Multilayered Zn-Ni alloy coatings for better corrosion protection of mild steel

A simple aqueous electrolyte for the deposition of anti-corrosive Zn-Ni alloy coatings was optimized using conventional Hull cell method. The corrosion protection value of the electrodeposited coatings at a current density (c.d.) range of 2.0-5.0 A dm−2 has been testified in 5 wt% NaCl solution, as representative corrosion medium. The electrochemical behavior of the coatings towards corrosion was related to its surface topography, elemental composition and phase structure using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses, respectively. Among the monolithic coatings developed at different c.d.'s, the coating obtained at 3.0 A dm−2 was found to be the best with least corrosion current (icorr) value. Further, the corrosion protection efficacy of the monolayer coatings were improved to many folds through multilayer coating approach, by modulating the cyclic cathode current densities (CCCD's). The composition modulated multilayer (CMM) Zn-Ni alloy coating with 60 layers, developed from the combination of CCCD's 3.0 and 5.0 A dm−2 was found to be the best with 3 fold enhancement in corrosion protection efficiency. The formation of multilayer coatings was confirmed using cross-sectional SEM, and the experimental results are discussed with tables and figures.