Cyanide-free silver bath composition and uses thereof

Abstract

The present invention relates to a cyanide-free silver plating bath composition comprising: silver methanesulfonate; a chelating agent selected from hydantoin, one of its derivatives, or a mixture thereof; a brightener selected from polyethyleneimine and bipyridine; a pH regulator selected from potassium or sodium hydroxide and water; the pH of the composition being between 8.5 and 13.5 and its electrolytic conductivity being greater than or equal to 10 mS / cm, characterized in that the composition does not comprise any brighteners other than the one selected from polyethyleneimine and bipyridine. It further relates to a method for preparing this composition, the use of this composition for the electrolytic deposition of silver onto a metallic substrate, and a method for electrolytic silver deposition using this composition.

Description

Technical Field

[0001] The present invention relates to the field of silver electroplating on a metal substrate using a cyanide-free silver plating bath. It relates in particular to the composition of this bath, its manufacturing process and its uses. Prior art

[0002] Electroplating silver onto a metal substrate using cyanide as a complexing agent was developed in the 19th century (in France by Maison Charles Christofle in Paris) and has become the only manufacturing process used in the field of industrial surface treatment to date. The technology offers advantages in terms of performance and robustness, but imposes significant constraints at the manufacturing site due to the high toxicity of the cyanide involved in the process.

[0003] For example, in the environmental and safety infrastructure of surface treatment plants, the collection and treatment networks for wastewater and discharged gases must be doubled, the first consisting of the neutralization of acid and alkaline substances and the second dedicated specifically to eliminating cyanide residues.

[0004] For decades, chemists and electrochemists around the world, both in basic and applied research, have been striving to find an alternative to silver plating with cyanide electrolyte. Their scientific work has led to the proposal of a number of cyanide-free silver plating baths, but none has been used to date in industrial applications, at least in the sector of electrical and electronic components for conductors and connectors.

[0005] In fact, it is not easy to find a satisfactory replacement solution, as there are many difficulties to overcome in order to achieve the substitution objective.

[0006] The major difficulty lies in the composition of the electrolyte, more commonly called the silvering bath formulation, which must include several essential chemical components. It is therefore important that these components are compatible with each other in the chemical sense of the term. It is through good synergy of all the chemical actions exerted by these components that a silvering of comparable quality to traditional silvering using cyanide can be achieved.

[0007] More specifically, the bath formulation must contain at least one silver salt, a so-called silver complexing agent, and if necessary a so-called silver chelating agent, as well as other agents such as conductor, wetting agent, brightener, etc. depending on the application.

[0008] The silver salt providing silver ions in the electroplating process must be easily soluble in water, forming an aqueous electrolytic bath. In the case of cyanide silver plating, this is silver cyanide AgCN.

[0009] The silver complexing agent, as its name suggests, allows the complexation of silver ions in the appropriate form so that they can not only be extracted at the anode and deposited on the surface of the cathode under the action of electrolysis, but also be transported without hindrance in the aqueous electrolytic medium from the anode to the cathode. In the case of cyanide silver plating, it is the so-called free cyanide CN- coming from sodium cyanide NaCN or potassium cyanide KCN dissolved in the aqueous electrolyte which fulfills this role, the compound thus complexed being Ag(CN). AgCN + KCN = Ag(CN) 2 -< + K +<

[0010] In some cases, when the action of the complexing agent proves insufficient, another type of agent called a chelator or chelating agent is generally used to accentuate the complexing effect. The chelator, considered as a highly coordinated ligand, is distinguished from the simple complexing agent by its cyclic molecular structure and by at least two coordination bonds.

[0011] In the prior art, cyanide-free silvering bath compositions are already described using nitrates, citrates, sulfates, chlorides and phosphates, all inorganic, as silver salts. The majority of them concern silver nitrate AgNO 3 .

[0012] The choice of silver nitrate would a priori be guided by its very high aqueous solubility. But good aqueous solubility alone is not sufficient to obtain a good performance silver plating electrolytic bath.

[0013] Other work has focused on the complexing agent, patent US5302278(1994) discloses the use of thiosulfate S2O3-2< prepared from sodium thiosulfate Na2S2O3 as a complexing agent in a silvering bath containing silver chloride as a salt.

[0014] Patent applications US2007 / 0284258A1(2007) and US2015 / 0184307A1(2015) disclose silver plating baths characterized by methanesulfonic acid CH3SO3H as a complexing agent. The first patent refers to the use of silver methanesulfonate AgCH3SO3 as the silver salt, and the second to silver nitrate AgNO3. Both baths are characterized by moderate acidity with a pH below 3.

[0015] Patent EP2431502 discloses another avenue of exploration in cyanide-free silver plating which mentions a co-presence of silver ions dissolved in a solution based on dimethyl hydantoin C5H8N2O2 and potassium nitrate KNO3. Without precise information regarding the chemical composition, reading the patent suggests that the source of silver ions would be relative to silver nitrate AgNO3. The patent also indicates that the pH of the baths is regulated at 9.5, therefore slightly alkaline.

