Oxidation behavior of Fe–16Cr alloy interconnect for SOFC under hydrogen potential gradient
Introduction
Solid oxide fuel cells (SOFC) has been extensively studied as an efficient power generation system. Since unit cell voltage is approximately 1 V, unit cells are connected electrically in series by interconnects in SOFC stacks to generate the desired power output. The property of the interconnect has to meet very severe demands as follows. Oxygen partial pressure of air side (cathodic condition) is about 2.1×104 Pa and that of fuel side (anodic condition) is 1.2×10−22–2.6×10−13 Pa when fuel gas is H2–H2O gas mixture in the temperature range of 873–1273 K. Under the reducing and oxidative conditions, the interconnect has to posses the following properties: chemical stability both in the air side and the fuel side at operation temperatures of 873–1273 K, high electronic conductivity, gas tightness, good machinability, and thermal expansion coefficient close to other ceramics components including yttria-stabilized zirconia electrolyte (≃10×10−6 K−1).
There are two major candidates for the interconnect of SOFC. The one is LaCrO3-based ceramic, and the other is Fe–Cr ferritic heat resisting alloy. The LaCrO3-based ceramic interconnect, as a matter of course, is stable at high temperature up to 1273 K, and its thermal expansion coefficient is similar to other ceramics components. Therefore LaCrO3-based ceramics have been studied extensively as the interconnect materials [1], [2], [3], [4].
Nowadays, alloy interconnect attracts a great deal of attention for commercial use of SOFC because of gas-tightness, machinability, and other advantages of alloys than ceramics [5] Alloy interconnect can be used below 1073 K where Fe–Cr alloys show good oxidation resistance [6]. Oxidation behavior of Fe–Cr alloys in reducing and oxidative conditions of SOFC operating atmospheres has been studied by several authors [7], [8], [9], [10].
In the previous work of our group, the oxidation kinetics of Fe–16Cr alloy (SUS430) has been studied under the conditions simulating the air side and the fuel side environments in SOFC, Ar–H2–H2O fuel gas mixtures with the values of PH2/PH2O=94/6 and 97/3, in the temperature range of 1023–1173 K [11]. The study revealed that oxide scale formed on the alloy consists of chromia and Mn–Cr double oxide in both environments, and the scale growth rates in the air side and the fuel side are almost the same at 1073 K.
Nakagawa et al. [12] have studied oxidation behavior of Fe–Cr ferritic steals (2.14–12.12 wt.%Cr) for a heat exchanger in steam power generation. The oxidation condition is the steam/steel/air environment. They measured the amount of hydrogen permeated through the samples from the steam side to the air side. The growth rates of the iron oxides at the air side were increased much higher than those in air because hydrogen permeated from the steam side affects the oxidation behavior. The interconnect of SOFC is exposed simultaneously both to air and fuel atmospheres, in which hydrogen potential gradient develops as shown in Fig. 1. We have confirmed the permeation of hydrogen through chromia forming alloy from the fuel side to the air side in the atmosphere simulating SOFC [13]. The permeating hydrogen may affect the oxidation behavior of the air side. It is important to clarify the effect of hydrogen on oxidation behavior of Fe–Cr alloy interconnect.
In the previous study [11], the samples were exposed to air or fuel conditions separately as shown in Fig. 2(A). In this study, the oxidation kinetics of SUS430 has been studied in an air/alloy/fuel atmosphere as shown in Fig. 2(B) where the samples were exposed to both air and fuel gas (Ar–H2–H2O gas mixture). The objective of this study is to make clear the oxidation behavior of Fe–16Cr alloy (SUS430) in the air/alloy/fuel atmosphere under the hydrogen potential gradient.
Section snippets
Experimental
SUS430 stainless steel was used in this study, and the chemical composition of the alloy is shown in Table 1. The samples were cut to the rectangular plates, 15×15×2 mm3, and were ground with SiC abrasive paper of #320–#2000 and were finally polished with 3–4 μm diamond paste. After polishing, the surfaces of the samples were cleaned ultrasonically with acetone and distilled water.
Experimental setup for oxidation under the air/alloy/fuel atmosphere is illustrated in Fig. 3. The apparatus is
Scale morphology and composition
Fig. 4 shows X-ray diffraction patterns of both sides of the sample oxidized at 1073 K for 86.4 ks. The scales on both sides of SUS430 are composed of Cr2O3, MnCr2O4 spinel and a small amount of FeCr2O4 spinel. Cristobalite was also identified in these X-ray diffraction patterns of both sides which peaks came from glass seal remaining on the sample surface.
Secondary electron micrographs and X-ray maps of both sides of SUS430 alloy after oxidation at 1073 K for 86.4 ks are shown respectively in
Microstructure of the scale
According to X-ray diffraction and microstructure analysis, the alloy was covered with dense oxide scale composed of Cr2O3, MnCr2O4 and a small amount of FeCr2O4. At the air side, MnCr2O4 crystals were observed on top of the scale layer which was mainly composed of Cr2O3. These crystals were precipitated on the surface of the scale corresponding to the alloy grain boundaries. At the fuel side, Cr2O3 whisker was observed on the top of the scale and MnCr2O4 was also precipitated on the surface of
Conclusions
The dense Cr2O3 scale including MnCr2O4 and a small amount of FeCr2O4 was observed on the surface of SUS430 after oxidation both in the air and fuel atmospheres of SOFC at 1073 K. The parabolic rate constant kp of the fuel side of the alloy is similar to that of the air side at 1073 K. The oxygen partial pressure dependence of the parabolic rate constant revealed that the growth rate of Cr2O3 scale is almost the same in various oxygen partial pressures (PO2≃4.1×10−17–2.1×104 Pa) at 1073 K.
Acknowledgements
The authors thank Dr. Harumi Yokokawa, National Institute of Advanced Industrial Science and Technology, for valuable discussions of thermodynamics in spinel.
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