Investigation of the oxide layers formed on a recycled high pressure die cast aluminium alloy: polished and as-cast surface conditions

The increased presence and accumulation of elements such as Cu and Fe can negatively impact the corrosion resistance of the recycled cast Al-Si alloys, and the corrosion resistance is strongly dependent on the surface oxide film formed on the surface of the alloys. In this study, the surface oxide film for two different surface conditions, as-cast and mechanically polished surfaces, of a recycled Al-Si alloy (EN-AC 46000) were analysed by using in situ synchrotron ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). Moreover, the microstructure and surface morphology were characterized by confocal microscopy and scanning electron microscope with energy-dispersive spectroscopy (EDS), and corrosion performance of the two surface conditions was evaluated by electrochemical potentiodynamic polarization measurements in 0.01 M NaCl solution. The synchrotron AP-XPS measurement was performed for each surface condition, in vacuum, at ambient pressure and after exposure to NaCl solution. The as-cast and polished Al-Si alloy possess vastly different heterogeneous microstructure, while the as-cast surface condition exhibits a slightly lower corrosion potential. The AP-XPS results show that Al- and Si-oxides and hydroxides are constituting the surface film for both surface conditions, with additional Mg-oxides and hydroxides observed for the as-cast condition. However, no Cu or Fe in the top surface layer could be detected by AP-XPS, although they are present in the alloy.