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Manufacturers use passivating services to remove unwanted contaminants from the surface of certain alloys (mainly steels), and in order to enhance the surface’s tendency to oxidize. The chemical passivation process does not involve electricity, but instead uses various chemical baths, such as nitric and citric acid solutions. Passivating also enhances the surface’s corrosion resistance and finish.
In the chemical passivation process, a work piece is treated by a mild oxidant, such as the solutions listed above. The oxidant eliminates all free iron from the surface, as well as a variety of other impurities. It also eliminates the surface’s chemical reactivity, and (on stainless steel) accelerates the generation of an oxide film. The process is particularly useful when certain contaminants on a work piece or component would negatively impact the system in which that component will be used. The acid solution used in passivation will dissolve a variety of contaminants produced by machining processes, such metal chips produced by cutting or bits of abrasive left by lapping, which often become embedded in work pieces.
Passivating chemicals, such as phosphoric acid solutions, are also used to remove rust from certain metals. Rust removal can be performed by either cold or hot dipping treatments, and can be used on a variety of materials. Although electropolishing is not a strictly chemical finishing method, it is considered a type of passivation treatment, since it renders the treated material passive.
Other types of passivation processes include bluing, a method of coating iron and steel; Iridite coating, which is used on aluminum; and Parkerizing. Many metal finishing companies provide chemical passivating services also provide electropolishing services and other finishing techniques, including anozing and electroplating. The proper selection of the right process is dependent on the nature of the application, the materials involved, and the desired properties of the treated piece.