The rusting of iron is formulated as ${\mathrm{Fe}}_2{\mathrm{O}}_3\cdot x{\mathrm{H}}_2\mathrm{O}$ which involves the formation of:

Iron rusts as a result of an increase in iron's oxidation state and an accompanying loss of electrons. Two distinct iron oxides with differing iron atom oxidation states make up the majority of rust. These oxides are ferrous oxide or iron(II) oxide.

The following reaction is obtained 

${\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{Fe}(\mathrm{OH}{)}_3\to {\mathrm{Fe}}_2{\mathrm{O}}_{3 }\cdot 3{\mathrm{H}}_{2 }\mathrm{O}$