Metastable and stable pitting events on Al induced by chlorate and perchlorate anions—Polarization, XPS and SEM studies

Abstract

The metastable and stable pitting events of Al were studied in 0.075 M deaerated acidic NaClO 3 and NaClO 4 solutions (pH 3) using potentiodynamic anodic polarization and potentiostatic measurements, complemented with SEM and XPS examinations of the electrode surface. Metastable pits (appeared here as oscillations in current in the nA range) form at potentials close to the pitting potential ( E pit). SEM examinations of the electrode surface showed that the current oscillations resulted in observable pits on sample surface. The repassivated metastable pitting sites are prone to become preferential sites for following metastable pits to nucleate, resulting in accumulated corrosion damages on the surface. Results showed that Cl − ions produced in solution via the reduction of ClO 3 − and ClO 4 − anions at sufficiently negative cathodic potentials as well as their decomposition at high anodic potentials. Chloride production induced via the reduction of perchlorates is much slower than induced by chlorates. XPS examinations of the electrode surface showed that the amount of ClO 4 − and Cl − anions detected on the electrode surface increases with both cathodic and anodic polarizations (even above E pit). Experimental results revealed that addition of Cl − ions to the ClO 4 − solution accelerates pitting corrosion, indicating that these anions cooperate together in passivity breakdown and initiation of pitting. The role of ClO 3 − and ClO 4 − ions, despite their large size, in pitting process is also discussed here. A point defect model (PDM) is employed to explain passivity breakdown induced by pitting corrosion as a result of the aggressive attack of Cl − ions.