Electrolytic deposition of gold and silver

Gold and silver belong to the family of noble metals (gold, silver, palladium, platinum, rhodium, ruthenium, osmium and iridium). They have sought-after physical and chemical properties for both technical and decorative applications. These properties are often only required on the surface of manufactured parts. Made from a material that is generally non-noble for technical (mechanical, thermomechanical, thermal, mass) or economic reasons, these parts are surface-treated and a layer of the chosen noble metal is then developed using a carefully selected process.

When the substrate is conductive (or rendered conductive), it is easy to obtain this surface metal layer electrolytically.

As the standard potential of noble metals is among the highest, the stability range of their oxides in aqueous solution is all the more reduced. In order to maintain the oxidized forms of these metals in solution, and thus be able to reduce them at the cathode, formulations combining complexing agents, pH buffering systems and other additives, stabilizers or deposition gas pedals have been developed. These formulations make it possible to obtain coatings whose properties are correlated to the electrolysis parameters. Indeed, while it is often possible to obtain coatings with properties close to those of annealed raw metal, it is also possible to modify certain mechanical or electrical properties, by acting on temperature, agitation, current density or the concentration of certain constituents, for example. The choice of metal, as well as the coating process, will therefore depend on the desired functionalization of the substrate to be coated.

Electrolytic coatings produced in aqueous media use electrolytes (or baths) composed of mixtures of chemicals, often harmful and sometimes toxic. This is all the more true for the electrolytic coating of gold and silver, as the conditions for solubilizing their oxidized forms often involve cyanide, or highly concentrated acids and bases. Although a few less dangerous solutions have been proposed, their industrialization is still in its infancy. Every precaution must therefore be taken to protect the operator and his environment.