The present invention relates to a process for galvanic electrodeposition and to the associated galvanic bath, and also to a yellow gold alloy and to a use thereof.

As is known, by means of a galvanic electrodeposition process it is possible to deposit a more or less thin metallic layer on an object, for decorative purposes, protective purposes, or others. In some cases the finished product is constituted by the very object coated by the electrodeposited layer. In others, which are in any case of considerable practical interest, the deposited metallic layer is subsequently detached from the object, allowing to obtain a sort of self-supporting shell (which is internally empty) which considers the actual goal of the process: in this case, the process takes the name of "electroforming" and the finished product is also known as "electroformed".

In any case, according to methods which are by now well established, the process provides an electrolytic reaction in a tank (galvanic bath), in which the object to be coated (which acts as a cathode) is immersed in an aqueous solution of the metal to be deposited. In this context, the anode can be constituted by the metal itself to be deposited or also by an inert metal or by graphite. In any case, by applying a potential difference to the two electrodes, a flow of ions of the metal to be deposited is generated and the metal progressively accumulates on the cathode, forming indeed the desired layer.

Among the various metals commonly used, in the field of fashion and jewelry it is customary to deposit metallic layers made of gold alloy, intended indeed to constitute a decorative (and/or protective) coating or a self-supporting shell. In particular, in this context yellow gold alloys which the reference standard (UNI EN ISO 8654) identifies with the codes IN (which corresponds to pale yellow), 2N (light yellow), and 3N (yellow) are well-known.

In greater detail, in the decorative field of plating, methods are known which are adapted to produce deposits of yellow gold (with a fineness greater than or equal to 12 carats), which are ductile to a thickness of 10 microns and highly resistant to the loss of luster. These deposits are obtained by electrolysis in an alkaline galvanic bath which contains, in addition to gold and copper compounds, cadmium in the form of alkaline cadmium cyanide. Cadmium facilitates the deposition of thick layers (between 1 and 800 pm) and allows to obtain the yellow coloring, reducing the quantity of copper contained in the alloy; however, this is an extremely toxic element, the use of which is by now forbidden in several countries.

Yellow gold alloys are also known which comprise gold, copper and silver (and therefore lack cadmium); however, these alloys are soft and therefore unsuitable for uses in the field of electroforming. Moreover, it is substantially impossible to obtain a production with a constant gold fineness and therefore they are not used in commerce.

Other known alloys which in any case avoid resorting to cadmium are those which comprise gold, copper and indium. In this case, if one maintains a sufficiently high percentage of indium (4%), one obtains an alloy that is fragile (before and after thermal treatment) and scarcely solderable, compromising the possibility to move on to subsequent treatments. Vice versa, when working with a lower indium percentage (2%), the resulting alloys ensure better performance but do not allow to obtain the desired INSIST yellow color.

The aim of the present invention is to solve the problems described above, by providing a process for galvanic electrodeposition and/or a galvanic bath that allow to provide layers made of a 1N-3N yellow gold alloy, which have good solderability and high mechanical strength.

Within this aim, an object of the invention is to provide a process for galvanic electrodeposition and/or a galvanic bath that allow to obtain by electroforming shells or self-supporting objects made of a 1N-3N yellow gold alloy, with adequate metallic properties.

Another object of the invention is to provide a 1N-3N yellow gold alloy that can be used to provide shells or self-supporting objects obtained by electroforming.

Another object of the invention is to provide a process for galvanic electrodeposition and/or a galvanic bath that allow to obtain by electroforming shells or self-supporting objects made of a 1N-3N yellow gold alloy without resorting to silver and without using cadmium or other toxic substances.

Another object of the invention is to provide a process for galvanic electrodeposition and/or a galvanic bath that ensure high reliability in operation.

Another object of the invention is to provide an alloy and propose a process for preparing said alloy that can be obtained and executed easily starting from commonly commercially available elements and materials.

Another object of the invention is to provide an alloy and a process for preparing said alloy that have low costs and are safe in application.

