TITLE

A GALVANIC AND THERMAL PROCESS TO OBTAIN THE COLORATION OF METALS, IN PARTICULAR PRECIOUS METALS

Technical field of the invention

The present invention refers to a technology with the purpose of obtaining particular colorations of objects, in particular on metal objects.

In particular, the invention refers to an innovative method which allows to obtain special color tones generally not obtainable with known techniques, in particular in metals and more particularly in precious metals for decorative purposes.

Background art

The creation of metallic objects can follow various known techniques depending on the object.

To make a nonrestrictive example, the production of precious objects can be processed by the known galvanic deposition process. For example, objects made of bronze or brass are processed by galvanic deposition in such a manner that mechanical and also technical characteristics are improved, especially if they are objects for the fashion industry or for jewelry industry in general.

By such known electrolytic process it is possible to deposit multilayers of material on the object with the external layer generally but not only of precious metal, such as gold and its alloys and/or palladium and its alloys.

Of course, depending on the layers of material which are deposited in terms of number of layers and type of used materials, different colorations are obtained.

However, determined colorations or color tones are not obtainable through this technique, as it has been tried to find the right combination of layers and material, making necessary the achievement of a classic coloration with a proper dye.

For example, colors such as orange, violet, red, blue and green cannot be obtained generally.

It is therefore necessary to foresee a coloration process for such colors, by using different technologies, such as painting or PVD deposition (Physical Vapour Deposition) .

In case of using paintings, there are considerable inconveniences, since the productive process is remarkably extended. In addition to the color deposition phase, which entails the use of solvent/s, it is necessary a further final step for fixing the color by means of a so-called "curing" phase or heat hardening. The high environmental impact in terms of pollution and disposal costs it to be highlighted.

In case of treatments such as PVD, the high cost of the needed machineries and the low number of processable pieces entail at the same time such high costs making said technology barely applicable. In addition, the range of colorations which are obtainable with said technology is very limited.

Disclosure of the invention

It is therefore the aim of the present invention to provide an innovative method which solves said technical inconveniences .

In particular, it is the aim of the present invention to provide a method which allows to obtain colorations and color shades on objects in general, on metallic or non-metallic base or hybrid/composite non- metallic - metallic, without the need to use final paintings . These and other aims are thus obtained with the present method for the coloration of and object, according to claim 1.

The method comprises a processing phase of the object by galvanic deposit, that is a standard electrolytic process and a heat treating phase which foresees the heating of the object to a predetermined temperature for a preset period.

In this manner, surprisingly, all said technical inconveniences are easily solved.

It has been surprisingly discovered that the combination of the two processes, individually well known on themselves, that is the galvanic process and heat thermal treatments and following cooling, allows the shade of colorations that instead cannot be obtainable when carried out in seguence on the object and according to predetermined parameters.

By varying the parameters of the sole galvanic process, it has not been possible to obtain colorations of a certain type.

The combination of a galvanic process with a thermal treatment allows to obtain colorations that were not obtainable differently. Depending on selected ranges of temperature and the permanence period, it is possible to obtain unlimited different shades, succeeding in covering the different shades of colors that until now have been obtainable only by an external additional coloration.

In a favorite realization form, said object is a metallic object; however, said object can also be an object with non-metallic base, for example, organic, organic-polymeric or mixed organic-metallic/organic- polymeric-metallic, which can be suitable to be undergone to a galvanic process and resist to high temperatures such as those temperatures used in the present method.

It has been tested that, the galvanic processing phase has preferably to be made before the thermal processing phase. In this manner, the galvanic phase allows the deposit of the precious material which is subsequently colored with the thermal treatment. In this way, various different color shades are obtained.

However, when it is preferred, the availability of a first process of a thermal treatment and a subsequent galvanic processing is not excluded. The above opposite method (that is the thermal-galvanic sequence) is certainly possible but it has some problems due to the "cleaning" and "pre-treating", necessary to the good bond of the galvanic coating, that can actually damage the surface, modifying the final obtainable coloration and allowing hardly to obtain green-orange-blue-violet colorations. Instead, such colorations are obtainable by the preferred sequence "galvanic + firing (that is thermal treatment)". In that sense, the thermal-galvanic sequence is functional but it allows a more limited number of colorations in respect with the galvanic-thermal sequence, which represents the preferred configuration of the invention .

