What Is Oxidation?
Oxidation is a chemical reaction in which a substance gains oxygen. It is one of the most important types of chemical reactions. Fast oxidation (with heat and light) is combustion. Slow oxidation (without flame) creates corrosion such as rusting.
In all oxidation reactions, a metal or non-metal combines with oxygen to form an oxide:
metal + oxygen → metal oxide
Examples:
- 2Mg + O₂ → 2MgO (magnesium burning in air — produces brilliant white light)
- 4Fe + 3O₂ → 2Fe₂O₃ (iron slowly forming rust)
What Is Rusting?
Rusting is the specific corrosion of iron (and steel, which contains iron). The product is iron(III) oxide (Fe₂O₃), commonly known as rust — a reddish-brown, brittle, flaky material.
The word equation is:
iron + oxygen + water → hydrated iron(III) oxide (rust)
Rusting is different from many oxidation reactions because it requires both oxygen and water to proceed. If only oxygen is present (completely dry conditions), iron rusts extremely slowly. If only water is present (with no dissolved oxygen), rusting is also very slow. Both together cause rapid rusting. Salt water accelerates rusting further because dissolved ions help conduct electricity between different parts of the metal surface.
Rusting Experiment
You can test what conditions cause rusting:
| Test tube | Contents | Result after 1 week |
|---|---|---|
| A | Iron nail + water + air | Rust forms |
| B | Iron nail + boiled water + oil seal (no air) | No rust |
| C | Iron nail + dry air (silica gel) | No rust |
| D | Iron nail + dry air + water | No rust (slow) |
Only test tube A, with both water AND oxygen (from air), shows significant rusting. This confirms both are needed.
Why Is Rusting a Problem?
Rust is a major problem because:
- It weakens steel structures (bridges, buildings, vehicles).
- Rust flakes off the surface, exposing fresh metal underneath, so the corrosion continues until the metal is completely destroyed.
- Rust damage costs billions of pounds each year worldwide.
When a metal reacts with oxygen, the oxide usually forms a protective layer. Aluminium and chromium do this well — their oxide layer sticks tightly to the surface and prevents further corrosion underneath. Iron is different: iron oxide (rust) flakes off the surface, exposing fresh metal to keep reacting.
Preventing Rusting
| Method | How it works |
|---|---|
| Painting or coating | Physical barrier keeps water and oxygen away |
| Galvanising | Coating iron with zinc; zinc corrodes preferentially, protecting iron |
| Stainless steel | Alloy with chromium, which forms a protective oxide layer |
| Greasing or oiling | Barrier method for moving parts like bicycle chains |
| Sacrificial protection | Attaching a more reactive metal (zinc or magnesium) which corrodes first |
Real world: Rusting cars and road salt
Cars rust faster in countries where road salt is used in winter. Salt water is a much better electrical conductor than pure water, accelerating the electrochemical corrosion process. Car manufacturers combat this with galvanised steel panels (zinc coating) and cavity wax. MOT tests in the UK check for excessive rust on structural components because rust weakens the steel and can make a car unsafe to drive.
Real world: Galvanised fences and scaffolding
Galvanised steel fences, guttering, and scaffolding poles are coated in zinc by dipping them in molten zinc (hot-dip galvanising). Even if the zinc layer is scratched, it still protects the iron underneath by sacrificial protection — zinc is higher in the reactivity series than iron, so it corrodes first, leaving the iron intact.
Real world: Aluminium cans and the protective oxide layer
Aluminium is a reactive metal, yet drinks cans do not corrode in the way iron does. When aluminium oxidises, it forms a thin, tightly bonded aluminium oxide (Al₂O₃) layer that clings to the surface and blocks further oxidation. This is very different from iron oxide (rust), which flakes off and exposes fresh iron. This is why aluminium is used in aircraft, window frames, and food packaging despite being highly reactive.
Where do you see this in real life? Rusty gates and car wheel arches (iron + water + oxygen), galvanised fence posts (zinc sacrificial protection), stainless steel kitchen sinks (chromium oxide layer), aluminium window frames (self-protecting oxide layer) — rust prevention is a billion-pound industry built on chemistry.
Practice Problems
An iron gate near the sea rusts much faster than an identical gate inland. Explain why.
Show answer
A student puts an iron nail in a test tube of cooled, boiled water and seals the tube with a layer of oil. Will the nail rust? Explain.