Lesson 8 of 8
Prevent rust by removing a condition or supplying protection
Rusting needs both oxygen and water.
SEC G2 Science K223 / K225 (2027)
I can explain rusting and barriers.Syllabus
Syllabus K223 / K225, 7.3(d). Explain rusting and barriers
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?
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.
Check your understanding
Iron kept in a tube of dry air does not rust. Does this prove that oxygen is not needed for rusting?
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.