Rusting and Its Prevention

Rusting questions are two-condition logic plus one explanation, and the failure pattern is stubborn: every series, students name water but not oxygen, or describe sacrificial protection as if it were paint. The chemistry takes an evening to learn; the phrasing discipline is what separates 3/4 from 4/4. This page covers both, within the wider Metals topic.

What rusting is and what it needs

Rusting is the corrosion of iron (and steel). The product, rust, is hydrated iron(III) oxide. Two conditions are required, and the word both belongs in your answer: oxygen and water.

The evidence is the standard three-tube experiment, a regular Paper 6 question:

Tube	Contents	Conditions	Result
1	Iron nail, tap water, open to air	Oxygen + water	Rusts
2	Iron nail, boiled water, oil layer on top	Water only (boiling removed dissolved oxygen; oil keeps air out)	No rust
3	Iron nail, anhydrous calcium chloride, sealed	Oxygen only (drying agent removes water vapour)	No rust

Each design detail earns marks: why the water is boiled, what the oil layer does, why the calcium chloride is there. Salt water speeds rusting up (relevant to ships and coastal structures), but salt is not a required condition.

Prevention strategy 1: barriers

Keep oxygen and water away from the iron surface and rusting cannot start. Painting, oiling/greasing, and plastic coating all work this way. The strength and the weakness belong together in your answer: barriers are cheap, but protection ends the moment the coating is scratched, because oxygen and water reach the iron again.

Match the barrier to the use when asked: oil for moving parts (paint would crack), paint for bridges and gates, plastic coating for fence wire.

Prevention strategy 2: sacrificial protection

Attach a metal more reactive than iron (almost always zinc, sometimes magnesium) and the more reactive metal corrodes in preference to the iron. Blocks of zinc bolted to ship hulls and underground pipes work this way, and keep working even though the iron is exposed to seawater.

(Supplement) The explanation is electron-based, and the mark scheme wants the comparison: zinc is more reactive than iron, so zinc atoms lose electrons more readily than iron atoms. The zinc is oxidised (Zn → Zn2+ + 2e−) in preference to the iron. This is the reactivity series doing real-world work: position predicts which metal gives up its electrons.

Galvanising is the hybrid and the favourite exam discriminator. Galvanised iron is coated in zinc, so it gets both protections: the zinc layer is a barrier, and when the layer is scratched, the zinc still protects sacrificially because it remains more reactive than the exposed iron. The comparison question (“explain why galvanising protects a scratched surface but tin plating does not”) turns on this: tin is less reactive than iron, so once tin plate is scratched, the iron corrodes preferentially and rusting accelerates.

Worked exam question

A steel ship’s hull has zinc blocks bolted to it below the waterline. (a) State the two substances that must both be present for the hull to rust. [2] (b) Explain how the zinc blocks protect the hull, in terms of electrons. [3] (c) Explain one advantage of this method over painting the hull. [1]

Model answer: (a) Oxygen (1) and water (1). (b) Zinc is more reactive than iron (1); zinc loses electrons more readily / is oxidised in preference to iron (1); so the zinc corrodes instead of the iron (sacrificial protection) (1). (c) Protection continues even if the surface is scratched / the paint layer would stop protecting once damaged (1).

Mark-by-mark: (a) both substances, two separate marks; “air and moisture” is accepted. (b) the three scoring ideas are the reactivity comparison, the electron loss, and the consequence; “zinc covers the iron” scores zero because these are blocks, not a coating. (c) must contain the scratch/damage comparison, not just “it lasts longer.”

The mistakes that cost marks

1. One condition instead of two. “Iron rusts when wet” loses the oxygen mark. Both oxygen and water: write both, every time.
2. Sacrificial protection described as a barrier. Zinc blocks do not cover the hull. The mechanism is preferential electron loss by the more reactive metal.
3. Choosing magnesium or zinc as “less reactive.” The sacrificial metal must be more reactive than iron. Students reverse this under pressure; check against the series.
4. Experiment details left vague. In the three-tube setup, “boiled water” without saying why (to remove dissolved oxygen) and “calcium chloride” without its job (to dry the air) leave marks on the table.

How examiners want it phrased

Student wording	Mark-scheme wording
”Iron rusts in the rain"	"Rusting requires both oxygen and water"
"The zinc protects the iron"	"Zinc is more reactive than iron, so it loses electrons / is oxidised in preference to the iron"
"Paint stops rust"	"Paint forms a barrier that prevents oxygen and water reaching the iron"
"Galvanising is a zinc cover"	"The zinc coating acts as a barrier and, if scratched, continues to protect sacrificially because zinc is more reactive than iron”

Rusting connects forwards too: corrosion-resistant alloys such as stainless steel are the design alternative to coatings. If your answers explain the right chemistry in the wrong words, one free trial lesson comparing your phrasing against live mark schemes will show you exactly what to change.

Test yourself

Both conditions, every time: answer all three before revealing the schemes.

Q1 (2 marks). In the rusting experiment, an iron nail is placed in boiled water with a layer of oil on top. Explain the purpose of (a) boiling the water and (b) the oil layer.

Show answer

• (a) boiling removes the dissolved oxygen from the water [1] • (b) the oil layer stops air (oxygen) re-dissolving into the water [1], so the nail has water but no oxygen and does not rust

Q2 (2 marks). A scratched tin-plated steel can rusts quickly, but a scratched galvanised steel sheet does not. Explain the difference.

Show answer

• tin is less reactive than iron, so once scratched the iron corrodes in preference to the tin [1] • zinc is more reactive than iron, so the zinc corrodes in preference to the exposed iron (sacrificial protection continues) [1]

Q3 (2 marks). Name the substance formed when iron rusts, and explain why painting protects iron only while the paint layer is intact.

Show answer

• hydrated iron(III) oxide [1] • paint is a barrier that keeps oxygen and water away from the iron; once scratched, both reach the iron surface and rusting starts [1]