[0016] On the other hand, all of the aforementioned patents only concern electrolytic silvering baths, none of which involve application to an industrially manufactured product or evaluation of functional properties such as adhesion, mechanical strength, thermal resistance and corrosion resistance.

[0017] The principle of industrial electrolytic silver plating used to date can be characterized by a continuous or discontinuous process comprising at least 3 successive stages as follows: Surface preparation Pre-silvering Silvering

[0018] This is the case for copper and its alloys which are widely used in the electrical components sector such as conductors and connectors.

[0019] For the silvering of aluminum alloys (pure aluminum is little used due to its poor mechanical properties) which are another class of electrical materials, a so-called pre-treatment step is often introduced given their poor suitability for electrolytic coating: Surface preparation Pre-treatment including pickling, activation, double zinc plating, chemical nickel plating Pre-silvering Silvering

[0020] All steps are individually followed by efficient water rinsing and / or drying to obtain a quality silver deposit.

[0021] For a copper or copper alloy substrate, the degreasing step is used because all the electrical components to be silver plated come from wire drawing for the conductors and machining for the connectors in the presence of lubricants of all kinds.

[0022] In the case of aluminum alloys, in addition to the degreasing step, the pre-treatment step includes several sub-steps such as pickling, activation, double zinc plating, chemical nickel plating. It is the set of sub-steps that makes it possible to break the possible surface oxidation of the metal and to modify the surface state of the metal by reducing the electrochemical potential difference between aluminum and silver which is in itself quite unfavorable in electrodeposition.

[0023] To date, the pre-silvering and silvering stages contain cyanide and involve environmental infrastructure constraints as previously indicated.

[0024] The present invention aims to provide cyanide-free pre-silvering and silvering baths having performance and quality comparable to current baths which contain cyanide.

[0025] Achieving such an objective requires firstly the formation of a cyanide-free silver plating bath and then a chemically stable and operationally robust implementation process.

[0026] The inventors surprisingly found that it was possible to achieve this objective by using a cyanide-free silvering bath in which silver methanesulfonate serves as both the silver salt and the complexing agent, the chelator is chosen from hydantoin or one of its derivatives, the brightener is chosen from polyethyleneimine and bipyridine, provided that the pH of the silvering bath is between 8.5 and 13.5 and that its electrolytic conductivity is greater than or equal to 10mS / cm through the use of a pH regulator chosen from potassium or sodium hydroxide.

[0027] Indeed, such a silvering bath makes it possible to avoid the use of other complexing agents since silver methane sulfonate has the dual role of silver salt and complexing agent, as well as the use of other chelators, which makes its chemical composition quite simple.

[0028] Furthermore, even though silver methanesulfonate AgCH 3 SO 3 has a lower solubility than silver nitrate, the judicious choice of pH and therefore of the pH regulator allows a strong chelation of the silver ions which prevents their precipitation which would thus destabilize the silvering bath. This high pH value also makes it possible to avoid any direct or indirect hydrolysis type reaction of chelators such as hydantoins and therefore their good stability. In addition, this choice of pH value also makes it possible to obtain an electrical conductivity necessary for the deposition of the silver layer.

[0029] The use of a brightener also allows the silver deposit to be aesthetically acceptable with a beautiful appearance. Indeed, the brightener assists the electrolytic process by creating and maintaining a fine crystalline structure of the silver deposit and thus leads to a visual brilliance of the deposit.

[0030] Patent applications US2022 / 205122 and DE102018120357 describe a silvering composition comprising several brighteners (primary brightener and secondary brightener) chosen from a very specific list not containing polyethylene imine or bi-pyridine. These documents do not describe or suggest that it is possible to use a much simpler composition not comprising both a brightener support and a brightener (i.e. a primary brightener and a secondary brightener).

[0031] Patent application EP2735033) describes a composition containing silver methane sulfonate and hydantoin (example 3). However, this composition does not include a brightener chosen from polyethyleneimine and bipyridine. Statement of the invention

[0032] The present invention therefore relates to a cyanide-free silvering bath composition comprising (advantageously consisting essentially of, more advantageously consisting of): silver methanesulfonate; a chelator chosen from hydantoin, one of its derivatives or a mixture thereof, advantageously chosen from hydantoin, 5,5-dimethyl-hydantoin, 1-methyl-hydantoin and their mixtures; a brightener chosen from polyethyleneimine and bipyridine, advantageously this is polyethyleneimine; a pH regulator chosen from potassium or sodium hydroxide, advantageously this is potassium hydroxide and water; the pH of the composition being between 8.5 and 13.5, advantageously between 11.0 and 13.0 and its electrolytic conductivity being greater than or equal to 10 mS / cm, advantageously greater than or equal to 50 mS / cm, characterized in that the composition does not comprise any other brighteners than that chosen from polyethyleneimine and bipyridine.

[0033] For the purposes of the present invention, the expressions "between... and...", "from... to...", "in the range...-..." must be understood to include the limits unless explicitly stated otherwise.

