On Metastable and Stable Pitting Corrosion of Zinc in Perchlorate- and Chlorate-Containing Sodium Hydroxide Solutions

Metastable and stable pitting corrosion behaviour of Zn in 0.5 M NaOH solutions containing various concentrations (0.01 - 0.1 M) of ClO3- or ClO4- anions was studied. Measurements were conducted by means of potentiodynamic anodic polarization technique, complemented with ex situ SEM and AFM examinations. Addition of either ClO3- or ClO4- stimulated general corrosion and ruptured the passive layer to an extent depending on the type and concentration of the tested anion. Experimental findings confirmed that metastable pitting events ( appeared as oscillations in current at potentials close to the pitting potential, E-pit) was mainly due to Cl- ions ( produced via the reduction of ClO3- and ClO4-). Thus, Cl- is the aggressive ion that caused metastable pitting and not ClO3- and ClO4-. However, ClO3- and ClO4- anions were found to induce stable pitting even in absence of Cl- ions, with ClO3- being more aggressive than ClO4-. A cooperative influence between Cl- and either ClO3- or ClO4- in passivity breakdown and pitting initiation is confirmed and discussed.