Electrodeposit of a pink gold alloy, its preparation and electroplating bath

Abstract

 Electrodeposition of a hard, bright, pore-free ductile gold alloy having a pink coloration, useful for decorative purposes, is effected using an aqueous cyanide-free electroplating bath essentially comprising:

(a) from 4 to 20 g/l of gold,

(b) from 0.1 to 3.0 g/l of copper,

(c) from 0.2 to 5.0 of palladium,

(d) from 5 to 200 g/l of free sulphite ion,

(e) from 0 to 150 g/l of one or more buffering agents and/or conducting salts,

(f) from 0.001 to 0.5 g/l of a brightening element selected from arsenic, antimony and thallium,

(g) from 0.01 to 5.0 g/l of a surface active agent, and

(h) water.

The bath has a pH in the range from 7 to 11.5, preferably about 9.5 and electrodeposition of the pink alloy can be carried out at a cathode current density of from 0.1 to 1.5 amp/dm² and a temperature of40 to 80°C, preferably with moderate agitation of the cathode during the electroplating process. The pink alloy will normally contain from 2 to 8 parts by weight copper, from 2 to 6 parts by weight palladium and the balance gold.
Such a bath is less prone to the colour stability and pollution problems encountered with conventional electroplating baths for producing pink gold electrodeposits.

Description

[0001] This invention relates to the electrodeposition of pink gold alloys and is concerned with an electroplating process and bath which can be used to obtain a pink gold alloy deposit. Such pink deposits are useful for decorative purposes, for example in the manufacture of spectacle frames.

[0002] Hitherto electroplating processes for obtaining pink gold alloy electrodeposits have generally been based on acid gold cyanide-containing electroplating baths, for example that described in British Specification No. 1,224,507, but bright gold alloy deposits having a pink coloration are generally difficult to obtain because the presence in the bath of copper, which is the main alloying metal producing the pink coloration, makes the bath sensitive to operation parameters, especially changes in concentration. Hence uniformly reproducible, hard, bright gold-copper alloy deposits having a pink coloration can normally only be obtained from such acid cyanide baths by careful regulation of the bath components and operating conditions. Another disadvantage of such acid cyanide baths arises from the necessity of treating the spent baths after use to remove any remaining cyanide and thereby provide a cyanide-free effluent which will not give rise to any pollution problems.

[0003] Recently there has been disclosed an electroplating bath for producing a pink gold-alloy deposit which is based on an amino-gold sulphite complex formed by reacting a trivalent gold compound with an amine and then reacting the resulting product with an alkali metal or ammonium sulphite in at least twice the molar quantity, as described in British Specification No. 1,325,352. However, in practice, this bath is not easy to operate and is moreover relatively expensive. It also has the disadvantage that it can easily be poisoned by airborne or dragged-in cyanide contamination which gives rise to colour stability problems.

[0004] It is therefore an object of the present invention to provide an electroplating process and bath capable of producing a hard, bright gold alloy electrodeposit having a pink coloration and which do not have the disadvantages of known acid gold cyanide-containing electroplating processes and baths for obtaining pink gold alloy deposits especially the pollution problems, or the disadvantages of the aforementioned amino-gold sulphite complex based bath.

[0005] According to one aspect of the invention, there is provided an aqueous cyanide-free electroplating bath for depositing a hard, bright gold alloy electrodeposit having a pink coloration, the bath comprising the following essential constituents:

(a) from 4 to 20 g/1 of gold,

(b) from 0.1 to 3.0 g/1 of copper,

(c) from 0.2 to 5.0 g/l of palladium,

(d) from 5 to 200 g/1 of free sulphite ion,

(e) from 0 to 150 g/1 of one or more buffering agents and/or conducting salts,

(f) from 0.001 to 0.5 g/1 of a brightening element selected from arsenic, antimony and thallium,

(g) from 0.01 to 5.0 g/1 of a surface active agent, and

(h) water.

