 applied, the other had no applied current. The samples were oxidized at 800oC
for 100 hours.
Figure 4 shows the XRD pattern of the oxide scale formed on the air side of the
Crofer 22 APU that was exposed in dual gas atmospheres, one sample with a current
density of 400 mA/cm2 applied, the other one without any current applied. Both samples
were oxidized at 800oC for 100 hours. The XRD patterns indicate that the oxide scale
formed on the air side of dual exposure with and without current flow comprised only
chromia and spinel phases. However, the intensity ratio of the MnCr2O4 peak to the
chromia peak on the air side with current flowing through the sample is smaller than that
of the same sample without any current applied suggesting that the applied current
suppresses the formation of MnCr2O4.
Conclusions
It was observed that the oxide scale formed on the fuel side of Crofer 22 APU in a
dual gas atmosphere with current flowing is significantly different from the oxide scale
formed on the fuel side of Crofer 22 APU in a dual exposure without any current applied.
In addition to Cr2O3 and MnCr2O4, the oxide scale also contained a substantial amount of
Mn2O3 and FeCr2O4. No significant effect of current flow on oxide scale formation was
observed on the air side of the Crofer 22 APU sample exposed to a dual atmosphere.
Investigations of the specific mechanisms responsible for the observed scale growth are
in progress.
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