Why does iron rust, and how can corrosion be prevented?
The rusting of iron and the conditions needed, and methods of preventing corrosion including barrier methods, galvanising and sacrificial protection.
A CCEA GCSE Chemistry answer on corrosion, covering the rusting of iron, the conditions of water and oxygen needed, and the methods used to prevent corrosion including barrier methods, galvanising and sacrificial protection.
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What this dot point is asking
CCEA wants you to state the conditions needed for iron to rust, describe an experiment showing both water and oxygen are required, and explain the methods used to prevent corrosion: barrier methods, galvanising and sacrificial protection.
What rusting is and what it needs
Rusting is an oxidation reaction: the iron gains oxygen (and loses electrons). The three-tube nail experiment proves both water and oxygen are needed, by removing one in each tube and seeing that the nail only rusts when both are present.
Barrier methods
Barrier methods work only while the coating is intact; if it is scratched, water and oxygen reach the iron and rusting begins at the gap. They are simple and cheap, which is why cars, railings and bridges are painted.
Galvanising and sacrificial protection
Sacrificial protection is used on ship hulls and underground pipes, where blocks of zinc or magnesium are attached and slowly corrode instead of the steel.
Worked example
Examples in context
- Example 1. Galvanised buckets and roofing
- Galvanised steel is used for buckets, fences and roofing because the zinc coating protects the iron both as a barrier and sacrificially. Even a scratched galvanised surface keeps resisting rust, which is why it lasts outdoors.
- Example 2. Protecting pipelines
- Buried steel pipelines have magnesium blocks attached at intervals to give sacrificial protection underground, where painting alone would fail. This long-term protection relies entirely on magnesium being more reactive than iron.
- Example 3. Tin cans and aluminium
- Steel food cans are coated with tin, which is less reactive than iron and acts as a barrier; if the tin is scratched the iron underneath rusts faster, so the coating must stay intact. Aluminium, by contrast, forms its own tough oxide layer that seals the surface and stops further corrosion. Comparing these shows why some metals resist corrosion naturally while iron must be protected.
Why iron corrodes so readily
Iron sits in the middle of the reactivity series, reactive enough to be oxidised by the water and oxygen all around us, yet not so unreactive that it resists like gold. Rust, unlike the protective oxide on aluminium, is flaky and crumbles away, exposing fresh iron underneath, so corrosion keeps going rather than sealing itself. This combination of moderate reactivity and a non-protective oxide is why so much engineering effort goes into protecting iron and steel, and why CCEA expects you to connect the corrosion problem back to the metal's position in the reactivity series.
Try this
Q1. State the two conditions needed for iron to rust. [2 marks]
- Cue. Water and oxygen.
Q2. Explain why the metal used for sacrificial protection must be more reactive than iron. [1 mark]
- Cue. So it is oxidised (corrodes) in preference to the iron.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20184 marksDescribe an experiment using iron nails to show that both water and oxygen are needed for rusting.Show worked answer →
Markers want three tubes that isolate the variables.
Set up three boiling tubes, each with an iron nail.
- Tube 1
- nail in water with air present (the control). The nail rusts, because both water and oxygen are present.
- Tube 2
- nail in water that has been boiled to remove air, with a layer of oil on top to keep air out. The nail does not rust, showing oxygen is needed.
- Tube 3
- nail in dry air, kept dry with a drying agent such as calcium chloride. The nail does not rust, showing water is needed.
Comparing the tubes shows rusting needs both water and oxygen.
Markers reward the three set-ups, the conditions removed in each, and the conclusion that both water and oxygen are required.
CCEA 20213 marksExplain how attaching blocks of zinc to the steel hull of a ship prevents the steel from rusting, even though both are exposed to seawater.Show worked answer →
The marks are for the sacrificial protection mechanism.
Zinc is more reactive than iron (the main metal in steel). When both are exposed to seawater, the zinc is oxidised in preference to the iron, because the more reactive metal loses electrons more readily.
So the zinc corrodes away instead of the steel, protecting the hull. This is called sacrificial protection, and the zinc blocks are replaced when they are used up.
Markers reward zinc being more reactive so it is oxidised/corrodes in preference to the iron, protecting the steel (sacrificial protection).
Related dot points
- The reactivity series of metals, the reactions of metals with water and acid, and displacement reactions of metals with metal salt solutions.
A CCEA GCSE Chemistry answer on the reactivity series, covering how metals are ranked by their reactions with water and acid, the order of the common metals, and how a more reactive metal displaces a less reactive one from its salt solution.
- Oxidation and reduction defined in terms of oxygen and of electrons (OIL RIG), identifying redox in displacement and other reactions, and writing half-equations.
A CCEA GCSE Chemistry answer on redox reactions, covering oxidation and reduction defined by the gain and loss of oxygen and of electrons (OIL RIG), how to identify oxidation and reduction in displacement and other reactions, and how to write half-equations.
- How the reactivity of a metal determines its extraction method, the extraction of iron by reduction with carbon in the blast furnace, and why reactive metals are extracted by electrolysis.
A CCEA GCSE Chemistry answer on the extraction of metals, covering how a metal's position in the reactivity series sets its extraction method, the reduction of iron oxide with carbon in the blast furnace, and why metals above carbon are extracted by electrolysis.
- Electrolysis of molten ionic compounds and of aqueous solutions including brine, predicting the products at each electrode, and writing electrode half-equations.
A CCEA GCSE Chemistry answer on electrolysis, covering how molten ionic compounds and aqueous solutions are split by an electric current, the rules for predicting the products at the cathode and anode including the electrolysis of brine, and writing electrode half-equations.
- Metallic bonding as positive ions in a sea of delocalised electrons, how this explains the properties of metals, and why alloys are harder than the pure metal.
A CCEA GCSE Chemistry answer on metallic bonding, covering the model of positive ions in a sea of delocalised electrons, how it explains conductivity, malleability and high melting points, and why alloys are harder than the pure metal.
Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)