Rusting costs the UK economy billions of pounds each year. Iron corrodes when oxygen and water both reach its surface, forming hydrated iron(III) oxide. Several protection methods — from barrier coatings to sacrificial anodes — exploit chemistry to slow or stop this reaction.
What is corrosion and why does iron rust?
Corrosion is the gradual destruction of a metal by chemical reaction with substances in its environment. For iron and steel, the specific form of corrosion is rusting — the formation of hydrated iron(III) oxide (Fe₂O₃·xH₂O), the familiar red-brown material.
Conditions required for rusting:
Rusting requires BOTH:
- Oxygen (from air)
- Water
Neither oxygen alone nor water alone causes iron to rust at any appreciable rate. This is demonstrated by the classic three-test-tube experiment:
| Test tube | Contents | Does iron rust? |
|---|---|---|
| A | Iron nail in dry air (silica gel removes moisture) | No |
| B | Iron nail in boiled, sealed water (no dissolved O₂) | No |
| C | Iron nail in ordinary water open to air | Yes |
The rusting reaction involves the oxidation of iron:
4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃·xH₂O(s)
Salt water accelerates rusting because dissolved ions increase the conductivity of the water, speeding up the electrochemical process. This is why cars corrode faster in coastal regions or on salted winter roads.
Unlike aluminium oxide (which forms a thin, dense, adherent layer that protects the underlying metal), rust is porous and flaky — it does not stick tightly to the iron surface and easily falls off, exposing fresh iron to further attack. Rusting is therefore a progressive, self-perpetuating process.
How does aluminium resist corrosion?
Aluminium is a reactive metal (higher in the reactivity series than iron), yet aluminium objects — aeroplanes, drinks cans, window frames — last for decades without visible corrosion. The reason is that aluminium reacts rapidly with oxygen in air to form a thin, tightly bonded layer of aluminium oxide (Al₂O₃). This oxide layer is:
- Chemically very stable and unreactive
- Non-porous and tightly adherent — it does not flake off
- Self-repairing — if scratched, the exposed aluminium immediately re-oxidises
This is why "anodising" aluminium (deliberately thickening the oxide layer by electrolysis) is used to improve its corrosion resistance further. Iron lacks this self-protecting oxide property.
What are barrier methods of corrosion protection?
Barrier methods work by physically preventing oxygen and water from reaching the iron surface:
| Method | Description | Advantages | Limitations |
|---|---|---|---|
| Painting | One or more layers of paint coat the surface | Cheap; can be decorative; suitable for large structures (bridges, ships) | Must be maintained — any scratch or chip exposes iron; paint weathers and must be reapplied |
| Oil or grease | Applied to metal surfaces | Easy to apply; penetrates joints | Wears off; not suitable for structural metal |
| Plastic coating | Polymer layer over the metal | Durable; colourful | Expensive to apply to complex shapes; must be complete or corrosion starts at gaps |
| Tin plating (tinning) | Thin layer of tin over steel | Tin is non-toxic; used for food cans | If the tin layer is scratched, the exposed steel rusts rapidly (tin is less reactive than iron and provides no sacrificial protection) |
All barrier methods share a critical weakness: once the barrier is breached, corrosion begins at the exposed point.
What is galvanising and how does it provide double protection?
Galvanising involves coating iron or steel with a layer of zinc. This can be done by hot-dipping (immersing the iron in molten zinc) or electroplating.
Galvanising provides two distinct forms of protection simultaneously:
-
Barrier protection: The zinc layer physically prevents oxygen and water from reaching the iron surface — exactly as paint would.
-
Sacrificial protection: Zinc is more reactive than iron (zinc is higher in the reactivity series). If the zinc layer is scratched or chipped, exposing the iron beneath, the zinc preferentially corrodes rather than the iron. Zinc acts as the sacrificial anode and iron as the cathode — the zinc is oxidised (sacrificed) and the iron is protected. This continues until all the local zinc is consumed.
This is why galvanised buckets, farm gates, and motorway crash barriers continue to resist corrosion even when scratched — the zinc sacrifices itself to protect the iron beneath. Tin plating does NOT offer this advantage: tin is less reactive than iron, so if a tin-plated can is scratched, the iron beneath corrodes preferentially while the tin remains intact.
What is a sacrificial anode?
