Intergranular Corrosion of Copper-Containing AA6xxx AlMgSi Aluminum Alloys

AlMgSi (AA6xxx-series) aluminum alloys are generally resistant to intergranular corrosion (IGC). However, copper may introduce susceptibility to IGC; its role was investigated by using model alloys with 0.02, 0.18, and 0.7 wt % Cu. The lowest copper-containing alloy was resistant to IGC in accelerated corrosion testing. The 0.18 wt % copper alloy showed superficial etching in the naturally aged condition and was highly susceptible to IGC in the underaged temper, but was only slightly susceptible in the peak aged or overaged condition. High-resolution field emission scanning electron microscopy imaging showed no visible grain boundary precipitation in the T4 and underaged tempers, whereas the T6 and overaged tempers had grain boundaries decorated with Cu-containing precipitates. Field emission transmission electron microscopy investigation of the underaged material showed a copper-enriched grain boundary layer and an adjacent copper-depleted zone. The reduced susceptibility to IGC upon extended artificial aging was attributed to the consumption of the copper-rich grain boundary film by the growth of grain boundary precipitates.