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July 23, 2008

Metal Coating Use of Electroplating

Electroplating is a method of producing a smooth, compact and fairly uniform film of metal from an aqueous solution of one of its chemical compounds by means of a direct electric current. The current flows from the aqueous solution to the article being electroplated, which must of course itself be electrically conducting, and which forms the cathode. The current enters the aqueous solution through another metallic electrode, called the electroplating anode. The rest of the circuit is of coarse metallic. The anode is usually made of the metal being plated out at the cathode; it dissolves under the action of the current and thus replenishes the solution, so that the net overall effect is to transfer metal from the anode and distribute it more or less uniformly over the cathode, leaving the composition of the solution unchanged. It is, however, incorrect to think of the metal passing at any appreciable speed between the electrodes in combined form. The metal is produced from the solution in one place (the cathode) and returned to it at another (the anode); in continued practical operation if the solution is not stirred it will become unduly concentrated around the anode, and impoverished around the cathode.

Electroplating dates from the 1930s, and is closely associated with the name of Michael Faraday. Gold, silver and copper were first electroplated soon after the invention of the Daniel cell, the first reliable source of continuous electric current. Electromagnetic generation of electricity by rotating machinery was not available at that time, dynamos; in fact were first applied commercially in the electroplating industry, but not until 1841.

To understand the electroplating it is necessary to recall the general principles of electrolysis and the nature of the simpler chemical compounds and of their aqueous solutions. Metals are chemical element (alloy steel are mixtures of elemental metals). Metals are distinctive among the elements because the atoms of which they are composed have one or two (occasionally three) outer electrons which are rather loosely attached to the rest of the atom. In the solid metal these outer electrons are shared by all the atoms present, and are very mobile. Their mobility is the cause of the distinctively high electrical and thermal conductivity of metals.

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