What causes rust to form on metal surfaces?

Rust forms when iron or steel reacts with oxygen and water in a chemical process called oxidation. When moisture contacts the metal surface, it acts as an electrolyte that allows electrons to flow between different areas of the metal. Oxygen dissolved in the water reacts with iron atoms, producing iron oxide, which is the reddish-brown substance we call rust. The process accelerates in the presence of salt, acids, or pollutants, which make water more conductive and speed up the electrochemical reaction. Rust is also porous and flaky, meaning it doesn't form a protective barrier like the oxide layers on aluminum or copper do, so it continues spreading deeper into the metal until the structure is completely degraded.