Rusting as an electrochemical cell, and why scratched galvanised iron survives but scratched tin plate doesn't

6 answers & replies

• [+][–]Kabir Joshi11thSample· 2mo ago

  Reactions (NCERT), with standard potentials:

  Anode: $2\text{Fe} \to 2\text{Fe}^{2+} + 4e^-$, with $E^\circ(\text{Fe}^{2+}/\text{Fe}) = -0.44$ V Cathode: $\text{O}_2 + 4\text{H}^+ + 4e^- \to 2\text{H}_2\text{O}$, with $E^\circ = +1.23$ V Overall: $2\text{Fe} + \text{O}_2 + 4\text{H}^+ \to 2\text{Fe}^{2+} + 2\text{H}_2\text{O}$, $E^\circ_{cell} = 1.23 - (-0.44) = 1.67$ V

  So it's strongly spontaneous. Then in the air: $4\text{Fe}^{2+} + \text{O}_2 + 4\text{H}_2\text{O} \to 2\text{Fe}_2\text{O}_3 + 8\text{H}^+$, and Fe₂O₃ picks up water as $\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}$ - rust. The H⁺ in plain water comes from dissolved CO₂ forming carbonic acid (and water's own tiny ionisation).

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• [+][–]Dev Agarwal12thSample· 2mo ago

  Tin is less reactive than zinc, so it's actually the better coat, it's very stable and doesn't react. Galvanised needs zinc to keep reacting and get used up. So tin is better protection, I'd say. If it's scratched it still covers most of the surface anyway. Zinc keeps getting consumed, so the coating wears off and you have to renew it, which sounds like a worse deal to me.

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  • [+][–]Neel Chatterjee12thSample· 2mo ago

    It is more stable, true, but only as long as the coating is intact. Once scratched, iron is exposed to air and water next to tin, and a cell forms. Look at potentials: $E^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76$ V, $E^\circ(\text{Fe}^{2+}/\text{Fe}) = -0.44$ V, $E^\circ(\text{Sn}^{2+}/\text{Sn}) = -0.14$ V. The more negative one is oxidised. With zinc coating scratched, zinc is more easily oxidised than iron, so Zn corrodes (sacrificial protection) and iron is protected. With tin scratched, iron is more easily oxidised than tin, so iron becomes the anode and corrodes faster than it would without a tin coating. That's why a damaged tin can is worse.

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    • [+][–]Farhan ShaikhDropperSample· 2mo agoAuthor

      Scratched tin accelerating rust, that is the line I was missing. Thanks for the potentials, I can actually reason it out now, not memorise.

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• [+][–]Ishaan Bose11thSample· 2mo ago

  Sea: salty water is a better electrolyte (more ions carry the current), so the little corrosion cells work faster. Same reason bikes near the coast rust like crazy. (Mine rusted in two monsoons, don't ask.) Also acidic rain and CO₂ help because H⁺ ions take part in the cathode reaction.

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• [+][–]Nisha Verma12thSample· 2mo ago

  Prevention, in the way it's asked:

  • Barrier: paint, grease, oil, enamel, plastic coatings.
  • Galvanising: zinc coating (works even when scratched).
  • Electroplating with Cr, Ni, Sn: a barrier only.
  • Sacrificial anode / cathodic protection: blocks of Mg or Zn connected to underground pipes or ship hulls, the Mg corrodes instead of the iron.
  • Alloying: stainless steel (Cr and Ni in iron) forms a protective oxide film.

  Mnemonic: 'the more reactive metal sacrifices'.

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