When iron is exposed to oxygen and moisture for a long period of time,then rusting of the iron occurs. Now when iron reacts with oxygen and water then iron oxide is formed. Iron oxide(Ferric oxide) is the chemical name for rust. Rust constitutes of hydrated Ferric oxide. Only iron or alloys that contain iron can rust. Water acts as a catalyst in the rusting process of iron.
First oxidation of the ferrous ion takes place. The ferrous ion gets converted to ferric ions. This is a redox reaction.
4 Fe2+ + O2 → 4 Fe3+ + 2 O2−
where Fe2+ is the ferrous ions and Fe3+ are the ferric ions.
Then the ferric ions react with the water molecules to form ferric hydroxide or rust.
Fe3+ + 3 H2O ⇌ Fe(OH)3 + 3 H+
where Fe(OH)3 is ferric hydroxide also called as rust.
