Corrosion Prevention Methods

Corrosion prevention is tested in the Cambridge 0620 syllabus under metals. The most commonly tested form of corrosion is rusting of iron and steel. Questions frequently ask candidates to compare methods and explain why some methods continue to protect after damage while others do not.

What is rusting?

Rusting is the corrosion of iron. It requires both water and oxygen. The iron is oxidised:

4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃.xH₂O (hydrated iron(III) oxide — rust)

If either water or oxygen is removed, rusting cannot occur. All prevention methods work by eliminating one or both conditions, or by using an electrochemical mechanism.

Barrier methods

Barrier methods physically prevent water and oxygen from contacting the iron surface.

Painting

Feature	Detail
How it works	Layer of paint seals the surface
Advantages	Cheap, easy to apply, decorative
Disadvantages	Chips and scratches expose iron; must be reapplied
If damaged	Rusting begins at the exposed area
Examples	Cars, bridges, railings, ships

Oiling and greasing

Feature	Detail
How it works	Oil/grease forms a water-repelling layer
Advantages	Easy to apply; good for moving parts
Disadvantages	Messy; wears off with use; must be reapplied
If damaged	Rusting begins where oil is worn away
Examples	Bicycle chains, machine parts, tools

Plastic coating

Feature	Detail
How it works	Object is dipped in or sprayed with molten plastic
Advantages	Durable, waterproof, electrically insulating
Disadvantages	Can crack or peel; limited to objects that can be coated
If damaged	Rusting begins at exposed area
Examples	Dishwasher racks, garden furniture, fridge shelves

Electroplating

Coating with a less reactive metal (tin, chromium, nickel) by electrolysis. See electroplating applications.

Important: if the plating metal is less reactive than iron (e.g. tin), a scratch accelerates rusting because iron becomes the anode in a tiny electrochemical cell. This is the opposite of galvanising.

Sacrificial protection

A more reactive metal is placed in contact with the iron. The more reactive metal corrodes instead of the iron because it loses electrons more easily.

How it works

The more reactive metal (Zn or Mg) is oxidised preferentially:

Zn → Zn²⁺ + 2e⁻

Electrons flow from the zinc to the iron, keeping the iron in its reduced (metallic) state. The iron is protected even where it is exposed to water and oxygen.

Examples of sacrificial protection

Application	Metal used	Why that metal
Ship hulls	Zinc or magnesium blocks	Bolted to steel hull; protect in seawater
Underground pipelines	Magnesium bars	Connected by wire; protect in damp soil
Oil rig legs	Zinc or aluminium anodes	Protect submerged steel structures
Galvanised steel	Zinc coating	Dual barrier + sacrificial protection

Why the metal must be more reactive

The sacrificial metal must be above iron in the reactivity series. If a less reactive metal is used (e.g. copper), the iron would corrode preferentially instead — the protection would fail.

Galvanising — the best of both

Galvanising is coating iron or steel with zinc. It combines barrier and sacrificial protection:

1. Intact coating: zinc acts as a physical barrier, keeping water and oxygen away
2. Damaged coating: zinc corrodes preferentially (sacrificial protection), protecting the iron at the scratch

This is why galvanising is widely used for outdoor steel in Malaysia’s tropical climate — corrugated roofing, fencing, scaffolding, and road barriers.

Comparison table

Method	Type	If damaged?	Cost	Durability
Painting	Barrier	Rusting starts	Low	Moderate
Oiling	Barrier	Rusting starts	Low	Short
Plastic coating	Barrier	Rusting starts	Moderate	Good
Tin plating	Barrier	Rusting accelerated	Moderate	Good
Chromium plating	Barrier	Rusting starts	High	Very good
Galvanising (zinc)	Barrier + sacrificial	Still protected	Moderate	Good
Sacrificial Mg/Zn blocks	Sacrificial	Still protected (while metal remains)	Moderate	Needs replacing
Alloying (stainless steel)	Neither — inherent resistance	Still resistant	High	Excellent

Stainless steel

Stainless steel is an alloy of iron with chromium (and often nickel). The chromium forms a tough, self-healing oxide layer on the surface that prevents further corrosion. Unlike other methods, stainless steel resists corrosion inherently — no coating is needed.

Used for: surgical instruments, kitchen sinks, cutlery, food processing equipment.

Worked exam question

A steel gate and a steel ship hull both need protection from rusting. Suggest a suitable method for each and explain your choice. [4]

Gate: galvanising — coating with zinc provides both barrier protection and sacrificial protection; if scratched, the zinc corrodes instead of the iron, and it is cost-effective for outdoor structures [2].

Ship hull: sacrificial protection using zinc or magnesium blocks bolted to the hull — the blocks are more reactive than iron and corrode preferentially in seawater, protecting the steel; they can be replaced when consumed [2].

Common exam mistakes

1. Saying tin plating provides sacrificial protection — tin is less reactive than iron, so it only provides barrier protection. If scratched, the iron rusts faster.
2. Not distinguishing between barrier and sacrificial methods when the question asks you to compare.
3. Forgetting that the sacrificial metal must be more reactive than iron — stating “any metal” protects iron is incorrect.
4. Not explaining how sacrificial protection works in terms of electron transfer and reactivity.