Process for electroplating matte silver on surface of red copper

Abstract

The invention provides a process for electroplating matte silver on the surface of red copper, which comprises the following steps: deoiling, pickling and activating a red copper workpiece in sequence, then pre-plating a layer of nickel substrate or zinc substrate, and pickling solution comprising sulfuric acid and glyoxylic acid; silver methanesulfonate, allylthiourea, polyethylene glycol, ethylenediaminetetraacetic acid and auxiliaries are added into a plating tank, and a plating solution is obtained; and the current density, the plating solution temperature, the pH value and the electroplating time are controlled. During pickling, glyoxylic acid forms a non-uniform organic adsorption film on the surface of red copper to realize selective corrosion inhibition, sulfuric acid is slightly dissolved in gaps of a film layer, and the contact area is increased to strengthen coating combination; the lattice constant of the pre-plated nickel / zinc substrate is between that of silver and that of copper, so that internal stress generated by mismatching of lattices of the silver and the copper can be buffered. In the plating solution, potassium iodide and Ag < + > form a stable complex, the crystallization speed is slowed down, plating crystal grains are refined, interface stripping caused by stress concentration is reduced, and finally potassium iodide cooperates with allylthiourea and polyethylene glycol to form a uniform matt silver plating layer with high adhesive force.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of surface treatment, more particularly, relates to a process for electroplating matte silver on the surface of red copper. BACKGROUND

[0002] In the field of electronics, decoration and precision instruments, red copper is widely used due to its excellent electrical conductivity and thermal conductivity. However, the surface of red copper is prone to oxidation and blackening, which not only affects the appearance quality, but also reduces its electrical conductivity and corrosion resistance, limiting its application in high-end scenarios. To solve this problem, the industry often uses electroplating process to treat the surface of red copper, among which electroplated silver is widely used due to its good electrical conductivity, corrosion resistance and decorative properties. Although traditional bright silver plating has high gloss, in some specific application scenarios, such as light shielding parts of precision instruments and matte texture demand in the decoration field, the high light reflection characteristics of bright silver plating become a disadvantage, so matte silver electroplating process has gradually attracted attention.

[0003] However, when directly electroplating matte silver on the surface of red copper, there are technical defects that make it difficult to meet high quality requirements. Although red copper (lattice constant 0.361 nm) and silver (lattice constant 0.408 nm) belong to the same face-centered cubic structure, the difference in lattice constant is large, and direct electroplating is prone to internal stress due to lattice mismatch, which makes the adhesion of matte silver plating layer insufficient and easy to peel off; and the matte silver plating layer is a cathodic plating layer, which further weakens the interfacial bonding force due to the difference in electrochemical activity between the red copper workpiece. For example, if the electroplating solution is contaminated by metal impurities or organic matter, if the oil and oxide layer on the substrate are not completely removed during pretreatment, or if the current density and temperature control are not proper, all of these will exacerbate the peeling of the plating layer. In addition, matte silver plating layer is prone to generate silver sulfide and silver chloride in sulfur / chlorine-containing environment, which leads to discoloration of the matte silver plating layer; if the oxide layer on the surface of red copper is not completely removed during pretreatment of the plating layer, the residual oxide layer will cause corrosion risk at the interface between the plating layer and the red copper. SUMMARY

[0004] The purpose of the embodiments of the present application is to provide a process for electroplating matte silver on the surface of red copper, in order to solve the technical problems of insufficient adhesion and poor corrosion resistance of matte silver plating layer on the surface of red copper in the prior art.

[0005] To achieve the above-mentioned purpose, the technical solution adopted by the present application is to provide a process for electroplating matte silver on the surface of red copper, comprising the following steps: pretreatment: sequentially performing oil removal treatment, pickling treatment and activation treatment on the red copper workpiece, and then pre-plating a layer of nickel or zinc; plating solution preparation: adding silver methanesulfonate, allyl thiourea, polyethylene glycol, potassium iodide and additives into the plating bath to prepare the plating solution; Electroplating: the red copper workpiece is placed in the plating tank for electroplating, and the current density, plating solution temperature, plating solution pH value and electroplating time are controlled within a preset range.

[0006] Optionally, the pickling treatment specifically comprises placing the red copper workpiece in the pickling solution and soaking at 25-30°C for 3-5 minutes. The pickling solution comprises 0.5-1 mol / L sulfuric acid and 0.01-0.05 mol / L glyoxalic acid.

[0007] Optionally, the activation treatment specifically comprises activating the surface of the red copper workpiece with 0.1 mol / L dilute hydrochloric acid for 10-20 seconds and adding 0.001 mol / L citric acid to adjust the zeta potential of the surface to -16 to -20 mV.

[0008] Optionally, the specific steps of pre-plating a layer of nickel bottom are as follows: Use nickel sulfamate or watt nickel electroplating solution, control current density 1-5 A / dm 2 , plating solution temperature 40-60°C, electroplating time 5-10 minutes, nickel bottom thickness 3-5 µm.

[0009] Optionally, the specific steps of pre-plating a layer of zinc bottom are as follows: Use alkaline zincate or acidic zinc chloride electroplating solution, control current density 1-3 A / dm 2 , plating solution temperature 20-40°C, electroplating time 3-5 minutes, zinc bottom thickness 2-4 µm.

[0010] Optionally, the auxiliary agent comprises ethylenediaminetetraacetic acid, urea, boric acid.

[0011] Optionally, the plating solution comprises 45-55 g / L silver methanesulfonate, 0.05-0.2 g / L allyl thiourea, 0.2-0.5 g / L polyethylene glycol, 3.5-8.0 g / L ethylenediaminetetraacetic acid, 105-125 g / L potassium iodide, 12-18 g / L urea and 30-40 g / L boric acid.

[0012] Optionally, in the electroplating step, the current density is controlled to be 0.5-2 A / dm 2 , the plating solution temperature is controlled to be 20-30°C, the plating solution pH value is controlled to be 4.0-4.5, the silver layer thickness is controlled to be 5-10 µm, and the electroplating time is controlled to be 5-15 minutes.

[0013] Optionally, current-temperature linkage control is adopted: During the 0-10% electroplating period, the current density is 0.5-0.8 A / dm 2 , and the plating solution temperature is 22-24°C. During the period of 10%-80% of electroplating, the current density is 1.5-2.0 A / dm 2 , and the plating solution temperature is 28-30℃. During the period of 80%-100% of electroplating, the current density is 1.0-1.3 A / dm 2 , and the plating solution temperature is 25-26℃.

