Formation of alloys upon the simultaneous electrochemical deposition of gold and tin from ethylene glycol and aqueous electrolytes

An ethylene glycol (EG) solution containing Au(III) and Sn(IV) compounds, and conditions for the electrochemical deposition of Au–Sn alloy based on AuSn and Au₅Sn intermetallics with total tin content of 30–55 at % are proposed. Fundamental difficulties of the deposition of alloys with high tin content, (including eutectic Au–Sn alloy) from aqueous electrolytes are revealed. It is determined via voltammetry that the simultaneous deposition of gold and tin from aqueous and EG electrolytes proceeds with the depolarization effect of both Au(III) and Sn(IV) as a result of the formation of the alloy, the increase in the rate of tin cathodic reduction being more noticeable in case of EG solution. Formation of SnCl₂EG(H₂O)₂⁺ complex upon the dissolution of SnCl₄ · 5H₂O in glycol, the stability of the composition of tetracyanoaurate ions upon the dissolution of KAu(CN)₄], and the weakening of intermolecular interactions in EG with small amounts of water were revealed via IR spectroscopy. It is suggested that the depolarization effect is due not only to alloy formation, but also to the formation of SnCl₂EG(H₂O)₂⁺ cations, their association with Au(CN)₄^- anions, and a change in the mechanism of Au(III) and Sn(IV) reduction.