Effects of copper and anions on zinc-nickel anomalous codeposition in plating and electrowinning

Anomalous codeposition describes inhibition of the reduction of a metal such as Ni by the presence of a less noble metal such as Zn. This phenomenon is observed in two important industrial applications. In the case of plating, it is shown that the addition of Cu leads to the formation of a ternary alloy which is enriched in Ni in the chloride bath, whereas little enrichment is observed in the sulfate, and none in the nitrate and acetate baths. In the case of Zn electrowinning with diluted metallic impurities, a description of the anomaly based on limiting current density is given, and once again the effect of Cu, which is to increase the content of the Ni impurity in the Zn, is much more important in the chloride than in the sulfate media.