Topic 7 of 7
Prevent rust by removing a condition or supplying protection
Rusting needs both oxygen and water.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Prevent rust by removing a condition or supplying protection
Rusting needs both oxygen and water.
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 understandingIron in a dry tube of air does not rust. Does this prove oxygen is unnecessary?Think it through, then reveal the answer
No. Oxygen is present but water is absent, showing that oxygen alone is insufficient. Compare with wet iron exposed to air to establish the need for both conditions.