Rusting and Corrosion of Metals — CIE IGCSE Chemistry Revision

What you'll learn

This topic examines how iron and other metals deteriorate when exposed to environmental conditions, with particular focus on the rusting of iron and steel. Understanding the conditions necessary for rusting, prevention methods, and the differences between various corrosion processes is essential for CIE IGCSE Chemistry examinations. Questions regularly appear testing experimental design, interpretation of results, and explaining protection methods.

Key terms and definitions

Corrosion — the gradual destruction of a metal by chemical reaction with substances in its environment, such as oxygen, water, or acids.

Rusting — the specific corrosion of iron and steel in the presence of oxygen and water, forming hydrated iron(III) oxide (Fe₂O₃·xH₂O).

Oxidation — the loss of electrons by a substance; in corrosion, metal atoms lose electrons to form positive ions.

Sacrificial protection — a method of preventing corrosion by attaching a more reactive metal that corrodes preferentially, protecting the iron or steel.

Galvanising — coating iron or steel with a layer of zinc to prevent corrosion through both barrier protection and sacrificial protection.

Electroplating — depositing a thin layer of one metal onto another using electrolysis, often to prevent corrosion or improve appearance.

Rust — the reddish-brown flaky substance formed when iron corrodes, consisting mainly of hydrated iron(III) oxide.

Core concepts

Conditions required for rusting

Iron requires both oxygen and water to rust. Neither alone is sufficient. This can be demonstrated through three standard test tube experiments:

Test tube 1 (both oxygen and water present):

Iron nail exposed to air and water
Rust forms on the nail surface
Confirms both substances are necessary

Test tube 2 (water only, no oxygen):

Iron nail in boiled distilled water with oil layer on top
Boiling removes dissolved oxygen; oil prevents more dissolving
No rust forms

Test tube 3 (oxygen only, no water):

Iron nail with calcium chloride or anhydrous calcium chloride (drying agent)
Air present but water removed
No rust forms

The presence of salt (sodium chloride) accelerates rusting significantly. This explains why cars in coastal areas or roads treated with salt in winter corrode faster. Salt increases the rate of electron transfer in the oxidation process.

Acidic conditions also speed up rusting. Acid rain containing dissolved sulfur dioxide and nitrogen oxides increases corrosion rates of iron structures.

The chemistry of rusting

Rusting is a redox reaction involving both oxidation and reduction:

Oxidation (at the metal surface): Iron atoms lose electrons to form iron(II) ions: Fe(s) → Fe²⁺(aq) + 2e⁻

Reduction (at a different point on the surface): Oxygen and water gain electrons: O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)

Overall reaction: The Fe²⁺ ions are further oxidised to Fe³⁺ ions and combine with hydroxide ions and water molecules to form hydrated iron(III) oxide: 4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃·xH₂O(s)

The rust formed is porous and flaky, meaning it does not adhere to the metal surface. This allows oxygen and water to continue reaching the iron beneath, so rusting continues until all the iron is converted. This distinguishes rust from the protective oxide layers on aluminium or chromium.

Methods of rust prevention

Barrier methods work by physically separating iron from oxygen and water:

Painting:

Applied to car bodies, bridges, railings
Effective if coating remains intact
Any scratch exposes metal and allows localised rusting
Requires regular maintenance

Oiling or greasing:

Used on machinery, tools, bicycle chains
Prevents water contact
Must be reapplied regularly as it wears off

Plastic coating:

Applied to steel wire, dishwasher racks, garden furniture
Durable and long-lasting
Damage to coating exposes metal underneath

Galvanising (zinc coating):

Iron or steel dipped in molten zinc or electroplated with zinc
Used for corrugated roofing, buckets, nails, crash barriers
Provides dual protection: barrier method AND sacrificial protection
If scratched, zinc continues to protect the exposed iron
Zinc is more reactive than iron in the reactivity series, so oxidises preferentially

Tin plating:

Thin layer of tin applied to steel (e.g., food cans)
Tin is less reactive than iron
Only provides barrier protection
If scratched, iron rusts rapidly as tin does not offer sacrificial protection

Electroplating with other metals:

Chromium plating (car bumpers, taps) — decorative and protective
Nickel plating — corrosion resistance
Silver plating (cutlery) — appearance
All function as barrier methods only

Sacrificial protection

Sacrificial protection relies on a more reactive metal corroding instead of iron. The more reactive metal acts as the anode and is oxidised (loses electrons), while iron acts as the cathode and is protected.

