5.3C - Preventing rusting of iron
Rust prevention works by stopping the reactions that damage iron. You will interpret evidence about the conditions for rusting and compare barrier methods with sacrificial protection.
Testing what rusting needs
Iron rusts when both oxygen and water reach its surface. Rust is hydrated iron(III) oxide. Removing either required substance prevents rusting.
A useful investigation compares clean iron nails in three conditions:
| Tube | Conditions | Expected observation |
|---|---|---|
| A | Air and water | Rust forms |
| B | Dry air, with a drying agent to remove water | No rust |
| C | Boiled water under oil, excluding air | No rust |
Boiling removes dissolved oxygen from water; the oil layer prevents oxygen from the air dissolving back into it. Use similar nails, the same temperature and the same observation time so that the conditions being tested explain the difference.
Barrier methods
Paint, plastic, oil and grease form a barrier that stops oxygen and/or water reaching iron. Paint suits outdoor gates; oil or grease can protect moving steel parts while lubricating them. Dry storage works by excluding water even though oxygen is still present.
A barrier protects only where it remains effective. A deep scratch in paint exposes iron to oxygen and water, allowing rust to start there.
[DIAGRAM: asset_name: Rust prevention methods - diagram 1; asset_slug: 122_5_3c_rusting_of_iron_can_be_prevented_by_a_exclusion_of_oxygen_b_exclusion_of_w_diagram1; file: diagram_assets/122_5_3c_rusting_of_iron_can_be_prevented_by_a_exclusion_of_oxygen_b_exclusion_of_w_diagram1.png; recommended_method: deterministic_drawn; description: Monochrome assessed-style comparison panel showing left an iron/steel surface covered by a barrier coating that blocks labelled oxygen and water arrows, and right an iron/steel object attached to a more reactive zinc or magnesium block that reacts/corrodes instead of iron. The diagram teaches that exclusion of oxygen/water prevents rusting by removing required reactants, while sacrificial protection uses a more reactive metal. Exact labels should include iron/steel, barrier coating, oxygen, water, blocked, zinc or magnesium, more reactive metal reacts instead, and iron protected. Accessibility: nearby prose states all labels and conclusions.]
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