Why metal rusts
Iron rusts when oxygen and water react with it, slowly turning the metal into flaky iron oxides.
It's often said that water alone causes rust — in fact, rust needs both water and oxygen, and dry metal can avoid the reaction even when oxygen is present.
Rust is part of a larger natural process called corrosion, in which materials gradually return to more chemically stable forms; engineers design bridges, cars, and tools to slow that process down.
Think of iron as a pile of tiny building blocks that oxygen and water can quietly rearrange into a weak, crumbly pile called rust.
Understanding rust helps whenever you choose outdoor tools, maintain a car or bicycle, or decide whether a coating is doing its job.
Imagine an iron nail left outside after rain: a thin film of water covers it, oxygen dissolves into that film, and the two help pull iron atoms from the nail's surface into a new compound. As more of the compound forms, the surface becomes the familiar rough, reddish-brown rust.
Salt makes rust faster
Saltwater carries charged particles more easily than plain water, helping the electrochemical reactions that move iron atoms and electrons along.
Rust does not protect iron
Unlike the tightly attached oxide layers on some metals, rust tends to crack and flake away, exposing fresh iron underneath.
Coatings block the ingredients
Paint, oil, plating, and other protective layers slow rust by keeping water and oxygen from reaching the iron surface.