Preparation of Ni Aluminide/Ni Bilayer Coating on Nb–W Alloys by Molten Salt Electrodeposition and Oxidation Resistance

Coatings consisting of Ni aluminide/Ni bilayer on Nb–W and Nb–W–Mo alloys were prepared by the electrodeposition of Al using a molten salt electrolyte after the Ni electrodeposition. The coating specimen with a thin Ni layer showed a vigorous oxidation during the initial period. On the contrary, the coating specimen with a thick Ni layer showed a high oxidation resistance. For the coating specimen with a thick Ni layer, a surface layer consisting of NiAl was formed after the oxidation test. For this coating specimen, further, intermediate layers consisting of Ni–Nb alloys were formed by the diffusion of the Ni layer and the alloy substrate during the oxidation test. These intermediate layers contributed to the maintenance of a high concentration of Al in the surface layer and the suppression of spallation of the surface layers, leading to the improvement in the oxidation resistance of the coating.