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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