Sacrificial Protection – IGCSE Chemistry Definition

Sacrificial protection is a Supplement topic that connects the reactivity series to a real-world application. Examiners consistently test whether candidates can explain why the sacrificial metal must be more reactive than iron, not less.

The 0620 definition

Sacrificial protection is a method of preventing the corrosion of iron or steel by attaching a more reactive metal. The more reactive metal corrodes instead of the iron.

How it works

1. A more reactive metal (typically zinc or magnesium) is attached to the iron/steel structure
2. When exposed to water and oxygen, both metals could potentially corrode
3. The more reactive metal loses electrons more easily
4. Electrons flow from the more reactive metal to the iron, keeping the iron in a reduced (metallic) state
5. The sacrificial metal gradually corrodes and must eventually be replaced

The key idea: the more reactive metal is oxidised instead of the iron.

Real-world examples

Application	Sacrificial metal	Why
Ships’ hulls	Zinc or magnesium blocks bolted to hull	Protects steel hull in seawater
Underground pipelines	Magnesium bars connected to pipeline	Protects steel pipes in damp soil
Galvanised steel	Zinc coating	Even if scratched, zinc corrodes first
Oil rigs	Zinc or aluminium anodes	Protects legs submerged in seawater

Sacrificial protection vs barrier methods

Feature	Sacrificial protection	Barrier (paint/oil/plastic)
How it works	Electrochemical — more reactive metal corrodes instead	Physical — keeps water and oxygen away
If coating is damaged	Still protects (zinc corrodes at scratch)	No longer protects (rust starts at scratch)
Metal must be…	More reactive than iron	Any material that seals the surface
Needs replacing?	Yes, sacrificial metal is consumed	Yes, coating wears or chips

Worked exam question

Zinc blocks are attached to a ship’s steel hull. (a) Name this method of rust prevention. [1] (b) Explain how the zinc blocks prevent the steel from rusting. [2] (c) State why magnesium could also be used but copper could not. [1]

Mark scheme

(a) Sacrificial protection [1]

(b) Zinc is more reactive than iron [1]; zinc corrodes/reacts preferentially instead of the iron/steel [1]

(c) Magnesium is more reactive than iron so would corrode instead; copper is less reactive than iron so the iron would corrode first [1]

Common exam mistakes

• Suggesting copper as a sacrificial metal — copper is less reactive than iron, so it would not corrode preferentially. The sacrificial metal must be above iron in the reactivity series.
• Confusing sacrificial protection with simply coating the iron — the protection is electrochemical, not a barrier.
• Forgetting to state that the sacrificial metal is “more reactive” — this phrase is essential for the mark.