Pitting Corrosion of Zn Peculiarly Caused by Acetate Anions

Abstract

The corrosion behaviour of zinc metal was studied in acetate solutions. The potentiodynamic polarization curve in 0.1 M acetate solution displays an anodic peak (A1) owing to the anodic dissolution of zinc followed by a passive layer formation region. Breakdown of the passive film and the initiation of pitting corrosion were observed beyond the pitting potential (Epit) and confirmed by SEM images. The reverse scan of the cyclic voltammogram shows three cathodic peaks; CI, CII and CIII. CI was attributed to the reduction of the pitting corrosion product formed at potentials more anodic than Epit, while the overlapped CII and CIII peaks were attributed to the reduction of the exterior and the interior passive layers, respectively. The potentiostatic current time transients at different applied potentials (around the pitting potential) for different electrolyte concentrations involved three stages. The first stage exhibited rapid current decrease with time till a minimum value (im) at the incubation time (ti). The second and the third stages, where the current increases again linearly with time through two different slopes, are correlated to the pit nucleation and growth, respectively. An increase of the pit nucleation rate and a decrease in its growth were observed with increasing the anodic potential limit (Ea). Electrochemical impedance spectroscopy (EIS) measurements, at different potentials before and after Epit were performed. The effect of the acetate concentration was studied as well. A suggested equivalent circuit was used to fit the EIS data. The impedance results confirmed that the thickness of the formed passive layer increases with proceeding of the anodic potential reaching a maximum value at Epit. The passive film thickness decreased again at more anodic potentials.