[0014] Optionally, the method further comprises the following steps: The finished red copper workpiece is cleaned with water to remove the residual plating solution on the surface, then immersed in a chromate solution or an organic passivation agent for passivation, and then dried.

[0015] The red copper surface electroplating matte silver process provided by the application has the beneficial effects that, compared with the prior art, the application uses a pickling solution prepared from sulfuric acid and glyoxylic acid in pickling treatment. Sulfuric acid has strong acidity and can quickly dissolve the oxide layer on the surface of red copper, but it is easy to cause excessive corrosion of the workpiece when used alone, resulting in a rough surface. Glyoxylic acid has weak reducing property and corrosion inhibition property, which can assist in dissolving stubborn oxide layers on one hand, and can form a thin and uneven organic adsorption film on the surface of red copper to achieve selective corrosion inhibition on the other hand. This adsorption film is not completely uniform and will leave small "active points" in areas with weak adsorption such as crystal defects on the surface of red copper, so that sulfuric acid can slightly and locally dissolve at these sites to form nanoscale small pits and protrusions. This can not only avoid the macroscopic roughness caused by overall excessive corrosion, but also increase the surface contact area through this microscopic concave-convex structure, so that the plating layer is combined more tightly. The lattice constant of nickel or zinc is between that of silver and copper, which can be used as a transition layer to buffer the internal stress caused by the mismatch of silver and copper lattices, thereby fundamentally solving the problem of poor adhesion of matte silver plating layer and easy peeling. + The lattice constant of nickel or zinc is between that of silver and copper, which can be used as a transition layer to buffer the internal stress caused by the mismatch of silver and copper lattices, thereby fundamentally solving the problem of poor adhesion of matte silver plating layer and easy peeling. + The lattice constant of nickel or zinc is between that of silver and copper, which can be used as a transition layer to buffer the internal stress caused by the mismatch of silver and copper lattices, thereby fundamentally solving the problem of poor adhesion of matte silver plating layer and easy peeling. + The lattice constant of nickel or zinc is between that of silver and copper, which can be used as a transition layer to buffer the internal stress caused by the mismatch of silver and copper lattices, thereby fundamentally solving the problem of poor adhesion of matte silver plating layer and easy peeling. + The lattice constant of nickel or zinc is between that of silver and copper, which can be used as a transition layer to buffer the internal stress caused by the mismatch of silver and copper lattices, thereby fundamentally solving the problem of poor adhesion of matte silver plating layer and easy peeling. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the accompanying drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the accompanying drawings in the following description only represent some embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative labor.

[0017] Figure 1 A process flow chart of the red copper surface electroplating matte silver provided by the embodiment of the present application. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the embodiments of the present application. Obviously, the described embodiments only represent some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0019] Please refer to Figure 1 , a red copper surface electroplating matte silver process provided by the embodiment of the present application will be described. The red copper surface electroplating matte silver process comprises the following steps: pretreatment: the red copper workpiece is sequentially subjected to oil removal treatment, acid pickling treatment and activation treatment, and then a layer of nickel or zinc is pre-plated, the acid pickling solution used in the acid pickling treatment is a mixed solution prepared by sulfuric acid and glyoxylic acid; plating solution preparation: silver methanesulfonate, allyl thiourea, polyethylene glycol, potassium iodide and additives are added to the plating bath to prepare a plating solution; electroplating: the red copper workpiece is placed in the plating bath for electroplating, and the current density, plating solution temperature, plating solution pH value and electroplating time are controlled within a preset range.

[0020] It should be noted that in the pre-treatment step, the oil is usually removed by using alkaline degreasing agent (such as 2%-3% sodium hydroxide, 1%-2% fatty alcohol polyoxyethylene ether, 0.2%-0.5% benzotriazole, and the rest is deionized water) or organic solvent degreasing agent (such as 90%-95% isopropyl alcohol, 1%-2% fatty alcohol polyoxyethylene ether-5, and the rest is ethanol) to remove the rolling oil, fingerprints, dust and other oil stains on the surface of red copper. If the oil stains remain, it will isolate the silver ion from contacting the workpiece, directly leading to the defects of peeling and peeling of the subsequent plating layer. The pickling treatment in the pre-treatment step uses a pickling solution prepared by sulfuric acid and glyoxylic acid. Sulfuric acid has strong acidity and can quickly dissolve the oxide layer (CuO, Cu2O) on the surface of red copper, but it is easy to over-corrode the workpiece when used alone, resulting in a rough surface. Glyoxylic acid has corrosion resistance, and by forming a thin and uneven organic adsorption film (carboxyl groups complex with copper ions on the surface of red copper, and aldehyde groups are physically adsorbed by van der Waals force) on the surface of red copper, selective corrosion is achieved. This adsorption film is not completely uniform and will leave small "active points" in the areas where the adsorption is weak on the crystal defects of the red copper surface, so that the sulfuric acid can slightly and locally dissolve at these sites to form nanoscale micro-pits and protrusions (i.e. nanoscale rough structure), which not only avoids the macroscopic roughness caused by overall over-corrosion, but also increases the surface contact area through this microscopic concave-convex, making the combination more closely. At the same time, the aldehyde group of glyoxylic acid has weak reducing property and can be oxidized to carboxyl group, so as to remove the oxidizing agent (such as dissolved oxygen, copper ions generated by the dissolution of the oxide layer) in the system, avoid the formation of new oxide layer, and ensure the activity of the surface after pickling.

[0021] The lattice constant of nickel (lattice constant 0.352 nm) or zinc (lattice constant 0.266 nm) is between that of silver and copper, which can be used as a transition layer to buffer the internal stress generated by the mismatch of silver and copper lattices, and fundamentally solve the problem of poor adhesion of matte silver plating layer and easy peeling; the nickel / zinc bottom is a dense metal layer, which can isolate the direct contact between silver ions and red copper in the subsequent silver plating process, and avoid the interface corrosion caused by the difference in electrochemical activity between copper and silver. Industrial silver plating layer cannot be completely dense, which is easy to make the corrosion medium penetrate to the surface of red copper workpiece, and pre-plating a layer of nickel / zinc bottom reduces the risk of corrosion medium penetration caused by the porosity of the subsequent silver plating layer.

[0022] Among them, silver methanesulfonate provides Ag + ions required for electroplating, which is the metal source of silver plating layer; compared with traditional silver nitrate, the release of silver ions of silver methanesulfonate is more stable, and the methanesulfonate has no oxidizing property, which will not have side reactions with red copper / other components, avoid turbidity of the plating solution or produce impurities, and ensure the purity of the plating layer.