[0006] The pH of the bath should be in the range from 7 to 11-.5, advantageously 8.5 to 11.5 and preferably abot 9.5. To maintain the pH at the required value, sufficient of an alkali, for example potassium hydroxide, or of a weak acid, for example citric acid, is added as appropriate, for example in the form of a 10% aqueous solution thereof.

[0007] According to another aspect of the invention, there is provided a process for electrodepositing a hard, bright gold alloy having a pink coloration on to a conductive substrate, which comprises electroplating the conductive substrate as cathode in an aqueous cyanide-free electroplating bath as defined above at a cathode current density of from 0.1 to 1.5 amp/dm² (amperes per square decimetre) and a temperature of from 40 to 80°C. Advantageously the cathode current density is about 0.5 amp/dm2 and the temperature about 60°C., whilst the cathode should be moderately agitated during the electroplating process.

[0008] In formulating the electroplating bath of the invention, the gold is normally added in the form of an ammonium or alkali metal gold sulphite complex, for example potassium gold sulphite. The alloying element copper is normally added as a water-soluble salt or complex, for example copper sulphate, copper tartrate or copper citrate, whilst the alloying element palladium is normally added in the form of a water-soluble complex or salt, for example sodium, potassium, or ammonium palladium sulphite, palladium diammine dinitrite or palladium citrate. The free sulphite ion (SO

) may be added as an ammonium or alkali metal sulphite, for example sodium, potassium or ammonium sulphite. The buffering agent and/or conducting salt, when present, may be selected from alkali metal, alkaline earth metal or ammonium phosphates, borates, sulphates, carbonates, acetates, citrates, gluconates and tartrates, and boric acid. The brightening element, preferably arsenic, may be added as a soluble compound or complex, for example, arsenic trioxide which is advantageously added in the form of an aqueous solution, potassium antimony tartrate or thallium sulphate. The type of surface active agent present in the bath is not critical and any anionic, cationic or non-ionic surfactant effective within the stated pH range can be employed. For example, the surfactant can be an alkyl phosphate ester or an alcohol alkoxylate.

[0009] In carrying out the electroplating process of the invention, the anode employed is advantageously a platinum or platinised titanium anode.

[0010] The bright, hard gold alloy deposit having a pink coloration which can be obtained by means of the invention is a gold/copper/palladium alloy in which the elements are present in parts by weight ranges of 86-94 Au - 8-2 Cu - 6-2 Pd, advantageously 88-90 Au - 8-7 Cu - 4-3 Pd. Generally the alloy will contain more copper than palladium.

[0011] The following Examples illustrate the invention.

EXAMPLE 1

[0012] An electroplating bath was prepared by dissolving in demineralised water the following constituents:-

[0013] The commercially available surface active agent Triton Q.S.-44 (sold by Rohm and Haas (U.K.) Limited) is an anionic phosphate surfactant in free acid form.

[0014] The pH of the bath was adjusted to a value of 9.5 by the appropriate addition of potassium hydroxide or citric acid.

[0015] A brass panel was electroplated to a thickness of i 3 microns in the foregoing bath at a temperature of 60°C. and a cathode current density of 0.5 amp/dm², with moderate agitation and using a platinum coated titanium mesh anode.

[0016] The deposit obtained was bright and pink, extremely hard (340 HV-Vickers hardness number), ductile and pore-free. Analysis showed the deposit to be the alloy 90 Au - 7 Cu - 3 Pd.

EXAMPLE 2

[0017] The procedure of Example 1 was repeated except that the Triton Q.S.-44 was replaced by the commercially available surface active agent Cu 84 (sold by LPW Neuss, West Germany), which is an alkyl phosphate ester.

EXAMPLE 3

[0018] An electroplating bath was prepared by dissolving in demineralised water the following constituents:-

[0019] The pH of the bath was adjusted to a value of 9.5 by the appropriate addition of potassium hydroxide or citric acid.

