Blackening of Electrodeposited Nickel Films by Adding Nitrate Ions and Its Mechanism

Nickel electrodeposited films with a black appearance were produced by adding nitrate ions to a nickel sulfate (Watt’s) bath. The blackening of the deposited films was attributed to both the formation of a Ni2O3 layer on their surface and the refinement of their structure. The addition of nitrate ions increased the pH in the vicinity of the cathode through their reduction reaction, thereby promoting the formation of nickel hydroxide. This hydroxide suppressed Ni2+ diffusion during Ni deposition and induced refinement of the deposited films.