Electroplating

electroplating, process of coating with metal by means of an electric current. Plating metal may be transferred to conductive surfaces (metals) or to nonconductive surfaces (plastics, wood, leather) after the latter have been rendered conductive by such processes as coating with graphite, conductive lacquer, electroless plate, or a vaporized coating.

Figure 1 shows a typical plating tank containing copper sulfate (CuSO₄) solution. A dynamo supplies electric current, which is controlled by a rheostat. When the switch is closed, the cathode bar, which holds the work to be plated, is charged negatively. Some of the electrons from the cathode bar transfer to the positively charged copper ions (Cu²⁺), setting them free as atoms of copper metal. These copper atoms take their place on the cathode surface, copperplating it. Concurrently, as shown in the drawing, the same number of sulfate ions (SO₄^2-) are discharged on the copper anodes, thereby completing the electrical circuit. In so doing, they form a new quantity of copper sulfate that dissolves in the solution and restores it to its original composition. This procedure is typical of nearly all ordinary electroplating processes; the current deposits a given amount of metal on the cathode and the anode dissolves to the same extent, maintaining the solution more or less uniformly. If this balance is perfect and there are no side reactions or losses, a 100 percent cathode efficiency and 100 percent anode efficiency could possibly be realized.

If the metal surface of the cathode is chemically and physically clean, the discharged atoms of copper are deposited within normal interatomic spacing of the atoms of the basis metal and attempt to become an integral part of it. In fact, if the basis metal is copper, the new copper atoms will frequently arrange themselves to continue the crystal structure of the basis metal, the plate becoming more or less indistinguishable from and inseparable from the basis metal.

If suitable solutions of different metals are mixed, it is possible to plate a wide variety of alloys of metals. By this means plated brass can be made more or less indistinguishable from cast brass. It is also possible, however, to deposit alloys or compounds of metals that cannot be produced by melting and casting them together. For example, tin-nickel alloy plate has been used commercially for its hardness and corrosion resistance, which are superior to that of either metal alone. The deposit consists of a tin-nickel compound (Sn-Ni) that cannot be produced in any other way.

Other common alloy plates include bronze and gold, with varying properties, such as different colours or hardnesses. Magnetic alloy plates of such metals as iron, cobalt, and nickel are used for memory drums in computers. Solder plate (Sn-Pb) is used in printed circuit work.