Composition of a cyanide-free silver plating bath and its uses

Abstract

Cyanide-Free Silvering Bath Composition and Uses The present invention relates to a cyanide-free silvering 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. 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

Title of the invention: Cyanide-free silver plating bath composition and its uses. Technical field

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

[0002] Silver electrodeposition on a metallic substrate using cyanide as a complexing agent was developed in the 19th century (in France by the Charles Christofle company in Paris) and has remained the only manufacturing process used in industrial surface treatment to date. The technology offers advantages in terms of performance and robustness, but imposes significant constraints on the manufacturing site due to the high toxicity of the cyanide involved in the process.

[0003] For example, in 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 acidic and alkaline substances and the second dedicated specifically to the elimination of cyanide residues.

[0004] For decades, chemists and electrochemists worldwide, in both basic and applied research, have been striving to find a substitute for silver plating with cyanide electrolyte. Their scientific work has led to the proposal of several cyanide-free silver plating baths, but none has yet been used in industrial applications, at least not in the electrical and electronic components sector 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 objective of substitution.

[0006] The major difficulty lies in the composition of the electrolyte, more commonly called the silver plating bath formulation, which must include several essential chemical components. It is therefore important that these components be chemically compatible with each other. It is through the effective synergy of all the chemical actions exerted by these components that a silver plating of comparable quality to that using traditional cyanide plating 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, thus than other agents such as driver, wetting, brightening, etc... depending on the application.

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

[0009] The silver complexing agent, as its name indicates, allows the complexation of silver ions in a suitable form so that they can not only be extracted at the anode and deposited on the surface of the cathode by electrolysis, but also be transported unimpeded in the aqueous electrolytic medium from the anode to the cathode. In the case of cyanide silver, it is the so-called free cyanide CN- from sodium cyanide NaCN or potassium cyanide KCN dissolved in the aqueous electrolyte that fulfills this role, the compound thus complexed being Ag(CN).

[0010] AgCN + KCN = Ag(CN)2 + K+

[0011] In some cases, when the action of the complexing agent proves insufficient, another type of agent, called a chelating agent, is generally used to enhance the complexing effect. The chelating agent, considered a highly coordinated ligand, is distinguished from a simple complexing agent by its cyclic molecular structure and by at least two coordinate bonds.

[0012] In the prior art, cyanide-free silver plating bath compositions using inorganic nitrates, citrates, sulfates, chlorides, and phosphates as silver salts have already been described. The majority of these involve silver nitrate, AgNO3.

[0013] The choice of silver nitrate would initially be guided by its very high aqueous solubility. However, good aqueous solubility alone is not sufficient to obtain a high-performance silver plating electrolytic bath.

[0014] Other work has focused on the complexing agent; US patent 5302278 (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 the salt.

[0015] Patent applications US2007 / 0284258Al(2007) and US2015 / 0184307Al(2015) disclose silver plating baths characterized by methanesulfonic acid (CH3SO3H) as a complexing agent. The first patent describes the use of silver methanesulfonate (AgCH3SO3) as the silver salt, and the second, silver nitrate (AgNO3). Both baths are characterized by moderate acidity with a pH below 3.

[0016] Patent EP2431502 discloses another avenue of exploration for cyanide-free silvering, which mentions the 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 This suggests that the source of silver ions is related to silver nitrate (AgNO3). The patent also indicates that the pH of the baths is regulated at 9.5, therefore slightly alkaline.

[0017] On the other hand, all the aforementioned patents relate only to electrolytic silvering baths, none of them involving application to an industrially manufactured product or evaluation of functional properties such as adhesion, mechanical strength, thermal strength and corrosion resistance.

[0018] The principle of industrial electrolytic silver plating used to date can be characterized by a continuous or discontinuous process comprising at least 3 successive steps: - Surface preparation - Pre-silvering - Silver plating

[0019] This is indeed the case for copper and its alloys which are widely used in the electrical components sector such as conductors and connectors.

[0020] For silver plating aluminum alloys (pure aluminum is rarely used due to its poor mechanical properties), which are another class of electrical materials, a 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 - Silver plating

[0021] All steps are individually followed by effective water rinsing and / or drying to obtain a quality silver deposit.

[0022] 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 conductors and machining for connectors in the presence of lubricants of all kinds.

