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Strictly speaking, rust is a reaction between iron and oxygen that’s catalyzed by moisture. The end product is an iron oxide that’s red. Aluminum doesn’t undergo this reaction and hence won’t ever rust.

As an aside, I took this picture in a store in California (USA) in 2018. It’s a shower caddy made of aluminum. The packaging proudly proclaims this part will never rust. Well, of course it won’t, because it’s made of aluminum! And while they’re at it, their marketing department may as well also state this product is gluten free and low cholesterol. That wouldn’t be a lie either.Having dispensed with the technicalities, I think the question is really asking what causes aluminum to corrode. Rust is one form of corrosion germane to iron but not to aluminum. However, aluminum does corrode by other means.

Aluminum intrinsically forms a protective oxide. As long as this oxide isn’t breached, aluminum is protected against corrosion, which accounts for its robustness in different atmospheres and fresh waters.[1] Thus, many of the modes of corrosion aluminum is susceptible to involve failure of its protective oxide. The oxide is stable for pH’s from 4.0 to 9.0;[2] thus, either nitric acid with a pH of 1 or ammonium hydroxide with a pH of 13 can induce corrosion in aluminum, for example.