[0023] Among them, allyl thiourea and polyethylene glycol are used as matte agents in the plating solution. The sulfur atom in the thioureido group of allyl thiourea has a lone pair of electrons, which can form a coordinate bond with Ag +or Ag on the surface of silver crystal + Forming coordination bond, adsorbed on the high activity growth sites of silver crystal (such as {111} crystal surface) through coordination bond, the adsorption film formed will reduce the binding capacity of these sites to silver ions, Ag + The deposition rate at these sites is significantly slowed down. Ag + Naturally gathered to the low activity sites which are not strongly adsorbed, and dispersed growth to more low activity sites. Ag + Deposition is more dispersed, only a large number of small, disordered microcrystalline can be formed. The size of these microcrystalline is usually in nanometer to micrometer level, and the arrangement is disorderly, when light irradiates to the surface of the coating, diffuse reflection will occur, thus reducing the gloss, showing the matt effect. Polyethylene glycol 400 (PEG-400) is a non-ionic surfactant, used to adjust the interfacial tension between the plating solution and the red copper workpiece and cooperate with allyl thiourea to refine the crystal. When the interfacial tension between the plating solution and the surface of the red copper workpiece is too high, Ag + Easy to gather in local area (such as micro convex), leading to local grain coarsening, Ag + Deposition is uneven. The hydrophilic group (ether bond, -O-) of PEG-400 can form hydrogen bond with water molecules, and the hydrophobic chain (methylene-CH2-) is directed to the surface of the red copper workpiece, effectively reducing the interfacial tension between the plating solution and the red copper workpiece, reducing Ag + Local gathering, laying the foundation for uniform crystallization. At the same time, PEG-400 can be weakly adsorbed on the surface of silver coating through van der Waals force, forming steric hindrance, hindering the migration and merging of newly deposited silver atoms to the existing grains.

[0024] Among them, potassium iodide as the main complexing agent, can form stable [AgI2] + - Complex ions in the plating solution, greatly reduce the free Ag + Concentration, effectively slow down the reduction deposition speed of silver ions, promote the coating crystalline particles more refined, close arrangement and uniform distribution, improve the compactness and adhesion of the coating. Potassium iodide inhibits Ag + Hydrolysis reaction in the plating solution, avoid the formation of insoluble impurities such as silver hydroxide precipitation, so as to maintain the stability of the plating solution composition. At the same time, if there is excess iodine ion in the plating solution, a uniform physical adsorption layer will be spontaneously formed on the surface of the matte silver coating, which can effectively hinder the preferred orientation growth of silver grains, avoid the formation of continuous smooth mirror reflection structure of the coating, finally strengthen the matt feeling of the coating, make the coating present a uniform and delicate matte appearance. Potassium iodide also has a significant effect of enhancing conductivity in the plating solution, as a strong electrolyte, can completely ionize K + and I - ​The two ions can effectively increase the carrier concentration in the plating solution, reduce the ohmic resistance of the plating solution, and thus improve the overall conductivity of the plating solution.

[0025] Meanwhile, some additives, such as buffering agents and complexing agents, need to be added to the plating solution to ensure the quality and performance of the matte silver coating and make the electroplating process more stable, so as to meet the consistency of the batch quality of the matte silver coating.

[0026] In addition, the current density, plating solution temperature, plating solution pH value, and electroplating time are controlled to ensure the stable quality of the matte silver coating: the current density is controlled to prevent the coating from being rough and loose; the plating solution temperature is controlled to help maintain the activity of the plating solution and prevent the components from deteriorating; the plating solution pH value is controlled to ensure the stable complexation of silver ions and avoid abnormal deposition; and the electroplating time is controlled to accurately match the target silver layer thickness, so as to ultimately achieve the effect of a uniform, dense, and performance-standardized coating.

[0027] In another embodiment of the present application, the pickling treatment specifically involves placing the red copper workpiece in the pickling solution and soaking it at 25-30°C for 3-5 minutes. The pickling solution contains 0.5-1 mol / L sulfuric acid and 0.01-0.05 mol / L glyoxalic acid.

[0028] It should be noted that the 0.5-1 mol / L sulfuric acid concentration in the pickling solution can quickly react with CuO and Cu2O on the surface of the red copper (to generate soluble copper sulfate), ensuring that the oxide layer is fully stripped in a short time, while avoiding excessive corrosion of the red copper workpiece due to a too high concentration of sulfuric acid, which would cause the macroscopic surface to be rough. The 0.01-0.05 mol / L glyoxalic acid can be oxidized to a carboxyl group through the weakly reducing aldehyde group, removing oxidizing agents in the system, and can form a thin and uneven organic film on the surface of the red copper through the complexation of the carboxyl group with copper ions on the surface of the red copper and the physical adsorption of the aldehyde group on the surface of the red copper, which can induce the formation of a nano-scale rough structure while inhibiting excessive corrosion, and will not cause the adsorption film to be too thick due to a too high concentration, hindering the dissolution of the oxide layer by sulfuric acid.

[0029] Operationally, the normal temperature condition of 25-30°C does not require additional heating, which reduces energy consumption and avoids higher temperatures (such as more than 30°C) that can accelerate the decomposition of glyoxalic acid, destroy its corrosion inhibition and nano-scale rough structure inducing effect, and ensure that the sulfuric acid dissolution reaction proceeds smoothly; the soaking time of 3-5 minutes forms a synergy with the concentration and temperature, which ensures that the oxide layer is completely stripped and the nano-scale rough structure is stably formed, laying a good foundation for the subsequent silver plating process, and avoiding incomplete removal of the oxide layer or damage to the red copper workpiece caused by the accumulation of local slight dissolution.

[0030] In another embodiment of the present application, the activation treatment is specifically using 0.1 mol / L dilute hydrochloric acid to activate the surface of red copper workpiece for 10-20 seconds, and adding 0.001 mol / L citric acid to adjust the surface dynamic potential to -16 to -20 mV.

[0031] It should be noted that the core role of 0.1 mol / L dilute hydrochloric acid is to deeply clean the surface and eliminate pickling residues: after pickling, the surface of red copper may be attached with a small amount of incomplete rinsing sulfate residues or a very thin newly formed oxide film (formed by air contact), and the low-concentration hydrochloric acid of 0.1 mol / L can quickly dissolve these impurities within 10-20 seconds, avoiding excessive corrosion of the red copper workpiece due to too high concentration of hydrochloric acid, and preventing macroscopic roughness on the surface through short-time soaking, thereby ensuring clean and smooth substrate.

