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ELECTROPLATING
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ELECTROPLATING , the See also:art of depositing metals by the electric current. In the See also:article See also:ELECTROLYSIS it is shown how the passage of an electric current through a See also:solution containing metallic ions involves the deposition of the See also:metal on the See also:cathode. Sometimes the metal is deposited in a pulverulent See also:form, at others as a See also:firm tenacious film, the nature of the See also:deposit being dependent upon the particular metal, the concentration of the solution, the difference of potential between the electrodes, and other experimental conditions. As the durability of the electro-deposited coat on plated wares of all kinds is of the utmost importance, the greatest care must be taken to ensure its See also:complete See also:adhesion. This can only be effected if the See also:surface of the metal on which the deposit is to be made is chemically clean. Grease must be removed by potash, See also:whiting or other means, and tarnish by an See also:acid or See also:potassium See also:cyanide, washing in plenty of See also:water being resorted to after each operation. The vats for depositing may be of enamelled See also:iron, See also:slate, glazed earthenware, See also:glass, See also:lead-lined See also:wood, &c. The current densities and potential See also:differences frequently used for some of the commoner metals are given in the following table, taken from M`Millan's See also:Treatise on See also:Electrometallurgy. It must be remembered, however, that See also:variations in conditions modify the electromotive force required for any given See also:process. For example, a rise in temperature of the See also:bath causes an increase in its conductivity, so that a See also:lower E.M.F. will suffice to give the required current See also:density'; on the other See also:hand, an abnormally See also:great distance between the electrodes, or a diminution in acidity of an acid bath, or in the strength of the solution used, will increase the resistance, and so require the application of a higher E.M.F. Large See also:objects are suspended in the tanks by hooks or wires, care being taken to shift their position and so avoid See also:wire-marks. Small objects are often heaped together in perforated trays or ladles, the cathode connecting-See also:rod being buried in the midst of them. These require See also:constant shifting because the objects are in contact at many points, and because the See also:top ones See also:shield those below from the depositing See also:action of the current. Hence processes have been patented in which the objects to be plated are suspended in revolving drums between the anodes, the rotation of the See also:drum causing the constant renewal of surfaces and affording a burnishing action at the same See also:time. Care must be taken not to expose goods in the plating-bath to too high a current density, else they may be " burnt "; they must never be exposed one at a time to the full anode surface, with the current flowing in an empty bath, but either one piece at a time should be replaced, or some of the anodes should be transferred temporarily to the See also:place of the cathodes, in See also:order to distribute the current over a sufficient cathode-See also:area. Burnt deposits are dark-coloured, or even pulverulent and useless. The strength of the current may also be regulated by introducing lengths of See also:German See also:silver or iron wire, See also:carbon rod, or other inferior conductors in the path of the current, and a See also:series of such resistances should always be provided See also:close to the tanks. Ammeters to measure the See also:volume, and volt-meters to determine the pressure of current supplied to the See also:baths, should also be provided. Very irregular surfaces may require the use of specially shaped anodes in order that the distance between the electrodes may be fairly See also:uniform, otherwise the portion of the cathode lying nearest to the anode may receive an undue See also:share of the current, and therefore a greater thickness of coat. Supplementary anodes are sometimes used in difficult cases of this See also:kind. Large metallic surfaces (especially See also:external surfaces) are sometimes plated by means of a " See also:doctor," which, in its simplest form, is a See also:brush constantly wetted with the electrolyte, with a wire anode buried amid the hairs or bristles; this brush is painted slowly over the surface of the metal to be coated, which must be connected to the negative terminal of theelectrical generator. Under these conditions electrolysis of the solution in the brush takes place. Iron See also:ships' plates have recently been coated with See also:copper in sections (to prevent the adhesion of barnacles), by See also:building up a temporary trough against the See also:side of the See also:ship, making, the thoroughly cleansed See also:plate See also:act both as cathode and as one side of the trough. Decorative plating-See also:work in several See also:colours (e.g. "See also:parcel-See also:gilding ") is