Answer: Corrosion is the destruction or deterioration of a metal by chemical or electrochemical reaction with its environment. In the oxygen absorption mechanism, iron is oxidised at the anode (\(\mathrm{Fe\rightarrow Fe^{2+}+2e^-}\)) and dissolved oxygen is reduced at the cathode (\(\mathrm{\tfrac12O_2+H_2O+2e^-\rightarrow 2OH^-}\)); the products combine to give \(\mathrm{Fe(OH)_2}\), which is oxidised to rust.
Definition: Destruction or deterioration of a metal, starting at its surface, by unwanted chemical or electrochemical reaction with its environment is called corrosion. Example: rusting of iron.
Oxygen absorption mechanism (wet corrosion in neutral or slightly alkaline aqueous medium containing dissolved oxygen, e.g. iron in NaCl solution):
- Iron is normally covered by a thin oxide film. If this film develops cracks, the exposed small area of iron acts as the anode and the large film-covered surface acts as the cathode. The aqueous solution (e.g. NaCl) acts as the electrolyte.
- At the anode (oxidation): \(\mathrm{Fe\rightarrow Fe^{2+}+2e^-}\). The \(\mathrm{Fe^{2+}}\) ions go into solution and the electrons flow through the metal to the cathode.
- At the cathode (reduction of dissolved oxygen): \(\mathrm{\tfrac12O_2+H_2O+2e^-\rightarrow 2OH^-}\).
- \(\mathrm{Fe^{2+}}\) and \(\mathrm{OH^-}\) ions diffuse towards each other and combine: \(\mathrm{Fe^{2+}+2OH^-\rightarrow Fe(OH)_2}\) (ferrous hydroxide).
- If enough oxygen is present, ferrous hydroxide is oxidised to ferric hydroxide: \(\mathrm{4Fe(OH)_2+O_2+2H_2O\rightarrow 4Fe(OH)_3}\). This yellowish-brown product is rust, \(\mathrm{Fe_2O_3\cdot xH_2O}\).
- If the oxygen supply is limited, the product is black anhydrous magnetite, \(\mathrm{Fe_3O_4}\).
- Because the anodic area is small and the cathodic area is large, the large cathode demands many electrons, so the small anode corrodes rapidly (pitting).
Diagram: an iron surface covered with an oxide film having a crack; the iron exposed at the crack is labelled anode (Fe → Fe2+ + 2e-); the surrounding oxide-covered surface is labelled cathode (½O2 + H2O + 2e- → 2OH-); arrows show electron flow through the metal from anode to cathode, the NaCl solution with dissolved O2 above, and rust deposited between the anode and cathode.
