High-sulfur-resistance cyanide-free composite silver plating brush plating solution and cyanide-free composite silver plating brush plating method

Abstract

The invention provides a high-sulfur-resistance cyanide-free composite silver plating brush plating solution and a cyanide-free composite silver plating brush plating method. The brush plating solution comprises the following raw materials: potassium iodide, nicotinic acid, 5, 5-dimethyl hydantoin, ammonium acetate, NH3.H2O, silver nitrate, potassium carbonate, graphene, carbon nanotubes, a dispersing agent and a solvent. On the basis of the volume of the brush plating solution, the content of potassium iodide is 100-300 g.L <-1 >, the content of 5, 5-dimethyl hydantoin is 1-150 g.L <-1 >, the content of nicotinic acid is 2-160 g.L <-1 >, the content of ammonium acetate is 8-85 g.L <-1 >, and the content of NH3.H2O is 5-50 g.L <-1 >; the ratio of the total molar weight of the potassium iodide, the nicotinic acid and the 5, 5-dimethyl hydantoin to the molar weight of the silver nitrate is (5-20): 1. The technical method provided by the invention is suitable for field repair, and the composite silver coating with excellent sulfur resistance, binding force, wear resistance and arc ablation resistance can be obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of electric contact material preparation, and in particular relates to a high-sulfur-resistant cyanide-free composite silver-plating brush plating solution and a cyanide-free composite silver-plating brush plating method. Background Art

[0002] At present, the moving and static contact protection parts of outdoor high-voltage disconnectors are mostly designed with ordinary silver plating and hard silver plating. This kind of electrical contact material can provide good electrical conductivity, thermal conductivity and low and stable contact resistance. Although the traditional tank plating electrochemical deposition method can obtain a composite coating with good performance, the process is complicated and the cycle is long, which is not suitable for rapid on-site repair. In contrast, brush plating technology is more suitable for emergency situations such as field repairs due to its lightweight equipment, simple operation, fast plating speed and high-quality coating. Over the years, brush plating technology has played an important role in the field of equipment maintenance and remanufacturing, and has a deep commercial background.

[0003] During operation, the silver coating of the contact may deteriorate or even fall off due to friction, ablation, etc. Traditionally, these problems are usually repaired by replacing the contact or cyanide silver plating. However, in actual applications, some equipment is difficult to disassemble, and replacing the contact not only wastes a lot of resources but also has low efficiency. Although cyanide silver plating has a simple process and good coating performance, the cyanide used in the process is a highly toxic substance, which poses a threat to the environment and public safety in actual applications. Therefore, researchers have conducted in-depth research on cyanide-free silver plating technology, but so far there has been no major breakthrough in the process technology of cyanide-free silver plating, and it is still difficult to meet the performance requirements of high-voltage power grid applications. There are three main problems with the current cyanide-free silver plating technology: (1) Compared with the cyanide silver plating layer, the overall performance of the cyanide-free silver plating layer is obviously insufficient and fails to meet the established indicators, which is specifically manifested in the poor sulfur resistance of the coating, insufficient gloss, easy discoloration, weak bonding, low conductivity and purity. This poses a serious obstacle to the development of cyanide-free silver plating, especially in the fields of power, military industry, aviation, etc., which have strict requirements on coating performance. (2) The current cyanide-free silver brush plating solution is difficult to meet the needs of on-site brush plating parts, especially when the current density range is narrow or the dispersibility of the plating solution is poor, and the phenomenon of missing plating and insufficient plating often occurs. In addition, the low cathode current density and poor anode solubility can easily lead to passivation during the brush plating process. (3) The composition stability of the cyanide-free silver brush plating solution is poor, and the main ligand is easily decomposed and precipitated when exposed to light. In addition, the metal ions and organic impurities in the plating solution will reduce the electroplating efficiency and coating quality, increase the application cost, and bring many inconveniences to the management and operation of the electroplating process. Therefore, it is urgent to develop a more suitable formula for on-site repair with high sulfur resistance and cyanide-free composite silver brush plating solution and its key preparation process. Summary of the invention

[0004] The purpose of the present invention is to provide a highly sulfur-resistant cyanide-free composite silver-plating brush plating solution and a cyanide-free composite silver-plating brush plating method for on-site repair, so as to obtain a composite silver-plated layer with excellent sulfur resistance, bonding strength, wear resistance and arc ablation resistance.

