The Electrocrystallization of Metals. Investigation of Zinc

In electrodeposition of zinc depending on the condition of deposition, various effects of electrocrystallization can be observed. In acid solution of zinc sulfate the considerable surface diffusion overpotential appears at lower values of the total overpotential, indicating the surface diffusion of actions as the rate determining step. An interesting dependence of exchange current density on pH with a minimum at pH - 2 indicate the probable change in structure of the reacting particle. The mechanism of the reaction is evaluated. In alkaline zincate solutions no surface diffusion limitations can be detected. However, due to the rather low zincate ion concentration and high exchange current density zinc deposition proceeds practically always under the diffusion control. Morphology of zinc deposited depends considerably on the value of the electrode potential. This can be seen from the experiments in which the deposition morphology was observed with a scanning electron microscope. It was found possible to disrupt the deposition repeatedly for microscopic observation and than continue deposition. At low overpotentials (up to 50 mV) epitaxial layer type growth was observed with the linear increase of the microstep width with time. For this effect a theory is proposed which accounts also for the observed effects of substrate orientation. At larger overpotentials (50-100 mV) boulder type deposit was observed, most of which were not epitaxial with the substrate. They are suggested to be originated by nucleation. The boulder density per cm2 is first sharply increasing, and then slowly decreasing with time. A statistical calculation for this is given based on the model that large boulders consume the smaller ones. With further deposition a small fraction of the boulders develop into dendrites, their number being limited by the available total current