Electrolytic deposition of gold-containing alloys

Abstract: A gold-copper alloy of good corrosion resistance is electrolytically deposited from a bath containing a cyanide compound of gold (e.g. KAu(CN)₂), a cyanide compound of copper and tellurium or bismuth in the form of a soluble salt (e.g. sodium tellurite), the bath being substantially free of any other metal in an electrolytically depositable form. The bath generally also contains a cyanide salt (e.g. KCN) and may also contain a surface-active agent. Foreign metals, e.g. zinc, may be chelated or complexed to prevent deposition thereof.

Description: Field of the Invention The present invention relates to a bath for the electrolytic deposition of a gold-copper alloy, to a method of electroplating articles using such a bath and to electroplated articles obtained by that method. Background to the Invention There have been a number of proposals in the art to plate articles with an alloy of gold, copper and at least one other metal by electrolytic deposition. The alloys, in general, are yellow to pink in colour and, although applied mainly for decorative purposes, it is clearly desirable that the deposited alloy should exhibit a good resistance to corrosion. A number of proposals have involved the deposition of a gold-copper-cadmium alloy. DE-A-2,221,159, for example, discloses the use of electrolytic baths which contain cyanide complexes of gold and copper, cadmium, and a chelating agent, in the presence of free cyanide, and/or a polyoxyalkylene compound, which baths may also contain a soluble selenium or tellurium salt and a buffer salt such as a carbonate or phosphate. US-A-4,687,557 discloses a bath for the electrolytic deposition of a gold-copper-cadmium-zinc alloy, which bath contains cyanide complexes of gold, copper, cadmium and zinc, together with at least one complexing agent and a surface-active agent. These baths may also contain, as an inorganic brightener, a soluble salt containing an element selected from selenium, tellurium, vanadium, arsenic, antimony, thallium, bismuth, titanium, germanium, zirconium, tantalum and niobium. Such brighteners, examples of which include sodium selenite and potassium vanadate, are used at low concentrations, in particular 0.01 to 100 mg/l. There is, however, increasing concern over the toxicity of cadmium. Reflecting such concern, it is proposed in US-A-4,358,351 to employ an aqueous alkaline bath comprising soluble gold cyanide, soluble copper cyanide, potassium carbonate (or potassium bicarbonate) and, instead of cadmium, a complex zinc cyanide, eg. K₂(Zn(CN)₄), in an amount of from 7.5 to 40g/l. More recently, it has been proposed in GB-A-2,151,661 to deposit a yellow to pink-coloured gold-copper-zinc alloy of low carat value from a bath containing at least 2 g/l gold, at least 10 g/l copper and at least 5 g/l zinc in the form of an alkali metal-zinc chelate. The concentration of chelate-formers is above 20 g/l, the bath having a pH value within the range 9 to 12 and also containing sodium ions at a concentration greater than 20 g/l and below the solubility limit thereof. US-A-4,617,096 teaches that light yellow, glossy and ductile gold-indium alloys which have a good resistance to gradual corrosion by silver sulfide are obtained from electrolytic (galvanic) baths containing 1 to 20 g/l of gold in the form of alkali metal or ammonium tetracyanoaurate (III), 0.5-50 g/l of indium in the form of a water-soluble indium salt, a buffer or conducting salt and 0.5 to 10 mg/l of selenium and/or tellurium. Japanese patent application JP62-164889 discloses an electrolytic bath containing 7-16 g/l gold, 2-4 g/l silver and 10-50 g/l copper (each of these metals being in the form of an alkali metal cyanide), 10-100 g/l alkali metal cyanide and, as a brightener, 0.0001-5 g/l tellurium in the form of a water-soluble compound thereof. European patent application EP-A-0 304 315 (a document falling within the terms of Art. 54(3) EPC), discloses a bath for the electrolytic deposition of a gold-copper-zinc alloy, which bath contains gold, copper and zinc, each in the form of a cyanide complex thereof, at least one surface-active agent and a water-soluble tellurium salt and/or a water-soluble bismuth salt. The said bath may also comprise a non-cyanide organic complex of zinc, a conductive salt, a depolarizing agent and an alkali metal (or ammonium) cyanide. The presence of the tellurium and/or bismuth is believed to improve the crystallization of the gold-copper-zinc alloy deposit such that it resists corrosion, in particular as measured by the nitric acid test (ISO Standards 4524/1 and 4524/4). The disclosure in that European patent application is incorporated herein by reference. Summary of the