Electroplating Technology

Electroplating Technology

 

a field of applied electrochemistry that deals with processes of electrolytic deposition on the surfaces of metallic and nonmetallic articles. Electroplating technology encompasses electro-deposition, which is the production on articles of strongly adherent thin metal coatings, and galvanoplastics, which is the production of easily removed, relatively thick, accurate copies (so-called matrices) of various articles. The discovery and technical development of electroplating technology belong to the Russian scientist B. S. Iakobi, who announced his discovery on Oct. 5, 1838, at a meeting of the St. Petersburg Academy of Sciences.

Electroplating technology is based on the phenomenon of electrocrystallization, which is the deposition on the cathode (in electrodeposition, the article being coated; in galvanoplastics, the matrix) of positively charged ions of metals from aqueous solutions of their compounds by passing a direct current through the solution. The processes of electroplating technology are governed quantitatively by Faraday’s laws, with allowance for side effects, which most frequently consist in the release of hydrogen along with the metal on the surface of the article being coated. Qualitatively, these processes are controlled by the type and composition of the electrolyte and by the mode of electrolysis—that is, current density—as well as temperature and the intensity of mixing.

A distinction is made between electrolytes based on simple or complex compounds. The former are much cheaper and simpler, and by using intensive mixing (most frequently by means of air), it is possible to use high current densities, thus accelerating electrolysis. Thus, for example, in electrodeposition, when coating articles of simple shape using an electrolyte based on zinc sulfate with colloid additions, current densities up to 300 amps/m2 are used, but with air mixing, densities of up to 30 kiloamps/m2 are used. In galvanoplastics, solutions of simple salts—most often sulfates—are usually used without any organic additives, since in thick layers they have a negative effect on the mechanical properties of the resultant copies. The current density used is less than in electrodeposition: in iron galvanoplastics baths it is not over 10-30 amps/m2, whereas in iron plating (electrodeposition) it reaches 2,000-4,000 amps/m2.

Electroplated coverings should be microcrystalline and of uniform thickness at various parts of the coated articles (projections and cavities). In electrodeposition this requirement is particularly important when coating articles of complex shape. In this case the electrolytes are based on complex compounds or on simple salts with added surface-active agents. The process of depositing tin from a stannic sulfate electrolyte is an example of the favorable effect of such agents on the structure of the coating; without the addition of these agents, isolated crystals that resemble tinsel and are useless as a coating form on the surfaces of the articles being coated. If phenol, cresol, or other aromatic compounds are added to the electrolyte with a small amount of a colloid (glue or gelatin), a dense, strongly adherent coating with an entirely satisfactory structure is formed. At 65°-70° C, alkaline stannic electrolytes in which the tin is present as a negative complex ion (SnO3)4- and which contain no surface-active agents yield microcrystalline coatings that adhere well. This difference in behavior between acidic and alkaline electrolytes is due to the fact that the simple divalent tin ions in the former, in the absence of a surface-active agent, discharge without any appreciable retardation (polarization), whereas with alkaline electrolytes the tin is present as complex ions, which discharge with appreciable retardation. Alkaline cyanide or other complex zinc salt baths are used in zinc-plating articles of complex shape. Cyanide electrolytes are usually used for cadmium, silver, gold, and brass plating.

Anodes, whose main purpose is to replace in the electrolyte the ions discharged on the articles being coated, play an essential role in electroplating-technology processes. Anodes should not contain impurities that adversely affect the appearance and structure of the coating. In some cases the anodes are made in the same shape as the articles being coated. Chromium, gold, platinum, and rhodium plating are done with insoluble metal or alloy anodes that are resistant to the particular electrolyte. The constant composition of the electrolyte is maintained through the periodic addition of salts or other compounds of the metal being plated out.

