WO2004027120A1 - Dark layers - Google Patents

Abstract

The invention relates to an alloy containing 1 - 15 % tin, 19 - 60 % copper and 30 - 80 % bismuth, an electrolytical method and electrolytic compositions for the production and use thereof.

Description

 Dark layers

The present invention relates to dark bismuth-tin-copper alloys, ionic bismuth, tin and copper compounds containing electrolyte compositions and a method for producing the alloys. The invention also relates to the use of the alloys for decorative and functional purposes.

It has long been known to change the color of metal surfaces or electrochemically deposited layers by treatment with chemical solutions. This process is called "dyeing" [Non-metallic inorganic coatings, W. Machu, Springer Verlag, Vienna, 1952; recipes for metal coloring and metal coatings without a power source, OP Krämer, Eugen G. Leuze Verlag, Saulgau, 1990]. In the past, mostly for decorative applications

desired gray or black color tone achieved with different methods. For example, the black dyeing of copper, brass or bronze with a solution of copper nitrate (so-called "black brandy"), with a solution of potassium hydroxide and potassium persulfate (so-called "persulfate pickling") or with a selenium-containing dyeing process, the black dyeing of iron with a solution is known Arsenic trioxide and iron sulfate in hydrochloric acid (so-called "arsenic staining") and the blackening of silver by the so-called "sulfur liver staining".

In these conventional dyeing processes there is the disadvantage that the surface chemical conversion of the substrate to be colored only produces a thin surface layer of paint, which is not only in the layer thickness to be achieved but also in adhesive strength or durability, e.g. with mechanical stress, is limited. Another disadvantage is the high reactivity of the dyeing solutions, which only allow short downtimes and make automation more difficult.

In addition to dyeing by chemical treatment of the surface to be colored, electrolytic processes for producing layers with a gray or black intrinsic color are known, such as black nickel, black zinc, black chrome, black ruthenium or tin-nickel alloys.

EP-B-0 808 921 describes the electrolytic production of a Sn-Cu-Pd alloy with a white luster and the use thereof for decorative purposes.

JP-A-59023895 describes the electrolytic generation of a black coating from a ternary Sn-Ni-Cu alloy.

The electrolytic coloring of anodized aluminum with metal salt solutions containing one or more metal salts selected from nickel, cobalt and tin is described in US-A-4,022,671 or US-A-4,066,816.

These processes sometimes require complicated process control, a high energy requirement or the use of expensive materials. Despite improvements in the layer thickness and the adhesive strength compared to layers produced by superficial chemical conversion, there are still disadvantages with regard to the resistance of the layers produced to mechanical stress, for example by using or cleaning the coated objects. A well-known phenomenon is exfoliation of the color layers of zippers or buttons during washing.

The invention was therefore based on the object of providing a coating material and a method for its production in which the problems mentioned can be avoided.

Surprisingly, it was found that this problem is solved by an alloy that contains certain amounts of tin, copper and bismuth.

Ternary alloys of bismuth, copper and tin, or alloys containing these metals, among others, have been described in the past. The properties of these materials vary greatly with the proportions of the metals.

For example, US-A-5, 368,814 describes a lead-free solder with a low solidus line, which mainly consists of bismuth and tin (ie in each case approx. 50%, based on the total amount of bismuth and tin) and in addition an effective amount of one contains physical and mechanical properties reinforcing third component, such as indium, copper, silver or combinations thereof (eg 4% copper). The alloy is described as a particularly useful joining material in microelectronic applications.

A scratch-resistant bronze alloy based on copper as the main component, which also contains 1-13% by weight of tin, not more than 18% by weight of zinc, 0.5-6% by weight of bismuth, 0.05-3% by weight of antimony contains as 1% by weight of phosphorus and less than 4% by weight of lead is described in US-A-6,419,766.

Copper-based casting alloys containing small amounts of bismuth and tin e.g. in US-A-5,487,867 (0.1-7 wt% bismuth and optionally 2-6 wt% tin), US-A-4,789,094 (1.5-7 wt% bismuth and 1-12 wt% tin ) and in US-A-5,330,712 (0.1-7 wt% bismuth and up to 16 wt% tin).

However, there is no indication in the cited documents that an alloy containing 1-15% by weight tin, 19-60% by weight copper, 30-80% by weight Bismuth and 0 - 10 wt .-% have oxidic oxygen, the present object is achieved.

