Conversion coatings are chemical or electrochemical processes that physically convert metal surface components into the desired finish, offering corrosion and wear resistance or a receptive layer for paints, dyes, lubricants, or adhesives. The most common conversion processes are oxide, phosphate, and chromate. Oxide conversions are used on various metals as a decorative and corrosion-resistant finish. Phosphate and chromate conversions offer more specialized finishes, improve wear resistance, and act as conductive elements, paint primers, and adhesives or lubricant reservoir layers.
A conversion coating is a metal surface treatment that offers decoration, corrosion and wear resistance or a receptive layer for paints, dyes, lubricants or adhesives. These coatings are achieved by chemical or electrochemical processes that physically convert metal surface components into the desired finish. The most common of these conversion processes are oxide, phosphate and chromate. Oxide conversions are used on various metals as a decorative and corrosion resistant finish and include bluing, black oxide and anodizing treatments. Phosphate and chromate conversions offer more specialized finishes, improve wear resistance, and act as conductive elements, paint primers and adhesives, or lubricant reservoir layers.
Metal parts are often treated with surface enhancement or surface alteration processes to improve the appearance or machining characteristics of the metal. The conversion plating family of treatments is a widely used example of this type of process and can be applied to a range of metals including steel, copper, aluminum and brass. Achieved by chemical or electrochemical processes, conversion coatings alter and improve existing surface characteristics.
Oxide conversion plating, one of three common variants, is typically applied to iron alloys such as steel although it is sometimes used on copper and brass. Oxide coatings are achieved by exposing the metal surface to chemicals that react with particular components of the metal to form an enhanced oxide layer. These coatings give the treated metal corrosion resistance, decorative qualities, dimensional stability and in some cases a receptive layer for paints. Examples of oxide conversion coatings include gun burnishing, black oxide, dip burnishing, and anodizing. Oxide conversion coating treatments are commonly found on mass-produced parts such as fasteners, bearings, firearms, railroad track sections, and tools.
The second member of the conversion coating family is the phosphating treatment. Also known by trade names such as Lubrite and Parkerizing, this dull gray conversion coating involves a phosphate dipping or spraying process in which a crystalline layer of iron, zinc or manganese is “grown” on the metal surface. Iron phosphating is applied as a corrosion resistant layer and primer for painting. Zinc layers are used as a primer for paint on car or truck bodies and appliances. Zinc coatings also applied to cold drawing dies to extend their life. Manganese phosphate layers are applied to high-stress engine parts such as pistons, rings, camshafts and gears where they act as a friction reducer and a reservoir for lubricating oil.
The third type of conversion coating, the chrome treatment, imparts a clear or yellow coating on metal parts. Color conversion coatings can be used in conjunction with other processes such as anodizing, where they are applied as a masked layer to form conductive paths. Also known as Iridite or Alodine, these finishes offer good corrosion resistance and conduct electricity well. They can also be applied as a primer for paints and dyes. Chrome coatings are typically applied as a protective layer on aluminum alloys that cannot be anodized, finishes for electronic ballasts, and as a touch-up treatment for damaged painted or anodized parts.
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