Describe methods of metal extraction and explain corrosion (rusting) of iron and methods used to prevent it.

What this dot point is asking

You must describe extraction methods, explain the electrochemistry of rusting, and evaluate corrosion-prevention methods.

Extraction by reduction

Most metals occur in ores as positive ions, so extraction means reduction - gaining electrons to form the metal. The method depends on how reactive the metal is.

• Reactive metals (e.g. aluminium, sodium) hold their ions strongly, so carbon cannot reduce them; they are extracted by electrolysis of the molten compound (aluminium from molten alumina).
• Less reactive metals (e.g. iron, zinc) can be reduced by heating the ore with carbon (coke), as in the blast furnace.

Corrosion of iron (rusting)

Rusting is an electrochemical process. Different parts of the iron surface act as anode and cathode, with a film of water as the electrolyte.

At the anode, iron is oxidised:

At the cathode, oxygen is reduced in the presence of water:

The $\text{Fe}^{2+}$ is then further oxidised by oxygen to hydrated iron(III) oxide - rust.

Preventing corrosion

• Barrier methods keep out oxygen and water: painting, oiling, greasing or coating with plastic. They fail once scratched.
• Galvanising coats the iron with zinc. The zinc both acts as a barrier and, being more reactive, corrodes in preference to the iron even where the coating is scratched.
• Sacrificial protection attaches a more reactive metal (zinc or magnesium) to the iron. Being more easily oxidised, it corrodes instead of the iron, protecting it as long as the sacrificial metal lasts.