Electrochemical investigation of rolled-in subsurface layers in commercially pure aluminium alloys with the micro-capillary cell technique

Rolling alters the surface properties of aluminium alloys and introduces disturbed oxide-rich subsurface layers. The presence of these layers alters the electrochemical characteristics of the aluminium surface and has an influence on its corrosion behaviour. In order to reach a better understanding of these rolled-in layers, the electrochemical behaviour of areas in the (sub)surface with and without these layers was characterised with a local electrochemical technique: the micro-capillary cell. The effect of these layers on pit initiation in a chloride containing environment was investigated by performing potentiodynamic polarisation measurements over areas including and excluding rolled-in layers and deriving the respective local breakdown potentials. GDOES was used as sample preparation method, which enabled us to compare the electrochemical behaviour at depths of 0.5 μm and 10 μm to the as-received surface. SEM/EDX+BEI was used to characterise the surface before and after the micro-electrochemical measurements. Differences in breakdown potential, as well as differences in local attack have been observed for regions including and excluding rolled-in oxides. It was concluded that areas containing rolled-in oxides exhibit lower breakdown potentials, thus indicating these areas to be more susceptible to localised attack.