Galvanising and Barrier Methods

Iron and steel rust when both oxygen and water are present. Salt water speeds up rusting. To protect metal, we can stop oxygen and water from reaching the surface, or make another metal corrode instead.

Barrier methods

Barrier methods put a coating on iron/steel so oxygen and water cannot touch it.

• Painting: common on gates and car bodies.
• Greasing/oiling: used on bike chains and machinery; easy to reapply.
• Plastic coating: thick, tough layer on wire racks and tools.
• Plating with another metal (e.g., tin or chromium) for shiny appearance and protection.

Key idea: Barrier methods work only while the coating is unbroken. A scratch lets in oxygen and water and rusting starts underneath. Tin-plated steel protects well, but if scratched, the exposed iron can rust quickly.

Galvanising

Galvanising is coating iron/steel with zinc, usually by dipping it into molten zinc. It protects in two ways:

• Barrier: the zinc layer keeps out oxygen and water.
• Sacrificial protection: if the coating is scratched, zinc still protects because it is more reactive than iron. Zinc loses electrons (is oxidised) in place of iron: $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$. Iron stays safe.

Zinc can also form a tight, protective surface layer (zinc oxide/carbonate) that further slows corrosion.

Why sacrificial protection works

In the reactivity series, zinc is above iron. More reactive metals lose electrons more easily. When zinc and iron touch in moist air, zinc supplies electrons and corrodes first, so iron does not rust.

Real-world examples

• Galvanised steel: street lights, fences, buckets, roofing.
• Similar idea: zinc “sacrificial anodes” on ships and underground pipes.

Common misconceptions

• Galvanising is not just a barrier; it still protects if scratched.
• Painting and plastic coating only work while intact.
• Stainless steel resists corrosion for a different reason (a chromium oxide layer), not galvanising.