Influence of Ageing on the Intergranular Corrosion of an Al–Mg–Si Alloy

Abstract

This report investigates the influence of ageing on the susceptibility to intergranular corrosion (IGC) of an Al–Mg–Si alloy. The selected AA6063 Al-alloy has been exposed to ageing treatments at a predetermine temperature of 423 K for durations of 1 to 168 h to achieve the wide range of ageing states covering under- to peak- to over-aged. The ageing behaviour of the selected alloy has been determined based on the results of hardness and tensile properties. The susceptibility to IGC has been evaluated following the British standard (BS ISO-11846) method B and assessed based on the average and maximum corrosion depths apart from in-depth surface and sub-surface characterizations of corroded specimens. It has been observed that the depths of corrosion increase monotonically with increase in ageing time. With progress of artificial ageing, the dominant mode of corrosion gradually changes in the order of minor pitting, moderate pitting, pitting plus localized IGC, localized IGC and uniform IGC. The phenomenon of preferential dissolution of Mg from the β″ (Mg5Si6) and β (Mg2Si) precipitates as well as the existence of Fe-rich intermetallic particles at the grain boundaries cause the pitting and intergranular corrosion because of the formation of local galvanic cells between the lower corrosion potential of Mg-depleted precipitates or precipitate free zones and to the adjacent anodic grain boundary precipitates.