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 characte rized by size and chemistry using scanning electron microscopy and ass ociated electron-beam microanalysis methods. It was found that approxi mately 60% of par tides greater than about 0.5 to 0.7 mu m were Al2CuM g (the S phase). This fraction corresponded to 2.7% of the total surfa ce area. S phase particles appeared to be active with respect to the m atrix phase, consistent with open-circuit potentials reported in the l iterature 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 peripher y 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 o r solution movement. This observation suggests that nonfaradaic libera tion of Cu from corroding 2024-T3 surfaces is possible, and provides o ne plausible explanation for how Cu can be redistributed across the su rface by a pitting process which occurs at potentials that are hundred s of millivolts negative of the reduction potential for Cu.