Degradation behavior of a commercial 13Cr ferritic stainless steel (SS405) exposed to an ambient air atmosphere for IT-SOFC interconnect applications

The objective of this work was to verify the feasibility of using low Cr ferritic stainless steels to improve IT-SOFC interconnect properties in terms of electrical conductivity and Cr evaporation resistance by taking advantage that low Cr ferritic stainless steels may form conductive and moisture-stable oxide layers of chromite spinels. In this context, surface degradation of a commercial 13Cr ferritic stainless steel (SS405) was studied at 700 °C in ambient air (containing ca. 3% H2O by volume) for up to nearly 5000 h. The results showed a slow, nearly linear oxidation rate indicating that the oxide scale could not act as a fully protective barrier during a prolonged exposure at 700 °C. Oxidation-induced degradation was mainly due to the effects of Cr depletion in the subscale matrix region and internal oxidation attack. However, analysis of the oxide scale after long-term exposure revealed the presence of an interesting example of all-spinel structure consisting of outer layer of iron-substituted Mn-Cr spinel particles on the top of an inner and continuous Fe-Cr spinel layer. Since SS405 steel contains also Al and Si as minor additions, the distribution of these elements in the spinel oxide scale was studied and found to be rather different. Whereas Si was present exclusively in the inner oxide regions, Al migrated also to the outermost part of the oxide scale. Possible explanations for this unexpected Al surface enrichment are discussed.