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Chromate Conversion Coating

Chromate Conversion Coating

 

Metal part manufacturers can perform a variety of finishing operations in order to complete products. A surface treatment changes the exterior of metal in order to obtain desired properties, such as better electrical conductivity, an improved aesthetic appearance or enhanced corrosion resistance. Applying a chemical conversion coating assists manufacturers in achieving desired surface changes in some situations.

A conversion coating deposits material on the surface which then undergoes electrical or chemical reactions with the metal composing the workpiece. This transformation results in the formation of outer surface material providing some desired properties, such as greater hardness, or a distinctive color. Today, many conversion coatings used during finishing rely upon chemical reactions to produce surface changes.

 

The Chromate Conversion Coating Process

Sometimes referred to by specific proprietary product names, such as "Iridite", "Chem Film", "Chemical Film" or "Alodine", chromate conversion coatings provides a surface treatment alternative to electroplating for finishing metal parts composed of magnesium, aluminum or aluminum alloys in order to provide better corrosion resistance. An acid or acids in the conversion coating typically dissolve some of the metal on the surface of the part, reacting to form new chromium compounds. Chromate applied to the surface of the part undergoes a chemical reaction and produces a microscopic yellow-greenish tinted surface layer of corrosion-resistant material. The specific formulations utilized by different coating manufacturers vary in composition.

In some cases, metal part manufacturers also sometimes apply chromate conversion coatings over phosphate conversion coatings to augment the properties of the phosphate coatings, or over steel previously electroplated with zinc. Proprietary chromate conversion coatings won't offer corrosion protection when directly applied to steel or ferrous metal surfaces, however. Since the chemical formulations of chromate conversion coatings vary, the methods for applying these surface treatments may also depend upon the specific brand's application directions. Some manufacturers apply chromate coatings as gels which gradually harden on the surface over time, while others recommend the application of the coating via immersion baths.

 

Yellow Chromate

Metal parts manufacturers may use proprietary formulations of yellow chromate as a primer to protect aluminum surfaces exposed to harsh environments. Yellow chromate coating gives aluminum exteriors a light yellow to brown hue, and applies in liquid form via a submersion bath or directly via a spray. Manufacturers also sometimes use powdered yellow chromate conversion coatings. Some yellow chromate products contain carcinogenic substances, so anyone applying these chemicals to aluminum surfaces must adhere closely to product safety instructions during industrial application and cleanup processes.

 

Materials And Applications

The materials required to perform chromate conversion coating in a metal parts fabrication environment vary based upon the requirements of specific proprietary coatings. Frequently, due to the specialized nature of proprietary coatings, manufacturers may outsource part electroplating and chromate conversion coating to companies specializing in the electroplating field. Products designers have discovered multiple applications for the use of metal parts treated with chromate conversion coatings:

Materials

Today, aluminum likely provides the chief raw material for parts protected by chromate conversion coating. Companies can use some proprietary formulations, including Alodine, to conversion coat alloys of magnesium. Manufacturers also sometimes apply chromate conversion coatings over certain electroplated surfaces, such as zinc or cadmium. The use of a specific chromate conversion coating formulation may depend upon the composition of the metal part receiving the surface treatment, and any applicable electroplating materials requirements.

Applications

Harsh marine environments sometimes require the use of aluminum components coated with chromate conversion coatings. Additionally, manufacturers often use this type of surface treatment to provide attractive colored surfaces for lightweight aluminum consumer and decorative products. Military, aerospace, aviation and automotive applications also exist for aluminum or magnesium parts finished with the assistance of chromate conversion coatings.

 

Advantages Offered by Chromate Conversion Coatings

The chromate conversion coating process does impose environmental and safety responsibilities in some manufacturing settings. However, as a finishing treatment, this process also provides several advantages:

  1. Many proprietary chromate conversion coatings significantly change the color of aluminum parts. Manufacturers may achieve yellow, golden, greenish, olive or even brown surface hues for aluminum-colored metal components by utilizing this finishing process, for instance. Chromate conversion coatings applied over electroplated zinc or cadmium produce vivid colors. A variety of proprietary formulations offer specific color changing capabilities.
  2. Chromate conversion coatings prepare some metal surfaces to accept coatings of paint. They function as excellent primers without producing unattractive porous surface textures. A metal parts manufacturer may decide to utilize a chromate conversion coating as part of a finishing process in order to facilitate subsequent painting, for instance.
  3. Chromate conversion coatings frequently enhance the ability of aluminum or magnesium surfaces to resist environmental corrosion. If a manufacturer anticipates a metal component will function under harsh conditions, subjecting the part to this finishing treatment may help extend the commercial product's anticipated lifespan. For instance, the aluminum casing of a wrist watch worn during scuba diving might withstand the effects of salt water better if the manufacturer employs chromate conversion coating during the fabrication process.
  4. Some manufacturers rely on chromate conversion coatings to help give extra sheen and a finely polished, bright appearance to certain cadmium, copper or zinc electroplated surfaces. This factor may enhance the market appeal of some lightweight decorative items and consumer goods in retail sales settings, for instance. Manufacturers of metal components may choose to utilize this finishing surface treatment because of its aesthetic contributions to the product. Using chromate conversion coating may offer a cost-effective manufacturing option.

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