Electrochemical Synthesis and Characterization of Ag-Re Coatings

Silver-white, matte, smooth, and durable deposits of silver-rhenium, with thicknesses ranging from 2.0 to 13.7 μm and containing 0.15 to 13.5 wt.% Re, were obtained with a current efficiency of 66-98% from a developed dicyanoargentate-perrhenate bath based on a borate-phosphate-carbonate silver-plating electrolyte. The study was focused on the influence of bath composition, the [Ag(I)]:[ReO4-] ratio, surfactant additives, applied current density, temperature, and stirring, on the alloys composition, structure, morphology, microhardness, adhesion, and porosity. A voltammetric analysis was conducted, considering the influence of ethanolamines on electrode processes. In baths with TEA, coatings similar to a silver matrix with rhenium doped in mass fractions are likely achievable. MEA is recommended due to its process-activating properties. All coatings were nanocrystalline (τ = 28.5 - 35 nm). For deposits containing less than 10 wt.% Re, characteristic silver XRD peaks were observed, while other deposits, additional peaks attributed probably to Re(VII) and Re(VI) oxides. A linear relationship, typical for Hall-Petch plots, was obtained, confirming that grain boundaries play a crucial role in mechanical properties of coatings. The conditions for stable electrochemical synthesis of promising functional Ag-Re coatings of predetermined composition (0.7-1.5 wt.% Re) were proposed for practical use in power electronics and energy sectors, for manufacturing electrical contacts operating across a wide temperature range. This was realized by deposition from an Ag-rich bath in the area of mixed electrochemical kinetics, at potential values corresponding to the region of half the limiting current: j = 2.5 ‒ 6 mA cm-2, t = 19 - 33°C.