Electrodeposition of zinc–nickel compositionally modulated multilayer coatings and their corrosion behaviours

Zinc–nickel compositionally modulated multilayer (CMM) coatings were produced by successive deposition from dual baths containing a zinc sulphate electrolyte and a nickel–ammonia complexed electrolyte. The surface and cross-sectional morphologies of zinc–nickel CMM samples were examined using scanning electron microscopy (SEM). Corrosion resistance evaluated by means of neutral salt spray test showed that zinc–nickel CMM coatings were more corrosion-resistant than the monolithic coating of zinc or nickel of similar thickness. Result obtained from corrosion potential measurement and anodic polarisation suggest that pores and perhaps microcracks existed in nickel sublayer, which played an important role in achieving improved corrosion resistance for zinc–nickel CMM coatings. Based on results from electrochemical measurement and the micrographic characteristics of zinc–nickel CMM coatings after corrosion testing, a probable corrosion mechanism of zinc–nickel CMM coatings was proposed.