Aluminum-copper-magnesium alloys also containing manganese and iron (commercial designation 2024) are susceptible to marine corrosion and stress corrosion cracking. Susceptibility depends on heat treatment, and is thought to involve redistribution of copper from within the microstructure onto the surface of the corroding alloy, but few direct observations of the mechanism have been made. Copper can be distributed in several ways throughout the microstructure, which complicates mechanism studies. The primary age-hardening phase is Al2CuMg (orthorhombic; a=4.01, b=9.25, c=7.15 Å) which appears as more or less large equiaxed S particles at equilibrium, and as metastable S’ plates after aging for shorter times at lower temperature (∼190°C). In addition, ubiquitous so-called “dispersoids” containing copper and manganese (prolate spheroid morphology) or iron (irregular “blocky” morphology) do not go into solution when the alloy is solid state homogenized (-495 °C) and are always present in the microstructure. All of these phases are copper-rich sources for surface redistribution relative to the matrix during corrosion.