EP3841233A1

EP3841233A1 - Electrolyte for the cyanide-free deposition of silver

Info

Publication number: EP3841233A1
Application number: EP19758675.3A
Authority: EP
Inventors: Sascha Berger; Klaus Bronder; Mario Tomazzoni; Uwe Manz
Original assignee: Umicore Galvanotechnik GmbH
Current assignee: Umicore Galvanotechnik GmbH
Priority date: 2018-08-21
Filing date: 2019-08-20
Publication date: 2021-06-30
Also published as: US11846036B2; WO2020038948A1; JP2023174667A; SG11202013166YA; JP2021535276A; CN112469847A; KR20210044866A; US20220205122A1; JP7405827B2

Abstract

The present invention relates to an electrolyte and to a method for the electrolytic deposition of silver coatings and silver alloy coatings. The electrolyte according to the invention is cyanide-free, storage-stable and ensures the deposition of high-gloss, brilliant and white silver and silver alloy layers for technical and decorative applications.

Description

Electrolyte for the cyanide-free deposition of silver

description

The present invention relates to an electrolyte and a method for the electrolytic deposition of silver and silver alloy coatings. The electrolyte according to the invention is cyanide-free, stable in storage and ensures the deposition of high-gloss, brilliant and white silver and silver alloy layers for technical and decorative applications.

In industry, silver is often electrodeposited from cyanide electrolytes. However, due to the toxicity of cyanide, there is a need for cyanide-free electrolytes. In currently known cyanide-free silver electrolytes, silver is generally used in the form of an organic complex, or the organic silver complex is formed in situ. For many of these cyanide-free silver electrolytes, the bath stability is insufficient. Furthermore, the deposited silver coatings are often not white enough and / or the gloss is insufficient. Therefore, there is still a need to develop stable, cyanide-free silver electrolytes for technical and decorative applications.

Hydantoin derivatives are often used as organic complexing agents for silver. For example, US 2005/0183961 A1 discloses a galvanic bath for the deposition of silver. Silver is used in the form of a non-precipitating, water-soluble salt. 5,5-Dimethylhydantoin or a derivative thereof is used as the organic complexing agent, and pyridyl derivatives serve as brighteners. The baths have a pH of 9 to 13. It is particularly advantageous if the bath contains both 2,2-dipyridyl and a substituted pyridine compound as a brightening agent and also a wetting agent. Advantageous wetting agents are substituted glycine derivatives, which are known commercially as Hamposyl®, and sulfonated naphthalene-formaldehyde condensates, which are commercially available as Blancol N or Rhodacal N. Hamposyl® is N-acyl sarcosinate, i.e. condensation products of fatty acid acyl residues and N-methylglycine (sarcosine). Silver coatings that are deposited with these baths are white and shiny to high-gloss.

US 5,601,696 discloses a galvanic bath for the deposition of silver and a method for deposition of silver using this bath. The bath includes silver salts of inorganic acids, such as silver nitrate and silver oxide, and a complex-forming agent, which is a hydantoin derivative. Furthermore the bathroom can optionally contain a brightener. This is at least one organic sulfur compound which contains an SH group or a carboxyl group, an amino acid containing sulfur or sulfite ions. Examples include thiosalicylic acid, thiamine hydrochloride, thiamine nitrate and potassium sulfite as brighteners. The bath can also contain conductive salts. These are preferably inorganic salts such as potassium chloride, potassium formate and carboxylates. The pH of the bath is between 8 and 13, the bath temperature during the deposition is 30 ° C to 90 ° C, and the current density between 1 and 150 A / dm ² depending on the application. The baths described in US Pat. No. 5,601,696 produce shiny silver layers and can be used for up to three throughputs.

WO 2008/043528 A2 discloses a cyanide-free electrolyte composition for the deposition of a silver or silver alloy layer, which has a silver ion source, a sulfonic acid or a derivative of a sulfonic acid, a wetting agent and a hydantoin derivative. The electrolyte composition is used to deposit crack-free and ductile silver and silver alloy layers. At least one silver salt of a sulfonic acid is used as the source of silver ions. Additional silver ion sources selected from silver oxide, silver nitrate and silver sulfate can optionally be included. If silver alloy layers are to be deposited, appropriate sources of alloy metal ions are used, advantageously sulfonic acid salts, oxides, nitrates or sulfates. The hydantoin derivative for complexing the silver has two substituents at position 5 of the heterocyclic ring, which are selected independently of one another from hydrogen, an alkyl group with 1 to 5 carbon atoms and substituted and unsubstituted aryl groups. Optionally, the electrolyte composition can have a wetting agent, for example a naphthalenesulfonic acid-formaldehyde polycondensate and / or a sulfopropylated polyalkoxylated naphthol. Furthermore, an alkali bromide, preferably potassium bromide, and / or a thiosulfate, preferably an alkali thiosulfate such as sodium thiosulfate, can optionally be added to the electrolyte. Both alkali bromide and thiosulfate serve as matting agents. The alkali bromide also results in more uniform color deposition results. The electrolyte compositions have a pH of 8 to 14. In fact, without the addition of alkali bromide or thiosulfate, shiny silver layers seem to be deposited, with no statements being made about the degree of gloss. Electrolyte compositions with alkali bromide and / or thiosulfate produce matt silver coatings. WO 2008/043528 A2 makes no statements regarding the color of the silver layers. US 201 1/0062030 A1 describes an electrolyte composition for the deposition of metals, especially silver, on solar cells. The silver is used, for example, in the form of its methanesulfonate. Imidosuccinate derivatives serve as complexing agents for the metal ions. Optionally, a hydantoin derivative can be used as the second complexing agent. The composition advantageously contains an additive to increase the conductivity, preferably a citrate, and a wetting agent, preferably with a polyalkylene oxide chain. The pH of the electrolyte composition is between 8 and 12. Apart from metals to be deposited, which may be in the form of their methanesulfonates, the composition preferably contains neither further sulfonic acid derivatives nor cyanides. No statements are made regarding the color and gloss of the deposited layers.

US 2012/0067733 A1 describes a method for depositing a silver layer on a nickel layer from a cyanide-free electrolytic bath. Suitable silver sources for the bath are silver oxide, silver nitrate, silver sodium thiosulfate, silver gluconate, silver-amino acid complexes such as e.g. Silver-cysteine complexes, silver alkyl sulfonates, e.g. Silver methanesulfonate, silver hydantoin compounds and silver succinimide complexes. The bath contains at least one imide, for example a succinimide, maleimide, phthalimide or a hydantoin derivative. The silver sources are present in comparatively low concentrations of 0.1 to 5 g / l silver, while the imide is used in a concentration of 40 g / l to 120 g / l. The bath optionally contains amidosulfonic acid or an alkylsulfonic acid. Furthermore, the bath can optionally contain surface-active substances, which can be anionic, cationic or amphoteric. The electrolytic bath has a pH of 8 to 12 and produces mirror-like silver layers on nickel.

US 2012/0067735 A1 describes a cyanide-free electrolyte for the deposition of silver, in which silver is complexed with at least one complexing agent selected from hydantoin, hydantoin derivatives, succinimide and succinimide derivatives. Suitable silver sources are, for example, silver oxide, silver nitrate, silver sodium thiosulfate, silver gluconate, silver-amino acid complexes such as, for example, silver-cysteine complexes, silver alkyl sulfonates, for example silver methanesulfonate, silver hydantoin compounds and silver succinimide complexes. Furthermore, the bath contains at least one pyridylacrylic acid and at least one organic sulfide, selected from dialkyl sulfides and dialkyl disulfides. The combination of pyridyl acrylic acids and organic sulfides results in mirror-like silver deposits, and the deposition can also take place at high currents and high bath temperatures. The electrolyte can also contain conductive salts and buffer substances. Its pH is between 8 and 14. With the electrolyte according to US 2012/0067735 A1, mirror-like silver layers can be galvanically be deposited. However, this disclosure makes no statement about the color of the deposited layers.

WO 2015/018654 A1 discloses a cyanide-free, acidic and aqueous electrolyte for the deposition of silver-palladium alloys containing predominantly silver as well as a method for the deposition of these layers. In addition to silver and palladium compounds, the electrolyte contains a tellurium or selenium compound, urea or an amino acid and a sulfonic acid. The amount of tellurium or selenium influences the silver concentration in the deposited alloy. The urea or amino acid complexes the palladium and increases the stability of the electrolyte. The electrolyte ensures uniform deposition of a corresponding silver-palladium alloy over a wide current density range, which is particularly advantageous for the industrial production of contact materials. The process for the deposition of the silver-palladium alloys is advantageously carried out in the strongly acidic pH range.

