Electroplating and characterization of Zn-Ni, Zn-Co and Zn-Ni-Co alloys

Zn-Ni, Zn-Co and Zn-Ni-Co coatings were electrodeposited on mild steel from an acidic chloride bath containing p-aminobenzenesulphonic acid (SA) and gelatin. These additives changed the phase content in the coatings, most likely as a result of their adsorption at the surface of the cathode. The effect of gelatin was more pronounced than that of SA. The Faradaic efficiency was higher than 90%. As the current density was increased or the bath temperature was decreased, the concentration of the nobler metal in the coating increased. Both concentrations of Ni and Co in the ternary alloy increased as the applied current density was increased. Nickel and cobalt were found to have a synergistic catalytic effect. The thickness of all coatings increased as the applied current density was increased. The hardness increased with current density to a peak value, and then decreased. The rate of Zn deposition was heavily influenced by mass-transport limitation at high applied current densities, while the rates of Ni and Co deposition were not. The anomalous codeposition was explained by the great difference between the exchange current densities of Zn and the iron-group metal. Potentiodynamic polarization scans and electrochemical impedance spectroscopy showed that the corrosion resistance of the ternary Zn-Ni-Co alloy coatings was approximately 10 times higher than that of Zn-Ni and 7 times higher than that of Zn-Co. The improved corrosion resistance of the ternary alloy was attributed to its surface chemistry, phase content, texture, and surface morphology. The ternary Zn-Ni-Co coating may thus replace the conventional Zn-Ni and Zn-Co coatings in a variety of applications.