Rusting of Iron
Summary: Rusting requires BOTH oxygen AND water — remove either and rusting stops. Rust is hydrated iron(III) oxide (Fe₂O₃·xH₂O), a flaky, porous solid that does NOT protect the iron underneath. Prevention methods: barrier (paint, oil, plastic, tin plating), sacrificial protection (galvanising with zinc, magnesium blocks), and alloying (stainless steel). Salt and acid accelerate rusting. Tags: igcse chemistry metals rusting corrosion redox Created: 2026-07-14 Last Updated: 2026-07-26
What Is Rust?
Rust = hydrated iron(III) oxide: Fe₂O₃·xH₂O
- It is a reddish-brown, flaky, porous solid
- It is NOT protective — unlike Al₂O₃ on aluminium, rust flakes off, exposing fresh iron to further corrosion
- Rusting is an electrochemical redox process — iron is oxidised (loses electrons) while oxygen is reduced (gains electrons)
- Rusting is a slow reaction under normal conditions, accelerated by salt and acid
Why Aluminium Doesn’t “Rust”
Aluminium also reacts with oxygen: 4Al + 3O₂ → 2Al₂O₃. But Al₂O₃ forms a thin, tough, transparent, impermeable layer that bonds tightly to the metal underneath and prevents further reaction. Rust (Fe₂O₃·xH₂O) is flaky and porous — it falls off, exposing fresh iron.
Conditions Required for Rusting
Both OXYGEN AND WATER Are Essential
The classic experiment uses iron nails in test tubes:
| Condition | Setup | Result After ~1 Week | Why? |
|---|---|---|---|
| Air + Water | Nail in tap water, open to air | Rust forms | Both O₂ and H₂O present |
| Dry air only | Nail + anhydrous CaCl₂ (desiccant), sealed | No rust | Water absent |
| Water only (no air) | Nail in boiled water (removes dissolved O₂), oil layer on top | No rust | Oxygen absent |
| Salt water + air | Nail in salt solution, open to air | Heavy rust (faster than plain water) | Salt acts as an electrolyte — accelerates the electrochemical process |
Boiled Water
Water is boiled before use to remove dissolved gases (especially O₂). A layer of oil is floated on top to prevent air from re-dissolving.
Anhydrous Calcium Chloride (CaCl₂)
A desiccant/drying agent — absorbs water vapour from the air inside the sealed test tube, keeping the iron nail in a dry atmosphere.
Factors That Accelerate Rusting
| Factor | Effect | Real-World Example |
|---|---|---|
| Salt (NaCl) | Strongly accelerates rusting | Cars rust faster in coastal areas or where roads are salted in winter |
| Acidic conditions (acid rain, SO₂ pollution) | Accelerates — H⁺ ions increase the rate of the electrochemical reactions | Industrial areas — faster corrosion of bridges, buildings, and vehicles |
| Higher temperature | Increases reaction rate (kinetic theory) | Hot, humid climates — faster rusting |
| Contact with less reactive metals (Cu, Ag) | Accelerates rusting of Fe (Fe becomes the sacrificial anode) | Iron bolts in copper plates rust faster |
| Scratches and surface damage | Exposes bare metal and creates sites for electrochemical cells | Scratched car paint → rust starts at scratches |
The Electrochemistry of Rusting (Simplified for IGCSE)
Rusting is an electrochemical process requiring an electrolyte (water with dissolved ions):
At the anode (iron is OXIDISED): Fe(s) → Fe²⁺(aq) + 2e⁻
- Iron atoms lose electrons and enter solution as Fe²⁺ ions
At the cathode (oxygen is REDUCED): O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
- Dissolved oxygen gains electrons in the presence of water
Further reaction: Fe²⁺ ions are further oxidised to Fe³⁺, which then precipitates as hydrated iron(III) oxide (rust): 4Fe²⁺ + O₂ + (2+4x)H₂O → 2Fe₂O₃·xH₂O + 8H⁺
The role of salt: NaCl(aq) provides mobile ions that complete the circuit — it acts as an electrolyte, dramatically increasing the rate of electron transfer.
