Bath composition and operating conditions were compared between the electrowinning in hydrometallurgical production of Zn and the electroplating of single Zn and Zn alloys with iron-group metals in the production of corrosion resistant steel sheets. Both electrochemical processes were equivalent in a sense that Zn deposition proceeded on the metal cathode using insoluble anode from the sulfate baths where Zn existed in its simplest form of hydrated ion although the acidity of the bath was greatly different from each other. The electrochemical meaning of the critical current density, at which Zn began to deposit, was clarified first on the basis of the requirement permitting the deposition of less noble Zn in the presence of more noble hydrogen ions in the bath. Then the codeposition behavior of secondary element, i.e. an impurity in electrowinning and an alloying constituent in alloy plating, was explained by the theory of regular and anomalous type alloy deposition. According to its electrochemical meaning, the effect of secondary element in the bath on the magnitude of the critical current density was predicted, which agreed well with the experimental results. Further, the dissolution behavior of deposited Zn was discussed in relation to the mechanism in which the critical current density was changed in the presence of impurity in Zn electrowinning bath.
