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

Nickel electrodeposited films with a black appearance could be produced by adding nitrate ions to a nickel sulfate bath (Watts bath).The blackening of the deposited films is attributed to formation of a NiO layer on the surface of the films and the refinement of its structure.The addition of nitrate ions seems to cause an increase in pH near the cathode due to the reduction reaction of nitrate ions and to increase the formation of nickel hydroxide, which suppresses the diffusion of Ni and induces the refinement of the deposited films.