Corrosion and Rusting
Corrosion is the degradation of metals by oxidation reactions with atmospheric oxygen and moisture. Rusting specifically refers to the corrosion of iron and iron-containing alloys (steel).
Rusting of Iron
Rust is hydrated iron(III) oxide, Fe2O3·xH2O. The process involves electrochemical cells set up on the metal surface:
Redox Half-Equations of Rusting
\(\text{Fe}^{2+}\) and \(\text{OH}^-\) form \(\text{Fe(OH)}_2\), which is further oxidised to hydrated iron(III) oxide (\(\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}\), rust).
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 |
Sacrificial Cathodic Protection
In sacrificial protection, a more reactive metal (e.g. \(\text{Zn}\) or \(\text{Mg}\)) with a more negative \(E^\circ\) oxidises preferentially, donating electrons to the iron structure and keeping iron in its reduced metallic state.
Dual Protection of Galvanised Iron
Galvanised iron: Even when the zinc coating is scratched, the remaining zinc continues to protect the exposed iron sacrificially because \(\text{Zn}\) has a more negative \(E^\circ\) than \(\text{Fe}\).
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