Pulse-plating of copper-nickel alloys from a sulfamate solution

In pulse-plating of alloys, a displacement reaction can take place during the off-time, involving dissolution of the less noble component and deposition of the more noble component. The extent of the reaction is dependent on the length of the off-time. The influence of the displacement reaction between copper and nickel on the composition of copper-nickel alloys pulse-plated from a sulfamate electrolyte has been studied and the results are compared with those previously obtained for citrate solutions. Copper-nickel alloys were pulse-plated on a rotating cylinder cathode using a wide range of pulse-plating parameters and the resulting deposit composition was measured by X-ray fluorescence. Experimental results were compared with the predictions of two theoretical models, previously described in the literature, assuming a mass-transport-controlled displacement reaction and the absence of displacement reaction, respectively. Mass transport properties needed for this comparison were measured by independent techniques. The data show that a displacement reaction similar to that observed in citrate solution occurs between copper and nickel. Mass-transport-controlled corrosion of nickel is observed up to longer pulse-off times than in citrate. This excludes the hypothesis that, in sulfamate solution, passivation or an 'irreversible mechanism' prevents dissolution of nickel in the presence of copper(II) species.