Abstract
Most commercial aluminum alloys, when heat treated in air at 600°C, become activated anodically in chloride solution as a result of segregation of a nanometer size Pb-rich film between the aluminum matrix and thermally-formed γAl2O3. However, certain recycled alloys, such as AA8006, can be activated at lower annealing temperatures. The purpose of this work was to explore the cause of low temperature activation. Alloy 8006 rolled samples in the fully-annealed T0 temper were used in the as-received condition and also mechanically polished to remove the grain-refined surface layer (GRSL) and subsequently annealed for 1 h at 450°C, at which maximum activation was attained. FEG-TEM analysis of cross-sectional sample foils and XPS depth profiling of the polished samples showed the presence of a nanometer size film at the metal-oxide interface, which was enriched in metallic Pb and Sn and which lead to the low temperature activation of the alloy. Pb and Sn segregation was also detected along the grain boundaries of the GRSL on the as-received samples, as interface segregant associated with oxide particles incorporated during hot rolling. Nanofilm segregation of Pb and Sn, and consequently activation, did not occur on the GRSL-free samples prepared by annealing in O2-free environment.
