GB2323853A - An electrodeposit free of cobalt, cadmium and nickel - Google Patents

Abstract

An electrodeposit free of cobalt, cadmium and nickel contains 1.25 to 1.55% w/w iron, 1 to 2 ppm titanium; and 97.7 to 98.2% gold and has a pale yellow colour less than 3N on the NIHS scale. Also provided is an electroplating bath, free of cobalt, cadmium and nickel comprising gold, as cyanide, iron as a soluble salt or complex, a soluble titanium salt or complex, a citrate, a weak acid, and optionally a heterocyclic sulphonate, which in a preferred form comprises gold as cyanide in an amount of 2.5 to 3.5 g/l gold; iron as iron nitrate in an amount of 0.6 to 0.8 g/l, titanium as titanium sulphate in an amount of 0.15 to 0.9 g/l, diammonium hydrogen citrate in an amount of 75 to 125 g/l, citric acid in an amount of 40 to 80 g/l, and 3-(1-pyridino)-1-propane sulphonate in an amount of 1 to 3 g/l.

Description

ELECTROPLATING PROCESSES COMPOSITIONS AND DEPOSITS The present invention relates to gold-iron alloy electroplating processes, compositions for use therein and gold-iron alloy electrodeposits produced therefrom.

Gold alloy electrodeposits are extensively used for decorative and functional deposits. Gold alloys with copper, cadmium, cobalt, indium, zinc or tin or mixtures thereof are well known. Examples of patent literature giving details of such compositions revealed by searches by the applicants are JP 53-58023 (Matsushita), JP 51-56241 (Citizen Watch), DE 1696087 (OMF), US 3926748 (AMP), GB 1445395 (Schering), GB 1375611 (Lea-Ronal), GB 1279141 (Degussa), GB 2151661 (LPW-Chemie), EP 193848 (Emmenegger), US 4470886 (OMI), US 2724687 (Spreter), JP 57-120686 (Suwa Seikosha), JP 57-120685 (Suwa Seikosha), JP 56-136994 (Nippon Mining), JP 56-105494 (Nippon Mining) and EP 140832 (H.E. Finishing).

An article in Galvanotechnik vol 83 (1992) pp 808-817 and 1180-1184 by F. Simon mentions gold-iron electroplating using cyanide baths. It refers to gold cyanide complex baths containing cobalt, nickel, indium, iron (it is not clear whether these are present together or separately) in a weak acid bath at pH 3 - 6.

A search by the UK Patent Office revealed the following cases: GB2242200 (Enthone); GB 1426849 (Deutsche Gold und Silber); EP-A-0480876 (Metaux Precieux); EP-A-0037534 (Degussa); US-A4687557 (Emmenegger); US A-4358351 (Degussa); JP-7018484 (Seiko); and US-A4075065 (Handy & Harman).

Gold-iron baths have the advantage of not inducing allergic reactions in contact with skin such as can be caused by gold alloys containing nickel or- cobalt, and do not contain cadmium which is a toxic metal.

it is very desirable to use gold alloy electrodeposits which do not contain nickel or cobalt for skin contacting products, such as rings and spectacle frames.

Gold-iron alloy electrodeposits however are thought to be brittle and to be liable to crack damaging the corrosion resistance of the Product. In addition they tend to be too warm a yellow for decorative uses and a paler colour is desired. Colour for gold alloy electrodeposits can be assessed on the (NIHS 03-50) standards scale. NIHS is Normes de l'industrie horlogere Suisse or Swiss watch industry standards. This provides a colour scale ranging from 5N (red), via 4N (pink) to 3N, which is too warm yellow colour of conventional gold-iron alloy electrodeposits, to 2N-18 to lN-14. The colours are made from gold-silver-copper alloys containing the following amounts for the relevant colours.

Colour SN 4N 3N 2N-18 lN-14 Ingredient gold 750 750 750 750 585 silver 45 90 125 100 265 copper 205 100 125 90 150 The NIHS 03-50 standard states that for gold articles the colour lN-14 is not obtainable for an alloy of more than 14 carats and for the colour 2N-18 for an alloy of more than 18 carats.

