If we can put it this way: aluminium reacts in a really smart way. It protects itself from corrosion by reacting with the oxygen in the air to form a natural oxide layer. This layer coats the material like a protective shield, protecting it from environmental influences. Therefore, unlike iron,
aluminium does not rust, but oxidizes. If this oxide layer is scratched away, a new surface immediately forms again. In addition, aluminium oxide is almost the same colour as the aluminium itself, and is therefore not noticeable.