EP2669407A1

EP2669407A1 - Galvanic baths for obtaining a low-carat gold alloy, and galvanic process that uses said baths

Info

Publication number: EP2669407A1
Application number: EP13170067.6A
Authority: EP
Inventors: Lorenzo Cavaciocchi; Elena Banchelli; Vincenzo Danilo Canelli; Massimo Innocenti
Original assignee: Bluclad Srl
Current assignee: Bluclad SpA
Priority date: 2012-06-01
Filing date: 2013-05-31
Publication date: 2013-12-04
Anticipated expiration: 2033-05-31
Also published as: EP2669407B1; PT2669407T; ITFI20120103A1; ES2918576T3

Abstract

What is described are galvanic baths containing gold salts, copper salts and indium salts, which make it possible to obtain a precise carat value in the 12 - 18-carat range, suitable thicknesses, uniform deposit and colour tending towards the yellow typical of a gold deposit.

Description

Field of the invention

The invention relates to the field of compounds used in galvanic processes, in particular galvanic baths for obtaining a low-carat gold alloy, and to the galvanic process for obtaining said alloy.

Prior art

Galvanic baths containing considerable amounts of toxic substances, such as cyanides, and heavy metals, which are also toxic and difficult to remove, for example cadmium, in the matrix (where matrix means an aqueous solution of organic and inorganic acids and salts thereof) have been used for decades to obtain a gold alloy containing less than 75% by weight of gold as metal (corresponding to a gold alloy of 18 carats or less).
These baths make it possible to effectively obtain gold deposits having the desired carat value and considerable thicknesses, however the presence therein of highly poisonous substances makes them virtually unacceptable for practical use.
In order to overcome the aforementioned disadvantages, alternatives to the formulations have been sought that in particular would substitute cyanide, as a complexing agent, and cadmium, as a binder, in combination with copper.
Previous alternatives in use, however, utilise products that have only partly solved the known problems encountered with the use of the prior galvanic baths and, in addition, do not demonstrate the same suitability for practical use and the same quality of the results: in fact, these are generally formulations which only partly eliminate the toxic products used and which have restricted periods of use or with which an alloy is obtained having characteristics of carat value, thickness or appearance that fail to meet the needs of the intended purpose: in particular, nonuniform deposits or deposits having colourations that are too inclined towards pink and red for the aesthetic purposes for which they are used are obtained. However, baths of this type also include baths that utilise cyanide in the matrix or that, although they do not contain cyanide in the matrix, contain other doped elements of a certain toxicity.
For example, US4358351 describes a bath containing gold, copper, zinc, free cyanide and arsenic as gloss additive; US4487664 describes a bath containing gold-silver and free cyanide; US5085744 describes a bath containing gold, copper, zinc, free cyanide and antimony as gloss additive; US4687557 describes a bath containing gold, copper, and cadmium devoid of free cyanide in solution.
It is therefore obvious in the light of the foregoing that it would be greatly desirable, from the industrial point of view, to make available galvanic baths that are practically free from the above-mentioned toxic substances, are practical in use and make it possible to obtain a precise carat value in the 12 - 18-carat range, suitable thicknesses, uniform deposit and colour tending towards the yellow typical of a gold deposit.

