in the electromeallury of aluminum oxygen gas is eloloved at anode `O_(2)` reacts with graohite or carbon (graphite electrodes ) to from carbon monoxide incase if case if some other metal electrodes is used as anode , them oxygen will aluminum formaed during the process to form alumoium oxide `(Al_(2)O_(3))` which will pass into the reaction mixture resulting into wastage of Al Since qraphite is cheaper than aluminum , its wastage orcan be tolerated .