Electroplating

ELECTROPLATING, the art of depositing metals by the electric current. In the article ELECTROLYSIS it is shown how the passage of an electric current through a solution containing metallic ions (electrically charged atoms of a metal) involves the deposition of the metal on the cathode (the negative electrode). Under certain conditions, however, the metal is deposited in a non adherent and pulverulent form. The main factors which influence the nature of the deposit are the following: chemical composition and temperature of the electrolyte, current density, concentration and circulation of the electrolyte, and the nature of the metal receiving the deposit (Watts, Metal Indust., 1913). As the dur ability 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 complete adhesion. This can only be effected if the surface of the metal on which the deposit is to be made is chem ically clean. If the surface is coated with rust or oxide, a pre liminary cleaning is given by a sandblast or wire brushes, if the size of the object allows. Grease is removed by dipping into a hot alkaline solution. The last traces of oxide are dissolved and a bright metallic surface produced by means of a "pickling" solu tion, which is generally acid and varies with the metal treated. A cleaning treatment, which can generally be employed very ad vantageously, is to join the article as cathode in a suitable electro lyte, when the gas evolution removes much of the scale or grease mechanically.

General Methods.

The vats for depositing are generally wooden, lead-lined vessels, but may be enamelled iron, slate or particularly for small sizes, glazed earthenware. Large objects are suspended in the tanks by hooks or wires, the position of support being occasionally displaced so as to avoid wire-marks. Small objects are often heaped together in perforated trays or ladles, the cathode connecting-rod being buried in the middle. These require constant displacement since the objects are in contact at many points, and the top ones shield those below from the de positing action of the current. Processes have been devised in which the objects to be plated are suspended in revolving drums between the anodes, the rotation of the drum causing a constant renewal of surfaces, and at the same time imparting a burnishing action. As a galvanic deposit must be regular, tenacious, and usually capable of a high polish, its crystalline structure should be fine and as even as possible. With some metals, like nickel and iron, such deposits can be obtained from simple salt solutions. With others, like silver and lead, coarsely crystalline deposits are produced from such solutions, whilst finely grained deposits may generally be obtained by using electrolytes in which the metal is mainly present in the form of a complex ion. Solutions which would deposit their metal on the metal to be coated by simple dis placement on immersion should not generally be used, as the re sulting deposit is usually non-adherent. For this reason the acid copper bath is not used for iron or zinc objects, but is replaced by one containing copper cyanide or oxide dissolved in potassium cyanide. The formation of the complex ions reduces to a very high degree the concentration of the free metal ions, and, in accordance with the Nernst relation (see ELECTROLYSIS), the po tential required for deposition is considerably raised; and the separation of the metal by simple displacement is precluded and is only brought about by secondary chemical interaction with some more electro-positive element, such as sodium or potassium, the ions of which are present in much larger numbers and are first separated.

Processes to Secure Regular Deposition.

It is found that an adherent, smooth, fine grained deposit is more readily ob tained from concentrated solutions. Consequently, the current density should be kept low, in general about 0.02 amp. per sq.cm. (or 20 amp. per sq.ft.), because otherwise the concentration of the electrolyte is too rapidly decreased in the neighbourhood of the cathode. A high temperature and stirring also helps to pre vent a decrease in concentration near the cathode. When possible the electrolyte should, as a general rule, be kept slightly acid, to prevent precipitation of basic salts or hydroxides at the cathode. The character of the deposit is frequently improved by the addi tion of a small quantity of an organic colloid to the electrolyte.

Hydrogen overvoltage is an important factor in the electrolytic deposition of metals. Although the separation potentials of zinc and cadmium from their solutions are higher than that of hydro gen, these metals can be deposited electrolytically before the hydrogen from a solution containing hydrogen ions; whilst iron, which has a lower separation potential than either metal, cannot. This is because the liberation of hydrogen is prevented by the high hydrogen overvoltage of zinc and cadmium, while iron has a very low hydrogen overvoltage.

A consideration of much practical importance in electroplating is the so-called throwing power of the electrolyte. With an object of irregular shape there is a tendency for a thicker layer to be deposited on parts of it which project towards the anodes, whereas in hollows and indentations very little metal may be deposited. With some solutions, however, under suitable conditions, this unevenness of the layer' on different parts of the surface at dif ferent distances from the anode is very small; such a solution is said to possess good throwing power. The factors which deter mine this important property have recently been elucidated by Arndt and Clemens (Chem. Zeit., 1922) and by Haring and Blum (Trans. Amer. Electrochem. Soc., 1923).