[0023] In the case of aluminum alloys, in addition to the degreasing step, the pretreatment step includes several sub-steps such as pickling, activation, double zinc plating, and electroless nickel plating. It is the combination of these sub-steps that makes it possible to break down any surface oxidation of the metal and to modify the surface condition of the metal by reducing the electrochemical potential difference between aluminum and silver, which is itself quite unfavorable in electrodeposition.

[0024] To date the pre-silvering and silvering stages contain cyanide and involve environmental infrastructure constraints as previously indicated.

[0025] The present invention aims to provide cyanide-free pre-silvering and silvering baths with performance and quality comparable to current baths that contain cyanide.

[0026] Achieving such an objective requires first the formation of a cyanide-free silver bath and then a chemically stable and operationally robust implementation process.

[0027] The inventors were surprised to discover that it was possible to achieve this objective by using a cyanide-free silvering bath in which silver methane sulfonate 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 polyethylene-imine and bi-pyridine, 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.

[0028] Indeed, such a silver 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.

[0029] Furthermore, even though silver methanesulfonate (AgCH3SO3) has a lower solubility than silver nitrate, the judicious choice of pH, and therefore of pH regulator, allows for strong chelation of silver ions, thus preventing their precipitation which would destabilize the silver plating bath. This high pH value also prevents any direct or indirect hydrolysis of chelating agents such as hydantoins, thereby preserving their stability. Moreover, this pH value also provides the electrical conductivity necessary for the deposition of the silver layer.

[0030] The use of a brightener further ensures that the silver deposit is aesthetically pleasing and has a beautiful appearance. Indeed, the brightener aids the electrolytic process by creating and maintaining a fine crystalline structure of the silver deposit, thus resulting in a visually brilliant deposit. Description of the invention

[0031] The present invention therefore relates to a cyanide-free silvering bath composition comprising (advantageously consisting essentially of, more advantageously consisting of): -silver methane sulfonate; -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 polyethylene-imine and bi-pyridine, advantageously it is polyethylene-imine; - a pH regulator chosen from potassium or sodium hydroxide, advantageously 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.

[0032] For the purposes of the present invention, the expressions "between... and...", "from... to...", "within the range..." shall be understood to include limits unless explicitly stated otherwise.

[0033] The composition according to the present invention therefore comprises silver methanesulfonate, AgCH3SO3 (MSAg). The silver methanesulfonate is both the silver salt that provides the silver ions to the composition and 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 only one product is sufficient. As previously stated, the complexing agent allows the silver ions to be complexed in a suitable form so that they can not only be extracted at the anode and deposited on the surface of the cathode by electrolysis, but also be transported unimpeded 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. 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.

[0034] The silver ion (Ag+) content of the composition according to the invention is advantageously between 10 and 80 g / L, more advantageously between 15 and 70 g / L, and even more advantageously between 20 and 60 g / L, in particular between 20 and 55 g / L. Depending on the operating parameters of the silvering process, the chelating agent, and the brightening agent 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.

[0035] The composition according to the present invention further comprises a chelating agent selected from hydantoin, one of its derivatives, or a mixture thereof. Hydantoin derivatives are well known to those skilled in the art. In particular, they may These are methylated and / or hydroxymethylated derivatives, such as, for example, 5,5-dimethylhydantoin, 1-methylhydantoin, 1,3-dimethylhydantoin, and 1-hydroxymethyl-5,5-dimethylhydantoin. Advantageously, the hydantoin derivative is selected from 5,5-dimethylhydantoin, 1-methylhydantoin, and mixtures thereof. Particularly advantageously, the chelating agent of the composition according to the invention is selected from hydantoin (HYD), 5,5-dimethylhydantoin (D-HYD), 1-methylhydantoin (M-HYD), and mixtures thereof.

[0036] For the purposes of this invention, "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, in this case, the silver ion. Due to their two lactam functional groups, hydantoin and its derivatives according to the invention act as chelators with respect to silver methanesulfonate. Furthermore, although organic, hydantoin and its derivatives are highly soluble in aqueous media, which offers another significant advantage in the formulation of industrially usable baths.

[0037] Advantageously, the chelator content chosen from hydantoin, one of its derivatives or a mixture of these 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.

[0038] In an advantageous embodiment, the hydantoin content of the composition according to the invention is between 40 and 150g / l, more advantageously between 50 and 100 g / l, in particular it is 90g / l.

[0039] 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.

[0040] In yet another advantageous embodiment, the 1-methyl-hydantoin content is between 50 and 250 g / l.

