Corrosion as a Redox Reaction

  1. Corrosion of metal = metal loses its electrons to form positive ions.
    Example
    Corrosion of iron
    Fe → Fe2+ + 2e

    Corrosion of Magnesium
    Mg → Mg2+ + 2e

  2. The higher the position of a metal in the electrochemical series, the more electropositive (reactive) the metal is. The metal has a greater tendency to give away electrons to form the metal ion, that is the metal is more easily corroded. 
  3. For instance, the metal magnesium corrodes more easily than copper because magnesium is more electropositive than copper.
  4. Corrosion of metal is a redox reaction as the metal loses electrons to oxygen and water, which act as the oxidising agents to receive the electrons.
  5. Corrosion of iron is also called rusting.
    Fe (s) → Fe2+ (aq) + 2e
  6. Rusting of iron can only occur if both oxygen and water are present.


Rusting of Iron

  1. In the rusting of iron, iron acts as the reducing agent and oxygen gas as the oxidising agent.
  2. The process or rusting of iron can be explained by using figure below.
  3. When the surface of the iron is exposed to water droplets, the centre of the water droplets undergoes the process of oxidation and is known as the anode.
  4. The edge of the water droplets undergoes a process of reduction and is known as the cathode. (The edge of the water droplets acts as the cathode because of the concentration of soluble oxygen is higher on the edge of the water droplets than in the centre.)
  5. At the anode, the metal iron undergoes oxidation to form the iron(II) ion with the loss of electrons.
    Fe → Fe2+ + 2e
  6. Electrons that are free at the anode flow through the metal iron to the cathode area where soluble oxygen in the water accepts electrons to form hydroxide ions.
    O2 + 2H2O + 4e → 4OH–
  7. The iron(II) ions are then combines with the hydroxide ion to form iron(II) hydroxide.
    Fe2+ + OH- → Fe(OH)2
  8. Iron(II) hydroxide is then oxidised by oxygen to form iron(III) hydroxide.
    4Fe(OH)2 + 2H2O + O2 → 4Fe(OH)3
  9. The iron(III) hydroxide is then decomposed to form hydrated iron(III) oxide, Fe2O3•xH2O by oxygen in the air.
    4Fe(OH)3 → Fe2O3•xH2O
  10. The hydrated iron(III) oxide is brown in colour and is known as rust.
  11. The overall equation for the rusting of iron is
    4Fe + 3O2 +2xH2O → 2Fe2O3•xH2O