Galvanising & Sacrificial Protection

• 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
• Iron can be prevented from rusting using the reactivity series
• Diagram to show the use of zinc bars on the sides of steel ships as a method of sacrificial protection

  A) steel

  B) zinc

  
• Zinc is more reactive than iron therefore will lose its electrons more easily than iron and is oxidised more easily:

  • Zn → Zn2+ + 2e-
• The iron is less reactive therefore will not lose its electrons as easily so it is not oxidised; the zinc is sacrificed to protect the steel

  • For continued protection, the zinc bars have to be replaced before they completely corrode
• 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
• You may be asked to explain why a metal is/is not suitable as a method of preventing an iron/steel object from rusting. Remember that if it is higher in the reactivity series than iron, it will be suitable for sacrificial protection as it will be oxidised instead of iron. If it is lower in the reactivity series than iron, it would not be suitable as iron would be oxidised, causing it to rust
• Galvanising is the process of dipping a steel object into zinc which is more reactive than iron in the steel

  • the thin layer of zinc corrodes and loses electrons to the iron so protecting it
  • this still works even if zinc is scratched as electrons move from zinc to iron surface and are picked up by H ions in water to form hydrogen gas
  • iron is never oxidized
• Sacrificial protection is when zinc is attached to hulls of ships and will corrode in prefernce to iron as it is more reactive

  • zinc loses more electrons more easily
  • as long as zinc is present, iron will not rust
  • magnesium can be used instead of zinc