Both iron and aluminium combine slowly with oxygen at room temperature. Why is this reaction a problem for iron but not for aluminium?

Explanation

Aluminium, on reacting with oxygen forms an impervious coat of `Al_2O_3` which protects the metal from further corrosion. In contrast, iron forms iron rust `(Fe_2O_3.xH_2O)` which flakes off exposing fresh surface to corrosion.