It is desired to produce a gold-iron alloy electrodeposit which has a colour of 2N-18 to lN-14 on the NIHS scale and which is free of cobalt, cadmium and nickel, and which has good corrosion resistance, and which has a high carat value, in excess of 18.

The applicants conducted extensive research to modify the colour of conventional gold-iron alloy deposits.

These deposits contain 2.1k iron, 97.9% gold and have a colour of 3N(+).

Addition of zinc sulphate at from 50-200 mg/l gave a colour of 3N to 3N(+); at 300 mg/l the colour becomes too yellow-gray.

Addition of ammonium monovanadate at from 100 mg/l to 1500 mg/l only gave a colour of 3N.

Addition of cadmium acetate on its own or with diethylene triamine penta-acetic acid (DTPA) chelate only gave a colour of 3N.

Addition of vanadium (IV) oxidesulphate in amounts up to 150 mg/l only gave a colour of 3N to 3N(+).

Addition of ammonium bismuth citrate with DTPA only gave a colour of 3N to 3N(+).

Addition of sodium tungstate dihydrate at from 0.55 to 4.45 g/l of tungsten at current densities of 1 to 4 A/dm2 and at pH values from 3.5 to 4.45 only gave a colour of 3N.

Addition of 5 g/l of nicotinic acid allowed one to increase the current density to 4 A/dm2 without burnt deposits but the colour remained at 3N(+).

Bismuth and lead both acted as a metallic impurity and only brown and matt deposits were produced. Lead was added as lead nitrate. Bismuth was added as bismuth III nitrate pentahydrate.

Addition of potassium stannate 1 g/l at current densities of 1 to 3 A/dm2 only gave a colour of 3N(+).

Addition of cerium (III) nitrate hexahydrate at 1 g/l gave a colour of between 3N and 2N-18. Cerium (III) sulphate, cesium nitrate and cesium sulphate all had no effect on the colour of the deposit.

EP-A-0193848 is concerned with gold-copper-cadmiumzinc cyanide baths and refers to a number of inorganic brighteners. Baths B1 to B5 show the use of selenium as sodium selenite, arsenic as sodium arsenite and zirconium as the sodium zirconium hydroxy ethyl-imino-diacetate, as inorganic brighteners in B2-B5, no brighteners being used in B1.

Col. 13 1. 38-42 of EP-A-0193848 states that all these deposits are pale yellow and give a colour of approximately 1N-14. There is no teaching of any effect on colour produced by the presence of zirconium. Bath B2 contains zirconium as the inorganic brightener, bath B1 does not contain an inorganic brightener.

In addition it is extremely difficult to obtain a constant colour in the range 1N-14 to 2N-18 with goldcopper-cadmium or gold-copper-cadmium-zinc systems.

According to the present invention an electrodeposit is provided which contains 1.25 to 1.55 % w/w iron, 1 to 2 ppm titanium; and 97.7 to 98.2% gold and has a pale yellow colour less yellow than 3N on the NIHS scale, and preferably at or near 2N-18.

It will be recognised that such a deposit is also of high carat. It is preferred that the deposit be of 2323.6 carat.

The gold-iron-titanium deposits of the present invention are free of toxic and allergy causing ingredients, have high carat values and corrosion resistance and at the same time a desirable pale yellow colour.

The invention also extends to an electroplating bath, desirably free of cobalt, cadmium or nickel comprising gold, as cyanide, iron as a soluble salt or complex, a soluble titanium salt or complex, a citrate, a weak acid, and optionally a heterocyclic sulphonate such as PPS. The function of the PPS is to allow higher cathodic current densities and to improve the macrodistribution a little.

The gold is preferably present as gold potassium cyanide preferably in an amount of 1.0 to 10 g/l especially 2.5 to 3.5 g/l of gold.