[0032] The dynamic potential reflects the surface charge state, which can be measured by electrophoretic light scattering method. The surface of red copper after pickling and dilute hydrochloric acid activation is usually weakly positive or low negative, which is not conducive to electrostatic adsorption with the positively charged silver ions (or silver-complexes) in the plating solution. Citric acid can form negatively charged citrate ions by complexing with copper surface ions through carboxyl groups in aqueous solution, and the surface charge can be adjusted to a specific range of negative potential by controlling the addition amount of citric acid. The electrostatic attraction provided by the potential range of -16 to -20 mV can not only enhance the attraction between the surface of red copper and Ag + + (or silver-complexes) in the plating solution to improve the subsequent coating adhesion, but also avoid too high negative potential leading to excessive adsorption of impurity ions on the surface, while stabilizing the surface colloidal state to prevent the agglomeration of small particles, thereby further ensuring the uniformity of the deposited coating.

[0033] In another embodiment of the present application, the specific steps of pre-plating a layer of nickel bottom are as follows: Use nickel sulfamate or watt nickel plating solution, control the current density 1-5 A / dm 2 , the plating solution temperature is 40-60℃, the electroplating time is 5-10 minutes, and the thickness of the nickel bottom is 3-5 µm.

[0034] ​It should be noted that when pre-plating a nickel bottom on the surface of red copper, the nickel sulfamate or watt nickel electroplating solution is selected, relying on the excellent stability and deposition characteristics of the two plating solutions to form a dense and uniform nickel transition layer. Among them, the nickel sulfamate plating solution is a plating solution composed of nickel sulfamate as the main salt, boric acid as the buffer, and additives such as leveling agent, brightener, etc. And the watt nickel plating solution is a plating solution composed of nickel sulfate as the main salt, nickel chloride as the conductive salt, boric acid as the buffer, and additives such as wetting agent, stress reliever, etc. The current density is controlled at 1-5 A / dm when plating nickel bottom in order to prevent the nickel layer from being loose and having high porosity due to too low current, or the nickel layer from being rough and having decreased adhesion due to too high current, ensuring the density of the nickel layer; the plating solution temperature is 40-60°C, which is the best reaction activity interval of the nickel sulfamate and watt nickel plating solution, which can maintain the stability of the plating solution composition, ensure uniform deposition of nickel ions, and avoid temperature deviation leading to plating solution failure or abnormal deposition rate; the electroplating time is 5-10 minutes in order to precisely control the thickness of the nickel bottom to 3-5 µm, and thickness that is too thin will not effectively play the role of transition and isolation, and thickness that is too thick is easy to accumulate internal stress leading to nickel layer cracking, finally through the cooperation of the above operations and parameters, a flat and stable substrate is provided for the subsequent matte silver plating layer, ensuring the adhesion and corrosion resistance of the silver plating layer.

[0035] In another embodiment of the present application, the specific steps of pre-plating a zinc bottom are as follows: Using alkaline zincate or acidic zinc chloride electroplating solution, controlling the current density at 1-3 A / dm 2 , the plating solution temperature is 20-40°C, the electroplating time is 3-5 minutes, and the zinc bottom thickness is 2-4 µm.

[0036] It should be noted that by selecting alkaline zincate or acidic zinc chloride plating solution to pre-plate zinc base on the surface of red copper, a uniform and dense zinc transition layer can be formed by virtue of the good covering capacity and adaptability of the two plating solutions to the surface of red copper. Among them, the alkaline zincate plating solution is a plating solution composed of zinc oxide as the main salt, sodium hydroxide as the complexing agent (also as the conductive salt), and additives such as brightener, leveling agent, etc.; the acidic zinc chloride plating solution is a plating solution composed of zinc chloride as the main salt (also as the conductive salt), and additives such as buffer, stabilizer, etc. The current density of 1-3 A / dm2 is controlled during zinc plating to prevent the zinc layer from depositing slowly, having loose structure, high porosity due to too low current, or the zinc layer surface from being scorched, rough, and even dendrite growth due to too high current, so as to ensure that the zinc layer is dense and firmly combined with the workpiece. The plating solution temperature of 20-40°C is the best reaction interval for the two zinc plating solutions. In the alkaline system, zinc ions need to form stable complexes with complexing agents to deposit uniformly. When the temperature is too low, the complexation reaction equilibrium moves to the dissociation direction of the complex, the complex stability constant decreases, and the concentration of free zinc ions increases, resulting in too fast local deposition rate and rough plating layer. When the temperature is too high, additives are prone to decompose. In the acidic system, zinc ions are mainly in the free state, and this temperature range can maintain the stability of the plating solution composition and ensure uniform deposition of zinc ions. The plating time of 3-5 minutes is to accurately control the thickness of the zinc base to 2-4µm. Thickness that is too thin cannot effectively play a transition and isolation role, and thickness that is too thick is prone to cracking or falling off due to stress accumulation in the zinc layer. Finally, through the cooperation of the above operations and parameters, a smooth and stable substrate is provided for the subsequent matte silver plating layer, and the adhesion and corrosion resistance of the silver plating layer are ensured.

[0037] In another embodiment of the present application, the additives include ethylenediaminetetraacetic acid, urea, and boric acid.

[0038] It should be noted that ethylenediaminetetraacetic acid (grain refiner) can form stable complexes with Ag + ions in the plating solution through the coordination of nitrogen atoms and oxygen atoms, and control the release rate of Ag + . When Ag + is released at a stable rate and participates in electrodeposition, the reduction process of Ag + on the cathode surface is more orderly, and the competition relationship between crystal nucleus formation and growth is balanced, avoiding the deposition of Ag + too fast leading to coarse crystallization and loose plating layer. At the same time, ethylenediaminetetraacetic acid and Ag +The formed complex also has selective adsorption properties, which can weakly adsorb on the high-activity crystal surface of silver crystal nucleus, inhibit the rapid growth of the crystal in this direction through steric hindrance effect, promote the uniform development of the crystal grain to other low-activity crystal surface, further reduce the size difference of the crystal grain, improve the uniformity and fineness of the plated layer crystallization, and form a fine-grained structure with more grain boundaries, which can greatly reduce the overall internal stress of the plated layer by dispersing stress through grain boundaries, reduce the interface peeling caused by stress concentration, and enhance the adhesion of the plated layer. Urea as an adsorption additive has good water solubility and adsorption. The amino and carbonyl groups in the molecule can be adsorbed on the cathode surface through polar action to form a steric hindrance layer, inhibit the rapid growth of silver crystal nucleus, and promote the formation of small equiaxed crystals. At the same time, the adsorption of urea can reduce the local pH value fluctuation caused by hydrogen evolution near the cathode, and assist in maintaining the stability of the deposition environment. In addition, urea can also adjust the buffer capacity of the plating solution, and reduce the influence of local pH value fluctuation on the quality of the plated layer. Due to the hydrogen evolution reaction near the cathode during electroplating, the local pH value will increase. As a weak electrolyte, boric acid can effectively buffer the local pH value fluctuation of the plating solution through ionization equilibrium - H3BO3+ H2O H + +[B(OH4)] - , stabilize the pH value of the plating solution, prevent the generation of silver hydroxide precipitate due to too high pH value, or cause the plated layer to be rough due to too low pH value. At the same time, the presence of boric acid can stabilize the pH value of the plating solution, maintain the stability of the ion form in the plating solution, and indirectly improve the stability of the electroplating process, so as to ensure that the silver plated layer is tightly combined with the substrate from the initial stage of deposition, avoid the formation of intermediate defect layer, and thus enhance the adhesion between the silver layer and the substrate.

