Passivation of Zinc Anodes in Alkaline Electrolyte: Part I. Determination of the Starting Point of Passive Film Formation

In the present contribution we demonstrate a new testing cell, which allows simultaneous in situ optical and electrochemical investigations of passivation processes on zinc anodes in quiescent alkaline electrolytes. By combination of microscopy and galvanostatic impedance spectroscopy it was possible to detect the starting point of passive film formation, which has not been achieved so far. We found that formation of the so-called type 1 passive film is not dependent on the anodic current density. This passive film appears at electrode overpotentials of <0.15 V after an amount of zincate ions of approx. 8.2·10−4 mol cm−2 has accumulated at the anode in a 30 wt% KOH electrolyte with 2 wt% ZnO at room temperature. Consequently, the passivation of zinc in quiescent electrolyte cannot be avoided even at very low dissolution currents. On the other hand, the so-called type 2 passive film appears below the first film due to direct oxidation of zinc. At overpotentials of ≥0.15 V the direct oxidation of zinc is favored and the potential-dependent type 2 passive film appears before type 1 film is formed.