Galvanizing Iron with Zinc — A Coat That Rusts So the Iron Will Not

Iron's great weakness is rust. Left in the weather, an iron roof or bucket flakes away to nothing in a few years. In 1837 the French engineer Stanislas Sorel patented a cure that is still used on a vast scale today: dip the iron in a bath of molten zinc. The zinc bonds to the surface and shields the iron beneath, and galvanized iron suddenly lasted for decades outdoors.

The zinc protects in two ways at once. It is a barrier, sealing the iron from air and water — but more cleverly, it is a sacrifice. Zinc is more reactive than iron, so even where the coating is scratched and bare iron shows, the surrounding zinc corrodes first and spares the iron. The coating gives up itself to save the metal underneath.

Hot-dip galvanizing clad the Industrial Revolution's wire, nails, pipes, and the corrugated iron roofs that sheltered half the world. It is simple in principle — clean the iron, flux it, and dip it in molten zinc — but every step matters, because a missed patch of grease or rust leaves a hole in the armour.