Passivation of zinc in concentrated alkaline solution—II. Role of various experimental factors and the distinction between the solid-state and dissolution—precipitation mechanisms

Abstract

Pseudo steady-state current—potential measurements have been made during the dissolution and passivation of zinc electrodes in 6.8 mol dm−3 (30% w/w) KOH solutions over a wide potential range. Thepre-passive characteristics are very sensitive to impurities,viz. iron. Similarly,ir drop has an appreciable effect in this region necessitating full correction if measured potentials are to have any quantitative significance. Amalgamation exerts a considerable influence but only in the pre-passive region where the pre-passive film is considerably more inhibiting than that formed on unamalgamated zinc. It is suggested that this film could be derived from intermediates of the form Zn (OH)−2 when the potential reaches a critical value. A high degree of reproducibility observed in thetransition region upon sweep reversal indicates that the potential is also the important parameter controlling the initiation of thepassive Zno/Zn (OH)2 film and not supersaturation/solubility effects. Active—passive characteristics are identical on single crystal and polycrystalline electrodes except in the transition region, where cosiderably less stability exists in the case of the polycrystal. This is interpreted in terms of an electrochemical spectrum for passive film formation/removal on different crystal planes. Polarization data from rotating disc experiments have been analysed in respect of the formation of both thepre-passive andpassive films. Evidence has been sought which would support a solid-state mechanism (constant potential of passivation under conditions where the supersaturationdecreases with increase in electrode rotation speed) or a dissolution—precipitation mechanism (constant supersaturation at the point of passivation under conditions where the passivation potentialincreases with increase in electrode rotation speed). Only evidence in favour of the former mechanism has been obtained for both film types.