Investigation of Surface Oxide Layer Structure to Improve Durability of Stainless Steels Under Humidified Hydrogen

Abstract

We study degradation mechanism of stainless steels under SOFC operating conditions to improve durability of SOFC stack. To reduce material cost, a low-cost stainless steel is preferred because metal interconnect occupies considerable amount in the stack. From commercially available stainless steels, four types of alloy composition were chosen as model materials, which are widely used in the other applications. The alloy test pieces were exposed to hydrogen with 10%-50% H2O at 600oC-700oC, and mass gain after the test was measured. The mass gain increased with time, indicating that steam oxidation proceeded. The oxidation rate was smaller for JIS SUS430J1L than the other three types of the alloy for all the test conditions. The proficient durability came mainly from the thin Cr-rich oxide layer covering the alloy uniformly with little defect or segregation. The additives in the alloy such as Nb also may have an effect on suppressing elemental diffusion through the oxide layer. Because morphology of the surface oxide layer initially formed on the surface should affect the process of steam oxidation during SOFC operation, we investigated influence of pre-heat treatment condition (700oC-900oC) of the alloy. The surface oxide layer formed by the pre-heat treatment at 900oC in air for 2 h had an effect of reducing steam oxidation rate. Microstructure of the surface oxide layer before and after the durability test was analyzed by STEM. From the difference in the microstructure of the surface oxide layer, predominant factor to improve durability will be discussed. This paper is based on results obtained from a project, (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1