For this study, Nb₃Sn superconducting films were prepared on Nb substrates using Sn electroplating and electroless Sn plating, followed by high-temperature annealing. After Sn layers were deposited on electropolished Nb substrates, subsequent annealing at 950 ℃ or 1100 ℃ enabled Nb₃Sn formation through solid-state diffusion. Structural analyses using X-ray diffraction and X-ray photoelectron spectroscopy confirmed Nb₃Sn phase formation by both plating methods, with electrodeposition yielding more uniform Sn layers and more reproducible Nb₃Sn formation. Sn deposition exhibited poorer uniformity, but appropriate annealing conditions still enabled Nb₃Sn formation. Repeated annealing cycles promoted deeper Sn diffusion into the Nb substrate, resulting in Nb₃Sn layer formation beneath the surface, which might mitigate the mechanical fragility inherent in Nb₃Sn. Superconducting measurements revealed transition temperatures of 17.9-18.0 K for electroplated samples and 17.5 K for electroless plating samples, including those with subsurface Nb₃Sn layers. These results demonstrate wet-chemical plating combined with thermal treatment as a promising and versatile approach to achieve Nb₃Sn coating formation for superconducting radio-frequency cavities.

Measurements of electrode impedance diagrams during electropolishing and SEM observations of specimen surfaces after electropolishing were taken at transpassive potentials on SUS316L stainless steel in a 63.8%H₃PO₄ - 24.5%H₂SO₄ - 11.7%H₂O solution. The relation between the type of impedance diagram and the smooth condition of the specimen surface was examined to elucidate transpassive dissolution processes that produce an excellent smooth surface. The smoothest surface was obtained when the electrode impedance diagram showed component diagrams of four types: a capacitive hemicycle ①, which is attributable to a charge transfer resistance and an electrical double layer for a transpassive dissolution reaction; a capacitive hemicycle ③, which is attributable to transpassive film formation; an inductive hemicycle ④, which is attributable to change in the coverage of intermediate adsorbates from transpassive film dissolution; and a curved line ⑥, which is attributable to the finite diffusion of dissolution products. The consecutive occurrence of ①-③-④-⑥ was observed specifically at potentials higher than the O₂ evolution potential. Finite diffusion of dissolution products is important for producing an exceedingly smooth surface.