EFFECT OF CHLORATE AND PERCHLORATE ANIONS ON LEAD PASSIVITY IN NAOH SOLUTIONS UNDER GALVANOSTATIC POLARIZATION

Anodic galvanostatic polarization technique has been used to study the electrochemical behavior of lead in NaOH solutions containing various concentrations of ClO3- or ClO4- as aggressive anions. It has been demonstrated that lower concentrations of these aggressive anions have no effect on the passive film formed on the metal surface. The potential starts to oscillate within the oxygen evolution region with an increase in the concentration of these aggressive anions. This suggests interference of such anions with the oxygen evolution. Further increases in the concentrations of these aggressive anions are associated with impaired Pb passivity that might indicate pitting attack. The ClO3- anion is more aggressive than the ClO4- anion. This might be explained in terms of differences in their adsorbability on the lead (or oxide-covered lead) surface. Addition of increasing concentrations of chromate, tungstate, phosphate, or molybdate anions causes a shift of the pitting potential in the noble direction, indicating the inhibitive effect of the added anion on the pitting attack.