[0039] In another embodiment of the present application, the plating solution comprises 45-55 g / L silver methanesulfonate, 0.05-0.2 g / L allyl thiourea, 0.2-0.5 g / L polyethylene glycol, 3.5-8.0 g / L ethylenediaminetetraacetic acid, 105-125 g / L potassium iodide, 12-18 g / L urea, and 30-40 g / L boric acid.

[0040] It should be noted that 45-55 g / L silver methanesulfonate as a source of silver ions can provide sufficient Ag + ensure the deposition rate and target thickness of the plated layer, and avoid too high concentration of Ag +Too fast reduction triggers rough crystallization; 0.05-0.2 g / L allyl thiourea and 0.2-0.5 g / L polyethylene glycol synergistically as matte control agents, the concentration range is matched and optimized to control the matte effect and the compactness of the coating: the sulfur atom in the allyl thiourea molecule can be adsorbed on the high-energy crystal face of the silver grain by forming a coordination bond, if the concentration is lower than 0.05 g / L, the adsorption is insufficient, it is difficult to effectively inhibit the preferred orientation of the grain, resulting in weakening of the matte effect; if the concentration is higher than 0.2 g / L, it may be excessive adsorption, hindering the normal reduction of silver ions, causing the coating to be gray or the adhesion to decrease. As a high molecular additive, polyethylene glycol forms an adsorption layer on the cathode surface to produce a steric hindrance effect, when the concentration is lower than 0.2 g / L, the steric hindrance is insufficient, it cannot effectively limit the grain growth, which is easy to lead to coarse crystallization; higher than 0.5 g / L will increase the viscosity of the plating solution, hinder the migration of silver ions, and may cause the porosity of the coating to rise. The synergistic effect of the two in the above range can significantly enhance the matte feel of the coating, while ensuring the uniformity and compactness of the coating; 3.5-8.0 g / L ethylenediaminetetraacetic acid is the complexing agent amount precisely matched with the concentration of silver methanesulfonate, which can just form a stable complex with Ag + to assist in controlling the release rate of Ag + , avoiding side reactions; the high concentration design of 105-125 g / L potassium iodide can fully combine with Ag + to form [AgI2] - , slow down the deposition and refine the grains, and also form an adsorption layer on the surface of the coating through excessive I - , synergize with allyl thiourea and polyethylene glycol to enhance the matte effect, while avoiding the problems of insufficient complexation due to low concentration or iodide precipitation due to high concentration; the concentration of 12-18 g / L urea can ensure the formation of small equiaxed crystals through weak complexation and steric hindrance effect, improve the uniformity of the coating, and also will not increase the viscosity of the plating solution or introduce organic impurities due to high concentration; 30-40 g / L boric acid can stably maintain the pH of the plating solution, while improving the stability of the plating solution, avoiding pH fluctuations that damage the effects of other components or cause coating defects.

[0041] In another embodiment of the present application, in the electroplating step, the current density is controlled to be 0.5-2 A / dm 2 , the plating solution temperature is 20-30°C, the plating solution pH value is 4.0-4.5, the silver layer thickness is 5-10 µm, and the electroplating time is 5-30 minutes.

[0042] It should be noted that the control of various parameters in the electroplating step is the key to ensuring that the performance of the matte silver plating layer meets the standards. Controlling the current density to be 0.5-2 A / dm2 can avoid slow deposition of silver ions due to low current, loose and porous plating layer due to high current, local overheating, and rough crystallization of the plating layer, and balance the deposition efficiency and crystallization quality; the silver ion diffusion rate is moderate at a plating solution temperature of 20-30°C, which can ensure uniform deposition, while avoiding decomposition of the auxiliary agent due to high temperature, and affecting the stability of the complex due to low temperature; the pH value of the plating solution is 4.0-4.5, which matches the buffer range of the buffer agent suitable for the silver methanesulfonate system, and can prevent Ag + hydrolysis to form silver hydroxide precipitate, and can maintain the complex balance of ethylenediaminetetraacetic acid and silver ions, ensuring stable release of silver ions; the thickness of the silver layer is 5-10 µm, which is a relatively optimal range considering corrosion resistance and adhesion, and too thin will result in high porosity of the plating layer, and corrosion medium will easily penetrate into the workpiece, and too thick will result in stress accumulation in the plating layer, which is easy to peel off from the pre-plated nickel / zinc bottom; under the condition that the cathode current efficiency is 90%, the electroplating time of 5-30 minutes is accurately set according to the current density and target thickness, to ensure that the thickness of the silver layer meets the standards and is uniform, and to avoid insufficient thickness due to insufficient time or excessive thickness and performance imbalance due to excessive time.

[0043] In another embodiment of the present application, current-temperature linkage control is adopted: During the 0-10% electroplating period, the current density is 0.5-0.8 A / dm 2 , and the plating solution temperature is 22-24°C; During the 10%-80% electroplating period, the current density is 1.5-2.0 A / dm 2 , and the plating solution temperature is 28-30°C; During the 80%-100% electroplating period, the current density is 1.0-1.3 A / dm 2 , and the plating solution temperature is 25-26°C.

[0044] It should be noted that the current-temperature linkage control is adopted to match the requirements of different growth stages of the plating layer, and ultimately improve the comprehensive performance of the matte silver plating layer. Among them, the 0-10% electroplating period is the 0-10% stage of the total electroplating time, the 10%-80% electroplating period is the 10%-80% stage of the total electroplating time, and the 80%-100% electroplating period is the 80%-100% stage of the total electroplating time. The specific effects of each period are as follows: The initial stage of 0-10% of the electroplating is the initial stage of the nucleation of the matte silver plating layer. Low current density (0.5-0.8 A / dm2) and low temperature (22-24 ℃) are adopted to control the slow and orderly reduction of silver ions on the surface of the pre-plated nickel / zinc base, because low current can avoid uneven distribution of crystal nuclei caused by too fast initial deposition (prevent local crystallization from being too thick), and low temperature can slow down the diffusion rate of silver ions, so that the crystal nuclei are uniformly attached to the surface of the base, laying a foundation for the close combination of the subsequent plating layer and the base and reducing the interface gap.

