What rusting is
Definition
Corrosion
The gradual destruction of a metal as it reacts with substances around it, such as oxygen and water.
Definition
Rusting
The corrosion of iron or steel by reaction with oxygen and water to form hydrated iron(III) oxide.
- Rust is hydrated iron(III) oxide, an orange-brown, crumbly solid.
- Only iron and steel rust, because steel is mostly iron, so the corrosion of other metals is not called rusting.
- The word equation is 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 gains oxygen as it rusts, so rusting is an oxidation reaction.
- Rust is weak and flakes away from the surface, which exposes fresh iron underneath to further rusting.
- Left unprotected, rusting continues until the iron is eaten right through, which weakens car bodies, bridges, ships and railings.
Conditions for rusting
- Iron and steel rust only when both oxygen and water are present at the surface of the metal.
- The oxygen comes from the air, or from air that has dissolved in the water.
- If either oxygen or water is missing, rusting does not happen.
- Iron rusts faster in salty water, which is why cars on gritted winter roads and ships at sea rust quickly.
Practical
Finding the conditions needed for rusting
- Aim: to find out whether both oxygen and water are needed for iron to rust.
- Apparatus: four boiling tubes in a rack, four clean iron nails, emery paper, tap water, distilled water that has just been boiled, a little cooking oil, salt solution, anhydrous calcium chloride, cotton wool, a bung and labels.
- Method, setting up:
- Rub each nail with emery paper so that all four start with a clean, shiny surface.
- Tube A, oxygen and water: stand a nail in tap water and leave the tube open to the air.
- Tube B, water but no oxygen: stand a nail in freshly boiled distilled water, which has had its dissolved air driven out, and pour a layer of oil on top to stop air dissolving back in.
- Tube C, oxygen but no water: put anhydrous calcium chloride in the bottom of the tube, cover it with a plug of cotton wool, rest the nail on top and seal the tube with a bung.
- Tube D, oxygen and salty water: stand a nail in salt solution and leave the tube open to the air.
- Method, observing:
- Leave the four tubes in the rack at room temperature for about a week.
- Compare the amount of orange-brown rust on each nail and record your observations in a table.
- Results: the nail in tube A rusts, the nails in tubes B and C stay shiny, and the nail in tube D is the most heavily rusted.
- Conclusion: iron rusts only when oxygen and water are both present, and dissolved salt speeds up rusting.
- Watch out: add the oil to tube B as soon as the boiled water has cooled, because air starts to dissolve back into the water straight away, and keep tube C sealed so that the calcium chloride can keep the air inside dry.
- Safety: wear eye protection, take care with the boiling water, and wash your hands after handling anhydrous calcium chloride, which irritates the skin.
Barrier methods
Definition
Barrier method
A way of preventing rusting by covering iron or steel with a layer that stops oxygen and water from reaching the metal.
- Barrier methods prevent rusting by excluding oxygen and water from the surface of the iron or steel.
- Painting
- A coat of paint seals the surface of large objects such as bridges, ships, car bodies and railings.
- Paint must be renewed from time to time, because it chips and flakes.
- Greasing or oiling
- A layer of grease or oil protects moving parts, such as bicycle chains, hinges and machine gears, where paint would be scraped off.
- The grease also lubricates the parts, but it wears away and must be reapplied.
- Coating with plastic
- A tough plastic layer protects objects such as garden furniture, wire fences, dish racks and fridge shelves.
- A plastic coating needs no repainting, but it can split or tear.
- A barrier method only works while the coating is unbroken.
- If the barrier is scratched or chipped, oxygen and water reach the bare iron, and rusting starts at that point.
- Rust can then spread underneath the surrounding paint and lift it away from the metal.
Example
Choosing a barrier method
- A bicycle chain has moving parts that rub together, so paint would soon wear off, and greasing is used instead because it keeps out oxygen and water while also lubricating the chain.
- A steel bridge is huge and fixed in place, so painting is the most practical way to seal its whole surface.
- A wire garden fence is coated with plastic, because the coating is tough, lasts for years and never needs repainting.
Exam technique
Explaining how a method prevents rusting
- State what the coating does, writing that it stops oxygen and water reaching the iron, because naming the method alone, such as "it is painted", does not explain it.
- When asked for the conditions for rusting, give oxygen and water together, because one of them alone does not earn the mark.
- When asked to choose a method, link it to the object, so moving parts are greased while large fixed structures are painted.
Common Mistake
- Do not describe rust simply as iron oxide, because it is hydrated iron(III) oxide.
- Do not say that rusting needs only air or only water, because both oxygen and water are needed.
- Do not use the word rusting for other metals, because aluminium and zinc corrode but only iron and steel rust.
Self review
- What is the chemical name for rust?
- Which two substances must be present for iron to rust?
- Why is the water in tube B boiled and then covered with oil?
- Name three barrier methods.
- How does a barrier method prevent rusting?
