CORROSION AND PASSIVATION OF TIN ANODE IN NA2CO3 SOLUTIONS AND THE EFFECT OF HALIDE-IONS

The corrosion and passivation of tin anode in Na2CO3 (0.01 to 1.0 M) w as investigated using potentiodynamic and cyclic voltammetry technique s and complemented by X-ray and scanning electron microscopy. The pola rization curves exhibit two anodic peaks assigned to the electroformat ion of Sn(II) and Sn(IV) species, respectively, prior to permanent pas sive region. The anodic dissolution reactions are controlled by diffus ion. X-Ray diffraction showed that the permanent passive layer is dupl ex and consists of SnO and SnO2. A multiplicity of cathodic peaks is r elated to the electroreduction of the anodically formed compounds. Add ition of Cl-, Br- or I- ions into the carbonate solutions enhances the anodic dissolution of tin to some extent depending upon the sodium ca rbonate concentration. In the passive region, addition of the halide i ons causes pitting corrosion at a critical pitting potential. The pitt ing potential decreases with increasing both the halide ion concentrat ion and the scanning rate but increases with increasing the sodium car bonate concentration. SEM examination confirms the occurance of pittin g corrosion.