Using resources: corrosion and its prevention

What you'll learn

This revision guide covers everything you need to know about corrosion and its prevention for AQA GCSE Chemistry. You'll understand why metals corrode, the specific conditions required for rusting of iron, and the various methods used to prevent corrosion. These concepts link directly to real-world applications in construction, transport and manufacturing industries.

Key terms and definitions

Corrosion — the destruction of materials by chemical reactions with substances in the environment, particularly the oxidation of metals

Rusting — the corrosion of iron (or steel) in the presence of both oxygen and water, forming hydrated iron(III) oxide

Oxidation — the loss of electrons or the gain of oxygen; in corrosion, metal atoms lose electrons to form metal ions

Sacrificial protection — using a more reactive metal to protect a less reactive metal from corrosion by corroding preferentially

Galvanising — coating iron or steel with a layer of zinc to prevent corrosion

Electroplating — using electrolysis to coat one metal with a thin layer of another metal

Barrier method — preventing corrosion by physically separating a metal from oxygen and water using a coating

Alloy — a mixture of two or more elements, at least one of which is a metal, often more resistant to corrosion than pure metals

Core concepts

The chemistry of corrosion

Corrosion occurs when metals react with substances in their environment. For most metals, this involves oxidation reactions where metal atoms lose electrons to form positive metal ions.

The general equation for metal corrosion is:

• Metal → Metal ions + electrons

Different metals corrode at different rates depending on their position in the reactivity series:

• More reactive metals (e.g. sodium, potassium, calcium) corrode very rapidly
• Moderately reactive metals (e.g. iron, zinc) corrode at moderate rates
• Less reactive metals (e.g. copper) corrode slowly
• Unreactive metals (e.g. gold, platinum) do not corrode under normal conditions

When metals corrode, they typically react with oxygen to form metal oxides. Some metals, like aluminium, form a protective oxide layer that prevents further corrosion. Iron, however, forms a flaky rust layer that does not protect the metal underneath.

Rusting of iron and steel

Rusting is the specific corrosion process that affects iron and steel (an alloy containing mostly iron). Understanding the exact conditions required for rusting is essential for GCSE exams.

Requirements for rusting: Iron rusts only when both oxygen and water are present. Either substance alone will not cause rusting.

The chemical process involves:

1. Iron atoms lose electrons (oxidation): Fe → Fe²⁺ + 2e⁻
2. Oxygen and water accept these electrons
3. Hydrated iron(III) oxide (rust) forms gradually

The word equation is:

• Iron + oxygen + water → hydrated iron(III) oxide

Factors affecting the rate of rusting:

• Salt (sodium chloride) accelerates rusting — this is why cars rust faster in coastal areas or regions where roads are salted in winter
• Acid rain increases the rate of rusting
• Higher temperatures generally increase the rate of rusting
• Contact with more reactive metals can accelerate rusting (the iron acts as a cathode)

Experimental evidence: You should be familiar with experiments using iron nails in different conditions:

• Nail in dry air (with anhydrous calcium chloride) — no rusting
• Nail in boiled water with oil layer (no oxygen) — no rusting
• Nail in air and water — rusting occurs
• Nail in salt water — faster rusting

Barrier methods of rust prevention

Barrier methods work by preventing oxygen and water from reaching the iron surface. If either substance cannot reach the metal, rusting cannot occur.

Painting:

• Applies a layer of paint to separate iron from the environment
• Effective if the coating remains intact
• Commonly used for large structures like bridges, gates and vehicles
• If the paint chips or scratches, the exposed iron will rust
• Requires regular maintenance and repainting

Oiling or greasing:

• Covers iron with a layer of oil or grease
• Used for moving parts in machinery where paint would wear off
• Needs regular reapplication
• Can be messy and attracts dirt

Plastic coating:

• Applies a plastic polymer layer to iron
• Very effective barrier against water and oxygen
• Used for everyday items like bicycle handlebars, dish racks and garden furniture
• Durable and low maintenance
• Can be applied by dipping or spraying

Coating with another metal (non-sacrificial):

• Tin plating: Coating steel with tin creates "tin cans" for food storage
• The tin provides a barrier while being non-toxic
• If the coating is broken, the iron underneath rusts rapidly
• Chrome plating: Chromium provides an attractive, shiny finish
• Used for car parts, bathroom taps and bicycle components
• Chromium is less reactive than iron and provides barrier protection only

Galvanising and sacrificial protection

Galvanising involves coating iron or steel with a layer of zinc. This method provides two levels of protection and is particularly important for GCSE exams.

