Effect of chromium(VI) on electrocrystallisation of cobalt from sulphate solution

The effect of chromium(VI) on current efficiency, deposit quality, polarisation behaviour, crystallographic orientations and surface morphology during the electrocrystallisation of cobalt from sulphate solution has been studied as a function of pH, chromium concentration, temperature and cobalt ion concentration. The behaviour of chromium is found to be very sensitive to the pH of the solution. Addition of Cr(VI) to cobalt electrolyte results in a decrease in current efficiency and this effect is greater at higher pH. The deposit quality is severely affected at higher pH and cobalt deposition stops when Cr(VI) concentration exceeds 25 mg/L. At lower pH (1.5), it is possible to achieve cobalt deposits in the presence of Cr(VI) concentration as high as 100 mg/L. The current efficiency decreases at higher current density and with increasing temperature. The presence of chromium also affects the crystal orientations, surface morphology and polarisation behaviour of the cobalt deposition process.