Microstructure and diffusion behavior in the multilayered oxides formed on a Co–W electroplated ferritic stainless steel followed by oxidation treatment

Numerous attempts have been made to improve the oxidation resistance and electrical conductivity of the interconnectors in solid oxide fuel cells. A Co–W alloy coating on ferritic stainless steel has attracted attention because the Co–W oxide layer formed by the oxidation treatment of the Co–W alloy coating has proven effective in reducing the outward diffusion of Cr and improving oxidation resistance. This study was designed to elucidate the diffusion behavior of elements and the barrier mechanism of the CoWO4 layer. After oxidation in air at 750°C, a dense, multilayered oxide formed, comprising (from the stainless steel substrate to the outer layer) Cr2O3, (Cr,Fe,Co)3O4, CoWO4, (Co,Fe)3O4, and Co3O4 layers. The CoWO4 layer and neighboring oxide layers were carefully analyzed by scanning electron microscopy and transmission electron microscopy with electron energy-loss spectroscopy, confirming the absence of trivalent cations (Co3+, Fe3+, and Cr3+) and the presence of Fe2+ ions in the CoWO4 layer; thus, CoWO4 functions as a selective diffusion barrier to trivalent cations, as hypothesized. The Cr-containing oxide layer grows based on the reaction between the metallic cations from the substrate and the inward-diffusing oxide ions, whereas Fe and Co can diffuse outward through CoWO4 as Fe2+ and Co2+ ions.