Metastable and stable pitting events at zinc passive layer in alkaline solutions

The electrochemical behavior of Zn in 0.5 M NaOH solutions containing various concentrations (0.01-0.1 M) of ClO3 (-) or ClO4 (-) anions was studied with potentiodynamic anodic polarization and chronoamperometry techniques. Microstructural and topographical characterization of the pitted surfaces was carried out by ex situ scanning electron microscopy and atomic force microscopy examinations. Addition of either ClO3 (-) or ClO4 (-) stimulated general corrosion and ruptured the passive layer (stable pitting), with ClO3 (-) being more aggressive than ClO4 (-). Metastable pitting events appear as current oscillations (spikes) at potentials close to the pitting potential when Cl- ions are produced by cathodic reduction of ClO3 (-) and ClO4 (-) before passive layer growth. Current-time measurements are performed at fixed potential after production of Cl- ions and show that the rate of metastable pitting and the intensity of current spikes increase with the potential and the concentration of aggressive anions. Concepts of thin film growth are applied to the passive layer formation in order to explain those results. Metastable events are related to the presence of defects in the passive layer because their frequency and intensity are enhanced in conditions that favor defect formation and roughening in growing films, while stable pitting typically occurs at regions of high metal disorder.