[0034] The composition according to the present invention therefore comprises silver methanesulfonate, AgCH 3 SO 3 (MSAg). Silver methanesulfonate is both the silver salt that provides the silver ions to the composition and also the silver complexing agent. This is one of the advantages of the present invention: it is not necessary to use two different products (a silver salt and a complexing agent) in the composition according to the present invention, since a single product is sufficient. As indicated previously, the complexing agent allows the complexation of the silver ions in an appropriate form so that they can not only be extracted at the anode and deposited on the surface of the cathode under the action of electrolysis, but also be transported without hindrance in the aqueous electrolytic medium from the anode to the cathode.The composition according to the present invention is therefore simpler than those described in the prior art. Thus, advantageously, silver methanesulfonate is the only silver salt in the composition according to the invention. Advantageously, it is also the only complexing agent in the composition according to the invention.

[0035] The silver ion content (Ag +< ) of the composition according to the invention is advantageously between 10 and 80 g / l, more advantageously between 15 and 70 g / l, even more advantageously between 20 and 60 g / l, in particular between 20 and 55 g / l. Depending on the operating parameter of the silvering process, the chelator and the brightener used, the advantageous silver ion content of the composition may differ. It may thus be, for example, between 10 and 30 g / l, between 20 and 55 g / l and / or between 20 and 40 g / l. It is obviously possible to combine these different ranges.

[0036] The composition according to the present invention further comprises a chelator chosen from hydantoin, one of its derivatives or a mixture thereof. Hydantoin derivatives are well known to those skilled in the art. They may in particular be methylated and / or hydroxymethylated derivatives, such as for example 5,5-dimethyl-hydantoin, 1-methyl-hydantoin, 1,3-dimethyl hydantoin and 1-hydroxymethyl-5,5-dimethyl hydantoin. Advantageously, the hydantoin derivative is chosen from 5,5-dimethyl-hydantoin, 1-methyl-hydantoin and mixtures thereof. Particularly advantageously, the chelator of the composition according to the invention is chosen from hydantoin (HYD), 5,5-dimethyl-hydantoin (D-HYD), 1-methyl-hydantoin (M-HYD) and mixtures thereof.

[0037] For the purposes of the present invention, the term "chelator" means an agent capable of carrying out a physicochemical process called chelation during which a chelate is formed between one or more organic ligands and a metal ion, here the silver ion. Due to their two lactam functions, the hydantoin and its derivatives according to the invention play a chelating role with respect to silver methanesulfonate. In addition, although organic, the hydantoin and its derivatives are highly soluble in an aqueous medium, which offers another major advantage in the formulation of an industrially exploitable bath.

[0038] Advantageously, the content of chelator chosen from hydantoin, one of its derivatives or a mixture thereof in the composition according to the invention is between 20 and 500 g / l, advantageously between 40 and 400 g / l, advantageously between 50 and 400 g / l.

[0039] In an advantageous embodiment, the hydantoin content of the composition according to the invention is between 40 and 150 g / l, more advantageously between 50 and 100 g / l, in particular it is 90 g / l.

[0040] In another advantageous embodiment, the 5,5-dimethyl-hydantoin content of the composition according to the invention is between 50 and 300g / l, in particular between 100 and 300g / l.

[0041] In yet another advantageous embodiment, the 1-methyl-hydantoin content is between 50 and 250 g / l.

[0042] Advantageously, the composition according to the present invention does not comprise any chelators other than that chosen from hydantoin, one of its derivatives or a mixture thereof.

[0043] The composition according to the present invention further comprises a brightener chosen from polyethyleneimine (PEI) and bipyridine (D-PYR). Advantageously, the brightener is polyethyleneimine. In particular, the polyethyleneimine according to the invention is linear. More particularly, it has a number-average molecular weight of between 100 and 100,000 u, more advantageously between 200 and 1000 u, in particular between 500 and 700 u, in particular it is 600 u.

[0044] Advantageously, the content of brightener chosen from polyethyleneimine and bipyridine of the composition according to the invention is between 0.2 and 3g / l, in particular between 0.3 and 1.5g / l.

[0045] The composition according to the present invention does not comprise any other brighteners than that chosen from polyethyleneimine and bipyridine, advantageously the only brightener in the composition is polyethyleneimine.

[0046] The composition according to the present invention further comprises a pH regulator chosen from potassium or sodium hydroxide, advantageously this is potassium hydroxide. Advantageously, the composition according to the present invention does not comprise any pH regulators other than potassium or sodium hydroxide, advantageously potassium hydroxide.

[0047] The pH regulator content is chosen so that: the pH of the composition according to the invention is between 8.5 and 13.5, advantageously between 9.0 and 13.0, more advantageously between 10.0 and 13.0, even more advantageously between 11.0 and 13.0, in particular between 11.0 and 12.5, and the electrolytic conductivity of the composition according to the invention is greater than or equal to 10 mS / cm, advantageously greater than or equal to 50 mS / cm, in particular between 50 and 120 mS / cm.

[0048] The composition according to the invention therefore presents: a pH between 8.5 and 13.5, advantageously between 9.0 and 13.0, more advantageously between 10.0 and 13.0, even more advantageously between 11.0 and 13.0, in particular between 11.0 and 12.5, and an electrolytic conductivity greater than or equal to 10 mS / cm, advantageously greater than or equal to 50 mS / cm, in particular between 50 and 120 mS / cm.

