Effect of Cl− anions on zinc passivity in borate solution

Abstract The anodic behaviour of zinc in Na 2 B 4 O 7 solutions in the absence and presence of Cl − anion as an aggressive anion has been investigated by the galvanostatic polarization technique. In the absence of Cl − anion, the polarization curves are characterized by one distinct arrest corresponding to Zn(OH) 2 or ZnO, after which the potential increases linearly with time before reaching the oxygen evolution region. Addition of low concentrations of Cl − anions, has 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 the Cl − anion. This suggests interference of Cl − anion with oxygen evolution. Further increases in the concentration of Cl − anions are associated with breakdown of Zn passivity, this denoting the destruction of the passivating film and initiation of pits. The susceptibility of zinc to pitting corrosion is enhanced with increasing Cl − anion concentration but decreases with an increase of both the Na 2 B 4 O 7 concentration and current density. Addition of increasing concentrations of tungstate, phosphate or molybdate anions causes a shift of the breakdown potential in the noble direction, indicating the inhibitive effect of the added anion on the pitting attack. In contrast, addition of the sulphate anion enhances the pitting attack.