Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 178 of 707

electroplating solutions DECORATIVE CHROMIUM PLATING BY DONALD L. SNYDER Atotech USA, Inc., Rock Hill, SC; Electrodeposition of decorative chromium is the principal means of imparting the physical and chemical properties of chromium to the surface of less expen- sive 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 lubricity and resistance to tarnish, corro- sion, wear and scratches. Decorative chromium is almost exclusively plated over a nickel electrode- posit, which can be plated over substrates such as plastics, steel, aluminum, copper alloys, and zinc die castings. Nickel is preferred because it protects the sub- strate from corrosion, helps to give chromium a pleasing bluish-white decorative appearance, 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 nick- el and copper can precede the chromium deposit depending upon the intended use of the part. The appearance of the chromium deposit can be significantly altered by using bright, dull, satin or mechanically modified nickel deposits preceding the chromium deposit. Decorative chromium deposits typically are plat- ed in the 2-20-millionths-of-an-inch range. Thicker deposits are usually referred to as functional chrome and tend to be duller and contain visible cracks. The traditional chromium deposit is produced from an electroplating elec- trolyte containing hexavalent chromium ions. About 1975, a chromium elec- trolyte containing the less toxic and less hazardous trivalent chromium ion was introduced to replace decorative hexavalent chromium electroplating. Depending upon the process, trivalent chromium electrolytes can either produce a metal- lic white deposit almost identical in appearance to the bluish white hexavalent chromium deposits, a deep-looking pewter or stainless steel appearing deposit or almost black decorative deposits. 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. For example, the cost for hexavalent chromium solutions is typically lower than that for trivalent. However, the fluoride used in many hexavalent chromium process- es will etch substrates, such as copper and steel, resulting in metallic contami- nation of the plating bath which could increase operating problems and rejects. Trivalent chromium processes typically do not contain fluoride but will dissolve copper and iron. However, metallic contamination is easier to remove from a trivalent chromium electrolyte. This makes it possible to produce nickel, free nickel/chrome looking decorative parts by plating trivalent chromium direct- ly over white copper alloys. Doing this with hexavalent chromium solutions would be problematic. HEXAVALENT CHROMIUM PROCESSES Hexavalent chromium electrolytes require a source of chromium and one or more 177

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