Plating in Rochelle Cyanide Baths

Rochelle electrolytes with lower metal concentrations can be used both for striking applications and, with higher metal concentrations, for plating applications. Rochelle salts produce some grain refinement, reduce the effects of some metallic contaminants, and aid in anode corrosion by increasing the anode current density range before anode polarization occurs. The Rochelle electrolyte can also be used for periodic-reverse plating with good results. Barrel plating with a Rochelle bath requires a variation in the chemistry. When plating parts that tend to nest or stick together during the barrel rotation, it is necessary to increase the free cyanide to 25 to 30 g/L (3 to 4 oz/gal) or slightly higher to obtain adequate coverage on the nested parts.

Rochelle baths usually are operated at a current density between 2 and 5 A/dm2 (20 and 50 A/ft2). Substituting potassium salts for sodium salts in the baths with higher metal concentration, up to 38 g/L (5 oz/gal) copper, can increase the allowable current density to 6 A/dm2 (60 A/ft 2), with the penalty of lowering the cathode efficiency. The Rochelle baths are usually operated at between 54 and 71 °C (130 and 160 °F) for best efficiency. The rate of deposition is higher at the higher temperatures. A high-efficiency electrolyte having a higher metal concentration can be operated at up to 77 °C (170 °F). For copper plating zinc-base die castings, the electrolyte is best operated at 60 to 71 °C (140 to 160 °F), provided the pH of the bath is maintained between 11.6 and 12.3. An increase in the operating temperature of Rochelle cyanide baths increases the efficiency of the anode and cathode; however, free cyanide decomposes more rapidly, which increases carbonate formation. An increase in agitation causes an increase in anode efficiency, but this also increases carbonate formation. Carbonates are always present in cyanide copper solutions from oxidation of the cyanide and, also, from adsorption of carbon dioxide from the air that reacts with the alkali in solution. Carbonates from a sodium copper cyanide plating solution can be removed by cooling the solution, which precipitates the less soluble sodium carbonate. High carbonate concentrations lower the anode efficiency, which accelerates additional carbonate formation in addition to producing rough or porous plated deposits (Ref 6).

Rochelle copper baths should be maintained at a pH between 12.2 and 13.0. Anode efficiency may be prohibitively low if the pH is too high. Raising the pH also decreases the voltage drop across the anode film. Figure 1 shows a buffer curve for adjusting the pH of Rochelle electrolytes.

To lower pH. add dilute tartaric acid (10% by volume)

To lower pH. add dilute tartaric acid (10% by volume)

To raise pH, add NaOH 0.75 g/L NaOH = 2 mL/gal 1.83 spgr HjSO.,

Fig. 1 Buffer curve for adjusting the pH of Rochelle electrolytes. Source: Ref 7

Conductivity of the bath is improved by raising the free alkali cyanide and the concentration of the copper complexes. When depositing copper directly on steel, brass, or copper, conductivity can be improved by the addition of 2 to 15 g/L

(4 to 2 oz/gal) of sodium hydroxide. Sodium hydroxide concentrations should be reduced if the electrolyte is used to deposit copper onto zinc-base die castings, aluminum, or magnesium.

Rochelle baths can become contaminated during plating of zinc-base die castings. Zinc contamination can be removed by electrolysis of the bath at room temperature, at the current density that produces the most brassy or off-color deposit, usually 0.2 to 0.3 A/dm2 (2 to 3 A/ft2). Iron, which forms complexes with cyanide, cannot be removed readily from the bath and causes a reduction in current efficiency. Drag-in of chloride ion from acid dips must be kept very low to prevent iron buildup due to dissolution of steel equipment. Bipolarity of steel tanks or heat exchangers should be avoided.

The Rochelle bath is susceptible to organic contamination, which can be controlled by the use of wetting agents. Organic contaminants should be removed by periodic batch treatment of the electrolyte with activated carbon, followed by filtration. Organic contamination is especially high in barrel plating. A low-foaming, free-rinsing surfactant or a dispersion agent must be used in barrel plating baths to prevent organic contamination from adversely affecting the quality of the plated deposit. Organic contamination can be controlled with carbon treatment methods. Continuous filtration of cyanide electrolytes is recommended to eliminate particulate matter or salts, which can result in rough deposits.

Increase in the current density or the presence of lead in the Rochelle cyanide bath causes an increase in the stresses of copper plate. These stresses can be reduced by increasing the concentration of copper in Rochelle baths. The addition of 15 g/L (2 oz/gal) of potassium thiocyanate produces an expansion stress instead of the usual contraction stress. Figure 2 shows stress in thin copper electrodeposits plated from a cyanide solution onto stainless steel.

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Fig. 2 Stress in thin copper plate deposited on stainless steel spirals. Stainless steel spirals are 0.127 mm (0.005 in.) thick. Source: Ref 8

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