Electrochemical study of the effects of additives and pH on the pulse-electrodeposition of high molybdenum content Ni-Mo alloys

To achieve ultrahigh hardness in Ni-Mo binary alloy, it is essential to maximize the incorporation of Mo while avoiding the emergence of intermetallic secondary phases. In the present paper, pulse current electrodeposition was employed to synthesize a Ni-Mo single-phase solid solution with elevated molybdenum content. This was accomplished by systematically optimizing the bath composition, including sodium molybdate concentration, additive selection, and pH adjustment, within a Ni-rich aqueous sodium citrate solution. It is demonstrated that inclusion of saccharin and 2-butyne-1, 4-diol as additives adversely affects the Mo atomic percentage in the Ni-Mo deposit. This is attributed to the intensified adsorption competition between additive molecules and the ternary intermediate ions on the cathode surface during the off-time of pulse current electrodeposition, as corroborated by in situ layer formation analysis using an electrochemical quartz crystal microbalance. Nevertheless, by modulating the pH value towards 7.7 through the controlled ammonia solution in the bath, the concentration of the ternary intermediate precursors was progressively enhanced. This optimization facilitated the successful synthesis of a supersaturated solid solution with a composition of Ni-26 at% Mo. An unprecedented hardness up to 12.8 ± 0.4 GPa can be achieved in this high-molybdenum Ni-Mo alloy through appropriate annealing.