What is the chemical formula of rust?

The common chemical formula for rust is Fe₂O₃·nH₂O (hydrated iron(III) oxide), often written as hydrated ferric oxide.

Rust is not one exact compound but a mix of hydrated iron oxides and hydroxides, mainly Fe₂O₃·nH₂O and sometimes FeO(OH) and Fe(OH)₃. It forms when iron reacts with oxygen and water (or moisture) in the presence of salts or acids.

• Basic reaction (overall): iron + oxygen + water → hydrated iron(III) oxides (rust).
• Prevention: painting, galvanizing (zinc coating), alloying (stainless steel), oiling, and cathodic protection.
• Why it flakes: Rust occupies more volume than the original iron, causing cracks and peeling.