Potentiodynamic Characteristics of Zinc Elektrodeposition from Chloride Solutions

Abstract

The major goal of this potentiodynamic study was to explore the characteristics of zinc electrodeposition from chloride (ZnCl2 – HCl) media. The influence of various operating parameters such as zinc concentration in the 30 – 150 g/dm 3 range, acidity level (pH in the 1.5 – 5.5 range) and rotation speed (0 – 950 r.p.m.) were investigated at room temperature. Deposit morphologies were recorded by a digital camera during cathodic polarization. The physical quality of the electrode surface seems to be one of the important parameters not only for the deposit structure but also for the side reaction of H2 evolution and also for the development of the polarization curves. There are three kinds of cathodic depositions occurring. It generally starts with a uniform and stable structure, followed by a sponge-like deposit and finally it turns into a dendritic growth due to concentration changes near the surface. However, electrolyte agitation can influence the deposition pattern. With increased rotation speeds (above ~ 500 r.p.m.), resulting in higher available concentrations of electro-active ions at the surface, there is not much sponge-like deposit, yet the dendrite formation at the edges could not be avoided. The strong increase in the actual specific surface area of the cathode, due to the sponge-like deposit and the dendrite formation, caused irregularities in the potential change executed by the computer controlled potentiostat. Although this instrument was designed and produced at the institute specifically for high speed measurements. By lowering the pH in the electrolytes of the lowest Zn concentrations, the side reaction of H2 evolution soon appeared, after the limiting current of Zn deposition was reached. At higher Zn concentrations though, instead of a clear limiting current, the slope of the polarization curve persisted all across the examined potential range, indicating a continual co-deposition of H2 together with that of Zn.