1.
When the surface of the iron is exposed to water droplets, the centre of the water droplets
undergoes the process of oxidation and is known as the anode.
2. The edge of the water droplets undergoes a process of reduction and is known as the cathode.
(The edge of the water droplets acts as the cathode because of the concentration of soluble
oxygen is higher on the edge of the water droplets than in the centre.)
3. At the anode, the metal iron undergoes oxidation to form the iron(II) ion with the loss of
electrons.
Fe → Fe2+ + 2e
4. Electrons that are free at the anode flow through the metal iron to the cathode area where
soluble oxygen in the water accepts electrons to form hydroxide ions.
O2 + 2H2O + 4e → 4OH-
5. The iron(II) ions are then combines with the hydroxide ion to form iron(II) hydroxide.
Fe2+ + OH- → Fe(OH)2
6. Iron(II) hydroxide is then oxidised by oxygen to form iron(III) hydroxide.
4Fe(OH)2 + 2H2O + O2 → 4Fe(OH)3
7. The iron(III) hydroxide is then decomposed to form hydrated iron(III) oxide, Fe2O3•xH2O by
oxygen in the air.
4Fe(OH)3 → Fe2O3•xH2O
8. The hydrated iron(III) oxide is brown in colour and is known as rust.
9. The overall equation for the rusting of iron is
4Fe + 3O2 +2xH2O → 2Fe2O3•xH2O
1. The rusting of iron can be made faster if the iron is in contact with a metal that is less
electropositive than it. Corrosion of metals such as this is known as electrochemical corrosion.
2. Electrochemical corrosion is a process where a metal corrodes through losing of electrons
(oxidation) to form cations in the presence of an electrolyte when the metal is in contact with
another metal that is less electropositive than it.
3. The rusting of iron is made slower if the iron is in contact with a metal that is more
electropositive than it.
4. The higher the difference of the electropositivity between the two metals, the faster the
corrosion of the metals that occurs on the metal which is more electropositive.
5. For example, iron rusts more easily when it pairs with copper metal than when it pairs with tin.
On the other hand, iron is less likely to rust when it pairs with magnesium than when it pairs
with zinc.
The use of a metal which is less electropositive
1. The plating of iron with a thin layer of a metal which is less electropositive such as tin, silver or
copper will prevent the iron underneath it to react with water and air, and so prevents the iron
from rusting.
2. However, the rusting of iron will occur faster if the protective layer is scratched. This is because
iron is more electropositive than tin, silver or copper. The plating of iron by tin is used a lot in
the making of tinned food.
The use of a metal which is more electropositive
1. Metals which are more electropositive are used as a sacrificial metal to prevent corrosion on
metals which are less electropositive. The metal which is more electropositive corrodes and acts
as the anode.
2. The less electropositive acts as the cathode and is protected from corroding. This method is
known as the cathode protection or electrochemical protection.
