Cyanide Alkaline Electrolyte for Copper Plating

An electroplating line can be described as a series of functional stations operating in different types of reactor, each containing an appropriate electrolyte. Each station is dedicated to a single operation, such as degreasing the parts (to give them perfect hydrophilicity); pickling, or more accurately, "activating" them (to eliminate surface hydroxides); the actual metal deposition, to produce a coating in compliance with a specification (particularly in terms of thickness); and finally, final rinsing, possibly completed by a cosmetic or protective (anti-corrosion) finishing treatment. On leaving the reactor, the parts are damp. They therefore carry electrolyte to the next station, and are thus a source of chemical pollution of the first reactor in the downstream function by electrolyte from the last reactor in the upstream function. For this reason, it is necessary to interpose rinsing functions between two successive unit operations. This type of pollution can be attributed to rinsing when the management of this function is faulty. The ensuing disorder is then known as "deferred corrosion", as it affects the quality of the protective coating and, in particular, its adhesion to the substrate. In a cyanide-alkaline copper-plating line, in addition to this disorder, other sources of disorder affect the copper-plating station. Experience shows that these are attributable to a lack of control in the management of this electrochemical reactor. In fact, over time, this management can become increasingly delicate, difficult and, consequently, polluting, and in some extreme cases almost impossible to manage, necessitating essential and costly "bath shutdowns". This is due to a series of secondary reactions, both chemical and electrochemical, which can develop both on the electrodes and within the electrolyte.

The stakes are high in terms of quality, operating costs, production and waste limitation. We therefore focus on the proper management of the copper-plating reactor alone. Starting with a typical formulation, this article identifies the reactions and other chemical or electrochemical equilibria likely to occur (thermodynamic aspect). The kinetic aspects then enable us to understand the origin and diversity of untimely drifts. Finally, by means of an exhaustive material balance, we discuss the conditions necessary for the proper management of a cyanide alkaline copper plating reactor.

At the end of the article, readers will find a table of acronyms used, as well as a table of free enthalpy of formation values.