Effect of thermomechanical history on intergranular corrosion of extruded AlMgSi(Cu) model alloy

The effect of cooling rate after extrusion, solution heat treatment, mechanical deformation and aging on intergranular corrosion (IGC) of two model AlMgSi alloys with different Cu content was investigated using an accelerated corrosion test. The low Cu extrusions (0.02 wt%) were resistant to IGC while the high Cu (0.17 wt%) variants were susceptible in certain tempers. Slow cooling in air introduced IGC. The IGC susceptibility was reduced and finally removed by artificial aging. Water quenching prevented IGC, but severe IGC was introduced by slight aging (underaging). Peak aging (T6) generally reduced IGC susceptibility regardless of cooling media. Overaging further reduced IGC, usually at the expense of introducing pitting. IGC susceptibility was related to the microgalvanic coupling between enriched Cu on the grain boundaries (noble) and the adjacent solute depleted zone (active). Cu enrichment was caused either by slow cooling in air or slight aging of water quenched samples. High IGC resistance was obtained either by keeping the Cu content low or by applying proper heat treatment to high Cu samples. For the present high Cu samples, artificial aging to peak strength provided at the same time the necessary IGC resistance.