4HF H3BO3 HBF43H2OEq 6

where HBF4is the resultant fluoboric acid. The reaction is reversible if the stoicheiometric amount of boric acid (H3BO3)is used to react with the hydrofluoric acid (HF) present. As the amount of boric acid in the above reaction is increased, the reaction is driven far to the right so that the reaction becomes irreversible and no free hydrofluoric acid is regenerated. This is important because if free hydrofluoric acid were present, then insoluble fluorides, especially lead fluoride, would precipitate. Thus, all fluoborate concentrates and plating baths contain free boric acid. Although 25 g/L of boric acid is optimum (close to its solubility), any amount of free boric acid is acceptable to prevent the formation of fluorides.

Free fluoboric acid is maintained in the solution to provide the requisite acidity and to raise conductivity. In conjunction with peptone, it can prevent "treeing" and give a fine-grain deposit. Free fluoboric acid can be added in amounts ranging between 100 to 500 g/L (13.4 to 67 oz/gal), depending on the solution formula used.

Peptone is added to the plating bath to promote formation of fine-grain adherent deposits and prevent treeing. Peptone solution is available commercially, and proper amounts can be measured and poured directly into the plating bath. The bath can then be used immediately, after it is gently stirred to ensure complete mixing of peptone. During the operation of the plating bath, a loss of peptone can result because of dragout, chemical breakdown, and codeposition with the metal. As peptone is depleted, it must be replenished. Replenishment amounts should be determined by experience. As a guide, 1 L (2.1 pints) of peptone solution per 380 L (100 gal) of plating bath can be used per week. A Hull cell operated at 1 A for 10 min can be used to control the peptone content of a plating solution.

Tin-lead fluoborate solutions containing peptone should be filtered through activated carbon at least four times per year to ensure removal of organic breakdown products and avoid buildup of peptone from indiscriminate additions. The plating bath should be treated with about 4.5 kg. (10 lb) of activated carbon per 380 L (100 gal) of bath until, after filtration, the solution is water white. The bath should not be heated during carbon treatment, because stannous will be oxidized to stannic tin. Fresh peptone is added after carbon treatment. Because there is no simple analytical method for determining peptone concentration in this solution, carbon treatment and replenishment of peptone every 3 or 4 months ensures proper amounts of peptone in a solution.

Proprietary synthetic wetting agents can be purchased for grain refinement, as a substitute for peptone. Because these components are synthetic, they are less susceptible to algae and bacterial growth.

Metallic impurities are removed by low-current-density electrolysis, but in a tin-lead bath, low current density favors deposition of lead, which may unbalance the solution. Metallic impurities can be removed by dummying a bath at a current density of 0.2 A/dm2 (2 A/ft2) for at least 8 h. The solution should then be analyzed and brought up to specification with stannous or lead fluoborate. Iron, nickel, and other metals above hydrogen in the electromotive series are not removed by dummying, although copper is easily removed.

Filtration. A tin-lead fluoborate plating solution should be filtered constantly, to keep the bath clear. If constant filtration is not used, a bath can turn cloudy because of sulfates entering the solution and precipitating as lead sulfate.

Stannic salts can also precipitate out of the solution. Anode sludge or breakdown products from peptone solutions can contribute to a cloudy appearance as well. Polypropylene filter spools or cartridges can be used as filters, but they must first be leached in hot water (65 °C, or 150 °F) to remove organic agents used in their manufacture. The end of the return hose from the filter must be submerged in the bath to prevent aeration of the bath.

Anodes. Tin-lead alloy anodes of at least 99.9% purity must be used. The most objectionable anode impurities are arsenic, silver, bismuth, antimony, copper, iron, sulfur, nickel, and zinc. Extruded anodes are preferred over cast anodes, because cast anodes have a larger grain size and suffer from intergranular corrosion, which causes large pits or depressions to form on the anode surface. The finer grain size of extruded anodes provides uniform and efficient corrosion during plating. Tin-lead anodes should be left in an idle tin-lead fluoborate bath because they exercise a reducing effect on tin in solution, thus helping to maintain the bath in a stannous valence state. Tin-lead anodes should be bagged with acrylic or polypropylene cloth to contain any anode sludge that may form. Anode sludge suspended in solution can cause rough deposits.

Materials of construction for tin-lead plating equipment include:

• Steel lined with rubber or polypropylene or made entirely of polypropylene, used for tanks and pumps

• Monel metal, used for anode hooks

• Polypropylene, used for anode bags and filter spools

• Pure paper pulp (alpha cellulose), used for filter aid

The following materials should not be used in contact with fluoborate solutions: glass; quartz or other silicated materials; nylon; neoprene; or titanium. Equipment in contact with fluoborates should have the recommendation of the manufacturer for use in a fluoborate solution.

Lead disposal for fluoborate and MSA solutions is most commonly accomplished by diverting the lead-containing rinse water (or dilute concentrate) to a pH-controlled holding tank, filtering the solution, and then running it through an ion exchanger. The fluoborate rinse will have to be regenerated with MSA before going to a holding tank and then to electrowinning. The MSA rinse can go directly to electrowinning, where the lead is plated out on a reusable cathode. The water can then be carbon treated, ion exchanged, and reused for certain rinses.

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