Deterioration of a metal by chemical or electrochemical reaction with its environment is called corrosion. It occurs when a metal reacts with substances around it, such as oxygen, water, acids, salts, or other chemicals.
Corrosion is usually an oxidation process: atoms at the metal surface lose electrons and enter a chemical reaction. In electrochemical corrosion, different areas of the metal act as anodic and cathodic regions, while an electrolyte such as damp air or seawater allows ions to move. This process gradually weakens or consumes the material.
Rusting is the most familiar example. When iron or steel is exposed to oxygen and moisture, it forms hydrated iron oxides, commonly called rust. Rust is corrosion, but the two words are not interchangeable in every context: copper can develop a green patina, silver can tarnish, and aluminum forms a thin protective oxide layer.
Corrosion is distinct from purely mechanical wear, such as abrasion or bending. Engineers reduce it through painting, plating, galvanizing, alloy selection, cathodic protection, and corrosion-resistant materials such as stainless steel.