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wise otter.Chemistry
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K324 / 2027
Periodicity and Metals

Lesson 8 of 8

Prevent rust by removing a condition or supplying protection

Rusting needs both oxygen and water.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can explain rusting and barriers.Syllabus

    Syllabus K324, 8.4(f). Explain rusting and barriers

  • I can explain sacrificial protection.Syllabus

    Syllabus K324, 8.4(g). Explain sacrificial protection

Prevent rust by removing a condition or supplying protection

Rusting needs both oxygen and water.

Make a guess. You are not marked.

An iron nail is placed in water that was boiled to remove dissolved air, with a layer of oil on top. Does it rust?

  1. Yes, because the nail is surrounded by water
  2. Yes, faster than usual, as the water was hot
  3. No, as there is little oxygen in the water
Show the answer

No, as there is little oxygen in the water

Rusting needs both oxygen and water. Boiling removes the dissolved oxygen and the oil stops more air dissolving.

Iron rusts when oxygen and water are both available. In a fair comparison, iron exposed to dry air lacks water; iron in boiled water protected by an oil layer lacks fresh oxygen; iron in ordinary water exposed to air has both. The last sample rusts. The controls isolate the conditions rather than showing that water alone is sufficient.

Painting, greasing and plastic coating provide a barrier to oxygen and water. Protection fails at a scratch if the exposed iron can contact both. The coating must remain intact and suit the object: grease can protect moving metal parts where paint would be unsuitable.

Pure only

Galvanising coats iron with zinc. It forms a barrier, and zinc is also more reactive than iron, so zinc can corrode preferentially even near a scratch. Sacrificial protection deliberately attaches a more reactive metal to iron; magnesium blocks attached to underwater pipes lose electrons and are consumed instead of the iron. They must be replaced periodically.

Check your understanding

Iron kept in a tube of dry air does not rust. Does this prove that oxygen is not needed for rusting?

  1. No. Oxygen is needed, but rusting in dry air takes weeks rather than days to appear.
  2. No. Oxygen is present, so it shows oxygen alone is not enough; water is needed too.
  3. Yes. The iron had plenty of air, so the result shows rusting does not depend on oxygen.
Show the answer

No. Oxygen is present, so it shows oxygen alone is not enough; water is needed too.

Dry air contains oxygen but no water. The result shows water is necessary. Compare with wet iron in air, and wet iron without oxygen, to show both are needed.