Effect of Impurities in Solution on the Lightness and Surface Morphology of Ni Deposited from Chloride Electro-Winning Solutions

Abstract

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.