Electroplated copper from cyanide-based plating solutions has long been used both as an engineering or decorative finish and as an undercoat for other plated metals. Cyanide copper solutions are used on a wide variety of base materials.
An important property of copper deposited from cyanide-based systems is its ability to form strong adherent bonds to most base metal and alloy substrates. Because acid copper plating solutions attack most basis metals and/or produce immersion deposits with poor adhesion, cyanide copper plating is used as a thin undercoating or strike to produce good adhesion.
Following the strike, a high-speed acid copper plating formulation can be used to produce the desired film thickness. Cyanide plating solutions are highly poisonous, and the rinses and spent bath solutions must undergo complete destruction before disposal.
Allowable cyanide in wastewaters is very small, and waste treatment is required in all areas. However, it should be recognized that cyanide destruction technology is well established and proven effective.
Because deposits from cyanide plating solutions exhibit good throwing power, more complicated shapes can be completely covered, with a uniform film thickness. This is a result of the relationship of efficiency and current density.
As the current density increases, the bath efficiency decreases. There is a drawback that must be addressed. The drop in plating efficiency yields an increase in the formation of hydrogen gas. This evolved gas can be absorbed by the base metal. The result can be a reduction of the metal’s fatigue strength by hydrogen embrittlement.
To relieve hydrogen embrittlement when plating on hardened steel (Rockwell C35 or higher), the parts are baked within 4 hr after the final plating operation at 325 ± 25°F (162 ± 14°C) for up to 24 hr.22,26,31 Periodic reverse plating cycles in cyanide copper baths have been used with some success in producing fine-grain deposits with good leveling.
Plating of parts with cyanide copper will continue with little change in total volume until a suitable and environmentally acceptable system can replace it. The ability to plate an initial coating with good adhesion to light metal base materials with good throwing power and coverage are the key features of cyanide copper systems.
The associated cyanide destruction, disposal costs, and safety measures required in cyanide handling can be accomplished with proper education and training.
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