Corrosion and its prevention — AQA GCSE Chemistry
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Corrosion and its prevention explained
Corrosion and its prevention covers 6 specification points, including Corrosion is the destruction of materials by… and Both air and water are necessary for iron to….
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Corrosion is the destruction of materials by chemical reactions with substances in the environment. Rusting is an example of corrosion.
Corrosion is the breakdown of a material caused by chemical reactions with substances around it, such as oxygen, water or acids. The material is destroyed because it is chemically changed into new substances. Rusting is the corrosion of iron and steel. It needs both oxygen and water, and it forms hydrated iron(III) oxide, which is the familiar orange-brown rust. The rust flakes away, exposing fresh iron, so the metal gradually weakens and is destroyed. Corrosion is not only about iron: copper reacts slowly with oxygen and water to form a green layer, and acid rain can corrode limestone and some metals. Understanding corrosion helps explain why prevention methods such as painting, oiling, galvanising or sacrificial protection are used.
Both air and water are necessary for iron to rust.
Rusting is the corrosion of iron, forming hydrated iron(III) oxide. It requires both oxygen, supplied by air, and water. Remove either and rusting stops. Classic experiments compare four tubes: nail in water with air; nail in boiled water under oil, so no air; nail in dry air with a desiccant, so no water; nail in salt water, which rusts fastest. Only the first and fourth rust, showing both air and water are needed. Salt water speeds rusting because ions help electrons move, but salt is not itself required. The overall idea is that iron is oxidised, losing electrons to oxygen in the presence of water.
Corrosion can be prevented by applying a coating that acts as a barrier, such as greasing, painting or electroplating.
A barrier coating stops oxygen and water reaching the metal surface, so corrosion cannot start. Greasing protects tools and machine parts, painting protects large steel structures such as bridges and car bodies, and electroplating coats a metal with another metal using electrolysis. Electroplating also improves appearance and conductivity. The coating must be complete and unbroken; a scratch exposes iron and rusting begins there. This method is physical, not electrochemical, because the coating simply blocks contact. Choosing a method depends on cost, durability, appearance and whether the object moves or is handled.
Aluminium has an oxide coating that protects the metal from further corrosion.
Aluminium reacts quickly with oxygen in air to form a thin, tough layer of aluminium oxide, Al₂O₃, on its surface. This layer is strongly bonded and unreactive, so it seals the metal underneath from oxygen and water. Unlike rust on iron, which flakes away and exposes fresh metal, the aluminium oxide layer stays intact and stops further corrosion. This is why aluminium resists corrosion despite being more reactive than iron. The coating can be thickened by anodising, which uses electrolysis. If the layer is scratched, it reforms immediately in air.
Some coatings are reactive and contain a more reactive metal to provide sacrificial protection, eg zinc is used to galvanise iron.
Sacrificial protection uses a metal more reactive than iron, such as zinc or magnesium. The more reactive metal loses electrons in preference to iron, so it corrodes instead. Zinc coating on iron is called galvanising. Even if the coating is scratched, the zinc still protects the exposed iron because the two metals are in contact and zinc is higher in the reactivity series. Electrons flow from zinc to iron, preventing iron from being oxidised. This differs from a barrier coating, which only works while intact. Magnesium blocks attached to ship hulls and underground pipelines work in the same way.
Students should be able to: describe experiments and interpret results to show that both air and water are necessary for rusting explain sacrificial protection in terms of relative reactivity.
Rusting is the corrosion of iron, forming hydrated iron(III) oxide. To show that both air and water are needed, set up three sealed tubes containing an iron nail. Tube 1 (the control) contains water and air. Tube 2 contains boiled water under oil (no air). Tube 3 contains dry air with a desiccant such as anhydrous calcium chloride (no water). Only the nail in Tube 1 rusts, proving both reactants are required. Interpreting results means identifying which tubes rust and explaining why. Sacrificial protection uses a more reactive metal, such as zinc or magnesium, attached to iron. The more reactive metal is oxidised preferentially, so iron does not corrode. This explains galvanising and blocks on ship hulls.
Your focus
- Define corrosion as the destruction of materials by chemical reactions with substances in the environment.
- Describe rusting as an example of corrosion that requires oxygen and water.
- Give another example of corrosion and state the substance formed.
Show all 18 objectives
- State that iron rusts only when both air and water are present.
- Describe an experiment showing the need for air and water.
- Explain why salt water increases the rate of rusting.
- Describe how barrier coatings prevent corrosion.
- Give examples of barrier coatings and their uses.
- Explain why a damaged coating allows rusting.
- Describe the formation of the aluminium oxide layer.
- Explain how the layer protects aluminium from corrosion.
- Compare aluminium oxide protection with rusting of iron.
- Define sacrificial protection and name an example.
- Explain why zinc protects iron even when scratched.
- Compare sacrificial protection with barrier coatings.
- Describe an experiment that shows both air and water are necessary for rusting.
- Interpret results from rusting experiments, including the role of a control.
- Explain sacrificial protection using the relative reactivity of metals.
Corrosion and its prevention exam tips
Marking Points
- Corrosion is the destruction of a material by chemical reactions with substances in the environment.
- The material is chemically changed into new substances, so corrosion is a chemical process, not just physical wear.
- Rusting is the corrosion of iron and steel and is one example of corrosion.
- Rusting requires both oxygen and water, and produces hydrated iron(III) oxide.
