Plating in Dilute Cyanide Baths

In the dilute cyanide bath, corrosion of the anodes increases with increasing concentration of free cyanide. Low free-cyanide content may cause rough deposits due to anode polarization; however, excessive free cyanide lowers cathode efficiency, resulting in thinner deposits per unit of time. Modifications of the pH, or alkalinity, of the strike compositions are used for striking various substrates. For use on steel, additional NaOH or KOH improves the conductivity of the solution and aids in protecting steel anode baskets, tanks, and other steel fixtures from corrosion. For use on zinc-base die castings, the hydroxide concentration is kept in the range of 1.3 to 3.8 g/L (0.2 to 0.5 oz/gal). For use on zincated aluminum alloys, the pH should be reduced to approximately 9.7 to 10.0 with sodium bicarbonate. The operator should keep adding tartaric acid or sodium bicarbonate to the solution to maintain the desired pH range (e.g., 10.0 to 10.5 for plating on aluminum alloys.

The dilute copper cyanide bath can be operated at room temperature, but the general practice is to operate the bath between 32 and 49 °C (90 and 120 °F) to increase the rate of deposition and to improve anode dissolution. This electrolyte is usually operated with a cathode current density of 1 to 1.5 A/dm2 (10 to 15 A/ft2). The tank voltage is normally between 4 and 6 V.

Agitation of the bath produces more uniform composition throughout the electrolyte, more uniform anode corrosion, and an increase in current densities where the brightest deposits are obtained. Current densities in excess of 5 A/dm2 (50 A/ft2) have been applied successfully by using air agitation of the solution and agitating the work.

Continuous filtration is preferred for dilute cyanide baths. Organic contamination or suspended matter in the strike is frequently responsible for roughness of copper plate subsequently deposited in the cyanide copper plating bath. Hexavalent chromium in the strike causes blistering of the deposit. Proprietary additives can be used to improve the bath operation, as well as aid in the control of organic and inorganic contaminants. These proprietary additives consist of organic complexing agents, such as tartrate salts. Organic reducing agents are used to control impurities such as hexavalent chromium. Wetting agents (surfactants) are used to control organic contaminants and to lower the surface tension of the plating solution, to allow better throwing power of copper over substrate irregularities, and to aid drainage and rinsing.

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