Corrosion, Rusting and How to Fight it.
Cairney McAteer
What is Corrosion
• Corrosion is the changing of
the surface of the metal from
an element into a compound.
Corrosion is an example of
oxidation because it involves a
loss of electrons.
• Fe ----------> Fe2+ + 2e•
Rusting
• Rusting is the special name given to the
corrosion of iron. As iron, in the form of
steel, is the most commonly used metal in
the world, the corrosion of iron is
important.
Speeding up Rusting
• Rusting can be sped up by the use of
sulphur dioxide which causes acid
rain.
• The quantity of air and water affects
the speed.
Iron and Other Metals
•
If a nail and a carbon rod are connected by a wire and dipped into
salt solution containing ferroxyl indicator, a blue colour quickly
appears around the nail, and a pink colour around the carbon rod.
This means that electrons are flowing from the nail to the carbon.
When electrons flow from iron, it rusts.
• In Topic 10 it was found that metals higher in the electrochemical
series could push electrons onto metals lower in the
electrochemical series.
Magnesium stops iron rusting, while copper makes iron rust quicker.
• Metals that push electrons onto iron stop rusting, but metals that
let electrons flow from iron increase the speed of rusting.
Cathodic Protection
• Iron has to lose electrons in order to
rust.
The negative terminal of the battery
is pushing electrons onto this nail and
this prevents this nail from losing any
electrons. This nail cannot rust.
Electrons flowing to the nail stop
rusting.
Galvanising
• Galvanising is when galvanised iron in
made by dipping iron into molten zinc
which coats the iron with zinc. It is
used to protect dustbins, car
exhausts and special nails.
Sacrificial Protection
• Sacrificial protection is when a more
reactive metal sacrifices itself to
protect the less reactive metal in the
electrochemical series.
Ways to Protect Iron
• painting e.g. the Forth Rail Bridge.
• greasing or oiling - protects moving
parts of machinery.
• coating with plastic - dish drainers
have a metal core and a plastic
coating.
• coating with other metals such as tin,
zinc, silver, gold.
Rusting
• Corrosion is an example of oxidation because it
involves a loss of electrons.
• Fe ----------> Fe2+ + 2e• The rusting process continues when iron(II) ions
lose another electron to form iron(III) ions.
• Fe2+ ----------> Fe3+ + e• Oxygen and water accept the electrons lost by the
iron.
• 2H2O + O2 + 4e- ----------> 4OH-