Corrosion and Alloys

Metals are among the most widely used materials in the modern world, but they are vulnerable to corrosion — a chemical process that weakens and destroys them over time. Understanding corrosion and how to prevent it, as well as how alloys are used to improve metal properties, is essential for AQA GCSE Chemistry: Using Resources.

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What Is Corrosion?

Corrosion is the destruction of a metal by chemical reactions with substances in the environment. It is an oxidation reaction — the metal atoms lose electrons and form metal oxide (or other compounds).

The most familiar example is the rusting of iron, but all metals can corrode.

Term	Definition
Corrosion	The gradual destruction of a material (usually a metal) by chemical reaction with its environment
Rusting	The specific corrosion of iron (or steel) in the presence of water and oxygen, producing hydrated iron(III) oxide
Oxidation	A reaction where a substance gains oxygen or loses electrons

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Rusting of Iron

Rusting is the corrosion of iron. For rusting to occur, both water and oxygen must be present. If either is absent, rusting does not occur.

The word equation is:

iron + water + oxygen -> hydrated iron(III) oxide (rust)

Rust is a flaky, orange-brown substance that does not protect the metal underneath — it flakes off, exposing fresh metal to further corrosion. This is why rusting is so destructive.

graph TD
    A[Iron / Steel Object] --> B{"Are both water AND<br>oxygen present?"}
    B -->|Yes| C[Rusting occurs]
    B -->|No| D[No rusting]
    C --> E["Hydrated iron III oxide<br>Rust forms on surface"]
    E --> F["Rust flakes off,<br>exposing more metal"]
    F --> C
    style A fill:#bbdefb,stroke:#1565c0
    style C fill:#ef9a9a,stroke:#c62828
    style D fill:#c8e6c9,stroke:#2e7d32
    style E fill:#ffcc80,stroke:#e65100

Experimental Evidence

The classic experiment to demonstrate rusting uses three test tubes:

Test Tube	Conditions	Result
A	Iron nail in water with dissolved oxygen (ordinary tap water)	Nail rusts — both water and oxygen are present
B	Iron nail in boiled water (to remove dissolved oxygen) sealed with oil on top	Nail does not rust — oxygen has been removed
C	Iron nail in a dry test tube with anhydrous calcium chloride (desiccant) to absorb moisture	Nail does not rust — water has been removed

Exam Tip: This experiment is a classic AQA question. You must explain that only the nail in tube A rusts because both water and oxygen are present. The boiled water in tube B has had its dissolved oxygen removed, and the oil prevents oxygen re-entering. The desiccant in tube C removes moisture. Both water AND oxygen are required.

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Preventing Corrosion

Since rusting requires both water and oxygen, prevention methods work by creating a barrier between the iron and these substances, or by using a more reactive metal to protect the iron.

Barrier Methods

Method	How It Works	Examples
Painting	A layer of paint prevents water and oxygen reaching the metal	Cars, bridges, railings
Oiling or greasing	A layer of oil or grease acts as a barrier	Bike chains, moving machinery parts
Plastic coating	A layer of plastic prevents contact with water and oxygen	Fridge shelves, garden furniture, paper clips
Electroplating	A thin layer of another metal (e.g. chromium, tin, nickel) is deposited on the iron by electrolysis	Tin cans (tin-plated steel), chrome bumpers

Sacrificial Protection (Galvanisation)

Sacrificial protection uses a more reactive metal to protect iron. The more reactive metal corrodes instead of the iron — it "sacrifices" itself.

Galvanisation is the most common form of sacrificial protection. Iron or steel is coated with a layer of zinc. Zinc is more reactive than iron, so:

1. The zinc layer acts as a physical barrier (like paint)
2. Even if the zinc layer is scratched, the zinc corrodes preferentially because it is more reactive
3. The iron is protected as long as some zinc remains in contact with it

Protection Method	How It Works	Advantage	Disadvantage
Galvanisation (zinc coating)	Zinc is more reactive; corrodes instead of iron	Provides both barrier and sacrificial protection; cheap	Zinc layer may eventually corrode away
Sacrificial blocks (zinc or magnesium)	Blocks of reactive metal are attached to iron structures	Protects large structures (ships, pipelines, oil rigs)	Blocks must be regularly replaced

Exam Tip: Sacrificial protection works because of the reactivity series. The more reactive metal (zinc or magnesium) is oxidised preferentially, protecting the less reactive iron. If asked to explain why this works, you must reference reactivity. Simply saying "the zinc corrodes instead" is not a complete answer.

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Corrosion of Other Metals

While rusting specifically refers to iron, other metals also corrode — but some form a protective oxide layer:

Metal	What Happens When It Corrodes	Protective?
Aluminium	Forms aluminium oxide (Al2O3) on the surface	Yes — the oxide layer is tough, adherent and prevents further corrosion
Copper	Forms a green layer of copper carbonate (verdigris / patina)	Yes — the patina slows further corrosion
Iron	Forms rust (hydrated iron(III) oxide)	No — rust is flaky and falls off, exposing more metal
Gold	Does not corrode (very unreactive)	Not applicable

Exam Tip: A common comparison question asks why aluminium does not appear to corrode despite being more reactive than iron. The answer is that aluminium forms a thin, tough oxide layer that adheres to the surface and prevents further reaction. Iron oxide (rust) is flaky and falls off, so corrosion continues.

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What Are Alloys?

An alloy is a mixture of two or more elements, at least one of which is a metal. Most alloys are mixtures of two or more metals, but some contain non-metals (e.g. steel contains iron and carbon).

Why Are Alloys Useful?

Pure metals are often too soft for many uses. In a pure metal, the atoms are arranged in regular layers that can slide over each other easily.

In an alloy, atoms of different sizes disrupt the regular arrangement, preventing the layers from sliding. This makes alloys harder and stronger than pure metals.

Property	Pure Metal	Alloy
Atom arrangement	Regular layers of identical atoms	Irregular arrangement of differently-sized atoms
Hardness	Often soft	Harder
Strength	Relatively weak	Stronger
Malleability	Very malleable	Less malleable (but can be tailored)

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Common Alloys