Metal Finishing Guide Book


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Page 302 of 903

Dilute conventional formulations consist of approximately 250 g/L (33 oz/gal) chromic trioxide (���chromic acid���), and 2.5 g/L (0.33 oz/gal) sulfate. Concentrated conventional baths contain approximately 400 g/L (53 oz/gal) chromic trioxide and 4.0 g/L (0.53 oz/gal) sulfate. In practice, however, concentrations in between these are widely used. The dilute formulation offers good coverage, moderate nickel substrate activation, and consistent current efficiency. It also has a lower solution cost, plates faster, and produces less waste to treat. The concentrated formulation gives better coverage and greater resistance to impurities and requires lower operating voltages; however, it is more expensive to make up because of its higher chromium concentration and more chromium is dragged out resulting in higher waste treatment coat. A critical point in all bath formulations is the requirement for close control of the CrO3/SO4 weight ratio needed to produce consistent plating results. Ratios of between 80:1 and 130:1 are typical, with a ratio of around 100:1 being common for conventional processes. A low ratio results in relatively poor throwing and covering power and an increased limiting current density. Higher ratios result in slower deposition rates, duller deposits, increased covering power, and decreased limiting current density. The addition of a fluoride catalyst necessitates an adjustment in the CrO3/SO4 weight ratio. Ratios of 170:1 to 210:1 are required, with 190:1 being most typical. The chromium concentrations for both the dilute and concentrated mixedcatalyst formulations are the same as for conventional baths; however, the sulfate concentration is lowered due to the use of the secondary catalyst. Unlike most plating baths in which the metal, M, is present in solution as a cation, M+, chromium is present as an anion complex, Cr2O7-2, which very likely undergoes further complexing with ions such as the sulfate catalyst to permit chromium deposition. Fluoride or silicofluoride ions are extensively used today in mixed-catalyst formulations, especially in self-regulating baths. Self-regulating formulations are obtained by using sparingly soluble salts of the fluoride catalysts. This controls the catalysts��� concentration because only the required amount of catalyst will dissolve. For this reason, less frequent catalyst analysis is required. Much more routine analyses and chemical control of the catalysts are needed in non-self-regulated formulations since the catalyst must be added to the bath as they are depleted. However, this offers a much wider range of operating conditions. Chromic acid concentrations may be maintained by making frequent specific gravity determinations of the operating solution using a hydrometer. Occasionally, more accurate chromium analyses should be conducted by an analytical method specific for chromium. Chromium is consumed and dragged out of the bath. Since lead anodes are used, the chromium must be replenished by additions of chromic acid (chromic trioxide). Sulfate concentrations should be determined frequently by using a centrifuge to measure the volume of the sulfate precipitated by the addition of barium. Occasionally, gravimetric analyses should be conducted to confirm this quick, less accurate method. Sulfate concentrations may be raised by adding sulfuric acid. To add 0.05 g/L sulfate (SO4), add 0.0295 ml/L concentrated (66�� B��) H2SO4. Sulfate can be dragged into as well as dragged out of the tank. To lower the concentration of sulfate by 0.05 g/L of H2SO4, add 0.01 g/L barium carbonate to pre295

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