Rust and corrosion

In short: Rust forms when iron reacts with oxygen and water. More generally, the gradual destruction of metals by their surroundings is called corrosion.

In more detail: Chemically, rusting is an oxidation: iron atoms give up electrons and, together with oxygen and water, form hydrated iron oxides. This layer is porous and flakes off, so fresh iron underneath keeps being attacked.

In detail

• Two ingredients: Iron only rusts when oxygen and water are present at the same time. In completely dry air or oxygen-free water it hardly rusts.
• Salt speeds it up: Dissolved salts make water more conductive and speed up the reaction. That is why cars rust from road salt in winter and ships rust quickly in seawater.
• More volume: Rust takes up more space than the original iron. It can crack concrete when the steel reinforcement inside it rusts.
• Protective layer: Aluminium, chromium and zinc form a thin, dense oxide layer in air that protects the metal beneath. Copper gradually develops a green patina.
• Stainless steel: Stainless steel contains chromium, which forms a self-healing protective layer. Even so it is not invulnerable, for example to chloride.
• Galvanising: A zinc coating protects iron in two ways. It covers it, and if damaged the zinc is attacked first because it is less noble.
• Sacrificial anode: On the same principle, pieces of magnesium or zinc are fitted to boat hulls or hot-water tanks so that they corrode instead of the part itself.
• Protection at home: Paint, oil, grease and dry storage keep water and oxygen away. Simply drying tools after use helps a lot.
• Basics: Corrosion is a redox reaction between a metal and its surroundings (chemische-reaktionen-grundlagen, metalle-und-legierungen).