INFLUENCE OF CHLORIDE-IONS ON THE ANODIC-DISSOLUTION OF TIN IN BICARBONATE AND PHOSPHATE SOLUTIONS AT PH-8

The influence of Cl- ions on passivated tin in 0.3M and 0.5M bicarbonate or phosphate solutions of pH 8 has been studied. The growth of a passive film was carried out in deaerated chloride free solution at +1 V(SCE) to a constant thickness of 4 mC cm-2 and the film breakdown potential was determined potentiodynamically in Cl- containing solutions. The film breakdown potential was found to increase linearly with the potential scan rate. The critical pitting potential (breakdown potential at zero scan rate) decreased linearly with log [Cl-]. The bicarbonate and phosphate ions inhibited the chloride attack, the effect being greater for bicarbonates. A rough, bare tin surface in the corrosion zones was observed in the phosphate solutions, whereas the pits were covered by corrosion products in the bicarbonate solutions. The current oscillations observed during experiments on the preoxidized electrode were very regular in the case of i-E and i-t: sine waveform with the same frequency of approximately 0.3 Hz. However, the variations became chaotic when the applied potential was more positive than the breakdown potential.