Prevention Methods
1. Barrier Protection
Prevents oxygen and/or water from reaching the iron surface:
| Method | How It Works | Examples | Limitation |
|---|---|---|---|
| Painting | Physical barrier excludes O₂ and H₂O | Bridges, ships, cars, buildings | Must be maintained — if paint chips, rust starts |
| Oil / Grease | Water-repellent coating | Moving engine parts, tools, machinery | Needs reapplication; not suitable for visible surfaces |
| Plastic coating | Impermeable polymer layer | Wire racks, garden furniture, tool handles, dishwasher racks | Can be scratched off |
| Tin plating | Layer of tin (Sn) over iron | Steel food cans (“tin cans”) | If scratched → iron rusts FASTER (Fe is more reactive than Sn — Fe acts as the sacrificial anode) |
| Chrome plating | Layer of chromium for decorative + corrosion-resistant finish | Car bumpers, bathroom fittings, motorcycle parts | Expensive — chromium is costly |
| Enamelling | Glass-like coating fused to the metal surface | Cookers, washing machines, baths, signs | Brittle — can chip |
2. Sacrificial Protection
A more reactive metal is electrically connected to the iron. The more reactive metal corrodes instead of the iron — it is “sacrificed.”
Why it works: The more reactive metal has a greater tendency to lose electrons (M → Mⁿ⁺ + ne⁻). It acts as the anode in the electrochemical cell, and iron becomes the cathode (protected). Electrons flow FROM the sacrificial metal TO the iron, preventing Fe from oxidising.
| Method | Sacrificial Metal | Application | How It Works |
|---|---|---|---|
| Galvanising | Zinc (Zn) | Corrugated iron roofing, buckets, dustbins, lamp posts, car underbodies | Zinc coating — Zn is more reactive than Fe. Even if scratched, the Zn corrodes instead of Fe |
| Magnesium blocks | Magnesium (Mg) | Ship hulls, underground pipelines, oil rigs, water heaters | Mg blocks bolted to the steel — Mg is oxidised instead of Fe; blocks are replaced periodically |
| Zinc anodes | Zinc (Zn) | Boat propellers, submarine hulls, bridge supports | Zn corrodes sacrificially — “cathodic protection” |
Galvanising vs Tin Plating — The Scratching Test
This is a classic IGCSE exam question:
- Tin-plated iron scratched: Iron RUSTS FASTER. Fe is more reactive than Sn → Fe becomes the anode and is oxidised → Fe²⁺ while Sn is protected (Fe is “sacrificed” for Sn)
- Galvanised iron scratched: Iron is PROTECTED. Zn is more reactive than Fe → Zn becomes the anode and is oxidised → Zn²⁺ while Fe is protected
Key point: Galvanising protects even when scratched. Tin plating only protects while the coating is intact.
3. Alloying
Changing the composition of the metal itself to make it corrosion resistant:
- Stainless steel (Fe + Cr + Ni): Chromium forms a thin, protective Cr₂O₃ surface layer — analogous to Al₂O₃ on aluminium. This layer is self-healing — if scratched, Cr reacts with O₂ to reform the layer
- Weathering steel (COR-TEN): Contains Cu, Cr, Ni — develops a stable rust-like patina that protects the underlying metal (used in sculpture and some bridges)
- More expensive than barrier methods but permanent — no maintenance required
Key Concepts from Past Papers
Definitions You MUST Know
- Rust: hydrated iron(III) oxide (Fe₂O₃·xH₂O) — the corrosion product of iron and steel
- Corrosion: the chemical deterioration of a metal by reaction with substances in the environment
- Sacrificial protection: the protection of iron/steel by connecting it to a more reactive metal that corrodes instead
- Galvanising: coating iron/steel with a layer of zinc for both barrier and sacrificial protection
- Barrier method: preventing corrosion by physically excluding oxygen and/or water from the metal surface
- Alloy: a mixture of a metal with one or more other elements to improve properties including corrosion resistance
Recurring Mark Scheme Answers
- “Both oxygen AND water are required for rusting — if either is absent, rusting does not occur”
- “Salt accelerates rusting because it acts as an electrolyte / increases the conductivity of water”
- “Galvanising protects iron even when scratched because zinc is more reactive than iron / zinc corrodes sacrificially”
