Factors determining the critical current density for zinc deposition in sulfate solutions

The critical current density at which Zn began to deposit from sulfate solutions was studied an the basis of the inherent deposition overpotential of hydrogen. The polarization curves for hydrogen evolution indicated that the hydrogen overpotential, which polarized the cathode to reach the equilibrium potential of Zn, was composed of two types of overpotential. One was the minimum hydrogen overpotential, which mainly depended an the cathode material, and the other was the extra-overpotential appearing in the presence of Zn Ions in the solution. The measurement of pH in the vicinity of the cathode revealed that the extra-overpotential was caused by the inhibitory Zn hydroxide adsorbed an the deposition sites for hydrogen. By considering the current density-dependence of the extra-overpotential, the electrochemical meaning of the critical current density was found to be the minimum hydrogen evolution rate which generated the necessary magnitude of the extra-overpotential to polarize the cathode to reach the equilibrium potential of Zn. The effect of factors such as cathode materials, the buffer capacity of the solution and the different metal ions in the solution, etc., an the magnitude of the critical current density was then discussed according to the meaning of the critical current density.