How galvanising works:

1. Barrier protection: The zinc coating prevents oxygen and water from reaching the iron
2. Sacrificial protection: If the coating is scratched or damaged, the zinc corrodes instead of the iron

Zinc provides sacrificial protection because it is more reactive than iron (check the reactivity series). When both metals are exposed to oxygen and water, the more reactive zinc corrodes preferentially, protecting the iron.

Uses of galvanising:

• Steel structures (bridges, pylons, railings)
• Roofing materials (corrugated iron sheets)
• Buckets, dustbins and storage containers
• Car bodies (before painting)
• Wire fencing

Other sacrificial protection methods:

• Attaching blocks of magnesium or zinc to iron structures
• Used on ships' hulls, underground pipes and oil rigs
• The sacrificial metal (magnesium or zinc) must be more reactive than iron
• These blocks need replacing periodically as they corrode away

Electroplating

Electroplating uses electrolysis to coat an object with a thin layer of metal. While primarily used for decorative purposes or to improve properties, it can also prevent corrosion.

The electroplating process:

1. The object to be plated is made the cathode (negative electrode)
2. The plating metal is made the anode (positive electrode)
3. The electrolyte is a solution containing ions of the plating metal
4. During electrolysis, metal ions move to the cathode and are deposited as metal atoms

Example — copper plating:

• Cathode: Object to be plated (e.g. iron key)
• Anode: Pure copper
• Electrolyte: Copper sulfate solution

At the cathode: Cu²⁺ + 2e⁻ → Cu At the anode: Cu → Cu²⁺ + 2e⁻

Uses of electroplating:

• Silver plating cutlery (improves appearance)
• Chromium plating car parts (shiny, corrosion-resistant finish)
• Gold plating jewellery (valuable appearance at lower cost)
• Nickel plating steel (corrosion protection)

The thickness of the plated layer depends on:

• Current used (higher current = faster plating)
• Time of electrolysis (longer time = thicker layer)
• Concentration of electrolyte

Alloying to prevent corrosion

Many alloys are more resistant to corrosion than pure metals. The AQA specification requires you to understand stainless steel as a key example.

Stainless steel:

• An alloy of iron with chromium (and often nickel)
• Contains approximately 10-20% chromium
• Chromium forms a tough, invisible oxide layer on the surface
• This protective layer prevents oxygen and water from reaching the iron
• Self-repairing — if scratched, the chromium oxide layer reforms

Properties and uses:

• Does not rust in normal conditions
• Strong and hard-wearing
• Can withstand high temperatures
• More expensive than ordinary steel

Common applications:

• Cutlery and cookware
• Sinks and kitchen equipment
• Surgical instruments
• Chemical plant equipment
• Building facades and architectural features

The chromium content is crucial — below 10% chromium, the alloy will still rust like ordinary steel.

Worked examples

Example 1: Identifying conditions for rusting

Question: A student sets up four test tubes to investigate rusting, as shown below:

• Tube A: Iron nail in air only (with anhydrous calcium chloride)
• Tube B: Iron nail in boiled water with oil layer on top
• Tube C: Iron nail in tap water
• Tube D: Iron nail in salt water

Predict which tubes will show rusting after one week. Explain your answer. [4 marks]

Answer:

• Tubes C and D will show rusting [1 mark]
• Tube A contains no water / only dry air [1 mark]
• Tube B contains no oxygen / the water has been boiled to remove oxygen and the oil prevents more dissolving [1 mark]
• Both oxygen AND water are needed for rusting / Tubes C and D contain both oxygen and water [1 mark]

Examiner note: Questions often test whether students understand BOTH conditions are essential.

