Abstract
Ohyama's critical current density, at which Zn began to deposit on pure Zn cathode from the electrowinning solutions containing an impurity, was studied by comparing its magnitude with Tainton's critaical current density at which Zn began to deposit on impurity metal cathode from the impurity-free solutions. According to the electrochemical requirement, previously reported by the present authors, for the initiation of Zn deposition in the presence of more noble hydrogen ions, the critical current density unequivocally depends on the minimum hydrogen overpotential, η0H, of cathode substrate. Therefore, the impurity in the solutions should change the magnitude of the critical current density either by depositing to alter the η0H or by acting as a catalyst to reduce the η0H of original Zn cathode. Then the electrochemical behavior of the impurities was estimated, and it was found that the Ohyama's critical current density was closely related to the behavior of the impurities at lower current densities than the critical current density.
