Ch 5 · Corrosion of Metals · IGCSE Chemistry 0620
Topic 5 of 6 in Metals — IGCSE Chemistry 0620 — 48 lessons.
Introduction
Rusting destroys a large fraction of the world's steel every year, from car bodies to bridges and ships. Chemically it is a slow oxidation of iron that needs both oxygen and water. This lesson covers the classic three-tube experiment that proves it, the everyday barrier methods used to stop it, and, for Extended candidates, the difference between a barrier and sacrificial protection explained in terms of the reactivity series and electron loss.
Key ideas — The conditions for rusting
Rusting is the corrosion of iron and steel to form hydrated iron(III) oxide, an orange-brown flaky solid often written Fe_2O_3 · xH_2O. It requires both oxygen and water. Neither on its own is enough. Salt (sodium chloride) and acids speed rusting up but are not required.
🧑🏫 Teacher's explanation
Imagine you are trying to burn a piece of wood. You need fuel (the wood) and oxygen from the air. If you put the wood in a vacuum, it won't burn. If you put it in pure water without air, it won't burn. Rusting is essentially a slow, 'wet' version of combustion. It is a chemical reaction where iron reacts with oxygen and water to form a new substance.
📖 Definitions
🔍 Worked example — Easy
A piece of iron is placed in a test tube containing only boiled, distilled water (which has had all dissolved oxygen removed) and a layer of oil on top. Will the iron rust?
🔍 Worked example — Standard
Calculate the mass of hydrated iron(III) oxide, Fe_2O_3· H_2O, produced if 11.2 g of iron reacts completely with oxygen and water.
🔍 Worked example — Exam-style
A steel ship is protected by attaching blocks of magnesium (Mg) to the hull. Explain why this prevents the steel from rusting in terms of electron loss.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Tube 1: Iron nail + water + air Tube 2: Iron nail + boiled water + oil layer Tube 3: Iron nail + anhydrous calcium chloride (absorbs moisture) + air Tube 4: Iron nail + boiled water + air + salt
Key ideas — The experiment showing the conditions
Set up three test tubes, each containing a clean iron nail, and leave them for about a week.
🧑🏫 Teacher's explanation
Think of rusting like a fire. To have a fire, you need fuel (the iron), heat, and oxygen. If you take away the oxygen (by using a lid) or the fuel (by coating it), the fire goes out. Rusting is a chemical reaction where iron reacts with oxygen and water to form a flaky, reddish-brown substance called hydrated iron(III) oxide.
📖 Definitions
🔍 Worked example — Easy
Identify the two essential conditions required for iron to rust.
🔍 Worked example — Standard
A piece of iron is protected by a layer of zinc (galvanising). If the zinc layer is scratched, why does the iron not rust immediately?
🔍 Worked example — Exam-style
A sample of 5.6 g of iron reacts completely with excess oxygen and water to form hydrated iron(III) oxide, Fe_2 extO_3 · xH_2 extO. If the molar mass of Fe_2O_3 is 160 g/mol, calculate the mass of Fe_2O_3 produced (assume x=0 for this calculation).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Barrier methods
A barrier method puts a coating between the iron and the surroundings. Common examples are:
🧑🏫 Teacher's explanation
Imagine you are wearing a raincoat. As long as the coat is intact, your shirt stays dry because the water (the reactant) cannot touch your skin (the iron). This is a barrier method. However, if you tear the coat, the water hits your shirt immediately. This is exactly how paint or plastic coatings work on metal.
📖 Definitions
🔍 Worked example — Easy
A steel bridge is coated with a layer of plastic to prevent rusting. Explain why the bridge might start rusting even if the plastic coating is still mostly intact.
🔍 Worked example — Standard
Iron is more reactive than tin. A food can is made of tin-plated steel. Explain why the can might rust faster if the tin coating is scratched.
🔍 Worked example — Exam-style
A piece of iron is coated with zinc (galvanised). If the zinc layer is scratched, explain why the iron does not rust.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Sacrificial protection
🧑🏫 Teacher's explanation
Imagine you are playing a game of 'protect the king'. The iron in a ship is the king. If oxygen and water attack, the king gets captured (corrodes). Sacrificial protection is like hiring a bodyguard who is much more aggressive and eager to fight than the king. This bodyguard (a more reactive metal like Zinc) steps in front of the king and takes all the hits. Because the bodyguard is more 'eager' to lose electrons, they get destroyed instead of the king.
📖 Definitions
🔍 Worked example — Easy
Identify which metal would provide sacrificial protection for Iron (Fe) from the following list: Copper (Cu), Magnesium (Mg), and Silver (Ag). Use the reactivity series order: Mg > Fe > Cu > Ag.
🔍 Worked example — Standard
Write the half-equation for the oxidation of Zinc (Zn) when used as a sacrificial anode to protect Iron.
🔍 Worked example — Exam-style
A ship's hull is made of Iron. It is protected by attaching blocks of Zinc. If the Zinc is completely consumed, what will happen to the Iron structure?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. A steel ship's hull is painted, and blocks of zinc are also bolted to the hull below the waterline. (a) Explain how the paint protects the hull. [2] (b) Explain how the zinc blocks continue to protect the hull even where the paint is scratched. [3]
🔍 Worked example — Easy
Identify the two essential substances required for iron to rust.
🔍 Worked example — Standard
A piece of magnesium is used to protect a piece of iron from rusting. Write the half-equation for the oxidation of magnesium.
🔍 Worked example — Exam-style
A steel bridge is protected by attaching blocks of magnesium to it. Explain why the magnesium protects the steel even if the magnesium is partially covered by salt water.
⚠️ Common errors
Common mistakes
Practice
Answers Hydrated iron(III) oxide [B1]; oxygen [B1]; water [B1]. Three tubes each with an iron nail [M1]; tube 1 with water open to air, tube 2 with boiled water under a layer of oil (water but no oxygen), tube 3 with anhydrous calcium chloride and a stopper (oxygen but no water) [M1 for correct setups]; leave for several days [M1]; only the nail in tube 1 rusts, so both oxygen and water are required [A1]. Any two of painting, greasing or oiling, coating with plastic [B1 each]. Magnesium is more reactive than iron (higher in the reactivity series) [B1]; magnesium loses electrons in preference to iron and corrodes instead, protecting the iron [B1]; copper is less reactive than iron, so iron would lose electrons instead and corrode faster [B1].