The corrosion of metals may be considered as the material returning to its original form as a metal oxide. However some metals corrode more rapidly than others in the same environment. Iron ore for example is an oxideofiron that is converted into the steels and irons used in engineering. If conditions are correct for moisture, acids, salts etc.- the tendency is for the material to revert back to an oxide of iron by combination with oxygen. An oxide is an element combined with oxygen, hence oxygen must be present for the transformation to take place.) Following a reaction with the oxygen in the atmosphere, some metals form a protective oxide film upon their surface which can prevent any further corrosion taking place. If this film is broken destroyed it can or in the case of certain metals be replaced very rapidly. For example, chromium, which is used in the alloy stainless st can form a microscopic film of chromium oxide upon the surface of the steel which further corrosion. Furthermore aluminium prevents which corrodes very rapidly is quickly rendered non-corrosive owing to the passive oxide film which forms.