Perchlorate Pitting Corrosion of a Passivated Silver Electrode

Abstract

The passivation and pitting breakdown of a silver electrode in sodium hydroxide solutions containing sodium perchlorate was studied using potentiodynamic and potentiostatic techniques. In perchlorate-free alkali solution, the voltammogram exhibits three anodic peaks prior to oxygen evolution. The ®rst two peaks correspond to the oxidation of Ag and formation of a passive ®lm of Ag2O on the electrode surface, the third to the conversion of Ag2O to AgO. In the presence of ClOÿ 4 , the voltammogram depends considerably on perchlorate concentration. ClOÿ4 increases the height of the three anodic peaks, and at potentials above a limiting value breakdown of the anodic passivity and initiation of pitting corrosion occurs. The pitting potential decreases linearly with ClO4 concentration but increases with scan rate. The potentiostatic current/time transients show that pitting corrosion can be described in terms of an instantaneous three dimensional growth under diffusion control.