Detailed description of the invention

The present invention makes it possible to satisfy the above-mentioned needs due to alkaline galvanic baths containing: gold salts, copper salts and indium salts, organic polycarboxylic acid salts, organic amines, and possibly complexing agents, surfactants and other metals in minor amounts, such as gloss additives and deposit refiners.
In accordance with the present invention, "galvanic bath" means the aqueous solutions in which the surfaces to be treated by means of the galvanic process are immersed.
In accordance with the present invention, the gold salts are preferably selected from: potassium dicyanoaurate, potassium tetracyanoaurate, ammonium cyanoaurate, or a combination thereof.
It is noted that, by contrast with the above-cited prior art, cyanide is not present in the matrix of the bath in the present invention and its content in the galvanic solution is caused by its counterion function in the gold salts alone and is therefore practically irrelevant, although necessary, for the stability of the gold in solution. The amount of gold, calculated on the basis of the respective salts, is between 0.7 and 4 g/L, preferably between 1.5 and 3 g/L, in order to achieve the desired efficiency and thicknesses.
In accordance with the invention, copper salts means, for example: sulfate, phosphate, pyrophosphate, chloride, or other salts that are stable under the working conditions of the bath.
Copper, calculated on the basis of the respective salts, is present in an amount between 0.3 and 2.5 g/L, preferably between 0.5 and 2 g/L, in relation to the desired carat value in the 12 - 18 carat range.
Indium salts useful in accordance with the invention are, for example: chloride, sulfate, citrate, tartrate, gluconate, or any other organic or amine complex compatible with the working conditions of the bath.
The concentration of indium, calculated on the basis of the respective salts, ranges from 0.1 to 2 g/L, preferably from 0.5 to 1,5 g/L, in relation to the desired carat value in the 12 - 18 carat range.
The matrix of the bath is formed by polycarboxylic acid salts having both buffer characteristics and complexing power, such as citrates, tartrates, gluconates, maleates, malonates, possibly in combination with the corresponding acids for creation of the expedient buffer, and possibly mixed together in order to achieve the expedient complexing power.
These acids and the respective salts are each used in an amount ranging from 30 to 150 g/L, more commonly from 50 to 100 g/L.
Complexing agents that can be expediently used in order to improve the stability of the metals in solution and to regulate the properties thereof in alloy include, for example, ethylenediaminetetraacetic acid and salts thereof, etidronic acid and salts thereof, ethylenediamine tetramethyl phosphonic acid and salts thereof, iminodiacetic acid and salts thereof, nitrilotriphosphonic acid and salts thereof, and nitrilotriacetic acid and salts thereof; these complexing agents normally being used in amounts ranging from 1 to 50 g/L according to the complexing power of each one thereof, a person skilled in the art knowing which complexing agent will be the most suitable for use and in what amounts and combinations, according to the desired carat value in the 12 - 18 carat range.
A complexing and regulatory action of the alloy metals is due to the amine component of the bath.
In particular, it has been found that some types of amine enable finer regulation of the percentage of indium in alloy.
In particular, the amines used for this purpose are triethylenetetramine and diethylenetetramine, but other organic amines, such as ethylenediamine, tetraethylenepentamine and other amines of similar structure, may also perform the same role.
The amine in question is used in amounts ranging from 0.05 to 1 g/L, more expediently from 0.1 to 0.7 g/L, depending on the desired percentage of indium in alloy, within the carat range from 12 to 18 carats.
The bath may also obviously contain other components normally used in galvanic processes for their glossing and surfactant action.
In particular due to the glossing action, it is expedient to add elements, such as silver, tellurium, bismuth, iron, zinc, iridium, rhenium, vanadium, molybdenum, tungsten, in such a form so as to be soluble and stable under the working conditions; the amounts to be used range from 0.005 to 0.2 g/L of element as such, and more in particular from 0.010 to 0.1 g/L.
The formulations according to the invention may also comprise sulfurated compounds, such as organic thiols belonging to the classes of mercaptotetrazoles, mercaptopyrimidines, mercaptopyrrols, mercaptoimidazoles and mercaptotriazoles, possibly containing a sulfur atom in the aromatic ring, possibly having a nitrogen ring condensed with a benzene ring, having one or more thio groups.
It has been observed that the general effect of these substances is that of regulating the deposition of the gold, improving the uniformity of the deposit and reducing the tendency thereof towards chemical deposition and stain formation. The amounts to be used expediently range from 0.01 to 0.500 g/L, and more in particular from 0.05 to 0.200 g/L.
The surfactants have both glossing and surfactant action, and those used normally within galvanic scope in baths having matrices of this type are used in this invention: a person skilled in the art will be able to identify the most suitable type to use; however, the amounts used are never greater than 0.005 L/L of an aqueous solution thereof (10% w/v): higher doses lower the efficiency of the bath and create interference with the regular properties of the metals in alloy.
The bath preferably functions in a pH range between 7 and 12, and more expediently between 8 and 10; the pH is regulated with organic acids belonging to the selected buffer pair, or with mineral acids such as sulfuric acid, phosphoric acid, sulfamic acid, methanesulfonic acid, pyridinesulfonic acid, or with alkali metal hydroxides, such as potassium and sodium.
The pH is not a critical parameter in this formulation for obtaining a precise carat value, but it is fundamental that the pH is alkaline for the stability of the components in solution.
The working temperature of the galvanic bath in question is preferably between 50°C and 70°C; however, this is not a critical parameter for obtaining a precise carat value.
The current to be applied to the galvanic bath according to the invention is normally between 0.5 - 3 A/cm², preferably 0.7 - 2 A/cm².
The application times vary according to the amperage imposed and the desired thickness: with the bath according to the invention, it is possible to obtain deposits having thicknesses up to 500 microns.
By expediently varying the applied current, it is possible to obtain alloys of different carat value and colour, since increasing the current applied causes a reduction of the gold in alloy and increases the copper, thus resulting in lower carat values and colours tending more greatly towards red, whereas, vice versa, reducing the current applied causes an increase of the gold in alloy and therefore higher carat values and colours tending more greatly towards yellow.
It is therefore possible to achieve any intermediate percentage in the 12 - 18-carat value range in a precise manner by imposing the expedient current.
Using the baths as described above, gold alloys in which the gold varies from 50% to 75% (percentages expressed in weight) are obtained by deposit at the cathode of the galvanic cell.

