Intergranular corrosion and grain boundary microstructure of extruded AA6005 aluminium

AlMgSi aluminium alloys (AA6XXX series) are normally resistant to intergranular corrosion (IGC). However, the material can become susceptible as a result of an undesirable combination of alloying and processing conditions. IGC of Extruded AA6005 alloy with varying tempers was investigated and compared by use of accelerated and outdoor exposure tests. Corrosion of alloy AA6005 depended on the cooling rate right after the extrusion process. Extrusions quenched in water did not suffer appreciable IGC, while those cooled in air were susceptible to IGC. Subsequent aging to the T6 condition decreased the susceptibility to IGC on air cooled specimen, while it increased the susceptibility on water quenched specimen. SEM and TEM investigations revealed the presence of precipitates along the grain boundaries of air-cooled AA6005 samples. These particles were identified as the Q-phase, containing Al, Mg, Si and Cu. Microgalvanic coupling between the particles and the depleted zone is believed to be the cause of IGC on this alloy. Quenched samples did not exhibit grain boundary precipitates. Grain boundary precipitation of Si or a MgSi phase occurred as a result of subsequent artificial aging heat treatment. This introduced an active depleted zone, which is assumed to cause IGC.