[0005] In order to achieve the above object, the present invention provides the following technical solutions.

[0006] In a first aspect, the present invention provides a highly sulfur-resistant and cyanide-free composite silver-plating brush plating solution, wherein the raw material composition of the brush plating solution includes:

[0007] Potassium iodide, nicotinic acid, 5,5-dimethylhydantoin, ammonium acetate, NH3·H2O, silver nitrate, potassium carbonate, graphene, carbon nanotubes, dispersant, solvent;

[0008] Based on the volume of the brush plating solution, the content of potassium iodide is 100-300g·L -1 , 5,5-dimethylhydantoin content is 1-150g·L -1 , niacin content is 2-160g·L -1 , the content of ammonium acetate is 8-85g·L -1 、NH3·H2O content is 5-50g·L -1 ;

[0009] The ratio of the total molar amount of potassium iodide, nicotinic acid and 5,5-dimethylhydantoin to the molar amount of silver nitrate is 5-20:1.

[0010] The high sulfur resistance and cyanide-free composite silver-plating brush plating solution provided by the present invention uses potassium iodide, nicotinic acid, 5,5-dimethylhydantoin, ammonium acetate, and NH3·H2O in specific contents as composite complexing agents. The composite complexing agent can greatly improve the polarization degree of the composite silver-plating brush plating solution. The composite brush plating can be carried out at a higher brush plating voltage, which is conducive to the formation of ultrafine crystals and dense composite silver-plating layers. Combined with the blocking effect of graphene in the composite plating layer and the hydrophobic structure formed on its surface by carbon nanotubes, the composite silver-plating layer has excellent sulfur resistance. The composite complexing agent in the high sulfur resistance and cyanide-free composite silver-plating brush plating solution provided by the present invention adds ammonium acetate and NH3·H2O. The regulation of ammonium acetate and NH3·H2O can effectively regulate the anode solubility and greatly improve the cathode current density and current efficiency of on-site composite brush plating. The high sulfur resistance and cyanide-free composite silver-plating brush plating solution provided by the present invention can effectively ensure the stability, dispersibility, deep plating ability, current efficiency, etc. of the composite silver-plating brush plating solution under the cooperation of a specific ratio of a special composite complexing agent and silver nitrate. The high-sulfur-resistant and cyanide-free composite silver-plating brush plating solution provided by the invention can obtain a composite silver-plated layer with excellent sulfur resistance, bonding strength, wear resistance and arc ablation resistance under the cooperation of a special composite complexing agent, silver nitrate, potassium carbonate, graphene and carbon nanotubes.

[0011] According to a preferred embodiment of the first aspect, preferably, the molar ratio of potassium carbonate to silver nitrate is 1.5-3: 1. This preferred technical solution can better ensure the conductivity and brightness of the composite silver-plated layer.

[0012] According to a preferred embodiment of the first aspect, preferably, based on the volume of the brush plating solution, the content of graphene is 1-3 g·L -1 .

[0013] According to a preferred embodiment of the first aspect, preferably, based on the volume of the brush plating solution, the content of the carbon nanotubes does not exceed 3 g·L -1 .

[0014] According to a preferred embodiment of the first aspect, preferably, the ratio of the total molar amount of ammonium acetate and NH3·H2O to the molar amount of silver nitrate is 3-8:1; this preferred embodiment can better ensure the brightness, dispersion ability and deep plating ability of the composite silver-plated layer, and better prevent anode passivation.

[0015] According to a preferred embodiment of the first aspect, preferably, the solvent is water.

[0016] According to a preferred embodiment of the first aspect, preferably, the dispersant includes at least one of sodium carboxymethyl cellulose, N-acetyl-L-proline, sodium dodecyl sulfate and sodium dodecylbenzene sulfonate.

[0017] In a second aspect, the present invention provides a method for preparing the highly sulfur-resistant and cyanide-free composite brush plating solution provided in the first aspect, the preparation method comprising:

[0018] Potassium iodide, nicotinic acid, 5,5-dimethylhydantoin, ammonium acetate, NH3·H2O, silver nitrate, potassium carbonate, graphene, carbon nanotubes and a dispersant are uniformly dispersed in a solvent to obtain the highly sulfur-resistant and cyanide-free composite brush plating solution.

