Electrodeposition of Zn–Ni protective coatings from sulfate–acetate baths

The electrodeposition conditions for Zn–Ni alloys from sulfate–acetate electrolytes have been studied with the view of preparing protective coatings. The influence of electrolyte composition (different [Ni(II)]/[Zn(II)] ratios, pH, buffer), cathode current density, cathode potential and hydrodynamic conditions on the composition of coatings, cathode current efficiency and corrosion resistance were determined. For all of the conditions examined, strong inhibition of nickel reduction with simultaneous increase in the rate of zinc discharge characteristic of an anomalous system, have been observed. The Zn(II) discharge becomes diffusion-controlled at more negative cathode potentials, whereas the partial nickel current densities are independent of electrode rotation speed. Consequently, nickel content and current efficiency are reduced with decreasing thickness of the diffusive layer. An increase in pH above 3.3 causes a significant catalysis of Zn–Ni deposition with a simultaneous decrease of the nickel in coatings. This effect may be related to the formation and increasing concentration of Zn(II) and Ni(II) acetate complexes in this condition. The Zn–Ni coatings obtained (5–18% Ni) characterise improved corrosion resistance in comparison to Zn layers deposited under the same conditions.