The first point to emphasise is that the resistance of aluminium and aluminium alloys to normal, ambient environmental corrosion is excellent. Its natural corrosion resistance is one reason why aluminium alloys find such diverse applications, from beverage cans, via architectural uses to aircraft construction.
The chief source of the protection to corrosion, comes from the adherent, self-healing oxide film which is always present in an ambient air atmosphere.
Although aluminium is reactive, the oxide film that forms on it is inert and protects it from further oxidation. The oxide film is about 2.5nm thick on fresh rolled sheet and it builds up over several years to some 10-20nm (1nm = 10-9 m) in thickness. There are two important points to remember about corrosion:
1. It can cost money
2. Component may no longer be able to perform.