Some potentiostatic studies on zinc

Abstract

The over-all passivation phenomenon in the case of zinc is very rapid and cannot be fully studied by galvanostatic techniques alone. The constant overvoltage technique has thus been applied to zinc. The d.c. amplifier used had a gain of 100,000 without feedback. Current/potential curves were obtained in 6 N, N, 0·1 N and 0·01 N KOH and zincate solutions for the range of −1·3 to about +2·0 V with respect to Hg/HgO/KOH as reference electrode. It has been found that in the first truly active region the main electrode reaction is the formation of zinc ions; after passivation, it changes to gas evolution. Potentiostatic techniques reveal intermediary stages, undisclosed by constant current methods, of pseudo-passivation and current-plateau regions in which the anodic layer thickens, controlled by high-field cation transport. These observations are supplemented by current/time transients obtained by suddenly dropping the potential from higher to lower values, when the currents were found to cut off. Some anomalies and sudden reversals of currents with increasing overvoltage are also discussed. The influence of other factors, e.g. concentration, stirring, sudden changes in overvoltage, presence of zincate, sulphate etc. is considered.