SOLUTION AND PROCESS FOR THE ELECTRODEPOSITION OF GOLD AND GOLD ALLOYS

Abstract: Gold and gold alloys products are electrodeposited from a solution free from cadmium and from free cyanide ions in which EDTA, DTPA or a similar chelating agent is used as complexing agent; the solution can comprise ceramic material that is co-deposited with the gold ions to impart improved mechanical and physical properties to the final products.

Description: SOLUTION AND PROCESS FOR THE ELECTRODEPOSITION OF GOLD AND GOLD ALLOYS. Field of the Invention The present invention relates to a solution and a process for plating of pieces and parts with gold and gold alloy and for eiectroforming items in gold and gold alloys. Description of the prior art Plating solutions (or baths) are known in the art and are used to electrodeposit a layer of a gold alloy on a substrate, e.g. electronic contacts, or to electroform pieces such as earrings and jewelry in general. A widely known and used solution for eiectroforming comprises Au, Cu and Cd ions and free CN ions that are used as complexing agents. In a known process to electroform pieces from these solutions, plastic and low melting models are made conductive and eventually immersed in the solution. A current is then passed through the solution bath to provide a deposition of Au on the models. This process has several drawbacks. The quantity of Au deposited with respect to the other species present in the solution is greatly dependent on the operating conditions, i.e. shape of the pieces and their position with respect to the anode(s), current distribution, flow conditons etc. . To check that the deposited content of gold is within the range requested for the final product, the manufacturer has to stop the deposition process and weigh the cathode. This means that, in order to comply with the declared Au content of the final product, the manufacturer has to use operating conditions that in practice give a higher content of gold to the products, i.e. a higher cost. A further drawback is the presence of cadmium in the products. Cd has been banished from many products because of its toxicity: earrings and several jewels are in constant contact with the user's skin. Even more important is to have cadmium-free products for use in dentistry. The presence of Cd in the bath is also undesirable and is a serious health hazard for workers. Unfortunately it is not possible to remove cadmium from the above r.n disclosed solutions without jeopardizing their efficiency. Another problem is due to presence of free cyanide ions in the solution; this makes the bath dangerous in more than one way, e.g. when the solution pH is adjusted a small error could result in cyanide gas being freed from the solution. A further problem is that electrodeposited Au-Cu alloys undergo severe decay of their mechanical properties, with typical failures such as cracking, under simple storage at room temperature for some days. Summary of the Invention It is an aim of this invention to solve the above problems by means of a solution formulation and a process for electrodepositing and eiectroforming gold and gold alloys. In the following description the wording "electrodepositing" and "electrodeposited product" will be used to mean eiectroforming and electroplating, as well as an electroformed product (e.g. jewels or products for dentists) and an electroplated product (e.g. electronic contacts). It is an object of the invention a solution for gold electrodeposition according to claim 1. In a preferred aspect of the invention, the complexing agent is selected from EDTA and DTPA (most preferably DTPA), or mixtures thereof, and the solution is substantially free from cadmium ions and from free cyanide ions. According to another embodiment of the invention, cetylpyridinium chloride is added to the bath. It was surprisingly found that it is possible to use solutions (i.e. baths) free from Cd ions and free cyanide ions if EDTA, DTPA or similar chelating agents are used as a complexing agent for the electroactive ions in said solution. The use of tri, tetra and, preferably, pentadentate chelating agents and salts thereof, namely EDTA, DTPA and salts thereof, also results in a solution that is less subject to changes of operating conditions. It is therefore possible to operate closer to the required content of gold in the final product, with considerable savings, and it is no longer necessary to interrupt the process to weigh the products for control purposes. The solution contains Au + and Cu ++ and the plated or formed product will comprise an alloy Au/Cu, free from cadmium. It was also found that the alloy deposition is substantially homogeneous and that accordingly the product is also homogeneous and substantially free from dramatic discontinuities in the composition of Au-containing product. More particularly, the addition of cetylpyridinium chloride monohydrate reduces drastically the amount of hydrogen embedded into the deposited metal. The use of DTPA also concurs to this effect, thus removing the above- mentioned embrittlement problems. Description of the preferred embodiment EDTA (Edetic acid, ethylene-diaminetetraacetic acid, CAS registry number [60-00-4]) is a well known and widely used tetradentate chelating agent. DTPA is a pentadentate chelating agent (pentetic acid, diethylenetriaminepentaacetic acid, CAS registry number [67-43-6]). EDTA and DTPA are generally used in the form of salts, particularly sodium salts. The concentration of EDTA or DTPA is preferably within the range of 1/1 to 2.5/1 (moles) with respect to the content of Cu of the solution; i.e. there is from an equimolar amount of Cu and EDTA (EDTA/Cu=l ) to EDTA/Cu = 2.5, the same range applying also for DTPA. Cetylpyridinium chloride monohydrate (1-hexadecylpyridinium chloride, CAS