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/m² and 7.2×10⁵ C·m^-2 of charge, in an unagitated chloride solution containing Mn²⁺, Cr³⁺ and SO₄^2- ions as impurity with pH 1 to 3, at 333 K. In solution containing 10 g·dm^-3 of Mn²⁺, 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 Mn²⁺ exceeded 1 g·dm^-3. When the Ni deposition was performed using soluble Ni anode to prevent the formation of MnO₂ at anode, the lightness of Ni was higher than that using insoluble anode, which suggesting that MnO₂ resulting from insoluble anode caused the decrease in lightness of Ni. In solution containing Cr³⁺, the current efficiency of Ni gradually decreased with increasing concentration of Cr³⁺, and significantly decreased above the Cr³⁺ concentration of 0.1 g·dm^-3. The lightness of deposited Ni greatly decreased with increasing concentration of Cr³⁺ above 0.001 g·dm^-3. In solution containing Cr³⁺, 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 SO₄^2-, the current efficiency of Ni somewhat decreased at SO₄^2- concentration above 50 g·dm^-3, and significantly decreased above 100 g·dm^-3. The lightness of deposited Ni somewhat increased at SO₄^2- 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 SO₄^2-, the surface of deposited Ni becomes smooth, resulting in increase in lightness.