High temperature oxidation of Co-W electroplated type 430 stainless steel for the interconnect of solid oxide fuel cells

•Co-W alloy coating was electroplated on ferritic stainless steel for SOFC interconnects.•The CoWO4 layer was found to act as an outward diffusion barrier of Cr.•Formation mechanism of the CoWO4 can be explained by the Gibbs formation free energy of each oxide.•The mechanism of the diffusion barrier property of CoWO4 can be explained by the valence numbers of cation sites in the lattice. Cobalt–tungsten alloy layer containing 2.4 at% W was electroplated on type 430 stainless steel (Fe–16 mass% Cr), and it was subjected to high-temperature oxidation at 800 °C in Air–3 vol% H2O atmosphere for 1000 h. A five-layered oxide of Co3O4, FeCo2O4, CoWO4, (Co,Fe,Cr)3O4, and Cr2O3 formed within 1 h, and the structure retained up to 1000 h. The outward diffusion of Cr3+ was effectively prevented by the CoWO4 layer. In the oxide, Co and W exist as divalent and hexavalent ions respectively, and there is no trivalent cation sites or trivalent cation vacancy through which Cr3+ diffuses.