Pitting of High Purity Zinc and Pitting Potential Significance

The passivity breakdown of high purity zinc in aqueous solutions containing different aggressive anions (Cl⁻, I⁻, SO₄⁼, and NO₃⁻) was studied. A critical evaluation of different methods for measuring the breakdown potential was made. Reproducible passivity breakdown potential measurements were given only by those methods producing mechanical disruption of the passive film. The passivity breakdown potential of high purity zinc was found to be independent of the nature of the anions tested, except for nitrates, and very close to the equilibrium potential of the reaction Zn = Zn²⁺ + 2e. The passivity breakdown potential in all the cases studied was found to be equal to the corrosion potential of zinc in the acid solution formed inside the pits. It is concluded that the pitting potential is the minimum potential at which an acidified solution can be produced and maintained in contact with the dissolving metal.