[0041] Advantageously the composition according to the present invention does not include any chelators other than the one chosen from hydantoin, one of its derivatives or a mixture thereof.

[0042] The composition according to the present invention further comprises a brightener selected from polyethylene imine (PEI) and bipyridine (D-PYR). Advantageously, the brightener is polyethylene imine. In particular, the polyethylene imine according to the invention is linear. More particularly, it has a number-average molecular weight between 100 and 100,000 u, more advantageously between 200 and 1,000 u, in particular between 500 and 700 u, in particular 600 u.

[0043] Advantageously, the gloss content chosen from polyethylene-imine and bi-pyridine in the composition according to the invention is between 0.2 and 3 g / l, in particular between 0.3 and 1.5 g / l.

[0044] Advantageously the composition according to the present invention does not include any brighteners other than the one chosen from polyethylene-imine and bi-pyridine, even more advantageously the only brightener in the composition is polyethylene-imine.

[0045] The composition according to the present invention further comprises a pH regulator selected from potassium or sodium hydroxide, advantageously 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.

[0046] 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.

[0047] The composition according to the invention therefore has: - 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.

[0048] The composition according to the invention is free of cyanide.

[0049] The composition according to the present invention may include other additives, such as than wetting agents or grain refiners, but advantageously it does not include any.

[0050] 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 - of iminodisuccinate derivatives, in particular as described in application US2011 / 0062030, more specifically tetrasodium iminodisuccinate, and / or - of niacin and / or - of ammonium sulfate and / or - of 1,4-butynediol and / or - of phosphine (in particular aliphatic or aromatic, more specifically tris(3-hydroxypropyl)phosphine) and / or - of azole compounds, in particular as described in US application 2007 / 0284258, more specifically triazoles such as 1,2,3-triazole, 3-amino-1,2,4-triazole, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole and benzotriazole or benzimidazoles such as 2-mercaptobenzimidazole, and / or - of benzoic acid derivatives, in particular as described in US application 2015 / 0184307, more specifically 2-sulfobenzic acid and 3,4-dinitrobenzoic acid, - of sulfonic acid or one of its derivatives, in particular as described in US application 2010 / 0044239, more specifically potassium methanesulfonate and poly condensate formaldehyde naphthalene of sulfonic acid, and / or - of potassium bromide, and / or -of sodium thiosulfate, and / or - tripotassium citrate monohydrate.

[0051] Thus advantageously, the composition according to the invention does not include other chelators and / or other complexing agents.

[0052] The present invention further relates to a method for 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 methane sulfonate 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 polyethylene-imine, 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.

[0053] Indeed, the inventors were surprised to find that if the pH and 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 less than 8.5, the composition lost its stability.

[0054] Maintaining the pH and electrolytic conductivity during steps b), c), and d) is achieved 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 by using a pH meter. meter. The electrolytic conductivity of the composition according to the invention during these steps can be verified and controlled by using a conductivity meter.

[0055] "Ambient temperature" means a temperature between 20 and 25 °C.

[0056] The present invention further relates to the use of the composition according to the invention for the electrolytic deposition of silver on a metallic substrate, advantageously the electrolytic deposition of a silver layer having a thickness between 0.1 pm and 200 pm on the metallic substrate.

[0057] Advantageously the metallic substrate is a substrate made of copper or copper alloy such as brass or copper-beryllium, aluminum alloy or nickel.

[0058] Advantageously the aluminium alloy is chosen from aluminium silicon, aluminium magnesium or aluminium magnesium silicon alloys such as for example the 1000 series alloys in particular alloy 1050, the 4000 series (aluminium silicon), in particular alloys 4047 and 4032, more particularly from the 4047 series, the 5000 series alloys (aluminium magnesium), in particular alloys 5083 and 5754, more particularly from the 5083 series and the 6000 series alloys (aluminium magnesium silicon), in particular alloy 6061, even more particularly the aluminium alloy is chosen from alloy 1050 and alloy 6061.

[0059] 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 of Pb for machinability) or brass.

[0060] In an advantageous embodiment, the metallic substrate is an electrical component, advantageously a conductor, a metallic contact, a connector, or a housing. More particularly, it is an electrical wire, especially a round wire.

[0061] The inventors were surprised to discover that the silver layer present on the surface of the metallic substrate has properties of homogeneity, brightness, thickness, adhesion, and solderability similar to those obtained using a silver plating bath containing cyanide.

