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What Is a Conversion Coating?

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  • Written By: Paul Scott
  • Edited By: R. Halprin
  • Last Modified Date: 26 November 2016
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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 which physically convert components of the metal surface into the desired finish. The most common of these conversion processes are oxide, phosphate, and chromate. Oxide conversions are used on various metals as decorative and corrosion resistant finishes and include gun bluing, black oxide and anodized treatments. Phosphate and chromate conversions offer more specialized finishes, improve wear resistance, and serve as conductive elements, primers for paints and adhesives, or reservoir layers for lubricants.

Metal parts are frequently treated with surface enhancing or altering processes to improve the appearance or working characteristics of the metal. The conversion coating family of treatments is a widely utilized example of this type of process and may be applied to a range of metals including steel, copper, aluminum, and brass. Achieved by chemical or electrochemical processing, conversion coatings alter and enhance existing surface characteristics.

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The oxide conversion coating, one of the 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 which react with particular components in the metal to form an enhanced oxide layer. These coatings lend 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 bluing, black oxide, immersion bath bluing, and anodizing. Oxide conversion coating treatments are commonly found on mass produced parts such as fasteners, bearings, firearms, railway track sections, and tools.

The second member of the conversion coating family is the phosphate treatment. Also known by trade names such as Lubrite and Parkerizing, this matte grey conversion coating involves a phosphate power spraying or immersion process during which a crystalline layer of iron, zinc, or manganese is “grown” on the metal surface. Iron phosphating is applied as a corrosion resistant and primer layer for painting. Zinc layers are utilized as a paint primer on automobile or truck bodies and domestic appliances. Zinc coatings also applied to cold form drawing dies to extend their life. Manganese phosphate layers are applied to high stress engine parts such as pistons, rings, cam shafts, and gears where they act as a friction reducer and a reservoir for lubricating oil.

The third conversion coating type, chromate treatment, imparts a clear or yellow coating on metal parts. Chromate 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 Irridite or Alodine, these finishes offer good corrosion resistance and conduct electricity well. They may also be applied as primers for paints and dyes. Chromate coatings are typically applied as protective layers on aluminum alloys which can't be anodized, finishes for electronic power supplies, and as a touch-up treatment for damaged painted or anodized parts.

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