A Mathematical Model for Anomalous Codeposition of Nickel‐Iron on a Rotating Disk Electrode

A mathematical model for anomalous alloy deposition onto a rotating disk electrode has been developed. The convective diffusion equations, coupled with the homogeneous dissociation reactions of water and metal‐hydroxide ions, are numerically solved, and the calculated interfacial concentrations are used to describe the electrode processes. The role of metal‐hydroxide ions in the deposition of single metals, reported by other researchers, is extended to codeposition. The kinetic parameters of the alloy deposition are assumed to be the same as those of the single metals involving the discharge of both metal and metal‐hydroxide ions; however, during codeposition, the metal‐hydroxide ions are allowed to discharge only at a fraction of surface sites proportional to their interfacial concentration. The simulation was applied to the electrodeposition of nickel‐iron. In agreement with the experimental observations reported in the literature, the percentage of iron in the deposit is much higher than in the electrolyte, and goes through a maximum as a function of applied potential. The estimated effect of agitation on alloy composition and current efficiency is in qualitative agreement with experiments. The model also predicts that while the polarization behavior of nickel is shifted to more cathodic values during code‐position, the iron behavior shows no significant change.