Rusting of Iron
Specification Point 5.3 C:
Explain how rusting of iron can be prevented by:
a) exclusion of oxygen
b) exclusion of water
c) sacrificial protection
- Rusting is a chemical reaction between iron, water and oxygen to form the compound iron (III) oxide.
- Oxygen and water must be present for rust to occur:
4Fe + 3O2 + xH2O → 2Fe2O3.xH2O
- Therefore any barrier method that excludes oxygen or water from getting into contact with the surface of iron will effectively prevent rusting.
Experiment to show that O2 and H2O are needed for rusting: only the nail on the left rusts
Exclusion of O2 and H2O
- Barrier methods include painting which is useful for large and visible structures such as bridges and pylons.
- Grease and oil are used to barrier protect moving parts such as parts of motors and bicycle chains.
- Vacuum containers and the use of desiccants also help to keep oxygen and water away and are used in special situations such as laboratories and for small machine parts.
- Iron can be prevented from rusting using the reactivity series.
- Galvanising is a process where the iron to be protected is coated with a layer of zinc.
- ZnCO3 is formed when zinc reacts with oxygen and carbon dioxide in the air and protects the iron by the barrier method.
- If the coating is damaged or scratched, the iron is still protected from rusting by the sacrificial method (magnesium can also be used).
- This is because zinc is more reactive than iron so it oxidises more readily:
Zn → Zn2+ + 2e–
- The iron stays protected as it accepts the electrons released by zinc, remaining in the reduced state.
- The electrons donated by the zinc react with hydrogen ions in the water producing hydrogen gas:
2H+ + 2e– → H2
- Zinc is therefore acting as the anode in the redox process.
- Zinc galvanising is an important process and is used in household items, transport, manufacturing and construction industries.
Zinc blocks are attached to the hull of ships as they corrode instead of the hull
- A similar process occurs with aluminium which is a curious metal in terms of its reactivity.
- It is placed high on the reactivity series yet it doesn´t react with either water or acids.
- This is because the surface of aluminium metal reacts with oxygen in the air forming a protective coating of aluminium oxide:
4Al + 3O2 → 2Al2O3
- The aluminium oxide layer is tough, unreactive and resistant to corrosion.
- It adheres very strongly to the aluminium surface and protects it from reaction with other substances, hence making it appear unreactive.
Rust Prevention by Electroplating
Specification Point 5.4C:
Explain how electroplating can be used to improve the appearance and/or the resistance to corrosion of metal objects.
- Electroplating is a process where the surface of one metal is coated with a layer of a different metal.
- The metal being used to coat is a less reactive metal than the one it is covering.
- It is performed in an electrolytic cell where the anode is made from the pure metal used to coat.
- The cathode is the object to be electroplated.
- The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode.
Diagram of electroplating iron with tin and SnCl2 electrolyte, a water soluble salt of tin
- Electroplating is used to make metals more resistant to corrosion or damage, e.g. chromium and nickel plating.
- It is also done to improve the appearance of metals, e.g. silver plating cutlery.
Edexcel GCSE Chemistry Notes
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