The co-deposition of Au-30at.%Sn alloy in a non-cyanide alkaline Au + -Sn 2+ bath was successfully achieved with sodium sulfite (Na 2 SO 3 ) and potassium pyrophosphate (K 4 P 2 O 7 ) 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(SO 3 ) 2 ] 3− , while the Sn(II) is mainly complexed in the form of [Sn(P 2 O 7 ) 2 ] 6− . The reduction potentials of both Sn(II) and Au(I)

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... 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 Au 5 Sn phases was successfully co-deposited.