Local dissolution phenomena associated with S phase (Al2CuMg) particles in aluminum alloy 2024-T3

Second-phase particles in Al-4.4Cu-15Mg-0.6 Mn (2024-T3) were characterized by size and chemistry using scanning electron microscopy and associated electron-beam microanalysis methods. It was found that approximately 60% of par tides greater than about 0.5 to 0.7 mu m were Al2CuMg (the S phase). This fraction corresponded to 2.7% of the total surface area. S phase particles appeared to be active with respect to the matrix phase, consistent with open-circuit potentials reported in the literature for Al2CuMg. The compound exhibited severe dealloying which resulted in the formation of Cu-rich particle remnants. Some particle remnants remained largely intact and induced pitting at their periphery once ennobled by dealloying. Other particle remnants decomposed into 10 to 100 nm Cu clusters that became detached from the alloy surface and were dispersed by mechanical action of growing corrosion product or solution movement. This observation suggests that nonfaradaic liberation of Cu from corroding 2024-T3 surfaces is possible, and provides one plausible explanation for how Cu can be redistributed across the surface by a pitting process which occurs at potentials that are hundreds of millivolts negative of the reduction potential for Cu.