Effect of grain boundary characteristics on intergranular corrosion resistance of 6061 aluminum alloy extrusion

The intergranular corrosion (IGC) behavior of 6061 aluminum alloy extrusions was investigated. After the IGC test in accordance with ISO/DIS 11846 (method B), heavy IGC was observed at the surface of the extrusion. However, little IGC occurred at the center plane of the extrusion thickness. It was considered that IGC was caused by the existence of precipitate-free zones (PFZs) because PFZs were clearly observed in the surface layer of the extrusion but were not clearly observed in the center position of the extrusion thickness. Furthermore, it was considered that the formation of PFZs was associated with the grain boundary characteristics. That is, most of the grain boundaries had random high angles in the surface layer of the extrusion, but 60 pct of the boundaries were lower than 0.26 rad (15 deg) in the center plane of the thickness. To verify this hypothesis, the center plane was cold rolled, recrystallized, and examined using the IGC test. As a result, heavy IGC was observed, while the center plane, in this case, had almost all random high-angle boundaries.