Rusting of Iron

Overview

Rust is the flaky, red-brown solid that forms when iron or steel reacts with oxygen and water. Rust is hydrated iron(III) oxide.

What is rust?

Word idea: iron + oxygen + water → rust (hydrated iron(III) oxide).

Balanced idea:

$$4\text{Fe} + 3\text{O}_2 + x\text{H}_2\text{O} \rightarrow 2\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}$$

Conditions required

• Oxygen from air
• Water (liquid water or moisture)

Both must be present. In dry air (no water) or under water with no dissolved oxygen, rusting is very slow.

Why both are needed

Rusting is a slow redox reaction. Iron atoms lose electrons (oxidation) while oxygen gains electrons (reduction) in the presence of water, which helps ions move.

Simple investigation (iron nails)

• Dry nail + drying agent (no water): no rust
• Nail under boiled water with oil on top (no oxygen): no rust
• Nail in tap or salty water (oxygen and water): rust forms faster (salt speeds it up)

Preventing rust

• Barrier methods stop oxygen or water touching iron: painting, greasing/oiling, coating with plastic. If the barrier is unbroken, rusting cannot start.
• Galvanising: coating iron or steel with zinc. This gives a barrier and also sacrificial protection.
• Sacrificial protection: attach a more reactive metal (like zinc or magnesium). It corrodes instead of iron. In electron terms: $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$. Zinc loses electrons more easily (higher in the reactivity series) and supplies electrons to iron, preventing $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$.

Real-world examples

• Bikes, cars, and bridges are painted or galvanised.
• Ship hulls use sacrificial zinc blocks to protect steel in seawater.

Common misconceptions

• Only iron and steel “rust.” Other metals corrode but do not form rust.
• Rusting needs both oxygen and water; oxygen alone is not enough.
• Scratches in paint allow rust to start under the coating.