Corrosion 😊

Corrosion slowly coats a metal surface with oxides or other salts. It ruins bridges, ships, machinery, and even household items, wasting crores of rupees every year! 🔧🌧️

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What really happens?

  • The metal loses electrons (oxidation) and becomes ions.
  • Oxygen picks up those electrons (reduction) and forms water.
  • Water and air start the process, so damp environments speed it up. 💧
  • The metal surface splits into tiny “anode” and “cathode” spots, acting like a mini-battery. ⚡

Rusting of iron step-by-step 🔩

Anode (oxidation spot):
$$2\,\text{Fe(s)} \;\rightarrow\; 2\,\text{Fe}^{2+}(aq) + 4e^- \qquad E^{\circ}_{\text{Fe}^{2+}/\text{Fe}} = -0.44\,\text{V}$$

Cathode (reduction spot):
$$\text{O}_{2}(g) + 4\text{H}^+(aq) + 4e^- \;\rightarrow\; 2\text{H}_{2}\text{O}(l) \qquad E^{\circ}_{\text{O}_{2}/\text{H}_{2}\text{O}} = +1.23\,\text{V}$$

Overall reaction:
$$2\,\text{Fe(s)} + \text{O}_{2}(g) + 4\text{H}^+(aq) \;\rightarrow\; 2\,\text{Fe}^{2+}(aq) + 2\text{H}_{2}\text{O}(l) \qquad E^{\circ}_{\text{cell}} = +1.67\,\text{V}$$

The fresh Fe2+ ions keep reacting:

$$2\,\text{Fe}^{2+}(aq) + 2\text{H}_{2}\text{O}(l) + \tfrac12\text{O}_{2}(g) \;\rightarrow\; \text{Fe}_{2}\text{O}_{3}\cdot x\text{H}_{2}\text{O}(s) + 4\text{H}^+(aq)$$

The reddish-brown Fe2O3·xH2O is the familiar rust. 🧱

Stopping corrosion 🛡️

  • Block air: Paint, oil, or special chemicals (e.g., bisphenol) create a barrier. 🎨
  • Coat with another metal: Tin or zinc plating covers the surface and keeps oxygen out. 🧲
  • Sacrificial protection: Attach a more active metal such as Mg or Zn; it corrodes first and saves the main structure. ⚙️

Important Concepts for NEET 📝

  1. Corrosion acts as a tiny galvanic cell with separate anode and cathode spots.
  2. Key half-reactions: iron oxidation (E° = –0.44 V) and oxygen reduction (E° = +1.23 V).
  3. The overall positive cell potential (+1.67 V) drives rusting forward.
  4. Water, dissolved CO2, and H+ ions accelerate the process.
  5. Protection methods: painting, metal plating, and sacrificial anodes (cathodic protection).