What's difference between iron rust and protective film of aluminum?
Generally speaking, iron won't rust in dry air. But there's always vapour in the air. When vapour is absorbed by the surface of iron, electrochemical corrosion happens. The texture of iron rust generated thereby is loose which makes it easy to absorb more moisture and cause corrosion. Aluminum has good corrosion resistance. The main reason is that aluminum reacts with the oxygen in the air and generates aluminium oxide forming a layer of protective film to prevent the aluminium from further oxidation.
The iron rust is red. Aluminium is ligher than iron but it's easy to react with salt and vinegar and separate out toxin.