Effect of limiting current density on corrosion performance of Ni–Mo, Ni–Cd and Ni–Mo–Cd alloy coatings

Abstract

In this article an attempt was made to increase the corrosion resistance of Ni–Mo alloy coatings with the addition of small quantity of CdCl2 into its bath. The limiting of limiting current density (iL) of Ni in both Ni–Mo and Ni–Cd baths due to inherent induced and normal type of codeposition has been successfully alleviated by addition of 1 g/L of CdCl2. The advent of induced and normal type of codeposition of individual binary baths has been used to optimize the Ni content of the ternary deposit for better corrosion stability. The composition vs. current density plots of all coatings have been studied, and thereby optimal iL of Ni in all baths were assessed. The content of Mo was found to be decreased with the small addition of Cd to the bath. Results revealed that (Ni–Mo–Cd)6.0 Adm−2 coating showed better corrosion resistance by reducing iL of Ni, on addition of Cd+2 ions into the bath and was explained in the light of diffusion limited deposition of Ni+2 ions. The results were supported by SEM (scanning electron microscopy), XRD (X-ray diffraction) and AFM (atomic force microscopy) study of Ni–Mo, Ni–Cd and Ni–Mo–Cd coatings at optimal current densities.