Development of silver, gold and gold-based alloys electrolyte compositions

The high electrical and thermal conductivity of gold and silver, and the high oxidation resistance of gold, make them necessary for many applications. They are produced especially in the form of coating due to their high cost. Gold and silver depositions are mostly used in electronics, sensors, biomedical and aerospace industries, and jewelry and decorative applications. Owing to the high amount of wastage in the vacuum coating systems, the coatings are produced with the electrochemical deposition technique which is a conventional method proper to selectivity and automation. One of the most important parameters in this technique is the electrolyte content. With many years of research and development studies and investments of certain companies in the world, electrolyte compositions mostly protected by patents have been developed. Even though the main components and salts contained in these electrolytes are known, the organic and inorganic components referred to as brighteners or additives to produce the fine-grained, smooth, adhesive and bright coatings are still unknown. In Turkey, the gold electrodeposition electrolytes required for the jewelry industry are imported over 3 million Euro per year (excluding the gold price). It is obvious that we need know-how about this subject. With this motivation in the thesis, silver and gold plating electrolyte compositions were investigated. Researching the effects of brighteners and cyanide concentration on the surface quality and deposition mechanisms were aimed while focusing on pure silver, cobalt hardened gold, gold-silver and gold-copper alloy coatings. Semi-bright and mirror bright silver, cobalt-hardened gold, gold-silver alloy, and gold-copper alloy coatings were developed aiming their use in different industries. A large number of brightener couples suitable for these electrolytes were assigned. For each coating type, one electrolyte composition was selected, and application conditions and deposit properties were studied. Subsequently, the effect of additives on microstructure and reduction mechanism was examined and type of the mechanism of nucleation and growth occurring on the surface was found. In the light of the results obtained, it was found that potassium antimony tartrate and 2-mercaptobenzothiazole brightener couple are the most effective brighteners among thiourea, 2-thiobarbituric acid, imidazole, potassium selenocyanate, ethylene glycol for silver electrodeposition. Moreover, the concentration of cyanide is very critical for these coatings. It was shown that pyrophosphate electrolytes with low cyanide content can only be applied to produce homogeneous semi-bright deposits. This improved coating has good adhesion and is feasible to use in the electronics industry because of having no free cyanide damaging photoresist polymers. The levelling effect required to obtain a bright coating was achieved only with a high concentration of cyanide. The high cyanide electrolyte composition is suitable for use in a variety of areas, notably decorative applications and accessories, which is not affected by cyanide. It was shown that the utilization of a combined structure as cyclic organic structure and an aliphatic open-chain structure has a significant brightening effect on gold electrodeposition. Among the several cyclic compounds such as cyanopyridine, 8-hydroxyquinoine, nicotinic acid, glycine, uracil, nitrobenzene, nitrobenzene sulfonate, imidazole, imidazolidin-2-thione, the most efficient brightener couples are as combination of tetraethylenepentamine and imidazole or imidazolidin-2-thione or nitrobenzene or nitrobenzene sulfonate. Alternatively, it was also demonstrated that using a polymeric structure polyethyleneimine (Mw=600) and an inorganic compound (potassium selenocyanate) together can be benefited for brightening action during electrodeposition. Further, the operating conditions of the electrolytes were detected. With proper operating conditions, yellow cobalt-hardened gold coatings with the content of 99.9 wt.