Topic 7 of 7
Prevent rust by removing a condition or supplying protection
Rusting needs both oxygen and water.
O-Level 6092 (2026) / SEC G3 K324 (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.
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.