Corrosion is a metal reacting with its surroundings
Definition
Corrosion
The gradual destruction of a metal by chemical reactions with substances in its surroundings, such as oxygen and water.
- Most metals corrode by reacting with oxygen, often helped by water, to form metal oxides or other compounds.
- The metal gains oxygen, so it is oxidised.
- The more reactive a metal, the faster it tends to corrode, while gold and platinum hardly corrode at all.
- Aluminium is more reactive than iron but corrodes very little, because a thin layer of aluminium oxide forms on its surface and seals the metal underneath.
- Corrosion weakens metal structures, so bridges, cars and ships need regular protection and repair.
Note
Only iron and steel rust, so zinc and aluminium corrode but do not rust.
Iron rusts only with both oxygen and water
Definition
Rusting
The corrosion of iron or steel by reaction with oxygen and water to form hydrated iron(III) oxide.
- Iron reacts with oxygen and water to form hydrated iron(III) oxide, which is rust: iron+oxygen+water→hydrated iron(III) oxide\text{iron} + \text{oxygen} + \text{water} \rightarrow \text{hydrated iron(III) oxide}iron+oxygen+water→hydrated iron(III) oxide
- Iron in dry air does not rust, and neither does iron in water that contains no dissolved oxygen.
- Salt water makes iron rust faster, which is why ships at sea and cars driven on gritted winter roads rust quickly.
- Acidic conditions, such as acid rain, also speed up rusting.
- Rust is weak and flakes off, exposing fresh iron underneath, so rusting carries on until the iron is used up.
Practical
Investigating the conditions for rusting
- Aim: show which conditions iron needs to rust.
- Apparatus: four test tubes in a rack, four clean iron nails, emery paper, tap water, boiled and cooled water, oil, anhydrous calcium chloride, salt, a bung and a label for each tube.
Method
- Clean four iron nails with emery paper and put one into each test tube.
- Tube 1: cover the nail with tap water and leave the tube open, so that both air and water reach it.
- Tube 2: cover the nail with boiled water and add a layer of oil, because boiling removes dissolved air and the oil stops more dissolving.
- Tube 3: add anhydrous calcium chloride and seal the tube with a bung, because calcium chloride absorbs water vapour from the air.
- Tube 4: cover the nail with salty water and leave the tube open.
- Leave the tubes for about a week, then compare the nails.
Accuracy and safety
- Clean every nail the same way, so that no nail starts with a layer of rust or grease.
- Only the nails in tubes 1 and 4 rust, which shows that both oxygen and water are needed.
- The nail in salty water rusts fastest, which shows that salt speeds up rusting.
- Wash your hands after handling calcium chloride, because it is an irritant.
A physical barrier stops oxygen and water reaching iron
- Paint protects large structures such as bridges, ships and car bodies.
- Oil or grease protects moving parts, such as bicycle chains, where paint would rub off.
- A plastic coating protects items such as garden furniture, fences and dish racks.
- Tin plating protects the steel of food cans, because tin is unreactive and not toxic.
- A barrier protects the iron only while it is unbroken, so a scratch lets oxygen and water reach the iron and rusting starts.
Common Mistake
Do not assume that a tin coating protects scratched steel, because tin is less reactive than iron, so the exposed iron rusts even faster.
In sacrificial protection a more reactive metal corrodes instead
Definition
Sacrificial protection
Protecting iron from rusting by attaching a more reactive metal, which corrodes instead of the iron.
- A block of a metal more reactive than iron, such as zinc or magnesium, is attached to the iron or steel.
- The more reactive metal forms its positive ions more readily than iron, so it reacts with oxygen and water instead of the iron.
- The iron does not rust while the sacrificial metal remains, so the blocks are replaced once they have corroded away.
- Magnesium blocks protect underground steel pipes, and zinc blocks protect the steel hulls of ships.
- Sacrificial protection still works where the iron is exposed, which a barrier alone cannot do.
Example
A ship's steel hull is painted as a barrier, and zinc blocks fixed to it give sacrificial protection wherever the paint is damaged.
Galvanising uses zinc as a barrier and sacrificial metal
Definition
Galvanising
Coating iron or steel with a layer of zinc to protect it from rusting.
- Galvanised steel is coated with a layer of zinc, usually by dipping it in molten zinc.
- While the coating is unbroken, the zinc acts as a physical barrier, keeping oxygen and water away from the steel.
- If the coating is scratched, the zinc still protects the steel by sacrificial protection, because zinc is more reactive than iron.
- The zinc corrodes instead of the iron, so the exposed steel does not rust.
- Galvanised steel is used for buckets, nails, fences and motorway crash barriers.
Exam technique
When a question asks why galvanised steel still resists rusting after it is scratched, say that zinc is more reactive than iron, so the zinc corrodes instead of the iron.
Self review
- Which two substances must be present for iron to rust?
- What is the chemical name for rust?
- Why does aluminium corrode very little even though it is more reactive than iron?
- How does a coat of paint prevent rusting?
- Why does a scratched galvanised bucket not rust?