Example 1

A bath having the following composition was prepared:

Citric acid: 50 g/L
Potassium citrate: 120 g/L
Tetrasodium EDTA: 30 g/L
Gold: 1.2 g/L as metal, introduced as potassium dicyanoaurate
Copper: 0.7 g/L as metal, introduced as copper sulfate
Indium: 0.3 g/L as metal, introduced as indium sulfate
Ethylenediamine 0.2 mL/L as pure substance, introduced as aqueous solution 50% w/v
Silver 0.05 g/L as metal, introduced as silver cyanide salt 80.6%

The bath was brought to pH 8.5 with potassium hydroxide and heated to 60°C.
A current of 0.7 A/cm² was applied for 3 minutes to a sheet of nickel-plated brass having a surface area of 0.5 cm², suitably de-greased and re-rinsed with deionised water.
At the end of the period, the sheet was rinsed with deionised water and dried by compressed air.
The colour coordinates measured in accordance with CIELab parameters were as follows: L= 79.2 a= 1.8 b= 13.6
Corresponding to an alloy definable as 14 carats.

Example 2

A bath having the following composition was prepared:

Sodium gluconate: 60 g/L
Iminodiacetic acid: 80 g/L
Gold: 2.5 g/L as metal, introduced as potassium dycanoaurate
Copper: 1.8 g/L as metal, introduced as copper sulfate
Indium: 0.35 g/L as metal, introduced as indium sulfate
Diethylenetetramine 0.1 mL/L as a pure substance, introduced as aqueous solution 50% w/v
Rhenium 0.05 g/L as metal, introduced as sodium perrhenate 2-(2-pyrazine)ethane thiol 0.01 g/L as pure substance, introduced in the bath as aqueous solution 10% w/v

The bath was brought to pH 9.5 with potassium hydroxide and heated to 60°C.
A current of 1.5 A/cm² was applied for 5 minutes to a sheet of nickel-plated brass having a surface area of 0.5 cm², suitably de-greased and re-rinsed with deionised water.
At the end of the period, the sheet was rinsed with deionised water and dried by compressed air.
The colour coordinates measured in accordance with CIELab parameters were as follows: L= 84.8 a= 5.1 b= 16.2
The surface of the alloy was analysed by electronic scan microscope; the following percentages were found
Au: 74% In: 5% Cu: 22%
Corresponding to an alloy definable as 18 carats.

Claims

Galvanic bath consisting of aqueous solutions comprising gold salts, copper salts and indium salts, organic polycarboxylic acid salts, organic amines, and possibly complexing agents, surfactants and other metals.
Galvanic bath according to claim 1, wherein said gold salts are selected from: potassium dicyanoaurate, potassium tetracyanoaurate, ammonium cyanoaurate, or a combination thereof; said copper salts are selected from: sulfate, phosphate, pyrophosphate, chloride, or other salts that are stable under the working conditions of the bath itself, and said indium salts are selected from chloride, sulfate, citrate, tartrate, gluconate, or an organic or amine complex.
Galvanic bath according to claim 2, wherein the amount of gold ranges from 0.7 to 4 g/L, preferably from 1.5 to 3 g/L, the amount of copper is between 0.3 and 2.5 g/L, preferably between 0.5 and 2 g/L, and the amount of indium is between 0.1 and 2 g/L, preferably between 0.5 and1.5 g/L.
Galvanic bath according to claims 1 - 3, wherein said polycarboxylic acid salts are selected from: citrates, tartrates, gluconates, maleates, malonates, possibly in combination with the corresponding acids for creation of the suitable buffer, and possibly in a mixture of mixtures thereof in amounts between 30 and 150 g/L, and preferably between 50 and 100 g/L.
Galvanic baths according to claims 1 to 4, wherein said amines are triethylenetetramine and diethylenetetramine in amounts that vary from 0.05 to 1 g/L, preferably from 0.1 to 0.7 g/L.
Galvanic baths according to claims 1 - 5, further comprising elements selected from: silver, tellurium, bismuth, iron, zinc, iridium, rhenium, vanadium, molybdenum, tungsten, in such a form so as to be soluble and stable under the working conditions in amounts between 0.005 and 0.2 g/L of element as such, and more in particular between 0.010 and 0.1 g/L.
Galvanic baths according to claims 1 - 6, further comprising sulfurated compounds selected from: mercaptotetrazoles, mercaptopyrimidines, mercaptopyrrols, mercaptoimidazoles and mercaptotriazoles, possibly containing a sulfur atom in the aromatic ring and in which the nitrogen ring may be condensed with a benzene ring, having one or more thio groups, said compounds being present in amounts between 0.01 and 0.500 g/L, and more in particular between 0.05 and 0.200 g/L.
Galvanic process for the plating of objects with gold alloys from 12 - 18 carats, wherein the baths according to claims 1 to 7 are used.
Galvanic process according to claim 8, wherein said process is carried out in a pH range between 7 and 12, at a temperature between 50°C and 70°C, and with a current applied between 0.5 - 3 A/cm².
Objects plated in 12 - 18-carat gold obtained by applying a galvanic process according to claims 8 and 9.