Chromating

Chromating, or chromate conversion coating, is a chemical surface treatment used to form a thin, protective chromium-containing film on metal substrates. The coating improves corrosion resistance, reduces metal ion migration, and enhances the adhesion and durability of paints, sealants, and other finishes. It is commonly applied to aluminum, magnesium, zinc, and certain steels.

The coating is produced by immersing the metal in acidic solutions that contain chromium salts, historically hexavalent chromium compounds. After appropriate cleaning or degreasing, the substrate is exposed to the chromating bath, which converts the surface to a passivating layer that incorporates chromium species. The resulting film is very thin and adherent, forming a barrier that slows corrosion and provides a receptive layer for subsequent coatings. Aluminum chromating is often referred to as alodining, while zinc-coated steel can receive a yellow to olive chromate finish such as zinc chromate or related coatings.

Environmental and health considerations have driven changes in chromating chemistry. Hexavalent chromium is toxic and carcinogenic, leading to regulatory restrictions in many regions. As a result, there is ongoing use of trivalent chromium-based chromates and non-chromate conversion coatings, which aim to provide similar protective benefits with lower risks. Nevertheless, Cr(VI)-based chromating remains widespread in many aerospace, automotive, and electronics applications where high-performance coatings are required.

Applications and limitations: chromating is valued for improving corrosion resistance, paint adhesion, and electrical insulation on treated parts. It is not a substitute for proper material selection and design against corrosion, and coating performance depends on substrate, bath formulation, process control, and subsequent finishes.