Investigation of Alloy Grain Boundary Effects in the High-Temperature Oxidation & Cr Volatilization of 22 wt.% Cr Ferritic Stainless Steel Using 3D EBSD Analysis

Solid Oxide Fuel Cell (SOFC) attracts great attention among various types of fuel cells because of the high efficiency and the flexibility of fuel selection. The high operating temperature above 1000 °C was an obstacle to the commercialization of SOFC, but with the advent of the new conductive electrolytes (e.g., doped lanthanum gallate), the operating temperature was reduced to below 800 °C. These developments made metallic interconnects applicable to SOFC stacks instead of expensive ceramic interconnects.<br/>The development of metallic interconnects has been one of the main interests of material scientists over the past two decades. For improving oxidation resistance, 22 wt.% Cr containing ferritic steels were introduced to make the protective Cr₂O₃ layer on the surface. However, this Cr₂O₃layer produces the volatile Cr species when reacted with the humid air in the high temperature, and the reaction between these volatile Cr species and electrolytes caused the degradation of SOFC stack performance. Therefore, controlling the oxidation resistance and the Cr volatilization becomes the crucial point to the development of metallic interconnects.<br/>In this study, the influence of alloy grain size of the Ferritic stainless steel interconnects was evaluated by the weight gain and Cr evaporation amounts during long-term oxidation. Small grain-sized specimens show a higher oxidation rate, but lower Cr evaporation rate. Intensive microstructural analyses were conducted to obtain the interrelation between the overall oxidation behavior and the grain size. It reveals the intergranular oxide formation at the grain boundaries is the main factor to make the differences in the oxidation behaviors of the interconnect specimens. Especially, cross-sectional 3D EBSD analyses were carried out to investigate the role of alloy grain boundaries. The different grain boundary oxide density results in different oxidation and evaporation behaviors.