Critical Current Density for Zinc Electrowinning on Various Kinds of Metal Electrodes.

Factors determining the magnitude of critical current density, at which Zn began to deposit from impurityfree electrowinning bath, were studied on the basis of extra-overpotentials peculiar to hydrogen evolution. Electrochemical condition, which permitted codeposition of electrochemically less noble Zn with more noble hydrogen, revealed a kinetic requirement that the deposition overpotential, nH, for hydrogen should be generated at the critical current density to polarize the cathode to reach the equilibrium potential of Zn. The nH was not caused by the diffusion overpotential but by the extra-overpotential, ηH, appearing due to the inhibitory Zn hydroxide adsorbed on the cathode in addition to the inherent hydrogen overpotential, ηH, on the substrate. The dependence of ηInhH on the total current density revealed that the critical current density, which was observed at various metalel ectrodes, should be identical to the hydrogen evolution rate to genarate necessary magnitude of ηInhH to satisfy above requirement.Therefore, the magnitude of the critical current density depends essentially on the η0H and any other factors affecting the depletion rate of hydrogen ions in the cathode layer. The theory predicts that the critical current density is unequivocally dependent on the riH in the bath of fixed composition while it depends on the buffer capacity of the baths when the electrode material is specified. These predictions were in good agreement with experimental results.