[0049] The composition according to the invention is free of cyanide.

[0050] The composition according to the present invention may comprise other additives, such as wetting agents or grain refiners, but advantageously it does not.

[0051] In particular, the composition according to the present invention does not comprise amino acids such as aminodiacetic acid and / or imino diacids and / or succinimides and / or iminodisuccinate derivatives, in particular as described in application US2011 / 0062030, more particularly tetrasodium iminodisuccinate, and / or niacin and / or ammonium sulfate and / or 1,4-butynediol and / or phosphine (in particular aliphatic or aromatic, more particularly tris(3-hydroxypropyl)phosphine) and / or azole compounds, in particular as described in application US2007 / 0284258, more particularly triazoles such as, for example, 1,2,3-triazole, 3-amino-1,2,4-triazole, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole and benzotriazole or benzimidazoles such as for example 2-mercaptobenzimidazole, and / or benzoic acid derivatives, in particular as described in application US 2015 / 0184307, more particularly 2-sulfobenzic acid and 3,4-dinitrobenzoic acid, sulfonic acid or one of its derivatives,in particular as described in application US 2010 / 0044239, more particularly potassium methanesulfonate and poly condensate formaldehyde naphthalene of sulfonic acid, and / or potassium bromide, and / or sodium thiosulfate, and / or tripotassium citrate monohydrate and / or boric acid and / or glyoxilic acid hydrate and / or cysteine ​​and / or nicotinic acid.

[0052] Thus advantageously, the composition according to the invention does not comprise other chelators and / or other complexing agents.

[0053] The present invention further relates to a method of manufacturing the composition according to the invention comprising the following successive steps: a- addition of the pH regulator according to the invention, advantageously potassium hydroxide, in demineralized water at room temperature so as to obtain a pH between 8.5 and 13.5 and an electrolytic conductivity greater than or equal to 10 mS / cm; b- addition of the chelator according to the invention at room temperature while maintaining the pH between 8.5 and 13.5 and the electrolytic conductivity at a value greater than or equal to 10 mS / cm; c- addition of silver methanesulfonate at room temperature while maintaining the pH between 8.5 and 13.5 and the electrolytic conductivity at a value greater than or equal to 10 mS / cm; d- addition of the brightener according to the invention, advantageously polyethyleneimine, while maintaining the pH between 8.5 and 13.5 and the electrolytic conductivity at a value greater than or equal to 10 mS / cm.

[0054] Indeed, the inventors surprisingly noticed that if the pH and the electrolytic conductivity of the composition were not maintained in the desired range during steps b), c) and d), more advantageously during step c), and in particular if the pH had a value lower than 8.5, the composition lost its stability.

[0055] Maintaining the pH and electrolytic conductivity during steps b), c) and d) is implemented by adding the pH regulator according to the invention. The pH of the composition according to the invention during these steps can be checked and controlled using a pH meter. The electrolytic conductivity of the composition according to the invention during these steps can be checked and controlled using a conductivity meter.

[0056] “Room temperature” means a temperature between 20 and 25°C.

[0057] The present invention further relates to the use of the composition according to the invention for the electrolytic deposition of silver on a metal substrate, advantageously the deposition of a silver layer having a thickness of between 0.1 µm and 200 µm by electrolytic means on the metal substrate.

[0058] Advantageously, the metal substrate is a substrate made of copper or a copper alloy such as brass or copper-beryllium, an aluminum alloy or nickel.

[0059] Advantageously, the aluminum alloy is chosen from aluminum silicon, aluminum magnesium or aluminum magnesium silicon alloys such as, for example, the alloys of the 1000 series, in particular alloy 1050, the 4000 series (aluminum silicon), in particular alloys 4047 and 4032, more particularly the 4047 series, the alloys of the 5000 series (aluminum magnesium), in particular alloys 5083 and 5754, more particularly the 5083 series and the alloys of the 6000 series (aluminum magnesium silicon), in particular alloy 6061, even more particularly the aluminum alloy is chosen from alloy 1050 and alloy 6061.

[0060] Advantageously, the copper alloy is a copper-beryllium alloy, such as copper-beryllium alloy type 33, also called C17300 (1.8% Be, 0.2% Co and 0.2% minimum Pb for machinability) or brass.

[0061] In an advantageous embodiment, the metal substrate is an electrical component, advantageously a conductor, a metal contact, a connector or a housing. More particularly, it is an electric wire, in particular a round wire,

[0062] The inventors surprisingly discovered that the silver layer present on the surface of the metal substrate has the properties of homogeneity, brilliance, thickness, adhesion and solderability similar to those obtained using a silvering bath containing cyanide.

[0063] In particular, adhesion complies with the NF EN ISO 2819 standard dated March 2018 and weldability complies with the IPC / JEDEC J-STD-002C standard dated December 2007.

