Study on Co-Electrodeposition Mechanism of Au-30at.%Sn Eutectic in Non-Cyanide Bath by Electrochemical Methods

Abstract

The co-deposition of Au-30at.%Sn alloy in a non-cyanide alkaline Au+-Sn2+ bath was successfully achieved with sodium sulfite (Na2SO3) and potassium pyrophosphate (K4P2O7) as green complexing agents, ethylenediamine tetraacetic acid (EDTA) as bath stabilizer and catechol as additive. The Au(I) is mainly complexed with sulfite in the form of [Au(SO3)2]3−, while the Sn(II) is mainly complexed in the form of [Sn(P2O7)2]6−. The reduction potentials of both Sn(II) and Au(I) shift to a negative value (more negative than −0.70 V vs. SCE) in the presence of sulfite and pyrophosphate complexing agents, resulting in the onset reduction potential gap between Au(I) and Sn(II) decreased from 1.828 V (for standard deposition potential) to 100 mV, and consequently the co-deposition of Au and Sn is realized. The cyclic voltammetry and chronoamperometry characterizations revealed that the co-deposition of Au-Sn alloy is an irreversible and diffusion-controlled process and the nucleation mechanism is represented by the progressive model. The addition of catechol, which can be adsorbed on the cathode surface, increased the co-deposition overpotential, inhibited the co-deposition process and refined the grain size of the electrodeposits. Finally, Au-Sn eutectic film consisting of AuSn and Au5Sn phases was successfully co-deposited.