Plating
Generally, plating refers to the deposition of one or more metals onto a base metal, or other material activated to behave as a metal surface. Our capabilities are limited to metallic substrates. The purpose of plating is either to prevent corrosion, make the part esthetically pleasing or to enhance the physical characteristics of the surface (such as increasing hardness). Electroplated metals applied by Kaehr are copper, nickel, tin, and zinc. We can also apply electroless nickel. Since no external power is applied to the work, problems such as high current density buildup are avoided and all surfaces receive the same thickness of plating.
Nickel is one of the earliest metals plated, dating back at least 100 years. One of the first applications was a decorative coating over polished brass, primarily to eliminate the need for frequent polishing. Today, it is used sparingly as a decorative coating, unless it is the base coat for chromium or brass. It is used for corrosion protection in environments (such as highly alkaline or acid ones) where zinc would be unacceptable. As with most modern plating solutions, additives are used to refine the grain structure. This can result as bright deposits with the ability to level, or fill in minor surface imperfections. The nickel deposit may easily be soldered so it finds much use in the electronics industry. It is common practice to apply a thin deposit of nickel over brass to prevent migration of the zinc into tin plate, or over copper as a base for gold plate.
Electroless nickel plating takes place without an external power source, such as is required for electro-plated nickel. As a result, the high current density effects of electroplating are avoided and all surfaces in contact with the plating bath receive the same thickness of plating. Most deposits of electroless nickel are alloys of nickel and phosphorous (typically 4-12%). This results in excellent corrosion inhibiting properties for steel and aluminum parts. This can result in substantial savings over manufacturing in stainless steel. Combined with an attractive decorative appearance, electroless nickel is a good choice for plating such household items as bottle and can openers. Occasionally, electroless nickel is used for its hardness which as plated can range from 50-57 Rockwell C. With a post plate heat treat is can exceed Rockwell 60. Some companies will plate heavy deposits of low porosity nickel, then polish to obtain extremely flat, defect free mirror surfaces.
Copper plating is used for corrosion protection, as a base for other plating, for masking purposes and for decorative finishes. As to corrosion protection, zinc plating has largely supplanted copper plating because of the galvanic protection afforded to ferrous materials. Occasionally however, copper is preferred for its electrical properties, as in the case of grounding spikes. As a base for other plating, such as nickel, it adds to the corrosion protection and if it has leveling properties, helps to smooth the surface for subsequent plating. For years, copper was plated to insure good adhesion of the next plating operation, and is still used as a barrier plate, such as is the case when plating over zinc die-castings. It is still used for a masking layer for the carburizing of certain steels. After the heat treat operation is completed, the copper is stripped for the part. Because of its pink color, copper is not used as much as brass for decorative finishes. It has many attractive finish variations. It may be oxidized and highlighted for an antique look, or polished for a very reflective finish. In almost all cases, decorative copper finishes are coated with an organic, such as laquer, to keep the surface from oxidizing.
Because of its low toxicity, tin has usually been associated with the food processing industry. Although it still is, most tin today is used in the electronics industry by itself, or alloyed with other elements such as lead. The primary advantage of tin and tin alloys are their ease of solderability. The deposits, when properly applied, will "flow" when heated to the melting point, hence very good solder connections can be obtained. Tin is also used on copper buss connections. Tin is deposited from several types of solutions. One of the most common is bright acid tin which gives it the appearance of a hot dipped deposit. Tin is seldom used for corrosion protection, not only due to the relatively high metal cost, but also because of galvanic corrosion concerns.
Zinc electroplate, or electro-galvanize, is most likely applied to steel surfaces to provide an inexpensive protective coating. The metal is sacrificial thereby corrosion is preferential for the zinc coating and spares the steel surface somewhat. To extend the life of the zinc coating, post treatments, most commonly chromates, are used. These react with and become part of the zinc surface. They are very stable and help the zinc withstand a wide variety of corrosive atmospheric conditions. Clear chromate gives the zinc a white or blue-chrome like appearance. Of the chromates it has the least protective value because of the lower hexavalent chromium content. Yellow chromate has a much better protective value due to a higher hexavalent chromium level. It usually has a "rainbow" appearance which varies depending on the depth of the chromate coating. Variations of the yellow chromate produce olive or black coating. The prior is used on military hardware or as a paint base, while the black is used commonly in the automotive industry. Bright decorative zinc deposits coated with a clear organic material are often used on such items as refrigerator trays because they provide looks and protective values almost equal to chrome plating at much less cost.