High-Temperature Oxidation Behavior of Fe-10Ni Alloy in Air/Ar-H2-H2o Dual Atmosphere
Authors: Yoneda Suzue , Abe Sota , Kawada Yasutaka , Imai Kiyoshi , Hayashi Shigenari 
Published: December 2024

The utilization of hydrogen is expected to achieve carbon neutrality. Solid Oxide Electrolysis Cell (SOEC) is gaining attention for hydrogen production. Each cell stack is connected by stainless tube, and SOEC is typically operated at high temperatures of 500 to 900°C. High-efficiency energy conversion can be achieved by operating at such high temperatures, however stainless tubes suffer severe degradation due to high temperature oxidation. The stainless tubes are exposed to a dual atmosphere of H2-H2O on the inside and air on the outside in SOEC system. A protective Cr2O3 scale is formed on the stainless tube in single air, but a thick Fe-rich oxide formation on the air side under dual atmosphere is reported. Hydrogen and water vapor on the H2-H2O side are considered to affect severe oxidation on the air side, but the details are not understood. It is difficult to evaluate the effect of hydrogen and water vapor by using Cr2O3-forming alloys because the oxide scale formed on the alloys is different. Fe-10Ni alloys is known to form similar oxide scale with or without water vapor. In this study, high-temperature oxidation behavior of Fe-10at.%Ni alloy in single air and air/Ar-H2-H2O dual atmosphere was investigated, and the effect of hydrogen and/or water vapor was evaluated.
Fe-10at.%Ni alloy was prepared by Ar-arc melting and the alloy ingot was hot rolled to approximately 1.3 mm thickness. Samples (approximately φ20 mm × 1 mm thick) were cut from homogenized alloy sheets and ground to a 4000-grit using SiC abrasive paper. The samples were then polished using 3-μm diamond paste, followed by ultrasonic cleaning in acetone. High-temperature oxidation tests were conducted using a horizontal furnace, as illustrated in Fig. 1. The sample was mechanically clamped using bolts and nuts, and gaskets used in SOFC were used to seal the sample tightly to the sample holder. During the oxidation tests, Ar gas was continuously flushing inside of the quartz tube to avoid degradation of the sample holder. Samples were heated to 800°C at a rate of 10°C/min in Ar, and flowing gas was immediately switched air or Ar-H2-H2O after heating to 800°C, followed by isothermal oxidation in single air and air/Ar-H2-H2O dual atmosphere for up to 16 h. The flow rate of air and Ar-H2-H2O was 25 ml/min. After the oxidation test, cross sections of oxide scale were observed by Optical Microscope and FE-SEM. The element distributions were analyzed by EPMA.
Figure 2 shows cross-sectional FE-SEM images of air side after oxidation for 16 h in (a) single air and (b) air/Ar-10H2-10H2O dual atmosphere. An outer Fe oxide and subscale were formed on both atmospheres. From EPMA analysis, the outer scale consists of Fe2O3 and Fe3O4 in single air, and the subscale was also identified as Fe3O4. In dual atmosphere, FeO was formed in the outer scale in addition to Fe2O3 and Fe3O4, and the oxide in subscale was also FeO. Ni content at the alloy surface in dual atmosphere was found to be about 50 at.%, indicating that partial pressure of oxygen at alloy surface on the air side decreased by diffusion of hydrogen from Ar-H2-H2O side. Thus, the formation of FeO was possible in dual atmosphere. The thickness of the outer scale and the subscale was found to be thicker in dual atmosphere, and this was especially pronounced for the subscale. In the subscale region, many voids were observed, and the number of voids decreased in dual atmosphere. Additionally, large gap was formed in the outer scale in dual atmosphere. These results indicate that the dissociation of FeO occurred in dual atmosphere. Figure 3 shows the proposed model for oxidation behavior of Fe-10Ni in air/Ar-10H2-10H2O dual atmosphere. At the interface Ⅰ, FeO is reduced by the reaction FeO + H2 → Fe + H2O, resulting in outward Fe supply and the formation of H2O. The released Fe diffuses toward the outer scale surface and form a new oxide. H2O might diffuse inwardly and react with Fe diffused from the alloy substrate to form a new FeO at the interface II and in the subscale, resulting in a thickening of FeO below the gap and a reduction of voids in the internal oxide layer. Higher growth rate of the outer scale and subscale in dual atmosphere might be caused by additional outward Fe supply and H2O generation due to the FeO dissociation.

Figure 1