Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/46—Electroplating: Baths therefor from solutions of silver
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated

Definitions

• 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.
• 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.
• 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.
• 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.
• 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.
• the silver salt providing silver ions in the electroplating process must be easily soluble in water, forming an aqueous electrolytic bath.
• this is silver cyanide AgCN.
• the silver complexing agent 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.
• 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 that fulfills this role, the compound thus complexed being Ag(CN).
• AgCN + KCN Ag(CN) 2 - + K +
• 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.
• 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 .
• the 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 C 5 H 8 N 2 O 2 and potassium nitrate KNO 3 . Without precise information regarding the chemical composition, reading the patent suggests that the source of silver ions would be relative to silver nitrate AgNO 3 . The patent also indicates that the pH of the baths is regulated at 9.5, therefore slightly alkaline.
• 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.
• 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.
• pre-silvering and silvering stages contain cyanide and involve environmental infrastructure constraints as previously indicated.
• the present invention aims to provide cyanide-free pre-silvering and silvering baths having performance and quality comparable to current baths which contain cyanide.
• the use of a brightener also allows the silver deposit to be aesthetically pleasing and have a beautiful appearance.
• the brightener assists the electrolytic process by creating and maintaining a fine crystalline structure of the silver deposit, thus resulting in a visual shine of the deposit.
• 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).
• the 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.
• 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.
• 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.
• composition according to the present invention is therefore simpler than those described in the prior art.
• silver methanesulfonate is the only silver salt in the composition according to the invention.
• it is also the only complexing agent in the composition according to the invention.
• 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.
• 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.
• 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. It may in particular, it may 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.
• the hydantoin derivative is chosen from 5,5-dimethyl-hydantoin, 1-methyl-hydantoin and mixtures thereof.
• 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.
• the term "chelator” means an agent capable of carrying out a physicochemical process known as 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 here 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.
• 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.
• 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.
• 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.
• the 1-methyl-hydantoin content is between 50 and 250 g/l.
• 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.
• composition according to the present invention further comprises a brightener chosen from polyethyleneimine (PEI) and bipyridine (D-PYR).
• the brightener is polyethyleneimine.
• 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.
• 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.
• 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.
• composition according to the present invention further comprises a pH regulator chosen from potassium or sodium hydroxide, advantageously this is potassium hydroxide.
• a pH regulator chosen from potassium or sodium hydroxide, advantageously this is potassium hydroxide.
• the composition according to the present invention does not comprise any pH regulators other than potassium or sodium hydroxide, advantageously potassium hydroxide.
• composition according to the invention is free of cyanide.
• composition according to the present invention may comprise other additives, such as wetting agents or grain refiners, but advantageously it does not.
• composition according to the invention does not comprise other chelators and/or other complexing agents.
• the inventors surprisingly realized 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.
• 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 by using a pH meter.
• the electrolytic conductivity of the composition according to the invention during these steps can be checked and controlled by using a conductivity meter.
• Root temperature means a temperature between 20 and 25°C.
• 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 ⁇ m and 200 ⁇ m by electrolytic means on the metal substrate.
• the metal substrate is a substrate made of copper or a copper alloy such as brass or copper-beryllium, an aluminum alloy or nickel.
• 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.
• 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.
• 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.
• 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,
• 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.
• the process according to the invention may be continuous or discontinuous.
• 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.
• 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).
• HNO 3 nitric acid
• a metallic underlayer such as nickel, zinc or copper (nickel plating, zinc plating and/or copper plating).
• 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).
• a metal underlayer for example zinc: double zinc plating
• 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.
• the silver ion (Ag + ) content 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.
