Effects of long-term natural aging on the altered surface layer on an Al-Zn-Mg-Cu alloy and on corrosion properties

The microstructure evolution in the abrasion-induced altered surface layer (ASL) during long-term natural aging and its effect on localized corrosion in AA7055 were studied using scanning transmission electron microscopy and potentiodynamic polarization measurements. Surface abrasion of AA7055 in the peak-aged (T6) temper creates an ASL that is around 300 nm thick and contains ultrafine subgrains. Cu-rich particles including Al2Cu phase were also found to precipitate at the extreme surface and at subgrain boundaries in the ASL, as well as in the underlying bulk substrate close to the ASL on AA7055-T6. These large Cu-rich particles grew during 12 months of natural aging following surface abrasion, then stopped growing. Some of them then dissolved during the subsequent 48 months of natural aging. Zn solute enriched at the subgrain boundaries in the ASL, and was depleted in the matrix solid solution of the ASL after 60 months of natural aging, but depletion of Cu was not evident on AA7055-T6. The breakdown potentials of the ASL were 50 mV-80 mV higher for the abraded and naturally aged AA7055-T6 samples compared to the as-abraded condition, indicating that long-term natural aging improved the corrosion resistance of the ASL. The depletion of Zn in the matrix of the ASL and the increased matrix Cu content due to the dissolution of large Cu-rich particles might be responsible for the increase in breakdown potential of the ASL.