Blocks of zinc or magnesium attached to:

Ship hulls and propellers
Underground pipelines
Oil rigs and offshore structures

The sacrificial metal must be more reactive than iron in the reactivity series:

Magnesium (most commonly used)
Zinc
Aluminium

These metals are replaced periodically once corroded away.

Why less reactive metals don't work: Coating iron with copper or tin provides only barrier protection. If the coating is damaged, iron corrodes preferentially because it is more reactive. With tin-plated steel, electrons flow from iron to tin, actually speeding up iron corrosion at the exposed area.

Alloying to prevent corrosion

Stainless steel is an alloy of iron containing chromium (and often nickel). The chromium forms a tough, invisible, protective oxide layer (Cr₂O₃) that adheres tightly to the surface and prevents oxygen and water reaching the iron beneath.

Stainless steel is used for:

Cutlery and kitchen equipment
Medical instruments
Industrial chemical containers
Architecture and building cladding

Unlike rust, the chromium oxide layer is non-porous and self-repairing — if scratched, the chromium immediately reacts with oxygen to reform the protective layer.

Corrosion of other metals

Aluminium also corrodes when exposed to oxygen, but forms a protective layer of aluminium oxide (Al₂O₃) that adheres strongly to the surface. This thin, transparent layer prevents further corrosion. Aluminium appears resistant to corrosion despite being a reactive metal high in the reactivity series.

Copper corrodes very slowly, forming a green layer of basic copper carbonate (also called verdigris) on exposure to moist air containing carbon dioxide. This layer is seen on copper roofs and bronze statues. The layer protects underlying copper from further corrosion.

Gold and platinum do not corrode under normal conditions because they are very unreactive metals at the bottom of the reactivity series. This property, combined with their rarity, contributes to their value.

Worked examples

Example 1: Experimental design (6 marks)

Question: A student sets up three test tubes to investigate the conditions needed for rusting.

Test tube A: Iron nail in tap water exposed to air Test tube B: Iron nail in boiled distilled water with a layer of oil on top Test tube C: Iron nail with anhydrous calcium chloride, sealed with a rubber bung

(a) Predict which test tube(s) will show rusting after one week. [1] (b) Explain why test tube B has boiled distilled water with an oil layer. [2] (c) State the purpose of the anhydrous calcium chloride in test tube C. [1] (d) Explain why both oxygen and water are needed for rusting to occur. [2]

Model answer: (a) Only test tube A will show rusting. [1]

(b) Boiling removes dissolved oxygen from the water [1]. The oil layer prevents atmospheric oxygen dissolving back into the water [1].

(d) Oxygen acts as the oxidising agent that accepts electrons [1]. Water is needed as a medium for ion movement / for the formation of hydroxide ions that react with iron ions [1].

Example 2: Prevention methods (5 marks)

Question: Ships' hulls are protected from corrosion using blocks of magnesium attached to the steel.

(a) Name this method of rust prevention. [1] (b) Explain why magnesium protects the steel hull from rusting. [2] (c) State what happens to the magnesium blocks over time. [1] (d) Suggest why copper blocks would not protect the steel hull. [1]

Model answer: (a) Sacrificial protection / sacrificial method [1]

(b) Magnesium is more reactive than iron (in the reactivity series) [1]. Magnesium corrodes preferentially / loses electrons in preference to iron / acts as the anode [1].

(c) The magnesium blocks corrode away / are converted to magnesium ions / magnesium oxide [1]. (They need to be replaced)

(d) Copper is less reactive than iron [1], so iron would corrode preferentially / copper would not act sacrificially.

