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.
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.
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.
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:
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.
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.
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:
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