registry number [6004-24-6]) is a known disinfectant and preservative; the amount of cetylpyridinium is within the range of 10 to 200 ppm. The other conditions of the electroplating process are as follows: [Au + ] within the range of 2 to 50 g/l; [Cu 2+ ] within the range of 0.5 to 50 g/l; current density within the range of 2 to 100 mA/cm 2 and pH within 5 to 8. A typical Au containing compound to be used for preparing the solution according to the invention is KAuCN 2 ; a Cu containing compound for the solution is typically CuO. The temperature of the bath is preferably within the range of 30 to 70°C. In a preferred aspect, the products are preferably vacuum degassed, to improve their resistance by removing hydrogen possibly entrapped within the deposited material. Alternatively, degassing can be carried out by heating of the products: thermal treatments of this kind are known and commonly used in the art. DTPA is preferably used as chelating agent, especially in combination with cetylpyridinium: with the combined use of these compounds in the solution for the plating bath the degassing step can be eliminated; in any case a traditional thermal treatment will be more than sufficient to completely degas the products thus obtained. The efficacy of cetylpyridinium and of DTPA was confirmed through tests carried out by analyses of the gas evolved from electroformed Au-Cu alloys. Analyses of the gas evolved from traditionally electroformed Au-Cu alloys subjected to isothermal heat treatments show a release of hydrogen in two temperature ranges: the first at about 400-500 °C (low activation energy trap), the second at about 650-700°C (high activation energy trap). The hydrogen is embedded within the alloy during electrodeposition of same. The effect of the surfactant (cetylpyridinium) was evaluated by monitoring the amount of incorporated hydrogen evolved during high temperature heat treatments with mass-spectro etric and LECO techniques. Cetylpyridinium chloride additions reduce drastically the amount of hydrogen present in low temperature traps: from about 50 ml/100 g to 3-4 ml/100 g. The action of cetylpyridinium chloride is extremely specific: several cationic and anionic surfactants of very similar structure were proved to be ineffective as far as hydrogen entrapment in the low- temperature trap is concerned. In some cases, the anionic surfactant itself and/or its decomposition products can be incorporated into the electrodeposited material. For more information concerning these analyses techniques see e.g. B.Bozzini et al, Eng.Fail.Ana 1999,vol.6, page83. When DPTA is added to the solution to be used as the chelating and complexing agent for electroactive ions, a complete disappearance of hydrogen incorporation in the high temperature trap occurs. The combined action of DTPA and cetylpyridinium chloride reduces the overall content of embedded hydrogen to levels of less than about 10 ml/ 100 g. It was also found that it possible to dramatically increase the mechanical properties of electrodeposited gold and gold alloys products if a ceramic material is dispersed in the solution. The ceramic material is preferably a carbide and most preferably is Boron carbide (B 4 C); a preferred form of the ceramic material is particles (powder) or fibers. In practice, the ceramic material is such that it is compatible with the complex of Au and/or Cu, i.e. the ceramic material is such that the electroactive complex containing gold and/or copper (i.e. the gold and/or copper complex that will deposit from the solution) can adsorb onto the surface of the ceramic material. The preferred concentration of Boron carbide in the solution is within 0.5 to 60 g/l, preferably from 2 g/l to 50 g/l; the particle size of the ceramic material - and in particular for boron carbide - is preferably within the range of 0.1 to 10 μm. The adsorbed electroactive complex will then be electrodeposited from the solution together with the particle of ceramic material. The resulting product has improved physical and mechanical properties, especially wear resistance, with respect to the traditionally obtained products, and a better ageing behaviour. In practice it was found that the thickness of a product obtained according to the invention can be approximately half the thickness of a corresponding product obtained according to known processes, while giving the same performances. The presence of boron carbide or other corresponding suitable ceramic material can impart improved characteristics also to products obtained from Au/Cu/Cd solutions containing also free CN- ions (i.e. from known solutions). A further object of the invention is therefore a gold electroplating solution according to claim 6. Another object of the invention is a process according to claim 10. As above mentioned, the concentration range of EDTA or DTPA is from equimolar to 2.5 times the moles of Cu and that of boron carbide is within 0.5 to 60 g/l. The other conditions of the electroplating process are as follows: [Au + ] within the range of 2 to 50 g/l; [Cu 2+ ] within the range of 0.5 to 50 g/l; current density within the range of 2 to 100 mA/cm 2 and pH within 5 to 8, with a preferred bath temperature of about 30 to 70 °C. In a preferred aspect, the products are degassed, preferably vacuum degassed, to improve their resistance by removing hydrogen possibly entrapped within the deposited material. Alternatively, degassing can be carried out by heating of the products. Further objects of the invention are products, in particular Au-Cu and Au-Cu-Cd products, according to claim 14 or 15. A further feature of the process is that the solution is stirred in order to impart a correct flow to the solution. More specifically, it was found that in order to obtain good results, especially if the solution contains a ceramic