GB2039532A - Electrolyte for the electrodeposition of white gold - Google Patents

Abstract

White gold coatings are electrodeposited from an electrolyte containing an alkali gold (I) cyanide, nickel, an aliphatic hydroxy-carboxylic acid and thioglycollic acid. Preferably, the electrolyte also contains a non-ionogenic nitrogen-containing compound of the ethoxylated fatty amine type, such as that commercially known as "Stockomin". Colour modifiers, such as cobalt, iron or indium, may be added to the bath.

Description

SPECIFICATION Electrolyte for the electrodeposition of white gold This invention lies within the field of electrolytic deposition of so-called "white golds," that is to say alloys of gold with metals that act to whiten or bleach the yellow colour of electrodeposited gold. The metal with which this invention is concerned is nickel.

It is known that gold and nickel can be deposited from electrolytes containing alkali cyanide, to yield a gold-nickel alloy. The deposition of the nickel takes place, however, only when the limiting current density for gold deposition is exceeded, and the deposits are not solutions of nickel in gold but mixtures of predominantly gold and predominantly nickel phases. Such deposits have neither the stability nor the resistance to corrosion which characterises single-phase solution alloys of gold, and are unacceptable commercially.

Attempts have been made to produce acceptable white gold coatings from electrolytes containing alkali gold (I) cyanide, a nickel salt and a hydroxy-carboxylic acid, with or without additives to improve the nature of the deposited coatings, but, as far as the applicant is aware, these attempts have not been fully successful.

The object of this invention is to provide an electrolyte from which white gold can be deposited to form commercially acceptable coatings as regards colour, brightness, resistance to tarnishing and other physical properties.

According to the present invention, we therefore provide an electrolyte for the electrodeposition of white gold comprising an alkali gold (I) cyanide, preferably potassium gold (i) cyanide, a water-soluble nickel salt, preferably nickel sulphate, and an organic carboxylic acid, preferably an aliphatic hydroxy-carboxylic acid, advantageously an aliphatic hydroxy di- or tri- carboxylic acid, with the further addition thereto of thiodiglycollic acid.

Further according to the invention, the electrolyte also contains a non-ionogenic nitrogen-containing compound of the ethoxylated fatty amine type.

In general, the basic aqueous electrolyte is made from potassium gold (I) cyanide, nickel in the form of nickel sulphate, citric acid and potassium hydroxide added in sufficient quantity to bring the pH of the bath to between 3.5 and 4.2. A suitable composition is: Potassium gold(l) cyanide 1 to 4.5 glC, and preferably 2 to 3 g/6.

Nickel sulphate (NiSO4) 16 to 55 g/t, and preferably 35 to 40 g/.

Citric acid 40 to 100 glC, and preferably about 50 glut.

Potassium hydroxide to bring the pH to between 3.5 and 4.2.

(usually about 35 to 40 glue.) To this bath is added thiodiglycollic acid between 0.2 to 3 g/(.

It has been found that the quality of the coatings is further improved by the addition of a few drops (say 0.3my') to 2 mt of the non-ionogenic nitrogen-containing compound of the ethoxylated fatty amine type.

This may, for instance, be in the form of a compound of the type known under the trade name "Genamin C or 0" derived from oleylamine by a medium degree of ethoxylation (for example with 5 to 8 mole of ethylene oxide). One such compound is known commercially as "Stokomin," and its use in the bath described above, in an amount of 0.3 mPlt to 2m(/t, , has proved particularly suitable.

Malic acid can be substituted for citric acid, but is objectionable because of the generation of unpleasant odour.

It is pointed out that, since the two additions to the basic bath are consumed during the plating process, they must be replenished from time to time or continuously.

Suitable operating conditions are: temperature of the bath, between 40 and 60"C, (and preferably 60"C), current densities, between 0.5 and 3.0 Ad/m2, and preferably 1.5 A/dm2, movement of the electrolyte relatively to the cathode, between 4 and 20 cm/sec and preferably 8 cm/sec.

