Perchlorate pitting corrosion of tin in Na 2CO 3 solutions and effect of some inorganic inhibitors

Abstract

The anodic behaviour of tin electrode in Na 2CO 3 solutions containing different concentrations of Na 2ClO 4 was studied by potentiodynamic technique and complemented by scanning electron microscope. In perchlorate free carbonate solutions; the polarization curves exhibit two anodic peaks assigned to the electroformation of Sn(II) and Sn(IV) species, respectively, prior to the permanent passivation region. The passivity is due to the presence of SnO and SnO 2 layers on the electrode surface. Addition of ClO 4 - to the carbonate solution breaks down the passive layer and initiate pitting corrosion at a certain critical pitting potential. The pitting potential decreases with an increase in ClO 4 - concentration but increases with increasing both Na 2CO 3 concentration and scan rate. Addition of increasing concentration of MoO 4 2 - , WO 4 2 - or NO 2 - causes a shift of the pitting potential in the positive direction indicating the inhibition effect of added anions, while addition of NO 3 - anion accelerates the perchlorate pitting corrosion.