[0062] In particular, adhesion complies with standard NF EN ISO 2819 dated March 2018 and weldability complies with standard IPC / JEDEC J-STD-002C dated December 2007.

[0063] The present invention further relates to a method for electrolytic silver deposition on a metallic substrate comprising the following successive steps: A- surface preparation of the metallic substrate; B- Pre-silvering of the metallic substrate obtained in step A), advantageously by using a pre-silvering bath whose composition includes silver methane sulfonate, a chelating agent selected from hydantoin, one of its derivatives or a mixture thereof, in particular selected from hydantoin, 5,5-dimethyl-hydantoin, 1-methyl-hydantoin and mixtures thereof, a pH regulator selected 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 metallic substrate obtained in step B) by using the composition according to the invention; D- recovery of the metallic substrate coated with a layer of silver.

[0064] The process according to the invention may be continuous or discontinuous.

[0065] Advantageously the metallic substrate, the chelator, the pH regulator, the pH and the electrolytic conductivity are as described above within the framework of the composition according to the invention and its uses.

[0066] Advantageously, step A) of the process according to the invention includes degreasing and / or surface activation, in particular by use of nitric acid (HNO3), 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).

[0067] Advantageously, all steps of the process are followed by water rinsing and / or drying.

[0068] In an advantageous embodiment, the metallic substrate is made of copper or copper alloy, in particular as described above, and step A) of the process according to the invention includes, advantageously, degreasing and optionally adding a metallic underlayer (for example zinc: double zinc plating).

[0069] In another advantageous embodiment, the metallic substrate is made of aluminum alloy and step A) of the process according to the invention comprises degreasing, surface activation, pickling, and the addition of a metallic undercoat. In particular, this step comprises the following substeps: - A1) degreasing; - A2) stripping; - A3) surface activation; - A4) double zinc plating; - A5) chemical nickel plating.

[0070] Indeed, these sub-steps make it possible to break down any superficial oxidation of the metal and to modify the surface state of the metal by reducing the electrochemical potential difference between aluminum and silver.

[0071] In another advantageous embodiment, the metallic substrate is brass and step A) of the process according to the invention comprises degreasing, surface activation, and the addition of a metallic underlayer. In particular, this step comprises the following substeps: - A1') degreasing; - A2') surface activation; - A3') copper plating.

[0072] In particular, the degreasing step is implemented by using electrolytic or chemical baths well known to those skilled in the art and available commercially 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.

[0073] Advantageously, step B) of the process according to the invention is carried out at a temperature between 25 and 45 °C, advantageously at a temperature of 30 °C, with a voltage between 0.5 and 3 V, in particular 1.5 V, for a duration between 10 and 60 seconds, in particular 45 seconds.

[0074] Particularly advantageously, the silver ion (Ag+) content of the pre-silvering bath in step B) of the process according to the invention is in the range of 0.5 to 5 g / l, in particular it is 2g / l.

[0075] Even more advantageously, the chelator of the pre-silvering bath in step B) of the process according to the invention is 5,5-dimethyl-hydantoin (D-HYD).

[0076] Preferably, the chelator content of the pre-silvering bath in step B) of the process according to the invention is in the range of 2 to 30 g / l, in particular it is 10 g / l.

[0077] Thus advantageously the composition of the pre-silvering bath and the conditions of step B) of the process according to the invention are summarized in Table 1 below:

[0078] [Tables 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 between 25 and 65 °C, advantageously at a temperature between 30 °C and 55 °C, with a current density (CDD) between 0.2 and 8 A / dm², in particular between 0.5 and 5.0 A / dm², for a duration between 1 and 60 minutes, in particular between 2.0 and 30.0 minutes. These conditions vary in particular depending on the brightener and chelating agent 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 summarized in Table 2 below:

[0082] [Tables2] Type D-HYD g / 1 M-HYD g / 1 HYD g / 1 Ag g / 1 D-PYR g / 1 PEI g / 1 DDC A / dm2 t min T °CI 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

[0083] Advantageously the silver layer obtained in step C) has a thickness between 0.1 pm and 200 pm.

[0084] The invention will be better understood by reading the following examples, which are given by way of non-limiting illustration. Examples Example 1: Type I bath on round copper wire

[0085] The process according to the invention is applied to a round copper wire with a diameter of 1.2 mm.

