Effect of impurities in solution on the lightness and surface morphology of Ni deposited from chloride electro-winning solutions

To investigate the effect of impurities in solution on the lightness, surface morphology and current efficiency of deposited Ni, Ni electrodeposition was performed at a current density of 300 A/m ² and 7.2×10 ⁵ C·m ^-2 of charge, in an unagitated chloride solution containing Mn ²⁺ , Cr ³⁺ and SO ₄ ^2- ions as impurity with pH 1 to 3, at 333 K. In solution containing 10 g·dm ^-3 of Mn ²⁺ , the current efficiency for Ni deposition decreased slightly, and crystal size of deposited Ni became small. The lightness of deposited Ni decreased when the concentration of Mn ²⁺ exceeded 1 g·dm ^-3 . When the Ni deposition was performed using soluble Ni anode to prevent the formation of MnO ₂ at anode, the lightness of Ni was higher than that using insoluble anode, which suggesting that MnO ₂ resulting from insoluble anode caused the decrease in lightness of Ni. In solution containing Cr ³⁺ , the current efficiency of Ni gradually decreased with increasing concentration of Cr ³⁺ , and significantly decreased above the Cr ³⁺ concentration of 0.1 g·dm ^-3 . The lightness of deposited Ni greatly decreased with increasing concentration of Cr ³⁺ above 0.001 g·dm ^-3 . In solution containing Cr ³⁺ , it is presumed that Cr(OH) ₃ formed at cathode layer suppresses the Ni deposition, resulting in some codeposition of NiO and Ni(OH) ₂ with Ni, which causes the decrease in current efficiency and lightness of Ni. On the other hand, in solution containing SO ₄ ^2- , the current efficiency of Ni somewhat decreased at SO ₄ ^2- concentration above 50 g·dm ^-3 , and significantly decreased above 100 g·dm ^-3 . The lightness of deposited Ni somewhat increased at SO ₄ ^2- concentration of 20 g·dm ^-3 , and greatly increased above 20 g·dm ^-3 . Since the overpotential for Ni deposition increases with increasing concentration of SO ₄ ^2- , the surface of deposited Ni becomes smooth, resulting in increase in lightness.