Corrosion is a process that occurs when metals react with substances in the environment, which weakens the metal. There are different types of corrosion, one of which is rusting.
Corrosion occurs only when the metal is exposed to the environment, meaning it takes place on the metal’s surface.
Rusting is a chemical reaction that occurs when Iron reacts with oxygen and water, forming hydrated Iron (III) Oxide (Fe2O3.xH2O), which is known as rust.
The equation for this reaction is:
Iron + Oxygen + Water → Hydrated iron(III) oxide
4Fe (s) + 3O2 (g) + xH2O (l) → 2Fe2O3.xH2O (s)
Rust is an orange-brown substance that flakes off, exposing the iron beneath it to further rusting. Rusting weakens the metal’s structure, making it unusable.
Aluminium is another metal that undergoes corrosion. However, when aluminium is exposed to air, it reacts with oxygen to form a tough protective outer layer of aluminium Oxide (Al2O3).
Unlike the soft layer formed during rusting, this layer acts as a shield and prevents the aluminium underneath from further corrosion.
Both oxygen and water are required for rusting to occur, as demonstrated in the following experiment.
There are three test tubes:
1. Iron nail in distilled water, with the nail exposed to air.
2. Iron nail in distilled water, with the water boiled to remove dissolved air and covered in oil to prevent air from dissolving in the distilled water.
3. Iron nail in the presence of calcium chloride, which is a drying agent that absorbs any water molecules present.
If we leave this for several days and observe the changes:
The only nail that rusted was in the test tube containing both air and water, which means that rusting requires the presence of both oxygen and water.