Coloration by zinc-nickel composite films electrodeposited from a chloride solution

Zn-Ni composite films were electrodeposited under coulostatic (515-5 145 C·m^-2) and galvanostatic (5 A·m^-2) conditions onto brass plates with a Ni precoating in an unagitated chloride solution at 23°C to investigate the color and structure of the deposited films. The chromaticity of the deposited Zn-Ni composite films changed in a clockwise direction in each quadrant of the a*b* diagram of the L*a*b* color space with increasing amount of charge. The deposited films contained S, Zn, and Ni, and their content decreased in the order of S > Zn > Ni. X-ray photoelectron spectroscopy revealed that the deposited films were primarily composed of ZnS, ZnO, and Ni and contained small amounts of Zn(OH)₂ and Zn. The color of the deposited films is attributed to interference of light, and the index of refraction of the deposited films composed of ZnS, ZnO, and Ni is assumed to be large. When Ni was excluded from the electrolytic solution, ZnS did not codeposit and the amount of Zn deposited in the metallic state increased. This result suggests that the Ni deposition induces the codeposition of ZnS.