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
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
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
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
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