Local electrochemical studies of the microstructural corrosion of AlCu4Mg1 as-cast aluminium alloy and influence of applied strain

The microstructure of AlCu4Mg1 as-cast aluminium alloy was first determined by means of field emission-scanning electron microscope with an integrated electron dispersion spectrometer, secondary ion mass spectroscopy and atomic force microscopy. Large precipitates (Al₂Cu, Al–Si–Mn–Fe–Cu, oxides) were located at grain boundaries, whereas small particles (aluminium, magnesium and copper) were present in grains. The electrochemical response and pitting susceptibility of sites containing precipitates were then investigated after polishing using the electrochemical microcell technique. After straining, big scattering was observed in the electrochemical response. The most active places corresponded to the sites containing wide microcracks and severe damages in the matrix. In this case, the corrosion potential was around −1,000 mV vs. Ag/AgCl, and the current in the passive domain was five times higher than on the strained matrix. In the absence of severe damage in the matrix or wide microcracks, the corrosion potential was more anodic and the current density in the passive range was around 0.5 mA cm⁻². Local polarisation curves carried out in sites containing large precipitates and no defects induced by straining were very close to those obtained in grains far from precipitates.