9.5 Corrosion of metals

2026 Syllabus Objectives

Core:

  1. State the conditions required for the rusting of iron and steel to form hydrated iron(III) oxide
  2. State some common barrier methods, including painting, greasing and coating with plastic
  3. Describe how barrier methods prevent rusting by excluding oxygen or water

Supplement: 4. Describe the use of zinc in galvanising as an example of a barrier method and sacrificial protection 5. Explain sacrificial protection in terms of the reactivity series and in terms of electron loss

What is Corrosion? 🔑

https://cambridgeassistant.com

When a metal is attacked by air, water or other surrounding substances, it is said to corrode. Most metals corrode by undergoing a chemical reaction with oxygen and other gases in the air to form compounds that collect on the surface of the metal.

Reactivity and Corrosion

Different metals corrode at different rates depending on their reactivity:

  • Very reactive metals such as sodium and potassium need to be stored under oil to keep them away from air and water
  • Moderately reactive metals such as calcium will slowly corrode away to powder over time
  • Less reactive metals like aluminium form protective oxide layers

Rusting of Iron and Steel ⚡

https://cambridgeassistant.com

In the case of iron and steel, the corrosion process is specifically known as rusting.

Definition

Rusting: The corrosion of iron and steel to form rust (hydrated iron(III) oxide)

What is Rust?

Rust is a red-brown powder consisting mainly of hydrated iron(III) oxide, with the chemical formula:

Fe2O3⋅xH2O\mathrm{Fe_2O_3 \cdot xH_2O}

where xx represents a variable number of water molecules.

Conditions Required for Rusting 🔑

Water and oxygen are essential for iron to rust. Both must be present for rusting to occur.

Additional factors that increase the rate of rusting:

  • Salt (seawater): Increases the rate of corrosion significantly, as can be seen on shipwrecks around the world
  • Acid rain: Also increases the rate at which iron objects rust

Economic Impact

Rusting is a serious economic problem. Large sums of money are spent each year replacing damaged iron and steel structures or protecting structures from such damage.

Sign in to view full notes