US 2016/0122890 A1 discloses a cyanide-free electrolyte for the deposition of silver or silver alloys and a method for the deposition of such layers. The electrolyte according to the invention comprises at least one silver ion source, a sulfonic acid and / or a sulfonic acid derivative, a wetting agent and a hydantoin. The silver or silver alloy coatings that can be deposited from this electrolyte are matt and ductile.

WO 2017/067985 A1 describes an electrolyte which contains suitable reducing agents for adjusting the composition of silver-palladium layers. The reducing agents also contribute to improving the layer optics and increasing the brightness (L value, CIE-Lab) of the deposited layers. WO 2017/067985 A1 also discloses a process for the electrolytic deposition of silver-rich silver-palladium alloys. In addition to silver and palladium compounds, the electrolyte contains a tellurium and / or selenium compound, urea or an amino acid as well as a sulfonic acid and also a reducing agent. The amount of tellurium and / or selenium influences the silver concentration in the deposited alloy. The urea or the amino acid complexes the palladium. As the reducing agent content increases, the palladium content in the deposited layer increases, so that the reducing agent serves to adjust the layer composition. The method also disclosed for the deposition of silver-palladium alloys is advantageously carried out in the strongly acidic pH range. Due to the palladium, the deposited silver-palladium alloys are darker than pure silver layers. JP 2018-009227 A discloses a method for the deposition of palladium-silver alloy layers, wherein the weight ratio of Pd to Ag in the layers can range from 1: 9 to 9: 1. The deposited layers are easy to solder and are suitable as electrical contact materials. The electrolytes for the deposition of the Pd-Ag alloy layers contain, in addition to palladium and silver salts, at least one diamine compound and one heterocyclic compound. The diamine compounds are advantageously alkylated diamines, preferably ethylenediamine and 1,3-propanediamine. The heterocyclic compound is hydantoin or a derivative thereof. 1 - (Hydroxymethyl) -5,5-dimethylhydantoin and 5,5-dimethylhydantoin are particularly advantageous. The pH of the electrolyte is between 7.0 and 14.0, most preferably between 10.0 and 1 1.0. The electrolyte can optionally contain conductive salts, buffers, brighteners and wetting agents. There are no statements about the color and gloss of the deposited layers. However, due to the palladium content of at least 10 percent by weight, it can be assumed that the deposited layers are darker than pure silver layers.

Despite the numerous, already known electrolytes for the electrolytic deposition of silver and silver alloys, there is still a need to offer electrolytes which are superior to the electrolytes of the prior art with regard to the whiteness and gloss of the deposits, bath stability and throughput behavior (metal turn over). For industrial use, such electrolytes should have a sufficiently high stability and allow stable alloy compositions to be deposited over the largest possible current density range. The electrolytes should remain fully functional even after a high current density load, and the deposits produced with these electrolytes should be homogeneous and advantageous with regard to their use in technical and decorative applications.

The above-mentioned objects are achieved by means of an electrolyte with the features of claim 1. Sub-claims dependent on claim 1 are directed to preferred embodiments of an electrolyte according to the invention. Claims 9-13 are directed to a method for electrolytic deposition in which an electrolyte according to the invention is used.

By specifying an aqueous, cyanide-free electrolyte for the electrolytic deposition of silver and silver alloy coatings, which in dissolved form has the following constituents: a) at least one silver compound in a concentration of 0.1 to 150 g / l silver, b) at least one compound of an alloy metal in a concentration of 0 to 100 g / l alloy metal, c) at least one compound of the formula (I)

wherein

R1, R2, R3 and R4 independently of one another represent hydrogen, a linear or branched alkyl group having 1 to 5 carbon atoms, an alkoxy group

1 to 5 carbon atoms, a hydroxyalkyl group with 1 to 5 carbon atoms or an aryl group, and wherein the at least one compound of the formula (I) is present in a concentration of 1 to 350 g / l, d) at least one brightener selected from i. at least one amino acid in a concentration of 0.0001-5 mol / l, in particular 0.01-5 mol / l, and / or ii. at least one pyridinecarboxylic acid in a concentration of 0.01-5 mol / le) at least one brightener selected from sulfonamide, 2,2'-sulfanediyldiethanol, cysteine, methionine, aliphatic and aromatic heterocyclic compounds with 5 to 7 ring atoms, the ring being the aliphatic and aromatic heterocyclic compounds contains at least one heteroatom selected from nitrogen and sulfur, and wherein the aliphatic and aromatic heterocyclic compounds optionally contain one or contain several further heteroatoms, selected from nitrogen, oxygen and sulfur, and mixtures of these brighteners, the concentration of the brightener or the mixture of brighteners being 0.005-25 g / l, in the event that the at least one brightener is selected from cysteine and / or methionine and at least one amino acid according to d) i. is chosen as gloss carrier, the gloss-bearing amino acid is neither cysteine nor methionine, f) an alkali metal hydroxide selected from lithium hydroxide, sodium hydroxide, potassium hydroxide and mixtures thereof in a concentration of 1-200 g / l, g) wherein the electrolyte If the pH value is greater than or equal to 7, the tasks set are solved.

Surprisingly, it was found that brilliant, shiny and white silver and silver alloy coatings can be deposited on electrically conductive substrates with the electrolyte described here over a wide current density range. Furthermore, the electrolyte according to the invention has high bath stability as well as high deposition rates and deposition rates, which makes it appear particularly advantageous in industrial application. With the present electrolyte, high-quality electrical contact materials can also advantageously be produced in rack and high-speed coating systems. The electrolyte preferably contains only the components specified above.

The electrolyte according to the invention can be used in a current density range of 0.1 to 100 A / dm ² . A current density range of 0.5 to 20 A / dm ² is preferred.

The person skilled in the art knows that the color and brightness of metallic coatings can be determined using the so-called L ^* a ^* b ^* measurement according to CIEL ^* a ^* b (www.cielab.de), the L ^* value being the brightness indicates. The brightness values (L ^* values) of the silver layers according to the invention are between 95 and 99 L ^* a ^* b ^* (Konica Minolta CM-700 measuring device, light type D65 / 10). The a ^* values are between -0.5 and + 0.5, the b ^* values are between 1.5 and 5.0.

A measurement of the reflection can be used to assess the gloss. The silver layers according to the invention have values in the range from 91 to 93.5. The reflection was measured with the BYK Gardner - mirror TRI-gloss measuring device. The measurement was carried out at 20 ° angle of incidence and 20 ° angle of reflection of the light beam according to EN ISO 7668 (latest version on the filing date). The gloss measurement of surfaces is known to the person skilled in the art and can be found, for example, in the “Series of Electroplating and Surface Treatment. Testing of functional metallic layers, chap. 4.3: Gloss and reflection measurement on surfaces ”, Eugen G. Leuze-Verlag, Saulgau, 1, edition 1997, pp. 1 17-125“.

Galvanic baths are solutions that contain metal salts, from which electrochemical metallic deposits (coatings) can be deposited on substrates (objects). Such galvanic baths are often referred to as “electrolytes”. Accordingly, the cyanide-free and aqueous galvanic baths according to the invention are referred to below as “electrolytes”.

The electrolyte according to the invention for the electrolytic deposition of silver and silver alloy coatings and the method for the deposition of such silver and silver alloy coatings are explained below, the invention comprising all of the embodiments listed below individually and in combination with one another.

The present electrolyte is a cyanide-free, aqueous electrolyte. It makes sense here if all the substances present in the electrolyte are as completely dissolved as possible in order to avoid contamination during the deposition of the layer with undissolved material. In the context of the present invention, a substance is considered water-soluble if at least 0.1 g of this substance dissolves in one liter of water at 25 ° C. Such substances are also referred to below as “soluble compounds” or “soluble substances”.

The silver compound contained in the electrolyte according to the invention is advantageously a silver salt which is soluble in this electrolyte. The silver salts are preferably selected from the group consisting of silver methanesulfonate, silver carbonate, silver phosphate, silver pyrophosphate, silver nitrate, silver oxide, silver lactate, silver fluoride, silver bromide, silver chloride, silver iodide and silver sulfate. Silver nitrate, silver carbonate, silver methanesulfonate, silver chloride and silver oxide are particularly preferably used in the electrolyte according to the invention. Here, the person skilled in the art should use the phrase that as little additional substances as possible should be added to the electrolytes. Therefore, the person skilled in the art will most preferably choose the silver methanesulfonate, the silver carbonate or the silver oxide as the silver salt to be added. Compounds made of silver and the other electrolyte components (e.g. silver hydantoinate) can also be used. In the concentration of the silver compound used, the person skilled in the art will have to orientate himself on the limit values given above. The is preferred Silver compound in a concentration of 0.1-150 g / l silver, more preferably 2-100 g / l silver and very particularly preferably between 4-40 g / l silver in the electrolyte.

