Galvanising – IGCSE Chemistry Definition and Key Facts

Galvanising is tested in the Cambridge 0620 syllabus under methods of preventing rusting. It is a particularly important example because it combines two different protection mechanisms in one process — a point that regularly carries 3 marks on Paper 4.

The 0620 definition

Galvanising is the process of coating iron or steel with a layer of zinc. The zinc protects the iron from rusting.

How galvanising works — two mechanisms

1. Barrier protection

The zinc layer physically prevents water and oxygen from reaching the iron surface. As long as the coating is intact, the iron cannot rust because the conditions for rusting are not met.

2. Sacrificial protection

Even if the zinc coating is scratched or damaged, the iron is still protected. Zinc is more reactive than iron (higher in the reactivity series), so it corrodes preferentially. The zinc atoms lose electrons instead of the iron atoms.

At the scratch: Zn → Zn²⁺ + 2e⁻ (zinc is oxidised)

The electrons flow to the iron, keeping it in the metallic (reduced) state and preventing rust.

This dual mechanism is what makes galvanising superior to paint alone — paint only provides barrier protection.

Methods of galvanising

Method	Process	Used for
Hot-dip galvanising	Iron/steel is dipped into a bath of molten zinc (~450 °C)	Fencing, roofing sheets, car bodies
Electrogalvanising	Zinc is deposited by electroplating	Thin, uniform coatings for precision parts
Sherardising	Object heated with zinc dust in a sealed rotating drum	Small components — nuts, bolts

Hot-dip galvanising is the most common industrial method and produces a relatively thick zinc layer.

Galvanising vs other rust prevention methods

Method	Type of protection	Still protects if damaged?	Cost
Galvanising (zinc)	Barrier + sacrificial	Yes	Moderate
Painting	Barrier only	No	Low
Oiling/greasing	Barrier only	No	Low
Plastic coating	Barrier only	No	Moderate
Electroplating with chromium	Barrier only	No (Cr is less reactive than Fe)	High
Sacrificial protection (Mg blocks)	Sacrificial only	Yes (while Mg remains)	Moderate-High

The critical exam point: galvanising is the only common method that provides both barrier and sacrificial protection simultaneously.

Real-world applications in Malaysia

Galvanised steel is widely used across Malaysia. Corrugated zinc roofing (the traditional “atap zink”) is galvanised iron sheeting, common in older buildings. Road barriers, lamp posts, and construction scaffolding are galvanised to resist corrosion in humid tropical conditions. The high humidity and rainfall in Malaysia make corrosion protection especially important.

Galvanising vs tin plating

This comparison is a common exam trap. Tin is less reactive than iron. A tin-plated can protects the iron only while the coating is intact (barrier). If scratched, the iron corrodes preferentially — and tin actually accelerates the corrosion because iron is now the more reactive metal losing electrons. This is the opposite of galvanising.

Worked exam question

A car body is made from steel that has been galvanised. (a) Explain what galvanising means. [1] (b) Explain why the steel does not rust even if the zinc layer is scratched. [3]

(a) Coating the steel with a layer of zinc [1]

(b) Zinc is more reactive than iron [1]. Where the scratch exposes the iron, zinc reacts/corrodes preferentially instead of the iron [1]. This is because zinc loses electrons more readily, and these electrons flow to the iron, keeping it in a reduced (metallic) state [1].

Common exam mistakes

1. Saying galvanising protects only as a barrier — candidates must also mention sacrificial protection for full marks.
2. Confusing galvanising (zinc coating) with tin plating (tin coating). Tin is less reactive than iron, so a scratched tin-plated can will rust faster than untreated iron.
3. Not explaining why zinc corrodes instead of iron — the answer must reference the reactivity series or state that zinc is more reactive.