Process of cathodic protection

Outline the process of cathodic protection, describing examples of its use in both marine and wet terrestrial environments

  • Cathodic Protection: The protection of a metal from corrosion by making it the cathode in a galvanic cell.
  • Sacrificial Anode: A metal that oxidises preferentially to protect another metal from corrosion, thus providing cathodic protection.
  • Sacrificial anodes of zinc or magnesium can be attached to the hull of a ship or to a buried tank or pipeline.
  • Such sacrificial anodes only work in marine and wet terrestrial environments, as the sacrificial anode and the potential sites for oxidation need to be connected by a moderately conducting medium.
  • The rate of corrosion of the sacrificial anode needs to be moderately slow, otherwise they will not provide cathodic protection for a very long period of time.
  • A sacrificial anode is only able to protect a steel surface within a particular distance from it, hence, on larger ships, more sacrificial anodes must be used.
  • Another method of cathodic protection is to use an inert anode and apply a suitable voltage between it and the steel being protected, with the latter being positive.
  • This voltage forces electrons into potentially active sites on the full and so prevents oxidation in the same way as a sacrificial anode.
  • This method can be used to protect hulls of ships, underground storage tanks and pipelines.
  • Galvanising: The cathodic protection of a metal, generally iron or steel, by coating it with zinc, which acts as a sacrificial anode.