Corrosion is a chemical process resulting the progressive destruction of a metal by the interaction of the local environment.    The process results in the eventual failure of the metal with resultant high additional lifetime costs.  

Three factors required involved in the corrosion of iron and steel are moisture, oxygen and a potential difference between adjacent areas on/in the surface under consideration.   Partially rusted metal in a moist atmosphere satisfies these conditions.   The corrosion may be uniformly distributed over the surface or highly localised (pitting).   It may also take place as a result of contact between the steel and another metal (bi-metallic corrosion).    The basic mechanism in these types of corrosion is electrochemical and involves the passage of electrons from the area of high to lower electron density(negative to positive).

On partially rusted steel in moist atmosphere a large number of small corrosion cells are set up.    In these the iron forms the anode and the rust or scale the cathode.  At the anode, atoms of iron go into solution as ferrous ions at the same time liberating electrons.

Fe -> Fe²⁺ + 2 e^- (Anode reaction).

The electrons confer a negative charge on the iron but immediately flow away to the less negative cathode areas (rust or scale).   Here they react with water and oxygen to form hydroxyl ions.

2e^- + Fe + H₂O + ½ O₂ -> 2 O

H^- (Cathode reaction).

The ferrous and hydroxyl ions react together in the surface moisture to form ferrous hydroxyl which in turns becomes oxidised to hydrated ferric oxide (rust).   This rust is formed away from the surface of the anode and offers no protection .    This process will continue as long as a there is moisture and oxygen available.   The rust products have a higher volume than the metal and the rust products therefore tend destroy any protective surface which is in place.

In the presence of dissolved salts e.g. sodium chloride the process is accelerated but the result is the same.

In an industrial environment there will be sulphur dioxide present and this will react with iron to form ferrous sulphate .   If the humidity is high the ferrous sulphate reacts with water and oxygen to form hydrated iron oxide (rust) and liberates sulphuric acid which regenerates ferrous sulphate.