Zinc coating by electrodeposition has been utilized in the production of corrosion-resistant steel sheets. In addition, binary Zn alloys such as Zn-iron-group metal, Zn-Mn and Zn-Cr electrodeposited from the sulfate type baths provide excellent corrosion-resistant coatings for the steel sheets used especially as automotive body panels. In the sulfate type baths, single Zn and Zn alloys can be deposited at higher current densities than the critical one. In this paper, based on that hydrogen does not evolve at its equilibrium potential, the electrochemical meaning of the critical current density is discussed. The anomalous deposition behavior of Zn-iron group metal alloys, in which electrochemically less noble Zn is deposited preferentially, is explained according to the hydroxide suppression mechanism. Further, the characteristic deposition behavior of Zn-Mn and Zn-Cr alloys is also referred briefly.