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Electroplating

Electroplating is the process of producing a metallic coating on a surface by electrodeposition - i.e., by the action of an electric current. Such coatings may perform a mainly protective function, to prevent corrosion of the metal on which they are deposited: e.g., plating with zinc (electrogalvanizing) or with tin; or a decorative function: e.g., gold and silver plating; or both functions: e.g. chromium plating. The principle of electroplating is that the coating metal is deposited from an electrolyte - an aqueous acid or alkaline solution - on to the base: i.e., the metal to be coated. The latter forms the cathode (negative electrode). A low-voltage direct current is used; the anode is gradually consumed. Various substances (addition agents) are added to the electroplating bath to obtain a smooth and bright metal deposit. These are principally organic compounds, usually colloidal. Sometimes the objects to be plated are coated with two or more layers of different metals; for example, chromium plating cannot suitably be applied directly to a zinc-sprayed base; a coating of copper followed by a coating of nickel must be applied intermediately before the chromium is deposited.

To obtain a good and firmly adhering coating it is necessary to subject the objects or components to a thorough cleaning.. This may be achieved by mechanical treatment - e.g., sandblasting, grinding, wire brushing, scraping, etc.; or by physical methods such as degreasing with organic solvents; or by chemical methods such as pickling with acid or degreasing by the action of alkalizes (saponification); or by electrocleaning, which is a method of cleaning by electrolytic action (more particularly the scrubbing action exercised by the evolution of gas at the surface of the metal). Wetting agents or emulsifiers may be added. The vats for electroplating baths differ greatly in size, shape and lining material (glass, lead, etc.), depending on the size and shape of the components to be plated and on the chemical character of the bath. Electroplating is normally done with direct current. However, particularly with cyanide copper baths, improved smoothness and uniformity of the coating can be obtained by means of the so-called periodic-reverse process, in which the polarity is periodically reversed, so that the metal is alternatively plated and deplated.

Steep strip is plated with zinc or with tin by continuous and largely automated high-speed processes. The electrolytic tin-plating process illustrated schematically comprises the following operations: electrolytic cleaning in dilute sulfuric acid, pickling, electrodeposition of tin, melting of the coating to give it a brilliant surface, chemical dipping in chromate solutions, oiling, shearing. The steel strip travels through the installation at a speed of about 25 m/mm. (80 ft./min.).

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