New insights into the cathodic dissolution of aluminium using electrochemical methods

The cathodic dissolution of aluminium in neutral medium was studied by electrochemical methods. It is shown that oxygen reduction at the surface of the metal is kinetically controlled and that the diffusion regime is never reached due to the poor conductivity of the oxide film and consequently a high potential drop across the layer. Nevertheless, at E < −1.4 V/MSE, the rate of hydroxyl production during oxygen reduction is high enough to raise the interfacial pH above 9, causing chemical dissolution of the oxide layer. The oxide film was characterized by electrochemical impedance spectroscopy in the potential range where it is stable and does not undergo chemical dissolution. The analysis based on the use of the power law model and a Cole-Cole graphical representation allows extraction of the oxide film thickness and provides the resistivity profile in the layer. In the potential range close to the open-circuit potential (−1.4 V < E < Ecorr), for the oxide layer formed during 2 hours in 10−2 M Na2SO4 solution, a constant thickness between 8 and 9 nm was determined and the resistivity varied between 1011 Ω cm−1 at the oxide/metal interface and 105 Ω cm−1 at the electrolyte/oxide interface. At more negative potential, the thickness of the layer and the resistivity at the electrolyte/oxide interface drop significantly due to alkalinisation of the interface which induces chemical dissolution of the oxide.