Electrodeposition of Zn-Ni alloys in sulfate electrolytes .1. Experimental approach

The mechanism of Zn-Ni alloy deposition in acidic sulfate electrolytes is analyzed essentially from polarization curves and impedance plots, using a rotating disk electrode. The anomalous deposition of the zinc-rich phases gamma and delta occurs at high cathodic polarizations, nickel deposition being inhibited and zinc deposition being stimulated. At low polarizations, nickel-rich deposits are formed, on which the diffusion-controlled hydrogen evolution predominates, at pH 1.5. The transition is shown to be related with both an increase in the interfacial pH and the presence of sulfur in the deposit. Impedance data reveal the reversibility of the charge-transfer reactions involved in hydrogen evolution and in anomalous deposition. They also show the existence of four relaxation processes: one inductive process due to zinc deposition, and three capacitive processes associated to nickel deposition and hydrogen evolution on a nickel-rich surface. They also reflect the anion adsorption.