Electrochemical study on Ni–P electrodeposition

The kinetics of P incorporation during Ni–P electrodeposition was investigated by means of interfacial pH measurements, cathodic polarization curves and electrochemical impedance spectroscopy. This process is strongly affected by solution pH. A buffering effect provided by sodium hypophosphite in the solution was observed at pH 1.5. By contrast, at pH 4.0, this effect was not observed and an increase in the degree of alkalination at the interface was verified. At pH 1.5, Ni2+ reduction was strongly disfavored during Ni–P deposition. The impedance measurements indicated that the mechanism of Ni deposition was modified by the presence of NaH2PO2·H2O in the solution. The characteristic time constants for the individual cathodic processes, H+, Ni2+ and NaH2PO2 reduction, could be identified during Ni–P electrodeposition. Moreover, at pH 4.0, Ni–P deposition occurs by means of a different mechanism than that observed at pH 1.5. These findings cannot be described by any of the models for Ni–P electrodeposition found in literature.