Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 179 of 707

catalysts in order to plate. The formulation of the traditional process (called the conventional, Sargeant or single catalyst process) contains hexavalent chromium and sulfate as the only catalyst. When fluoride is added as an additional catalyst to the conventional hexavalent chromium plating bath formulation to enhance particular plating operations or the deposit's properties it is typically called a mixed-catalyst or dual catalyst bath. Proprietary organic additives can also be used to produce dual or triple catalyst baths to further enhance the plating operation and deposits. Mixed-catalyst baths are used when their special properties are required, but they are more expensive and slightly more difficult to operate than conven- tional processes. They typically plate at faster speeds, have better coverage, have wider bright ranges, and are more tolerant to impurities. They are also less sen- sitive to current interruptions and can be plated over more passive surfaces. The basic formulations of hexavalent chromium processes are very similar. They consist of chromic trioxide (CrO3), which when combined with water forms chromic acid (H2CrO4), and the sulfate ion (SO4 +2) which is added in the form of sulfuric acid or a sulfate salt. Even though chemically incorrect, solid chromic oxide is commonly referred to as chromic acid, its hydrated form. A number of sometimes conflicting theories have been proposed to explain the very complex mechanism of hexavalent chromium electroplating. In a simplified form, they all contain the following multiple reactions. CrO3+H2OjH2CrO4 CrO4j--2 +2H+ 2H2CrO4jH2Cr2O7 +H2OjCr2O7j-2 +2H++H2O Deposition reaction: Cr2O7 -2+14H++12(e)+CAT 2Cr0 +7H2O (1) (2) (3) CAT is the required catalyst, e.g., sulfate in addition to fluoride and/or an organic catalyst, while (e) refers to an electron. The plating efficiency of reaction (3) is approximately 20% or less depending upon the catalyst, bath chemistry and the current density. Side reactions: 2H+ + 2(e)jH2(gas) + a mist(H2SO4 +Cr+6+H2O) (4) This side reaction consumes most of the available power not used by reaction (3). The mist, a major HSE concern, can be trapped within the plating tank by the use of mist control agents or surface tension reducers. Trivalent chromium ions are created and oxidized by: Cr2O7 -2 +14H+ +6(e) 2Cr+3+7H2O 2Cr+3+3O2 -6(e) 2CrO3 anode. 178 (5) (6) Reaction (6) is catalyzed by a PbO2 film that naturally forms on the lead

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