- “Aluminium does not rust because it has a protective layer of aluminium oxide that is impermeable”
- “Sacrificial protection works because the more reactive metal loses electrons in preference to the iron”
- “Stainless steel resists corrosion because chromium forms a protective oxide layer”
Common Mistakes
- Saying “rust is iron oxide (Fe₂O₃)”: Must say “hydrated iron(III) oxide” or Fe₂O₃·xH₂O — the water of hydration is essential
- Confusing rust with Al₂O₃: Rust is flaky and non-protective; Al₂O₃ is an impermeable protective layer
- Thinking salt reacts chemically with iron to cause rusting: Salt acts as an electrolyte — it speeds up the electrochemical corrosion process
- Forgetting galvanising protects even when scratched: This is the defining advantage over tin plating
- Saying “sacrificial protection = painting”: Painting is a barrier method, NOT sacrificial protection
- Writing aluminium “rusts”: Only iron and steel rust — aluminium corrodes (forms Al₂O₃) but doesn’t “rust”
Common Question Types
Type 1: “Describe an experiment to show that both oxygen and water are needed for rusting” (4–6 marks)
- Frequency: ~35% of Paper 5/6
- Must describe: 3 test tubes (air + water, dry air, boiled water + oil), control variables, observations after ~1 week
Type 2: “Explain why galvanising protects iron even when the zinc layer is scratched” (3 marks)
- Frequency: ~40% of papers
- Model answer: “Zinc is more reactive than iron. When scratched, zinc corrodes/loses electrons in preference to iron. Iron is therefore protected by sacrificial protection.”
Type 3: “Suggest and explain a method to prevent rusting for [specific object]” (2–3 marks)
- Frequency: ~30% of papers
- Examples: Ship hull → magnesium blocks (sacrificial); car body → paint + galvanising; cutlery → stainless steel
Type 4: “Explain why salt spread on roads increases rusting of cars” (2–3 marks)
- Frequency: ~25% of papers
- Model answer: “Salt dissolves in water to form an electrolyte solution. This increases the electrical conductivity of the water, accelerating the electrochemical process of rusting.”
Key Facts to Memorize
- Rust = hydrated iron(III) oxide (Fe₂O₃·xH₂O) — flaky, porous, NOT protective
- Both O₂ AND H₂O required — remove either = no rust
- Salt and acid ACCELERATE rusting (electrolyte effect)
- Barrier methods: paint, oil, plastic, tin plating, chrome plating, enamelling
- Sacrificial protection: galvanising (Zn), magnesium blocks
- Galvanising protects even when scratched — Zn is more reactive than Fe
- Tin plating does NOT protect when scratched — Fe is more reactive than Sn
- Stainless steel: Fe + Cr + Ni — Cr forms protective Cr₂O₃ layer
- Al₂O₃ on Al is protective — unlike rust, it’s impermeable and self-healing
- Rusting is a redox reaction: Fe → Fe²⁺ + 2e⁻ (oxidation); O₂ + 2H₂O + 4e⁻ → 4OH⁻ (reduction)
Related Notes
- Reactivity of Metals — The reactivity series explains sacrificial protection
- Redox Reactions — Rusting is a redox process (Fe oxidised, O₂ reduced)
- Metal Extraction — Why iron is so widely used despite rusting
- Uses of Metals — Choosing metals based on corrosion resistance
- Alloy — Stainless steel and other corrosion-resistant alloys
- Sacrificial Protection — Detailed mechanism
- Barrier Method — Types of barrier protection
- Galvanising — The galvanising process and its advantages
- IGCSE-Chem-Index
Past Paper Sources
- 0620/62 M/J 2022 Q5: Describe experiment to investigate conditions for rusting (6 marks)
- 0971/32 O/N 2023 Q7(c): Explain why galvanising protects iron even when scratched (3 marks)
- 0620/43 M/J 2019 Q6(b): Why does salt accelerate rusting? (2 marks)
- 0620/42 M/J 2021 Q8(d): Compare tin plating and galvanising — which protects when scratched? (3 marks)
- 0971/52 M/J 2023 Q4: Suggest methods to prevent rusting for four different objects (4 marks)
- 0620/32 O/N 2018 Q3(d): Explain why aluminium resists corrosion but iron rusts (3 marks)
IGCSE Chemistry (0620/0971) wiki. Rusting is estimated to cost ~3% of global GDP annually — understanding and preventing it is one of the most economically important topics in chemistry.