Role of prior cathodic polarization in the pitting corrosion of pure aluminium in acidic chloride solution

Role of prior cathodic polarization in the pitting corrosion of pure aluminium (Al) has been investigated in aqueous 1 M HCl solution (pH 0) using a potentiostatic current transient technique combined with an electrochemical quartz crystal microbalance technique, impedance spectroscopy and scanning electron microscopy (SEM). From the analysis of the first period of combined cathodic current transient and electrogravimetric curves, it is suggested that the hydrogen evolution current starts obeying the Tafel relation via proton absorption, formation of such hydrogen-containing species as OH− ions and/or H2O molecules within the native oxide film and proton reduction at the metal/oxide interface. During the second period appearing at potentials −1.5 and −1.7 VSCE (SCE, saturated calomel electrode) hydrogen evolution may occur at an enhanced rate at the bottom of micro-pits, formed due to local breakdown of the oxide film. This breakdown was ensured by the more abrupt decrease in oxide film resistivity ρox values with increasing prior cathodic polarization, calculated at −1.5 and −1.7 VSCE, from the analysis of the measured impedance spectra as compared to that reduction in ρox calculated at −1.0 VSCE. The surface area of the pitted specimens was quantitatively estimated from the measured capacitance values to increase with rising prior cathodic polarization, as evidenced by SEM. This means that the exposed metal surface at the bottom of micro-pits formed during prior cathodic polarization serves as a preferential site for Cl− ion attack during the following anodic polarization. Hence, a sufficient prior cathodic polarization for the formation of the micro-pits is a necessary condition for the promotion of pitting corrosion of pure Al.