📘 IB Understanding
Corrosion is the oxidation of metals by reaction with oxygen and water. Rusting of iron is the most common example and requires both oxygen and water.
Rusting of Iron
Rust is hydrated iron(III) oxide, Fe2O3·xH2O. The process involves electrochemical cells set up on the metal surface:
Half-equations
- Oxidation (anodic area): Fe(s) → Fe2+(aq) + 2e-
- Reduction (cathodic area): O2(g) + 2H2O(l) + 4e- → 4OH-(aq)
Methods of Prevention
| Method | How It Works | Example |
|---|---|---|
| Painting / oiling | Physical barrier prevents O2/H2O contact | Car bodies, bridges |
| Galvanising | Zinc coating acts as barrier AND sacrificial metal | Steel roofing, fences |
| Sacrificial protection | More reactive metal oxidises preferentially | Zinc blocks on ship hulls |
| Electroplating | Corrosion-resistant metal layer (Cr, Ni) | Cutlery, taps |
| Alloying | Stainless steel (Fe + Cr + Ni) forms protective Cr2O3 layer | Kitchen equipment |
❗ Key Point
Sacrificial protection works because the more reactive metal (e.g. Zn) is oxidised instead of the iron. The zinc acts as the anode in an electrochemical cell, protecting the iron cathode even if the coating is scratched.
⚠️ Exam Tip
If the question says "explain how galvanising protects iron", mention both the barrier effect AND the sacrificial protection. Zinc oxidises preferentially because it has a more negative E° value than iron.