2024 is high in copper, 3.8-4.9 wt %, plus 0.5% max Fe.
These form intermetallics such as CuAl2 within the Al solid solution matrix. These intermetallics
1) interfere with the formation of protective oxide films, both native & from anodizing, as well as chemical conversion coatings.
2) Are cathodic to the Al matrix, thus causing galvanic corrosion in the presence of moisture.
One word Copper, the is in the form that allows it to become part of an electrolytic cell that causes the rapid corrosion of the Aluminum Matrix.
Though a little more involved than this it the primary mode of failure for 2024 by severe pitting. Though 2024 is used in some applications it usually clad with pure Al to prevent these cells from forming.
The Al 2024 has copper as the principal alloying element added for strength purposes. This alloy is used for aircraft frames and structures where their strength is valued. On the other hand, Al 6082 has Mg and Silicon as its principal alloying elements. The addition of Cu in Al 2024 reduces its corrosion resistance more than any other alloying elements. Therefore, Al 6082 can be considered as superior to Al 2024 in terms of corrosion resistance. If Al 2024 to be used in corrosive environment, one way around is to use it with clad of pure aluminium or Al-Mg-Si alloys.