[0064] The present invention further relates to a method for electrolytically depositing silver on a metal substrate comprising the following successive steps: A- surface preparation of the metal substrate; B- pre-silvering of the metal substrate obtained in step A), advantageously by using a pre-silvering bath whose composition comprises silver methanesulfonate, a chelator chosen from hydantoin, one of its derivatives or a mixture thereof, in particular chosen from hydantoin, 5,5-dimethyl-hydantoin, 1-methyl-hydantoin and their mixture, a pH regulator chosen from potassium or sodium hydroxide, in particular potassium hydroxide, and water, the pH of the composition being between 8.5 and 13.5, advantageously between 11.0 and 13.0 and its electrolytic conductivity being greater than or equal to 10 mS / cm, advantageously greater than or equal to 50 mS / cm; C- silvering of the metal substrate obtained in step B) by using the composition according to the invention; D- recovery of the metal substrate coated with a silver layer.

[0065] The process according to the invention may be continuous or discontinuous.

[0066] Advantageously, the metal substrate, the chelator, the pH regulator, the pH and the electrolytic conductivity are as described above in the context of the composition according to the invention and its uses.

[0067] Advantageously, step A) of the process according to the invention comprises degreasing and / or surface activation, in particular by using nitric acid (HNO 3 ), and / or pickling and optionally the addition of a metallic underlayer, such as nickel, zinc or copper (nickel plating, zinc plating and / or copper plating).

[0068] Advantageously, all process steps are followed by water rinsing and / or drying.

[0069] In an advantageous embodiment, the metal substrate is made of copper or copper alloy, in particular as described above, and step A) of the method according to the invention comprises, advantageously consists of, degreasing and optionally the addition of a metal underlayer (for example zinc: double zinc plating).

[0070] In another advantageous embodiment, the metal substrate is made of aluminum alloy and step A) of the method according to the invention comprises degreasing, surface activation, pickling and the addition of a metal underlayer. In particular, this step comprises the following sub-steps: A1) degreasing; A2) pickling; A3) surface activation; A4) double zinc plating; A5) chemical nickel plating.

[0071] In fact, these sub-steps make it possible to break any possible surface oxidation of the metal and to modify the surface state of the metal by reducing the difference in electrochemical potential between the aluminum and the silver.

[0072] In another advantageous embodiment, the metal substrate is made of brass and step A) of the method according to the invention comprises degreasing, surface activation and the addition of a metal undercoat. In particular, this step comprises the following sub-steps: A1') degreasing; A2') surface activation; A3') copper plating.

[0073] In particular, the degreasing step is carried out using electrolytic or chemical baths well known to those skilled in the art and commercially available, such as the Slotoclean EL DCG F electrolytic bath supplied by Schlôtter or chemical degreasing of the Oxidite C5 type or the Aluclean 250 type supplied by MacDermid.

[0074] Advantageously, step B) of the method according to the invention is carried out at a temperature of between 25 and 45°C, advantageously a temperature of 30°C, with a voltage of between 0.5 and 3 V, in particular 1.5 V, for a duration of between 10 and 60 seconds, in particular 45 seconds.

[0075] Particularly advantageously, the silver ion content (Ag +< ) of the pre-silvering bath of step B) of the process according to the invention is in the range 0.5 to 5 g / l, in particular it is 2 g / l.

[0076] Even more advantageously, the chelator of the pre-silvering bath of step B) of the process according to the invention is 5,5-dimethyl-hydantoin (D-HYD).

[0077] Preferably, the chelator content of the pre-silvering bath of step B) of the process according to the invention is in the range 2 to 30 g / l, in particular it is 10 g / l.

[0078] Thus advantageously the composition of the pre-silvering bath and the conditions of step B) of the process according to the invention are gathered in table 1 below: [Table 1] D-HYD (g / l) Ag (g / l) Temperature (°C) Time (s) Voltage (V) pH Conductivity mS / cm 10 2 30 45 1,5 11,5 10

[0079] The Ag concentration expressed here in g / l is obtained by dissolving MSAg with an Ag concentration of 275 g / l. KOH is added to ensure a pH in the range of 11-13.

[0080] Advantageously, step C) of the process according to the invention is carried out at a temperature of between 25 and 65°C, advantageously at a temperature of between 30°C and 55°C, with a current density (CD) of between 0.2 and 8 A / dm 2, in particular between 0.5 and 5.0 A / dm2, for a duration of between 1 and 60 minutes, in particular between 2.0 minutes and 30.0 minutes. These conditions vary in particular depending on the brightener and the chelator used.

[0081] Thus advantageously the composition of the silvering bath according to the invention and the conditions of step C) of the process according to the invention are gathered in table 2 below: [Table 2] Kind D-HYD g / l M-HYD g / l HYD g / l Ag g / l D-PYR g / l PEI g / l DDC A / dm 2< t min T °C I 100-200 20-55 0,5-3 1.0-3,5 3,0-30 25-35 II 100-300 20-55 0,2-1,0 1,0-3,5 3,0-30 25-40 III 50-200 10-50 0,2-1,0 0,5-1,5 2,0-30 25-55 IV 50-150 20-40 0,2-1,0 0,5-3,0 3,0-10,0 25-35 V 50-300 40-100 20-40 0,5-1,5 1,0-5,0 3,0-20,0 25-55

[0082] Advantageously, the silver layer obtained in step C) has a thickness of between 0.1 µm and 200 µm.