[0045] The main deposition stage of 10%-80% of the electroplating is to improve the deposition efficiency under the premise of ensuring the quality of crystallization by increasing the current density (1.5-2.0 A / dm2) and the temperature (28-30 ℃): higher current accelerates the reduction of silver ions, and higher temperature promotes the diffusion rate of silver ions and silver-complexes in the plating solution, so as to ensure that the plating layer thickens quickly and uniformly, and avoid thickness deviation caused by insufficient deposition rate. The temperature of 28-30 ℃ can enhance the activity of allyl thiourea, polyethylene glycol, urea and other additives, and ensure that the matte effect is consistent during the thickening process of the plating layer.

[0046] The finishing stage of 80%-100% of the electroplating is to reduce the surface stress of the plating layer by adjusting the current density (1.0-1.3 A / dm2) and the temperature (25-26 ℃): low current avoids the surface from being excessively deposited and blackened or appearing “dendritic growth” caused by the continuous action of high current, and moderate temperature reduces the thermal stress caused by sudden temperature change, while the remaining silver ions on the surface of the plating layer slowly deposit to fill the small pores, and the surface flatness and compactness are optimized.

[0047] The three stages of matte silver electroplating not only consider the deposition efficiency and the thickness of the plating layer, but also avoid the problems of initial poor adhesion, uneven thickness in the middle stage and surface defects in the later stage by stage-by-stage regulation, so as to finally realize the effect of strong adhesion, uniform matte and dense structure of the plating layer.

[0048] In another embodiment of the present application, the following steps are further included: The finished red copper workpiece is cleaned with water to remove the residual plating solution on the surface, immersed in a chromate solution or an organic passivation agent for passivation, and then dried.

[0049] It should be noted that the acidic components (such as methanesulfonate) in the residual plating solution on the surface of the red copper workpiece can slowly corrode the matte silver plating layer or the pre-plated nickel / zinc base, causing local discoloration and peeling of the plating layer; the residual allyl thiourea and polyethylene glycol may react with oxygen and moisture in the air to generate more sulfides, which can accelerate the discoloration of the matte silver plating layer due to sulfidation, and cleaning the red copper workpiece with water can significantly reduce the probability of subsequent problems.

[0050] Passivation is the core of post-treatment. A dense passivation film is formed on the surface of matte silver plating layer by immersing in chromate solution or organic passivation agent, solving the defects of easy oxidation and easy sulfuration of silver plating layer. The chromate solution can form a chromium-containing oxidation composite film (mainly composed of Cr2O3 and Ag2O) on the surface of matte silver plating layer. The film layer is tightly attached to the surface of the plating layer, can isolate air, moisture, sulfur ions and other corrosive media from contacting silver, greatly reduces the oxidation rate and sulfuration rate of silver, and at the same time, the film layer itself has certain acid resistance and can resist slight environmental corrosion. The organic passivation agent (such as methyl benzotriazole and benzotriazole reagent) forms a uniform organic adsorption film on the surface of the plating layer by forming a coordination bond between the active groups (such as amino and mercapto) in the organic molecules and silver ions. The film layer not only can block the penetration of corrosive media, but also can maintain the original appearance of matte silver plating layer, which is suitable for scenes with higher appearance requirements.

[0051] Drying is to remove the water on the surface of the workpiece. The drying temperature is usually controlled at 60-80℃, and the drying time is 10-20 minutes. The residual water can be evaporated by moderate heat to prevent water stains and rust from occurring in the early stage of storage or use of the plating layer, and to ensure that the chemical components in the passivation film remain stable. If the temperature is too high or the time is too long, the chromate passivation film may become more brittle and have lower adhesion due to excessive dehydration, and the organic passivation film may be broken or decomposed, losing its protective effect. If the temperature is too low or the time is too short, the water on the surface of the red copper workpiece is not completely removed.

[0052] The present application is a kind of red copper surface electroplating matte silver process, which uses non-toxic and harmless cyanide-free electroplating solution to reduce the safety risk and environmental protection treatment cost in the production process. At the same time, by adjusting the concentration of matte agent and the electroplating time, the gloss of matte silver plating layer surface can be effectively controlled. In the selection of pre-plating bottom, if nickel bottom is used to plate matte silver, the adhesion of the plating layer is good and the corrosion resistance is excellent, which is especially suitable for high requirement environment; if zinc bottom is used to plate matte silver, the adhesion of the plating layer is better and the corrosion resistance is good, which is suitable for indoor environment. The red copper surface electroplating matte silver process can be applied to electronic industry (such as connector, terminal and other products), decoration industry (such as matte silver jewelry, handicraft and other products) and industrial field (such as corrosion-resistant parts and other parts).

[0053] The present application is further described by specific examples as follows: Example 1 (1) The oil removal treatment is to immerse the red copper workpiece in an alkaline oil removal agent (3% sodium hydroxide, 2% fatty alcohol polyoxyethylene ether, 0.5% benzotriazole, 94.5% deionized water) for 10 minutes, and then rinse with water. The pickling treatment is to immerse the red copper workpiece in a pickling solution containing 1 mol / L sulfuric acid and 0.05 mol / L glyoxylic acid at 25°C for 4 minutes. The activation treatment is to activate the surface of the red copper workpiece using 0.1 mol / L dilute hydrochloric acid for 15 seconds, and add 0.001 mol / L citric acid to adjust the surface dynamic electric potential to -18 mV. The surface dynamic electric potential is measured by electrophoretic light scattering method. The nickel sulfamate plating solution (containing 350 g / L nickel sulfamate, 35 g / L boric acid, 1 g / L sodium saccharin, and 0.03 g / L 2-mercaptobenzimidazole) is used for plating, and the current density is controlled at 3 A / dm 2 , the plating solution temperature is 50°C, the plating time is 8 minutes, and the nickel thickness is 4 µm.

[0054] (2) The silver methanesulfonate, allyl thiourea, polyethylene glycol, ethylenediaminetetraacetic acid, potassium iodide, urea, and boric acid are added to the plating tank to prepare a plating solution, so that the plating solution contains 50 g / L silver methanesulfonate, 0.1 g / L allyl thiourea, 0.3 g / L polyethylene glycol, 5.0 g / L ethylenediaminetetraacetic acid, 120 g / L potassium iodide, 15 g / L urea, and 35 g / L boric acid.

