The influence of some sulphur-containing anions on the anodic behaviour of zinc in an alkaline medium

The anodic behaviour of Zn in 0.1 M NaOH containing various concentrations of Na2SO4, Na2SO3, Na2S, Na2S2O3 or NH4SCN was studied by means of the potentiodynamic technique, complemented by X-ray diffraction analysis and scanning electron microscopy. In the absence of sulphur-containing anions in solution, the cyclic voltammogram displays two anodic peaks in the forward scan prior to reaching the oxygen evolution potential. The first anodic peak A(1) is related to the electroformation of Zn(OH)(2), while the more positive peak A, is assigned to the formation of ZnO2. The reverse scan exhibits a reactivated anodic peak A(3) and one cathodic peak C-1 prior to reaching the hydrogen evolution potential. The presence of either SO42- or SO32- stimulates the active dissolution of Zn while the presence of S2- (and/or SH-), S2O32- or SCN- inhibits it, presumably as a result of electroformation of sulphur-containing solid phases preceding the formation of Zn(OH)(2). Also, the presence of one of the cited anions studied in the alkali solution produces pitting of Zn at a certain specific pitting potential. The existence of pitting is confirmed by scanning electron microscopy. The aggressiveness of the sulphur species decreases in the order SCN- > SO42- > SO32- > S2O32- > S2-. The pitting potential decreases with increasing concentration of the sulphur species.