[0020] A brass panel was electroplated using the foregoing bath in the manner described in Example 1 to obtain a bright, pink, pore-free ductile and extremely hard deposit. Analysis showed the deposit to be the alloy 94 Au - 2 Cu - 4 Pd.

EXAMPLE 4

[0021] An electroplating bath, which contained no conducting salt or buffering agent, was made up by dissolving in demineralised water the following constituents:-

[0022] The pH of the bath was adjusted to a value of 7-7.5 by the appropriate addition of potassium hydroxide or citric acid.

[0023] A brass panel was electroplated using the foregoing bath in the manner described in Example 1 to obtain a bright, pink, ductile and extremely hard deposit consisting of the alloy 91 Au - 4 Cu - 5 Pd.

EXAMPLE 5

[0024] An electroplating bath was prepared by dissolving in demineralised water the following constituents:-

[0025] The pH of the bath was adjusted to a value of 9.5 by the appropriate addition of potassium hydroxide or citric acid.

[0026] A brass panel was electroplated using the foregoing bath in the manner described in Example 1 to obtain a bright, pink, pore-free ductile and extremely hard deposit which on analysis was found to be the alloy 88 Au - 8 Cu - 4 Pd.

EXAMPLE 6

[0027] An electroplating bath was prepared by dissolving in demineralised water the following constituents:-

[0028] The pH of the bath was adjusted to a value of 9.5 by the appropriate addition of potassium hydroxide or citric acid.

[0029] A brass panel was electroplated using the foregoing bath in the manner described in Example 1 to obtain a bright, pink, pore-free ductile and extremely hard deposit which on analysis was found to be the alloy 88 Au - 8 Cu - 4 Pd.

EXAMPLE 7

[0030] An electroplating bath was prepared by dissolving in demineralised water the following constituents:-

[0031] The pH of the bath was adjusted to a value of 9.5 by the appropriate addition of potassium hydroxide or citric acid.

[0032] A brass panel was electroplated using the foregoing bath in the manner described in Example 1 to obtain a bright, pink, pore-free ductile and extremely hard deposit which on analysis was found to be the alloy 88 Au - 8 Cu - 4 Pd.

Claims

1. An aqueous cyanide-free electroplating bath for depositing a hard, bright gold alloy electrodeposit having a pink coloration, the bath comprising as essential constituents:-

(a) from 4 to 20 g/1 of gold,

(b) from 0.1 to 3.0 g/1 of copper,

(c) from 0.2 to 5.0 g/1 of palladium,

(d) from 5 to 200 g/1 of free sulphite ion,

(e) from O.to 150 g/l of one or more buffering agents and/or conducting salts,

(f) from 0.001 to 0.5 g/l of a brightening element selected from arsenic, antimony and thallium,

(g) from 0.01 to 5.0 g/1 of a surface active agent, and

(h) water,

the bath having a pH in the range from 7 to 11.5.

2. A process for electrodepositing a hard, bright gold alloy having a pink coloration on to a conductive substrate, which comprises electroplating the conductive substrate as cathode in an aqueous cyanide-free electroplating bath comprising as essential constituents:-

(a) from 4 to 20 g/l of gold,

(b) from 0.1 to 3.0 g/1 of copper,

(c) from 0.2 to 5.0 g/1 of palladium,

(d) from 5 to 200 g/1 of free sulphite ion,

(e) from 0 to 150 g/l of one or more buffering agents and/or conducting salts,

(f) from 0.001 to 0.5 g/1 of a brightening element selected from arsenic, antimony and thallium,

(g) from 0.01 to 5.0 g/l of a surface active agent, and

(h) water,

the bath having a pH in the range from 7 to 11.5 and the electroplating being carried out at a cathode current density of from 0.1 to 1.5 amp/dm² and a temperature of from 40 to 80°C.

3. A hard bright gold alloy electrodeposit having a pink coloration, said alloy consisting essentially of from 2 to 8 parts by weight copper, from 2 to 6 parts by weight palladium and the balance gold.