Potentiodynamic passivation of zinc in aqueous KOH solutions

The potentiodynamic oxidation of an upward-facing, horizontal zinc electrode in unstirred KOH solutions at concentrations in the range 1.0–5.0 M has been studied. Besides electrochemical treatment of the experimental data, the morphological evolution of the electrode surface during the oxidation has been followed by means of a scanning electron microscope. In the initial dissolution region, the Tafel slopes are near 40 mV dec⁻¹(25°C) and the reaction orders with respect to the OH⁻ion about 3, these results being consistent with the mechanisms already proposed by Bockris and by Chang and Prentice. The experimental results suggest that the passivation of zinc takes place by a dissolution-precipitation mechanism. However, the rate determining step for passivation appears to be the resistance of the solution in the pores formed by the precipitation and spreading of a film presumably composed of Zn(OH)₂, which is supposed to be the result of a local supersaturation with Zn(OH)²⁻₄.