Corrosion and its prevention

What is Corrosion?

Corrosion is the process by which metals are slowly broken down by reacting with substances in their environment, such as oxygen and water.
Rusting is a specific type of corrosion where iron reacts with oxygen and water to form hydrated iron oxide, commonly known as rust.

The Redox Chemistry of Rusting

During rusting, iron atoms each lose three electrons to form iron(III) ions (Fe³⁺), meaning iron is oxidised.
Each oxygen molecule gains four electrons in total, with each oxygen atom becoming an oxide ion (O²⁻), meaning oxygen is reduced.
Because both oxidation and reduction occur in the same reaction, rusting is classified as a redox reaction.

2. The Redox Chemistry of Rusting AQA

Notes

During rusting, iron atoms each lose three electrons to form iron(III) ions (Fe³⁺), meaning iron is oxidised.
Each oxygen molecule gains four electrons in total, with each oxygen atom becoming an oxide ion (O²⁻), meaning oxygen is reduced.
Because both oxidation and reduction occur in the same reaction, rusting is classified as a redox reaction.

Conditions Required for Rusting

Both oxygen and water must be present simultaneously for rusting to occur; neither alone is sufficient.
A nail in boiled water sealed with oil will not rust because dissolved oxygen has been removed and no new oxygen can enter.
A nail in dry air containing calcium chloride will not rust because all water vapour has been absorbed, leaving no moisture.
Only a nail exposed to both air (oxygen) and water will rust, confirming that both conditions are essential.

3. Conditions Required for Rusting AQA

Notes

Both oxygen and water must be present simultaneously for rusting to occur; neither alone is sufficient.
A nail in boiled water sealed with oil will not rust because dissolved oxygen has been removed and no new oxygen can enter.
A nail in dry air containing calcium chloride will not rust because all water vapour has been absorbed, leaving no moisture.
Only a nail exposed to both air (oxygen) and water will rust, confirming that both conditions are essential.

How Corrosion Affects Different Metals

Only the surface of a metal corrodes because only the surface is exposed to oxygen and water in the environment.
When iron rusts, the rust flakes off the surface, exposing fresh metal underneath, which can then rust in turn, eventually destroying the entire piece.
When aluminium corrodes, the aluminium oxide formed on the surface creates a tough, adherent protective layer that prevents further oxidation of the metal beneath.

4. How Corrosion Affects Different Metals AQA

Notes

Only the surface of a metal corrodes because only the surface is exposed to oxygen and water in the environment.
When iron rusts, the rust flakes off the surface, exposing fresh metal underneath, which can then rust in turn, eventually destroying the entire piece.
When aluminium corrodes, the aluminium oxide formed on the surface creates a tough, adherent protective layer that prevents further oxidation of the metal beneath.

Barrier Methods of Rust Prevention

Barrier methods work by physically preventing oxygen and water from coming into contact with the iron surface.
Painting is a common barrier method used for large iron structures, such as the hulls of ships.
Oiling or greasing is used where moving parts are involved, such as a bicycle chain, as it provides a flexible protective coating.
Electroplating uses electrolysis to deposit a thin layer of another metal over the iron, forming a durable protective barrier.

5. Barrier Methods of Rust Prevention AQA

Notes

Barrier methods work by physically preventing oxygen and water from coming into contact with the iron surface.
Painting is a common barrier method used for large iron structures, such as the hulls of ships.
Oiling or greasing is used where moving parts are involved, such as a bicycle chain, as it provides a flexible protective coating.
Electroplating uses electrolysis to deposit a thin layer of another metal over the iron, forming a durable protective barrier.

Sacrificial Methods of Rust Prevention

The sacrificial method involves attaching a more reactive metal, such as aluminium or zinc,
to the iron so that it is oxidised preferentially instead of the iron.
Because the more reactive metal sacrifices itself by reacting with oxygen first, the iron underneath is protected from rusting.

6. Sacrificial Methods of Rust Prevention AQA

Notes

The sacrificial method involves attaching a more reactive metal, such as aluminium or zinc,
to the iron so that it is oxidised preferentially instead of the iron.
Because the more reactive metal sacrifices itself by reacting with oxygen first, the iron underneath is protected from rusting.

Galvanising: Combining Both Methods

Galvanising involves coating an iron object in a layer of zinc, which acts as a barrier method by forming a protective coating over the entire surface.
If the zinc coating is scratched and the underlying iron is exposed, the zinc still acts sacrificially, reacting with oxygen in preference to the iron due to its greater reactivity.
Galvanising is therefore an example of both barrier protection and sacrificial protection working together.

7. Galvanising: Combining Both Methods AQA

Notes

Galvanising involves coating an iron object in a layer of zinc, which acts as a barrier method by forming a protective coating over the entire surface.
If the zinc coating is scratched and the underlying iron is exposed, the zinc still acts sacrificially, reacting with oxygen in preference to the iron due to its greater reactivity.
Galvanising is therefore an example of both barrier protection and sacrificial protection working together.

AQA student objectives

All · Most · Some

Key knowledge

What is Corrosion?

Notes

The Redox Chemistry of Rusting

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Conditions Required for Rusting

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How Corrosion Affects Different Metals

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Barrier Methods of Rust Prevention

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Sacrificial Methods of Rust Prevention

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Galvanising: Combining Both Methods

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