The alloy metals are tin, palladium, antimony, cobalt, indium, iron, nickel, ruthenium, rhodium, platinum, copper, zinc, selenium, tellurium, bismuth, iridium, germanium, gallium, gold, rhenium, tungsten, molybdenum , Dysprosium and cerium. They become the bath according to the invention in the form of soluble compounds of Sn ²⁺ , Sn ⁴⁺ , Pd ²⁺ , Sb ³⁺ , Co ²⁺ , ln ³⁺ , Fe ²⁺ , Fe ³⁺ , Ni ²⁺ , Ru ³⁺ , Ru ⁴⁺ , Rh ³⁺ , Pt ²⁺ , Pt ⁴⁺ , Cu ²⁺ , Cu ⁺ _, Zn ²⁺ , Se ²⁺ and Se ⁴⁺ , Te ²⁺ , Bi ³⁺ , Ir ³ *, Ir ⁴ *, Ge ² *, Ge ⁴ *, Ga ³ *, Au ³ *, Re ³ *, Re ⁴ *, W ⁶ *, Dy ³ * and Ce ³ * added.

Suitable soluble compounds of the alloy metals mentioned are known to the person skilled in the art and can be used without leaving the scope of protection of the patent claims. Soluble compounds of the alloy metals to be used advantageously are mentioned below, the invention also encompassing soluble compounds of these metals which are not explicitly listed.

The divalent tin compound is selected from tin (II) fluoride, tin (II) chloride, tin (II) bromide, tin (II) iodide, tin (II) hydroxide, tin (II) oxide, tin (II) pyrophosphate, tin (II) sulfate, tin (II) methane sulfonate. The divalent tin compound is advantageously selected from tin (II) pyrophosphate, tin (II) sulfate and tin (II) methane sulfonate.

The tetravalent tin compound is selected from sodium hexahydroxostannate (IV), potassium hexahydroxostannate (IV) and mixtures thereof.

The divalent palladium compound is selected from tetraamine-palladium (II) chloride, tetraamine-palladium (II) bromide, palladium hydroxide, palladium chloride, palladium sulfate, palladium pyrophosphate, palladium methanesulfonate, palladium nitrate, palladium phosphate, palladium bromide, diamminedinitritopalladium (ll) chloride (di) iodomium (ll) chloride (ll) chloride (ll) -dihallophenadium (II), Diamine dinitritopalladium (II) sulfate, palladium glycinate, potassium di-oxalatopalladate, palladium iodide, palladium (II) cyanide, palladium (II) pentacyanonitrosyl ferrate (III),

Tetraamine palladium (II) sulfate, bis (ethylenediamino) palladium (II) carbonate,

Bis (ethylenediamino) palladium (II) sulfate, bis (ethylenediamino) palladium (II) bromide,

Bis (acetylacetonato) palladium (II), diamine dichloropalladium (II), palladium oxide hydrate, tetraamine palladium (II) hydrogen carbonate, bis (ethylenediamino) palladium (II) chloride,

Palladium acetate, di-potassium tetracyanopalladate.

The trivalent antimony compound is selected from antimony (III) oxide, antimony (III) fluoride, antimony (III) chloride, antimony (III) bromide, potassium antimony oxide tartrate. That is advantageous trivalent antimony compound selected from antimony (III) oxide and potassium antimony oxide tartrate.

The divalent cobalt compound is selected from cobalt (II) chloride, cobalt (II) oxide, cobalt (II) nitrate, cobalt (II) sulfate, cobalt (II) thiocyanate, cobalt (II) acetate.

The trivalent indium compound is selected from indium (111) -chloride, indium (III) gluconate, indium (III) sulfate, and indium (III) oxide. The trivalent indium compound is advantageously selected from indium (III) sulfate, indium (III) chloride and indium (III) gluconate.

The divalent iron compound is selected from iron (II) sulfate hydrate, iron (II) chloride, iron (II) citrate, iron (II) methane sulfonate, ammonium iron (II) citrate, iron (II) chloride hexahydrate, Iron (ll) pyrophosphate, ammonium iron (ll) oxalate, iron (ll) - phosphonic acid complexes, iron (ll) fluoride, iron (ll) bromide, iron (ll) nitrate, iron (ll) thiocyanate, iron (II) hydroxide.

The trivalent iron compound is selected from iron (III) sulfate hydrate Fe ₂ (S0 ₄ ) ₃ , FeCI ₃ , Fe (III) citrate, Fe (III) methane sulfonate, ammonium iron (III) citrate, Fe (III) chloride hexahydrate, Fe (III) pyrophosphate, ammonium iron (III) oxalate, Fe (III) phosphonic acid complexes, Fe (III) fluoride, Fe (III) bromide, Fe (III) nitrate, Fe (III) thiocyanate, Fe (III) hydroxide.

The nickel compounds are selected from nickel (II) sulfate heptahydrate, nickel (II) chloride hexahydrate, nickel (II) sulfamate, nickel (II) nitrate hexahydrate and nickel (II) ethylenediamine complex.

The ruthenium compounds are selected from ruthenium (III) fluoride, ruthenium (III) chloride, ruthenium (III) bromide, ruthenium (III) iodide, ruthenium (III) nitrosyl nitrate, ruthenium (III) acetate, ruthenium-isonitrile complexes, Ru-nitrido Halo complexes of the general formula [Ru ₂ N (H ₂ 0) ₂ X ₈ ] ³ , in which X is a halide ion selected from fluoride, bromide, chloride and iodide, for example [Ru2N (H20) 2Cl8] ³ , ruthenium - Nitrido-hydroxo complexes and Ru-Nitrido-Oxalato complexes.

The trivalent rhodium compounds are selected from rhodium (III) fluoride, rhodium (III) chloride, rhodium (III) bromide, rhodium (III) iodide, rhodium (III) oxide hydrate, rhodium (III) methanesulfonate and rhodium (III) sulfate.

The divalent platinum compound is selected from platinum (II) chloride, tetrachloroplatinic (II) acid H2 (PtCI ₄ ), platinum (II) bromide, dinitrosulfatoplatinic (II) acid and its salts, diamminodinitritoplatinum (II), tetraammine platinum (II) salts, platinum (ll) nitrate and platinum (ll) iodide. The tetravalent platinum compound is selected from hexachloroplatinic acid IV (H2ClCl6), platinum (IV) fluoride, hexahydroxoplatinic acid and their salts and platinum (IV) bromide.

The divalent copper compound is selected from copper (I) -su ifate, copper (I I) -f I ori, copper (I I) -chloride, copper (II) -bromide, copper (II) -iodide , Copper (II) hydroxide, copper (II) oxide, copper (II) oxalate, copper (II) carbonate, copper (II) nitrate, copper (II) phosphate, copper (II) - pyrophosphate, copper (II) methanesulfonate, copper (II) citrate, copper (II) acetate. The divalent copper compound is advantageously selected from copper (II) sulfate, copper (II) chloride and copper (II) pyrophosphate.

The divalent zinc compound is selected from zinc fluoride, zinc chloride, zinc bromide, zinc iodide, zinc sulfate, zinc oxide, zinc hydroxide, zinc pyrophosphate, zinc citrate, zinc methane sulfonate. The divalent zinc compound is advantageously selected from zinc pyrophosphate, zinc sulfate and zinc methanesulfonate.

Suitable selenium and tellurium compounds are those in which selenium or tellurium are present in oxidation states +4 or +6. Selenium and tellurium compounds are advantageously used in the electrolyte, in which selenium or tellurium are present in oxidation state +4. The selenium and tellurium compounds are particularly preferably selected from tellurites, selenites, partial acid, selenic acid, telluric acid, selenic acid, selenocyanates, tellurocyanates and selenate and tellurate. The use of tellurium compound is generally preferred over selenium compounds. The addition of the tellurium to the electrolyte in the form of a salt of the partial acid, e.g. in the form of potassium tellurite.

The trivalent bismuth compound is selected from bismuth (III) oxide, bismuth (III) hydroxide, bismuth (III) fluoride, bismuth (III) chloride, bismuth (III) citrate, bismuth (III) bromide, bismuth ( III) iodide, bismuth (III) methanesulfonate. The trivalent bismuth compound is advantageously selected from bismuth (III) citrate and bismuth (III) methanesulfonate.

The trivalent iridium compounds are selected from iridium (III) sulfate, iridium (III) chloride, iridium (III / IV) chloride, iridium (IV) chloride, potassium hexabromoiridate (IV),

Potassium hexachloroiridate (IV), sodium hexabromoiridate (IV), sodium hexachloroiridate (IV), hexachloroiridium (IV) acid, ammonium hexachloroiridate (IV), ammonium hexabromoiridate (IV).

