STUDIES ON ZN-NI-CD TERNARY ALLOY COATING AS A REPLACEMENT FOR CADMIUM COATINGS

摘要

A new Zn-Ni-Cd plating process was developed which offers a unique way of controlling and optimizing the Ni and Cd contents in the final deposit. Zinc-Nickel-Cadmium alloy was deposited from a 0.5 M NiSO_4+0.2 M ZnSO_4 bath in the presence of 0.015 M CdSO_4 and 1 g/L nonyl phenyl polyethylene oxide. Using this process a Zn-Ni-Cd ternary alloy, with a higher nickel content as compared to that obtained from conventional Zn-Ni baths, was synthesized. The Zn-Ni-Cd alloy coatings deposited from an electrolyte containing 0.015 M (0.3%) CdSO_4 has a Zn to Ni ratio of 5:2. The increase in nickel content accounts for the observed decrease in the corrosion potential to a value lower than that of Cd but higher than the corrosion potential of iron. The coatings have superior corrosion resistance and barrier properties than the typical Zn-Ni and cadmium coatings. Polarization studies and EIS analysis on Zn-Ni-Cd coatings show a barrier resistance that is ten times higher than that of a conventional Zn-Ni coating. Permeation studies done using Devanathan-Stachurski technique show complete inhibition of hydrogen permeation under normal corroding conditions in case of Zn-Ni-Cd alloy.