Degradation of Metals in the Environment

  • 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 increases the electrical conductivity of the water.
  • The equation for the rusting of iron is:

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 compromised.
  • This is a major concern as iron is used extensively in industries such as transport, construction and household objects.
  • 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 Conditions for Rusting

  • Oxygen and water must be present for rust to occur
  • Rusting is a redox process and it occurs faster in salty water since the presence of sodium chloride increases the electrical conductivity of the water.

Investigating Conditions for Rusting

Conditions for rusting, IGCSE & GCSE Chemistry revision notes

Diagram showing the setup for investigating the conditions for rusting

Method:

  • Set up the apparatus as shown in the diagram.
  • The water in test tube 2 is boiled to remove any dissolved oxygen.
  • Leave the apparatus for a few weeks 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 prove that both air and water must be present for rusting to occur.

Rust Prevention Methods

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.

Barrier Method Table, IGCSE & GCSE Chemistry revision notes

Galvanising / Sacrificial protection

  • Iron can be prevented from rusting using the reactivity series.
  • Galvanising is a process where the iron to be protected is coated with a layer of 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 the sacrificial method (magnesium can also be used).
  • This is because zinc is more reactive than iron and so it loses its electrons more readily:

Zn → Zn2+ + 2e-

  • The iron stays protected as it accepts the electrons released by zinc, remaining in the reduced state and thus it does not undergo oxidation.
  • The electrons donated by the zinc react with hydrogen ions in the water producing hydrogen gas:

2H+ + 2e → H2

  • Zinc therefore reacts with oxygen and water and corrodes instead of the iron.

AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.