Invention It has now been found that it is possible to omit electrolytically depositable zinc from baths of the type disclosed in the aforesaid European patent application and yet still obtain alloy coatings that are bright and resistant to corrosion, provided that tellurium (and optionally also bismuth) is (are) also present in the bath. This is unexpected for, as can be seen from the prior art discussed above, it has hitherto been considered essential to include a metal such as cadmium, zinc or silver in alloys containing gold and copper in order to obtain a satisfactory resistance to corrosion. Accordingly, the present invention provides an aqueous bath for the electrolytic deposition of a gold-copper alloy, which bath contains (a) gold in the form of a cyanide compound, (b) copper in the form of a cyanide compound, (c) tellurium in the form of a water-soluble compound and optionally (d) bismuth in the form of a water-soluble compound, the bath containing less than 0.1g/l, in total, of any other metal in an electrolytically depositable form. The present invention also provides a method of coating a substrate with a gold-copper alloy, wherein the said substrate and an electrode are immersed in a bath according to this invention and an electric current is passed through said bath between the substrate and electrode. The present invention also provides an article having at least one surface that has been coated with a gold-copper alloy by means of such a method. Description of Preferred Embodiments The bath according to the present invention contains gold preferably in the form of a cyanide complex. Preferred complexes are the alkali metal (eg. sodium or potassium) and the ammonium gold cyanides, eg. KAu(CN)₂. Preferably, the gold is present in an amount of from 0.5 to 20 g/l, more preferably from 3.0 to 6.0 g/l. The present baths also contain copper and this may be in the form of a cyanide complex, preferred complexes being the alkali metal (eg. sodium or potassium) and the ammonium copper cyanides, eg. K₂Cu(CN)₃. Copper oxide, CuO, copper carbonate, CuCO₃, or copper cyanide, Cu(CN)₂, may, for example, be employed as the source of copper although these may require the presence of sufficient cyanide to solubilize them. Copper cyanide is the preferred source of copper. Preferably, the copper is present in the bath in an amount of from 0.1 to 30 g/l, more preferably from 10.0 to 20.0 g/l. A further component of the baths according to the present invention is tellurium, in the form of a soluble salt thereof. Suitable salts include the alkali metal, ammonium or amine salts with tellurium-containing anions, eg. sodium tellurite or potassium tellurite. However, in this context, the expression "soluble salt" also extends to complex salts with carboxylic acids, amino acids, aminocarboxylic acids and the like. In general, the tellurium is present in an amount of from 1 to 2,000 mg/l, preferably from 1 to 100 mg/l when the tellurium is in the tetravalent form or from 200 to 2,000 mg/l when the tellurium is in a hexavalent form. It is particularly preferred that the tellurium should be in the tetravalent form and present in an amount of from 5 to 28 ppm (mg/l). In other embodiments the tellurium may be replaced, in part, by bismuth in the form of a water-soluble salt thereof. However, although bismuth has been found to impart corrosion resistance to the gold-copper alloys it has also resulted in alloys of poor (i.e. dull and brownish) appearance. For that reason it is preferred to employ tellurium in the substantial absence of bismuth. It appears that tellurium (and bismuth if present) may be co-deposited with the gold and copper and, although the Applicant does not wish to be bound or limited by any hypothesis herein, it is believed that this contributes to the corrosion resistance of the alloy deposit despite the substantial absence of zinc or cadmium. This effect could not be predicted from the known use of tellurium or bismuth as a brightener in certain alloys. Nevertheless, the amount of any co-deposited tellurium (and bismuth if present) is very small and the deposited coating therefore is essentially a gold-copper alloy. The baths of the present invention are essentially free of metals (other than gold, copper, bismuth and tellurium) which are in electrolytically depositable form. In particular, the bath contains less than 0.1 g/l, preferably less than 0.01 g/l, of total electrolytically depositable metal other than gold, copper, bismuth and tellurium. It is particularly preferred that there should be no deliberate addition to the bath of such other electrolytically depositable metal, eg. cadmium, zinc or silver, although, of course, it is possible that traces of such metals may be