[0086] Preparation:

[0087] The wire is degreased using a Slotoclean EL DCG F electrolytic bath supplied by Schlôtter under the conditions set out in Table 3 below:

[0088] [Tables3] Temperature T (°C) Time t (minutes) DDC (A / dm²) 60 5 5

[0089] Pre-silvering

[0090] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.

[0091] Silver plating

[0092] A silver plating bath is prepared with the composition indicated in Table 4 below:

[0093] [Tables4] D-HYD concentration (g / 1) Ag concentration (g / 1) B-PYD concentration (g / 1) 160 37 1

[0094] The Ag concentration in g / 1 is achieved by dissolving MSAg salt with an Ag concentration of 275 g / 1.

[0095] Then the silver plating is carried out on the pre-silvered wire under the conditions indicated in Table 5 below:

[0096] [Tables5] DDC (A / dm²) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1.5 10 30 10.94 55.7

[0097] A silver deposit with an average thickness of 6.5 pm is obtained as measured with Fischer XDV SDD, which corresponds to a cathode efficiency of 67.59%.

[0098] The coating has a homogeneous and glossy appearance, as well as adhesion conforming to standard NF EN ISO 2819 (March 2018). The weldability test on the coating according to standard IPC / JEDEC J-STD-002C (December 2007) and using a Menisco ST78 apparatus is also found to be compliant. Example 2: Type II bath on round copper wire

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

[0100] Preparation:

[0101] Degreasing is carried out on the wire under the same conditions as in Example 1, and then activation is performed in a bath called Metex M629 supplied by MacDermid at 35°C for 2 minutes.

[0102] Pre-silvering

[0103] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.

[0104] Silver plating

[0105] A silvering bath is prepared with the composition indicated in Table 6 below:

[0106] [Tableauxô] D-HYD concentration (g / 1) Ag concentration (g / 1) PEI concentration (g / 1) 200 40 1

[0107] The Ag concentration in g / 1 is achieved by dissolving MSAg salt with an Ag concentration of 275 g / 1.

[0108] Then the silver plating is carried out on the pre-silvered wire under the conditions indicated in Table 7 below:

[0109] [Tables7] DDC (A / dm²) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1.0 10 35 11.48 63.2

[0110] A silver deposit with an average thickness of 5.209 pm is obtained, corresponding to a cathodic efficiency of 81.55%. The deposit exhibits an appearance, adhesion, and weldability similar to that of Example 1. Example 3: Type III bath on round Cu wire

[0111] The process according to the invention is applied to a round copper wire with a diameter of 1.2 mm.

[0112] Preparation:

[0113] Degreasing is carried out on the wire under the same conditions as in Example 2.

[0114] Pre-silvering

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

[0116] Silver plating

[0117] A silvering bath is prepared with the composition indicated in Table 8 below:

[0118] [Tables8] M-HYD concentration (g / 1) Ag concentration (g / 1) PEI concentration (g / 1) 200 40 1

[0119] The Ag concentration in g / 1 is achieved by dissolving MSAg salt with an Ag concentration of 275 g / 1.

[0120] Then the silver plating is carried out on the pre-silvered wire under the conditions indicated in Table 9 below:

[0121] [Tables9] DDC (A / dm²) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1.5 17 55 11.50 63.6

[0122] A silver deposit with an average thickness of 10.01 pm is obtained, corresponding to a cathodic efficiency of 61.48%. The deposit exhibits an appearance, adhesion, and weldability similar to that of Example 1. Example 4: Type III bath on round Cu wire

[0123] The process according to the invention is applied to a round copper wire with a diameter of 1.2 mm.

[0124] Preparation:

[0125] Degreasing is carried out on the wire under the same conditions as in Example 2.

[0126] Pre-silvering

[0127] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.

[0128] Silver plating

[0129] A silver plating bath is prepared with the composition indicated in Table 10 below:

[0130] [TableauxlO] HYD concentration (g / 1) Ag concentration (g / 1) PEI concentration (g / 1) 90 40 0.5

[0131] The Ag concentration in g / 1 is achieved by dissolving MSAg salt with an Ag concentration of 275 g / 1.