% gold, and yellow-white colored gold-silver alloy coatings containing about 75 wt. % gold were achieved in submicron and micron thicknesses. Additionally, if the required hydrodynamic conditions were provided, three different colors of gold were acquired with gold-copper and gold-copper-cadmium alloy electrodeposition in homogeneous color visuality. The concentration of cyanide in gold-copper alloy coatings is highly critical and affects the alloy composition. Without free cyanide addition approximately 98 wt. % Cu containing coatings were obtained. With the addition of 10 g/L free cyanide and tetraethylenepentamine to electrolyte, rose colored gold-copper alloy coatings having about 80 wt.% Au were produced. Tetraethylenepentamine and nitrobenzene sulfonate addition resulted in 97 wt.% Au content in the presence of 10 g/L free cyanide. It has yellow color associated with the composition. On the other hand, tetraethylenepentamine and nitrobenzene sulfonate and CdSO4 addition built up coatings with 87 wt.% Au in green color in 10 g/L free cyanide electrolyte content. The electrolyte compositions developed during the course of this thesis are listed below; 1. Semi-bright silver coating electrolyte (without free cyanide): 80 g/L KAg(CN)2, 0.5 g/L KCN, 30 g/L sodium borate decahydrate, 0.05 g/L 2-mercaptobenzothiazole, 0.5 g/L potassium antimony tartrate, pH = 7.5 Application conditions: 25°C, 1 L/min. electrolyte circulation rate, 2-6 A.dm-2, until 6.9 m thickness. 2. Mirror-bright silver coating electrolyte (high level cyanide content): 80 g/L KAg(CN)2, 60 g/L KCN, 7.5 g/L KOH, 0.05 g/L 2-mercaptobenzothiazole, 0.5 g/L potassium antimony tartrate, pH = 13. Application conditions: 25°C, 1 L/min. electrolyte circulation rate, 0.5-6 A.dm-2, until 6.3 m thickness. 3. Cobalt-hardened gold coating electrolyte: 4 g/L KAu(CN)2, 1 g/L CoSO4, 90 g/L potassium citrate, 60 g/L citric acid, 3 g/L imidazolidin-2-tion, 1 g/L tetraethylenepentamine, pH = 4.2. Application conditions: 60°C, 350 rpm with magnetic stirrer, 2-5 mA.cm-2, until 3 m thickness. 4. 18 carat green gold-silver alloy coating electrolyte: 4 g/L KAu(CN)2, 3 g/L KAg(CN)2, 80 g/L tetra potassium pyrophosphate, 2.5 g/L polyethyleneimine (Mw=600), 5 ppm potassium selenocyanate, pH = 10. Application conditions: 60°C, 350 rpm with magnetic stirrer, 5-10 mA.cm-2, until 2.9 m. 5. 19 carat red gold-copper alloy coating electrolyte: 4 g/L KAu(CN)2, 60 g/L K2Cu(CN)3, 10 g/L KCN, 1 g/L tetraethylenepentamine, pH = 11. Application conditions: 70°C, 2 L/min. electrolyte circulation rate, 7-10 mA.cm-2, until 20 min with 8 mA.cm-2. 6. 23 carat fine gold-copper alloy coating electrolyte: 4 g/L KAu(CN)2, 60 g/L K2Cu(CN)3, 10 g/L KCN, 1 g/L tetraethylenepentamine, 3 g/L 3-nitrobenzenesulfonate, pH = 11. Application conditions: 70°C, 2 L/min. electrolyte circulation rate, 7-10 mA.cm-2, until 20 min with 8 mA.cm-2. 7. 21 carat green gold-copper alloy coating electrolyte: 4 g/L KAu(CN)2, 60 g/L K2Cu(CN)3, 10 g/L KCN, 1 g/L tetraethylenepentamine, 3 g/L 3-nitrobenzenesulfonate, 1 g/L CdSO4, pH = 11. Application conditions: 70°C, 2 L/min. electrolyte circulation rate, 8-12 mA.cm-2, until 20 min with 8 mA.cm-2. Also within the scope of this thesis, the developed cobalt-hardened gold electrolyte [4 g/L KAu(CN)2, 1 g/L CoSO4, 90 g/L potassium citrate, 60 g/L citric acid, 3 g/L imidazolidin-2-tion] with the content of 99.9 wt.% gold and nonporous microstructured deposit properties can be applicable for electronics industry. The above listed electrolytes for mirror bright coatings of silver, gold and gold alloys (24 carat cobalt-hardened gold, 19 carat and rose gold-copper coating, 23 carat fine gold-copper alloy coating, 21 carat green gold-copper alloy coating) are convenient for decorative and jewelry / accessory industries. This thesis is a pioneering study in the precious metal electrodeposition in Turkey. With the gained knowledge, the electrolyte compositions developed here established a well-suited basis to reduce imports and external dependence of the internal market.