material, the flow of the solution should be substantially perpendicular to the surface of the model. In other words, the flow should be substantially perpendicular to the surface area on which the gold or gold alloy should be deposited. In order to do this, on the average every portion of the surface is subjected to a substantially perpendicular flow of solution for a same amount of time, so as to give a substantially uniform deposition of the gold alloy. The invention will be further disclosed with reference to the following examples. The reactor was a cylinder provided with temperature control. Three basic solutions were prepared, namely Composition (g/l) A B C Au (as KAu(CN) 2 ) 7.5 7.5 7.5 Cu (as CuO) 2.5 2.5 2.5 EDTA disodic salt monohydrate 17 19.5 21 citric acid 60 40 30 ammonium citrate 20 40 60 For each bath three different concentrations of B C were prepared: B 4 C (1500 mesh) at 2 g/l; 10 g/l and 50 g/l. The deposition substrate (cathodic material) was a plurality of hollow roundish supports made of copper, previously obtained by lost-wax forming. The supports were mounted on a rotatable carrousel. The anode was platinized titanium. The solution was stirred by the combined action of a magnetic stirrer and by rotation of the cathode at about 80 rpm. The bath temperature was of about 50 °C and its pH was within the range of 6.5 to 6.7. The current density was 12-14 mA/cm 2 and deposition speed was 22/28 μm/h. The deposition time was set to obtain, in the above operating conditions, deposits 120-200 μm thick. Immediately after the deposition step is finished, all the cathodes were put in an electric tubular oven connected with a vacuum source. The oven was progressively heated from room temperature to 370+5 °C at a pressure of about 10- 7 mbar. This temperature and pressure were maintained for about three hours. The products were eventually removed from the supports by means of nitric acid. The amount of carbon boride particles incorporated within the deposit of gold alloy was within the range of 18% to 35% by volume and the specific weight of the products was within the range of 14.7 to 12.3 g/cm 3 . The matrix alloy title was not checked during the deposition process; nevertheless, the title of all the products was within the range of 750/1000 to 780/1000, i.e. consistent with the planned value of 750/1000 (18 carats). The same kind of products were obtained from baths A, B and C without addition of ceramic material. These products, free of Cd, showed a uniform distribution of the alloy components throughout their section and the same range of values of the matrix alloy title. The hardness (Vickers microhardness - HV0.025) of products with and without ceramic material was determined: products incorporating the ceramic material showed hardness values that were 20-25% higher than the values of corresponding products without the ceramic material. It was also found that vacuum degassing is preferable to degassing only by heating; vacuum degassed samples have better values of fracture toughness.

Claims: CLAIMS 1. A solution for electrodeposition of gold and gold alloys, free from cyanide free ions, comprising gold ions and at least one complexing agent characterised in that said at least one complexing agent is a metal chelating agent. 2. A solution according to claim 1 , wherein said agent is a tridentate and/or tetradentate and/or pentadentate chelating agents. 3. A solution according to claim 2, wherein said chelating agent is selected from EDTA, DTPA and their salts, or mixture thereof. 4. A solution according to any previous claim, comprising copper ions and being free from cadmium ions. 5. A solution according to any previous claim further comprising a ceramic material such that an electroactive gold and/or copper complex can adsorb on said ceramic material. 6. A solution for electrodepositing gold or gold alloys, comprising a complexing agent and gold ions, characterised in that it also comprises a ceramic material such that an electroactive gold and/or copper complex can adsorb on said ceramic material. 7. A solution according to claim 5 or 6, wherein said ceramic material is a carbide. 8. A solution according to claim 7, wherein said ceramic material is boron carbide. 9. A solution according to any previous claim, further comprising cetylpyridinium chloride. 10. A process of electrodepositing gold and gold alloys from a solution, characterised in using a solution according to any claim 1 to 9. 1 1. A process according to claim 10, wherein said solution is stirred and on average every portion of the surface is subjected to a substantially perpendicular flow of solution for a same amount of time, so as to give a substantially uniform deposition of the gold or gold alloy. 12. The use of tri, tetra and/or pentadentate chelating agents as complexing agents in a process of electrodeposition of gold or gold alloys according to claims 10 or 1 1. 13. The use of ceramic materials onto which electroactive gold- and/or copper-containing complexes can adsorb, in a process of electrodeposition of gold or gold alloys according to claims 10 or 1 1. 14. A gold or gold alloy electrodeposited product as obtainable through a process according to claims 10 or 1 1, characterised in that it is substantially free from cadmium and has a substantially homogeneous metallographic structure. 15. A gold or gold alloy electrodeposited product, characterised in that it contains a ceramic material. 16. A product according to claim 15, wherein said ceramic material is boron carbide.

Inventors: BOZZINI BENEDETTO, GIOVANNELLI GIUSEPPE, BOZZINI, BENEDETTO, GIOVANNELLI, GIUSEPPE
Applicants: BOZZINI BENEDETTO, GIOVANNELLI GIUSEPPE, HALF TONE LIMITED, BOZZINI, BENEDETTO, GIOVANNELLI, GIUSEPPE
Application year: 1999
Main classification: C25D3
IPC codes: C25D3