Rust formed on iron exposed to moist air is chiefly

In rusting, iron is oxidised to $\mathrm{Fe}{}^{2+}$ at anodic spots and oxygen is reduced at cathodic spots. The $\mathrm{Fe}{}^{2+}$ is further oxidised by atmospheric oxygen to $\mathrm{Fe}{}^{3+}$, which separates as hydrated ferric oxide, $\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot x\mathrm{H}{}_2\mathrm{O}$, the reddish-brown rust. $\mathrm{Fe}(\mathrm{OH}){}_2$ is only an intermediate and $\mathrm{Fe}{}_3\mathrm{O}{}_4$ (black) forms on heating iron in steam, not in ordinary rusting.