The invention thus relates to an alloy comprising a) 1-15% by weight tin, b) 19-60% by weight copper, c) 30-80% by weight bismuth and d) 0-10% by weight. % oxidic oxygen and its use for decorative or functional purposes.

The invention further relates to a coated article comprising a substrate and a layer of an alloy according to the present invention.

The invention further relates to an electrolyte composition for producing the alloy according to the invention, comprising a) at least one ionogenic tin compound, b) at least one ionogenic copper compound, and c) at least one ionogenic bismuth compound.

The invention also relates to an electrolytic method for producing dark alloy layers, comprising the steps a) providing a substrate to be coated as a cathode, b) providing an electrolyte composition according to the invention, c) electrolytically depositing a dark alloy layer according to the invention on the substrate, and d ) optionally aftertreatment of the dark alloy layer produced in step c).

The alloy according to the invention comprises 1-15% tin, 19-60% copper, 30-80% bismuth and 0-10% oxidic oxygen. All percentages based on the alloy are intended to mean% by weight below.

The alloys of the present invention are characterized by a dark color. The dark color can vary depending on the exact composition of the alloy between brown, gray, dark blue, gray-blue and black. The color can also be varied by increasing the oxygen content (e.g. gray / anthracite: 0% O; dark gray - black: 1 - 2% O).

The alloys of the present invention can also contain small amounts of other metals such as iron or aluminum. Under the label “Small amounts” are to be understood here as meaning amounts which are not more than 5% by weight, preferably 2% by weight and particularly preferably 0.5% by weight.

In a preferred embodiment, the alloy consists essentially of 1-15% tin, 19-60% copper, 30-80% bismuth and 0-10% oxidic oxygen and inevitable impurities. Such contaminants can e.g. impurities resulting from the manufacturing process or the starting materials used, such as nitrogen, phosphorus, carbon. Typically, these contaminants are present in the alloy in amounts that are no more than 3.0% by weight. Such impurities are preferably present in amounts which are not more than 1.0% by weight and particularly preferably not more than 0.5% by weight. Accordingly, the amount of inevitable impurities in the alloys of the present invention is about 0 to about 3.0% by weight, preferably 0 to 1.0% by weight, and particularly preferably 0 to 0.5% by weight.

In a further preferred embodiment of the invention, the alloy is essentially free of one or more elements selected from nickel, lead, selenium, chromium and antimony. It is particularly preferred that the alloy be substantially free of at least two elements, more preferably free of at least three of these elements, and most preferably free of all of the elements listed.

The expression "essentially free of" means that the amount of these elements in the alloy is less than 2% by weight. The amount is preferably less than 1% by weight, particularly preferably less than 0.5% by weight. %, and most preferably less than 0.1% by weight.

The alloys of this embodiment have the additional advantage that they contain no toxic or allergy-causing elements and accordingly the use of toxic or allergy-causing substances in the production of the alloys can be avoided.

In another preferred embodiment, the alloy is essentially free of one or more elements selected from silver, gold, palladium, platinum, ruthenium, rhodium or iridium. The expression "essentially free of" is as defined above.

The alloys of this embodiment have the additional advantage that they are particularly inexpensive to manufacture.

In a further preferred embodiment, the alloy is essentially free of zinc, the expression “essentially free of” as defined above. The alloys of this embodiment are distinguished by a particularly pronounced resistance to mechanical stress.

In the alloys of the present invention, the amount of tin is 1-15%, preferably 1-10% and particularly preferably 1-2%.

In the alloys of the present invention, the amount of copper is 19-60%, preferably 29-60% and especially 39-60%.

In the alloys of the present invention, the amounts of bismuth are 30-80%, especially 35-70% and particularly preferably 39-60%.

The metals forming the alloy, i.e. Tin, copper and bismuth, as well as any other metals that may be present, can also be in oxidic form. Accordingly, the amount of oxidic oxygen in the alloys according to the invention is 0-10% by weight, preferably 0-5% and particularly preferably 0-2%.

The alloys of the invention are obtainable by electrodeposition from an electrolytic solution. First, a substrate to be coated is provided and switched as a cathode.

The substrate is not limited and can consist of an electrically conductive but also an electrically non-conductive material. In the latter case, it is preferred to first coat the material to be coated with an electrically conductive material, for example a metal such as palladium or copper. Tin, copper, zinc or alloys of these metals, such as bronze or brass, are suitable as electrically conductive substrates.