This aim and these and other objects which will become better apparent hereinafter are achieved by a process according to claim 1, a galvanic bath according to claim 10, a yellow gold alloy according to claim 11, and a use of a yellow gold alloy according to claim 14.

Further characteristics and advantages of the invention will become better apparent from the description of a preferred but not exclusive embodiment of the process, of the bath and of the alloy according to the invention, illustrated by way of non-limiting example in the paragraphs that follow.

The galvanic electrodeposition process comprises a step in which at least one object immersed in a galvanic bath which in turn comprises an aqueous solution is coated with a layer of yellow gold alloy. According to per se known methods, the process is performed by applying a potential difference to two electrodes immersed in the bath: one electrode, the cathode, is constituted by the object to be coated, while the other one, the anode, can be constituted by the metal to be deposited, by an inert metal, by graphite, or others. In any case, the flow of ions of the metal generated by the application of the potential difference causes the accumulation of said ions on the object, so as to obtain progressively the forming of the desired coating layer. The possibility is provided to immerse a number of objects at will (of any shape and size), insofar as allowed by the dimensions of the tank filled with the aqueous solution and by the limitations and the technical requirements of each specific situation.

So far, in any case, these are traditional practices, which are well- known in the art and to the person skilled in the art, and therefore these aspects will not be dwelt upon further.

The process according to the invention can be used to deposit a protective layer intended to constitute an external layer or an underlayer (substrate) of the finished product that one wishes to obtain. At the same time, and as will be specified further hereinafter, in the preferred application (which enhances the particularities described in the paragraphs that follow) the process according to the invention is intended to be performed in the field of the technique also known as electroforming, according to which the object (core) that is coated during the process is subsequently removed, leaving a sort of self-supporting shell (electroformed product) which is indeed the actual goal of the process.

It is specified furthermore that the process according to the invention is intended to be have a preferred application in the field of fashion, leatherwear or jewelry, although other uses, which are in any case within the protective scope claimed herein, to provide finished products (or intermediate products) intended for other product markets are not excluded.

According to the invention, the aqueous solution comprises at least gold in a quantity comprised between 1 gram and 10 gram per liter of solution, in the form of alkaline cyanide. In particular, the gold is preferably in the form of a potassium dicyanoaurate complex, but other practical choices (tetrachloroaurate, potassium tetracyanoaurate, or others still) are not excluded.

Furthermore, according to the invention, the aqueous solution comprises at least copper in a quantity comprised between 30 grams and 70 gram per liter of solution, in the form of an alkaline cyanide compound (and preferably in the form of cuprous cyanide). In particular, the copper is preferably in the form of cuprous cyanide, but other practical choices (cuprous chloride, copper iodide, copper carbonate) are not excluded.

Furthermore, according to the invention, the aqueous solution comprises indium in a quantity comprised between 100 milligrams and 2 gram per liter of solution, in the form of a soluble compound (and preferably in the form of a soluble complex).

Furthermore, according to the invention, the aqueous solution comprises at least zinc in a quantity comprised between 100 milligrams and 2 grams per liter of solution, in the form of a soluble compound (and preferably in the form of a zinc complex).

Usefully, during the execution of the process according to the invention the bath (and therefore the aqueous solution) is kept at a constant temperature comprised between 50°C and 80°C.

Advantageously, furthermore, during the execution of the process the solution is kept at a pH between 8 and 12.

In a solution of considerable practical interest, which in any case does not limit the application of the invention, during the electrodeposition step the current density that is applied is comprised between 0,2 A/dm ² e 1,5 A/dm ² .

In addition to the four components cited above (gold, copper, zinc and indium), the bath (the aqueous solution) can be enriched with various other additives, some of which are mentioned by way of non-limiting example. With reference to these additives, it is specified that the solution can provide just one or several types among the ones that will be identified hereinafter, and that within the same type one or more different substances among the ones listed (or others still) might be present.

