The picture given shows the corrosion of iron. The chemical formula of rust is Fe2O3.H2O.
The rust on iron does not form by a simple reaction of oxygen with iron surface. The rust is formed by an electrochemical reaction.
Different regions on the surface of iron become anode and cathode.
Fe is oxidised to Fe2+ (anode)
Fe (s) → Fe2+ (aq) + 2e-
O2 is reduced to form water (cathode)
O2 (g) + 4H+ (aq) + 4 e- → 2H2O
When Fe2+ ions migrate from the anode region they react with water and further get oxidised to form Fe3+ ions.
A redish coloured hydrated oxide is formed from Fe3+ ions.
It is called rust which collects on the surface.
Due to various components of atmosphere, oxidation of metals takes place, consequently resulting in their damage. This is called ‘corrosion’.
Iron rusts and a redish coloured layer is collected on it. This is corrosion of iron.