Coloration by zinc-nickel composite electrodeposition from chloride solution

Zn-Ni composite electrodeposition was performed under coulostatic (515-5145C·m-2) and galvanostatic (5A·m-2) conditions on a brass with Ni precoating in an unagitated chloride solution at 23°C to investigate the color and structure of the deposited films. The chromaticity of deposited Zn-Ni composite films changed in a clockwise direction in each quadrant of a*b* diagram of La*b* color space with increasing the amount of charge. The deposited films contained S, Zn and Ni, and the content was in the order of S >Zn >Ni. XPS revealed that the deposited films were composed of ZnS, ZnO and Ni mainly and contained Zn(OH)2 and Zn slightly. The color of deposited films is attributed to interference of light, and the index of refraction of deposited films composed of ZnS, ZnO and Ni is assumed to be large. When the Ni was excluded from the electrolytic solution, ZnS did not codeposit and the deposition of Zn in metallic state increased. This suggests that the Ni deposition induces the codeposition of ZnS.