What causes iron and steel to rust?

Iron and steel rust due to a chemical reaction with oxygen and moisture, known as oxidation.

Rusting, or the formation of iron(III) oxide, is a chemical process that occurs when iron or steel comes into contact with oxygen and water. This process is a type of corrosion, which is a natural phenomenon where metals break down when exposed to the environment.

The process begins when iron reacts with oxygen in the air to form iron(II) oxide, a process known as oxidation. This is a slow process, which is why rust doesn't appear immediately on the surface of the iron. The iron(II) oxide then reacts with more oxygen and water to form iron(III) oxide, commonly known as rust.

The chemical equation for this process is: 4 Fe + 3 O2 → 2 Fe2O3. This means that for every four atoms of iron and three molecules of oxygen, two molecules of iron(III) oxide are produced.

Rust is porous and allows oxygen and water to reach the iron beneath, causing further rusting. This is why rusted objects continue to deteriorate over time if not treated.

The rusting process can be accelerated by certain factors. For example, the presence of salt can speed up the process, which is why iron and steel structures near the sea tend to rust more quickly. Similarly, acidic conditions can also speed up rusting.

To prevent rusting, iron and steel can be coated with a protective layer, such as paint or a layer of a less reactive metal. This prevents the iron from coming into contact with oxygen and water, thus stopping the rusting process.