top of page

15.1 Rusting

GCSE Chemistry · AQA · Chapter 15: Using Our Resources

A free GCSE Chemistry lesson for AQA (8462) from OBA School: corrosion, the conditions iron needs to rust, and how barrier methods and sacrificial protection stop it. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe corrosion and rusting, and name the product of rusting
  • Describe an experiment to show that both air (oxygen) and water are needed for iron to rust
  • Explain how barrier methods such as painting, oiling and electroplating prevent rusting
  • Explain sacrificial protection, including galvanising, using the reactivity of metals
  • Calculate percentage changes in mass, percentage of iron in iron(III) oxide and (Higher tier) moles of zinc in a coating

Key words

  • Corrosion: The destruction of a material by chemical reactions with substances in its surroundings, such as oxygen and water.
  • Rusting: The corrosion of iron and steel. Iron reacts with oxygen and water to form rust.
  • Rust: Hydrated iron(III) oxide, Fe₂O₃·xH₂O. It is brittle and flakes off.
  • Barrier method: A coating that keeps air and water away from the metal surface, e.g. paint, oil, grease, plastic or a layer of another metal.
  • Electroplating: Using electrolysis to coat an object with a thin layer of another metal, such as tin or chromium.
  • Sacrificial protection: Attaching or coating iron with a more reactive metal, which corrodes instead of the iron.
  • Galvanising: Coating iron or steel with a layer of zinc.

Corrosion and rusting

Many metals react slowly with oxygen and water in the air. This corrosion weakens metal objects over time. The corrosion of iron and steel is called rusting. Rust is hydrated iron(III) oxide.

iron + oxygen + water → hydrated iron(III) oxide

4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃·xH₂O(s)

Rusting is an oxidation reaction: the iron gains oxygen. In terms of electrons, each iron atom loses three electrons to become an Fe³⁺ ion: Fe → Fe³⁺ + 3e−. Rust is soft and crumbly and flakes off, so fresh iron underneath is exposed and the rusting carries on. Rust cannot be given one exact formula, because the amount of water (x) in it varies.

Salt (for example from sea spray or salt spread on winter roads) speeds rusting up, which is why cars and ships rust faster near the sea.

Key fact: Aluminium is more reactive than iron but hardly corrodes. Its surface quickly forms a thin, tough layer of aluminium oxide that sticks to the metal and stops air and water reaching the aluminium underneath.

What does iron need to rust?

You can show that iron needs both air (oxygen) and water by leaving identical iron nails in three test tubes for about a week.

  • A; How the condition is set up nail in ordinary tap water, open to the air; Why control: both air and water present
  • B; How the condition is set up water is boiled first, then a layer of oil is added and the tube is stoppered; Why boiling removes dissolved air; the oil stops air dissolving back in
  • C; How the condition is set up anhydrous calcium chloride at the bottom, cotton wool, stoppered; Why calcium chloride is a drying agent that absorbs water vapour from the air

Worked example: percentage change in mass

A nail had a mass of 3.20 g. After two weeks in tube A, its mass was 3.33 g.
Change in mass = 3.33 − 3.20 = 0.13 g
Percentage change = 0.13 ÷ 3.20 × 100 = 4.06…%
The mass goes up because oxygen and water from the surroundings have combined with the iron.

Answer: an increase of 4.1% (2 s.f.)

Barrier methods

A barrier keeps oxygen and water away from the surface of the iron. It works only while the barrier is unbroken.

  • Paint; Example of use car bodies, bridges, railings; Why it suits that use cheap to cover large areas; can be decorative
  • Oil or grease; Example of use bicycle chains, tools, machine parts; Why it suits that use also lubricates moving parts; paint would crack or rub off
  • Plastic coating; Example of use garden fencing wire, dish-drainer racks; Why it suits that use flexible, waterproof, comes in colours
  • Electroplating with tin; Example of use steel food cans; Why it suits that use tin is unreactive and not toxic in contact with food
  • Electroplating with chromium; Example of use taps, bike handlebars; Why it suits that use hard, shiny and attractive

A metal coating can be put on by electroplating or by dipping the object into the molten metal (hot-dip galvanising uses molten zinc at about 450 °C). Any barrier only works while it is complete: through even a tiny gap, water and oxygen reach the iron and the rust then spreads under the coating.