introduced as impurities in the main components of the bath, or may dissolve out of the substrate or workpiece to be coated. Notwithstanding the preceding discussion, it is, in principle, possible for a bath according to the present invention to contain more than a trace amount of a metal other than gold, copper, bismuth or tellurium, provided that it is in such a form that there is no significant deposition of the metal under the normal conditions of use of the bath. For example, it has been found that zinc can be held in solution in the bath by means of a sufficiently strong chelating agent, eg. nitrilotriacetic acid (NTA). Preferably, the baths according to the present invention also contain free cyanide salt selected from alkali metal and ammonium cyanides, amongst which potassium cyanide is particularly preferred. If employed, such cyanide salt is preferably present in a concentration of up to 20 g/l, more preferably from 5 to 15 g/l. Normally, the level of free cyanide is governed by the copper concentration: the higher the copper level the higher should be the level of free cyanide. It may be mentioned that cyanide ions constitute a powerful complexing agent. Thus the addition of free cyanide will generally convert simple cyanide salts, e.g. copper cyanide, into complex salts and may also serve to prevent unwanted deposition of foreign metals (e.g. small amounts of zinc and, possibly, tin or lead which may have entered into solution from a brass substrate). The baths of this invention preferably contain a surface-active or wetting agent. It is possible to employ, singly or in combinations, commercially available products selected from nonionic, anionic, cationic and amphoteric surfactants. In particular, amidopropyldimethylamino oxides of fatty acids, dimethylamino oxides of saturated fatty acids, dimethylalkylamino oxides and bis(2-hydroxyethyl) alkylamino oxides, and the like, may be advantageously employed. It is also possible advantageously to employ as a wetting agent an amidoaminopropionate derivative in the form of the ampholytic ion (zwitterion) having the following formula where R is a C₈ to C₂₀ alkyl group and R' is a hydrogen atom or a -CH₂COOH residue. These derivatives are known as amphopropionates; by way of examples there may be mentioned the cocoamphopropionate and cocoamphocarboxypropionate, which are marketed under the trademark "Miranol". The surface-active agent is preferably used in an amount of from 0.1 to 20 ml/l, more preferably from 0.5 to 10 ml/l, typically 2 ml/l. To increase the conductivity of the bath between the electrodes, a conducting salt may be added, for example sodium, potassium or ammonium salts of carboxylic, hydroxycarboxylic, amino or aminocarboxylic acids, such as acetic, formic, succinic, tartaric, citric, hydroxyacetic, glycolic, malonic, maleic, mandelic, gluconic or heptonic acid. A particularly preferred conducting salt is sodium potassium tartrate (Rochelle salt). The conducting salt, if used, is preferably present in an amount of up to 100 g/l, more preferably from 50 to 70 g/l, and typically 60 g/l. Although it has not been found to be particularly advantageous, the addition to the bath of one or more conventional brighteners is not precluded herein. Amongst these are such primary brightening agents as sodium rhodanide, urea, certain urea derivatives and saccharin and such secondary brightening agents as butynediol, pent-2-yne-1,4-diol and but-1-yne-3-ol. As indicated above, the chelating agent NTA may be included in the bath: in addition to, or instead of, NTA any of the other conventional chelating agents such as EDTA may also come into consideration. However, the use of a chelating agent to hold foreign metals in solution is often not necessary since the baths commonly contain free cyanide which, as mentioned above, can prove effective for such a purpose. Any of the components of the bath discussed above can, of course, be constituted by a mixture of compounds of the appropriate description. It is, of course, desirable that each component should, in the amount used, be completely soluble or solubilized in the aqueous medium of the bath. The pH of the bath is preferably from 7 to 12, more preferably from 9.5 to 10.5. The baths according to the present invention may be used in a conventional manner for the deposition of the gold-containing alloy onto the appropriate surfaces ofarticles to be plated. The bath will normally be operated at a temperature between 50° and 75°C, especially at a temperature of from 55 to 65°C. The current density will usually be from 0.5 to 2.5 A/dm², preferably from 0.6 to 1.1 A/dm². Preferably, the anode(s) will be of platinum or platinized titanium. With such anodes and when using the comparatively