Electrodeposition of Au–Sn alloys from acid Au(III) baths B. BOZZINI 1, *, A. FANIGLIULO 1 , G. GIOVANNELLI 2 , S. NATALI 2 and C. MELE 1 1 INFM – Dipartimento di Ingegneria dell’Innovazione, Universita ` di Lecce, v. Monteroni, I-73100 Lecce, Italy 2 Dipartimento ICMMPM, Universita ` di Roma ‘‘La Sapienza’’, v. Eudossiana 18, I-00185 Roma, Italy (*author for correspondence, fax: +39 0832 320525, e-mail: benedetto.bozzini@unile.it) Received 18 June 2002; accepted in revised form 11 April 2003 Key words: alloy electrodeposition, Au–Sn, in situ SERS, white gold, XRD Abstract A bath for the electrodeposition of white gold alloys of interest for the electroforming of hollow jewellery is proposed and investigated. The system was an acidic Au(III)–Sn(IV) bath for the electrodeposition of Au–Sn alloys. The electrochemical investigations were based on cyclic voltammetry, linear-sweep voltammetry, galvanostatic electrodeposition experiments and in situ Raman spectroscopy. The electrode kinetics of alloy formation were elucidated by stripping voltammetry. The effects of cathodically adsorbed CN ) were studied by in situ Raman spectroscopy. Electrodeposited foils were studied from the crystallographic, compositional and morphological points of view. Codeposition of Au and Sn gives rise to a single phase of approximately equiatomic composition over a current density interval of 10 to 40 mA cm )2 . This orthorhombic phase is structurally the same as the f¢ phase of the equilibrium Au–Sn system, but its stoichiometry and lattice parameters are different. The equilibrium two-phase d, f¢ structure can be obtained by heat-treatment. 1. Introduction The electrodeposition of white precious metals and alloys still requires extensive research and development efforts in order to achieve industrially acceptable quality standards [1, 2]. Alloying effects giving rise to commer- cially acceptable colours for white Au alloys are not straightforward. The human perception of the colour of metals derives from the following optical properties [3]: (i) spectral reflectivity, broadly related to brightness and (ii) light absorption, broadly related to hue. The former property derives from the mobility of conductance electrons and is related to electrical and thermal conductivity. The latter property depends on the pecu- liarities of the valence band structure. Alloying changes the colour of Au according to three different mecha- nisms: (i) modification of the valence band structure, (ii) reduction of the spectral reflectivity, (iii) formation of intermetallic compounds with their own electronic structure. We are currently investigating [4, 5] the electrodeposition of binary and ternary Au alloys with Sn and Zn. This approach is expected to give rise to whitening effects due to all three causes listed above. The literature on the electrochemical behaviour of the Au–Sn system is very limited [6–8]. Underpotential deposition of Sn on polycrystalline Au and Au(III) has been reported. Two anodic peaks were measured and assigned to the oxidation of Sn° to insoluble oxygenated Sn(II) species and to the oxidation of Sn(II) to soluble Sn 4+ . Three patents on high-Au Au–Sn alloys have been filed [9], stressing the bath chemistry, but giving no electrochemical or structural details. Recently a paper providing a sound description of electrodeposition processes from a weakly acidic sulphite bath appeared [10]. Bath preparation is thoroughly described and some morphological data are reported, but limited electrochemical details and no structural data are provided. In this work an acid Au–Sn bath without free cyanide is formulated and studied. Particular care was devoted to the determination of the crystalline structure. 2. Experimental details 2.1. Solutions A Au(III) bath was used of composition: Au (as KAu(CN) 4 )5gl )1 , Sn (as SnCl 2 6 ) 75 g l )1 , pH equals 0.5. Analytic grade chemicals were used. The solutions were prepared with ultra-pure water (resistivity 18 MW cm) obtained with a Milli-Q system. The Sn complex was prepared by dissolving the metal into aqua-regia. The excess nitrate was eliminated by boiling. The pH was adjusted by adding HCl. The Au(III) salt was dissolved in the Sn(IV) solution. The operating conditions were: current density (c.d.) 10 to 40 mA cm )2 , temperature 20 °C. Journal of Applied Electrochemistry 33: 747–754, 2003. 747 Ó 2003 Kluwer Academic Publishers. Printed in the Netherlands.