Additive-free electrolytic bath for pulse plated AuCu coatings

Abstract

The appearance and properties of gold have made it a very valuable metal since ancient times. It is used in catalysis and electronics, but its most common use is in jewellery. In this field it presents a problem of durability because it is a very soft material that wears out easily. To solve this problem, alloys are mixed with other metals that improve its mechanical and optical properties. This work is based on obtaining a gold-copper alloy of 18 karats (kts) (75% gold). The gold that is used in jewellery is produced using an electrodeposition technique. Most of the commercial electrodeposition baths are based on cyanide salts, a toxic compound that is harmful to people and the environment. To ensure the correct deposition of all the components of the alloy and to ensure its composition and brilliance, different additives are used, many of which are pollutants when reacting causing a shortening of bath lifetime. To obtain a bright 18 kts coatings without compromising the stability of the bath, the objective of this work is the optimization of an electrodeposition technique that allows to obtain coatings from electrolytic baths without additives. The pulsed direct current technique is used, which consists of the application of current poles alternated with rest periods, allowing an orderly growth of the deposit. To determine the most suitable parameters of this technique, a pre-pulsating study has been carried out with direct current, including the speciation of the electrolyte. The pre-pulsed studies have allowed to establish which copper complex is more favorable to electrodeposition, which is [Cu(CN)3]2, and the most favorable conditions to carry out the pulsed direct current study have been determined: [Cu+] = 6.86 g/L, [Au+] = 5.00 g/L, [CN-] = 0.40 M, w = 100 rpm, T = 70ºC and j = 2.5 – 5 A/dm2. In this work it has been possible to demonstrate that the pulsed direct current technique is effective and allows to obtain gold-copper bright coatings of 18 kts without additives in the bath