The iron is preferably present as a nitrate which may be hydrated. It is preferably present in an amount up to 5 g/l of iron e.g. 0.1 to 5 g/l preferably 0.2 to 3 g/l especially 0.6 to 0.8 g/l. The more iron there is in the bath the more there is in the deposit. However at a current density of 0.5 A/dm2 as the iron content of the bath increases from 0.25 g/l, at which the cathodic efficiency is 25 mg/A.min, to 2.0 g/l the cathodic efficiency falls to 7 mg/A.min.

Examples of other salts which may be used instead of iron nitrate are iron sulphate, iron (III) chloride, iron (III) citrate and iron (III) phosphate.

The titanium is preferably present as the sulphate.

The titanium is preferably present in an amount of 0.01 to 2 g/l of titanium e.g. 0.04 to 1.5 g/l or 0.1 to 1 g/l, especially 0.15 to 0.9 g/l.

The citrate is preferably diammonium hydrogen citrate (C6H14N207) or (NH4) 2C6H6O7 and is preferably present in an amount of 10 to 500 g/l e.g. 50 to 200 g/l especially 75 to 125 g/l. Diammonium hydrogen citrate is preferred to sodium or potassium citrate because it gives much higher macrodistribution of the gold layer e.g. as high as 90% as shown by tests in a Haring cell, as compared with about 50% when sodium or potassium citrate is used.

The weak acid is preferably a hydroxy carboxylic acid such as citric acid (HO(COOH) (CH2COOH)2.H2O, though other carboxylic acids such as oxalic, lactic, formic, thiomalic, gluconic, tartaric, acetic or malic acid could be used. Phosphoric acid could also be used instead of citric acid.

The weak acid is preferably present in an amount of 1 to 500 g/l e.g. 10 to 200 g/l e.g. 20 to 100 g/l especially 40 to 80 g/l.

The PPS is 3-(1-pyridino)-1-propane sulphonate (C8H1lNO3S). It is preferably present in an amount of 0.1 to 10 g/l e.g. 0.5 to 5 g/l especially 1 to 3 g/l.

Materials which can be used instead of PPS include for example pyridine-4-ethanesulphonic acid.

The bath can be used to plate gold-iron-titanium deposits directly on a range of substrates such as nickel undercoat, or one of the following when provided with a flash of pure gold, namely copper, palladium, palladiumnickel, palladium-cobalt, gold-silver or gold-coppercadmium.

The invention can be put into practice in various ways and a number of specific embodiments will be described to illustrate the invention with reference to the accompanying examples.

Examples 1 to 4 In each of these examples plating bath compositions were made up from gold potassium cyanide (KAu(CN)2) (4.39 g/l) to provide 3.0 g/l of gold, iron (III) nitrate nonahydrate (5.2 g/l) to provide 0.72 g/l of iron, a 15% by weight aqueous titanium sulphate solution having a pH of 1.3 (in varying volumes given in Table 1), diammonium hydrogen citrate (100 g/l), citric acid (60 g/l) and 3 (1-pyridino) -l-propane-sulphonate (C8H11NO3S) (2 g/l) in conventional manner except that the titanium sulphate solution was added before the gold and the solution adjusted at 400C to pH 3.5 electromeric and a density 80Be (Baume).

Rack plating using a brass cathode and a platinized titanium anode was then carried out at 400C using a current density of 4 A/dm2 for a plating time of 35 minutes at 0.8 A/litre, to give a thickness of about 1 micrometre (1 micron). The anode-cathode ratio was 4/1 and the solution was stirred vigorously with a magnetic stirrer. The cathode was agitated by revolution of the cathode at 7 rpm.

Table 1 gives the titanium content of the plating bath and the resultant NIHS colour.

Table 1 Example 1 2 3 4 Titanium content 0.17 0.37 0.57 1.0 g/l vol of titanium 4.5 9.85 15.2 26.6 sulphate solution (ml) colour 2N18- 2N18- 2N18+ 2N18 (NIHS) 3N 3N -3N The carat value of each of the examples was at least 23. Each of the examples had a good pale yellow colour lower than 3N. 2N18-3N means between 2N18 and 3N; 2N18+ means just yellower than 2N18 i.e. paler than 2N18-3N.