low current densities usual for decorative plating, it is possible to dispense with the use of a depolarising agent, although the use of such an agent (as described in EP-A-0 304 315) may be advisable when using anodes of stainless steel or the like and/or when using high current densities. A wide variety of articles may be used as substrates. For example, the pink alloys that can be obtained in the practice of this invention are suitable as decorative coating on spectacle frames. Examples The present invention is illustrated in and by the following specific examples. In each case, small polished brass panels were used as the substrates (cathodes) onto which the gold-containing alloy was electrolytically deposited, using a platinum anode. The corrosion resistance of the resultant alloy deposit or coating was measured by various tests, including the following. The nitric acid test was carried out by placing one drop of cold 50% v/v HNO₃ on the back of the test panel. To pass this test, there shall be no attack on the deposit after 30 seconds have elapsed. The copper chloride test is carried out by placing one drop of saturated copper chloride solution on the back of the test panel. To pass this test, there shall be no attack on the deposit after 30 seconds have elapsed. Example 1 An aqueous bath was prepared in accordance with the following formulation: Gold (added as potassium gold cyanide) 3 g/l Copper (added as copper cyanide) 14 g/l Potassium cyanide 25 g/l Sodium potassium tartrate 60 g/l Tellurium (added as sodium tellurite) 20 mg/l Cu 84 (surfactant) 2 ml/l A gold-copper alloy was deposited onto test panels under the following operating conditions: pH 10.5; current density 0.8 A/dm²; temperature 60°C. A 16.8 carat deposit was obtained that was pale pink and fully bright in appearance. The corrosion resistance in the nitric acid test and the copper chloride test was deemed to be a borderline pass. Example 2 The procedures of Example 1 were followed except that the concentration of gold (added as potassium gold cyanide) was 5 g/l. A 21.8 carat deposit was obtained which was fully bright in appearance but of a paler pink colour than the deposit obtained in Example 1. The deposit passed both the nitric acid test and the copper chloride test for corrosion resistance. Example 3 The procedures of Example 1 were followed except that the concentration of the gold (added as potassium gold cyanide) was 7 g/l. A 23.0 carat deposit of alloy was obtained, which was gold in colour. The deposit passed both the nitric acid test and the copper chloride test for corrosion resistance. Example 4 A bath of the following formulation was prepared: Gold (added as potassium gold cyanide) 7 g/l Copper (added as copper cyanide) 20 g/l Potassium cyanide 30 g/l Sodium potassium tartrate 60 g/l Zinc carbonate 9.5 g/l Nitrilotriacetic acid 20 g/l Cu 84 (wetting agent) 2 ml/l Tellurium (added as sodium tellurite) 15 mg/l The copper cyanide formed a complex with (and was hence solublized by) cyanide from the KCN, thereby reducing the free cyanide content in the bath to 2.5 g/l. Alloy was electrolytically deposited on test panels using a bath of the above formulation under the following operating conditions: pH 10.5; current density 0.8 A/dm²; bath temperature 60°C. No zinc was detected in the deposit, which was of 12.6 carat. The deposit was fully bright and pale pink in appearance and passed the copper chloride test for corrosion resistance. Example 5 The procedures of Example 4 were repeated except that the amount of zinc carbonate was increased to 19 g/l and the amount of NTA was increased to 40 g/l. These two ingredients were reacted in water to form a Zn-NTA complex which was then added to the bath, giving a concentration of zinc (Zn) of 5 g/l. The resultant bath had a pH of 10.0. No zinc was found in the resultant deposit of alloy, which was of 14.6 carat. The deposit was bright and was a paler pink in colour than the deposit obtained in Example 4. The deposit passed the copper chloride test for corrosion resistance. Example 6 The procedures of Example 4 were repeated, except that the content of free cyanide was increased from 2.5 to 16.5 g/l. No zinc was detected in the resultant alloy deposit, which was of 22.3 carat. The deposit was slightly bloomed and golden in appearance and it passed the copper chloride test for corrosion resistance. Example 7 An aqueous bath was prepared in accordance with the following formulation: Gold (added as potassium gold cyanide) 5 g/l Copper (added as copper cyanide) 21 g/l Potassium cyanide 35 g/l Sodium potassium tartrate 60 g/l Tellurium (added as sodium tellurite) 20 mg/l Cu 84 (surfactant) 2 ml/l A gold-copper alloy was deposited onto test panels under the following