Corrosion – IGCSE Chemistry Definition

Corrosion is the general principle behind rusting, and examiners expect you to know both the specific case (iron rusting) and the broader concept. Paper 2 may ask you to compare rusting with other forms of corrosion or to explain why aluminium resists corrosion despite being reactive.

The 0620 definition

Corrosion is the destruction of a metal by chemical reaction with substances in its environment, typically oxygen and water.

Corrosion is an oxidation process — the metal atoms lose electrons and form metal ions or metal oxide compounds.

Rusting: the most important example

Rusting is the corrosion of iron. It requires both water and oxygen.

iron + water + oxygen → hydrated iron(III) oxide (rust)

Rust (Fe₂O₃·xH₂O) is flaky and porous — it does not protect the metal underneath, so corrosion continues deeper into the iron.

Aluminium: a special case

Aluminium is high in the reactivity series but does not appear to corrode quickly. This is because:

1. Aluminium reacts instantly with oxygen in the air
2. A very thin layer of aluminium oxide (Al₂O₃) forms on the surface
3. This oxide layer is tough, non-porous, and strongly bonded to the metal
4. It acts as a barrier, preventing further reaction

Unlike rust on iron, the aluminium oxide layer is protective. This is why aluminium is used for window frames, aircraft, and kitchen foil despite being more reactive than iron.

Factors that speed up corrosion

Factor	Effect
Contact with water	Provides the medium for ion movement
Contact with salt water	Increases conductivity, speeds up electrochemical corrosion
Higher temperature	Increases rate of reaction
Contact with a less reactive metal	Galvanic corrosion — the more reactive metal corrodes faster

Worked exam question

Iron corrodes in moist air but aluminium does not appear to corrode. Explain this difference. [3]

Mark scheme

Iron forms rust / hydrated iron(III) oxide [1]; rust is flaky / porous / does not protect the surface, so corrosion continues [1]; aluminium forms a layer of aluminium oxide that is tough / non-porous / protective, preventing further reaction [1]

Common exam mistakes

• Saying aluminium does not react with oxygen — it does, but the oxide layer protects it from further reaction.
• Using “corrosion” and “rusting” interchangeably — rusting only applies to iron. Other metals corrode but do not rust.
• Forgetting that corrosion is an oxidation reaction — the metal loses electrons and forms positive ions.