Abstract
AlMgSi (6000-series) aluminium alloys are generally known to be re-sistant to intergranular corrosion (IGC). However, unfavourable alloy-ing and thermomechanical history may introduce susceptibility to IGC. A model alloy in the 6000-series, with 0.2 wt% Cu, was studied using an accelerated corrosion test. In the naturally aged T4 temper, the ma-terial did not exhibit localized corrosion, while in the underaged state (below peak strength), the alloy became highly susceptible to IGC. Samples aged to peak strength T6 temper or overaged were nearly re-sistant to IGC, exhibiting only shallow, localized IGC and pitting. TEM investigation of the IGC-susceptible material revealed nanosize AlMgSiCu particles along the grain boundaries, along with Cu-enriched nanofilm connecting the particles, indicating the cause of sharp IGC propagation. Further ageing caused coarsening of the grain boundary film thereby destroying its continuity and giving increased resistance against IGC.
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