Various types of plastics, ceramic materials or glass are suitable as electrically non-conductive substrates, for example.

The alloy components are provided in the form of an electrolyte composition which contains at least one ionogenic tin compound, at least one ionogenic copper compound and at least one ionogenic bismuth compound.

In addition, the electrolyte composition can have a suitable solvent or solvent mixture. An aqueous electrolyte composition is particularly preferably used.

The ionic tin, copper and bismuth compounds are in no way limited as long as they are suitable for electrolytic deposition. Typically, preference is given to those compounds which can be dissolved in the selected solvent.

Compounds in which tin is present in oxidation states +2 and / or +4 are preferably used as the tin compound. Compounds in which copper is present in oxidation states +1 and / or +2 are preferably used as the copper compound. Compounds in which bismuth is present in oxidation states +3 and / or +5 are preferably used as bismuth compounds.

The ionogenic tin compound is particularly preferably selected from tin sulfates, tin chlorides, tin sulfonates, tin oxalates, sodium stannates, potassium stannates and mixtures thereof.

The ionogenic copper compound is preferably selected from copper sulfates, copper chlorides, copper sulfonates, copper oxalates, copper oxides and copper cyanides, complex copper compounds and mixtures thereof. The ionic bismuth compound is preferably selected from bismuth nitrates, bismuth carbonates, bismuth citrates, complex bismuth compounds and mixtures thereof.

Other metals optionally contained in the alloy are also preferably used in the form of their salts.

Usual additives can be added to the electrolyte composition as required. These can be auxiliaries for adjusting the pH, such as alkalis (for example potassium, sodium or ammonium hydroxide), inorganic acids (for example hydrochloric acid, sulfuric acid, phosphoric acid or boric acid) and their salts, buffers or organic acids (for example hydroxycarboxylic acids, Citric acid or its salts) act. Conductive salts (e.g. alkali salts of hydrochloric acid, sulfuric acid, phosphoric acid), complexing agents (e.g. EDTA), wetting agents (e.g. anionic, cationic or zwitterionic surfactants) or brightening agents (e.g. organic nitrogen compounds such as polymeric reaction products from epichlorohydrin and amines) can be added as required ,

The electrolyte compositions can be selected, for example, from alkaline (cyanide-containing or cyanide-free), acidic or neutral electrolyte compositions.

An alkaline, cyanide-containing electrolyte composition according to the invention can, for example

a) 0.05 - 5 g / l copper as an ionogenic copper compound,

b) 5-50 g / l tin as an ionogenic tin compound,

c) 0.05 - 5 g / l bismuth as an ionic bismuth compound,

d) 10-200 g / l potassium or sodium hydroxide,

e) 10-200 g / l complexing agent

f) 5-50 g / l of a cyanide compound

g) 0.1-10 g / l brightener and h) 0.1 - 10 g / l wetting agent

exhibit.

The term "g / l metal compound" in the present invention refers to the amount of metal atom in the compound in g, counterions not being included.

The copper compound is preferably selected from sulfates, chlorides, sulfonates, oxalates or oxides.

Potassium or sodium stannate is preferably used as the tin compound.

A nitrate, carbonate or citrate compound is preferably used as the bismuth compound.

Sodium or potassium cyanide is preferably used as the cyanide compound.

Resistant surfactants, e.g. cationic surfactants, polyhydric alcohols, e.g. Glycol or organic nitrogen compounds, e.g. polymeric reaction products of epichlorohydrin and amines can be used.

The pH of the electrolyte composition is preferably adjusted to a strongly alkaline range (e.g. pH 11-13).

An alkaline, cyanide-free electrolyte composition according to the invention can, for example

a) 0.05 - 5 g / l copper as an ionogenic copper compound,

b) 5-50 g / l tin as an ionogenic tin compound,

c) 0.05 - 5 g bismuth as an ionogenic bismuth compound,

d) 10-200 g / l potassium or sodium hydroxide,

e) 10-200 g / l complexing agent,

f) 0.1-10 g / l brightener and g) 0.1 - 10 g / l wetting agent

exhibit.

Sodium or potassium stannate is preferably used as the tin compound.

A sulfate, chloride, oxalate or oxide compound is preferably used as the copper compound.

A nitrate, citrate or carbonate compound is preferably used as the bismuth compound.

