Yttrium, cobalt and yttrium/cobalt oxide coatings on ferritic stainless steels for SOFC interconnects

Ferritic stainless steels are being considered as potential interconnect materials for SOFCs, in part because of their low cost relative to alternatives. These materials are, however, susceptible to degradation over time. A primary source of degradation is an increase in the area specific resistance (ASR), which is due to the formation of poorly conducting oxides (Mn–Cr spinel and Cr2O3) on the surface. In this work, the influence of Y, Co and Y/Co oxide coatings on the oxidation behaviour of a ferritic stainless steel (16–18 wt% Cr) has been investigated. Samples were oxidized in air for up to 500 h at temperatures ranging from 700 to 800 °C. Coated and uncoated samples were characterized, before and after heat treatment, using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric (TG) analysis and four-point probe resistance measurements. Surface morphology investigations of coated and uncoated stainless steels showed differences for Y and Co in terms of oxide formation. In all cases, Cr–Mn spinel and Cr2O3 were the two main surface oxides; however, the morphology of the spinel phase was dependent on the type of coating. The lowest resistances were obtained for the Y/Co-coated samples, which had ASR values up to seven times lower than corresponding uncoated ferritic stainless steels.