📖 In-Depth Theory
Corrosion and Rusting
CORROSION is the destruction of materials by chemical reactions with substances in the environment.
RUSTING — corrosion of iron:
Iron reacts with oxygen AND water to form hydrated iron(III) oxide (rust).
Both air AND water are necessary — iron will not rust in dry air alone or in pure water without dissolved oxygen.
Chemical equation:
4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O (rust)
Rusting is an OXIDATION reaction — iron loses electrons (Fe → Fe²⁺ → Fe³⁺).
FACTORS THAT SPEED UP RUSTING:
Salt water (electrolyte) — increases electrical conductivity → speeds up electrochemical corrosion.
Acidic conditions — lower pH speeds up the reaction.
Contact with a more reactive metal — galvanic corrosion.
ALUMINIUM:
Aluminium is MORE reactive than iron but resists corrosion in air.
Reason: forms a thin, DENSE OXIDE LAYER (Al₂O₃) on the surface.
This oxide layer is IMPERMEABLE — prevents further reaction.
Protects the metal underneath — called PASSIVATION.
Methods of Preventing Corrosion
All prevention methods work by either:
(a) Creating a BARRIER between the metal and oxygen/water, or
(b) Using SACRIFICIAL PROTECTION — a more reactive metal corrodes instead.
1. PAINTING:
Barrier coating — widely used on cars, ships, bridges.
Must be maintained — if paint chips, corrosion begins.
2. GREASING/OILING:
Barrier coating for moving parts — chains, hinges, machinery.
Prevents water and oxygen reaching the surface.
3. ELECTROPLATING:
Coating with another metal (e.g. chromium, tin, zinc) by electrolysis.
Chromium plating: shiny, decorative and corrosion-resistant.
Tin plating: used in food cans — tin is less reactive than iron, non-toxic.
4. GALVANISING — coating with ZINC:
Zinc is MORE REACTIVE than iron.
Two effects: (a) barrier coating; (b) sacrificial protection.
If zinc coating is scratched, zinc still protects iron — zinc corrodes preferentially.
5. SACRIFICIAL PROTECTION:
Attach a MORE REACTIVE metal to the structure.
The reactive metal corrodes instead of the iron.
Examples: zinc blocks on ship hulls; magnesium blocks on underground steel pipes.
Must be replaced periodically as the sacrificial metal is consumed.
Choosing the Right Method
The choice of prevention method depends on:
COST — painting is cheap; electroplating more expensive.
WHERE the metal is used — underwater requires sacrificial protection; indoor items can use paint.
APPEARANCE — chromium plating gives shiny decorative finish.
DURABILITY — galvanising outlasts paint for outdoor structures.
FOOD CANS — tin plating:
Tin is less reactive than iron — acts as barrier.
If tin layer is scratched: iron corrodes faster (galvanic cell — iron = anode).
This is why dented cans should be discarded — exposed iron rusts rapidly.
SHIPS AND OFFSHORE STRUCTURES:
Paint + sacrificial zinc/magnesium blocks.
Blocks regularly inspected and replaced.
BRIDGES:
Regular repainting — Forth Bridge famously requires continuous painting.
New bridges: weathering steel (forms protective rust layer) or galvanised steel.
⚠️ Common Mistake
Iron needs BOTH oxygen AND water to rust — not just one of them. Aluminium doesn't corrode despite being reactive because it forms an impermeable OXIDE LAYER. Galvanising uses zinc — zinc is more reactive than iron and provides SACRIFICIAL PROTECTION even if the coating is scratched.