• the chelator of the pre-silvering bath of step B) of the process according to the invention is 5,5-dimethyl-hydantoin (D-HYD).
• 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.
• 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.
• 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 (DC) 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.
• DC current density
• the silver layer obtained in step C) has a thickness of between 0.1 ⁇ m and 200 ⁇ m.
• the method according to the invention is applied to a round copper wire with a diameter of 1.2 mm.
• Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.
• 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
• a silver deposit with an average thickness of 6.5 ⁇ m is obtained, measured with Fischer XDV SDD, which corresponds to a cathodic efficiency of 67.59%.
• 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
• the method according to the invention is applied to a round copper wire with a diameter of 1.2 mm.
• 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 is carried out on the degreased wire under the conditions indicated in Table 1 above.
• 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
• the Ag concentration in g/l is achieved by dissolving MSAg salt with an Ag concentration of 275 g/l.
• a silver deposit with an average thickness of 5.209 ⁇ m is obtained, which corresponds to a cathodic efficiency of 81.55%.
• the deposit has an appearance, adhesion and weldability similar to the case of Example 1.
• Example 3 Type III bath on round Cu wire
• the method according to the invention is applied to a round copper wire with a diameter of 1.2 mm.
• Degreasing is carried out on the wire under the same conditions as in Example 2.
• Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.
• 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
• the Ag concentration in g/l is achieved by dissolving MSAg salt with an Ag concentration of 275 g/l.
• a silver deposit with an average thickness of 10.01 ⁇ m is obtained, which corresponds to a cathodic efficiency of 61.48%.
• the deposit has an appearance, adhesion and weldability similar to the case of Example 1.
• Example 4 Type III bath on round Cu wire
• the method according to the invention is applied to a round copper wire with a diameter of 1.2 mm.
• Degreasing is carried out on the wire under the same conditions as in Example 2.
• Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.
• 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
• the Ag concentration in g/l is achieved by dissolving MSAg salt with an Ag concentration of 275 g/l.
• a silver deposit with an average thickness of 3.821 ⁇ m 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
• the method according to the invention is applied to a round copper wire with a diameter of 1.2 mm.
• Degreasing is carried out on the wire under the same conditions as in Example 2.
• Pre-silvering is carried out on the degreased wire under the conditions indicated in Table 1 above.
• 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
• the Ag concentration in g/l is achieved by dissolving MSAg salt with an Ag concentration of 275 g/l.
• a silver deposit with an average thickness of 2.275 ⁇ m is obtained, which corresponds to a cathodic efficiency of 71.24%.
• the deposit has an appearance, adhesion and weldability similar to the case of Example 1.
• Example 6 Type II bath on brass electrical contact
• the method according to the invention is applied to an electrical contact made of brass with a surface area of 0.17 dm 2 .
• 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 35 7 5
• Pre-silvering is carried out on the degreased contact under the conditions indicated in Table 1 above.
• a type II silvering bath is prepared identical to that defined in Example 2.
• a silver deposit with an average thickness of 8.775 ⁇ m is obtained, which corresponds to a cathodic efficiency of 31.22%.
• the deposit has an appearance, adhesion and weldability similar to the case of Example 1.
• Example 7 Type II bath on connector housing in AI 6061
• 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 compared to copper to cover with a metallic coating by electrolytic means.
• a surface preparation of the aluminum therefore proves necessary in the application of the method according to the invention, comprising in order a chemical degreasing of the Oxidite C5 type, an Alumon AC70 pickling, and a combination of a nitric acid activation HNO 3 at 53% and 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.
• 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 is carried out on the degreased case under the conditions indicated in Table 1 above.
• a type II silvering bath is prepared identical to that defined in Example 2.
• a silver deposit with an average thickness of 9.652 ⁇ m 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.
• 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
• the method according to the invention is applied to a wire with a diameter of 0.1 mm in 1050 aluminum.
• Example 18 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 is carried out on the degreased wire under the conditions indicated in Table 1 above.
• a type II silvering bath is prepared identical to that defined in Example 2.
• a silver deposit with an average thickness of 5.25 ⁇ m 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.
• 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. After 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
• the method according to the invention is applied to a round nickel wire with a diameter of 0.4 mm.
• 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 is carried out on the degreased wire under the conditions indicated in Table 1 above.
• a type II silvering bath is prepared identical to that defined in Example 2.

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Citations (9)

• 2023
  • 2023-11-06 FR FR2312011A patent/FR3155008B1/fr active Active
• 2024
  • 2024-11-04 DE DE24210591.4T patent/DE24210591T1/de active Pending
  • 2024-11-04 EP EP24210591.4A patent/EP4549633A1/de active Pending

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