Electrolysis has many commercial uses, for example:
extracting reactive metals such as aluminium from their ores or compounds
anodising aluminium
extraction of active non metals such as halogens
electroplating (including chrome plating and nickel plating)
electrorefining (including obtaining pure copper from an impure sample
Processes such as anodizing and galvanizing enhance the appearance of a metal or prevent corrosion. Corrosion occurs when a metal reacts with substances in the environment forming oxides and sometimes, sulphides, carbonates and hydroxides.
Anodising Aluminium
Anodising is the process by which aluminium is given a thick protective coat of aluminium oxide through electrolysis. When exposed to air, aluminium combines with oxygen to form aluminium oxide (Al2O3). This oxide forms a coat on the outside of the metal that protects it from further corrosion. Electrolysis is used to make this oxide layer thicker and tougher.
In anodizing, the aluminium is used as the anode of an electrolytic cell containing dilute sulphuric acid (or dilute chromic (VI) acid) as the electrolyte. Although, any electrolyte which releases oxygen at the anode may be used.
The reaction at the anode is as follows:
4OH⁻ → 2H₂O(l) + O₂ + 4e⁻
Thus, the liberated oxygen combines with the aluminium anode coating it with oxide. The outer coat can also absorb dyes.
Electroplating
Electroplating involves covering a metal with a layer of another through electrolysis. Usually, the outer layer is a less reactive metal. Electroplating metal objects serves several purposes, including:
enhancing visual appeal of the plated object
protecting the covered metal object from corrosion
avoiding using large quantities of expensive metals for the object
During electroplating, the object to be covered is used as the cathode (negative electrode) and the pure plating metal is used as the anode. The electrolyte must also contain ions of the plating metal.
In electroplating, ions pass into solution from the pure metal anode and are discharged and deposited as a thin layer on the cathode.
Electrorefining
Copper refining is an example of refining metals through electrolysis (electrorefining).
The electrolyte in copper electrorefining is a mixture of copper (II) sulphate and sulphuric acid. The impure sample of copper is used as the anode and the cathode is a strip of pure copper metal.
Copper atoms leave the anode and enter the electrolyte as ions. These ions are then discharged and deposited at the cathode. Usual impurities include zinc and gold. Zinc will lose electrons and pass into solution as zinc ions. Other unreactive impurities like gold collect at the bottom of the anode as a solid mixture known as anode mud.