White gold coatings deposited under these conditions from the electrolyte described above and with a thickness of 5 pm or micrometres, have been found to be free of stress cracks, with micro-Vickers hardness numbers of from 350 to 500. The nickel content, dependent upon the conditions of deposition, is between 8 and 25%, averaging between 14 and 16%. X-ray examination showed that the nickel was present in the white gold deposit in solid soiution in the gold. The coatings were highly resistant to tarnishing and in artificial perspiration and salt-spray tests they underwent no significant change.

In order to achieve special colour tints in the coatings, other metals may be added to the bath, for example cobalt, iron or indium.

The electrolytes of the invention can be used to deposit white gold coatings directly upon gold, copper or silver, upon gold, copper or silver alloys and upon nickel or silver intermediate coatings. Since the coatings are resistant to tarnishing and corrosion, they are especially suitable for application to jewellery and other articles such as spectacle frames. They can be used for both single and multilayer coatings.

Claims (20)

1. An electrolyte for the electrodeposition of white gold comprising an alkali gold (I) cyanide, a nickel salt and an organic carboxylic acid, with the further addition thereto of thiodigly-collic acid.

2. An electrolyte as claimed in claim 2, where the said alkali gold (i) cyanide is potassium gold (I) cyanide.

3. An electrolyte as claimed in either of claims 1 and 2, wherein the said water-soluble nickel salt is nickel sulphate.

4. An electrolyte as claimed in any one of the preceding claims, wherein the said organic carboxylic acid is an aliphatic hydroxy-carboxylic acid.

5. An electrolyte as claimed in any one of the preceding claims, wherein the said organic carboxylic acid is an aliphatic hydroxy di- ortri-carboxylic acid.

6. An electrolyte as claimed in any one of the preceding claims further comprising a non-ionogenic nitrogen-containing compound of the ethoxylated fatty amine type.

7. An electrolyte as claimed in claim 6, wherein the said nitrogen-containing compound is present in a concentration of from 0.3 to 2 ml/l.

8. An electrolyte as claimed in either of claims 6 and 7, wherein the said nitrogen-containing compound is that known commercially as "Stockomin".

9. The electrolyte of any of the above claims, in which the alkali gold (I) cyanide is potassium gold (I) cyanide and the nickel is added as nickel sulphate.

10. The electrolyte of Claim 9 containing from 1 to 4.5. g/l of potassium gold (I) cyanide, from 16 to 55 g/l nickel sulphate (NiS04) from 40 to 1 00'gil citric acid, about 35 to 40 g/l potassium hydroxide, and 0.2 to 3.0 g/l thiodiglycollic acid.

11. The electrolyte of Claim 10 in which the electrolyte contains from 2 to 3 g/l of potassium gold (I) cyanide, from 35 to 40 gil nickel sulphate (N iSO4). about 50 gil citric acid, from 35 to 40 gil of potassium hydroxide, and between 0.2 and 0.3 gil of thiodiglycollic acid.

12. An electrolyte as claimed in any of the preceding claims the pH whereof is between 3.5 and 4.2.

13. The electrolyte of any of the above claims, with the addition of a metal to modify colour.

14. The electrolyte of Claim 13 in which the metal is chosen from the group cobalt, iron and indium.

15. Electrolytes for electrodesposition of white gold, substantially as herein described.

16. A process for electrodepositing white gold using the electrolyte of any of the above claims, in which the bath is at a temperature between 40 and 60"C, the current density is between 0.5 and 3.0 A/dm2 and the movement of electrolyte relative to the cathode is between 4 and 20 cm/sec.

17. The process of Claim 16 in which the bath temperature is about 60"C, the current density about 1.5 Aidm2 and the movement of electrolyte relative to the cathode about 8 cm/sec.

18. A process for producing white gold coatings in which the electrolyte used is that claimed in any of claims 1 to 15.

19. Process for the electrodeposition of white gold, substantially as herein described.

20. Articles whenever electrolytically coated by the use of an electrolyte as claimed in any of claims 1 to 15.