PLATING

application of a plate, or coat, of metal to a surface for decoration, reflection of light, protection against corrosion, or increased wearing quality. The practice is of ancient origin: gilding was developed early; the Romans soldered silver plates to articles of baser metals; and in the 5th cent. a process was described where iron weapons were coated with copper by dipping them in a copper solution. In the 18th cent. the plating of copper or brass with silver by fusion was originated in England (see Sheffield plate). Plating by electrolysis, or electroplating, is the most common method because it permits the control of the thickness of the plating. Cadmium, zinc, silver, gold, tin, copper, nickel, and chromium are the most frequently used plating metals. Any of the common metals and some nonmetals, e.g., plastics, with suitably prepared surfaces can be used as a base. In electroforming, a mold made of specially prepared wax is electroplated. The wax is then melted and the plating itself constitutes the finished product. The part to be plated may be dipped in molten metal. For example, molten zinc is used in galvanizing; lead-tin alloy, in terneplating; and tin, in tinning. Without electricity, some metals can be precipitated from chemical solutions onto the surface of plastics or metals. A powdered, liquid, or gaseous plating metal can be made to diffuse into a heated base-metal surface. Sherardizing is the diffusion coating of zinc on steel; chromizing, chromium on steel; and calorizing, aluminum on steel. The application by welding or brazing of a sheet or plate to the base metal is called cladding. Decorative metal coatings are applied to plastic by vapor deposition. The metal, usually aluminum, and the plastic parts are put in a vacuum chamber. When the aluminum is vaporized by a surge of electricity, the particles settle on any exposed surface. Worn parts can be sprayed with molten metal to build them up to their original dimensions. Iron or steel is best protected from corrosion by a coating of cadmium or zinc, which oxidizes, creating a protective coat for the underlying material. Parts such as automobile bumpers usually have an undercoating of copper, a protective layer of nickel, and a thin, shiny surface layer of chromium for decoration. In the manufacture of phonograph records an original recording is made on a metal disk coated with lacquer, which is then plated. From the plated master a die is made to be used in a hydraulic press. The silvering of mirrors is a type of plating in which silver is precipitated chemically on glass.

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