AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al2O3 on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO4− ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (Epit) and the repassivation potential (Erp) to be determined. Epit decreases with increase in ClO4− concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (ti). The rate of pit nucleation (ti−1) increases with increase in ClO4− concentration and applied step anodic potential (Es,a). EIS measurements showed that at Es,a < Epit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at Es,a > Epit. An attempt is made to compare the values of Epit and Erp obtained through different methods and to determine the factors influencing these values in each particular method.