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To better understand particle-induced pitting corrosion in aluminum alloys, thin foil specimens of 7075-T6 and 2024-T3 aluminum alloys, with identified constituent particles, were immersed in aerated 0.5M NaCl solution and then examined by transmission electron microscopy (TEM). The results clearly showed matrix dissolution around the iron-and manganese-containing particles (such as Al 23 CuFe 4 ), as well as the Al 2 Cu particles. While Al 2 CuMg particles tended to dissolve relative to the matrix, limited local dissolution of the matrix was also observed around these particles. These results are consistent with scanning electron microscopy (SEM) observations of pitting corrosion and are discussed in terms of the electrochemical characteristics of the particles and the matrix.

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To better understand the role of constituent particles in pitting corrosion, analytical electron microscopic studies were performed on the constituent particles in commercial 7075-T6 and 2024-T3 alloys. Five phases, namely, Al 23 CuFe 4 and amorphous SiO 2 in 7075-T6 and Al 2 CuMg, Al 2 Cu, and (Fe,Mn) x Si(Al,Cu) y in 2024-T3, were identified. The crystal structure and chemistry of the Al 23 CuFe 4 , Al 2 CuMg, and Al 2 Cu phases in these alloys are in good agreement with the published data. Small deviations from their stoichiometric compositions were observed and are attributed to the influence of alloy composition on the phase chemistry. For the (Fe,Mn) x Si(Al,Cu) y (approximately, x ϭ 3 and y ϭ 11) phase, a rhombohedral structure, with lattice parameter a ϭ b ϭ c ϭ 1.598 nm and ␣ ϭ ␤ ϭ ␥ ϭ 75 deg, was identified and is believed to be a modified form of either Al 8 Fe 2 Si or Al 10 Mn 3 Si. Information from this study provided technical support for studying the electrochemical interactions between the individual particles (or phases) and the matrix. The corrosion results are reported in a companion article.

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