In situ confocal laser scanning microscopy of AA 2024-T3 corrosion metrology - I. Localized corrosion of particles

The morphology of attack at and around the intermetallic compounds (IMC) present on bare AA 2024-T3 was studied in situ using confocal laser scanning microscopy. Exposures were conducted in 0.1 M Na2SO4 + 0.005 M NaCl at pH 3, 6, and 10 as well as near-neutral 0.5 M NaCl. The types of attack observed could be categorized as matrix and IMC pitting, trenching adjacent to IMC, and matrix etching. The electrochemical behavior of bulk synthesized Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic compounds as well as that of AA2024-T3 was used to rationalize the observed attack metrology. The galvanic coupling between the AA2024-T3 matrix and the intermetallic particles controlled the attack rates. In Al-Cu-Mg, the strong polarization to the open-circuit potential of the alloy caused rapid dissolution (ca. 10 mA/cm(2)), whereas for the Al-Cu-Fe-Mn the dissolution rates were on the order of 100 mA/cm(2). The limited dissolution rates of the Al-Cu-Fe-Mn phase were due to the cathodic polarization of these particles by the matrix under open-circuit conditions. Several pits were initiated at large Al-Cu-Mg particles. These pits were stable within the Al-Cu-Mg phase, but could not form stable pits in the alloy matrix during open-circuit corrosion. Calculation of growth rates and pit stability products for the individual IMC emphasized the role of metastable pitting in the observed corrosion metrology, which developed on AA2024-T3 during open-circuit corrosion. (C) 2004 The Electrochemical Society.