Resistant surfactants, e.g. cationic surfactants, polyhydric alcohols, e.g. Glycol and as a brightener organic nitrogen compounds, e.g. polymeric reaction products of epichlorohydrin and amines can be used.

For example, organic acids and their salts, phosphonic acids, phosphonates, gluconates, glucoheptonic acids, glucoheptonates and ethylenediaminetetraacetic acid can be used as complexing agents.

The pH of the electrolyte composition is preferably adjusted to be strongly alkaline (e.g. pH 11-13).

A neutral electrolyte composition according to the invention can, for example

a) 0.05 - 5 g / l copper as an ionogenic copper compound,

b) 5-50 g / l tin as an ionogenic tin compound,

c) 0.05 - 5 g / l bismuth as an ionic bismuth compound,

d) 1-50 g / l potassium or sodium hydroxide,

e) 10-200 g / l complexing agent,

f) 0.1-10 g / l brightener and

g) 0.1 - 10 g / l wetting agent exhibit.

A sulfate, chloride, sulfonate, oxalate or oxide compound is preferably used as the copper compound.

A sulfate or chloride compound or sodium or potassium stannate is preferably used as the tin compound.

A nitrate, citrate or carbonate compound is preferably used as the bismuth compound.

Tetrasodium pyrophosphate, for example, is suitable as a complexing agent.

Resistant surfactants, e.g. cationic surfactants, polyhydric alcohols, e.g. Glycol and as a brightener organic nitrogen compounds, e.g. polymeric reaction products of epichlorohydrin and amines can be used.

The pH of the electrolyte composition is preferably adjusted to a neutral range (pH 6-8).

A further neutral electrolyte composition according to the invention can, for example

a) 0.05 - 5 g / l copper as an ionogenic copper compound,

b) 5-50 g / l tin as an ionogenic tin compound,

c) 0.05-20 g of bismuth as an ionic bismuth compound,

d) 10-200 g / l potassium or sodium hydroxide,

e) 10-200 g / l complexing agent,

f) 0.1-10 g / l brightener and

g) 0.1 - 10 g / l wetting agent

exhibit. Sodium or potassium stannate is preferably used as the tin compound.

A sulfate, chloride, oxalate or oxide compound is preferably used as the copper compound.

A nitrate, citrate or carbonate compound is preferably used as the bismuth compound.

Resistant surfactants, e.g. cationic surfactants, polyhydric alcohols, e.g. Glycol or organic nitrogen compounds, e.g. polymeric reaction products of epichlorohydrin and amines can be used.

For example, organic acids and their salts, phosphonic acids, phosphonates, gluconates, glucoheptonic acids, glucoheptonates and ethylenediaminetetraacetic acid can be used as complexing agents.

The pH of the electrolyte composition is preferably adjusted to a neutral range (pH 6-8).

An acidic electrolyte composition according to the invention can, for example

a) 0.005-0.5 g / l copper as an ionogenic copper compound,

b) 5-50 g / l tin as an ionogenic tin compound,

c) 0.5-15 g / l bismuth as an ionogenic bismuth compound,

d) 10-200 g / l carboxylic acid,

e) 0.1-10 g / l brightener,

f) 0.1 - 10 g / l wetting agent

exhibit.

A sulfate, chloride, sulfonate, oxalate or oxide compound is preferably used as the copper compound. A sulfate or chloride compound or sodium or potassium stannate is preferably used as the tin compound.

A nitrate, citrate or carbonate compound is preferably used as the bismuth compound.

The pH of the electrolyte composition is preferably adjusted to an acidic range (e.g. pH 1 to 5).

The ratio of the metals in the alloy can be influenced in a manner known to those skilled in the art by the ratio of the metals in the electrolyte composition, by the type and amount of any further additives, and by the deposition parameters, such as current density, temperature or flow rate.

Anodes made of insoluble materials, such as platinized titanium or graphite, can be used as the counter electrode. Soluble anodes made from bismuth, copper, tin or alloys made from these metals are also possible.

The temperature during the electrolytic deposition is preferably selected so that it is in a range from approximately 20-70 ° C., preferably in a range from 50-60 ° C.

The current density in the electrolyte solution is preferably set to a range of approximately 0.1-5 A / dm ² , preferably 0.2-0.5 A / dm ² .

The deposition rates are preferably in a range of approximately 0.01-0.5 μm / minute, preferably 0.05-0.1 // m / minute.

The process can produce layer thicknesses of 0.005 - 5 μm, preferably 0.5 - 1 μm.

