Role of intermetallic phases in localized corrosion of AA5083

Abstract

The presence of intermetallic inclusions very often plays a crucial role for the susceptibility of different aluminium alloys to localized corrosion attack. The intimate details of localized corrosion of 5083 aluminium alloy have been studied in the present work. Local techniques such as scanning Kelvin probe force microscopy, in situ atomic force microscopy, scanning electron microscopy coupled with energy dispersive spectroscopy were used to investigate the mechanisms and the kinetics of localized corrosion attack. The importance of iron-rich and Mg 2Si intermetallic phases in the initiation of corrosion processes is demonstrated in the paper. The Mg 2Si phase has a potential lower relatively to the matrix. Moreover, the high reactivity of magnesium leads to the dissolution and consequently to the fast dealloying of these intermetallics. However, hydroxide (Mg(OH) 2 and SiO 2· nH 2O) deposits formed during corrosion act as an additional diffusion barrier hindering the deep propagation of pits. The iron containing intermetallics have the potential higher with respect to the aluminum matrix playing the role of effective cathodic centers for oxygen reduction causing anodic polarization and pitting in the surrounding alloy matrix. Dealloying of such intermetallics with subsequent iron enrichment was also revealed. Pitting initiation seems to be statistical and independent of the composition of Fe-rich intermetallics. However, the active growth of the pits prevents initiation of localized corrosion attack in nearby sites. A new pit can start to grow only when a neighbor one becomes passivated.