Development of Nanolaminated Multilayer Ni–Co Alloy Coating of Better Corrosion Resistance

Abstract

Electrodeposition of composition modulated multilayer (CMM) coatings of Ni–Co alloy was accomplished on mild steel using single bath technique (SBT). Multilayer coatings, having layers of alternatively different compositions have been produced on the mild steel using the electrolytic bath having both Ni and Co ions by periodic pulsing of the current density between two set values, known as cyclic cathode current densities (CCCD’s). The deposition conditions, in terms of the composition and thickness of alternate layers were identified for best performance of those coatings against the corrosion. Electrochemical corrosion study of alloy coatings has been made using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization methods. It was found that CMM(Ni–Co)_1.0/3.0/60 coatings shows approximately three times better corrosion resistance than its monolayer counterpart, deposited from same bath for same duration. The experimental study demonstrated that corrosion protection efficacy of CMM Ni–Co alloy increased with number of layers up to 60 layers, and then started decreasing. The observed increase of corrosion rate at high degree of layering (after 60 layers) was attributed to the diffusion of layers, affected due to rapid change of current densities during deposition. Improved corrosion protection efficacy of multilayer coating is attributed to the alternate layers of alloys having different composition, confirmed by energy dispersion spectroscopy (EDS) and X-ray diffraction (XRD) techniques. The deposited coatings were analyzed for both surface and cross-sectional features using scanning electron microscopy (SEM), and results are discussed.