Pitting Corrosion of Zinc in Na2S2O3 Solutions. Part I. Polarization Studies and Morphology of Pitting

As a first step towards studying pitting corrosion of Zn in aerated neutral 1.0 M sodium thiosulphate solution (pH 6.6), we have reported the results of cyclic polarization measurements on the passivity and passivity breakdown of Zn in this solution. The recorded voltammograms involved two oxidation processes labeled as AI and All, and two reduction processes labeled as CH and Cl on the forward (anodic) and reverse (cathodic) scans, respectively. The cathodic peak CH was attributed to the reduction of the pitting corrosion products (process All). On the other hand, the cathodic process Cl was related to the reduction of Zn2+ species formed during the course of the forward scan (process AI and the subsequent formation of the passive region), and the reduction of the adsorbed S2O32- anions to S2- and SO32-. The reduction products of S2O32-, namely S2- and SO32- anions, were detected in solution as a function of starting potential and time of holding the electrode at a cathodic potential of -2.0 V (Ag/AgCl). These anions were found to cooperate with the aggressive (SO32-)-O-2 anion itself in passivity breakdown and initiation of pitting corrosion. The effect of solution temperature (15-70 degrees C) on the cyclic polarization behaviour of Zn was also studied, and metastable and stable pitting events observed at high temperatures were discussed. The morphology of pitting was also studied by ex situ scanning electron microscopy (SEM) as a function of applied anodic potential, bulk concentration of S2O32- and solution temperature. Cross-sectional view of pits revealed the formation of irregular deep and shallow pits. The aspect ratios (the ratio of pit width to pit depth) of the growing pits were estimated as a function of potential, electrolyte concentration and solution temperature. The aspect ratio of the growing pits was found to be potential and concentration independent, while it increased with temperature.