[0083] The invention will be better understood by reading the following examples which are given for non-limiting information purposes. Examples Example 1: Type I bath on round Cu wire

[0084] The method according to the invention is applied to a round copper wire with a diameter of 1.2 mm. Preparation :

[0085] Degreasing is carried out on the wire using a Slotoclean EL DCG F electrolytic bath supplied by Schlôtter under the conditions listed in Table 3 below: [Table 3] Temperature T (°C) Time t (minute) DDC (A / dm 2< ) 60 5 5 Pre-silvering

[0086] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above. Silvering

[0087] A silvering bath is prepared with the composition indicated in Table 4 below: [Table 4] D-HYD concentration (g / l) Ag concentration (g / l) B-PYD concentration (g / l) 160 37 1

[0088] The Ag concentration in g / l is achieved by dissolving MSAg salt with an Ag concentration of 275 g / l.

[0089] Then silver plating is carried out on the pre-silvered wire under the conditions indicated in table 5 below: [Table 5] DDC (A / dm 2< ) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1,5 10 30 10,94 55,7

[0090] A silver deposit with an average thickness of 6.5 µm measured with Fischer XDV SDD is obtained, which corresponds to a cathodic efficiency of 67.59%.

[0091] The deposit has a homogeneous and shiny appearance, as well as adhesion in accordance with the NF EN ISO 2819 standard (March 2018). The weldability test on the deposit according to the IPC / JEDEC J-STD-002C standard (December 2007) and using a Menisco ST78 device also proves to be compliant. Example 2: Type II bath on round Cu wire

[0092] The method according to the invention is applied to a round copper wire with a diameter of 1.2 mm. Preparation :

[0093] The wire is degreased under the same conditions as in Example 1, and then activated in a bath called Metex M629 supplied by MacDermid at 35°C for 2 minutes. Pre-silvering

[0094] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above. Silvering

[0095] A silvering bath is prepared with the composition indicated in Table 6 below: [Table 6] D-HYD concentration (g / l) Ag concentration (g / l) PEI concentration (g / l) 200 40 1

[0096] The Ag concentration in g / l is achieved by dissolving MSAg salt with an Ag concentration of 275 g / l.

[0097] Then silver plating is carried out on the pre-silvered wire under the conditions indicated in table 7 below: [Table 7] DDC (A / dm 2< ) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1,0 10 35 11,48 63,2

[0098] A silver deposit with an average thickness of 5.209 µm is obtained, which corresponds to a cathodic efficiency of 81.55%. The deposit has an appearance, adhesion and weldability similar to that of Example 1. Example 3: Type III bath on round Cu wire

[0099] The method according to the invention is applied to a round copper wire with a diameter of 1.2 mm. Preparation :

[0100] Degreasing is carried out on the wire under the same conditions as in Example 2. Pre-silvering

[0101] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above. Silvering

[0102] A silvering bath is prepared with the composition indicated in Table 8 below: [Table 8] M-HYD concentration (g / l) Ag concentration (g / l) PEI concentration (g / l) 200 40 1

[0103] The Ag concentration in g / l is achieved by dissolving MSAg salt with an Ag concentration of 275 g / l.

[0104] Then silver plating is carried out on the pre-silvered wire under the conditions indicated in table 9 below: [Table 9] DDC (A / dm 2< ) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1,5 17 55 11,50 63,6

[0105] A silver deposit with an average thickness of 10.01 µm is obtained, which corresponds to a cathodic efficiency of 61.48%. The deposit has an appearance, adhesion and weldability similar to that of Example 1. Example 4: Type III bath on round Cu wire

[0106] The method according to the invention is applied to a round copper wire with a diameter of 1.2 mm. Preparation :

[0107] Degreasing is carried out on the wire under the same conditions as in Example 2. Pre-silvering

[0108] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above. Arqenture

[0109] A silvering bath is prepared with the composition indicated in Table 10 below: [Table 10] HYD concentration (g / l) Ag concentration (g / l) PEI concentration (g / l) 90 40 0,5

[0110] The Ag concentration in g / l is achieved by dissolving MSAg salt with an Ag concentration of 275 g / l.

[0111] Then silver plating is carried out on the pre-silvered wire under the conditions indicated in table 11 below: [Table 11] DDC (A / dm 2< ) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1,5 5 30 11,48 63,2

[0112] A silver deposit with an average thickness of 3.821 µm is obtained, which corresponds to a cathodic efficiency of 80.02%. The deposit has an appearance, adhesion and weldability similar to the case of Example 1. Example 5: Type V bath on round Cu wire

[0113] The method according to the invention is applied to a round copper wire with a diameter of 1.2 mm. Preparation :

[0114] Degreasing is carried out on the wire under the same conditions as in Example 2. Pre-silvering

[0115] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above. Silvering

[0116] A silvering bath is prepared with the composition indicated in Table 12 below: [Table 12] D-HYD concentration (g / l) HYD concentration (g / l) Ag concentration (g / l) PEI concentration (g / l) 270 90 40 0,5

[0117] The Ag concentration in g / l is achieved by dissolving MSAg salt with an Ag concentration of 275 g / l.