[0132] Then the silver plating is carried out on the pre-silvered wire under the conditions indicated in Table 11 below:

[0133] [Tableauxll] DDC (A / dm²) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1.5 5 30 11.48 63.2

[0134] A silver deposit with an average thickness of 3.821 pm is obtained, corresponding to a cathodic efficiency of 80.02%. The deposit has an appearance, adhesion, and weldability similar to that of Example 1. Example 5: V-type bath on round copper wire

[0135] The process according to the invention is applied to a round copper wire with a diameter of 1.2 mm.

[0136] Preparation:

[0137] Degreasing is carried out on the wire under the same conditions as in Example 2.

[0138] Pre-silvering

[0139] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.

[0140] Silver plating

[0141] A silver plating bath is prepared with the composition indicated in Table 12 below:

[0142] [Tables 12] D-HY D concentration (g / 1) HYD concentration (g / D Ag concentration (g / D PEI concentration (g / D 270 90 40 0.5

[0143] The Ag concentration in g / 1 is achieved by dissolving MSAg salt with an Ag concentration of 275 g / 1.

[0144] Then the pre-silvered wire is silver-plated under the conditions indicated in Table 13 below:

[0145] [Tables 13] DDC (A / dm²) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1.0 5 30 12.06 68.1

[0146] A silver deposit with an average thickness of 2.275 pm is obtained, corresponding to a cathodic efficiency of 71.24%. The deposit exhibits an appearance, adhesion, and weldability similar to that of Example 1.

[0147] Example 6: Type II bath on brass electrical contact

[0148] The method according to the invention is applied to a brass electrical contact with a surface area of ​​0.17 dm2.

[0149] Preparation:

[0150] Since the substrate to be silver-plated is brass, the preparation comprises 3 steps which are in order: electrolytic degreasing Slotoclean EL DCG F (supplier Schlôtter), activation in a Metex M629 bath (supplier MacDermid) and copper plating in a bath called Cuprum 10 (supplier Schlôtter), under the following conditions indicated in Table 14 below:

[0151] [Tables 14] Degreasing Activation Coppering t (min.) DDC (A / d m2) T (°C) t (min.) DDC (A / d m2) T (°C) t (min.) DDC (A / d m2) 7 5 35 7 5 35 7 5

[0152] Pre-silvering

[0153] Pre-silvering is carried out on the degreased contact under the conditions indicated in Table 1 above.

[0154] Silver plating

[0155] A type II silver plating bath is prepared identical to that defined in Example 2.

[0156] Then the silver plating is carried out on the pre-silvered contact under the conditions indicated in Table 15 below:

[0157] [Tables 15] DDC (A / dm²) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 2.0 22 35 11.75 66.4

[0158] A silver deposit with an average thickness of 8.775 pm is obtained, corresponding to a cathodic efficiency of 31.22%. The deposit has an appearance, adhesion and weldability similar to the case of Example 1.

[0159] Example 7: Type II bath on Al 6061 connector housing

[0160] The process according to the invention is applied to a connector housing, with a surface area of ​​0.1636 dm2, made of 6061 aluminum. It is known that aluminum is more difficult compared to copper to coat with a metallic coating by electrolytic means.

[0161] Preparation:

[0162] Surface preparation of the aluminum is therefore necessary in the application of the process according to the invention, comprising, in order, chemical degreasing of the Oxidite C5 type, pickling with Alumon AC70, and a combination of activation with 53% nitric acid (HNO3) and zinc plating of the Bondal type, the latter being repeated and commonly referred to as double zinc plating. All these products are supplied by MacDermid.

[0163] Said preparation is carried out under the conditions indicated in the following Table 16:

[0164] [Tables 16] Oxidite C5 Alumon AC 70 Activation HNO 3 Bondal Activation H NO3 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

[0165] Given that chemical nickel plating on 6061 aluminium connector housing is currently a common practice and effectively protects the aluminium substrate as an underlayer, this step is added after preparation.

[0166] Nickel plating is carried out in a bath made from a mixture of 2 products supplied by the company 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.

[0167] Pre-silvering

[0168] Pre-silvering is carried out on the degreased housing under the conditions indicated in Table 1 above.

[0169] Silver plating

[0170] A type II silver plating bath identical to that defined in Example 2 is prepared.

[0171] Then the silver plating is carried out on the pre-silver-plated case under the conditions indicated in Table 17 below:

[0172] [Tables 17] DDC (A / dm²) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1.5 11 35 12.06 68.1

[0173] A silver deposit with an average thickness of 9.652 pm is obtained, corresponding to a cathodic efficiency of 91.60%. The deposit exhibits an appearance, adhesion, and weldability similar to that of Example 1.

