Only aluminum alloys that contain appreciable amounts of soluble alloying elements (copper, magnesium, silicon, and zinc) are susceptible to SCC. There is general agreement that the electrochemical theory of SCC describes the dominant corrosion mechanism of SCC for aluminum alloys.
SCC cracking in Al alloys is characteristically intergranular. According to the electrochemistry theory, this requires a condition along grain bourndaries that makes them anodic to the rest of the microstructure so that corrosion can propagate selectively along them. Such a condition is produced by the localized decomposition of solid solution, with a high degree of continuity of decomposition products, along the grain boundaries. The most anodic regions may be either the boundaries themselves (most commonly the precipitate formed in them) or regions adjoining the boundaries that have been depleted of solute.
Alloy 7075 contains a large amount of solute atoms-- 5.6% Zn, 2.5% Mg, and 1.6% Cu. After T6 heat treating, many precipitates are formed, especially along the grain boundaries, including CuMgAl2. This intermetallic phase is anodic to the grains, which allows for selective corrosion along the grain boundaries.
In order to improve the SCC resistance of 7xxx series alloys containing Cu, the T73 and T76 tempers were developed. These tempers require a two-stage artificial aging treatment, the second stage of which is performed at higher temperatures than the standard T6 treatment. The first stage nucleates a fine, high-density precipitation dispersion, which results in high strength. The second treatment develops SCC resistance, because instead of Guinier-Preston (GP) zones, which is the CuMgAl2 phase, the metastable transition form of MgZn2 (called the eta prime phase) forms. This phase is more resistant to intergranular corrosion that causes SCC.
