If elemental aluminum is exposed to the air, immediately. However, once this oxide forms it protects the rest of the metal quite well and any further corrosion is extremely slow.
Aluminum "rust", or oxidation, is not the red kind found on iron and steel. In fact, aluminum oxidation occurs extremely easily but is not a much different color, nor does it build up over time like...
The water dissolves the oxide coating that normally forms and protects the metal from further damage. This exposes more of it to be corroded which in turn is dissolved. The cycle repeats and so it...
Aluminum, like all metal, errode. The process is called oxidation, because oxygen is what pulls electrons slowly off the metal, that causes rust. Aluminum can errode but many people don't consider it...