Further advantages can be deduced from the independent claims.

Description of the invention

The present invention concerns a method to obtain the coloration of an object, preferably but not necessarily a metallic object. The method, comprising a phase of deposit of an external material on the object and a thermal treatment phase which foresees the heating of the object to a predetermined temperature for a preset period. The two phases are not necessarily in said sequential order.

More particularly, according to a first preferred configuration of the invention, the obtaining of particular colorations, that is various shades of color, on metals has been surprisingly obtained by coupling a galvanic process and a subsequent thermal process.

In this manner, the subsequent thermal process on the metal, depending on the selected ranges of temperatures and on the exposition period, allows to modify the color of the previous galvanic phase, thus managing to obtain shades and colors obtainable generally by the use of painting treatments (with a resulting value impoverishment/reduction of the treated object).

Entering into the descriptive process of the invention, it is here described in details the galvanic process .

The "electroplating" process allows to cover an object of non-precious metallic material (of a suitable non-metallic material or of a composite non-metallic- metallic composite as well) , with a thin layer or more layers of a more precious metal/s, by using the electrolytic plating.

In a bath, which constitutes the so-called galvanic bath, containing a water-based solution of salt of the metal to be deposited, two electrodes are immerged: the cathode is constituted by the object to be covered, while the anode can be constituted by the metal which has to be deposited, by another inert metal or by graphite. A difference of potential is imposed to such electrodes by means of a power generator. In such conditions, the cations of the metal to be deposited will move towards the cathode (negatively charged) while anions will move towards the anode (positively charged) .

The following phenomena occur to the two electrodes:

- Gaining of electrons to the cathode (reduction) Production of electrons to the anode (oxidation) . Therefore, cations are deposited on the cathode and they gain electron reducing themselves. In such manner, the cathode is slowly covered by a thin metallic layer while the anode, when is sacrificial, is slowly consumed thus releasing ions in solution. In connection with the metallic layer which has to be deposited and is usually several dozen microns or less, for a predetermined value of current density used by the bath and knowing the deposition speed, it is sufficient to set the necessary period in order to form the deposit of the desired thickness. Some baths, as those used for the deposit of noble metals such as silver and gold, use also a solution of cyanide ions and for this reason they are called "cyanide baths". This kind of bath reguires, by law, qualified operators for the use of cyanide, such qualification is recognized by a renewable license. Rhodium, nickel, copper, chrome, palladium, ruthenium, silver, copper, tin, zinc and their alloys are other commonly used metals in electroplating.

The technique is well known in itself and it is not further described in details here.

According to the invention, the thickness range of the depositing metal can be comprised from 0,001 (micron) ym and 5 (micron) μπι and preferably from 0,05 to 1 micron ( m) .

On the obtained plated products, precious covering/plating metals, generally Au, Pd, Ru, Rh, Pt, Ag, Ir, are usually deposited as pure metals or in alloy with selected metals from the group comprising Cr, Fe, Co, Ni, Cu, Zn, Cd, In, Sn, Pb, Bi, V, Ga, Y, Zr, Mo, W; preferably Au, Pd, Ru, Rh, Pt, Ag, as pure metals or in alloy with Fe, Co, Ni, Cu, Zn, Cd, In, Sn, Pb, Bi; more preferably in alloy with Co, Ni, Cu, Zn, In, Sn, Bi .

Non-precious metals, so-called base or underlayers, are usually selected from the group comprising Fe, Ni, In, Co, Sn, Cu, Zn, Cr, Bi, Al, V, Mn, Mo, Bi and their alloys; preferably comprising Fe, Ni, In, Co, Sn, Cu, Zn, Cr, Bi, Al; more preferably comprising Fe, Ni, In, Sn, Cu, Zn, Cr, Bi e Al .

In a fulfilling form of the invention it is also foreseen the use of suitable combinations/alloys of the above mentioned metals, in the more suitable amount, depending on the necessity of the problem to be solved.