After the desired layer thickness has been deposited, the alloys can optionally be subjected to a post-treatment step in order to further stabilize the color of the alloy. The alloy layer is treated with an oxygen-releasing substance. The coated substrate is preferably immersed in a solution of this substance. As Oxygen-releasing substances are suitable, for example, peroxide or persulfate compounds, such as hydrogen peroxide or ammonium persulfate.

The concentration of the aftertreatment solution is preferably 0.01 g / l - 5 g / l. Typical diving times are between 1 and 20 minutes.

The alloys according to the invention have a characteristic blue-gray to black color and are suitable for use for decorative purposes, e.g. as a covering material for clothing accessories, such as buttons or zippers. The coatings are characterized by good resistance to wear due to mechanical stress or cleaning. The dark alloy layers can also be used for functional purposes, e.g. can be used as absorbent layers in solar applications. In preferred embodiments of the invention, the alloys are also free of toxic or expensive substances and offer further advantages in terms of economy and usability.

The invention is illustrated by the following examples. However, the invention is not restricted to the preferred embodiments listed therein.

EXAMPLES

example 1

An alkaline, cyanide-containing electrolyte has the following composition:

0.35 g / l copper as copper cyanide

3 g / l tin as sodium stannate

0.5 g / l bismuth as ammonium bismuth citrate

20 g / l dipotassium tartrate 10 g / l potassium cyanide

2 ml / l methylene tetramine

0.1 ml / l polyethyleneimine

At 60 ° C and 0.5 A / dm ² , a brown-black alloy layer can be deposited with this electrolyte. The color can be additionally stabilized with the after-treatment listed under Example 6.

Example 2

An alkaline electrolyte has the following composition

22 g / l tin as sodium stannate

0.5 g / l bismuth as ammonium bismuth citrate

0.2 g / l copper as copper sulfate

100 ml / l 1-hydroxyethylene (1, 1-diphosphonic acid)

120 g / l potassium hydroxide

0.2 ml / l methylene tetramine

At 60 ° C and 0.5 A / dm ² , a blue-gray-black alloy layer can be deposited with this electrolyte. The deposition rate was approximately 0.05 μm / minute. The color can be additionally stabilized with the after-treatment listed under Example 6.

Example 3

A neutral electrolyte has the following composition

22 g / l tin as sodium stannate 0.5 g bismuth as ammonium bismuth citrate

0.2 g / l copper as copper sulfate

100 ml / l tetrasodium diphosphate

Potassium hydroxide to adjust the pH to 8 +/- 1

0.2 ml / l methylene tetramine

At 60 ° C and 0.5 A / dm ² , a black alloy layer can be deposited with this electrolyte. The deposition rate was approximately 0.05 μm / minute. The color can be additionally stabilized with the after-treatment listed under Example 6.

Example 4

An acidic electrolyte has the following composition

22g / l tin as sodium stannate

10 mg / l copper as copper sulfonate

2.5 g / l bismuth as ammonium bismuth citrate

2.5 ml / l methylene tetramine

1 ml / l surfactant

At 60 ° C and 0.5 A / dm ² , a black alloy layer can be deposited with this electrolyte. The pH of the electrolytes was 2.5 +/- 0.5. The color can be additionally stabilized with the aftertreatment listed under Example 5. Example 5

Another neutral electrolyte has the following composition

10 g / l bismuth as bismuth citrate

0.75 g / l copper as copper sulfate

22 g / l tin as sodium stannate

100 ml / l 1-hydroxyethylene (1, 1-diphosphonic acid)

50 g / l potassium hydroxide

2 ml / l methylene tetramine

20 ml / l cationic surfactants

At 50 to 60 ° C and 0.3 A / dm ² , a gray, anthracite-colored alloy layer can be deposited with this electrolyte. The deposition rate was approximately 0.05 μm / minute. There was no further stabilization of the color by post-treatment.

Example 6

The aftertreatment was carried out with

3 g / l perhydrite (e.g. Merck) = 0.1% H ₂ O ₂

With a treatment time of 5 to 10 minutes at 20-25 ° C., the color of the alloys obtained in Examples 1 to 4 can be further stabilized.