A sacrificial anode (or sacrificial protection) uses a more reactive metal, attached to or placed near the iron object, which corrodes preferentially:
- The more reactive metal (the anode) is oxidised and loses electrons
- The iron/steel (the cathode) is protected from oxidation because it receives electrons from the anode
Common applications:
- Ship hulls: Blocks of zinc or magnesium are bolted to the steel hull. They corrode slowly and are replaced periodically. This protects the hull without needing the entire hull to be covered in zinc.
- Offshore oil rigs and pipelines: Similar blocks protect steel underwater structures.
- Hot water tanks: A magnesium rod inside a steel hot water heater corrodes preferentially, protecting the steel.
Sacrificial anodes are particularly useful for structures that are difficult to paint or inspect, or for areas where the barrier coating has been damaged.
What is electroplating?
Electroplating uses electrolysis to deposit a thin layer of one metal over another. The object to be plated acts as the cathode in an electrolysis cell; the plating metal is the anode (or dissolved in the electrolyte). Metal ions from the electrolyte are reduced at the cathode and deposited as a thin metal coating.
Examples:
- Chrome plating on car bumpers and bathroom taps (decorative and protective)
- Gold plating on jewellery (decorative; gold is unreactive)
- Silver plating on cutlery
- Nickel plating as an undercoat for chrome (better adhesion)
- Zinc electroplating — an alternative to hot-dip galvanising for smaller, precision items
Electroplating produces a very uniform, controllable thickness of coating and can plate complex shapes evenly. However, if the plating is purely a barrier (like chrome or tin), a scratch removes the protection completely.
Frequently asked questions
Why does salt water make iron rust faster?
Rusting involves an electrochemical process in which iron atoms at the surface lose electrons (are oxidised) and oxygen gains electrons (is reduced). This process is a form of corrosion cell, which requires ions in solution to carry charge between the anodic (iron-dissolving) and cathodic (oxygen-reducing) regions. Pure water has very few ions and very low conductivity, so the electrochemical reaction is slow. Salt water is an excellent electrolyte — it is full of sodium and chloride ions — dramatically increasing conductivity and therefore the speed of the corrosion process. This is why bridges, vehicles, and ships in coastal environments corrode far faster than identical structures inland.
Why is galvanising better than tin-plating for protection against corrosion?
Both galvanising and tin-plating form a barrier layer that protects the iron underneath. The critical difference is what happens when the barrier is breached. Zinc is more reactive than iron: when a scratch exposes iron beneath a zinc coat, zinc corrodes sacrificially and the iron is protected. Tin is less reactive than iron: when a scratch exposes iron beneath a tin coat, iron corrodes first (faster than tin). This means a scratched tin-plated can actually corrodes faster than uncoated steel, because the presence of tin sets up a galvanic cell that accelerates iron's oxidation. Galvanising therefore provides superior long-term protection, especially for outdoor structures.
How does electroplating use electrolysis?
In electroplating, the item to be coated is connected as the cathode (negative electrode) in an electrolysis circuit. The anode (positive electrode) is made of the plating metal, and the electrolyte is a solution containing ions of that metal (e.g. copper sulfate solution for copper plating). When current flows: the anode dissolves (Cu → Cu²⁺ + 2e⁻, replenishing the electrolyte), and copper ions in solution are reduced at the cathode (Cu²⁺ + 2e⁻ → Cu), depositing as solid copper on the object. By controlling the current and time, the thickness of the deposit can be precisely controlled. The plated layer is typically only a few micrometres thick but can be built up for specific applications.
Can painted iron ever be protected if the paint is chipped?
If the iron is protected only by a paint barrier, then yes — a chip in the paint exposes bare iron to air and water, and rusting begins at that point. However, some paints contain primer coats that include corrosion inhibitors — chemicals that either react with the iron surface to form a protective layer (phosphoric acid-based primers convert iron to iron phosphate) or contain zinc particles in the paint (zinc-rich primers act as a local sacrificial anode at the damaged area). Modern automotive paints use multi-layer systems that combine phosphate conversion coatings, epoxy primers, and topcoats specifically to provide some sacrificial or inhibitor protection even at scratched areas.
For particle-model-first GCSE chemistry with Professor Curie — explaining every corrosion protection method from the electron-transfer level up — visit aitutors.me.