Intergranular corrosion behaviour of friction stir welded joints of 6005A aluminium alloy

The intergranular corrosion (IGC) behaviour of friction stir welded 6005A aluminium alloy was investigated. The results show that the base material(BM) is the most susceptible to IGC and the heat affected zone (HAZ) takes the second place,while the nugget zone (NZ) and thermo-mechanically affected zone (TMAZ) show high resistance to IGC. The corrosion behaviour in different zones could be well explained from microstructural observation by means of field emission scanning electron microscope (FE-SEM) and high-resolution transmission electron microscopy (HRTEM). IGC in the BM was caused by two microgalvanic couplings of precipitate free zone (PFZ,anode)/cathodic precipitates (Q and Si phases) and the PFZ (anode)/matrix. The coarsening of the intergranular precipitates as well as the precipitation of the intragranular Q' phases in the HAZ reduced the microgalvanic driving force causing IGC and introduced the pitting corrosion. The relatively homogeneous microstructure both in NZ and TMAZ restrained the occurrence of IGC.