Why Do Some Metals Rust?

Rusting is an electrochemical process specific to iron and its alloys (including steel). When iron is exposed to water and oxygen, it loses electrons (oxidises) to form iron(II) ions, which further oxidise to iron(III) and combine with water to form hydrated iron(III) oxide — the reddish-brown flaky substance we call rust. The process requires both water (which acts as an electrolyte, conducting ions) and oxygen (which accepts electrons). Salt dramatically accelerates rusting because dissolved sodium chloride ions increase the electrical conductivity of the water film. Unlike aluminium, which forms a thin, adherent, self-healing oxide layer (aluminium oxide) that protects the metal beneath, rust is porous and crumbly — it flakes off, exposing fresh metal to further corrosion. This is why iron eventually rusts through completely. Corrosion costs the global economy an estimated $2.5 trillion per year — about 3.4% of global GDP.