The role of the oxide layer in the corrosion of aluminium in acidic solutions

The role of the oxide layer in the corrosion of aluminium in hydrochloric, sulphuric(VI) and orthophosphoric acid is studied at pH 1. The oxide layer is stable in hydrochloric and sulphuric acids. The corrosion rate of aluminium in these solutions is 0.2 and 0.3 mm/year, respectively. The oxide layer is unstable in orthophosphoric acid and the corrosion rate is higher, 1.75 mm/year. The corrosion process is studied using scanning electron microscopy, immersion and electrochemical techniques. The mechanism of corrosion of aluminium in the orthophosphoric acid solution is proposed. It includes the adsorption of a reaction intermediate on the surface of the corroding metal and the formation of the oxide layer at anodic potentials with respect to the open-circuit potential. It is also explained when the inductive loop appears on the electrochemical impedance spectra of aluminium corroding in orthophosphoric acid solution. The mechanism of aluminium corrosion in hydrochloric and sulphuric acid is more complex compared to orthophosphoric acid, and is not analysed in detail.