Steady-state model for anomalous Co–Ni electrodeposition in sulfate solutions

A comprehensive steady-state model for Co–Ni alloy co-deposition in sulfate solutions at pH 3 that accounts for mass-transfer and kinetic effects is presented. It also incorporates empirical expressions describing the variation of the diffusion coefficients with concentration. The model is statistically fit to experimental polarization curves obtained at different NiSO4/CoSO4 concentration ratios and used to determine the kinetic parameters for the proposed reaction mechanism. It is subsequently used to predict the alloy composition and current efficiency at a different bath composition and found to agree very well with the experimental measurements. Various mechanisms previously proposed to explain the anomalous behaviour observed for the electrodeposition of iron-group alloys are also assessed. Analysis of the model reveals that a previously proposed reaction pathway involving the formation of a mixed adsorbed intermediate NiCo(III)ads on the electrode surface during alloy co-deposition is not required to explain the anomalous behaviour of the system. The model also indicates that the anomalous behavior cannot be attributed to the preferential surface coverage of the adsorbed intermediate Co(I)ads over that of Ni(I)ads. Instead, it appears that the main factor is the much faster charge-transfer of Co(II) reduction than that of Ni(II) reduction.