The germanium compounds are selected from the germanium (ll) or germanium (IV) halides, germanium (ll) selenide, germanium (ll) telluride and germanium (IV) oxide.

The gallium compounds are selected from gallium (III) fluoride, gallium (III) chloride, gallium (III) bromide, gallium (III) iodide and gallium (III) oxide. The gold compounds are selected from alkali gold (l) sulfite, ammonium gold (l) sulfite, tetrachlorogold (lll) acid, gold as gold (l) hydantoin complex or gold (lll) hydantoin complex, potassium dicyanoaurate (l), potassium tetracyanoaurate (lll), gold (l ) cysteine complex and gold (III) sulfate.

The rhenium compounds are selected from rhenium (III) chloride and rhenium (IV) oxide.

The tungsten compounds are selected from alkali tungstate, ammonium tungstate and tungsten oxide; the tungsten (VI) compounds are preferred.

The molybdenum compounds are selected from alkali molybdate, ammonium molybdate and molybdenum oxide; the molybdenum (VI) compounds are preferred.

The dysprosium compounds are selected from dysprosium (III) chloride and dysprosium (III) nitrite.

The cerium compounds are selected from cerium (III) chloride and cerium (III) sulfate hydrate.

The at least one compound of an alloy metal is present in the electrolyte in a concentration of 0 to 100 g / l. In a preferred embodiment, the concentration of the at least one alloy metal in the electrolyte is 0 g / l. In this case, there is no metal to be deposited other than silver, and the electrolyte is used to deposit coatings from pure silver. In a further more preferred embodiment, the at least one compound of the alloy metal is present in the electrolyte in a concentration of greater than 0 to a maximum of 100 g / l. In this case, there is at least one further metal to be deposited in addition to silver, and the electrolyte is used to deposit coatings from silver alloys. The concentration of the at least one alloy metal in the electrolyte is advantageously 0.05-100 g / l, preferably 0.5-20 g / l, particularly preferably 1-10 g / l.

For the purposes of the present invention, “at least one compound of an alloy metal” comprises the following variants: a) A single compound of a single alloy metal is used.

b) Multiple compounds of a single alloy metal are used, i.e. the cations of these compounds originate from the same alloy metal in all cases, while the anions differ. The cations can exist in different oxidation states.

c) Compounds of several alloy metals are used, where all compounds have the same anion, but the cations differ. d) Several compounds of several alloy metals are used, ie there are several different anions as well as several different cations.

In cases a) and b) there is an electrolyte for the deposition of binary silver alloys, in cases c) and d) an electrolyte for the deposition of at least ternary silver alloys.

In the present electrolyte, silver is complexed with at least one compound of the formula (I):

R1, R2, R3 and R4 independently of one another represent hydrogen, a linear or branched alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, a hydroxyalkyl group having 1 to 5 carbon atoms or an aryl group,

The linear or branched alkyl group with 1 to 5 carbon atoms is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, 2-methylpropyl, tert-butyl, n-pentyl, 2-pentyl, 3 -Pentyl, 2-methylbutyl, 3-methylbutyl, 3-methylbut-2-yl, 2-methylbut-2-yl, 2,2-dimethylpropyl.

The alkoxy group with 1 to 5 carbon atoms is selected from methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, iso-butoxy, 2-methylpropoxy, tert-butoxy, n-pentoxy, 2-pentoxy, 3-pentoxy, 2-methylbutoxy, 3-methylbutoxy, 3-methylbut-2-oxy, 2-methylbut-2-oxy, 2,2-dimethylpropoxy.

The hydroxyalkyl group with 1 to 5 carbon atoms is derived from the alkyl groups mentioned, a hydrogen atom of the respective alkyl group being replaced by a hydroxy group. The aryl group is selected from phenol, naphthol, benzene, toluene, xylene, cumene.

The compounds of the formula (I) are hydantoin and its derivatives. The at least one compound of the formula (I) is advantageously selected from 1-methylhydantoin, 1, 3-dimethylhydantoin, 5,5-dimethylhydantoin, 1-hydroxymethyl-5,5-dimethylhydantoin, 5,5'-diethylhydantoin and 5,5- Diphenylhydantoin and mixtures thereof. The at least one compound of the formula (I) is particularly preferably 5,5-dimethylhydantoin.

The complex of silver and the at least one compound of the formula (I) is formed in situ from the silver compound used and the at least one compound of the formula (I). The at least one compound of the formula (I) is used in a concentration of 1 -350 g / l, preferably 5-200 g / l, particularly preferably 10-100 g / l. In a preferred embodiment of the present invention, silver is used as a complex of the formula (I).

In the electrolyte according to the invention, the molar ratio of the silver to the compound of the formula (I) is 1: 2 to 1: 6. This applies regardless of whether a complex of silver and a compound of the formula (I) is used as the silver compound or whether another silver compound is used which is not a complex of silver with a compound of the formula (I) : Overall, the amount of the compound of formula (I) in the electrolyte according to the invention is twice to five times as much as the amount of silver, regardless of what proportion of the compound of formula (I) is present as a silver complex.

Furthermore, the electrolyte according to the invention contains at least one gloss carrier, selected from at least one amino acid in a concentration of 0.0001 -5 mol / l, preferably 0.001 -1 mol / l and particularly preferably 0.01 -0.5 mol / l and / or at least one pyridinecarboxylic acid in a concentration of 0.01 -5 mol / l, preferably 0.01 -1 mol / l and very preferably 0.1 -0.5 mol / l.

It is known to the person skilled in the art that amino acids are compounds having a carboxyl and an amino group. These can be essential or non-essential amino acids. Furthermore, they can be alpha, beta or gamma amino acids, it being known to the person skilled in the art that alpha amino acids have at least two, beta amino acids at least three and gamma amino acids have at least four carbon atoms. The at least one amino acid can be in the D form, the L form or as a racemate. If more than one amino acid is used, each individual amino acid can be present in the D form, the L form or as a racemate, independently of the other amino acids. The at least one amino acid is advantageously selected from alanine, arginine, asparagine, aspartic acid, cystine, cysteine, glutamine, glutamic acid, glycine, Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, valine, sarcosine and mixtures thereof. With regard to sulfur-containing amino acids, such as cysteine or the dimer cystine, it can be advantageous to use these only in concentrations of 0.0001-1 mol / l, preferably 0.0005-0.5 mo / l and particularly preferred 0.001-0.01 mol / l in the electrolyte.

In a particularly advantageous embodiment, the at least one amino acid is selected from glycine, alanine, proline, cysteine and sarcosine and mixtures thereof. The amino acid is very particularly preferably selected from glycine and sarcosine.

In an advantageous embodiment, the at least one pyridinecarboxylic acid is selected from picolinic acid, picolinic acid amide, nicotinic acid, nicotinic acid amide, isonicotinic acid, isonicotinic acid amide and mixtures thereof. The at least one pyridinecarboxylic acid is preferably selected from nicotinic acid, nicotinic acid amide, picolinic acid and picolinic acid amide. In the context of the present invention, both the free pyridinecarboxylic acids mentioned and their amides are referred to as “pyridinecarboxylic acids”. In a very particularly preferred embodiment, the electrolyte according to the invention does not contain pyridinecarboxylic acid, but instead at least one aminocarboxylic acid as mentioned above as a gloss carrier.

The at least one brightener is advantageously selected from the group of the sulfonamides. Sulfonamides are a group of chemical substances known to the person skilled in the art, some of which have an antibiotic effect (Beyer-Walter, Textbook of Organic Chemistry, S. Hirzel Verlag Stuttgart, 22nd edition, 1991, pp. 496, 497, 575-577, 784, 785). In a preferred embodiment, the sulfonamide on a structural element R1-SO2-NR2R3, wherein Ri, R2 and R3 are independently hydrogen, (Ci-Cio) alkyl, (C3-C10) - cycloalkyl, _(C6 -Cio) aryl , (C5-Cio) heteroaryl, (C5-Cio) heterocycloalkyl. R2 and R3 can also form a saturated or mono- or polyunsaturated ring which can have 4 or 5 further atoms, in particular C atoms or 1 or 2 nitrogen or oxygen atoms. This ring can also be substituted.

In this context, (Ci-Cio) alkyl are saturated or mono- or polyunsaturated alkyl radicals, which can be linear or branched as desired. These are preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, 2-methylpropyl, tert-butyl, n-pentyl, 2-pentyl, 3-pentyl, 2-methylbutyl, 3- Methylbutyl, 3-methylbut-2-yl, 2-methylbut-2-yl, 2,2-dimethylpropyl. The radicals considered here can be substituted with heteroatoms, the heteroatoms preferably being selected from the group consisting of oxygen, nitrogen or sulfur. The heteroatoms can also again be substituted with other organic radicals. These radicals are particularly preferably selected from hydrogen, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, 2-methylpropyl, tert-butyl, n-pentyl, 2-pentyl, 3-pentyl, 2- Methylbutyl, 3-methylbutyl, 3-methylbut-2-yl, 2-methylbut-2-yl, 2,2-dimethylpropyl.

