Barrier principle

A continuous coating prevents oxygen and water reaching iron. Because both are required, excluding either stops rusting.

Examples

Paint is used on bridges and vehicles, grease or oil on moving machinery and tools, and plastic coatings on racks and wire. The method must suit temperature, movement and appearance.

Limitations

A barrier protects only while intact. Scratches expose iron, and water may become trapped under damaged coatings. Inspection and maintenance are therefore important.

Choosing a method

Grease is useful where surfaces move but can be messy. Paint is convenient and decorative but may chip. Plastic is durable in mild conditions but may soften with heat.

High-value recall and connections

Essential recall: How does painting prevent rust? It blocks oxygen and water. Why can scratched paint fail? Exposed iron contacts oxygen and water. Give a suitable barrier for a moving chain. Grease or oil. Practical connection: Compare barrier methods for a bridge, bicycle chain and dish rack. Justify each selection using service conditions. Examination connection: Barrier methods work by excluding oxygen or water, not by making iron less reactive.

Review questions and suggested answers