The used underlying layers are preferably bright and/or glazed nickel (Ni), bright and/or glazed copper (Cu) and white and/or yellow bronze, brass or other Cu, Sn and/or Zn alloys based; otherwise they are constituted by layers of overlapping precious metals (for example selected from the previous mentioned ones), in order to create some diffusion barriers for the metal migration of the base material (for example but not only, selected from the previous mentioned ones, such as rhodium (Rh) , ruthenium (Ru) , tin (Sn) and/or their alloys).

The electrodes used for the electrolytic depositions of the desired metallic alloys are conveniently selected from those commonly used in background art, depending on the productive need. Only to exemplify and not to limit the aims of the present invention, electrodes can be selected from: soluble electrodes such as nickel or copper electrodes or inert electrodes such as graphite, mixed oxides and plated titanium electrodes.

In the same way, electrolytic baths/electrolytes suitable to realize depositions of the desired galvanic layers, are conveniently selected from those commonly used in background art, possibly by modifying them on necessity in order to obtain the desired alloy. Only to exemplify and not to limit the aims of the present invention, electrolytes can be selected from: electrolytes for the deposit of gold-iron alloys from citrate matrix, electrolytes for the deposit of gold-cobalt alloys from citrate matrix, electrolytes for the deposit of palladium- iron alloys from alkaline ammoniac matrix, electrolytes for the deposit of palladium-indium alloys from alkaline ammoniac matrix, electrolytes for the deposit of palladium-iron alloys from acid matrix, electrolytes for the deposit of gold-copper-cadmium, gold-copper-zinc and gold-copper-indium ternary alloys from alkaline matrix, acid electrolytes on sulphate and sulphammate base for the deposit of rhodium and ruthenium and their alloys. Other saline matrices for the deposit can be matrix with base of phosphate, pyrophosphate, cyanide, thiocyanate, carboxylates and ammines variously substituted.

The concentration of salts in solutions of said electrolytes is usually comprised between 1 g/L and 500 g/L; preferably, between 10 and 400 g/L; more preferably, between 20 and 300 g/L.

The concentrations of metals in solutions for the electrodepos ition are usually comprised between 0,0001 g/L and 500 g/L; preferably, between 0,001 and 300 g/L; more preferably, between 0,01 and 200 g/L.

The intensities of working currents are usually comprised between 0,001 A/dm ² and 100 A/dm ² ; preferably, between 0,01 and 50 A/dm ² ; more preferably, between 0,1 and 30 A/dm ² .

The thickness of deposited layers is usually comprised between 0, 0001 μπι and 200 pm for non-precious metals; preferably, between 0,001 and 100 ym; more preferably, between 0,1 and 50 ym.

In turn, the thickness of deposited layers is usually comprised between 0, 0001 and 50 μπι for precious metals; preferably, between 0,001 and 10 μπι; more preferably; between 0,01 and 5 μιη.

After the galvanic processing/deposit phase it is fulfilled the heat treatment which foresees generally a temperature increase to reach an operative temperature, so-called set point, a maintenance for a determined period of the reached temperature and a subsequent cooling. The heating is fulfilled in suitable heating devices such as, for example, stoves or ovens.

In a first embodiment, the heating cycle, so-called "firing", can foresee the insertion of the object, which is at room temperature, inside a stove/oven at room temperature as well. The stove/oven is turned on in order to bring it to the desired temperature (to the so-called set point temperature) , keeping it to such temperature for a predetermined period in order to cool it in calm air (but, possibly, also in moving air; for example, in forced air flow), for example, simply turning off the stove/oven.

In a second embodiment, the firing cycle comprises first a heating phase of the stove/oven to the desired temperature followed by the insertion of the object, which is at room temperature. In this manner, the metal is subjected to a sort of initial thermal shock.

The choice of one method or another one depends on desired chromatic effects.

In the case of progressive temperature increase the treated object will have a more uniform coloration, while the insertion of the piece inside the stove/oven, already heated to the set point temperature for the treatment, allows to modulate soft colorations and depending on the layout of the objects themselves.

