Effect of W Content on the Oxidation and Electrical Behaviors of Co–W Coatings for SOFC Interconnects Fabricated by Electrodeposition

Abstract

Vaporization of Cr species from solid oxide fuel cell (SOFC) interconnectors must be avoided to prevent the degradation of the electrocatalytic activity of cathodes. A diffusion barrier against Cr ions in the thermally grown oxide is required for the prevention of vaporization because the origin of Cr is an interconnector substrate made of ferritic stainless steel. Additionally, the diffusion barrier should not decrease the electron conductivity of the interconnector. We found that a CoWO₄ layer formed on type 430 stainless steel (18 mass% Cr) that was covered with a Co–2.4 at% W alloy via the electroplating process. The layer effectively blocked the outward diffusion of Cr because the trivalent Cr ion, Cr³⁺, cannot penetrate this layer. In this report, further understanding of Co–W electroplated ferritic stainless steels is described. First, a Co–W co-electroplating method with a high W content was developed. The W content increased with increasing W concentration in the bath and with the bath pH, and Co–30 at% W plating was obtained. Then, electroplating was conducted on types 430 and 445 (23 mass% Cr) for the comparative study of oxidation properties. A thick and compact CoWO₄ layer was formed on the Co–5 at% W electroplated steel. Excess W in the plating with more than 10 at% W resulted in the formation of a Fe–Co–Cr intermetallic layer, which may cause a decrease in substrate toughness. The structures of the multi-layered oxides that formed on both alloy substrates were similar to each other. The low specific resistance of CoWO₄ and the thin Cr₂O₃ that formed on the coated specimens resulted in the low area-specific resistance of the Co–W-coated specimen at 800°C, and the value was lower than for the uncoated steels. We concluded that Co–5 at% W electroplating on 23 mass% Cr stainless steels is optimal for SOFC interconnectors.