Corrosion of Iron
- Most metals react with substances when exposed to the environment for prolonged periods
- This causes degradation of the metal in a process called corrosion
- Corrosion occurs at the surface of the metal only
- Rusting is a chemical reaction between iron, water and oxygen to form the compound iron (III) oxide
- Oxygen and water must be present for rusting to occur
- Rusting is a redox process and it occurs faster in salty water since the presence of sodium chloride catalyses the process
- The equation for the rusting of iron is( x represents a varying amount of water):
4Fe (s) + 3O2 (g) + xH2O (l) → 2Fe2O3.xH2O (s)
- Rust is a soft solid substance that flakes off the surface of iron easily, exposing fresh iron below which then undergoes rusting
- This means that over time all of the iron rusts and its structure becomes weakened
- This is a major concern as iron is used extensively in industries such as transport and construction
- Aluminium is another metal that undergoes corrosion but in a slightly different way to iron
- Aluminium reacts with oxygen to produce aluminium oxide, Al2O3
- The aluminium oxide forms a tough protective layer that covers the aluminium, preventing further corrosion
Investigating Rusting
- Oxygen and water must be present for rust to occur
- You can investigate the conditions needed for rusting by setting up a series of control test tubes as shown below:
Diagram showing how the conditions for rusting can be investigated
Method:
- Set up the apparatus as shown in the diagram
- The water in the second test tube is boiled to remove any dissolved oxygen
- The oil provides a barrier to prevent oxygen diffusing into the boiled water
- Calcium chloride is a drying agent in the third test tube
- Leave the apparatus for a few days to give it time to react
Results:
- The nail on the left rusts as it is in contact with both air (which contains oxygen) and water
- The nail in the middle does not rust as it is not in contact with air
- The nail on the right does not rust as it is not in contact with water (calcium chloride absorbs any water molecules present due to moisture)
- The results show that both air and water must be present for rusting to occur