Example 2: Comparing rust prevention methods

Question: A manufacturer needs to prevent rusting on steel railings for a coastal playground.

(a) Suggest two suitable methods to prevent rusting. [2 marks] (b) Explain why galvanising is more effective than painting for this application. [3 marks]

Answer: (a) Any two from:

• Galvanising / coating with zinc [1 mark]
• Painting [1 mark]
• Plastic coating [1 mark]
• Using stainless steel instead [1 mark]

(b)

• Galvanising provides barrier protection / zinc coating prevents oxygen and water reaching the iron [1 mark]
• If scratched/damaged, galvanising still protects the iron / zinc corrodes instead of iron [1 mark]
• Paint only provides barrier protection / when paint is scratched, the iron underneath rusts / the salt air in coastal areas would damage paint faster [1 mark]

Examiner note: The context (coastal = salt exposure) requires you to consider durability.

Example 3: Electroplating calculations

Question: A steel spoon is silver-plated by electrolysis for 15 minutes using a current of 0.4 A.

Explain how the thickness of the silver layer could be increased. [2 marks]

Answer:

• Increase the current / use higher current [1 mark]
• Increase the time / electroplate for longer [1 mark]
• Also accept: increase the concentration of the electrolyte [1 mark] (any two points)

Examiner note: Simple recall question — factors affecting electroplating thickness appear regularly.

Common mistakes and how to avoid them

• Stating that either oxygen OR water alone causes rusting — Remember that BOTH oxygen AND water must be present for iron to rust. Either substance alone will not cause rusting.

• Confusing corrosion with rusting — Corrosion is the general term for metal degradation; rusting specifically refers to iron/steel corrosion. Not all corrosion is rusting, but all rusting is corrosion.

• Thinking all metal coatings provide sacrificial protection — Only more reactive metals (like zinc on iron) provide sacrificial protection. Tin and chromium provide barrier protection only — if the coating breaks, the iron underneath rusts faster.

• Forgetting that galvanising provides TWO types of protection — Many students only mention barrier protection. Always state both barrier AND sacrificial protection for full marks.

• Muddling the electrode positions in electroplating — The object being plated is always the cathode (negative electrode), where metal ions gain electrons and are deposited. The plating metal is the anode (positive electrode).

• Not linking prevention methods to real contexts — Exam questions often give scenarios (e.g. coastal areas, underground pipes, moving machinery). Choose the most appropriate method for the specific conditions described.

Exam technique for "Using resources: corrosion and its prevention"

• Command words matter: "Explain" requires reasons or mechanisms (e.g. zinc is more reactive than iron so corrodes instead). "Describe" needs observations or methods without detailed explanation. "Suggest" means apply your knowledge to unfamiliar contexts.

• Look for mark allocations: A 3-mark question on galvanising typically requires three distinct points — barrier protection, sacrificial protection, and why zinc works (more reactive than iron). Don't write the same point three different ways.

• Use correct chemical terminology: Write "hydrated iron(III) oxide" not just "rust" when explaining reactions. Use "oxidation" when discussing electron loss. Reference "oxygen AND water" not "air and moisture."

• Six-mark questions on prevention methods: Compare at least two methods, discuss advantages and disadvantages, link to the specific context given, and use scientific reasoning. Structure your answer in paragraphs for clarity.

Quick revision summary

Corrosion is metal destruction through environmental chemical reactions. Iron rusting requires both oxygen and water, forming hydrated iron(III) oxide. Prevention methods include barrier methods (painting, oiling, plastic coating) that physically separate iron from the environment. Galvanising coats iron with zinc, providing barrier and sacrificial protection since zinc is more reactive. Electroplating deposits thin metal layers using electrolysis. Stainless steel resists corrosion through chromium oxide layer formation. Understanding both the chemistry and practical applications is essential for exam success.