Effect of electrolysis condition on the lightness and surface morphology of Ni deposited from chloride electro-winning solutions

To investigate the effect of current density and pH in solution on the lightness and surface morphology of deposited Ni, Ni electrodeposition was performed at current density 100 to 500 A·m^-2 and the amount of charge 7.2 × 10⁵ C·m^-2 in unagitated chloride solution of pH 1 to 3 at 333 K. The composition and structure of the surface of the deposits were analyzed by radio frequency glow discharge optical emission spectroscopy and three dimensional SEM. The lightness of deposited Ni decreased with increasing pH, and became to be minimum at 300 A·m^-2. The concentration of the surface oxide of deposits increased with pH, and became high at 300 A·m^-2, which shows that the surface oxide of the deposits causes the decrease in the lightness. On the other hand, the surface of the deposited Ni was composed of both the pyramid microcrystals of several micrometers and the hill formed by the aggregations of pyramid microcrystals. To separate the two structural units of the pyramid microcrystals and the hill, the deposits were cut every 200 nm from the top to the thickness direction in three dimensional SEM, and the number and area of deposits in the cutting plane were measured, as a result, it was found that the lightness of the deposited Ni depended on the surface structure. The lightness of deposited Ni seems to depend on both the surface oxide and the microstructure of deposits.