Which Metal is Used for Galvanising Iron?

Galvanisation (also spelled galvanization) is a corrosion-protection method in which iron or steel is coated with a thin layer of zinc (Zn).

Key facts: • Named after Luigi Galvani, the Italian scientist. • Iron left unprotected reacts with atmospheric oxygen and moisture to form rust (iron oxide, Fe₂O₃·xH₂O). • A zinc coating prevents moisture and oxygen from reaching the iron surface. • Even if the zinc layer is scratched or damaged, zinc still protects the iron by acting as a sacrificial anode (explained below).

Chemical basis: • Iron is more reactive than copper but less reactive than zinc. • Zinc (higher up in the reactivity series) oxidises in preference to iron. • This galvanic protection is why zinc is chosen over other metals like tin.

There are two main industrial methods of galvanising iron/steel:

1. Hot-Dip Galvanising: • The iron/steel article is cleaned (degreased and pickled in acid to remove rust and scale). • It is then dipped into a bath of molten zinc at ~450°C. • The zinc bonds metallurgically with the iron, forming zinc-iron alloy layers at the interface. • The outermost layer is pure zinc. • Produces a thick, durable coating (typically 45–85 microns thick). • Used for: steel beams, pipes, guardrails, buckets, and galvanised sheets.

2. Electrogalvanising (Electroplating): • Iron/steel is made the cathode in an electrolytic cell containing a zinc salt solution. • Electric current causes zinc ions to deposit on the iron surface. • Produces a thinner, smoother coating than hot-dip galvanising. • Used for: automotive body panels, appliances, bolts, nuts.

3. Sherardising (dry galvanising): • Iron is tumbled with zinc powder at ~300°C. • Zinc diffuses into the iron surface. • Used for small parts like screws and fasteners.

The most important property of galvanisation is zinc's role as a sacrificial anode:

Why zinc protects iron even when damaged: • When a galvanised iron surface is scratched, both zinc and iron are exposed. • In the presence of moisture, zinc and iron form a galvanic cell (electrochemical cell). • Zinc is higher in the electrochemical series (more active metal) → zinc acts as the anode. • The anode oxidises: Zn → Zn²⁺ + 2e⁻ (zinc corrodes). • Iron acts as the cathode and is protected from corrosion. • This protection continues until all the zinc near the scratched area is consumed.

In contrast, tin-plated iron (tinplate): • Tin is less reactive than iron. • If the tin layer is scratched, iron becomes the anode and corrodes FASTER. • This is why galvanised iron is preferred for outdoor structures over tin-plated iron.

The reactivity (electrochemical activity) series of common metals (most reactive to least reactive):

Potassium (K) Sodium (Na) Calcium (Ca) Magnesium (Mg) Aluminium (Al) Zinc (Zn) ← Used for galvanising Iron (Fe) ← The metal being protected Nickel (Ni) Tin (Sn) Lead (Pb) Copper (Cu) Silver (Ag) Gold (Au)

Zinc is positioned ABOVE iron in the series: → Zinc is more reactive than iron. → Zinc oxidises in preference to iron. → Zinc sacrifices itself to protect iron.

This makes zinc the ideal choice for galvanising iron.

Galvanised iron (GI) is widely used because of its corrosion resistance and low cost:

Construction and infrastructure: • Roofing sheets (GI sheets) • Corrugated iron roofing • Steel pipes for water supply • Structural beams and columns • Fencing wire and chain-link fence • Highway guardrails and lamp posts

Household and industrial: • GI buckets and tubs • Nails and screws • Nuts and bolts • Electrical conduit • Air-conditioning ducts

Automotive: • Galvanised steel body panels to prevent rust • Underbody protection

Note: GI wire is the common term for galvanised iron wire used in construction and agriculture (for binding, fencing, etc.).