[0019] In a third aspect, the present invention provides a cyanide-free composite silver-plating brush plating method, the method comprising:

[0020] The highly sulfur-resistant and cyanide-free composite brush plating solution provided in the first aspect of the present invention is used to perform brush plating on the surface of a workpiece to form a composite silver plating layer on the surface of the workpiece.

[0021] According to a preferred embodiment of the third aspect, preferably, in the process of brush plating on the surface of a workpiece using the high sulfur-resistant and cyanide-free composite brush plating solution provided by the first aspect of the present invention, the working voltage of the brush plating is 2-5V, and the relative movement speed between the brush plating pen and the workpiece is 8-20m·min -1 , current density is 0.5-50A·dm -2 , the time is determined according to the required silver plating layer thickness.

[0022] According to a preferred embodiment of the third aspect, preferably, the thickness of the composite silver-plated layer formed on the surface of the workpiece is 8-50 μm.

[0023] According to a preferred embodiment of the third aspect, preferably, the cyanide-free composite silver brush plating method further comprises:

[0024] Before using the highly sulfur-resistant and cyanide-free composite brush plating solution provided by the first aspect of the present invention to perform brush plating on the surface of the workpiece, the surface of the workpiece is treated to remove the oxide scale, corrosion marks and insoluble substances adhering to the surface of the workpiece;

[0025] More preferably, surface treatment of the workpiece surface comprises:

[0026] The workpiece surface is polished, electro-cleaned and activated in sequence;

[0027] Further preferably, the grinding process includes: coarse grinding and fine grinding in sequence; more preferably, the coarse grinding is performed using 400# sandpaper, and the fine grinding is performed using 1000# sandpaper; by using 400# sandpaper to perform coarse grinding on the surface of the workpiece, insoluble substances, oxide scale and other impurities attached to the surface can be effectively removed to improve its surface flatness, and by using 1000# sandpaper to perform fine grinding on the surface of the workpiece, the glossiness of the workpiece surface can be improved, and the bonding strength between the workpiece surface and the coating can be improved;

[0028] Further preferably, the raw material composition of the electro-cleaning liquid used in the electro-cleaning treatment includes sodium hydroxide, sodium carbonate, sodium phosphate, sodium silicate and solvent; based on the volume of the electro-cleaning liquid, the content of sodium hydroxide is 10-45g·L -1 , the content of sodium carbonate is 11-18g·L -1 , the content of sodium phosphate is 25-35g·L -1 , the content of sodium silicate is 2-15g·L -1 ; More preferably, the solvent is water; More preferably, the pH value of the electro-cleaning solution is 10-13.5; More preferably, the working polarity of the electro-cleaning treatment is cathode, the working voltage is 8-13V, and the working time is 10-20s;

[0029] Electrocleaning is to dissolve the oil on the surface of the workpiece to ensure that the surface of the workpiece is clean and flawless.

[0030] Further preferably, the raw material composition of the activation solution used in the activation treatment includes hydrochloric acid, sodium chloride and a solvent; based on the volume of the activation solution, the content of hydrochloric acid is 15-30 g·L -1 , the content of sodium chloride is 110-145g·L -1 ; More preferably, the solvent is water; More preferably, the working polarity of the activation treatment is anode, the working voltage is 8-13V, and the working time is 10-20s;

[0031] Activation treatment can effectively remove the oxide film and fatigue layer on the metal surface, so that there is better bonding between the coating and the metal on the workpiece surface.

[0032] According to a preferred implementation of the third aspect, preferably, the surface material of the workpiece is copper.

[0033] In a fourth aspect, the present invention provides a composite silver-plated layer, wherein the composite silver-plated layer is prepared by a cyanide-free composite silver-plating brush plating method provided by the present invention.

[0034] In a fifth aspect, the present invention provides an application of the composite silver-plated layer provided in the fourth aspect of the present invention in an electrical contact material.

[0035] The technical solution provided by the technical solution of the present invention can quickly modify various conductor parts on site without disassembly and installation, and there is no pollution. The composite silver-plated layer has high sulfur resistance, high bonding strength, wear resistance and arc ablation resistance. The technical solution of the present invention is green and environmentally friendly and does not harm the health of the operator. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is an image of the composite silver-plated layer prepared in Comparative Example 1.

