[goldenkey]

Aluminum is very oxidation resistant once the initial thin oxide layer forms. I doubt that is the issue.

I would guess the reason is similar and would be weathering/erosion due to being outside and pelted with sand/rain/etc.

[Cthulhu_]

Yeah I was going to say, repeated rain (which is slightly acidic) and the like will keep pelting it. I mean the lifetime can probably be extended if they store it inside a building, but that's a lot of building space.