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Why iron rusts — Chemistry, 7–10 years

Rust is a new, crumbly material made when iron reacts slowly with oxygen and water. Knowing what rust needs helps us protect bridges, bicycles and tools from damage.

Rust needs iron, air and water

Rust forms on iron when oxygen and water reach its surface for long enough. It is not just dirt stuck to the metal: it is a new substance with a reddish-brown colour and a crumbly texture. Salt can make this change happen faster.

Why compare dry and wet iron?

A rusty bicycle suggested that water caused rust, but rain always brings air too. To find what matters, scientists compare iron kept dry, iron in water with air and iron protected from air. Changing one condition at a time turns a guess into a fair test.

Three test tubes

Put an iron nail in each of three labelled tubes. Leave one dry, cover one with boiled water and oil so air stays out, and put the third in ordinary water exposed to air. After several days, the third nail shows the most rust. The comparison suggests that both water and oxygen are needed.

Water alone is not enough

It is reasonable to blame water because a wet nail often rusts and a dry one usually does not. But a nail submerged in water can rust less at first if little oxygen reaches it. Rust needs the meeting of iron, water and oxygen, not water acting alone.

Protecting metal

Paint, oil, plastic coatings and galvanising can keep water or oxygen away from iron. Builders use these methods on bridges, cars and railings, then inspect them because a tiny scratch can expose fresh metal. Keeping tools dry also slows rust, but it does not repair metal already eaten away.

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