[0037] Figure 2 This is an image of the composite silver-plated layer prepared in Comparative Example 1 after being immersed in a Na2S solution with a Na2S concentration of 1 wt% for 48 hours.

[0038] Figure 3 This is an image of the composite silver-plated layer prepared in Example 1.

[0039] Figure 4 This is an image of the composite silver-plated layer prepared in Example 1 after being immersed in a Na2S solution with a Na2S concentration of 1 wt% for 48 hours. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention.

[0041] Example 1

[0042] This embodiment provides a highly sulfur-resistant and cyanide-free composite silver plating brush plating solution.

[0043] The high-sulfur-resistant and cyanide-free composite silver-plating brush plating solution is obtained by uniformly dispersing potassium iodide, nicotinic acid, 5,5-dimethylhydantoin, ammonium acetate, NH3·H2O, silver nitrate, potassium carbonate, graphene, carbon nanotubes, sodium dodecyl sulfate and sodium dodecylbenzene sulfonate in water.

[0044] Based on the volume of the brush plating solution, the content of potassium iodide is 200g·L -1 , 5,5-dimethylhydantoin content is 120g·L -1 , niacin content is 100g·L -1 , the content of ammonium acetate is 36g·L -1 、NH3·H2O content is 28g·L -1 , the content of silver nitrate is 30g·L -1 , the content of potassium carbonate is 50g·L -1 , the graphene content is 2g·L -1 , the content of carbon nanotubes is 1g·L -1 The contents of dispersants sodium dodecyl sulfate and sodium dodecylbenzene sulfonate were 0.6 g·L -1 .

[0045] This embodiment also provides a cyanide-free composite silver-plating brush plating method.

[0046] The method includes:

[0047] 1) Grinding the surface of the copper workpiece, specifically including: using 400# sandpaper to perform coarse grinding on the surface of the copper workpiece, and using 1000# sandpaper to perform fine grinding;

[0048] 2) Performing electro-cleaning treatment on the surface of the copper workpiece after grinding;

[0049] The electro-cleaning liquid used in the electro-cleaning treatment is obtained by dispersing sodium hydroxide, sodium carbonate, sodium phosphate, and sodium silicate in water; based on the volume of the electro-cleaning liquid, the content of sodium hydroxide is 13g·L -1 , the content of sodium carbonate is 12g·L -1 , the content of sodium phosphate is 28g·L -1 , the content of sodium silicate is 10g·L -1 ; The pH value of the electro-cleaning solution is 11;

[0050] The working polarity of the electro-cleaning treatment is cathode, the working voltage is 10V, and the working time is 15s;

[0051] 3) Activate the surface of the copper workpiece after electro-cleaning;

[0052] The activation solution used in the activation treatment is obtained by dispersing hydrochloric acid and sodium chloride in water; based on the volume of the activation solution, the content of hydrochloric acid is 16g·L -1 , the content of sodium chloride is 115g·L -1 ;

[0053] The working polarity of the activation treatment is anode, the working voltage is 9V, and the working time is 11s;

[0054] 4) using the highly sulfur-resistant and cyanide-free composite brush plating solution provided in this embodiment to perform brush plating on the surface of the copper workpiece after activation treatment to form a composite silver plating layer on the surface of the copper workpiece;

[0055] The working voltage of brush plating is 3V, and the relative movement speed between the brush plating pen and the workpiece is 9m / min. -1 , the current density is 0.8A·dm -2 , time is 5min;

[0056] The thickness of the composite silver-plated layer is 20 μm.

[0057] Example 2

[0058] This embodiment provides a highly sulfur-resistant and cyanide-free composite silver-plating brush plating solution.

[0059] The high-sulfur-resistant and cyanide-free composite silver-plating brush plating solution is obtained by uniformly dispersing potassium iodide, nicotinic acid, 5,5-dimethylhydantoin, ammonium acetate, NH3·H2O, silver nitrate, potassium carbonate, graphene, carbon nanotubes and sodium dodecyl sulfate in water.

[0060] Based on the volume of the brush plating solution, the content of potassium iodide is 250g·L -1 , 5,5-dimethylhydantoin content is 110g·L -1 , niacin content is 89g·L -1 , the content of ammonium acetate is 38g·L -1 、NH3·H2O content is 27g·L -1 , the content of silver nitrate is 30g·L -1 , the content of potassium carbonate is 37g·L -1 , the graphene content is 1.5g·L -1 , the content of carbon nanotubes is 1.5g·L -1 , the content of dispersant sodium dodecyl sulfate is 0.6g·L -1 .

