Corrosion of Metals

1. Conditions required for the rusting of iron and steel to form hydrated iron(III) oxide

• Rusting is the corrosion process of iron or steel in which the metal reacts with oxygen (O₂) and water (H₂O) from the environment to form hydrated iron(III) oxide Fe₂O₃·xH₂O (commonly known as rust).
• Essential conditions for rusting:
  • Presence of water (moisture): Without water, rusting will not occur.
  • Presence of oxygen: Oxygen from the air is required to oxidise the iron.
• Chemical equation (overall):
  • 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → Fe₂O₃·xH₂O (hydrated iron(III) oxide)
• Process:
  • In the presence of water containing dissolved oxygen, iron atoms lose electrons (oxidation) to form Fe²⁺ ions:
    • Fe → Fe²⁺ + 2e⁻
  • Oxygen dissolved in water is reduced to hydroxide ions:
    • O₂ + 2H₂O + 4e⁻ → 4OH⁻
  • The Fe²⁺ ions are further oxidised to Fe³⁺, which combine with OH⁻ to form hydrated iron(III) oxide.
• Electrochemical nature:
  • Rusting is an electrochemical process involving anodic and cathodic areas on the iron surface.
  • Areas with higher oxygen concentration act as cathodes, and areas with lower oxygen concentration act as anodes.

2. How barrier methods prevent rusting by excluding oxygen or water

• Barrier methods protect iron or steel by forming a physical layer that prevents oxygen and/or water from reaching the metal surface.
• These methods work on the principle of isolation—if either oxygen or water is missing, rusting cannot occur.
• Examples:
  • Paint coating: Creates a waterproof layer.
  • Grease/oil coating: Repels water and excludes oxygen.
  • Plastic coating: Forms an impermeable layer.
• Limitation:
  • If the coating is scratched or damaged, rusting can start underneath the coating, often progressing faster than in unprotected metal.

3. Common barrier methods

• Painting: Used for vehicles, bridges, gates. Aesthetic and protective.
• Greasing or oiling: Common for moving machine parts and tools.
• Plastic coating: Durable protection used for items like wire mesh, appliance surfaces.
• Powder coating: Fine powder sprayed and baked to form a hard protective layer.

4. Sacrificial protection in terms of the reactivity series and electron loss

• Sacrificial protection:
  • Attaching a more reactive metal to iron or steel so that the more reactive metal corrodes instead of the iron.
  • Works because metals higher in the reactivity series lose electrons more readily.
• How it works:
  • The more reactive metal acts as the anode and is oxidised, releasing electrons.
  • These electrons flow to the iron, preventing it from losing electrons and corroding.
• Example:
  • Zinc attached to an iron hull of a ship. Zinc corrodes instead of iron.
  • Magnesium used for protecting underground steel pipes.

5. Use of zinc in galvanising as both a barrier method and sacrificial protection

• Galvanising:
  • Coating iron or steel with a layer of zinc.
  • Zinc provides a double protection:
    • Barrier protection: The zinc coating prevents oxygen and water from contacting the steel.
    • Sacrificial protection: If the zinc layer is scratched, zinc still corrodes preferentially, protecting the exposed steel.
• Why zinc works well:
  • It is more reactive than iron (higher in reactivity series).
  • It forms a protective zinc oxide layer as it corrodes, slowing down further oxidation.
• Chemical explanation of sacrificial effect:
  • Zn → Zn²⁺ + 2e⁻ (oxidation at zinc)
  • The electrons flow to iron, making iron a cathode, where no oxidation occurs.