![]() Deficiency in sulfate is restored by sulfuric acid. Deficiency in chromic acid concentration is restored by the use of a proprietary salt mixture which restores the balance of the solution catalysts at the same time. Chromium-plating solutions are regularly analyzed for chromic acid, sulfates, catalyst and contamination. These PFOS-free alternatives are compliant with EPA regulations and can consistently and reliably control spray and mist in chrome electrolytes. Fortunately, commercially viable replacements are available for use in chromic acid solutions. Recent legislation, which was finalized in September 2012, mandates eliminating PFOS-based fume suppressants by September 21, 2015. PFOS elimination has been a hot topic for the EPA as well as state and local agencies and publicly owned treatment works (POTWs). However, these materials are themselves becoming subject to prohibition due to their environmental persistence and bio-accumulative effects in mammals. Proprietary chemical mist suppressants are usually based on perfluoro-octane sulfonates (PFOS). Occupational Safety and Health Administration (OSHA) standard for the permissible exposure limit (PEL) for chromic acid mist is 5 μg/cu m with an action level at 2.5 μg/cu m. During electrolysis of the chromic acid, a mist is produced which must be controlled by the use of a suitable chemical mist suppressant, usually in conjunction with local exhaust ventilation. This film is necessary to ensure that the trivalent chromium produced by electrochemical reduction of the chromium acid at the cathode is re-oxidized to ensure correct solution balance. When current is passing through the chrome solution gassing will occur at the anodes, which are normally covered with a dark-chocolate-colored film. The anode area should be about 20 percent greater than the area of the normal plating load. In a chromium plating tank, the lead anodes serve two purposes: as the positive electrode and to maintain the solution in satisfactory balance by re-oxidizing the trivalent chromium to chromic acid. Tin-lead alloy anodes are generally employed for chromium plating. The solution is heated by means of either silica-cased or Teflon electric immersion heaters or steam coils. Chromium plating solutions are usually contained in PVC-lined tanks. TABLE II - Plating Time (Min) for a 1-µM Average Chromium Deposit Current Density, asfĮquipment and anodes. In practice there will be considerable variation in current density over the surface and, therefore, the average thickness of chromium applied will be in excess of any minimum thickness requirements. ![]() These plating times are for average thickness values under uniform current density conditions. Table II shows the time needed to produce a deposit averaging 1 μm in thickness at various cathode efficiencies and current densities. These changes will also affect cathode efficiency. ![]() This mixed catalyst has the advantages of improving cathode efficiency, covering power and ability to plate on bright nickel layers when compared to the straight sulfate catalyst type.Īny change in current density will generally involve a corresponding adjustment in temperature to keep the solution within the optimum plating range. The most common electrolytes employed today are based on the dual sulfate/fluoride catalyst. Simple preparation Tolerant to contamination TABLE I - Decorative Hex Chrome Electrolyte Systems Type The primary differences among the systems are outlined in Table I. Decorative Hexavalent Electrolytesįour types of hexavalent chromium electrolytes are commonly employed: Typically, articles plated to service condition 1/2 would use regular chromium plate, whereas plating to service condition 3 or above would use a discontinuous layer (ASTM B456). Inducing micro-discontinuities (microcracks) across the surface spreads the corrosion current and slows the corrosion rate. Deep pits form at these points and rapidly penetrate through the nickel deposit and the component substrate. Because these cracks or pores are relatively widely spaced, the current developed by the chromium/nickel couple is concentrated at a few points. Corrosion is an electrochemical mechanism. Failure of bright chromium plate is often due to deep pits developing as a result of corrosion starting at random cracks or pores in the chromium surface. The selection of a continuous or discontinuous layer depends on the level of corrosion resistance required. Discontinuous deposits are formed by depositing the chromium on microporous (or microcracked) nickel plate. Henkel Adhesive Technologies Presents Solvent- and Chromium-free Pretreatmentīright chromium can be applied as continuous (regular) or discontinuous layers.
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