Creative Preparation of High-oxidation Resistant Coating on Nb-base Heat-resistant Alloys using Molten-salt Electrodeposition and Self-systematization

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Coatings 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, 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 the maintenance of a high concentration of Al in the surface layer and the suppression of spallation of the layers, leading to the improvement in the oxidation resistance of the coating.