1.19 - High Temperature Corrosion Issues for Metallic Materials in Solid Oxide Fuel Cells

Abstract

La-chromite-based ceramics as well as high temperature alloys are being considered as construction materials for interconnects in planar solid oxide fuel cell (SOFC) designs. Compared to the ceramics, metallic materials have the advantage of easier fabricability, lower costs as well as higher heat and electrical conductivity. Based on the requirements with respect to oxidation resistance, low thermal expansion coefficient and electrical conductivity of surface oxide scales, Cr-based alloys, and high-Cr ferritic steels seem to be the most promising metallic interconnector materials. A number of Cr-based dispersion-strengthened alloys have especially been developed for the SOFC application, and a large number of ferritic steels are commercially available in a wide range of compositions. However, due to the specific combination of properties required for the SOFC application and the high operating temperatures of 600–800 °C, hardly any of the commercial steels seems to be suitable as construction materials for interconnects. Therefore, a number of high-Cr ferritic stainless steels were developed, which possess the required combination of SOFC-relevant properties, especially with respect to thermal expansion coefficient, oxidation resistance, electrical conductivity of the surface oxide scale, machinability, and low cost. Further, SOFC-specific criteria which govern the suitability of the steels are the extent of interaction with the cathode and anode side contact materials as well as the glass-ceramic sealants in combination with resistance against carbonaceous gas species for the cases in which fossil fuels instead of hydrogen are being used for stack operation. Various types of coating systems have been studied to reduce the deleterious effect of volatile chromium species as well as adverse interactions of contacting materials with the ferritic steels.