The bath solution of Example 4 showed some slight instability and the deposit showed some signs of haziness and accordingly it may be desirable to use lower titanium concentrations which do not exceed 1 g/l, e.g. up to 0.7 g/l.

During use of the bath the gold metal content should be maintained at the recommended range of 2.5 to 3.5 g/l by periodic additions of gold potassium cyanide.

The gold will be consumed at a rate of about 100 g per 4500 ampere minutes, working at 2A/dm2, or for every 8330 ampere minute, working at 4A/dm2. A replenisher solution will also be used as is conventional to replace the other ingredients which are consumed during use of the bath.

When rack plating is being used as in the examples given above the current density is typically 2-4 A/dm2.

The ratio of the anode area to the cathode area is preferably 3:1 or 4:1 or higher. The solution density is preferably at least 90 Baume.

Claims (18)

1. An electrodeposit free of cobalt, cadmium and nickel which contains 1.25 to 1.55% w/w iron, 1 to 2 ppm titanium; and 97.7 to 98.2% gold and has a pale yellow colour less than 3N on the NIHS scale.

2. An electrodeposit as claimed in claim 1, in which the colour is in the range 3N to 2N-18.

3. An electrodeposit as claimed in claim 1 or claim 2 in which the deposit is of 23-23.6 carat.

4. An electroplating bath, free of cobalt, cadmium and nickel comprising gold, as cyanide, iron as a soluble salt or complex, a soluble titanium salt or complex, a citrate, a weak acid, and optionally a heterocyclic sulphonate.

5. An electroplating bath as claimed in claim 4 in which the gold is present as gold potassium cyanide in an amount of 1.0 to 10 g/l.

6. An electroplating bath as claimed in claim 4 or claim 5 in which there is 2.5 to 3.5 g/l of gold.

7. An electroplating bath as claimed in claim 4, 5 or 6 in which the iron is present as a nitrate which may be hydrated, iron sulphate, iron (III) chloride, iron (III) citrate or iron (III) phosphate.

8. An electroplating bath as claimed in claim 7 in which the iron content is in the range from 0.25 g/l to 5.0 g/l.

9. An electroplating bath as claimed in any one of claims 4 to 8 in which the titanium is present as the sulphate.

10. An electroplating bath as claimed in claim 9 in which the titanium is present in an amount of 0.1 to 0.9 g/l of titanium.

11. An electroplating bath as claimed in any one of claims 4 to 9 in which the citrate is diammonium hydrogen citrate.

12. An electroplating bath as claimed in claim 11 in which the citrate is present in an amount of 10 to 500 g/l.

13. An electroplating bath as claimed in any one of claims 4 to 12 in which the weak acid is a hydroxy carboxylic acid or phosphoric acid.

14. An electroplating bath as claimed claim 13 in which the weak acid is citric acid ( HO ( COOH) (CH2COOH)2.H2O, oxalic, lactic, formic, thiomalic, gluconic, tartaric, acetic or maleic acid.

15. An electroplating bath as claimed in claim 13 or claim 14 in which the weak acid is present in an amount of 1 to 500 g/l.

16. An electroplating bath as claimed in any one of claims 4 to 15 in which the heterocyclic sulphonate is 3 (1-pyridino)-1-propane sulphonate or pyridine-4ethanesulphonic acid.

17. An electroplating bath as claimed in claim 16 in which the heterocyclic sulphonate is present in an amount of 0.1 to 10 g/l.

18. An electroplating bath, free of cobalt, cadmium and nickel comprising gold as cyanide in an amount of 2.5 to 3.5 g/l gold, iron as iron nitrate in an amount of 0.6 to 0.8 g/l, titanium as titanium sulphate in an amount of 0.15 to 0.7 g/l, diammonium hydrogen citrate in an amount of 75 to 125 g/l, citric acid in an amount of 40 to 80 g/l, and 3- (1-pyridino) -1-propane sulphonate in an amount of 1 to 3 g/l.