effected by See also:painting a portion of an See also:object with a stopping-out (i.e. a non-conducting) See also:varnish, such as See also:copal varnish, so that this portion is not coated. The varnish is then removed, a different See also:design stopped out, and another metal deposited. By varying this process, designs in metals of different colours may readily be obtained. Reference must be made to the textbooks (see See also:ELECTROCHEMISTRY) for a See also:fuller See also:account of the very varied solutions and methods employed for electroplating with silver, See also:gold, copper, iron and See also:nickel. It should be mentioned here, however, that solutions which would deposit their metal on any object by See also:simple See also:immersion should not be generally used for electroplating that object, as the resulting deposit is usually non-adhesive. For this See also:reason the acid copper-bath is not used for iron or See also:zinc objects, a bath containing copper cyanide or See also:oxide dissolved in potassium cyanide being substituted. This solution, being an inferior conductor of See also:electricity, requires a much higher electromotive force to drive the current through it, and is therefore more costly in use. It is, however, commonly employed hot, whereby its resistance is reduced. Zinc is commonly de-posited by electrolysis on iron or See also:steel goods which would ordinarily be " galvanized," but which for any reason may not conveniently be treated by the method of immersion in fused zinc. The zinc cyanide bath may be used for small objects, but for heavy goods the sulphate bath is employed. Sherard See also:Cowper-Coles patented a process in which, working with a high current density, a lead anode is used, and powdered zinc is kept suspended in the solution to maintain the proportion of zinc in the electrolyte, and so to guard against the See also:gradual acidification of the bath. See also:Cobalt is deposited by a method analogous to that used for its See also:sister-metal nickel. See also:Platinum, See also:palladium and See also:tin are occasionally deposited for See also:special purposes. In the deposition of gold the See also:colour of the deposit is influenced by the presence of impurities in the solution; when copper is See also:present, some is deposited with the gold, imparting to it a reddish colour, whilst a little silver gives it a greenish shade. Thus so-called coloured-gold deposits may be produced by the judicious introduction of suitable impurities. Even pure gold, it may be noted, is darker or lighter in colour according as a stronger or a weaker current is used. The electro-deposition of brass—mainly on iron See also:ware, such as bedstead tubes—is now very widely practised, the bath employed being a mixture of copper, zinc and potassium cyanides, the proportions of which vary according to the See also:character of the See also:brass required, and to the mode of treatment. The colour depends in See also:part upon the proportion of copper and zinc, and in part upon the current density, weaker currents tending to produce a redder or yellower metal. Other See also:alloys may be produced, such as See also:bronze, or German silver, by selecting solutions (usually cyanides) from which the current is able to deposit the constituent metals simultaneously. Electrolysis has in a few instances been applied to processes of manufacture. For example, See also:Wilde produced copper See also:printing surfaces for See also:calico printing-rollers and the like by immersing rotating iron cylinders as cathodes in a copper bath. Elmore, See also:Dumoulin, Cowper-Coles and others have prepared copper cylinders and plates by depositing copper on rotating mandrels with special arrangements. Others have arranged a means of obtaining high conductivity wire from cathode-copper without See also:fusion, by depositing the metal in the form of a See also:spiral See also:strip on a See also:cylinder, the strip being subsequently See also:drawn down in the usual way; at present, however, the See also:ordinary methods of wire Amperes. Metal. Per s 9• decimetre Per sq. in. of VAno between of Cathode Cathode See also:Ana de and Surface. Surface. Cathode. See also:Antimony 0.4-0.5 0.02-0.03 1.0-1.2 Brass 0.5-a•8 0.•03-0.05 3.0-4.0 Copper, acid bath 1.0-1.5 0.065-0.10 0.5-1.5 alkaline bath . 0.3-0.5 0.02-0.03 3.0-5.0 Gold o•1 o•oo6 0.5-4.0 Iron 0.5 0.03 1•o Nickel, at first 1.4-1.5 0.09-0.10 5.0 „ after 0.2-0.3 0.015.0.02 1.5-2.0 on zinc o•4 0.025 4•o-5.0 Silver . . 0.2-0.5 0.015-0'03 0.75-1.0 Zinc 0.3-0.6 0.02-0.04 2.5-3.0 See also:production are found to be cheaper. J. W. See also:Swan (Journ. Inst. Elec. Eng., 1898, vol. See also:xxvii. p. 16) also worked out, but did not proceed with, a process in which a copper wire whilst receiving a deposit of copper was continuously passed through the draw-plate, and thus indefinitely extended in length. Cowper-Coles (Journ. Inst. Elec. Eng., 1898, 27, p. 99) very successfully produced true parabolic reflectors for projectors, by depositing copper upon carefully ground and polished glass surfaces rendered conductive by a film of deposited silver. 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