Comparison Between the Pitting corrosion of Pure Al and Al-Si Alloys in Deaerated Neutral Sodium Perchlorate Solutions and the Effects of some Inorganic Inhibitor

This paper reports the results of potentiodynamic polarization and chronopotentiometry potential/time measurements on the pitting corrosion of pure Al and three Al-Si alloys, namely (Al + 6% Si), (Al + 12% Si), and (Al + 18% Si) in deaerated neutral sodium perchlorate solutions, complemented by ex situ energy dispersive X-ray (EDX) examinations of the electrode surface. In all cases, the potentiodynamic anodic polarization curves do not exhibit an active dissolution region due to spontaneous passivation. The passivity is due to the presence of a thin film of Al2O3 on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by C104 ions. Polarization measurements showed that the breakdown potential (Eb) decreases with increase in C104 concentration, while it increases with increasing %Si in the alloy. 'The resistance of the four tested samples towards pitting corrosion increases in the order: Al < (Al + 6% Si) < (Al + 12% Si) < (Al + 18% Si). Chronopotentiometry potential/time measurements show that the incubation time for pitting initiation decreases with increasing C104 concentration and applied anodic current density, while it increases with increasing Si content in the alloy. Addition of W042- and Mo042-, as inorganic inhibitor anions, to the perchlorate solution inhibits pitting corrosion to an extent depending on the concentration and type of the added inhibitor.