Microstructure, Formation, and Diffusion Kinetics of 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 formation kinetics and diffusion barrier mechanism of the Co-W coating. After oxidation in air at 750 °C, a dense, multilayered oxide formed, comprising (from the stainless steel substrate to the outer layer) Cr oxide, Cr-Fe-Co oxide, Co-W oxide, Co-Fe oxide, and Co oxide layers. The CoWO₄ layer and neighboring oxide layers were carefully analyzed by scanning electron microscopy and transmission electron microscopy, which revealed that the innermost Cr₂O₃ layer grows via the reaction between Cr in the substrate and inward-diffusing oxygen, whereas outward diffusion of Fe and Co is involved in the formation and growth of (Co,Fe)₃O₄ and Co₃O₄. Analysis by electron energy-loss spectroscopy confirmed 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.