Gold Electropolishing Using Cyanide-Free Electrolytes

Abstract

Electropolishing or electrochemical polishing (EP), is an ultrafinishing process that improves the aesthetic, technical and functional properties of surfaces by reducing their roughness. This process is based on the anodic dissolution of the workpiece through an oxidation reaction. The polishing effect is due to the formation of a viscous layer that induces a preferential dissolution of the peaks of the surface profile at the expense of the valley. This results in the surface leveling and in brightness improvement. Compared to conventional mechanical finishing techniques, EP is a non-contact process that does not disturb the metallurgy of the surface layer. In addition, it allows the treatment of relatively complex geometries as well as the obtention of reproducible results.Regarding the electrolyte composition, it often includes oxidizing agents to activate the anodic dissolution (strong acids, hydrogen peroxide, ...) as well as depassivating agents to prevent the oxide film from becoming too stable (halides, alcohols, ...) and viscous layer promoting agents (glycerol, organic acids, ...). In the case of gold, the presence of a complexing agent lowers the oxidation potential and facilitates dissolution. However, most of the electrolytes used for EP of precious metals are toxic due to the presence of cyanides or thiourea. Furthermore, each alloy requires a specific electrolyte formulation and therefore suitable operating parameters. The complexity of the mechanisms involved and the associated environmental risks make it difficult to deploy this technique in the luxury and jewelry industry. Another concern is the lack of literature regarding the role of each component.A better understanding of the process could lead to an improvement of the electrolyte formulation and to the proposal of other options with a lower environmental impact. The aim of this work is to study the effect of various non-toxic and ecofriendly electrolytes on the electropolishing mechanisms of gold and gold alloys. Their efficiency will be evaluated in terms of the surface properties obtained after the treatment: roughness reduction (macro/micro) and gloss. Moreover, the effect of each component of the electrolytes is investigated using electrochemical measurements, in the perspective of describing EP mechanism and its consequences on surface properties. Figure 1