Corrosion of metals

O Level / IGCSE Chemistry · Metals. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Rusting is the corrosion of iron and steel. Iron reacts with oxygen and water together to form rust, which is hydrated iron(III) oxide. Both oxygen and water must be present: remove either one and the iron does not rust.

A barrier method covers the iron so that oxygen and water cannot reach it. Paint, grease and a plastic coating all work this way, but only while the coating is unbroken.

In sacrificial protection a more reactive metal, such as zinc or magnesium, is attached to the iron. The more reactive metal loses electrons more readily, so it corrodes instead of the iron. Galvanising, which is coating iron with zinc, works as a barrier and also as sacrificial protection.

Rules to remember

Common mistake

Thinking that any metal coating gives sacrificial protection. The protecting metal must be more reactive than iron, so that it loses electrons first; a less reactive metal such as copper cannot do this.

Worked example

One steel sheet is coated with zinc and another with copper. Both coatings are scratched through to the steel and the sheets are left outside in the rain. Which sheet rusts at the scratch?

  1. At the scratch, oxygen and water can reach the iron, so the coating no longer works as a barrier.
  2. Zinc is above iron in the reactivity series, so the zinc loses electrons (Zn → Zn2+ + 2e−) and corrodes instead of the iron.
  3. Copper is below iron, so the iron loses electrons instead and rusts.

Answer: Only the copper-coated sheet rusts at the scratch; the zinc-coated sheet is still protected.

Practice questions

Try each one, then open the answer.

1. Which method is most suitable to protect the moving chain of a bicycle from rusting?

  1. A
    oiling or greasing
  2. B
    sacrificial protection with a block of magnesium
  3. C
    coating with plastic
  4. D
    painting
Show answer

Answer: A. Paint and plastic would crack off a moving chain. Oil or grease keeps out water and oxygen and also lubricates.

2. Which two substances must both be present for iron to rust?

  1. A
    water and salt
  2. B
    nitrogen and water
  3. C
    oxygen and carbon dioxide
  4. D
    oxygen and water
Show answer

Answer: D. Rusting is the oxidation of iron by oxygen in the presence of water, forming hydrated iron(III) oxide. Salt speeds it up but is not needed.

3. What is rust?

  1. A
    anhydrous iron(III) chloride
  2. B
    iron(II) oxide with no water
  3. C
    iron(II) carbonate from carbon dioxide
  4. D
    hydrated iron(III) oxide
Show answer

Answer: D. Iron reacts with oxygen and water to form rust, hydrated iron(III) oxide.

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