In some instances a screening or vignetting of the surface of the cathode is employed to increase the regularity of the deposit, or specially shaped anodes are employed in order that the distance between the electrodes may be fairly uniform. Supplementary anodes are sometimes used in difficult cases of this kind. With large metallic external surfaces use is sometimes made of a brush which is constantly wetted with the electrolyte while the hairs or bristles are connected with a wire anode and is painted slowly over the surface of the metal to be coated, which is connected to the negative pole of current supply.

Under these conditions electrolysis of the solution in the brush takes place. Iron ships' plates have been coated with copper in sections (to prevent adhesion of barnacles) by building up a temporary trough against the side of the ship, making the thor oughly cleansed plate act both as cathode and as one side of the trough. Decorative plating-work in several colours (e.g., "parcel gilding") is effected by painting a portion of an object with a stopping out (i.e., a non-conducting) varnish, such as a copal varnish, so that this portion is not coated. The varnish is then removed, a different design stopped out, and another metal de posited. By varying this process, designs in metals of different colours may readily be obtained.

Special Methods.

Reference must be made to the text-books (see ELECTRO-CHEMISTRY) for a fuller account of varied solutions and methods used for electro-plating with different metals.

For the deposition of zinc, Sherard Cowper-Coles devised a proc ess in which a lead anode is used, and powdered zinc is suspended in the solution to maintain the proportion of zinc in the electro lyte, and so to guard against the gradual acidification of the bath. It was formerly considered essential for the satisfactory deposi tion of zinc to avoid the presence of any considerable amount of acid. In a process developed by Tainton and Pring, however, it has been established that the deposition of zinc either for plating or for recovery from solutions of ores can be efficiently conducted in solutions containing as much as 20-25% free sulphuric acid. This is effected by the use of current densities as high as I0o-500 amp. per sq.ft. and the presence of small amounts of a suitable colloid.

Cobalt is deposited by a method analogous to that used for nickel. Platinum, palladium and tin are occasionally deposited for special purposes. In the deposition of gold the colour of the deposit is influenced by the presence of impurities in the solution; when copper is 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 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 accord ing to the character of the brass required, and to the mode of treatment. The colour depends in 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 alloys may be produced, such as 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, Wilde produced copper printing sur faces for calico printing-rollers and similar articles by immersing rotating iron cylinders as cathodes in a copper bath. Elmore, Dumoulin, Cowper-Coles and others have prepared copper cylin ders and plates by depositing copper on rotating mandrels with special arrangements. Cowper-Coles was also successful in pro ducing true parabolic reflectors for projectors by depositing copper on glass surfaces.

The most noteworthy recent developments in electro-plating are in the deposition of iron and chromium and the use of electro plating for restoring worn components.

Iron Plating.

Processes of iron plating have been applied mainly at Grenoble and in the United States (Trans. Amer. Elec trochem. Soc., 1924) for the direct formation of articles such as boiler tubes of soft iron which on account of its purity has special properties.

In the process of Boucher in operation at Grenoble, which is used for producing iron tubes, an electrolyte is employed consist ing of ferrous sulphate or ferrous chloride solution. The elec trolyte is, during use, circulated over iron turnings and air is also blown in or iron oxide added. In this way the hydrogen ion con centration is kept low, and the dissociated air tends to depolarize the discharge of these ions. The current density is io–i z and the temperature about 80° C. The small hydrogen content of the metal is eventually removed by annealing at 900° C. The product contains 99.97% iron. About 4 kw. of current are re quired for one kilo of iron deposited.

Chromium Plating.

Chromium plating has been largely developed very recently for special purposes where surfaces are required which possess the highest possible resistance to abrasion. Chromium deposits can be produced which are considerably harder than the hardest steel. The main application has so far been in the surface coating of plates used in intaglio printing which are subjected to severe abrasion during use. Formerly these plates were made of case-hardened steel or nickel-plated. A procedure now adopted is to deposit a thin layer of chromium on the nickel surface of the finished printing plate (Chem. and Met. Eng., 1925). The thickness of the deposit need not exceed 0•0002 in. (0.005 mm.) in order to yield a greatly increased service. It is found that as a result of the poor throwing power in chromium plating, the lines are made deeper and the impressions are better than from the original plates. A smooth bright deposit is essential as subsequent polishing is impracticable. Further, after the chro mium plate has been worn almost through it can be "stripped" and a new surface deposited, without any loss of definition. The electrolyte employed is usually a solution of chromic acid with a small amount of chromic sulphate and chromium carbonate. The chromium carbonate neutralizes any excess acid and main tains the correct degree of acidity or hydrogen ion concentration which is essential for the deposition. The current density employed is i oo to 200 amp. per sq. foot (i i to 22 amp. per sq.dm.) , volt age, 7.9, and temperature of bath 4o°-50° C.

