Relation between microstructural aspects of AA2024 and its corrosion behaviour investigated using AFM scanning potential technique

The microstructure of the aluminium alloys has a large influence on the corrosion behaviour. In particular, the intermetallic particles play an important role in the corrosion resistance of aluminium alloys and they can give rise to localized corrosion, such as pitting and exfoliation, because of the formation of galvanic cells. The shape, size and chemical composition of the intermetallic particles are determined by the processing route (heat treatment and forming) carried out on the aluminium alloy. In recent work in our lab, it was found [9] that when the 2024 aluminium alloy undergoes a long quench delay time (time spent in air after the homogenisation treatment) shell-shaped particles precipitate. These are characterized by differences in chemical composition between the core and the surface. Simultaneously, the corrosion mechanism of the AA2024 changes from general to localized. Therefore, it was thought that the shell-shaped particles increase the pitting susceptibility of the 2024 aluminium alloy. In the present work, the surface potential of these particles was measured by means of atomic force microscopy (AFM) scanning potential technique. The shell-shaped particles show a large difference in potential between the core and the shell, which results in a highly localized galvanic coupling. Therefore, the formation of shell-shaped particles largely lowers the resistance to pitting attack of the AA2024.