Whatever firing procedure is selected, depending on the desired color effect, inside the stove/oven different types of atmosphere can be used and in particular:

- an oxidizing atmosphere, for example, oxygen (0 ₂ ) , air, air containing excess oxygen, nitric oxide (NOx) ;

- a reducing atmosphere, for example, hydrogen (H ₂ ) , ammonia (NH ₃ ) , carbon, carbon monoxide (CO) , molecular sulphur ;

an inert atmosphere, for example nitrogen (N ₂ ) , argon (Ar) , helium (He), vacuum.

From the interaction of the object surface with the compounds in the atmosphere of the stove/oven different colorations are produced, following various phenomena that can take place on the surface, such as diffusion of elements on the surface, formation of superficial compounds like oxides or nitrides or migration of elements between layers.

In order to develop/realize the desired final color, the following parameters, related to the stove/oven and to the heating phases, have to be adequately regulated:

- the operative temperature (the so called "set- point" temperature for the thermal treatment, determined on the basis of the required coloration, of the base material and of the galvanic cycle to which the object is submitted) ;

- the residence time of the specimen inside the stove/oven to the operative temperature (set-point) with stationing period of the specimen/product to such temperature preferably from 2 minutes (min) to 120 minutes (min) ;

- ramp temperatures (with a pre-heating from room temperature to 70% of the operative temperature, with variable periods from 1 to 10 min, followed by a heating to the final temperature to realize in maximum 5 min) heating/cooling speed.

The used preset temperature range, of set-point, will be connected to the temperature possibly reachable by base materials and will generally vary from 150°C to 1000°C, preferably, from 250°C to 800°C.

The residence of the specimen and/or product inside the stove/oven will be less than 240 min, preferably, less than 120 min, more preferably less than 60 min.

Cooling cycles can vary from minimum 5 min to maximum 2 hours depending on objects size, by using forced air flow, if necessary.

Colors obtained with the above described two phases method of the present invention, can be, for example, regulated from yellow to orange, red, brown, violet, from blue to green, by varying conveniently the alloy and/or deposited metal/s and their related thermal treatment.

In the following experimental section, a certain amount of preparations of products with different color shades is described, only as an example and not restrictively :

Example 1. - It is described a procedure for the preparation of a gold- blue object.

Base material: brass (a specimen for the test is a small brass plate of 5 x 3 cm with 0,3 cm thickness) .

Electrolytic treatment:

Electrolytic degreasing (for 1 min) ; cyanide-free degreasing, cathode treatment;

Plating with acid copper (10 pm) ; acid copper bath with solphuric acid base, with a 200 g/L copper sulphate concentration, anode soluble in copper, room temperature, 4A/dm ² electrical current density;

- Plating with white bronze (2 ym) ; alkaline bath with cyanide base, deposit of white lead-free bronze; deposited alloy: ternary alloy Cu-Sn-Zn. Operative conditions: 60°C, 1 A/dm ² ;

Plating with gold-iron alloy (0,2 ym; alloy composition: Au 98%, Fe 2% in p/p) obtained by weakly acid electrolyte with citrate matrix pH 4,5, gold from gold salt (I), cyanide and iron, as iron sulphate; operative conditions: 40°C, 1 A/dm ² .

Thermal treatment:

Made in oxidizing atmosphere (as enriched in oxygen air) ;

Heat point (set-point) : 600°C;

Heat period at the above temperature: 6 min;

Inserting of the specimen at the above heat point (without any pre-heating) . Obtained color: gold-blue.

Coloration parameters (as per coloration coordinates CIEL*a*b*) result as follows:

a = -6,76 ± 2

b = -28,8 ± 4

Examples 2-7. - By using the same base material of example 1, the same galvanic base and the same process, but varying the treatment period to the same set-point, it has been possible to obtain the following colorations:

Ex.2. - Obtained color: straw-yellow

Set-point residence time: 2 min

Coloration parameters (as per CIEL*a*b*) :

a = 9, 61 ± 2

b = 35, 1 ± 4

Ex .3. - Obtained color: Brown

Set-point residence time: 2,5 min

Coloration parameters (as per CIEL*a*b*) :

a = 21,4 ± 2

b = 45, 1 ± 4

Ex.4. - Obtained color: Orange

Set-point residence time: 3 min

Coloration parameters (as per CIEL*a*b*) :

a = 30, 6 ± 2

b = 44, 9 ± 4

Ex .5. - Obtained color: Violet

Set-point residence time: 4 min

Coloration parameters (as per CIEL*a*b*) :

a = 40, 6 ± 2

b = -15,3 ± 4

Ex.6. - Obtained color: Light blue

Set-point residence time: 5 min

Coloration parameters (as per CIEL*a*b*) :

a = -8,48 ± 2

b = -9, 8 ± 4

Ex.7. - Obtained color: Green Set-point residence time: 7 min

Coloration parameters (as per CIEL*a*b*) :

a = -5,50 ± 2

b = 27,7 ± 4

Example 8. - Processing a blue object

It has been used the same base material of example 1 and applied the same galvanic treatment (electrolytic degreasing; plating with acid copper; plating with white bronze; plating with gold-iron alloy 0,2 pm) , while the thermal treatment has been varied.

Thermal treatment:

ramp temperature from room temperature to 200°C with a heat speed of 5°C/min;

- thermal stay at 200°C for 10 min;

- further heating to the set-point temperature of

500°C;

- thermal stay at 500°C for 15 min.

Coloration parameters (as per CIEL*a*b) :

a = -7,2 ± 2

b = -29,4 ± 4

Example 9. - Processing a violet object

It has been followed the same procedure as per example 9 (the same base material and the same galvanic treatment: electrolytic degreasing; plating with acid copper; plating with white bronze; plating with gold-iron alloy 0,2 pm) , except for the thermal treatment.

Thermal treatment:

ramp temperature from room temperature to 500 °C (set-point) with a heat speed of 5°C/min;

- thermal stay at 500°C for 5 min.

Coloration parameters (as per CIEL*a*b) :

a = 40,2 ± 2

b = -16, 1 ± 4

Example 10. - Processing an orange object

The same galvanic treatment of example 1 has been applied to the same base material of the previous examples, except for the gold-iron alloy plating with 0,05 ym) thickness. Furthermore, the following thermal treatment has been applied:

ramp temperature from room temperature to 500 °C (set-point) with a heat speed of 5°C/min;

- thermal stay at 500°C for 5 min.

Coloration parameters (as per CIEL*a*b) :

a = 40,2 ± 2

b = -16, 1 ± 4

Example 11. - Processing a pink object

The following galvanic treatment has been applied to the same base material of the previous examples:

- electrolytic degreasing, as per example 1 ;

- plating with acid copper, as per example 1 ;

- plating with acid copper, satinized with acid bath containing copper sulphate and organic additives;

- plating with white brass, as per example 1 ;

- plating with gold-cobalt alloy 0,10 μπι thickness. In addition, the following thermal treatment has been applied :

ramp temperature from room temperature to 200°C with a heat speed of 5°C/min;

- thermal stay at 200°C for 10 min;

further heating to the set-point temperature of

500°C;

- thermal stay at 500°C for 15 min.

Coloration parameters (as per CIEL*a*b) :

a = 35, 5 ± 2

b = -21,3 ± 4

As disclosed in the introduction of the present invention, a variation of the invention itself foresees first the thermal treatment and later the galvanic treatment, after having removed the processing specimen/ob ect from the oven. As described above, the process of the present invention is applicable also to apt non-metallic objects, or not fully metallic, that are suitable to be subjected to the galvanic process of the present invention and resistant to expected high firing temperatures (set-point) and to the expected application period for themselves. Only for example and not restrictively, said non-metallic objects can be organic, organic-polymeric or mixed organic-metallic/organic-polymeric-metallic (such as a composite of an apt plastic material or a composite of an apt plastic material covered by one or more layers of metals or metallic alloys as those described above) , that can be subjected to the galvanic process of the invention and that are enough resistant to the high temperatures used in the thermal treatment of the present invention.

Eventually, even the preferred form of the invention concerns the arrangement of an external coating layer of the object by galvanic deposition (electroplating), however the availability to deposit said layers with fully equivalent deposit systems, such as well known "electroless" method and chemical deposition.