In this context, (C3-Cio) cycloalkyl are cyclic alkyl radicals, such as e.g. preferably cyclopropyl, cyclopentyl or cyclohexyl. These residues can have double bonds. The radicals considered here can be substituted with heteroatoms, the heteroatoms preferably being selected from the group consisting of oxygen, nitrogen or sulfur. The heteroatoms can in turn also be substituted with further organic radicals. These radicals are particularly preferably selected from hydrogen, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, 2-methylpropyl, tert-butyl, n-pentyl, 2-pentyl, 3-pentyl, 2- Methylbutyl, 3-methylbutyl, 3-methylbut-2-yl, 2-methylbut-2-yl, 2,2-dimethylpropyl.

(C ₆ -Cio) aryl are aromatic cyclic compounds which, if appropriate, are selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, 2-methylpropyl, tert.-butyl, n- Pentyl, 2-pentyl, 3-pentyl, 2-methylbutyl, 3-methylbutyl, 3-methylbut-2-yl, 2-methylbut-2-yl, 2,2-dimethylpropyl may be substituted. These are very preferably selected from the group consisting of phenol, naphthol, benzene, toluene, xylene and cumene. The radicals considered here can be substituted with heteroatoms, the heteroatoms preferably being selected from the group consisting of oxygen, nitrogen or sulfur. The heteroatoms can in turn also be substituted with further organic radicals. These radicals are particularly preferably selected from hydrogen, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, 2-methylpropyl, tert-butyl, n-pentyl, 2-pentyl, 3-pentyl, 2- Methylbutyl, 3-methylbutyl, 3-methylbut-2-yl, 2-methylbut-2-yl, 2,2-dimethylpropyl.

(C5-Cio) heteroaryl or (Cs-Cio) heterocycloalkyl are derived from the above-mentioned cycloalkyl and aryl radicals, with at least one C atom in the ring being replaced by a heteroatom. It is therefore preferably aliphatic or aromatic heterocyclic compounds having 5 to 10 ring atoms, the ring of the aliphatic and aromatic heterocyclic compounds containing at least one heteroatom selected from nitrogen and sulfur, and the aliphatic and aromatic heterocyclic compounds optionally having one or more contain further heteroatoms selected from nitrogen, oxygen and sulfur. These ring systems can in turn also be substituted with further organic radicals. These radicals are particularly preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, 2-methylpropyl, tert-butyl, n-pentyl, 2-pentyl, 3-pentyl, 2-methylbutyl, 3-methylbutyl, 3-methylbut-2-yl, 2- Methyl but-2-yl, 2,2-dimethylpropyl. The (C5-Cio) heteroaryl or (C5-Cio) heterocycloalkyl radicals can be substituted with heteroatoms, the heteroatoms preferably being selected from the group consisting of oxygen, nitrogen or sulfur. The heteroatoms can in turn also be substituted with further organic radicals. These radicals are particularly preferably selected from hydrogen, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, 2-methylpropyl, tert-butyl, n-pentyl, 2-pentyl, 3-pentyl, 2- Methylbutyl, 3-methylbutyl, 3-methylbut-2-yl, 2-methylbut-2-yl, 2,2-dimethylpropyl.

In a further advantageous embodiment, the at least one brightener is sulfonamides which, as radicals R 1, R ₂ and R _{3, are} aromatic or heterocyclic compounds which have 5 to 10 ring atoms, the ring having at least one nitrogen and / or one Has sulfur atom. Sulfonamides which have aromatic, optionally heterocyclic compounds as the radicals R 1, R ₂ and R ₃ and which can be used as brighteners in the context of the present invention preferably have 5 to 7 ring atoms. At least one ring atom is a nitrogen or sulfur atom. Optionally, one or more further heteroatoms selected from oxygen, nitrogen and sulfur can also be present. These further heteroatoms can also be components of the aromatic ring, but they can also be present in side chains and groups attached to the ring. Suitable aliphatic and aromatic heterocyclic compounds are derivatives of tetrahydrothiophene, thiophene, tetrahydrofuran, furan, pyrrolidine, pyrrole, imidazolidine, pyrazolidine, imidazole, pyrazole, oxazolidine, isoxazolidine, oxazole, isoxazole, thiazolidine, diazoleoleiazole, diazoleoleole, isothiazole , Furazane, Oxadiazole, Thiadiazole, Dithiazole, Tetrazole, Piperidine, Pyridine, Tetrahydropyrane, Pyrane, Thiane, Thiopyrane, Piperazine, Pyrimidine, Diazine, Morpholine, Oxazine, Thiomorpholine, Thiazine, Dioxane, Dioxine, Dithiane, 3-Dithiane, Dithiine , 5-triazine, triazine, trioxane, trithiane, tetrazine, pentazine, azepane, azepine, oxepane, oxepine, thiepane, thiepine, diazepane, diazepine, thiazepine. Aniline or pyrimidine, pyridine and pyrazine are particularly preferred in this context. Those skilled in the art are aware that organic compounds are those based on carbon. Organic hetero compounds in the sense of the present invention are compounds which contain at least one further atom in addition to carbon and hydrogen. This other atom is the so-called heteroatom. It is advantageously nitrogen, oxygen or sulfur. The aliphatic and aromatic heterocyclic compounds mentioned can carry functional groups. These functional groups are advantageously thio, thiol, carbonyl, carboxyl, alkyl, hydroxy. Sulfonyl and sulfonylalkyl groups. In a likewise particularly advantageous embodiment, the at least one brightener is selected from sulfonamides which are substituted by a (C ₆ -Cio) aryl radical and which carry an amino group preferably in the para position on the aryl radical, for example sulfanilamides (https: //de.wikipedia.orq / wiki / sulfanilamide) selected from the group, 4-aminobenzenesulfonamide, 4-amino- / V-pyridin-2-yl-benzenesulfonamide, 4-amino- / V- (2-pyrimidinyl) benzenesulfonamide, 4-amino- / V- (6-chloropyrazine-2yl) benzenesulfonamide, 4-amino -N- (6-chloro-3-pyridazinyl) benzenesulfonamide 4-amino- / V- (5-methoxy-2-pyrimidinyl) benzenesulfonamide, 4-amino- V- (1, 3-thiazol-2yl) benzenesulfonamide, 4-amino- / V- (5-methyl-1, 3,4-thiadiazol-2yl) benzenesulfonamide, 4-amino- / V- (4-methyl-1 , 3-thiazole

2yl) benzenesulfonamide, 4-amino- / V- (4-methoxy-1, 2,5-thiadiazol-3yl) benzenesulfonamide, 4-amino- / V- (5-methyl-3-isoxazolyl) benzenesulfonamide, 4-amino- / V- (4-methyl-2-pyrinidinyl) benzenesulfonamide, 4-amino- / V- (5-methylpyrimidinyl) benzenesulfonamide, 4-amino -N- (6-methoxypyridazin-3yl) benzenesulfonamide, 4-amino- / V- (3-methoxy-2-pyrazinyl) benzenesulfonamide, 4-amino- / V- (4,5-dimethyl-1, 3-oxazol-2yl) benzenesulfonamide, 4-amino- / V- (3,4-dimethyl-5 -isoaxazolyl) benzenesulfonamide, / V- (3,3-dimethylacroyl) sulfanilamide, 4-amino- / V- (4,6-dimethyl-2-pyrimidinyl) benzenesulfonamide, 4-amino- / V- (2,6-dimethyl -2- pyrimidin-4yl) benzenesulfonamide, 4-amino- / V- (6-methoxy-2-methylpyrimidine-

4yl) benzenesulfonamide, 4-amino- / V- (2,6-dimethoxy-4-pyrimidinyl) benzenesulfonamide, 4-amino- / V- (5,6-dimethoxy-4-pyrimidinyl) benzenesulfonamide, 4-amino- / V - (2-phenylpyrazole

3yl) benzenesulfonamide, 2-hydroxy-5 - ((2 - ((pyridinyl) sulfonyl) phenyl) azo) benzoic acid, 4- aminophenylsulfonylthiourea, 1 - (4-aminobenzenesulfonyl) urea, 4-

(Aminomethyl) benzenesulfonamide, / V- (p-aminophenylsulfonyl) acetamide, 4-amino -N-

(Diaminomethylene) benzenesulfonamide.

