Pulsed Electrodeposition of Thin Cobalt Coating Layer on Ferritic Stainless Steel for SOFC Interconnects

A thin, compact, and uniform cobalt layer was coated on ferritic stainless steel (Crofer 22 APU) by high frequency square pulsed electrodeposition in an aqueous solution containing cobalt chloride and boric acid. The effects of various electrodeposition parameters and post heat treatment on the morphology, thickness, roughness and adhesion of the coating layer were investigated. It was found that neutralizing with sodium hydroxide solution after acid washing of the polished sample surface prevented delamination of the sub-surface layer. The addition of boric acid had a strong influence on the nucleation and agglomeration behavior of the deposited Co, which consequently controlled both the thickness and the roughness of the coating layer. With increasing current density, the grain size decreased and, therefore, a more uniform microstructure could be achieved. The duty cycle of the square pulse strongly affected the morphology but not the roughness of the coating layer. Subsequent heat treatment at 800 ℃ in air greatly improved the adhesion of the Co layer to the substrate. By optimizing the aforementioned electrodeposition parameters and adopting the post heat treatment, it was possible to obtain a fine, uniform, compact, and thin (< 2 μm) Co layer with excellent adhesion properties.