Corrosion of metals (New)

Rusting is the chemical corrosion of iron in the presence of water AND oxygen.

Equation (simplified). $4\text{Fe} + 3\text{O}_{2} \to 2\text{Fe}_{2}\text{O}_{3}.$

(In reality, rust is hydrated iron(III) oxide $\text{Fe}_{2}\text{O}_{3} \cdot x\text{H}_{2}\text{O}$, with variable water content.)

Required conditions. BOTH water AND oxygen must be present. Iron can sit indefinitely in dry air (no rust) or under boiled water with no air (still no rust).

Classic experiment (Cambridge favourite). Three test tubes with iron nails:

1. Air + water → RUSTS.
2. Boiled water + oil layer (no oxygen) → does NOT rust.
3. Anhydrous calcium chloride (dries the air) + air (no water) → does NOT rust.

Conclusion. Both water and oxygen are NECESSARY for rusting.

Accelerators of rusting.

• Salt water: salt provides ions that conduct → speeds up the electrochemistry of rusting.
• Acidity: acid eats through any protective coatings AND speeds up the reaction.
• Higher temperature: accelerates (chemical kinetics).

Why does rust matter? Rust is FLAKY — it falls off, exposing fresh iron underneath. Iron objects gradually disintegrate: structures lose strength, vehicles fail. Costs trillions globally per year in replacement and damage.

1. Paint / lacquer. Coats the metal, blocks water and air contact.

• Pros: cheap, easy to apply.
• Cons: scratched paint exposes iron locally; rust forms there and spreads under the paint.

2. Oil / grease. Used on machinery, tools, locks.

• Pros: also lubricates moving parts.
• Cons: needs frequent reapplication; impractical for static structures.

3. Plastic coating. Used on garden furniture, dishwasher racks, fence posts.

• Pros: more durable than paint.
• Cons: still scratchable.

4. Tin plating ("tinning"). Steel cans for food are coated with tin.

• Pros: tin is non-toxic, doesn't react with food.
• Cons: if scratched, IRON RUSTS FASTER than uncovered (because tin is below iron in the reactivity series → iron is the more reactive partner → iron is "sacrificed" to protect tin). Hence: dented tin cans rust quickly.

Worked qualitative. Why is a chrome-plated bicycle handlebar still vulnerable to rust? Chromium plating is a barrier. If scratched, the underlying steel is exposed → can rust. AND because chromium is similar in reactivity to iron, no sacrificial protection.

Galvanising. Coating iron or steel with a thin layer of ZINC. Done by:

• Dipping in molten zinc.
• Electroplating with zinc.

Why zinc protects, even if scratched. Zinc is MORE REACTIVE than iron. So in any electrochemical cell formed by water/oxygen on the surface:

• Zinc oxidises preferentially (giving up electrons).
• Iron does NOT oxidise (stays metallic).

The zinc is "sacrificed" — it slowly corrodes away — while the iron remains intact.

Sacrificial protection (more general). Attach a block of a more reactive metal (zinc or magnesium) to the iron object. Zinc/Mg corrodes preferentially; iron stays intact.

Examples.

• Underground pipelines: magnesium blocks attached at intervals.
• Ship hulls: zinc plates bolted to the hull. The plates corrode and are replaced periodically.
• Galvanised steel buckets: zinc coating.

Worked qualitative. A car body is rust-protected by a zinc undercoat. A small stone chips the paint and zinc. Why doesn't the iron rust at the chip? Because the surrounding zinc is still in electrical contact with the iron AND in moist air → zinc oxidises in preference to iron. Iron remains protected.

Why aluminium and chromium don't need sacrificial protection. Both form a tough, IMPERMEABLE oxide layer on their surface ($\text{Al}_{2}\text{O}_{3}$, $\text{Cr}_{2}\text{O}_{3}$). The layer blocks O₂ and water from reaching the metal underneath. Doesn't flake off (unlike rust).

Stainless steel. Iron alloyed with chromium ($\sim 11\%+$) and nickel.

How it resists corrosion.

• The Cr atoms oxidise on the surface → form a thin, INVISIBLE Cr₂O₃ layer.
• The layer is impermeable to O₂ and water → underlying iron is protected.
• The layer is SELF-HEALING — if scratched, fresh Cr from below oxidises and reforms it.

Worked qualitative. Why is stainless steel surgical equipment preferable to mild steel?

• Resistance to rust + body fluids (corrosive).
• Easy to sterilise at high temperature without rusting.
• Hygienic — smooth, non-porous surface.
• Long-lasting — pays for itself.

Trade-off. Stainless steel is significantly more expensive than mild steel. For uses where corrosion isn't a problem (e.g. interior beams), mild steel + paint is cheaper.

Cambridge tip. Memorise three layers of corrosion protection:

1. BARRIER (paint, oil): blocks contact, but vulnerable to scratches.
2. SACRIFICIAL (galvanising, zinc anodes): more reactive metal corrodes first.
3. ALLOYING (stainless steel): chromium oxide self-protects.

Best answer: combine multiple methods (e.g. galvanised AND painted = more durable than either alone).