[0118] Then silver plating is carried out on the pre-silvered wire under the conditions indicated in table 13 below: [Table 13] DDC (A / dm 2< ) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1,0 5 30 12,06 68,1

[0119] A silver deposit with an average thickness of 2.275 µm is obtained, which corresponds to a cathodic efficiency of 71.24%. The deposit has an appearance, adhesion and weldability similar to that of Example 1. Example 6: Type II bath on brass electrical contact

[0120] The method according to the invention is applied to an electrical contact made of brass with a surface area of ​​0.17 dm 2. Preparation :

[0121] The substrate to be silvered being made of Brass, the preparation includes 3 steps which are in order: electrolytic degreasing Slotoclean EL DCG F (supplier Schlötter), activation in a Metex M629 bath (supplier MacDermid) and copper plating in a so-called Cuprum 10 bath (supplier Schlötter), under the following conditions indicated in table 14 below: [Table 14] Degreasing Activation Copper plating t (min.) DDC (A / dm 2< ) T (°C) t (min.) DDC (A / dm 2< ) T (°C) t (min.) DDC (A / dm 2< ) 7 5 35 7 5 35 7 5 Pre-silvering

[0122] Pre-silvering is carried out on the degreased contact under the conditions indicated in Table 1 above. Silvering

[0123] A type II silvering bath identical to that defined in Example 2 is prepared.

[0124] Then silver plating is carried out on the pre-silvered contact under the conditions indicated in table 15 below: [Table 15] DDC (A / dm 2< ) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 2,0 22 35 11,75 66,4

[0125] A silver deposit with an average thickness of 8.775 µm is obtained, which corresponds to a cathodic efficiency of 31.22%. The deposit has an appearance, adhesion and weldability similar to that of Example 1. Example 7: Type II bath on AI 6061 connector housing

[0126] The method according to the invention is applied to a connector housing, with a surface area of ​​0.1636 dm 2< , made of 6061 aluminum. It is known that aluminum is more difficult than copper to cover with a metallic coating by electrolytic means. Preparation :

[0127] A surface preparation of the aluminum is therefore necessary in the application of the process according to the invention, comprising in order a chemical degreasing of the Oxidite C5 type, an Alumon AC70 pickling, and a combination of an activation of nitric acid HNO 3 at 53% and a zinc plating of the Bondal type, the latter being repeated and commonly called double zinc plating. All these products are supplied by the supplier MacDermid.

[0128] The said preparation is carried out under the conditions indicated in the following table 16: [Table 16] Oxidite C5 Alumon AC70 HNO3 activation Bondal HNO3 activation Bondal T (°C) t (min) T (°C) t (min) T (°C) t (min) T (°C) t (min) T (°C) t (min) T (°C) t (min) 65 5 35 2 ambient 1 65 5 35 2 ambient 1

[0129] Considering that chemical nickel plating on 6061 aluminum connector housing is currently a common practice and allows to effectively protect the aluminum substrate as an undercoat, this step is added after the preparation.

[0130] Nickel plating is done in a bath made from a mixture of two products supplied by MacDermid Enthone, NIKLAD ELV 809 A and NIKLAD ELV 809 B. This mixture is heated to 90°C and the part is immersed in the bath for 10 minutes. Pre-silvering

[0131] Pre-silvering is carried out on the degreased case under the conditions indicated in Table 1 above. Silvering

[0132] A type II silvering bath identical to that defined in Example 2 is prepared.

[0133] Then silver plating is carried out on the pre-silvered case under the conditions indicated in table 17 below: [Table 17] DDC (A / dm 2< ) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1,5 11 35 12,06 68,1

[0134] A silver deposit with an average thickness of 9.652 µm is obtained, which corresponds to a cathodic efficiency of 91.60%. The deposit has an appearance, adhesion and weldability similar to the case of Example 1.

[0135] Additionally, a so-called thermal shock test, specifically designed for connectors, is added in this example. This involves placing a coated sample at 220°C for 15 minutes and then immersing it in cold water to reach room temperature. The coated sample is then observed under an optical microscope with X5 magnification to check that the coating is not peeling or blistering. The silver case made in this Example passes this test successfully. Example 8: Type II bath on AI 1050 wire

[0136] The method according to the invention is applied to a wire with a diameter of 0.1 mm made of 1050 aluminum. Preparation :

[0137] Since the substrate is made of 1050 aluminum, another preparation is applied. It differs from that of Example 7 and includes, in order, a chemical degreasing of the Aluclean 250 type and a double zinc plating. The operating conditions are summarized in Table 18 below. [Table 18] Aluclean HNO 3 Activation Bondal HNO 3 Activation Bondal T (°C) t (min) T (°C) t (min) T (°C) T (°C) t (min) T (°C) t (min) T (°C) 60 5 ambient 1 ambient 60 5 ambient 1 ambient Pre-silvering

[0138] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above. Silvering

[0139] A type II silvering bath identical to that defined in Example 2 is prepared.

