The biggest cause of corrosion on aluminum is galvanic action.

If you put a piece of a aluminum on a steel bench or against almost any other metal in moist conditions then a small current will be generated between teh two metals. over time this causes a gradual erosion of the aluminum.

Also, different alloys have different levels of resistance to corrosion.

Pure aluminum (1000 series) has excellent resistance, as does the 5000 series (often used in boats) but some - like 7000 and 6000 series are alloyed with more reactive metals (zinc and silicon) so they will corrode more rapidly (albeit still *much* more slowly than steel.