Metal Finishing Guide Book


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plating processes, procedures & solutions FUNCTIONAL CHROMIUM PLATING BY GENE BARLOWE ATOTECH USA INC., ROCK HILL, S.C.; INTRODUCTION Hard chrome plating, also known as functional chrome or engineering chrome is different than decorative chrome. Decorative chrome is used as a top coat typically over a nickel or copper-nickel layer and is used to lower corrosion, give tarnish protection and improve abrasion resistance of surfaces of parts such as automotive trim, faucets and appliances. With functional chrome the essential difference is that the focus is on the properties of the deposit as they relate to wear resistance, low coefficient of friction and its resistance to corrosion. In these applications, the deposit is typically applied in thicknesses greater than 2.5 microns (0.0001"). As a functional coating it is used in a variety of industries such as hydraulic devices; automotive shocks, and valves; steel and textile industries for rolls; mold dies for plastic and steel; medical instruments; aerospace application as well as a host of other industries that enable parts to mitigate billions of dollars of loss due to wear and corrosion. Functional chrome plating has been part of our industrial life for over 85 years and while under a great deal of pressure to find alternatives it remains an essential process for industry around the world. Over the past several years the process has come under critical review due to the toxic nature of the hexavalent electrolyte as well as some misunderstanding of some regulations. The European directives such as ELV for automobiles and RoHS, list "hexavalent" chrome as a hazardous substance. This reference is to hexavalent chromium ions within a coating: chromate on zinc or aluminum, for example, and not as it's used to produce chrome deposits. This leaves the main issue with hex chrome with the hexavalent chrome chemistry used to plate it. Both the Clean Air Act and the Clean Water Act limit the amount of chromium ions that can be discharged into the environment either through discharge of water or fumes discharged into the outside air. The Occupational Health and Safety Act (OHSA) also have restrictions on chrome to prevent the hazardous products from affecting workers that work in such environments. OSHA has done studies to determine the toxic levels and has set a Permissible Exposure Limit or PEL to ensure worker safety. The industry as a whole has embraced the regulations and as a group met their obligations. Through the efforts of the industry and its willingness to work together, the reported discharge of hex chromium ions to the water and to the atmosphere has been dramatically reduced. CHEMISTRY Hexavalent chromium or hex chrome is used as the electrolyte to plate functional chrome deposits. The typical bath runs between 150 to 450 g/l of chromic acid and uses one or more catalyst to create the reaction that deposits chrome. Three types of chemistries are commercially available: an ordinary chrome bath which contains chromic acid with a sulfate as a catalyst; a mixed catalyst bath which uses sulfate as well but also incorporates a fluoride compound as 261

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