Repair of Worn Components by Electro-deposition.—An important application of electro-deposition which has been made in recent years is to the restoration of worn parts such as the bearings of machines, wheel axles and the linings of guns and howitzers (J. P. McLare, Trans. Faraday Soc., 1924-25). Copper was applied at the outset for this purpose on account of ease of deposition, but it is not sufficiently hard for a rubbing or rolling bearing surface, and even when used merely as a "packing," its liability to extrude under pressure renders it unsuitable for use. Methods of depositing iron have subsequently been developed. Formerly it was only possible to obtain this in coherent layers of a maximum thickness of about 0.003 in. and a procedure was adopted which employed alternate thin layers of copper and iron. It was found that one of the main causes of the inability to obtain thick deposits of iron was the lack of adequate preparatory clean ing of the metal surface. Eventually the adoption of an anodic system of cleaning in an acid bath, following a prolonged im mersion of the articles in a boiling caustic soda solution, has en abled the development of a process whereby deposits of iron or nickel of any desired thickness may be applied.

The initial cleaning of the metal surface to be coated is effected by connecting as cathode and suspending in a hot electrolyte containing i lb. of commercial caustic soda, 2 lb. of washing soda, and i oz. of sodium cyanide per gallon of water. A current density of about ioo amp. per sq.ft. of cathode surface is applied and electrolysis continued for a period of from 3 to 20 minutes. The parts to be protected from deposition are then insulated by coating with a suitable wax composition and the metal is given a further anodic cleaning treatment for a short period in a cold alkaline cleaning bath. The object is then rinsed in clear cold water and transferred to an acid cleaning bath containing 24 lb. of sulphuric acid in r gal. of water, and by connecting as anode is subjected to an electrolytic current of at least 200 amp. per sq. foot. On applying the current a black film forms on the metal immediately and very little gas is formed; in about 3o sec. or so, however, the film is observed to break away and a free evolu tion of oxygen occurs. The gassing is allowed to continue for about 20 sec. when the surface of the article is seen to have a clear grey matte appearance. The acid is then washed off as rapidly as possible and the article transferred to the depositing tank. In some instances, as with the harder steels, the black film does not break away completely. In this event, the current is reversed for about a minute, making the article the cathode ; an evolution of hydrogen then takes place which assists in disintegrating the clinging film, and on again reversing the current the cleaning is completed. The metal, after rinsing with water, is then trans ferred rapidly to the electro-deposition bath. The electrolyte formerly considered essential consisted of a dilute solution of neu tral ferrous ammonium sulphate. With this bath, a current density not exceeding 5 amp. per sq.ft., agitation of the solution, and a mechanical means of removing clinging hydrogen bubbles from the deposit to prevent pitting were essential. It has subsequently been found possible to employ concentrated solutions, containing from 3 to 3, lb. of ferrous ammonium sulphate per gallon, and a cur rent density of 18 to 20 amp. per sq.ft. without the necessity for continuous agitation of the electrolyte. The concentrated solu tion should be neutral or only very slightly acid to obtain deposits of reasonable hardness. The anodes consist preferably of very soft Swedish iron wire, wound in the form of a cylinder on flat grids. The temperature should be maintained at about 18 ° C.

Thick deposits of nickel are obtained from an electrolyte con taining 2 lb. nickel sulphate, 3+ oz. nickel chloride, and 4.1 oz. boric acid per gallon of water. A current density of 20 amp. per sq.ft. of cathode surface can be employed at normal room temper atures, giving a rate of deposition of approximately o•ooi in. of thickness per hour. As with iron, good deposits free from pitting can be obtained without the necessity for agitation when a suffi ciently large bulk of liquid can be accommodated around the work.

BIBLIOGRAPHY.

A. J. Allmand and F. Ellingham, The Principles of Bibliography.—A. J. Allmand and F. Ellingham, The Principles of Applied Electro-Chemistry (1924) ; W. R. Barclay and C. H. Hains worth, Electroplating; J. Hughes, Modern Electroplating.

(J.

N. P.)