Favorably, therefore, the aqueous solution can comprise free cyanide in a quantity comprised between 10 grams and 50 grams per liter of solution.

In one solution of great practical interest, the aqueous solution can furthermore comprise a complexing agent, preferably in a quantity comprised between 5 grams and 30 grams per liter of solution. Preferably, said complexing agent is chosen from the group constituted by a carboxylic acid, an amino acid, a polyamine, an amine, and a phosphonic acid.

Positively, the aqueous solution can furthermore comprise a conducting salt, preferably in a quantity comprised between 10 grams and 100 grams per liter of solution. Preferably, the conducting salt is chosen in the group constituted by a citrate, a tartrate, an oxalate, a gluconate, a carbonate, a phosphate, and a sulfate.

Usefully, the aqueous solution may furthermore comprise a wetting agent, preferably in a quantity comprised between 0.05 grams and 10 grams per liter of solution. In particular, the wetting agent is chosen preferably in the group constituted by lauryl sulfate and a quaternary ammonium salt.

Advantageously, the aqueous solution can furthermore comprise also a grain refiner and/or a brightener. In greater detail, the grain refiner and/or the brightener are preferably chosen from the group constituted by zirconium, iridium, selenium, antimony, tin, gallium, tellurium, germanium, bismuth, titanium, and silver.

Furthermore, it is specified that the aqueous solution may furthermore comprise additional metals or elements in traces, which derive from the technical products used for its provision and preferably comprised in the group constituted by: silver, arsenic, selenium, germanium, and gallium.

Like the electrodeposition process, a subject matter of the present description is also a galvanic bath intended for the execution of a galvanic electrodeposition process (of the type just described). This galvanic bath comprises an aqueous solution and is adapted to coat with a layer of yellow gold alloy at least one object immersed in said solution.

According to the invention, and by analogy with what has been described in the preceding paragraphs, the aqueous solution comprises at least:

- gold in a quantity comprised between 1 gram and 10 grams per liter of solution, in the form of alkaline cyanide,

- copper in a quantity comprised between 30 grams and 70 grams per liter of solution, in the form of an alkaline cyanide compound,

- indium in a quantity comprised between 100 milligrams and 2 grams per liter of solution, in the form of a soluble compound,

- zinc in a quantity comprised between 100 milligrams and 2 grams per liter of solution, in the form of a soluble compound.

Furthermore, the bath according to the invention can comprise one or more of the specifications related to the bath itself and to the aqueous solution described in the preceding paragraphs in relation to the galvanic electrodeposition process.

Merely by way of example, one possible formulation of the aqueous solution of the galvanic bath, which has been found to be of extreme practical interest and maximally effective in achieving the goals that will be described in the pages that follow, is described hereinafter.

In this possible formulation, the aqueous solution thus comprises: gold in a quantity equal to 5 grams per liter of solution, copper in a quantity equal to 55 grams per liter of solution, indium and zinc in quantities (each) equal to 0.5 grams per liter of solution. Furthermore, in the electrodeposition process which uses a galvanic bath with the formulation indicated above said bath is kept at a constant temperature equal to 60 °C, the solution is kept at pH 10.5, and a current density equal to 0.5 A/dm ² is applied. Moreover, said formulation is enriched with:

- tetraethylenepentamine (1 milliliter per liter of solution),

- (2-hydroxy ethyl)iminodiacetic acid (10 grams per liter of solution),

- sodium glucoheptonate (20 grams per liter of solution),

- free potassium cyanide (25 grams per liter of solution).

In addition to the process and to the bath already described, the present description also relates to a yellow gold alloy, which (preferably but not necessarily) is obtained indeed by means of the process and/or the bath described in the preceding pages.

According to the invention, said alloy comprises at least:

- gold in a quantity by weight comprised between 30% and 95%,

- copper in a quantity by weight comprised between 1% and 65%,

- indium in a quantity by weight comprised between 2% and 6%,

- zinc in a quantity by weight comprised between 0.01% and 3%.

