The electrodeposition of low-Sn imitation gold Cu–Sn alloy from EDTA-tartrate double complexing agents

The electrodeposition of a Cu–Sn binary alloy on stainless steel from a cyanide-free alkaline electrolyte containing EDTA·2Na and C4H4O6KNa was investigated to explore the possibility of a cyanide-free double complexing system that maintains the decorative quality and coating performance. The Cu–Sn co-deposition occurred at −0.95 Vvs.Hg|HgO, and the height of cathode peak A from the BR bath was −0.0280 A. EDTA·2Na could simultaneously complex with Cu ions and Sn ions, while C4H4O6KNa could only complex with Cu ions, which was shown through the analysis of the cyclic voltammetry (CV) curves together with the nuclear magnetic resonance (NMR) results. The molar concentration ratio and sum of the two main salts or the two complexing agents, and the pH of the electroplating solutions were shown to influence the surface micromorphology, composition and phase structure of the Cu–Sn coatings. The scanning electron microscopy (SEM) analysis showed that the Cu–Sn coating obtained from the bath with EDTA-tartrate double complexing agents exhibited smaller grain sizes than that obtained from the bath with the single EDTA or tartrate complexing agent, and the smallest grain size of 0.2 μm was obtained from the BR bath. In addition to being smaller than the others, this coating contained uniform particle sizes. The energy-dispersive X-ray spectroscopy (EDS) analysis indicated that the composition of the Cu–Sn coating obtained from the BR bath was 92.8 wt% Cu and 7.2 wt% Sn, and this coating was golden in colour. X-ray diffraction (XRD) showed that the Cu–Sn coatings were crystalline and were composed of a mixture of Cu, Cu6Sn5, [Cu, Sn] and Cu10Sn3 phases, indicating the formation of Cu–Sn binary alloy. These results may provide theoretical guidance for the electrodeposition technology of other alloy coatings.