The kinetics of zinc deposition at low overpotentials in alkaline electrolytes

Abstract

In alkaline zincate electrolytes, the processes leading to the formation of irregular deposits at low cathodic overpotentials are analysed by means of potentiostatic current transients, ac impedance spectroscopy and scanning electron microscopy. In the absence of anodically dissolved zinc (ADZ) in electrolytes, it is shown that an inhibiting layer of oxidation products is slowly formed on the electrode surface and results in the formation of scattered packs of deposit on the blocked electrode surface. On the basis of a model of stratified layer interphase, it is shown that zinc deposition takes place through a conductive sub-layer and is controlled by the ion diffusion through a porous medium. With increasing overpotential, the results are consistent with both an activation of the conductivities of the two sub-layers and a lessened control of the electrode kinetics by the ion diffusion. The dynamic response of the layer appears to be strongly affected by the variation of the mass transfer coefficient with current density. The formation of ADZ species in electrolytes generates a decrease in the thickness of the porous medium which appears to control the growth of spongy deposits on the whole electrode.