Structure and protective properties of zinc and nickel alternate electrodeposits

The electrodeposition of coatings consisting of alternate zinc and nickel deposits produced by successive deposition from a revised zinc sulphate electrolyte and a new developed nickel bath has been investigated. Coated samples have been evaluated in terms of surface appearance, surface morphologies and cross-sectional morphologies using scanning electron microscopy (SEM). The corrosion resistance of laminar coatings has been studied by means of salt spray accelerated testing and anodic polarization measurement. The results showed that the corrosion performance of zinc-nickel laminar coatings was better than that of a single-layer zinc or nickel deposit with the same thickness, and the coatings with nickel sublayer adjacent to the steel substrate and zinc as the top layer had the best corrosion resistance. These results were in good agreement with the characteristics indicated in the morphologies of zinc-nickel laminar coatings after anodic polarisation measurement.