Effect of impurities in a nickel sulfate electrolyte on internal stress development morphology and adhesion to titanium of electrodeposited nickel

Abstract

Successful electrowinning and electrorefining processes for the production of high purity and compact electroplated metal require the optimization of the electrolyte composition and operating parameters. In nickel electrowinning and electrorefining plants the optimization of the electrolyte composition for good deposit morphology, optimal adhesion to the substrate, and minimum energy requirements remains a challenge. The influence of some impurities usually present in nickel sulfate electrolytes on the stress level, morphology and adhesion of nickel deposits were investigated using both quantitative and qualitative assessments, such as internal stress, adhesion, grain size and chemical composition measurements, complemented by visual observation of the morphology and condition of the plated nickel. The results indicate that both the internal stress and yield stress increased, but at different rates, with impurities such as copper, cobalt, chromium and aluminium in the electrolyte. The impurity level at which the internal stress exceeded the yield stress typically resulted in cracking and/or disbonding of the plated metal and could be used to define the allowable impurity levels. The impurity metal contents of the plated nickel increased proportionally to the concentrations of copper, cobalt and chromium in the electrolyte, but not so in the case of aluminium where a maximum was found at an intermediate concentration in the range of concentrations studied. The measured internal stress in the deposits followed the trend of impurity contents in the nickel for aluminium, chromium and copper, while a slight decrease in internal stress was observed at low cobalt concentrations in the electrolyte. It was also found that nickel adhesion to the titanium substrate is relatively weaker in the presence of impurities, with aluminum being the worse impurity, followed by copper, cobalt and chromium impurities, respectively.