[0174] In addition, a thermal shock test, specifically designed for connectors, is included 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 at 5x magnification to check for any peeling or blistering of the coating. The silver enclosure produced in this example passes this test successfully. Example 8: Type II bath on Al 1050 wire

[0175] The process according to the invention is applied to a 0.1 mm diameter wire made of 1050 aluminium.

[0176] Preparation:

[0177] Since the substrate is aluminum 1050, a different preparation is applied. This differs from that of Example 7 and comprises, in order, chemical degreasing with Aluclean 250 and double zinc plating. The operating conditions are summarized in Table 18 below.

[0178] [Tables l8] Aluclean Activation HNO3 Bondal Activation HNO3 Bondal T (°C) t (min n) T (°C) t (min n) T (°C) T (°C) t (min n) T (°C) t (min n) T (°C) 60 5 ambient 1 ambient 60 5 ambient 1 ambient

[0179] Pre-silvering

[0180] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.

[0181] Silver plating

[0182] A type II silver plating bath is prepared identical to that defined in Example 2.

[0183] Then the pre-silvered wire is silver-plated under the conditions indicated in Table 19 below:

[0184] [Tables 19] DDC (A / dm²) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1.0 6 45 12.51 69.9

[0185] A silver deposit with an average thickness of 5.25 pm is obtained, corresponding to a cathodic efficiency of 98.5%. The deposit exhibits an appearance, adhesion, and weldability similar to that of Example 1.

[0186] In addition, a supplementary test is performed, often used to check the porosity of coatings on aluminum conductors. This test consists of winding a coated aluminum wire around a cylinder and then immersing it in a 30% sodium hydroxide (NaOH) solution for 15 minutes. After these 15 minutes, without removing it from the solution, it is observed whether the wire has numerous bubbles on its surface or if it emits gas.

[0187] The silver wire in this example passes this test positively. Example 9: Type II bath on Ni wire

[0188] The process according to the invention is applied to a round nickel wire with a diameter of 0.4 mm.

[0189] Preparation:

[0190] 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.

[0191] Pre-silvering

[0192] Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.

[0193] Silver plating

[0194] A type II silver plating bath is prepared identical to that defined in Example 2.

[0195] Then the silver plating is carried out on the pre-silvered wire under the conditions indicated in Table 20 below:

[0196] [Tables20] DDC (A / dm²) Time t (minute) Temperature T (°C) pH Conductivity mS / cm 1.0 20 55 12.7 71.2

[0197] A silver deposit with an average thickness of 7.29 pm is obtained, corresponding to a cathodic efficiency of 57.07%. The deposit exhibits an appearance, adhesion, and weldability similar to that of Example 1.

Claims

Demands

1. Cyanide-free silvering bath composition comprising: - silver methane sulfonate; - a chelating agent selected from hydantoin, one of its derivatives or a mixture thereof, advantageously selected from hydantoin, 5,5-dimethyl-hydantoin, 1-methyl-hydantoin and mixtures thereof; - a brightener selected from polyethylene-imine and bi-pyridine, advantageously polyethylene-imine; - a pH regulator selected from potassium or sodium hydroxide, advantageously 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 brighteners other than that chosen from polyethylene-imine and bi-pyridine.

2. Composition according to claim 1, characterized in that the 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 comprise other chelators and / or other complexing agents.

4. A method for manufacturing the composition according to any one of claims 1 to 3 comprising the following successive steps: a- adding the pH regulator, advantageously potassium hydroxide, to 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- adding 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- adding 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 polyethylene-imine, 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 metallic substrate.

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

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

8. A process for electrolytic silver deposition on a metallic substrate comprising the following successive steps: A- surface preparation of the metallic substrate; B- pre-silvering of the metallic substrate obtained in step A), advantageously by using a pre-silvering bath whose composition comprises silver methane sulfonate, a chelator selected from hydantoin, one of its derivatives or a mixture thereof, in particular selected from hydantoin, 5,5-dimethyl-hydantoin, 1-methyl-hydantoin and a mixture thereof, a pH regulator selected 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 metallic substrate obtained in step B) by using the composition according to any one of claims 1 to 3; D- recovery of the metallic substrate coated with a layer of silver.

9. A process according to claim 8, characterized in that step A) includes degreasing and / or surface activation and / or pickling and optionally the addition of a metallic undercoat.

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