Claims

OMG AG & Co. KG Case: 020055 GV u. Z .: G 5385 DE patent claims 1. Alloy comprising a) 1 - 15% tin b) 19 - 60% copper and c) 30-80% bismuth d) 0-10% oxidic oxygen. 2. The alloy of claim 1, wherein the alloy has a dark color. 3. Alloy according to claim 1 or 2, wherein it is essentially free of one or more elements selected from nickel, lead, selenium, chromium and Is antimony. 4. Alloy according to one of claims 1 to 3, wherein it is essentially free of one or more elements selected from silver, gold, palladium, platinum, ruthenium, rhodium or iridium. 5. Alloy consisting essentially of a) 1 - 15% tin b) 19 - 60% copper c) 30-80% bismuth d) 0-10% oxidic oxygen and possibly unavoidable impurities. 6. Coated article comprising a substrate and a layer of an alloy according to one of claims 1 to 5. 7. An electrolyte composition comprising a) at least one ionogenic tin compound, b) at least one ionogenic copper compound, and c) at least one ionic bismuth compound. 8. An electrolyte composition according to claim 7 comprising a) 0.05 - 5 g / l copper as an ionogenic copper compound, b) 5-50 g / l tin as an ionogenic tin compound, c) 0.05 - 5 g / l bismuth as an ionic bismuth compound, d) 10-200 g / l potassium or sodium hydroxide, e) 10-200 g / l complexing agent f) 5-50 g / l of a cyanide compound, g) 0.1-10 g / l brightener and h) 0.1 - 10 g / l wetting agent. 9. The electrolyte composition of claim 7 comprising a) 0.05 - 5 g / l copper as an ionogenic copper compound, b) 5-50 g / l tin as an ionogenic tin compound, c) 0.05 - 5 g bismuth as an ionogenic bismuth compound, d) 10-200 g / l potassium or sodium hydroxide, e) 10-200 g / l complexing agent, f) ^" 0.1-10 g / l brightener and g) 0.1 - 10 g / l wetting agent. 10. The electrolyte composition of claim 7 comprising a) 0.05 - 5 g / l copper as an ionogenic copper compound, b) 5-50 g / l tin as an ionogenic tin compound, c) 0.05-20 g / l bismuth as an ionic bismuth compound, d) 1-50 g / l potassium or sodium hydroxide, e) 10-200 g / l complexing agent, f) 0.1-10 g / l brightener and g) 0.1 - 10 g / l wetting agent. 11. The electrolyte composition of claim 7 comprising a) 0.005-0.5 g / l copper as an ionogenic copper compound, b) 5-50 g / l tin as an ionogenic tin compound, c) 0.5-15 g / l bismuth as an ionogenic bismuth compound, d) 10-200 g / l carboxylic acid, e) 0.1-10 g / l brightener, f) 0.1 - 10 g / l wetting agent. 12. The electrolyte composition according to any one of claims 7 to 11, wherein the ionogenic tin compound is selected from tin sulfates, tin chlorides, tin sulfonates, tin oxalates, sodium stannates, potassium stannates and mixtures thereof. 13. Electrolyte composition according to one of claims 7 to 12, wherein the ionogenic copper compound is selected from copper sulfates, copper chlorides, copper sulfonates, copper oxalates, copper oxides, copper cyanides, complex copper compounds and mixtures thereof. 14. Electrolyte composition according to one of claims 7 to 13, wherein the ionic bismuth compound is selected from bismuth nitrates, bismuth carbonates, bismuth citrates, complex bismuth compounds and mixtures thereof. 15. The electrolyte composition according to any one of claims 7 to 14, further comprising a suitable solvent or solvent mixture. 16. Electrolytic process for producing dark alloy layers, comprising the steps a) providing a substrate to be coated as a cathode, b) providing an electrolyte composition according to one of the Claims 7 to 15, c) electrolytic deposition of a dark alloy layer according to one of claims 1 to 5 on the substrate, and d) optionally aftertreatment of the dark alloy layer produced in step c). 17. The method according to claim 16, wherein step c) is carried out at a temperature of 20 to 70 ° C. 18. The method according to any one of claims 16 or 17, wherein step c) is carried out at a current density of about 0.1 to 5 A / dm ² . 19. The method according to any one of claims 16 to 18, wherein step d) comprises treating the dark alloy layer produced in step c) with an oxygen-releasing substance. 20. The method of claim 19, wherein the oxygen-releasing substance is selected from peroxide compounds and persulfate compounds and mixtures thereof. 21. Use of the alloy according to one of claims 1 to 5 for decorative or functional purposes.