This thickness is sufficient to produce macrocracking. However, some earlier trivalent processes did produce thick deposits, over 1.3 ^m (50 ^in.). Process control, while plating at high current densities, is not a serious concern for trivalent processes, because they have less tendency to produce burnt deposits, compared with hexavalent processes. ![]() Hexavalent chromium processes fall short, particularly around holes and slots and in low current density areas. In general, wherever nickel can be plated, trivalent chromium can be plated. High current density spiking at the onset of plating increases the already excellent covering and throwing powers of trivalent chromium processes, when compared with those of hexavalent processes. ![]() The typical operating conditions for trivalent chromium processes are summarized in Table 9. Surfactants are added to reduce the surface tension of the solution for mist suppression, as well as to act as additives in the plating operation. Lower amperes but higher volts are required for trivalent chromium processes, compared with hexavalent chromium process requirements. In comparison to hexavalent chromium solutions, which have good conductivity, the conductivity of the relatively high-pH and low-metal-content trivalent plating solution is increased by the addition of conductivity salts/buffers. However, the stability process is not strong enough to interfere with the normal precipitation sequence used with chromium during waste treatment. These agents permit the trivalent chromium ion to be stable in solution until it is plated out at the cathode. It is introduced as a water-soluble salt and forms a stable specie upon combining with the stabilizing agents/catalysts. Depending on the process used and its operating conditions, the trivalent chromium ion content typically ranges from 5 to 20 g/L (0.67 to 2.67 oz/gal). The membrane keeps the trivalent chromium from contacting the anode, thereby preventing the formation of hexavalent chromium. Conventional lead anodes are used in a 10% sulfuric-acid electrolyte. ![]() This second technique, commonly referred to as the double-cell, or shielded anode, method, uses an ion-selective membrane to create a barrier around the anode. This technique is referred to as the single-cell process, in contrast to the second technique, which isolates the insoluble graphite anodes from the trivalent-chromium-containing plating solution to restrict the formation of hexavalent chromium. If it does manage to get into the plating solution, then it is reduced to the trivalent state, which eliminates it as a contaminant. Under normal operating conditions, hexavalent chromium cannot form. The oldest and most frequently used technique incorporates several lines of defense against hexavalent chromium ions. Two well-known proprietary approaches were developed to address the problem of hexavalent chromium formation during plating. They initially cause a poor deposit appearance and eventually result in the cessation of plating. Hexavalent chromium ions are a contaminant in trivalent chromium processes. One of the main difficulties with the development of trivalent chromium baths was the formation of hexavalent chromium at the insoluble anodes during plating. Increased throwing and covering powers, lack of burning, and tolerance to current interruptions and ripple also reduce rejects and can increase the allowable number of parts on a rack. This increases productivity in some shops. Some of the trivalent chromium processes plate up to three times faster than hexavalent chromium processes. Finally, the chromium in the rinse water is already in the trivalent state, which eliminates the expensive and sludge-volume-building reduction step required with hexavalent chromium ions. In addition, the solution drains faster, so that less solution is removed with the parts. ![]() Waste-treatment costs are reduced by a factor of 10, because less than one-tenth of the chromium contained in hexavalent processes is used in the trivalent process (8 to 23 g/L, or 1 to 3 oz/gal, versus 115 to 300 g/L, or 15 to 40 oz/gal). Trivalent chromium processes have reduced misting to the extent that scrubbers, such as those used with hexavalent chromium processes, are presently not required to meet federal and state air-quality discharge standards. The trivalent chromium ion is estimated to be about 100 times less toxic than hexavalent chromium ions. Hexavalent chromium ions are also considered carcinogenic and can cause skin ulcerations. Environmental, safety, and productivity advantages have been the driving forces for the commercialization of trivalent chromium processes. The first commercially successful decorative trivalent chromium process began in England in 1975 and in the United States in 1976. The use of the trivalent chromium ion, instead of the hexavalent ion, in solution to deposit chromium has been of interest for many years.
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