Corrosion is the decomposition of metals and alloys. This may e.g. involve air corroding the metal. Metals differ with regard to how corrosion resistant they are. Oxygen accelerates the corrosion of sodium, but less so when it comes to magnesium, aluminium, iron and copper.
This difference is based on how good the metals are at forming a protective seal of oxides, impeding the corrosion process. In dry air iron is not subjected to corrosion due to an invisible iron oxide seal, whereas humid air accelerates the process of corrosion.
Aluminium has a strong affinity to oxygen, but quickly develops a 0.02-1 μm thick Al2O3 seal and therefore does not corrode. Humid air accelerates the process of corrosion in most metals.
Iron protects itself by producing rust, while aluminium protects itself by oxidising. Natural oxidation gives aluminium a dull, grey look. Different surface treatments may be used to prevent this process, including powder lacquering, wet lacquering, anodising, chromium plating or metallising.