Influence of bath composition on the electrodeposition of amorphous Ni-Mo alloys using potassium-sodium tartrate as complexing agent

Currently, sodium citrate is the most used complexing agent for electrodeposition of Ni-Mo coatings, especially for studying the effect of bath temperature and current density on the proprieties of Ni-Mo coatings. This study evaluates the influence of the concentrations of nickel sulphate and sodium molybdate on the electrodeposition of Ni-Mo alloys in baths using tartrate as a complexing agent. Electrodepositions were carried out in a two-electrode cell. A 3(2) complete factorial design of experiments along with response surface methodology (RSM) was used for the optimization of bath variables for electrodeposition of Ni-Mo alloys. The coatings obtained were characterized in terms of chemical composition and surface morphology. The chemical composition of the coatings was most influenced by the sodium molybdate concentration. Ni-Mo coatings of good quality were obtained in potassium-sodium tartrate baths with Mo content in the range 18-44 wt%; some results present even higher Mo content than coatings obtained from citrate baths. The coating with the highest Mo content was deposited from the bath with a concentration ratio of nickel sulphate to sodium molybdate of 1:1. Surface morphology varied according to chemical composition, and microcracks were observed for the coating with 41 wt% Mo, for which scanning electron microscopy (SEM) images also show the presence of nodules. The most homogeneous coating surface was obtained at a concentration ratio of nickel sulphate to sodium molybdate of 10:3. X-ray diffraction analyses have shown that all coatings obtained in this work are amorphous.