[0140] Then silver plating is carried out on the pre-silvered wire under the conditions indicated in table 19 below: [Table 19] DDC (A / dm 2< ) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1,0 6 45 12,51 69,9

[0141] A silver deposit with an average thickness of 5.25 µm is obtained, which corresponds to a cathodic efficiency of 98.5%. The deposit has an appearance, adhesion and weldability similar to that of Example 1.

[0142] In addition, a complementary test is often used to check the porosity of the coating on an aluminum conductor. This test consists of wrapping a coated aluminum wire around a cylinder and then immersing it in a 30% NaOH sodium hydroxide solution for 15 minutes. At the end of these 15 minutes, without removing it from the solution, it is observed whether the wire has numerous bubbles on its surface or whether it emits gas. The silver wire in this example passes this test. Example 9: Type II bath on Ni wire

[0143] The method according to the invention is applied to a round nickel wire with a diameter of 0.4 mm. Preparation :

[0144] Degreasing is carried out on the wire under the same conditions as in Example 1, followed by activation with concentrated citric acid at 75 g / l at 30°C for 5 minutes. Pre-silvering

[0145] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above. Silvering

[0146] A type II silvering bath identical to that defined in Example 2 is prepared.

[0147] Then silver plating is carried out on the pre-silvered wire under the conditions indicated in table 20 below: [Table 20] DDC (A / dm 2< ) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1,0 20 55 12,7 71,2 A silver deposit with an average thickness of 7.29 µm is obtained, which corresponds to a cathodic efficiency of 57.07%. The deposit has an appearance, adhesion and weldability similar to that of Example 1.

Claims

1. Cyanide-free silvering bath composition comprising: - silver methanesulfonate; - a chelator chosen from hydantoin, one of its derivatives or a mixture thereof, advantageously chosen from hydantoin, 5,5-dimethyl-hydantoin, 1-methyl-hydantoin and mixtures thereof; - a brightener chosen from polyethyleneimine and bipyridine, advantageously it is polyethyleneimine; - a pH regulator chosen from potassium or sodium hydroxide, advantageously it is potassium hydroxide and - water; the pH of the composition being between 8.5 and 13.5, advantageously between 11.0 and 13.0 and its electrolytic conductivity being greater than or equal to 10 mS / cm, advantageously greater than or equal to 50 mS / cm, characterized in that the composition does not include any other brighteners than that chosen from polyethyleneimine and bipyridine.

2. Composition according to claim 1, characterized in thatthe silver ion content is between 10 and 80 g / l.

3. Composition according to any one of claims 1 or 2, characterized in that it does not include other chelators and / or other complexing agents.

4. A method of manufacturing the composition according to any one of claims 1 to 3 comprising the following successive steps: a- addition of the pH regulator, advantageously potassium hydroxide, in demineralized water at room temperature so as to obtain a pH of between 8.5 and 13.5 and an electrolytic conductivity greater than or equal to 10 mS / cm; b- addition of the chelator at room temperature while maintaining the pH between 8.5 and 13.5, and the electrolytic conductivity at a value greater than or equal to 10 mS / cm; c- addition of the silver methanesulfonate at room temperature while maintaining the pH between 8.5 and 13.5 and the electrolytic conductivity at a value greater than or equal to 10 mS / cm; d- addition of the brightener, advantageously polyethyleneimine, while maintaining the pH between 8.5 and 13.5 and the electrolytic conductivity at a value greater than or equal to 10 mS / cm.

5. Use of the composition according to any one of claims 1 to 3 for the electrolytic deposition of silver on a metal substrate.

6. Use according to claim 5, characterized in that the metal substrate is a copper or copper alloy, aluminum alloy, or nickel substrate.

7. Use according to any one of claims 5 or 6, characterized in that the metal substrate is an electrical component, advantageously a conductor, a metal contact, a connector or a housing.

8. A method for electrolytically depositing silver on a metal substrate comprising the following successive steps: A- surface preparation of the metal substrate; B- pre-silvering of the metal substrate obtained in step A), advantageously by using a pre-silvering bath whose composition comprises silver methanesulfonate, a chelator chosen from hydantoin, one of its derivatives or a mixture thereof, in particular chosen from hydantoin, 5,5-dimethyl-hydantoin, 1-methyl-hydantoin and their mixture, a pH regulator chosen from potassium or sodium hydroxide, in particular potassium hydroxide, and water, the pH of the composition being between 8.5 and 13.5, advantageously between 11.0 and 13.0 and its electrolytic conductivity being greater than or equal to 10 mS / cm, advantageously greater than or equal to 50 mS / cm;C- silvering of the metal substrate obtained in step B) by using the composition according to any one of claims 1 to 3; D- recovery of the metal substrate coated with a layer of silver.; 9. Method according to claim 8, characterized in that step A) includes degreasing and / or surface activation and / or stripping and optionally the addition of a metallic undercoat.

10. Method according to any one of claims 8 or 9, characterized in that it is continuous or discontinuous.

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