An approach to the first stages of cobalt-nickel-molybdenum electrodeposition in sulphate-citrate medium

The first stages of Co-Ni and Co-Ni-Mo deposition in sulphate-citrate medium at pH 4.0 were analysed. In both cases, the formation of non-hydrogenated nickel on the electrode before alloy deposition was detected by linear sweep voltammetry and inductively coupled plasma mass spectrometry. Co-Ni electrodeposition was anomalous since the Co/Ni ratio in the alloy was higher than the corresponding [Co(II)]/[Ni(II)] ratio in solution. The adsorption of Co(II) over the initial nickel could explain the anomalous codeposition, which persisted with the addition of molybdate to the Co-Ni bath. However, the formation of intermediate molybdenum oxides also took place. A mechanism has been proposed to describe the sequence of steps for Co-Ni-Mo electrodeposition. Under our conditions, the alloy is formed mainly from free Co2+ and Ni2+ cations, whereas molybdate is reduced firstly to molybdenum oxide from MoO4(H3Cit)2− and, secondly, NiCit− catalyses the subsequent reduction to molybdenum metal of the intermediate [MoO2-NiCit−]ads species.