All processes, both in galvanoplastics and in electrodeposition, take place in electroplating baths. The composition of the electrolyte in the bath is often also called the electroplating bath. Depending on the size of the bath and the aggressiveness of the electrolyte, baths can be made of ceramic material, enameled cast iron, organic glass, or steel lined with lead or polyvinyl chloride sheets. The volume of the bath varies from a fraction of a cubic meter for gold plating to 10 m3 or more. Baths are classified as stationary (in which the articles being coated are fixed), semiautomatic (in which the articles rotate or move in a circular or horseshoe-shaped pattern), and unit-type (in which the articles are automatically loaded, unloaded, and transported through a series of baths). The direct current for the electrolysis is generally obtained from selenium or silicon rectifiers, and the current density is controlled by means of a multistage transformer.

Electrodeposition is used more widely than galvanoplastics; its goal is to impart certain properties—improved corrosion resistance (galvanizing and cadmium, tin, and lead plating) and wear resistance of friction surfaces (chromium and iron plating)—to finished or semifinished articles. It is used for the protective and decorative finishing of surfaces (chromium or nickel plating or plating with precious metals). Compared with methods of applying coatings used since ancient times, such as immersion in molten metal, electrodeposition has a number of advantages, particularly when only a very thin coating is required. Thus, the electrolytic process for coating sheet metal with tin to produce food containers is displacing the old, hot method. In the USA electrolytic tin plate accounts for more than 99 percent of the total production (1966). The expenditure of tin is thus reduced manyfold, mainly because of the differentiation in the thickness of the tin coating between 0.2-0.3 and 1.5-2 microns depending on the aggressiveness of the food. In electrodeposition all coatings must adhere strongly to the articles coated, and with many coatings this requirement must be met at any degree of deformation of the metal base. Strong adherence of the coating to the base is ensured by the proper preparation of the surface of the article being coated, which involves the complete removal of oxides and grease impurities by pickling or degreasing. When applying protective and decorative coatings (silver, gold, and so on), roughness from previous operations must be removed by grinding and polishing.

In electrodeposition, technical progress is developing along the lines of direct preparation of shiny coatings that do not require additional polishing. Progress in the area of equipment consists in the development and introduction of mechanized and automated assemblies for mechanical preparation of the surface and application of coatings, including all subsidiary operations, up to the application of coatings to a continuous strip followed by stamping of the articles—for example, automobile bodies and food packaging containers. The key industries in which electrodeposition is important are automobile construction, aviation, radio, and electronics.

The difference between galvanoplastics and electrodeposition lies mainly in the methods of preparing the surfaces of the reverse images of the objects (matrices) being copied and in the greater thickness of the accumulated metal (dozens and hundreds of times greater). There are metal and nonmetallic matrices. The advantages of the metallic matrices are the easier preparation of the surface (most frequently by oxidation) and the possibility of making a large number of copies. A film of silver (tenths of a micron thick) or nickel (up to 2 microns thick) is applied to the metal matrices to form an intermediate layer. Both of these metals oxidize excellently upon immersion for three minutes in a 2-3 percent bichromate solution and permit easy removal of the built-up layer. Oxidized aluminum is a promising material for metal matrices. Coating with graphite is usually used to impart electrical conductivity to the face surfaces of nonmetallic matrices. For this purpose, fine flaked graphite free from impurities is applied to the surfaces of the matrices with soft hair brushes. Gypsum and gutta-percha matrices are most often used for large objects and objects with complicated relief—for example, statues and bas-reliefs. The matrices for such objects are made in parts. The direct galvanoplastic copies are soldered together without distortion.

Copper galvanoplastics is the most common; iron and nickel galvanoplastics are less common. Printing is the main field of application of galvanoplastics, which is also extensively used to make the matrices of phonograph records and in the manufacture of wave guides.

REFERENCES

Iakobi, B. S. Raboty po elektrokhimii. Moscow-Leningrad, 1957.
Lainer, V. I. Sovremennaia gal’vanotekhnika. Moscow, 1967.
Modern Electroplating. Edited by A. G. Gray. New York-London, 1953.
Modern Electroplating, 2nd ed. Edited by F. A. Lowenheim. New York-London-Sydney, 1963.

V. I. LAINER

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