The at least one brightener can also be selected from the compounds 2,2'-sulfanediyldiethanol, cysteine, methionine, aliphatic and aromatic heterocyclic compounds having 5 to 7 ring atoms, the ring of the aliphatic and aromatic heterocyclic compounds having at least one heteroatom selected from nitrogen and Contains sulfur, and wherein the aliphatic and aromatic heterocyclic compounds optionally contain one or more further heteroatoms selected from nitrogen, oxygen and sulfur, and mixtures of these brighteners,

Aliphatic and aromatic heterocyclic compounds which can be used as brighteners in this regard have 5 to 7 ring atoms. At least one ring atom is a nitrogen or sulfur atom. Optionally, one or more further heteroatoms selected from oxygen, nitrogen and sulfur can also be present. These further heteroatoms can also be components of the aromatic or aliphatic Rings, but they can also be present in side chains and functional groups attached to the ring.

Suitable aliphatic heterocyclic compounds in this connection are derivatives of tetrahydrothiophene, thiophene, tetrahydrofuran, furan, pyrrolidine, pyrrole, imidazolidine, pyrazolidine, imidazole, pyrazole, oxazolidine, isoxazolidine, oxazole, isoxazole, thiazolidine, diazolidine, isothiazole, isothiazole Triazoles, Furazane, Oxadiazole, Thiadiazole, Dithiazole, Tetrazole, Piperidine, Pyridine, Tetrahydropyrane, Pyrane, Thiane, Thiopyrane, Piperazine, Diazine, Morpholine, Oxazine, Thiomorpholine, Thiazine, Dioxane, Dioxine, Dithiane, Dithroid, 1-Dithiine, Dithiine 5-triazine, triazine, trioxane, trithiane, tetrazine, pentazine, azepane, azepine, oxepane, oxepine, thiepane, thiepine, diazepane, diazepine, thiazepine.

The aliphatic and aromatic heterocyclic compounds mentioned can carry functional groups. These functional groups are advantageously thio, thiol, carbonyl, carboxyl, alkyl, hydroxy. Sulfonyl and sulfonylalkyl groups.

In a particularly advantageous embodiment in this regard, the at least one brightener is an aliphatic or aromatic heterocyclic compound which has 5 to 7 ring atoms, the ring having a nitrogen and a sulfur atom.

In an extremely advantageous embodiment in this regard, the at least one brightener is selected from cysteine, 2,2'-sulfanediyldiethanol, 2-mercaptonicotinic acid, pyridine-3-sulfonic acid, thiomorpholine, 1, 2,3-thiadiazole, 1, 2,4-thiadiazole, 1, 2,5-thiadiazole and 1, 3,4-thiadiazole and their derivatives. Particularly preferred is thiomorpholine, also called tetrahydro-2H-1,4-thiazine according to the IUPAC nomenclature.

If the at least one brightener is selected from cysteine and / or methionine and if at least one amino acid is chosen as the gloss carrier, the gloss-bearing amino acid is neither cysteine nor methionine.

The use of cysteine and / or methionine in the electrolyte according to the invention as gloss carrier or brightener according to the above definition is explained below using some examples: a) The at least one brightener is selected from cysteine and / or methionine

Then: - If at least one amino acid is selected as the gloss carrier, this gloss-bearing amino acid is not cysteine or methionine.

- The electrolyte according to the invention can optionally contain at least one pyridinecarboxylic acid. b) The at least one brightener is selected from 2,2′-sulfanediyldiethanol or aliphatic and aromatic heterocyclic compounds as defined above.

Then:

- If at least one amino acid is selected as the gloss carrier, it can be essential or non-essential amino acids as defined above, including cysteine and / or methionine.

- The electrolyte according to the invention can optionally contain at least one pyridinecarboxylic acid.

If the at least one brightener is selected from cysteine and / or methionine, each of these two amino acids can be present independently of the other in the D form, the L form or as a racemate.

Furthermore, the following combination is also possible in the combination of gloss carriers and gloss formers: c) The at least one gloss carrier is at least one pyridinecarboxylic acid as defined above. No amino acid is used as a gloss carrier. The brightener is selected from 2,2'-sulfanediyldiethanol, cysteine, methionine and aliphatic and aromatic heterocyclic compounds as defined above.

The terms “gloss carrier” and “gloss former” are known to the person skilled in the art. Gloss carriers are also referred to as "primary gloss agents". In the electrodeposition of layers from an electrolyte, they bring about a certain gloss, but not a high gloss, and often only in a limited current density range. Gloss carriers often have a grain-refining effect. Brighteners are also used as "secondary brighteners" designated. They give the deposited layers a high gloss, but are also often only effective in a limited current density range. High gloss over a wide current density range is sometimes possible through the combination of suitable gloss carriers and gloss formers.

The brighteners or the mixture of brighteners are present in a concentration in the electrolyte of 0.005-25 g / l, preferably 0.01 -5 g / l and particularly preferably 0.05-1 g / l.

The electrolyte according to the invention further contains an alkali metal hydroxide selected from lithium hydroxide, sodium hydroxide, potassium hydroxide and mixtures thereof in a concentration of 1-200 g / l, preferably 5-150 g / l, particularly preferably 10-100 g / l. In an advantageous embodiment, the alkali metal hydroxide is potassium hydroxide. The pH of the electrolyte according to the invention is greater than or equal to 8; it is advantageously between 9 and 1 1.

The cyanide-free, aqueous electrolyte according to the invention optionally contains one or more wetting agents.

In a preferred embodiment, the at least one wetting agent is selected from nonionic wetting agents such as, for example, beta-naphthol ethoxylate potassium salt, fatty alcohol polyglycol ethers, polyethyleneimines, polyethylene glycols and mixtures thereof. Wetting agents with a molecular weight below 2,000 g / mol are particularly advantageous. anionic wetting agents such as N-dodecanoyl-N-methylglycine, (N-lauroylsarcosine) -Na salt, alkyl collagen hydrolyzate, 2-ethylhexyl sulfate Na salt, lauryl ether sulfate Na salt and mixtures thereof, cationic wetting agents such as 1 H-imidazolium 1 -ethenyl (or 3-methyl), methyl sulfate homopolymers

In the electrolyte according to the invention, anionic and nonionic surfactants can typically be used as wetting agents, e.g. Polyethylene glycol adducts, fatty alcohol sulfates, alkyl sulfates, alkyl sulfonates, aryl sulfonates, alkylarylsulfonates, heteroarylsulfates, betaines, fluorosurfactants and their salts and derivatives are used (see also: Kanani, N: Galvanotechnik; Hanser Verlag, Munich Vienna, 2000; page 84 ff).

In a further advantageous embodiment, the electrolyte according to the invention contains at least one further salt. The anions of these salts are selected from the group the sulfates, fluorides, chlorides, bromides, iodides, carbonates, formates, acetates, propionates, butyrates, valerates, nitrates, nitrites, sulfonates, alkyl sulfonates, especially methanesulfonates, amidosulfonates, sulfamates, anions of aminocarboxylic acids and N-heterocyclic carboxylic acids. The cations of these salts are selected from ammonium, lithium, sodium and potassium ions. In the case of polyprotonic acids, one or all of the hydrogen atoms can be replaced by the cations mentioned. If more than one hydrogen atom is replaced by one of the cations mentioned, these cations can be identical or different. The at least one additional salt is also referred to below as the “conductive salt”. The at least one conductive salt is selected from the sodium, potassium and ammonium salts of sulfuric acid, hydrochloric acid, methanesulfonic acid, carbonic acid, nitric acid and phosphoric acid. In an advantageous embodiment, the at least one conductive salt is a potassium salt, particularly preferably potassium methanesulfonate and / or potassium nitrate. In an advantageous embodiment, the at least one conductive salt is used in a concentration of 1 to 200 g / l, preferably 10-100 g / l.

The present invention also relates to a method for the electrolytic deposition of silver and silver alloy coatings from an electrolyte according to the invention, wherein an electrically conductive substrate is immersed in the electrolyte and a current flow is established between an anode in contact with the electrolyte and the substrate as the cathode. It should be mentioned that the mutatis mutandis preferred embodiments for the electrolyte also apply to the method mentioned here.

The temperature that prevails during the deposition of the silver and silver alloy coatings can be chosen at will by those skilled in the art. It will be based on a sufficient deposition rate and the applicable current density range on the one hand and on the other hand on economic aspects and the stability of the electrolyte. It is advantageous to set a temperature of 20 ° C. to 90 ° C., preferably 40 ° C. to 80 ° C. and particularly preferably 50 ° C. to 70 ° C.

The current density which is established between the cathode and the anode during the deposition process in the electrolyte according to the invention can be selected by the person skilled in the art in accordance with the efficiency and quality of the deposition. The current density in the electrolyte is advantageously set to 0.1 to 100 A / dm ² , depending on the application and coating system type. If necessary, the current densities can be increased or decreased by adapting the system parameters such as the structure of the coating cell, flow rates, anode, cathode ratios, etc. One is advantageous Current density of 0.1-100 A / dm ² , preferably 0.2-50.0 A / dm ² and very particularly preferably 0.5-30 A / dm ² .

