The Ellingham curve for aluminium lies below the curves of most metals such as chromium, iron, etc. This fact indicates that aluminium can be used as the reducing agent for oxides of all these metals. This result is illustrated as follows:
The free energies of formation of chromium(III) oxide and aluminium oxide per mole of oxygen consumed are -541 kJ and -827 kJ respectively. The processes are:
The second equation minus the first equation gives:
So aluminium oxide is more stable than chromium oxide (at least at normal temperatures, and in fact all the way up to the decomposition temperatures of the oxides). Since the Gibbs free energy change is negative, aluminium can reduce chromium oxide.
In pyrometallurgy, aluminium is used as a reducing agent in the alumino-thermic process or thermite process to extract chromium and manganese by reduction of their oxides.