A surface coverage model for pulse-plating of binary alloys exhibiting a displacement reaction

Abstract

Models for the prediction of the composition of pulse-plated copper-nickel alloys in the presence and absence of a displacement reaction during the pulse off-time are reviewed and critically evaluated. For very long and very short pulse off-times the composition of copper-nickel alloys deposited from electrolytes containing an excess of nickel ion is entirely controlled by mass transport of copper ion. At intermediate pulse off-times, however, the kinetics of nickel dissolution play a role. A modified model is proposed which takes into account the role of surface coverage by copper for the rate of nickel dissolution. Assuming a step function for the variation of surface coverage with off-time the model predictions can be fitted to experimental data using the critical off-time which corresponds to a change in surface coverage from zero to one as adjustable parameter. The model is used to estimate for different experimental conditions the amount of copper leading to a suppression of the displacement reaction. More copper is deposited by the displacement reaction in sulfamate than in citrate electrolytes.