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.
