The process of formation of hydrated iron oxide is called ________.

The reaction of iron (Fe) with oxygen (O₂₎ and moisture (H₂O) in the air resulting in the formation of reddish-brown coloured hydrated iron oxide(rust) is called rusting. Hydrated iron oxide is called 'rust'(Fe₂O₃.xH₂O).

$4\mathrm{Fe}\left(\mathrm{s}\right) + 3{\mathrm{O}}_2\left(\mathrm{g}\right)+ {\mathrm{xH}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{Fe}}_2{\mathrm{O}}_3.{\mathrm{xH}}_2\mathrm{O}\left(\mathrm{s}\right)$