Exfoliation corrosion of an extruded low-Cu-containing AA7003-T6 profile with a partially recrystallised grain structure

The effect of a partially recrystallised grain structure on the exfoliation corrosion susceptibility of a peak aged (T6) extruded low-Cu-containing AA7003 profile was evaluated using the ASTM G34 standard test method coupled with a metallographic examination. Both the as-extruded (recrystallised) and milled (non-recrystallised) starting surfaces were assigned the same ASTM G34 visual classification of EB (moderate exfoliation corrosion) after 48 h immersion. Damage initiated on the as-extruded starting surface as pitting corrosion in the recrystallised coarse-grained surface layer and propagated as exfoliation corrosion in the non-recrystallised fibrous interior once the surface layer was penetrated. A complementary scanning transmission electron microscopy (STEM) examination suggested that there was not likely a marked difference in the distribution of Mg, Zn and Cu across the precipitation free zone (PFZ) and grain-boundary precipitates between the recrystallised and non-recrystallised grain structures. This indicates that the transition from pitting corrosion to exfoliation corrosion in the peak aged AA7003 alloy was controlled more so by the variation in some other microstructure parameter across the grain-boundary region and less so by the variation in the microchemistry (Cu content in particular).