- Corrosion: The degradation of a metal so that it loses strength and becomes unable to fulfill its intended purpose.
- Rusting: The corrosion of iron to form hydrated iron (III) oxide, or rust.
- Iron will rust when it is in contact with both oxygen and water.
- Rusting is faster if the water involved contains an electrolyte, either an acid or a dissolved salt.
- The rusting is accelerated if the iron is:
- In contact with a less active metal.
- Under mechanical stress.
- In the process of the rusting of iron:
- The iron is the anode and is oxidized:
Fe → Fe2+ + 2e-
- The reduction of oxygen occurs at the cathode:
O2 + 2H2O + 4e- → 4OH-
- Electrons move from the anode to the cathode through the iron.
- The anode and the cathode may be separated by some distance.
- Insoluble iron (II) hydroxide is formed and the iron (II) ions are further oxidized to produce hydrated iron (III) oxide, or rust.