In the context of the present invention, low, medium and high current density ranges are defined as follows:

Low current density range: 0.1 to 0.75 A / dm ² ,

Medium current density range: greater than 0.75 A / dm ² to 5 A / dm ² ,

High current density range: greater than 5 A / dm ² .

The electrolyte according to the invention and the method according to the invention can be used for the electrolytic deposition of silver and silver alloy coatings for technical applications, for example electrical plug connections and printed circuit boards, and for decorative applications such as jewelry and watches. In an advantageous embodiment of the present invention, a low current density range is used in the electrolytic deposition of silver and silver alloy coatings, and the at least one gloss carrier contains 0.2-3 mol / l of the at least one amino acid and 0.01-0.5 mol / l of the at least one pyridinecarboxylic acid.

In an advantageous embodiment of the present invention, a medium current density range is used in the electrolytic deposition of silver and silver alloy coatings, and the at least one gloss carrier contains 0.1-1.5 mol / l of the at least one amino acid and 0.1-1 mol / l of the at least one pyridinecarboxylic acid.

In an advantageous embodiment of the present invention, a high current density range is used in the electrolytic deposition of silver and silver alloy coatings, and the at least one gloss carrier contains 0.01-0.1 mol / l of the at least one amino acid and 0.25-2.5 mol / l of the at least one pyridinecarboxylic acid.

As already indicated above, the electrolyte according to the invention is an alkaline type. The pH should be greater than or equal to 7 and particularly preferably between 8 and 11, better 9 and 10.5. There may be fluctuations in the pH of the electrolyte during electrolysis. In a preferred embodiment of the method in question, the person skilled in the art therefore proceeds by checking the pH during the electrolysis and, if necessary, adjusting it to the desired value. To adjust the pH value, potassium hydroxide or Methanesulfonic acid used. Alternatively, lithium or sodium hydroxide or mixtures of these alkali metal hydroxides can be used instead of potassium hydroxide.

Different anodes can be used when using the electrolyte. Soluble or insoluble anodes are just as suitable as the combination of soluble and insoluble anodes. If a soluble anode is used, it is particularly preferred if a silver anode is used.

The insoluble anodes used are preferably those made from a material selected from the group consisting of platinized titanium, graphite, mixed metal oxides, glass carbon anodes and special carbon material (“Diamond Like Carbon” DLC) or combinations of these anodes. Insoluble anodes made of platinized titanium or titanium coated with mixed metal oxides are advantageous, the mixed metal oxides preferably being selected from iridium oxide, ruthenium oxide, tantalum oxide and mixtures thereof. Iridium transition metal oxide mixed oxide anodes, particularly preferably mixed oxide anodes made of iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide, are also advantageously used for carrying out the invention. Others can be found at Cobley, A.J. et al. (The use uf insoluble anodes in Acid Sulphate Copper Electrodeposition Solutions, Trans IMF, 2001, 79 (3), pp. 1 13 and 1 14).

Depending on the embodiment, the deposited silver and silver alloy coatings can have a thickness of up to several millimeters, preferably 0.005-500 pm, particularly preferably 0.01-25 pm and very particularly preferably 0.5-10 pm.

Typically, thin layer thicknesses in the range from 0.1 to 0.3 pm silver are used, for example, for coating plastic caps in rack operation. Here low current densities in the range of 0.25 to 0.75 A / dm ² are used. Another application of low current densities is used in drum or vibration technology, for example when coating contact pins. Approx. 0.5 to 3 pm silver in the current density range from 0.25 to 0.75 A / dm ^{2 are} applied here. Layer thicknesses in the range from 1 to 10 pm are typically deposited in rack operation predominantly for decorative applications with current densities in the range from 1 to 5 A / dm ² . For technical applications, layers up to 25 pm thick are sometimes deposited. In continuous systems, layer thicknesses are deposited over a relatively large range from approx. 0.5 to approx. 5 pm with the highest possible separation speeds and thus the highest possible current densities between 5 and 30 A / dm ² . There are also special applications in which relatively high Layer thicknesses from a few 10 pm to a few millimeters, for example in the case of electroforming, are deposited.

Instead of direct current, pulsed direct current can also be used. The current flow is interrupted for a certain period of time (pulse plating). In reverse pulse plating, the polarity of the electrodes is changed so that the coating is partially anodized. The layer structure is controlled in this way in constant alternation with cathodic pulses. The use of simple pulse conditions such as 1 s current flow (t _on ) and 0.5 s pulse _pause (t _0ff ) at medium current densities led to homogeneous, shiny and white coatings.

Suitable substrate materials that are typically used here are copper base materials such as pure copper, brass or bronze, iron base materials such as e.g. Iron or stainless steel, nickel, gold and silver. The substrate materials can also be multi-layer systems that have been electroplated or coated with another coating technique. This applies, for example, to circuit board base material or iron materials that have been nickel-plated or copper-plated and then optionally gold-plated or coated with pre-silver. Another substrate material is a wax core that has been pre-coated with conductive silver varnish (electroforming).

A special case of a silver electrolyte in the sense of this invention is a pre-silver or silver strike electrolyte. This means an electrolyte that typically contains little silver and a lot of complexing agents. This means that silver cannot be deposited by charge exchange, but only by applying a voltage. Layers deposited by charge exchange adhere poorly, which is why a thin layer of pre-silver is often deposited before a thicker silver layer is applied with another electrolyte (see Example 4.

The electrolyte according to the invention is long-term stable and has a high anode solubility. The method according to the invention for the electrolytic deposition of silver and silver alloy coatings from this electrolyte produces very white coatings, the color of which is close to the white point of the L ^* a ^* b ^* color space. At the white point, L ^* = 100, and a ^* and b ^* are zero. The coatings are high-gloss, brilliant and very tarnish-resistant even at high layer thicknesses above 5 pm, ie there is no subsequent yellowish discolouration. Until now, such coatings could only be deposited from cyanide electrolytes. With the help of the electrolyte according to the invention, silver and silver alloy coatings can be deposited over a very large current density range. EXAMPLES

1 liter of the electrolyte mentioned below is heated to the temperature mentioned in the exemplary embodiment with the aid of a magnetic stirrer while stirring with a 60 mm long cylindrical magnetic stirrer bar at least 200 rpm. This agitation and temperature is also maintained during the coating.

After the desired temperature has been reached, the pH of the electrolyte is adjusted to the value mentioned in the exemplary embodiment using a KOH solution (c = 0.5 g / ml) and methanesulfonic acid (c = 70%).

Two plates made of fine silver with at least 99.9% purity serve as anodes. These anodes can also be covered with bags made of textiles, filter paper or a semi-permeable membrane such as Nafion.

A mechanically polished brass sheet with an area of at least 0.2 dm ² , which was previously coated with at least 5 pm nickel from an electrolyte which produces high-gloss layers, serves as the cathode. An approximately 0.1 pm thick gold layer can also be deposited on the nickel layer.

Before being inserted into the electrolyte, these cathodes are cleaned with the aid of an electrolytic degreasing agent (5-7 V) and a pickling containing sulfuric acid (c = 5% sulfuric acid). Between each cleaning step and before being introduced into the electrolyte, the cathode is rinsed with deionized water.

The cathode is positioned in the electrolyte between the anodes and moved parallel to them with at least 5 cm / second, the distance between anode and cathode must not change.

In the electrolyte, the cathode is coated by applying an electrical direct current between the anode and cathode. The current strength is chosen so that at least 0.5 A / dm ^{2 is} achieved on the surface. Higher current densities can be selected if the electrolyte mentioned in the application example is able to produce layers which can be used for decorative purposes.

The duration of the current flow is chosen so that at least a layer thickness of 1.5 pm is achieved on average over the area. Higher layer thicknesses can be produced if the electrolyte mentioned in the application example is able to produce it with technically and decoratively usable quality. After coating, the cathode is removed from the electrolyte and rinsed with deionized water. After coating, a customary treatment in hot water, complexing agent solution, pickling or treatment with tarnish protection, for example based on octadecanethiols, can be carried out. The cathodes can be dried by compressed air, warm air or centrifugation.

The area of the cathode, height and duration of the applied current and weight of the cathode before and after coating are documented and used to determine the average layer thickness and the efficiency of the deposition.

The color of the deposited layers is determined and documented by an L ^* a ^* b ^* measurement according to CIEL ^* a ^* b.

Tests and comparative tests according to the invention are shown in the table below.