[0055] (3) The plating solution pH value is controlled at 4.0, the silver layer thickness is 10 µm, and the plating time is 9 minutes during plating. The current-temperature linkage control is adopted: the current density is 0.8 A / dm 2 and the plating solution temperature is 24°C during the 0-1 min period of plating; the current density is 1.8 A / dm 2 and the plating solution temperature is 29°C during the 0-1 min period of plating; the current density is 1.2 A / dm 2 and the plating solution temperature is 25°C during the 7-9 min period of plating.

[0056] (4) The red copper workpiece after plating is cleaned with water to remove the residual plating solution on the surface, immersed in 20 mL / L chromic acid solution for 15 minutes for passivation, and then dried. The drying temperature is 70°C, and the drying time is 15 minutes.

[0057] Example 2 (1) The specific steps are the same as those in Example 1. The pickling treatment is to immerse the red copper workpiece in a pickling solution containing 0.5 mol / L sulfuric acid and 0.02 mol / L glyoxylic acid at 27°C for 4 minutes. The current density is controlled at 2 A / dm 2 , the plating solution temperature is 48°C, the plating time is 6 minutes, and the nickel thickness is 3.5 µm.

[0058] (2), the specific steps are the same as example 1, and the plating solution comprises 48 g / L silver methanesulfonate, 0.8 g / L allyl mercaptan and polyethylene glycol, 4.5 g / L ethylenediaminetetraacetic acid, 110 g / L potassium iodide, 12 g / L urea and 30 g / L boric acid.

[0059] (3), the specific steps are the same as example 1, and the plating time is 11 minutes. In the period of 0-1 min of plating, the current density is 0.5 A / dm 2 , the plating solution temperature is 24℃; in the period of 1-9 min of plating, the current density is 1.5 A / dm 2 , the plating solution temperature is 30℃; in the period of 9-11 min of plating, the current density is 1.0 A / dm 2 , the plating solution temperature is 26℃.

[0060] (4), the finished red copper workpiece is cleaned with water to remove the residual plating solution on the surface, immersed in 15 mL / L chromic acid solution for 15 minutes for passivation, and then dried. The drying temperature is 60℃, and the drying time is 15 minutes.

[0061] Example 3 (1), the specific steps are the same as example 1, and the alkaline zincate plating solution (including 13 g / L zinc oxide, 90 g / L sodium hydroxide and 8 mL / L triethanolamine) is used. The current density is controlled to be 2 A / dm 2 , the plating solution temperature is 30℃, the plating time is 5 minutes, and the zinc bottom thickness is 4 µm.

[0062] (2), (3), (4), the specific steps are the same as example 1.

[0063] Comparative Example 1, the difference between this comparative example and example 1 is that the pickling is treated with 1 mol / L sulfuric acid.

[0064] Comparative Example 2, the difference between this comparative example and example 1 is that the thickness of the pre-plated nickel layer is 0 µm.

[0065] Comparative Example 3, the difference between this comparative example and example 1 is that the plating solution composition is 50 g / L silver methanesulfonate, 0.1 g / L allyl mercaptan, 2.0 g / L ethylenediaminetetraacetic acid, 15 g / L urea and 35 g / L boric acid.

[0066] Adhesion test Test standard: ASTM D3359 (adhesion test standard) method B cross-cut adhesive tape test Sample preparation: the same red copper sheet (size 50mm×50mm, thickness 1mm) is prepared according to the sample preparation method of each example and comparative example, and the sample surface is free of scratches, bubbles and oil stains (wiped dry with anhydrous ethanol before testing).

[0067] Test procedure: Use a marker to select 3 non-edge areas (interval ≥10mm, avoid edge effect) on the sample surface, mark a test range of 20mm x 20mm in each area.

[0068] Along the marked range, use a craft knife to draw 6 parallel straight lines perpendicular to the sample surface, line interval 1mm, the depth of the grid needs to penetrate the matte silver plating layer and the pre-plating layer until the purple copper workpiece is exposed (color observation can confirm: silver layer is matte white, workpiece is purple red); Cross-draw 6 vertical straight lines in the same area to form 36 square grids, keep the force uniform during the grid drawing process to avoid grid edge skewing or breaking.

[0069] Use a soft brush to sweep along the diagonal direction of the grid to completely remove the plating layer debris generated during grid drawing, avoid residual debris getting stuck in the grid gap affecting the subsequent adhesive tape peeling results.

[0070] Take 3M 610 adhesive tape, tear off the release paper, and cover the grid area completely (the edge of the adhesive tape exceeds the grid range by at least 5mm), press the adhesive tape center to the edge with the finger pad for 10s (ensure that the adhesive tape and the plating layer are completely bonded without air bubbles, and the pressing force is uniform); After standing for 5s, quickly tear off the adhesive tape at a 45° angle (the tearing time is controlled within 0.5-1s, completed at one time, avoid slow peeling leading to result deviation), repeat the adhesion-peeling operation 1 time.

[0071] Grade determination: Observe the grid edge and internal plating layer peeling of the 3 test areas with a 10x magnifying lens, determine the grade according to the ASTM D3359 grading standard (take the worst grade of the 3 areas as the final adhesion grade of the sample): 0 grade: the grid edge is completely smooth without any plating layer peeling; 1 grade: single grid edge peeling area ≤5% (only the grid corner has very slight peeling); 2 grade: grid edge peeling area 5%-15%, or part of the grid internal plating layer peeling area ≤5%; 3 grade: grid edge peeling area 15%-35%, or part of the grid internal plating layer peeling area 5%-15%; 4 grade: grid edge peeling area 35%-65%, or part of the grid internal plating layer peeling area 15%-35%; 5 grade: plating layer peeling area >65% (large area peeling or grid internal plating layer is basically completely peeled off).

[0072] Corrosion resistance test: Test standard: ASTM B117 (salt spray test standard) Sample preparation: The same size samples prepared for adhesion test were used, the surface residual passivation solution was washed with deionized water, dried at 60°C for 1 h, and after cooling to room temperature, the sample number was marked (Examples 1-3, Comparative Examples 1-3).

[0073] Equipment: Salt spray test chamber (temperature control ±2°C, mist amount 1-2 mL / (h dm²); Reagent: Prepare a 5% (mass fraction) sodium chloride solution, pH 6.5-7.2, and stand for 24 h.

