Corrosion stability of electrodeposited cyclic multilayer Zn–Ni alloy coatings

Abstract

This paper reports on a study of electrodeposition and characterisation of cyclic multilayer coatings of Zn–Ni alloy from a sulphate bath. Cyclic multilayer alloy coatings were deposited on mild steel through the single bath technique by appropriate manipulation of cathode current densities. The thickness and composition of the individual layers of the CMA deposits were altered precisely and conveniently by cyclic modulation of the cathode current during electrodeposition. Multilayer deposits with sharp change in composition were developed using square current pulses, using thiamine hydrochloride and citric acid as additives. Laminar deposits with different configurations were produced and their corrosion behaviours were studied by AC and DC methods in 5%NaCl solution. It was observed that the corrosion resistance of the CMA coating increased progressively with the number of layers (up to certain optimal numbers) and then decreased. The decrease in corrosion resistance at high degree of layering was attributed to interlayer diffusion due to less relaxation time for redistribution of metal ions at cathode during deposition. The coating configurations have been optimised for peak performance of the coatings against corrosion. It was found that CMA coating developed at cyclic cathode current densities of 3·0/5·0 A dm−2 with 300 layers showed the lowest corrosion rate (0·112×10−2 mm/year) which is ∼54 times better than that of monolithic Zn–Ni alloy, deposited from the same bath. The protection efficacy of CMA coatings is attributed to the difference in phase structure of the alloys in successive layers, deposited at different current densities, evidenced by X-ray diffraction analysis. The formation of multilayers and corrosion mechanism were examined by scanning electron microscopy.