DECORATIVE CHROMIUM PLATING

摘要

Electrodeposition of chromium is the principal means of imparting the physical and chemical properties of chromium to the surface of less expensive and easier-to-form materials such as steel and plastics. The most desirable properties of chromium as a metal coating are its inherent protective and decorative characteristics. The deposit's high reflectivity is retained in service because of chromium's excellent tarnish, corrosion, wear, and scratch resistance. Decorative chromium is almost exclusively plated over a nickel electrodeposit, which can easily be plated over substrates such as plastics, steel, aluminum, copper alloys, and zinc die castings. Nickel is preferred because it protects the substrate from corrosion, helps give chromium a white color, and is protected from surface oxidation by the chromium. Stainless steel is the only substrate that is frequently plated directly with chromium, but a nickel preplate before chromium is also used. Multiple or single layers of nickel and copper can precede the nickel/chromium deposits depending upon the intended use of the part. Decorative chromium deposits typically are plated in the 2-20-millionths-of-an-inch range. Thicker deposits tend to be duller and contain visible cracks. The traditional chromium deposit is produced from an electroplating electrolyte containing hexavalent chromium ions and has a pleasing bluish white decorative appearance. About 1975, a chromium electrolyte containing the less toxic and less hazardous trivalent chromium ion began replacing many decorative hexavalent chromium electroplating installations. Depending upon the process, trivalent chromium electrolytes can either produce a metallic white deposit almost identical in appearance to the bluish white hexavalent chromium deposits, or a deep-looking pewter or stainless steel appearing deposit. In almost all chromium plating applications, both hexavalent and trivalent chromium deposits are interchangeable with each other; however, one or the other process might offer unique advantages.