Immersion coatings are applied without electricity by immersing parts in a chemical solution or bath containing the metal to be deposited. Deposition can take place either by displacement, when the metal in solution displaces the base metal, or by reduction, where the base metal does not enter into the reaction.
Although many metals can be deposited in immersion baths, comparatively few have proved acceptable for decorative or functional applications. These are nickel, tin, copper, gold, and silver.
Electroless Nickel Properties
Electroless nickel is generally more expensive than electroplated nickel. For this reason it is used primarily for its functional properties, although a very smooth, bright deposit can be obtained on buffed ferrous and nonferrous metals.
Because of its amorphous structure and phosphorous content (8 to 10%), the coatings are said to have better corrosion resistance than electrolytic or wrought nickel. Hardness of the coatings is relatively high — about 50 Rc — and can be raised to 64 Rc by heat treatment. Thickness of the coatings ranges from 1 to 5 mils, depending on end use.
Applications
The most important uses for electroless nickel are to protect parts from corrosion and to prevent product contamination. The coatings are widely used on tank-car interiors to protect caustic soda, ethylene oxide, tetraethyl lead, tall oil, and many other liquids from contamination. Other similar applications include oil refinery air compressors, missile fuel for plates, gas storage bottles for liquid, and pumps for petroleum and related products.
The hardness of the coatings is particularly valuable in increasing the life of rotating and reciprocating surfaces in gas compressors, pumps, hydraulic cylinders, sheaves, and armatures. The coatings are also used on aluminum electronic devices to facilitate soldering, on stainless steel to facilitate brazing, on moving metal parts to prevent galling, and on stainless steel equipment to prevent stress corrosion cracking.
Tin
Properties
Tin immersion coatings are especially noted for their low cost, bright appearance, good fric-tional properties, and ease of application to many common metals such as copper, brass, bronze, aluminum, and steel. However, their corrosion resistance is only fair.
As with some other immersion coatings, plating usually stops when the base metal is completely covered. Thus, thickness for common decorative uses is limited to about 0.015 mil. However, thicknesses up to 2 mils for heavy-duty applications have been produced by placing the base metal in contact with a dissimilar metal, thereby generating current and promoting additional plating.
Applications
Tin immersion coatings are popular for decorative finishing of small parts such as safety pins, thimbles, and buckles. They are also applied to copper tubing to prevent discoloration from water, and to aluminum engine pistons to provide lubrication during break-in periods.
Copper Properties
The most important characteristics of copper immersion coatings are their high electrical conductivity, good lubrication properties, and unique appearance. In addition to steel, they can be applied to brass and aluminum and to printed circuit boards. Usual thickness range is 0.1 to 1 mil.
Applications
Because of their conductivity, copper immersion coatings have proved particularly useful for printed circuits. They are not especially noted for their decorative appeal, but can be used in applications where a particular appearance is required, e.g., inexpensive, decorative hardware such as casket parts. Because of their good lubrication properties they can also be used on steel wire in die-forming operations.
Gold properties
Gold immersion coatings are relatively inexpensive because of their extreme thinness — about 0.001 mil. The coatings have good electrical conductivity and emissivity characteristics, and a bright, attractive appearance. As deposited, they are not especially resistant to discoloration and abrasion; however, they can be protected with a clear lacquer finish. They are used on a wide variety of ferrous and nonferrous metals, and on copper printed circuit boards.
Applications
Because of their good appearance gold immersion coatings are principally used on costume jewelry, trophies, auto trim, and inexpensive novelties. Their conductivity and solderability are used to advantage in electrical applications such as printed circuits, transistors, and connectors. Also, the unique emissivity properties of the coatings have proved useful in missile applications.
Silver properties
Like gold, silver immersion coatings are relatively inexpensive because of their extreme thinness. The coatings have a bright, attractive appearance when first deposited. Their resistance to tarnishing and abuse is poor; however, they can be protected somewhat with a clear lacquer coating.
Silver immersion coatings can be applied to most base metals except lead, zinc, aluminum, and very active metals. They perform best on copper, nickel, and steel. Usual thickness is about 0.001 mil, but 0.03 mil can be deposited in some cases.
Applications
Because of their poor durability, silver immersion coatings are not too popular; the only applications are cheap decorative products, minor electronic parts, and maintenance plating.
