What is rusting ? Describe the electrochemical theory of rusting of iron. List any two methods for the prevention of rusting. [5] GROUP: C Long answer questions 3 × 8 = 24

Rusting

Rusting is the slow, electro‑chemical corrosion of iron (or its alloys) in the presence of moisture and oxygen, producing hydrated iron(III) oxide $\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot n\mathrm{H}{}_2\mathrm{O}$ (commonly called rust). It is a red‑brown, flaky deposit that weakens the metal.

Electrochemical theory of rusting

Rusting proceeds as an electro‑chemical cell formed on the metal surface. Small, spatially separated areas act as anodes and cathodes, while the thin film of water containing dissolved salts serves as the electrolyte.

Half‑reactions

• Anodic (oxidation) reaction – iron loses electrons:

  $$\mathrm{Fe}(\mathrm{s})\to \mathrm{Fe}{}^{2+}(\mathrm{aq})+2\mathrm{e}{}^{-}$$

• Cathodic (reduction) reaction – dissolved oxygen is reduced in the presence of water:

  $$\mathrm{O}{}_2(\mathrm{g})+2\mathrm{H}{}_2\mathrm{O}(\mathrm{l})+4\mathrm{e}{}^{-}\to 4\mathrm{OH}{}^{-}(\mathrm{aq})$$

The electrons generated at the anodic sites travel through the metal to the cathodic sites, completing the circuit.

Overall process

Combining the two half‑reactions (multiply the anodic reaction by 2) gives

$$\begin{array}{rl}2\mathrm{Fe}(\mathrm{s})+\mathrm{O}{}_2(\mathrm{g})+2\mathrm{H}{}_2\mathrm{O}(\mathrm{l})& \to 2\mathrm{Fe}{}^{2+}(\mathrm{aq})+4\mathrm{OH}{}^{-}(\mathrm{aq})\\ \mathrm{Fe}{}^{2+}+2\mathrm{OH}{}^{-}& \to \mathrm{Fe}(\mathrm{OH}){}_2(\mathrm{s})\\ 4\mathrm{Fe}(\mathrm{OH}){}_2+\mathrm{O}{}_2+2\mathrm{H}{}_2\mathrm{O}& \to 2\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot 3\mathrm{H}{}_2\mathrm{O}(\mathrm{rust})\end{array}$$

Thus iron is first oxidised to $\mathrm{Fe}{}^{2+}$; the ferrous ions further react with hydroxide ions and oxygen to form hydrated ferric oxide, the characteristic rust.

Methods for preventing rusting

• Protective coating (painting, oiling, enamel):
  Forms a physical barrier that isolates iron from water and oxygen, interrupting the electro‑chemical circuit.

• Galvanisation (zinc coating):
  Zinc, being more electro‑positive, becomes the sacrificial anode. It oxidises preferentially, protecting the underlying iron.

Other viable methods (not required for the answer) include cathodic protection using a more active metal or the addition of corrosion inhibitors to the environment.