Corrosion and Its Prevention

(Separate Chemistry only) Topic 4.10.3 is assessed only in GCSE Chemistry, not in Combined Science.

Corrosion is the destruction of a material by chemical reactions with substances in its environment. It happens because many metals react with oxygen, water and other chemicals around them, gradually breaking down the metal.

The most familiar example is the corrosion of iron, which has its own name: rusting. Rust is a form of iron oxide, an orange-brown flaky solid that crumbles away from the surface. Because rust flakes off, it exposes fresh iron underneath, so the rusting continues until the whole object is eaten away.

Corrosion = destruction of a material by chemical reaction with its environment. Rusting is the specific corrosion of iron.

Not all corrosion is destructive in the same way. Some metals, such as aluminium, form a protective layer that stops the reaction going further, which is covered later in this lesson.

For iron to rust, both air (which supplies oxygen) and water must be present. Remove either one and rusting does not happen. This is the key fact examiners test, and it is shown by a classic three-tube experiment.

Iron nails are placed in three sealed test tubes set up to control what the iron is exposed to:

Test tube	Contents	Air present?	Water present?	Result
A	Nail in water plus air (open to the atmosphere)	Yes	Yes	Rusts
B	Nail in water that has been boiled, sealed with a layer of oil	No	Yes	Does not rust
C	Nail in dry air, with a drying agent to absorb any moisture	Yes	No	Does not rust

In tube B, boiling the water drives out dissolved air (oxygen), and the oil stops more air dissolving back in, so there is water but no air. In tube C, the drying agent (such as anhydrous calcium chloride) keeps the air dry, so there is air but no water. Only tube A, with both air and water, shows rusting.

The experiment proves rusting needs air and water by removing one factor at a time. Tube A is the control that has both.

The simplest way to stop rusting is to keep air and water away from the iron using a barrier coating. If the reactants cannot reach the metal, the reaction cannot happen.

Common barrier methods:

Method	How it protects	Example use
Greasing / oiling	A layer of oil or grease keeps air and water off the surface	Moving parts like bike chains
Painting	A paint layer seals the surface from air and water	Car bodies, railings, bridges
Electroplating	Electrolysis coats the iron with a thin layer of another metal	Steel food cans plated with tin

Electroplating uses electrolysis to deposit a thin, even layer of a metal over the object, both to protect it and to improve its appearance.

A barrier only works while it is intact. If paint chips or grease wears away, air and water reach the iron and rusting begins at that spot.

This weakness of barrier methods is why more reactive-metal coatings, covered next, can be better: they keep protecting even if the coating is scratched.

Aluminium is more reactive than iron, so you might expect it to corrode faster. It does not, and the reason is the oxide it forms.

When aluminium meets oxygen in the air, its surface reacts to form a thin layer of aluminium oxide. This layer is strongly bonded to the metal and does not flake off. It seals the aluminium underneath, acting as a barrier that stops air and water reaching the fresh metal, so the corrosion stops after this thin layer has formed.

This is different from rust. Rust is flaky and falls off, exposing new iron, so iron keeps corroding. Aluminium oxide stays put and protects the metal, so aluminium objects last a long time even when uncoated.

Metal	Oxide behaviour	Result
Iron	Rust flakes off, exposing fresh iron	Corrosion continues into the metal
Aluminium	Oxide layer sticks and seals the surface	Corrosion stops; metal is protected

Aluminium's own oxide layer is a built-in barrier. This is why aluminium is used for window frames and drinks cans without needing paint.

Sacrificial protection uses a more reactive metal to protect a less reactive one. The more reactive metal corrodes in place of the metal being protected, sacrificing itself.

Zinc is commonly used to protect iron because zinc is more reactive than iron. When the two metals are in contact, the environment attacks the more reactive zinc first, so the zinc corrodes and the iron does not. As long as some zinc remains, the iron is protected.

This works because of the reactivity series: the more reactive metal always reacts in preference to the less reactive one.

$\text{reactivity: }\text{zinc}>\text{iron}$

Sacrificial protection: a more reactive metal (zinc) corrodes instead of the metal you want to save (iron). Explain it in terms of relative reactivity.

Crucially, sacrificial protection keeps working even if the surface is scratched, because the zinc protects the iron by reacting first rather than only by covering it. This is its advantage over a plain barrier coating.

Galvanising means coating iron or steel with a layer of zinc. It is a common way to protect items such as steel buckets, corrugated roofing and outdoor fencing, and it protects in two ways at once.

1. As a barrier. While the zinc layer is intact, it keeps air and water off the iron, just like paint.
2. By sacrificial protection. If the zinc coating is scratched and the iron is exposed, the zinc still protects the iron, because zinc is more reactive and corrodes in preference to the iron underneath.

This double action is what makes galvanising better than a paint or grease barrier. A scratch in paint lets rusting start; a scratch in galvanising is still protected because the surrounding zinc sacrifices itself.

Protection type	Works when coating is intact?	Works when coating is scratched?
Painting	Yes	No: rusting starts at the scratch
Galvanising	Yes	Yes: zinc sacrifices itself

Galvanising = coating with zinc. It is both a barrier and sacrificial protection, so it protects even where the coating is damaged.

1. Naming only one condition for rusting

Iron needs both air (oxygen) and water to rust. Answers that give only water, or only air, do not score. State both.

2. Explaining tube B or C without saying what was removed

In the rusting experiment, tube B has no air (boiled water sealed with oil) and tube C has no water (drying agent). Say which factor was removed and how, not just that the nail did not rust.

3. Saying aluminium does not react with oxygen

Aluminium does react with oxygen. It just forms a thin, stuck-on oxide layer that seals the surface and stops further corrosion. The protection comes from the oxide, not from a lack of reaction.

4. Describing sacrificial protection as a barrier

Sacrificial protection works because the more reactive metal corrodes instead of the iron, even if the iron is exposed. Explain it using the reactivity series, not as a physical coating.

5. Forgetting galvanising protects a scratch

The strength of galvanising is that the zinc keeps protecting the iron even where the coating is scratched, because zinc is more reactive. Compare this with paint, which fails at a scratch.