Corrosion of MetalsCambridge IGCSE Chemistry: Revision notes
Section 1
What conditions are needed for iron and steel to rust?
Rusting is the corrosion of iron and steel, producing hydrated iron(III) oxide (Fe₂O₃·xH₂O), the reddish-brown substance seen on corroded metals.
Three conditions must be present simultaneously for rusting to occur:
- Oxygen must be present
- Water (or moisture/moisture vapour) must be present
- Iron or steel must be exposed to both
If any one of these conditions is removed, rusting cannot happen. This is why:
- Objects stored in dry conditions do not rust
- Objects sealed from air (under oil or grease) do not rust
- Objects without iron content (pure copper, aluminium) do not rust in the same way
The rusting process involves oxidation — iron loses electrons to form iron(III) ions, which then combine with oxygen and water to form the hydrated oxide coating.
Examiners expect you to state all THREE conditions (oxygen AND water AND iron) together. Saying 'air' is too vague — specify oxygen. Always mention that BOTH oxygen and water are required, not just one.
Think of rusting like baking a cake — you need flour AND eggs AND heat. Leave out one ingredient and the cake won't bake properly. Similarly, remove oxygen, water, or iron and rusting stops.
Section 2
How do barrier methods prevent rusting?
Barrier methods protect iron and steel by creating a physical barrier between the metal surface and the environment, preventing contact with either oxygen or water (or both).
Common barrier methods include:
| Method | How it works | Materials used |
|---|---|---|
| Painting | Creates a protective coat that blocks both oxygen and water | Paint, varnish, enamel |
| Greasing/oiling | Forms an oily layer that excludes water and reduces oxygen contact | Grease, oil, petroleum jelly |
| Plastic coating | Plastic layer physically separates metal from air and moisture | Plastic film, plastic powder coating |
The effectiveness of barrier methods depends on:
- Completeness — the barrier must cover the entire surface; any gaps or scratches allow rusting to start
- Durability — the barrier must remain intact over time; wear, damage, or peeling reduces protection
- Adhesion — the barrier must stick firmly to the metal surface
Barrier methods are preventative — they stop rusting from starting but cannot reverse rusting that has already begun.
Students often say barrier methods 'prevent oxygen' when they should say 'exclude oxygen' or 'prevent contact with oxygen'. Also, saying 'paint stops air' is imprecise — say 'paint prevents oxygen and water from reaching the metal'.
A car door is painted to exclude both oxygen and water. If the paint scratches and bare metal is exposed, rusting can begin at that point because oxygen and water can now reach the iron. This shows why barrier methods must cover the entire surface without gaps.
Section 3
What is sacrificial protection and how does zinc protect steel?
Galvanising is the coating of steel with a layer of zinc to prevent rusting. Unlike simple barrier methods, galvanising provides sacrificial protection — a special form of protection based on reactivity.
How sacrificial protection works:
Zinc is more reactive than iron (higher in the reactivity series). This means:
- Zinc loses electrons more readily than iron does
- Zinc preferentially oxidises (corrodes) instead of the iron beneath
- Even if the zinc coating scratches or wears, the exposed iron is still protected because zinc continues to corrode instead
The sacrificial process:
- Zinc atoms lose electrons: Zn → Zn²⁺ + 2e⁻ (oxidation)
- These electrons flow to the iron surface
- Oxygen and water react with these electrons on the iron, but the iron itself doesn't lose electrons
- The zinc acts as a sacrificial anode — it is 'sacrificed' (corroded away) to protect the steel
Key advantages over simple barrier methods:
- Provides protection even where coating is damaged
- Zinc corrosion products (zinc hydroxide, basic zinc carbonate) form a protective layer that slows further corrosion
- Long-lasting protection even with mechanical damage
Important distinction: Galvanising uses sacrificial protection, whilst painting uses barrier protection.
Examiners expect you to explain sacrificial protection in terms of the reactivity series and electron loss. State: 'Zinc is more reactive than iron, so zinc loses electrons more readily and oxidises preferentially, protecting the iron.'
