Corrosion & Its Prevention (AQA GCSE Chemistry)

• Corrosion is the destruction of materials by chemical substances in their environment which act on them over a period of time
• Most metals can corrode in the presence of oxygen to form the corresponding metal oxide
• Rusting is the name given specifically to the corrosion of iron in the presence of water and oxygen from the air:

iron + water + oxygen ⟶ hydrated iron(III)oxide

• 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

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
• Boiled water removes any dissolved oxygen and calcium chloride is a drying agent 

Investigating rusting

Diagram showing how the conditions for rusting can be investigated

• The nail on the left rusts
  • It is in contact with both air (which contains oxygen) and water
• The nail in the middle does not rust
  • It is not in contact with air because the oil provides a barrier to prevent oxygen diffusing into the boiled water
• The nail on the right does not rust
  • It is not in contact with water because 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

Barrier Methods

• Rust can be prevented by coating iron with barriers that prevent the iron from coming into contact with water and oxygen
• However, if the coatings are washed away or scratched, the iron is once again exposed to water and oxygen and will rust
• Unlike some other metals, once iron begins to rust it will continue to corrode internally as rust is porous and allows both air and water to come into contact with fresh metal underneath any barrier surfaces that have been broken or scratched
• Common barrier methods include: paint, oil, grease, and electroplating

Sacrificial Protection

• Iron can be prevented from rusting using the reactivity series
• A more reactive metal can be attached to a less reactive metal
• The more reactive metal will oxidise and therefore corrode first, protecting the less reactive metal from corrosion

Zinc bars on the side of steel ships

Diagram to show the use of zinc bars on the sides of steel ships as a method of sacrificial protection

• Zinc is more reactive than iron therefore will lose its electrons more easily than iron and is oxidised more easily
• For continued protection, the zinc bars have to be replaced before they completely corrode

Galvanising

• Galvanising is a process where the iron to be protected is coated with a layer of zinc
• This can be done by electroplating or dipping it into molten zinc
• ZnCO₃ is formed when zinc reacts with oxygen and carbon dioxide in the air and protects the iron by the barrier method
• If the coating is damaged or scratched, the iron is still protected from rusting by sacrificial protection

Aluminium 

• Aluminium has an oxide coating that protects the metal from further corrosion
• The oxide layer acts as a barrier