Example 3: Data interpretation (4 marks)

Question: A student measured the mass of iron nails left in different conditions for two weeks:

Nail in distilled water exposed to air: lost 12% mass as rust formed
Nail in salt water exposed to air: lost 28% mass as rust formed
Nail in distilled water with oil layer (no air): lost 0% mass

(a) State which condition causes fastest rusting. [1] (b) Explain why salt water increases the rate of rusting. [2] (c) Explain why the nail with an oil layer showed no mass loss. [1]

Model answer: (a) Salt water (exposed to air) [1]

(b) Salt / sodium chloride / dissolved ions increase the rate of electron transfer [1] in the oxidation reaction / make the water a better conductor [1].

(c) Oil prevents oxygen from dissolving in the water / reaching the nail surface [1], so one essential condition for rusting is absent.

Common mistakes and how to avoid them

• Mistake: Writing that oxygen OR water causes rusting. Correction: Both oxygen AND water must be present simultaneously. Neither alone is sufficient. Always state "both" in exam answers.

• Mistake: Describing rust as "iron oxide" without mentioning water. Correction: Rust is hydrated iron(III) oxide (Fe₂O₃·xH₂O). The water molecules are chemically bonded in the structure, which is why rust appears different from anhydrous iron(III) oxide.

• Mistake: Claiming any metal coating provides sacrificial protection. Correction: Only more reactive metals (magnesium, zinc, aluminium) provide sacrificial protection. Less reactive metals (tin, copper) provide barrier protection only — if damaged, they accelerate iron corrosion.

• Mistake: Stating that galvanising is only a barrier method. Correction: Galvanising provides both barrier protection (zinc coating blocks oxygen and water) and sacrificial protection (zinc corrodes preferentially if the coating is scratched). This dual protection makes galvanising particularly effective.

• Mistake: Confusing rusting (specific to iron) with general corrosion. Correction: Rusting specifically refers to iron and steel corrosion. Other metals undergo corrosion but not rusting. Use "corrosion" for aluminium, copper, etc.

• Mistake: Writing that stainless steel "doesn't react" or "doesn't corrode." Correction: Stainless steel does corrode, but the chromium in the alloy forms a protective oxide layer that adheres tightly and prevents further corrosion. The iron underneath is protected by this layer.

Exam technique for "Rusting and corrosion of metals"

• "State the conditions" questions (2-3 marks): Always write "both oxygen and water" or "oxygen and water (must be present)". Do not write "air" alone — be specific about oxygen. For accelerating factors, mention salt or acids if the context is given.

• "Explain" questions about sacrificial protection (2-3 marks): Your answer must include: (1) the sacrificial metal is more reactive than iron, (2) the sacrificial metal corrodes/oxidises preferentially, (3) reference to electron loss. Example: "Magnesium is more reactive than iron in the reactivity series, so it loses electrons preferentially and corrodes instead of the iron."

• Experimental questions (4-6 marks): When describing rust prevention experiments, clearly state what each test tube contains and what it demonstrates. Use precise terms: "boiled distilled water" not just "water", "anhydrous calcium chloride" not just "desiccant". Explain the purpose of each component (oil layer prevents oxygen dissolving, boiling removes dissolved oxygen).

• Comparison questions: When comparing methods (galvanising vs. painting, zinc vs. tin coating), structure your answer to contrast them point-by-point. Mention whether each provides barrier protection only or also sacrificial protection. Discuss what happens when the coating is damaged.

Quick revision summary

Rusting requires both oxygen and water; neither alone is sufficient. Rust is hydrated iron(III) oxide, formed through oxidation of iron. Prevention methods include barrier methods (painting, oiling, plastic coating) that separate iron from oxygen and water, and sacrificial protection using more reactive metals like zinc or magnesium. Galvanising provides both types of protection. Stainless steel resists corrosion due to a protective chromium oxide layer. Salt and acids accelerate rusting. Other metals corrode but form protective layers (aluminium, copper) or remain unreactive (gold).