Corrosion of Metals

Rust is hydrated iron(III) oxide, formula approximately Fe₂O₃·xH₂O. The word equation:

$\text{iron} + \text{oxygen} + \text{water} \rightarrow \text{hydrated iron(III) oxide (rust)}$

A classic three-tube experiment:

1. Iron nail in just water (boiled, then oil layer on top to keep oxygen out) → NO rust.
2. Iron nail in dry air (in a sealed tube with calcium chloride desiccant) → NO rust.
3. Iron nail in air + water → RUST.

Conclusion: BOTH oxygen and water are needed. Salt or acid accelerates the process (which is why cars in coastal areas or where roads are salted in winter rust faster).

In electron terms, iron LOSES electrons:

• Fe → Fe³⁺ + 3 e⁻ (oxidation).
• O₂ gains the electrons (reduction).
• Iron(III) ions combine with hydroxide and water → rust.

Rust is FLAKY — it doesn't stick to the iron underneath. So fresh iron is constantly exposed and the corrosion continues.

Aluminium is HIGH in the reactivity series — more reactive than iron. So why don't aluminium window frames rust?

When fresh aluminium is exposed to air, an instant reaction occurs: $4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3$

Aluminium oxide (Al₂O₃) is tightly bonded to the surface, non-porous and only a few nanometres thick — invisible but completely sealing the metal beneath. Once formed, no more O₂ can reach the unreacted Al. Corrosion essentially stops.

Iron oxide doesn't do this — it's flaky, not stuck, not waterproof. So iron rusts all the way through, while aluminium stays usable for decades.

This is why aluminium is so widely used in window frames, aircraft and food cans — despite being a reactive metal, in practice it lasts a very long time.

1. Barrier methods. Paint, oil, grease, plastic coatings — they all do the same thing: keep oxygen and water away from the iron. Cheap but easily damaged. Once scratched, rust can start at the scratch.

2. Galvanising. A coating of ZINC over the iron (e.g. by dipping or spraying). Zinc is MORE reactive than iron, so even if the coating gets scratched, the zinc oxidises preferentially (not the iron underneath). This is called sacrificial protection when applied locally, or 'cathodic protection' more broadly.

3. Sacrificial anodes. A LARGER piece of more reactive metal (zinc or magnesium) is attached directly to the iron object. Electrons flow from the more reactive metal to the iron, keeping iron in its metallic form. The sacrificial metal is gradually consumed and has to be replaced.

Examples:

• Bridges and pipelines: zinc blocks attached to the structure.
• Ship hulls: zinc or magnesium anodes bolted on; replaced every few years.
• Underground steel tanks: connected to magnesium electrodes.

MYP criterion D often asks for a comparison — e.g. galvanising vs paint. Galvanising lasts longer and protects even when scratched, but is more expensive. Paint is cheap and easy but needs maintenance.