Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 352 of 707

plating procedures MECHANICAL PLATING AND GALVANIZING BY ARNOLD SATOW ATOTECH USA INC., ROCK HILL, S.C.; The manufacturers of metal products recognize the need to keep fasteners from corroding. Mechanical plating is a method for coating ferrous metals, copper alloys, lead, stainless steel, and certain types of castings. The process applies a mal- leable, metallic, corrosion-resistant coating of zinc, cadmium, tin, lead, copper, silver, and combinations of metals such as zinc-aluminum, zinc-tin, zinc-nickel, tin-cadmium, and others. These combination coatings are often referred to as codeposits, layered deposits, or alloy mechanical plating. The mechanical plating process has been used internationally for over 50 years and is referred to by a vari- ety of names including peen plating, impact plating, and mechanical galvanizing. Mechanical plating and galvanizing can often solve engineering, economic, and pollution-related plating issues. It offers a straightforward alternative method for achieving desired mechanical and galvanic properties with an extremely low risk of hydrogen embrittlement. In some cases, it offers a potential cost advan- tage over other types of metal-finishing processes. Mechanical coatings can be characterized to some extent by the relative thick- ness of deposit.1 "Commercial" or standard plating is usually considered to be in a thickness range between 5 and 12.5 µm; however, coatings up to 25 µm are often utilized. The heavier deposits are often referred to as mechanical galvanizing and sometimes utilize the coating weight designation (g/m2 vanizing industry. Typical coating thicknesses range from 25 to 65 µm (179 to 458 g/m2 ) but can go as high as 110 µm (775 g/m2 ). The mechanical plating process is accomplished at room temperature, with- out an electrical charge passing through the plating solution that is necessary with electroplating. The metallic coating is produced by tumbling the parts in a mix- ture of water, glass beads, metallic dust or powder, and proprietary plating chemistry. The glass beads provide impacting energy, which serves to hammer or "cold-weld" the metallic particles against the surface of the parts. They per- form a number of functions including assisting cleaning through a mildly abra- sive scrubbing action; facilitating mixing and dispersion of the chemicals and met- al powders; impacting and consolidating the metallic coating; protecting and separating parts from one another; preventing edge damage and tangling; and helping impact the plating metal into corners, recesses, and blind areas. The glass beads or "impact media" are chemically inert and nontoxic, with high wear resistance. They are constantly recycled through the system and reused to ensure their cost effectiveness. The glass impact beads are considered the "driving force" in the mechanical plating and galvanizing process. The diameters of the most commonly used glass beads are 5 mm (0.187 in.), 1.5 mm (0.056 in.), 0.7 mm (0.028 in.), and 0.25 mm (0.010 in.). The ratio of glass bead mixture to parts in a particular load is about 1.5:1 by weight. The plating result is a tight, adherent metallic deposit formed by the building of fine, powdered metal particles to the surfaces of parts. Special advantages of the mechanical plating process are that it greatly reduces the part susceptibility to hydrogen embrittlement; consumes comparatively low 351 ) found in the hot-dip gal-

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