Gold and Gold Alloys Electropolishing Using HCl-Glycerol-Ethanol Electrolytes

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Electropolishing (EP) is an electrochemical process used in the final stage of metal processing to reduce surface roughness and enhance functional, technical, and aesthetical properties. Some of its advantages are the preservation of surface metallurgical properties and the ability of treating complex geometries.EP is based on a homogeneous dissolution of the surface, driven by a competition between oxidation reactions and dissolution. Compounds such as cyanides and thiourea, which was now classified as a classed 3 carcinogen, can form very stable soluble complex with precious metals like gold and its alloys. Consequently, they have been widely used in the formulation of EP electrolytes for these materials, but their toxicity has led to the research of alternative solutions.This work presents a study based on a cyanide and thiourea free electrolyte previously reported in the literature for pure gold electropolishing and composed by hydrochloric acid, glycerol, and ethanol1,2. The aim was to give new insights in the involved mechanisms to evaluate the electrochemical behavior and the influence of different parameters on pure gold, and to extend this process to 18k jewelry gold alloys (yellow, rose, red and gray). In the first part of the study, only two of the electrolyte components (HCl-glycerol) were used to study the influence of chloride concentration on pure gold EP. Electrochemical techniques and surface properties analysis were evaluated.In the second part, the effects of adding ethanol to electrolyte composition and of variating rotation speed were also evaluated on pure gold. The best results were obtained for a HCl 50%vol. - Glycerol 25%vol. - Ethanol 25%vol. electrolyte at a rotation speed of 1000 rpm. This solution was then used to electropolish 18K gold alloys. Mitigated results were obtained for silver-containing alloys (yellow, rose and red), which were not uniformly polished due to the formation of an AgCl film on the surface that partially masks the substrates, whereas better results were obtained with 18K grey gold3.(1) Peck, W. F.; Nakahara, S. Preparation and Electropolishing of Thin Gold Disc Specimens for Transmission-Electron-Microscope Examinations. Metallography 1978, 11 (3), 347–354. https://doi.org/10.1016/0026-0800(78)90047-2.(2) Verlinden, J.; Celis, J. P.; Roos, J. R. Passivation-depassivation behaviour of gold in a hcl-glycerol solution and its application to electropolishing. Thin Films Sci. Technol. 1983, 341–346.(3) Rodriguez J. ; Doche M-L ; Hihn J-Y ; Electropolishing of gold and gold alloys in HCl-glycerol-ethanol electrolytes App. Surf. Sci. Adv. 21(2024) 100604 https://doi.org/10.1016/j.apsadv.2024.100604