operating conditions: pH 10.0; current density 0.8 A/dm²; temperature 60°C. A gold-copper alloy deposit of 19.8 carat was obtained. The deposit was fully bright and pale pink in appearance and passed the copper chloride test for corrosion resistance. The alloy deposit also showed corrosion resistance in the nitric acid test, the conventional sweat test and the Leeds and Clark test. Example 8 The aqueous bath disclosed in any of the preceding Examples may be modified by the inclusion of saccharin in an amount of 2 g/l. This addition of saccharin has been found to reduce slightly the internal stress of the deposits obtained, to improve the appearance of the deposits and to widen slightly the bright plating range; it is especially useful in electroforming. It is envisaged that this additive will commonly be used in an amount of from 0.5 to 5 g/l. It will of course be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Claims: An aqueous bath for the electrolytic deposition of a gold-copper alloy, which bath contains (a) gold in the form of a cyanide compound, (b) copper in the form of a cyanide compound, (c) tellurium in the form of a water-soluble compound and optionally (d) bismuth in the form of a water-soluble compound, the bath containing less than 0.1 g/l, in total, of any other metal in an electrolytically depositable form. An aqueous bath according to claim 1, characterised in that the gold is present in the bath in the form of a cyanide complex, preferably an alkali metal or ammonium gold cyanide. An aqueous bath according to claim 1 or 2, characterised in that the gold is present at a concentration of 0.5 to 20 g/l. An aqueous bath according to claim 1, 2 or 3, characterised in that the copper is present in the bath in the form of a cyanide complex, preferably an alkali metal or ammonium copper cyanide. An aqueous bath according to any of claims 1 to 4, characterised in that the copper is present at a concentration of from 0.1 to 30 g/l. An aqueous bath according to any of claims 1 to 5, characterised in that the tellurium is introduced into the bath in the form of an alkali metal, ammonium or amine tellurite. An aqueous bath according to any of claims 1 to 6, characterised in that the tellurium is present at a concentration of from 1 to 2,000 mg/l. An aqueous bath according to any of claims 1 to 7, characterised in that it also contains a surface-active agent. An aqueous bath according to claim 8, characterised in that the surface-active agent is present at a concentration of from 0.1 to 20 ml/l. An aqueous bath according to any of claims 1 to 9, characterised in that it also contains a free cyanide salt selected from alkali metal and ammonium cyanides. An aqueous bath according to claim 10, characterised in that the free cyanide salt is present in the bath at a concentration of up to 20 g/l. An aqueous bath according to any of claims 1 to 11, characterised in that it contains a conducting salt selected from alkali metal and ammonium salts of carboxylic, hydroxycarboxylic, amino or aminocarboxylic acids, at a concentration of up to 100 g/l. An aqueous bath according to claim 12, characterised in that the conducting salt is sodium potassium tartrate. An aqueous bath according to any of claims 1 to 13, characterised in that it contains zinc which is sufficiently strongly chelated or complexed to prevent electrolytic deposition thereof. An aqueous bath according to any of claims 1 to 14, characterised in that the bath contains less than 0.01 g/l, in total, of electrolytically depositable metal other than gold, copper, tellurium or bismuth. An aqueous bath according to any of claims 1 to 15, characterised in that bismuth is absent or substantially absent. A method of coating a substrate with a gold-copper alloy, wherein the said substrate and electrode are immersed in a bath and an electric current is passed through said bath between the substrate and the electrode, characterised in that the bath contains (a) gold in the form of a cyanide compound, (b) copper in the form of a cyanide compound, (c) tellurium in the form of a water-soluble compound and optionally (d) bismuth in the form of a water-soluble compound, the bath containing less than 0.1 g/l, in total, of any other metal in an electrolytically depositable form. A method according to claim 17, characterised in that the bath is according to any of claims 2 to 16. An article having at least one surface that has been coated with a gold-copper alloy by a method according to claim 17 or 18.

Inventors: GREEN REBECCA VICTORIA, PEACEY SALLY ANN, WILKINSON PETER, GREEN, REBECCA VICTORIA, PEACEY, SALLY ANN, WILKINSON, PETER
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1990
Main classification: C25D3
IPC codes: C25D3