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

To investigate the effects of current density and pH of the solution on the lightness and surface morphology of Ni deposit, Ni electrodeposition was performed at a current density of 100 to 500 A•m^-2 and 7.2 × 10⁵ C•m^-2 of charge, in an unagitated chloride solution with pH 1 to 3, at 333 K. The composition and structure of the surface of deposits were analyzed by radio frequency glow discharge optical emission spectroscopy and three-dimensional scanning electron microscope (3D-SEM). The lightness of the Ni deposit decreased with increasing pH, and was also related to the current density with the minimum reached at 300 A•m^-2. The concentration of the surface oxygen in the deposits increased with pH, with the highest value obtained at 300 A•m^-2, which shows that the concentration of surface oxygen possibly affects the lightness. On the other hand, the surface of the Ni deposit was composed of several micrometers of pyramidal microcrystals and the mound formed by the aggregation of pyramidal microcrystals. To separate the two types of structural units, the deposits were cut every 200nm from the top in the thickness direction using 3D SEM. The number and area of the deposits in the cutting plane were measured and it was found that the lightness of the Ni deposit depended on the surface structure. Therefore, the lightness of the Ni deposit seems to depend on both the surface oxygen content and the microstructure of the deposits.