Electrolysis is the process of reducing metal ions (anode) or oxide ions (cathode) with electrons. In liquids, the ions are free to move but in solids they must be dissolved.
When aluminium is left in air, it very quickly acquires a thin layer of oxide to protect it from corrosion. However, this coating is strongly passivating - it prevents any further reaction between aluminium and oxygen.
To get rid of the oxide layer, aluminium must be reacted with an oxygen ion at the surface. In order to do this, the ion must be free to move, so that it can reach the appropriate electrode (cathode) to receive an electron and become molten aluminium.
A typical electrolysis cell carries out the Hall-Heroult process, which is an electric current through a solution of a metal and a non-metal ion. The metal ion moves from the negative cathode to the positive anode and the non-metal ion from the anode to the cathode.
In a galvanostatic process, the anodic potential increases until it reaches a critical value where the breakdown of the electrolyte/gas/oxide/electrode system takes place. This is accompanied by the partial formation of a gas film over the electrode, which increases the electric resistance in this system and inhibits the continuation of the current flow.
Large discharges that occur during a galvanostatic process can destroy the formed oxide coating. These deteriorating discharges should be avoided. Moreover, the onset of these discharges is strongly dependent on the process parameters.