In particular, in an embodiment of considerable practical interest, the yellow gold alloy comprises:

- gold in a quantity by weight comprised between 33% and 92%,

- copper in a quantity by weight comprised between 3% and 60%,

- indium in a quantity by weight comprised between 2% and 6%,

- zinc in a quantity by weight comprised between 0.5% and 1.5%.

Even more particularly, in a preferred embodiment option, which is in any case mentioned by way of non-limiting example of the invention, the alloy comprises:

- gold in a quantity by weight equal to 75%,

- copper in a quantity by weight equal to 19%,

- indium in a quantity by weight equal to 5%,

- zinc in a quantity by weight equal to 1%. The present description also relates to the use of a gold alloy as described in the preceding paragraphs to provide self-supporting shells which can be used in particular in the fields of clothing, jewelry, costume jewelry, shoes and leatherwear (and other sectors such as fashion, faucets, eyewear and also electronics).

It should be specified in any case that the protective scope claimed herein is extended to processes and baths (and alloys) which have the particularities described in the preceding pages, but independently of the specific field of application, which therefore may be any.

The use of the bath and of the alloy and in general the execution of the process and of the invention are in fact evident from what has been described so far.

It has in fact been shown that the process provides, according to per se traditional manners, for applying a potential difference to the electrodes immersed in the galvanic bath, wherein the object (each object) to be coated with a protective layer acts as a cathode. One of the particularities of the invention, which allows to overcome the limitations of known processes, is indeed the composition of the aqueous solution of the bath (in which the object to be coated is indeed immersed).

The choice to resort to a bath the aqueous solution of which comprises (in the indicated ranges) gold and copper in the form of alkaline cyanides, indium and zinc in the form of soluble compounds, in fact allows to deposit a layer of quaternary gold alloy (comprising gold, copper, indium and zinc) with high mechanical strength, both with low thicknesses and with high thicknesses, so as to be able to constitute a self-supporting shell (but also, if so desired, an effective coating layer). In this regard, especially resorting to zinc in the form of a soluble complex is of extreme practical interest.

The specific combination of components (copper, gold, indium and especially zinc) in the indicated ranges also allows to obtain a yellow (IN- 3N) deposit.

In greater detail, in addition to mechanical strength, the obtained layer has high solderability, understood indeed as the capacity to withstand successive soldering treatments (which are for example necessary to close the openings provided in order to remove the internal core and indeed obtain the self-supporting shell).

Furthermore, at the end of the galvanic electrodeposition process the deposited layer is free from residual tensions, proving itself resilient (not fragile) and therefore capable of withstanding impacts and stresses without generating cracks and failures.

The invention is furthermore very versatile and allows to obtain yellow gold alloys with a number of carats comprised between for example 8 and 23.

The chosen composition (and in particular the use of cyanides) ensures the capability to keep the fineness constant, as a further assurance of the reliability and repeatability of the process and of the strength of the alloy obtained.

Similar remarks can be developed in relation to the gold alloy according to the invention, wherein said alloy indeed has a peculiar quaternary formulation, which comprises at least (in the indicated ranges) gold and copper, zinc and indium as bonding metals. Said 1N-3N yellow gold quaternary alloy indeed has a high mechanical strength and solderability in addition to the other benefits illustrated earlier.

The invention allows to obtain the results described above without using and without resorting to silver (except in traces or as a grain refiner or brightener) and especially without cadmium or other metals and substances that are potentially toxic, thereby revealing itself to be of extreme practical interest.

The further additives and substances that can enrich the aqueous solution, described in the preceding pages, increase and further enhance the particularities described above.

The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims; all the details may furthermore be replaced with other technically equivalent elements.

In the exemplary embodiments shown, individual characteristics, given in relation to specific examples, may actually be interchanged with other different characteristics that exist in other exemplary embodiments.

In practice, the materials used, as well as the dimensions, may be any according to the requirements and the state of the art.