Passivity and pitting corrosion of 3003 aluminum alloy in ethylene glycol-water solutions

Aluminum (Al) alloys of 3xxx series, due to their favorable strength-to-weight property, high thermal conductivity and excellent formability, have been widely used in automobile heat exchange system. Ethylene glycol has been a heat coolant in chillers because of its low cost and excellent freeze and heat-protection over a wide temperature range. However, coolant in the automotive heat exchanger is always contaminated with chloride ions, resulting in pitting corrosion of Al alloy in an even fully inhibited coolant. In this work, passivity and pitting corrosion behavior of 3003 Al alloy was investigated in chloride-containing ethylene glycol-water solutions by various electrochemical and micro-electrochemical measurements, Mott-Schottky analysis and surface analysis techniques. Results demonstrated that the passive film formed on the Al alloy electrode is complex, containing both Al oxide and Al-alcohol film. The presence of Al-alcohol film enhances the stability of the passivated Al alloy, resulting in the film to show a p-type semiconductor behavior. Chloride ions would cause the initiation of corrosion pits, which are always associated with the second phase particles contained in 3003 Al alloy. An enrichment of Mn in the second phase particles forms a galvanic cell effect relative to the adjacent Al alloy substrate. The initiation of pitting corrosion is attributed to the local dissolution of Al alloy substrate around the second phase particles. When a sufficient amount of Al is dissolved away, the second phase particles drop off from the substrate, forming large pitting cavities that are usually linked each other. Furthermore, the effects of coolant chemistry on corrosion and pitting corrosion of 3003 Al alloy were determined.