The Role of Homogeneous Chemical Kinetics in the Anomalous Codeposition of Binary Alloys

Abstract

The role of finite-rate homogeneous chemistry during the electrodeposition of binary alloys is analyzed using a detailed math-ematical model. The model accounts for solute reactions and transport within the aqueous boundary layer as well as multistep reactions on the electrode surface. Calculations are carried out for nickel-iron deposition from a sulfate bath, using a previously proposed surface-reaction mechanism that accounts for the observed anomalous codeposition. The common assumption of homo-geneous chemical equilibrium is found to produce only modest errors in the computed deposition rates, but it distorts the relative importance of hydrolyzed metal ions. In contrast with previous literature claims, the present calculations show that these ions tend to have a greater role than is predicted by assuming equilibrium. The model also suggests that the precipitation of solid metal hydroxides may be slow, i.e., that substantial supersaturation of the plating solution may occur. The electrodeposition of binary alloys is a process of great prac-tical importance which has been studied extensively over the last few decades. One aspect of particular interest is anomalous codepo-sition, in which the alloy is found to be much richer in the less-noble component than would be expected on the basis of single-metal deposition rates. Many attempts to explain this phenomenon via mathematical models have been made, but there is still no consen-sus. One of the earliest efforts was the theory of Sysoeva and Rotinyan, 1 which attributed the anomalous behavior to changes in both the thermodynamic potential and the potential of zero charge upon formation of the alloy. In sharp contrast, Dahms and Croll 2