Potassium hydroxide and methanesulfonic acid, which are used to adjust and adjust the pH, are not explicitly listed in the tables. The expert knows how to set or adjust a pH value. In the production of the electrolytes according to the invention, it is possible first to prepare an aqueous solution of all the components mentioned in the table and then to adjust the pH to the desired value using potassium hydroxide or methanesulfonic acid. Alternatively, it is also possible to firstly introduce a potassium hydroxide solution, then add the hydantoin derivative, finally add all the other ingredients and finally adjust the pH to the desired value using potassium hydroxide or methanesulfonic acid. During the deposition of the silver or silver alloy layers, the pH can also be adjusted with potassium hydroxide or methanesulfonic acid.

Table 1: Exemplary embodiments according to the invention

Table 1: Exemplary embodiments according to the invention (continued)

Note 1:

a Beaker (stirring rod 60 mm; 200 rpm), cathode movement b Beaker (stirring rod 60 mm; 200 rpm), drum

c Jet plating (400 l / h)

d Hull cell (stirring rod 40 mm; 600 rpm)

Note 2

a silver anode

b Platinum-plated titanium

c Mixed metal oxide

Note 3

a homogeneous, white, shiny

b homogeneous, white, matt

c yellowish

Note 4

a no precipitation, stable over months, constant quality of the coatings b no precipitation, but no constant quality of the coatings possible c precipitation after a short time

nb not determined

Table 2: Exemplary embodiments according to the invention

Table 2: Exemplary embodiments according to the invention (forest setting) Table 2: Exemplary embodiments according to the invention (continued)

Note 1:

c Jet plating (400 l / h)

d Hull cell (stirring rod 40 mm; 600 rpm)

Note 2

a silver anode

b Platinum-plated titanium

c Mixed metal oxide

Note 3

a homogeneous, white, shiny

b homogeneous, white, matt

c yellowish

Note 4

nb not determined Table 3: Comparative examples

Table 3: Comparative examples (continued)

Note 1:

c Beaker (stirring rod 60 mm; 400 rpm), cathode movement d Jet plating (400 l / h)

Note 2

a silver anode

b Platinum-plated titanium

c Mixed metal oxide

Note 3

a homogeneous, white, shiny

b homogeneous, white, matt

c yellowish

Note 4

a no precipitation, stable over months, constant quality of the coatings b no precipitation, but no constant quality of the coatings possible c precipitation after a short time n.b. = not determined

Determination of color values

Color values were measured in accordance with the L ^* a ^* b color space for a silver layer deposited from an electrolyte according to the invention and for three comparative examples.

Test conditions: Volume: 1 liter

Magnetic stirrer: IKA RET CV

Stirring: 200 and 400 rpm; the higher stirring speed was used at current densities above 3 A / dm ² .

Stirring rod: 60 mm cathode: moves parallel to the anodes, 5cm / s

Cathode area: 0.2 dm2, brass

Cathodic current density: 0.5 to 3 A / dm2 layer thickness: 1.5 pm.

Anodes: Silver 99.9% Temperature: 40 ° C - 65 ° C, depending on the stability of the solution. Comparative example 4 was tested at 40 ° C. because more silver was deposited at 50 ° C. (original parameter Example 5 in US Pat. No. 5,601,696) than is theoretically possible electrochemically. This is a sign that chemical deposition takes place in addition to galvanic deposition, the latter being undesirable. pH: 9.5 to 10, depending on the example.

Measuring device: Konica Minolta Spectrophotometer CM-700, SCI 107D65 Determination of the L * values

The L ^* axis in the L ^* a ^* b ^* color space describes the brightness of the color with values from 0 (black) to 100 (white).

Layers of 1.5 μm thick were deposited at current densities of 0.5 to 3.0 A dm ₂ .

The results of the determination of the L ^* values are shown in FIG. 1.

Determination of the a * values

The a ^* axis describes the green or red portion of a color in the L ^* a ^* b ^* color space, with negative values for green and positive values for red.

The results of the determination of the a ^* values are shown in FIG. 2. Determination of the b * values

In the L ^* a ^* b ^* color space, the b ^* axis describes the blue or yellow component of a color, whereby negative values stand for blue and positive values for yellow.

Layers of 1.5 μm thickness were deposited at current densities of 0.5 to 3.0 A / dm ₂ .

The results of the determination of the b ^* values are shown in FIG. 3.

Claims

claims

1. Aqueous cyanide-free electrolyte for the electrolytic deposition of silver and silver alloy coatings, which in solution form has the following constituents: a) at least one silver compound in a concentration of 0.1 to 150 g / l silver, b) at least one compound of an alloy metal in a concentration from 0 to 100 g / l alloy metal, c) at least one compound of the formula (I)

wherein

R1, R2, R3 and R4 independently represent hydrogen, a linear or branched alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, a hydroxyalkyl group having 1 to 5 carbon atoms or an aryl group, and wherein the at least one compound of Formula (I) is present in a concentration of 1 to 350 g / l. D) at least one gloss carrier selected from i. at least one amino acid in a concentration of 0.0001-5 mol / l, in particular 0.01-5 mol / l, and / or ii. at least one pyridinecarboxylic acid in a concentration of 0.01-5 mol / le) at least one brightener selected from sulfonamide, 2,2'-sulfanediyldiethanol, cysteine, methionine, aliphatic and aromatic heterocyclic compounds with 5 to 7 ring atoms, the ring being the aliphatic and aromatic heterocyclic compounds contains at least one heteroatom selected from nitrogen and sulfur, and wherein the aliphatic and aromatic heterocyclic compounds optionally contain one or more further heteroatoms selected from nitrogen, oxygen and sulfur, and mixtures of these brighteners, the concentration of the brightener or the mixture of the brighteners is 0.005-25 g / l, and if the at least one brightener is selected from cysteine and / or methionine and at least one amino acid according to d) i) is chosen as the brightener, the brightener is chosen Amino acid neither for cysteine nor for Is methionine, f) an alkali metal hydroxide selected from lithium hydroxide, sodium hydroxide, potassium hydroxide and mixtures thereof in a concentration of 1-200 g / l, g) wherein the electrolyte has a pH greater than or equal to 7.

2. Electrolyte according to claim 1,

characterized in that

which has sulfonamide as structural element R1-SO2-NR2R ₃ ,

wherein

Ri, R2 and R ₃ independently of one another (C1-Cio) alkyl, (C ₃ - Cio) cycloalkyl, (C ₆ - C10) aryl, (C ₅ - Cio) heteroaryl, (C ₅ - Cio) - Is heterocycloalkyl.

3. Electrolyte according to claim 1 and / or 2,

characterized in that

the silver compound is selected from silver methanesulfonate, silver carbonate,

Silver phosphate, silver pyrophosphate, silver nitrate, silver oxide, silver lactate, silver fluoride, silver bromide, silver chloride, silver iodide and silver sulfate.

4. Electrolyte according to one of the preceding claims,

characterized in that

the connection of the at least one alloy metal is selected from Compounds of tin, palladium, antimony, cobalt, indium, iron, nickel, ruthenium, rhodium, platinum, copper, zinc, selenium, tellurium, bismuth, iridium, germanium, gallium, rhenium, tungsten, molybdenum, dysprosium, cerium and gold.

5. Electrolyte according to one of the preceding claims,

characterized in that

the at least one compound of the formula (I) is selected from 1-methylhydantoin,

1,3-dimethylhydantoin, 5,5-dimethylhydantoin, 1-hydroxymethyl-5,5-dimethylhydantoin, 5,5'-diethylhydantoin and 5,5-diphenylhydantoin and mixtures thereof.

6. Electrolyte according to one of the preceding claims,

characterized in that

the at least one amino acid is selected from glycine, alanine, proline and sarcosine and mixtures thereof.

7. Electrolyte according to one of the preceding claims,

characterized in that

the at least one pyridinecarboxylic acid is selected from picolinic acid,

Picolinic acid amide, nicotinic acid, nicotinic acid amide, isonicotinic acid, isonicotinic acid amide and mixtures thereof.

8. A method for the electrolytic deposition of silver and silver alloy coatings from an electrolyte according to claims 1 to 7,

characterized in that

an electrically conductive substrate is immersed in the electrolyte and a current flow is established between an anode in contact with the electrolyte and the substrate as the cathode.

9. The method according to claim 8,

characterized in that

the temperature of the electrolyte is 20 ° C to 90 ° C.

10. The method according to any one of claims 8 to 9,

characterized in that

the current density during the electrolysis is 0.2 to 100 A / dm ² .

1 1. The method according to any one of claims 8 to 10,

characterized in that the pH value is constantly adjusted to a range between 9 and 1 1 during the electrolysis.

12. The method according to any one of claims 8 to 1 1,

characterized in that

a soluble silver anode and / or an insoluble anode is used as the anode.