Perchlorate fitting corrosion of a passivated silver electrode

The passivation and pitting breakdown of a silver electrode in sodium hydro xide solutions containing sodium perchlorate was studied using potentiodyna mic and potentiostatic techniques. In perchlorate-free alkali solution, the voltammogram exhibits three anodic peaks prior to oxygen evolution. The fi rst two peaks correspond to the oxidation of Ag and formation of a passive film of Ag2O on the electrode surface, the third to the conversion of Ag2O to AgO. In the presence of ClO4-, the voltammogram depends considerably on perchlorate concentration. ClO4- increases the height of the three anodic p eaks, and at potentials above a limiting value breakdown of the anodic pass ivity and initiation of pitting corrosion occurs. The pitting potential dec reases linearly with ClO4- concentration but increases with scan rate. The potentiostatic current/time transients show that pitting corrosion can be d escribed in terms of an instantaneous three dimensional growth under diffus ion control.