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, ni ckel deposition being inhibited and zinc deposition being stimulated. At low polarizations, nickel-rich deposits are formed, on which the di ffusion-controlled hydrogen evolution predominates, at pH 1.5. The tra nsition is shown to be related with both an increase in the interfacia l pH and the presence of sulfur in the deposit. Impedance data reveal the reversibility of the charge-transfer reactions involved in hydroge n evolution and in anomalous deposition. They also show the existence of four relaxation processes: one inductive process due to zinc deposi tion, and three capacitive processes associated to nickel deposition a nd hydrogen evolution on a nickel-rich surface. They also reflect the anion adsorption.