[0074] Test steps: Hang the sample on the sample holder in the salt spray chamber, the sample forms a 15° angle with the vertical direction, the sample spacing is ≥20 mm, the temperature in the salt spray chamber is stabilized at 35°C, the relative humidity is maintained >95%, continuously spray 5% sodium chloride solution, the mist amount is controlled at 1.5 mL / (h dm²), and the total test time is 72 h.

[0075] At 24 h, 48 h, and 72 h of the test, respectively, take out the sample, gently rinse the surface salt with deionized water (water flow speed ≤0.5 m / s to avoid damaging the plating layer), and after drying at 60°C for 10 min, observe the sample surface corrosion and record the time: White rust appearance time: the first appearance time of white oxidation products generated by corrosion of the matte silver plating layer; Red rust appearance time: the first appearance time of red oxidation products generated by corrosion after exposure of the red copper substrate (if there is no red rust within 72 h, record as “>72 h”).

[0076] Table 1. Relevant data table for adhesion and corrosion resistance test According to the analysis in Table 1, the adhesion grades of Examples 1-3 are all at level 1 or below, which are significantly better than Comparative Examples 1-3. Among them, the adhesion of Examples 1 and 2 (nickel pre-plating bottom) is better than that of Example 3 (zinc pre-plating bottom), which indicates that the nickel bottom can more effectively enhance the adhesion effect of the plating layer compared with the zinc bottom. Comparative Example 1 lacks the mechanical embedding effect of nano concave-convex caused by glyoxylic acid, Comparative Example 3 lacks polyethylene glycol and potassium iodide, polyethylene glycol can improve the uniformity of silver layer deposition, and potassium iodide can complex with silver ions to refine the crystal grains and reduce the porosity, the absence of the two causes the surface flatness of the silver layer to decrease and the crystal grains to coarsen, and the internal stress to increase; the matte silver plating layer of Comparative Example 2 is directly combined with the red copper substrate, and there is no transition layer to buffer, which causes the internal stress to increase dramatically due to the mismatch of silver-copper crystal lattice, and the plating layer is prone to large-scale peeling.

[0077] In terms of corrosion resistance, Examples 1-3 also performed outstandingly, with white rust appearing at >66h and no red rust appearing within 72h. Comparative Example 1 had poor adhesion of the pre-plating layer to the red copper substrate due to the absence of glycolic acid cleaning, and Comparative Example 3 had uneven silver deposition and silver grain coarsening due to the absence of polyethylene glycol and potassium iodide; Comparative Example 2 had the fastest corrosion rate because, due to the absence of a pre-plating layer, the matte silver plating layer had high internal stress and porosity, the corrosion medium quickly penetrated the silver layer to reach the red copper substrate, and the matte silver plating layer had poor adhesion to the red copper substrate and was prone to peeling. This further shows that the composite plating layer prepared by the process of the examples has good protective effect on the red copper substrate and can effectively prevent the red copper substrate from being exposed and corroded.

[0078] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A process for electroplating a matte silver finish on a red copper surface, characterized in that, The method comprises the following steps: Pre-treatment: sequentially performing oil removal treatment, acid pickling treatment and activation treatment on the red copper workpiece, and pre-plating a layer of nickel or zinc bottom, the acid pickling solution used in the acid pickling treatment is a mixed solution prepared by sulfuric acid and glyoxylic acid; Preparation of plating solution: adding silver methanesulfonate, allyl thiourea, polyethylene glycol, potassium iodide and additives into a plating tank to prepare the plating solution; Electroplating: placing the red copper workpiece in the plating tank for electroplating, and controlling the current density, plating solution temperature, plating solution pH value and electroplating time within a preset range.

2. The process for electroplating matte silver on a surface of red copper as claimed in claim 1, wherein, The acid pickling treatment specifically comprises placing the red copper workpiece in the acid pickling solution and soaking at 25-30°C for 3-5 minutes. The acid pickling solution contains 0.5-1 mol / L sulfuric acid and 0.01-0.05 mol / L glyoxylic acid.

3. The process for electroplating matte silver on a surface of red copper as claimed in claim 1, wherein, The activation treatment specifically comprises activating the surface of the red copper workpiece with 0.1 mol / L dilute hydrochloric acid for 10-20 seconds, and adding 0.001 mol / L citric acid to adjust the surface dynamic electric potential to-16 to-20 mV.

4. The process for electroplating matte silver on a surface of red copper as claimed in claim 1, wherein, The specific steps of pre-plating a layer of nickel bottom are as follows: Electroplating was carried out using nickel sulfamate or Watts nickel plating solution, controlling current density 1-5 A / dm 2 , plating solution temperature 40-60°C, plating time 5-10 minutes, and nickel base thickness 3-5 µm.

5. The process for electroplating matte silver over a surface of red copper as claimed in claim 1, wherein, The specific steps of pre-plating a layer of zinc bottom are as follows: Electroplating using alkaline zincate or acidic zinc chloride plating solution, controlling current density 1-3 A / dm 2 , plating solution temperature 20-40°C, plating time 3-5 minutes, zinc base thickness 2-4 µm.

6. The process for electroplating matte silver over a surface of red copper as claimed in claim 1, wherein: The additives include ethylenediaminetetraacetic acid, urea and boric acid.

7. The process for electroplating matte silver over a surface of red copper as claimed in claim 6, wherein, The plating solution comprises 45-55 g / L silver methanesulfonate, 0.05-0.2 g / L allyl thiourea, 0.2-0.5 g / L polyethylene glycol, 3.5-8.0 g / L ethylenediaminetetraacetic acid, 105-125 g / L potassium iodide, 12-18 g / L urea and 30-40 g / L boric acid.

8. The process for electroplating matte silver over a surface of red copper as claimed in claim 1, wherein, In the electroplating step, the current density is controlled to be 0.5-2 A / dm 2 , the plating solution temperature is 20-30℃, the plating solution pH value is 4.0-4.5, the silver layer thickness is 5-10 µm, and the electroplating time is 5-15 minutes.

9. The process for electroplating matte silver over a surface of red copper as claimed in claim 8, wherein, Current-temperature linkage control is adopted: During the plating period of 0-10%, the current density is 0.5-0.8 A / dm 2 , and the temperature of the plating solution is 22-24℃. During the plating period of 10%-80%, the current density is 1.5-2.0 A / dm 2 and the temperature of the plating solution is 28-30℃. During the plating period of 80-100%, the current density is 1.0-1.3 A / dm 2 and the temperature of the plating solution is 25-26℃.

10. The process for electroplating matte silver over a surface of red copper as claimed in claim 1, wherein, The method further comprises the following steps: The red copper workpiece after electroplating is cleaned with water to remove the residual plating solution on the surface, immersed in a chromate solution or an organic passivation agent for passivation, and then dried.

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