What is Electroplating? What are the uses of Electroplating? Information on principles and equipments of Electroplating.
Electroplating; is the electrodeposition of an adherent coating on an object to obtain a surface different from that of the underlying material. Electroplated surfaces are most widely used for their protective and decorative properties. On a smaller scale they are used to provide other specific properties such as high electrical conductivity, high optical reflectivity, or low toxicity. Some of the metals most commonly used for plating are nickel, chromium, tin, copper, silver, and gold.
Electroplating is done for many purposes and on every scale, from the refmishing of a silver candlestick to high-speed electrotinning, where tin is plated on continuous 3-foot-wide strips of steel at speeds up to 2,000 feet per minute to produce the tin plate that goes into the manufacture of tin cans.
Silver is plated on tableware for its bright attractive appearance and because it is nontoxic. Gold commonly is plated on jewelry for its attractive, nontamishing finish. It is also plated on surfaces where its low electrical contact resistance is desired.
Steel is coated with various metals for protection against corrosion and wear. Tin is plated on steel, and coatings of nickel and copper covered with chromium are also used. Cadmium plating is particularly useful in marine atmospheres. Zinc provides the most economical corrosion protection for steel where it is not important to have a bright, shiny finish.
In some industries, such as the electronics industry, it is important that surfaces that are to be soldered are instantaneously wetted by the solder with the use of only a mild flux. In such cases electroplating with tin, tin-lead alloys, or gold is done to produce such surfaces.
Electroplates are applied for their physical and mechanical properties, and in such applications the deposits are usually much thicker than those used for decoration or protection. Chromium, which is hard and has a low coefficient of friction, is widely used for such purposes. As hard chromium it is plated on cutting tools, dies, and rolls. Hard chromium is no harder than decorative chromium; it is only a thicker deposit. Worn or mismachined parts can be plated back to their proper dimensions, with iron, nickel, or chromium at costs much below thosdl of replacement. Bearing properties can be improved by the application of a deposit of porous chromium, which has pits or channels in its surface that hold the oil. Tin is plated on piston rings to prevent scoring of the cylinder walls during the breaking-in period.
Copper, although occasionally used as a final finish, is generally used as a preplate that is over-plated with other metals, most commonly nickel and chromium. Copper is used as a preliminary coating because it is relatively easy to apply, it tends to smooth over defects in the substrate, and, when mechanical finishing is needed, it is much easier to buff than steel.
Rhodium plating is used on reflectors of high-intensity light and for electrical contacts. Several alloys can be plated, including brass (copper-zinc), bronzé (copper-tin), tin-lead, tin-nickel, and tin-zinc.
Electroplating is an electrolytic process in which a piece of the metal to be plated acts as the anode of the cell, the electrolyte is most commonly a solution containing ions of the metal to be plated, and the object to be plated acts as the cathode of the cell. When current enters the cell, metal ions from the electrolyte are attracted to the cathode, where they are reduced and become deposited on the object to be plated. The anode metal dissolves, thus providing more metal ions in the electrolyte. In some cases a nondissolving anode is used, and metal ions must be supplied by adding fresh solution of the electrolyte.
The basic equipment for electroplating includes a plating tank, a tank to hold the cleaning solutions, and several rinse tanks.The plating tank is equipped with bus bars to carry the electric current to the anodes, which are usually arranged around the sides, and the cathodes (the work being plated), which are usually down the center. Heating coils may be required to keep cleaners and plating solutions at their proper operating temperatures, and sometimes cooling coils are used to dissipate the heat evolved in the plating tank.
Whenever work is moved from one tank to another, it must go through a rinse tank to remove the solution adhering from the previous operation. The final rinse must be thorough to prevent salts from drying on the finished article. Articles may be transferred from one tank to another by hand, or the process may be partially or completely automated. Small parts, such as screws or nuts, that do not lend themselves to individual handling are placed in barrels that rotate under the surface of the solution.
In order for a satisfactory deposit to be formed, the basis metal must be clean and free of rust Or other interfering layers. Also, some metals must be treated to render them “active”—that is, receptive to an adherent deposit. Some basis metals are more difficult to plate than others. Ordinary steel is relatively easy to prepare for plating, whereas aluminum, magnesium, titanium, and tantalum require very specialized preplating treatments.
The basic cleaning procedure begins with the removal of gross soils and contamination by treatment with hot alkaline solutions or organic solvents. The next step is acid pickling to remove oxide films. A final cleaning is done with mild alkalies. There are many variations on this cycle, depending on the nature of the metal and how dirty it is.
Plating solutions are usually aqueous, although with some metals that cannot be plated from water, organic electrolytes are used. Other metals are plated from fused salts. Most plating solutions also contain a variety of chemicals that increase the conductivity of the solution, regulate the acid-base balance, modify the physical form of the deposit, and increase the rate at which the anode dissolves (to replace the metal ions plating out of the electrolyte).
Many plating solutions are patented, either as a whole or with regard to specific additives that permit the production of deposits that are bright as they come from the solution and do not have to be buffed. Most deposits from baths not containing these special additives are mat, or dull, and for decorative purposes must be brightened by mechanical means. Many baths require small amounts of organic additives, called addition agents, to produce satisfactory deposits.
Many plating solutions contain cyanide compounds and heavy metals, such as chromium, nickel, and zinc. Since these substances are highly toxic, disposing of the wastewaters in which they are present has become an increasingly important environmental problem.
Thickness of Electroplates
Because electroplating is a relatively expensive means of applying a coating, commercial plates are no heavier than necessary. Gold plate applied purely for decorative purposes may be as thin as 0.000002 inch (0.05 micron). The chromium layer in common copper/nickel/chromium platings is about 0.00001 inch (0.25 micron), while the nickel layer, when such coatings are used for outdoor applications such as automobile bumpers, is 0.001 to 0.002 inch (25-50 microns) thick. Zinc and cadmium, depending on the corrosion protection required, are plated to a thickness of 0.0001 to 0.0006 inch (2.5-15 microns). When tin is plated for solderability, it is about 0.0002 inch (5 microns) thick, while tin plate used on tin cans is only about 0.000015 inch (0.4 micron) thick. Hard chromium plated for engineering applications may be from 0.004 to 0.01 inch (100-300 microns). Electroforms are plated to a thickness of up to 0.2 inch (0.5 cm).