Watch out: If a tin-plated can is scratched, the iron underneath rusts even faster than normal. Iron is more reactive than tin, so the iron corrodes in preference to the tin.

Sacrificial protection

If iron is in contact with a more reactive metal such as zinc or magnesium, the more reactive metal loses electrons (is oxidised) instead of the iron. The more reactive metal is “sacrificed”: it corrodes away, and the iron is protected.

Zinc atoms give away electrons more easily than iron atoms (zinc is the stronger reducing agent): Zn → Zn²⁺ + 2e−. While zinc is there to be oxidised, the iron atoms are not oxidised. Magnesium and aluminium work in the same way.

  • Galvanising coats steel with zinc. The zinc is a barrier and gives sacrificial protection, so a scratch does not let the steel rust. Used for buckets, nails, crash barriers and car bodies.
  • Blocks of zinc or magnesium are bolted to ship hulls, oil rigs and piers, or joined by wires to buried pipelines. Salt water makes rusting much faster, so these are places where sacrificial protection is worth the cost. The blocks slowly corrode away, so they must be replaced from time to time.

Exam tip: In an explanation, always say the protecting metal is more reactive than iron, so it loses electrons / is oxidised (corrodes) instead of the iron.

Worked example: percentage of iron in iron(III) oxide

Ar: Fe = 56, O = 16. Mr of Fe₂O₃ = (2 × 56) + (3 × 16) = 112 + 48 = 160
Percentage of iron = 112 ÷ 160 × 100 = 70%
So 25 g of iron(III) oxide contains 25 × 0.70 = 17.5 g of iron.

Answer: 70% iron by mass; 17.5 g of iron

Worked example: moles of zinc in a galvanised coating (Higher tier)

A steel gate panel measures 1.5 m by 0.80 m. Both faces are galvanised with 250 g of zinc per m². Ar: Zn = 65
Area of one face = 1.5 × 0.80 = 1.2 m², so both faces = 2 × 1.2 = 2.4 m²
Mass of zinc = 2.4 × 250 = 600 g
Moles of zinc = mass ÷ Ar = 600 ÷ 65 = 9.23 mol (3 s.f.)

Answer: 9.23 mol of zinc

Common mistakes

  • Saying iron rusts in water alone, or in air alone. It needs both oxygen and water.
  • Thinking an oil layer is there to “keep the water in”. It stops air dissolving back into the boiled water.
  • Saying zinc protects iron because it is less reactive or “stronger”. Zinc is more reactive, so it is oxidised first.
  • Mixing up tin plating and galvanising. A scratch in tin makes rusting faster; a scratch in zinc does not matter.
  • Saying a rusting nail loses mass. It gains mass, because oxygen and water join onto the iron.

Revision questions

Try these, then check your answers below each one.

Question 1. Which substances must both be present for iron to rust? (Choose from: Oxygen and water, Oxygen and carbon dioxide, Water and salt, Nitrogen and water.)

Answer: Oxygen and water

Working: Iron needs both oxygen (from air) and water to rust. Salt only speeds rusting up; it is not essential.

Question 2. What is the chemical name for rust?

Answer: hydrated iron(III) oxide

Working: Rust is hydrated iron(III) oxide, Fe2O3·xH2O.

Question 3. An iron nail is placed in boiled water with a layer of oil on top, in a stoppered tube. Why does it not rust? (Choose from: There is no oxygen in contact with it, There is no water in contact with it, Oil reacts with the iron, Boiling the water made it acidic.)

Answer: There is no oxygen in contact with it

Working: Boiling removes dissolved air and the oil stops air dissolving back in, so there is no oxygen. Water alone is not enough.

Related lessons

Continue in the full course

This lesson is section 15.1 (AGCCH1501) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

bottom of page