The role of silicon alloying addition on the pitting corrosion resistance of an Al-12 wt.%Si alloy

The electrochemical behavior of Al-12 wt.%Si in neutral and slightly alkaline solutions was compared to that of pure Al in order to explore the effect of Si on the properties of the passive film. The film growth kinetics for the Al-Si alloy was slower than that corresponding to pure Al and XPS depth profile suggested that the oxide film was thinner. Potentiodynamic polarization measurements in borate solution with addition of 0.01 M NaCl revealed that Si has a beneficial effect on the pitting resistance of the passive film. The Mott-Schottky analysis indicated an n-type semiconductor behavior and that less noble flat-band potential for the alloy corresponded with higher pitting potential. XPS results suggested that the barrier layer on the alloy contains SiO₂ particles distributed within the Al₂O ₃/AlOOH film. The protectivity of the Al-12%Si alloy was attributed in part to its ability to retard the adsorption of chloride ion, and also because silicon oxide helps to block entry sites and restricts the transport of chloride ions through the passive film.