- Other examples include the green layer on copper and the action of acid rain on limestone or metals.
- Corrosion weakens a material because the new substance does not have the same useful properties as the original material.
- Rusting needs both oxygen from the air and water together; removing either prevents it.
- Rust is hydrated iron(III) oxide, formed when iron is oxidised.
- A control tube with both air and water rusts; tubes missing air or water do not.
- Salt water accelerates rusting but is not a requirement.
- Boiling removes dissolved air, and an oil layer stops air re-entering.
- A desiccant such as calcium chloride keeps air dry so no water is present.
- Barrier coatings work by preventing oxygen and water from contacting the metal.
- Greasing, painting and electroplating are named examples of barrier methods.
- Electroplating uses electrolysis to deposit a metal layer on the object.
- Damage to a coating exposes the metal and allows rusting to start.
- Barrier protection does not rely on a more reactive metal.
- Method choice depends on cost, appearance, durability and use.
- Aluminium reacts with oxygen to form aluminium oxide, Al₂O₃.
- The oxide layer is thin, tough and strongly bonded to the metal.
- The layer acts as a barrier, preventing oxygen and water reaching the metal.
- Unlike rust, the oxide layer does not flake away, so protection continues.
- Aluminium is more reactive than iron yet corrodes less because of this layer.
- Anodising thickens the oxide layer using electrolysis.
- A more reactive metal is used so it corrodes in place of iron.
- Zinc coating on iron is called galvanising.
- Sacrificial protection still works when the coating is scratched.
- The more reactive metal loses electrons, which flow to the iron.
- Magnesium blocks on ships and pipelines are further examples.
- This differs from barrier coatings, which must remain unbroken.
- States that rusting needs both oxygen (from air) and water, and that removing either prevents rusting.
- Describes a valid experiment: iron nails in three sealed tubes with water and air (control), boiled water under oil (no air), and dry air with a desiccant (no water).
- Interprets results by identifying that only the tube containing both air and water rusts, supporting the conclusion.
- Explains sacrificial protection: a more reactive metal, such as zinc or magnesium, corrodes instead of iron because it is higher in the reactivity series.
- Applies the idea to examples such as galvanised steel or sacrificial blocks on ships, linking the method to the relative reactivity of the metals.
Examiner Tips
- 💡Define corrosion in terms of chemical reactions with substances in the environment.
- 💡When explaining rusting, name both reactants, oxygen and water, and name the product, hydrated iron(III) oxide.
- 💡Use a second example, such as copper or acid rain on limestone, to show that rusting is only one type of corrosion.
- 💡Describe the control and each changed variable when explaining the experiment.
- 💡Use the phrase 'both air and water' explicitly in your answer.
- 💡Link salt water to faster rusting, not to a different product.
- 💡Name the specific coating and state that it blocks air and water.
- 💡For electroplating, mention electrolysis and the deposited metal.
- 💡Compare methods using cost, appearance or durability where asked.
- 💡Name the compound as aluminium oxide and give its formula Al₂O₃.
- 💡Contrast the behaviour of the oxide layer with rust on iron.
- 💡Mention that the layer reforms if scratched.
- 💡State that the sacrificial metal is more reactive than iron.
- 💡Use galvanising as your named example and explain the scratch case.
- 💡Link electron transfer to protection of iron.
- 💡Name each test tube and state what is present or absent, then link the result to the conclusion about air and water.
- 💡Use the reactivity series to justify why zinc protects iron, naming the more reactive metal and the metal it protects.
- 💡When describing sacrificial protection, state that the protective metal is oxidised or corroded instead of the iron, not that it blocks water.
Common Mistakes
- Saying rusting needs only water or only oxygen. Correction: rusting requires both oxygen and water.
- Writing that rust is iron oxide without the water of crystallisation. Correction: rust is hydrated iron(III) oxide.
- Treating corrosion as physical wear such as scratching. Correction: corrosion is a chemical reaction that forms new substances.
- Saying only water or only air causes rust; correction: both are needed together.
- Believing salt causes rust; correction: salt only speeds up rusting, which still needs oxygen and water.
- Thinking rust is iron oxide only; correction: it is hydrated iron(III) oxide.
- Confusing barrier coatings with sacrificial protection; correction: barrier coatings block air and water, while sacrificial methods use a more reactive metal.
- Thinking electroplating is painting with electricity; correction: it deposits a metal layer by electrolysis.
- Assuming any coating works even if scratched; correction: a break exposes the metal and rusting can begin.
- Saying aluminium does not react with oxygen; correction: it reacts readily, forming a protective oxide layer.
- Confusing the oxide layer with rust; correction: rust flakes and exposes iron, while aluminium oxide stays intact.
- Thinking aluminium is unreactive because it is low in the reactivity series; correction: it is reactive but protected by its oxide.
- Thinking galvanising is only a barrier; correction: zinc also protects sacrificially when scratched.
- Choosing a less reactive metal such as copper; correction: the sacrificial metal must be more reactive than iron.
- Saying the iron corrodes first; correction: the more reactive metal corrodes instead.
- Thinking that rusting needs only water or only air; correction: both oxygen and water are essential, so removing either one prevents rusting.
- Believing that a less reactive metal protects iron; correction: the sacrificial metal must be more reactive than iron, so it is oxidised in preference.
- Confusing rusting with burning or with the reaction of a metal with acid; correction: rusting is a slow oxidation of iron by oxygen and water, producing hydrated iron(III) oxide.