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USES OF ELECTROLYSIS AND CORROSION OF METALS: CORROSION TREATED AS A REDOX PROCESS
Some of the main applications of electrolysis are as follows:
- In the extraction of elements especially metals such as Na, K, Mg, Ca, Zn and non-metals (e.g.H2, F2, Cl2) are obtained either from their fused compounds or their aqueous solutions.
- Purification of metals (e.g. Cu, Hg, Ag, Au).
- Electroplating of one metal by another.
- Preparation of certain important compounds, such as sodium hydroxide and sodium trioxochlorate (v).
- EXTRACTION OF ELEMENTS USING ELECTROLYSIS METHOD
An example of extraction can be shown using extraction of aluminium from purified bauxite (Al2O3). Â Highly electropositive metals like sodium, potassium, and calcium are extracted by the electrolytic reduction of their fused chlorides. For example, electrolysis of molten sodium chloride liberates sodium at the cathode and chlorine gas at the anode. Elements like chlorine and hydrogen are extracted by electrolysis.
- Purification of metals: The less electropositive metals e.g copper, mercury, silver and gold, tend to occur in the un-combined from in Ores. These ores are usually contaminated with impurities such as sand, limestone and iron compounds. An example is the extraction of copper from copper ore. In this process, an electrolysis cell is used in which the impure ore series as the anode and a pure copper plate series as the cathode. The electrolyte can be any soluble copper salt, such as copper(ii) tetraxoxsulphate (vi).
- At the Anode: The metallic copper atoms in the ore give up two electrons each to go into solution as copper ions.
- Cu (s) Â Â Â Cu2+(aq) + 2e–
- At Cathode: The copper ions in the solution are attracted to the cathode where they receive two electrons each from the cathode to become neutral copper atoms which are then deposited on the cathode.
- Cu2+(aq) + 2e– Â Â Â Â Â Â Cu (s)
- The net result is that pure atoms are transferred from the impure ore (anode) to the cathode. Similar arrangements are used in the extraction and purification of silver, mercury, and gold. In all cases, the electrolysis is soluble salt of the metal being extracted or purified, the anode is metallic ore or impure metal, while the cathode is a pure piece of metal on which the pure metallic atoms can be deposited.
- ELECTROPLATING
Electroplating is a method of coating the surface of one metal with another metal, usually copper, silver, chromium, nickel, or gold by means of electrolysis, for decoration or protection against corrosion; Articles which are usually plated include table-ware, cutlery, jewellery and iron or steel objects such as the metallic parts of motor – cars, the arrangement of the electrolytic cell is essentially the same as that used in the extraction of metals. The cathode is the metallic object which is to be located or plated. The anode strips off the plating metals; and the electrolyte, a salt of the plating metal. Thus, in silver – plating a spoon, the cathode is the spoon, the anode is the silver rod and the electrolyte is a soluble silver salt, usually a silver trioxonitrate (v) solution. As the current passes through the cell, the plating metal dissolves at the anode and the ions produced migrate to the cathode where they are discharged and deposited a layer on the object.
Many articles are plated at the same time. The thickness of the plating may be controlled by adjusting the magnitude of the current and the duration of the process. Where the first material used for coating could not adhere, (like in the case of chromium which does not adhere well to steel), first coat with copper which provides adhesion, then with a layer of nickel for protection against corrosion and finally, with a layer of chromium for a tarnish free finish.
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