A galvanised bucket has a zinc coating. Even if the coating is scratched and bare steel is exposed, the steel doesn't rust because zinc (being more reactive) continues to lose electrons and oxidise instead. The zinc gradually wears away but protects the steel underneath until it is completely gone.
Section 4
How does the reactivity series explain sacrificial protection?
The reactivity series is central to understanding why sacrificial protection works.
Position in the reactivity series:
Zinc is positioned above iron in the reactivity series, meaning zinc is more reactive. The order is:
Zinc > Iron (simplified section showing relevant metals)
This ordering determines electron loss tendencies:
- More reactive metals (like zinc) lose electrons more readily
- Less reactive metals (like iron) lose electrons less readily
Electron transfer in sacrificial protection:
When zinc and iron are in contact in the presence of oxygen and water:
- Zinc atoms preferentially lose electrons: Zn → Zn²⁺ + 2e⁻
- These electrons cannot flow to zinc ions (they're already oxidised); instead they flow to the iron surface
- On the iron surface, oxygen and water accept these electrons: O₂ + 2H₂O + 4e⁻ → 4OH⁻
- Because iron is not directly losing electrons, iron does not oxidise and does not rust
Why reactivity order matters:
If a less reactive metal (like copper) were used to coat iron, it would not provide sacrificial protection because iron is actually more reactive than copper. The iron would oxidise preferentially and rust faster — this is why the correct choice of protective metal is essential.
When answering questions about sacrificial protection, always refer to the reactivity series and specifically state that zinc is more reactive than iron, therefore zinc loses electrons more readily. This is the chemical principle examiners want to see explained.
Students often forget to explain WHY zinc protects iron. Simply saying 'zinc is more reactive' is incomplete. You must explain that because zinc is more reactive, it LOSES ELECTRONS MORE READILY, causing zinc to oxidise instead of iron.
Section 5
How do barrier methods and sacrificial protection compare?
Both methods prevent rusting but work through fundamentally different mechanisms:
| Feature | Barrier Methods (paint, grease, plastic) | Sacrificial Protection (galvanising with zinc) |
|---|---|---|
| Mechanism | Physically block oxygen and/or water | More reactive metal oxidises instead of iron |
| Based on | Exclusion of corrosion conditions | Reactivity series and electron loss |
| Protection if coating damaged | Lost — exposed metal rusts | Maintained — zinc still protects nearby iron |
| How long it lasts | Until coating wears away | Longer — zinc is slowly consumed |
| Cost | Lower initial cost | Higher initial cost |
| Application | Wide range of metals and surfaces | Only practical for iron and steel |
| Maintenance | Requires repainting/recoating | Lower maintenance after application |
When to use each method:
- Barrier methods are suitable for items that are regularly maintained and where durability damage is acceptable
- Sacrificial protection is ideal for items exposed to harsh conditions (moisture, salt water) where coating damage is likely
Important point: Some applications use both methods together — for example, galvanised steel coated with paint provides maximum protection.
Exam questions often ask 'Why is galvanising better than painting?' The key answer is: galvanising provides protection even when the coating is damaged because zinc continues to corrode preferentially, whereas painting loses protection immediately if scratched.
Must Know
- Three conditions required for rusting: Iron/steel, oxygen, AND water must all be present; removing any one stops rusting
- Hydrated iron(III) oxide is the reddish-brown compound formed when iron rusts; formula is Fe₂O₃·xH₂O
- Barrier methods (painting, greasing, plastic coating) prevent rusting by excluding oxygen and/or water — protection is lost if the barrier is damaged
- Galvanising uses zinc as a protective coating because zinc is more reactive than iron in the reactivity series
- Sacrificial protection principle: Zinc loses electrons more readily than iron, so zinc oxidises preferentially and iron is protected even if the coating is scratched
- Electron transfer in sacrificial protection: Zn → Zn²⁺ + 2e⁻ (zinc oxidises); these electrons protect iron from oxidation
- Key advantage of sacrificial protection over barrier methods: Protection is maintained even when coating is damaged because the more reactive metal continues to corrode preferentially
That's the notes covered.
Carry on to the next subtopic.