Chemguide: Core Chemistry 14 - 16

The rusting of iron

This page looks at the rusting of iron, and the various ways it can be prevented or slowed down. This is the last page in the section about the reactivity series, and assumes you are confident with the earlier material.

You will definitely need to understand the page about oxidation and reduction in terms of electron transfer.

Rusting as a form of corrosion

Corrosion happens on the surface of a metal when the metal atoms react to form compounds such as oxides or carbonates or more complicated substances. Rusting is an example of this.

Rusting is the special name given to the corrosion of iron. Many metals react with oxygen to form a layer of metal oxide on the surface. The very strong, very thin, layer of aluminium oxide on the surface of aluminium helps to protect the aluminium.

Rusting is the special name given to the corrosion of iron. Rust can be thought of as a hydrated iron(III) oxide, Fe₂O₃.xH₂O. "x" shows a variable amount of water attached to the iron(III) oxide.

The problem with iron is that, unlike aluminium, the rust layer is very porous and flaky. So rather than protect the iron underneath, it still allows air and water to reach the iron.

The conditions for the rusting of iron

The video below gives you all the information you want. It is a bit slow to start with, but is clearer than anything else I have been able to find.

When iron rusts:

• You need the presence of both air and water.

• Rusting is accelerated by the presence of dissolved ionic substances like salt.

The chemistry of the rusting of iron

This is surprisingly complicated, and to understand it properly, you will need to do chemistry for a few more years. All I can do here is to give a hint of the main processes, but without detail.

The main thing is that rusting involves the initial formation of Fe²⁺ ions as the iron atoms lose electrons:

Fe     Fe²⁺ +2e^-

The iron is being oxidised by loss of electrons (OIL RIG).

The electrons are picked up by oxygen and water molecules to form hydroxide ions. These react with the iron(II) ions to give iron(II) hydroxide, which is further oxidised and rearranged to give rust. You do not need to know this at this level.

But you should know that rusting starts by the loss of electrons from iron atoms to give Fe²⁺ ions.