Effect of altered surface layer on localized corrosion of aluminum alloy 2024

The surface microstructure and localized corrosion behavior of AA2024 with T3 and overaged tempers in NaCl solution were studied using transmission electron microscopy and polarization measurements. After surface abrasion and polishing, an altered surface layer (ASL) containing ultrafine subgrains was observed on both samples. For the overaged sample, the pre-existing aging-induced S precipitates dissolved in the ASL during or after surface abrasion. The solute elements Cu and Mg from these precipitates went into the solid solution of the ASL. A new phase containing high contents of Al and Cu but no Mg formed at the subgrain boundaries in the ASL and at the grain boundaries in the region just below the ASL. Due to the higher Cu content in the matrix of the ASL than in the underlying substrate, the ASL exhibited slightly less susceptibility to pitting corrosion than the underlying substrate, which was revealed by retesting after etching away the ASL. For AA2024-T3, the susceptibility gap between the ASL and underlying substrate was so small that it could not be resolved by the polarization measurements. Unlike many other Al alloys, the ASL on the as-abraded AA2024 with T3 and overaged tempers did not dissolve preferentially during potentiodynamic polarization because of its protective nature.