Corrosion Behaviour of Borohydride Reduced Ni–B, Ni–B–W, Ni–B–Mo, Ni–B–W–Mo Coatings with Varying Composition: A Comparison

Abstract

The borohydride reduced binary (Ni–B), ternary (Ni–B–W, Ni–B–Mo) and quaternary (Ni–B–Mo) alloy coatings are deposited by chemical deposition method. The same coatings are used to study the variation in corrosion behavior of the coatings with the inclusion of W, Mo individually or together. The coatings are deposited over steel substrate with changing bath composition to study the role of coating bath parameters on corrosion resistance behavior. The coating characterization based on roughness, morphology and phase structure are performed using stylus type roughness tester, scanning electron microscopy (SEM) and x-ray difraction (XRD), respectively. A potentiostat is employed to measure corrosion response of the coatings by potentiodynamic polarisation (PDP) and electrochemical impedance spectroscopy (EIS) test method. The tests are carried out using a corrosive medium of 3.5% NaCl solution. All the coatings are observed to exhibit caulifower like surface morphology. The roughness of the coatings increases with bath parameter concentration. The corrosion resistance of the Ni–B–Mo coatings are found to be better than other three variants till mid-level of concentrations. The Ni–B coated specimens show higher corrosion resistance at higher level of concentrations compared to the ternary and quaternary coatings.