Perchlorate pitting corrosion of tin in Na2CO3 solutions and effect of some inorganic inhibitors

The anodic behaviour of tin electrode in Na2CO3 solutions containing different concentrations of Na2ClO4 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 SnO2 layers on the electrode surface. Addition of ClO4- 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 ClO4- concentration but increases with increasing both Na2CO3 concentration and scan rate. Addition of increasing concentration of MoO42-, WO42- or NO2- causes a shift of the pitting potential in the positive direction indicating the inhibition effect of added anions, while addition of NO3- anion accelerates the perchlorate pitting corrosion.