Production of Compositionally Modulated Nanolayer Zn–Ni Alloy Coatings and Their Corrosion Behaviours

Abstract

Compositionally modulated nanolayer alloy (CNA) coatings of Zn–Ni having alternatively the same compositions have been developed galvanostatically on mild steel from single bath technique (SBT) using square current pulses. Sulphanilic acid and citric acid (CA) were used as additives. Modulation in the composition of alloys were introduced by passing a different switched cathode current densities (SCCD’S) using sophisticated power sources. Variety of CNA coatings was developed under different conditions of cyclic cathode current densities, sequence and degree of layering. The corrosion behaviours of CNA coatings were studied by electrochemical DC and AC technique. CNA coating developed at 2.0–5.0 A/dm2 cyclic current densities having 300 layers showed least corrosion rate, and has been proposed as optimum condition. It was found that CNA coatings showed ~35 times better corrosion resistance compared to that of monolayer Zn–Ni alloy having same thickness. The improvement in corrosion resistance were attributed to the change in intrinsic properties, associated with changed dielectric constant at the interface of the metal and medium, evidenced by electrochemical impedance spectroscopy. The surface morphology, surface corrosion and cross-section view of Zn–Ni coatings were examined using Scanning Electron Microscopy (SEM).