[0061] This embodiment also provides a cyanide-free composite silver-plating brush plating method.

[0062] The method includes:

[0063] 1) Grinding the surface of the copper workpiece, specifically including: using 400# sandpaper to perform coarse grinding on the surface of the copper workpiece, and using 1000# sandpaper to perform fine grinding;

[0064] 2) Performing electro-cleaning treatment on the surface of the copper workpiece after grinding;

[0065] The electro-cleaning liquid used in the electro-cleaning treatment is obtained by dispersing sodium hydroxide, sodium carbonate, sodium phosphate, and sodium silicate in water; based on the volume of the electro-cleaning liquid, the content of sodium hydroxide is 28g·L -1 , the content of sodium carbonate is 17g·L -1 , the content of sodium phosphate is 32g·L -1 , the content of sodium silicate is 8g·L -1 ; The pH value of the electro-cleaning solution is 12;

[0066] The working polarity of the electro-cleaning treatment is cathode, the working voltage is 9V, and the working time is 13s;

[0067] 3) Activate the surface of the copper workpiece after electro-cleaning;

[0068] The activation solution used in the activation treatment is obtained by dispersing hydrochloric acid and sodium chloride in water; based on the volume of the activation solution, the content of hydrochloric acid is 25g·L -1 , the content of sodium chloride is 143g·L -1 ;

[0069] The working polarity of the activation treatment is anode, the working voltage is 12V, and the working time is 15s;

[0070] 4) using the highly sulfur-resistant and cyanide-free composite brush plating solution provided in this embodiment to perform brush plating on the surface of the copper workpiece after activation treatment to form a composite silver plating layer on the surface of the copper workpiece;

[0071] The working voltage of brush plating is 2.5V, and the relative movement speed between the brush plating pen and the workpiece is 12m / min. -1 , the current density is 0.55A·dm -2 , time is 6min;

[0072] The thickness of the composite silver-plated layer is 25 μm.

[0073] Example 3

[0074] This embodiment provides a highly sulfur-resistant and cyanide-free composite silver-plating brush plating solution.

[0075] The high-sulfur-resistant and cyanide-free composite silver-plating brush plating solution is obtained by uniformly dispersing potassium iodide, nicotinic acid, 5,5-dimethylhydantoin, ammonium acetate, NH3·H2O, silver nitrate, potassium carbonate, graphene, carbon nanotubes and sodium dodecylbenzene sulfonate in water.

[0076] Based on the volume of the brush plating solution, the content of potassium iodide is 280g·L -1 , 5,5-dimethylhydantoin content is 58g·L -1 , niacin content is 113g·L -1 , the content of ammonium acetate is 32g·L -1 、NH3·H2O content is 21g·L -1 , the content of silver nitrate is 30g·L -1 , the content of potassium carbonate is 45g·L -1 , the content of graphene is 1g·L -1 , the content of carbon nanotubes is 2g·L -1 , the content of dispersant sodium dodecylbenzene sulfonate is 0.6g·L -1 .

[0077] This embodiment also provides a cyanide-free composite silver-plating brush plating method.

[0078] The method includes:

[0079] 1) Grinding the surface of the copper workpiece, specifically including: using 400# sandpaper to perform coarse grinding on the surface of the copper workpiece, and using 1000# sandpaper to perform fine grinding;

[0080] 2) Performing electro-cleaning treatment on the surface of the copper workpiece after grinding;

[0081] The electro-cleaning liquid used in the electro-cleaning treatment is obtained by dispersing sodium hydroxide, sodium carbonate, sodium phosphate, and sodium silicate in water; based on the volume of the electro-cleaning liquid, the content of sodium hydroxide is 38g·L -1 , the content of sodium carbonate is 14g·L -1 , the content of sodium phosphate is 22g·L -1 , the content of sodium silicate is 7g·L -1 ; The pH value of the electro-cleaning solution is 13;

[0082] The working polarity of the electro-cleaning treatment is cathode, the working voltage is 12V, and the working time is 13s;

[0083] 3) Activate the surface of the copper workpiece after electro-cleaning;

[0084] The activation solution used in the activation treatment is obtained by dispersing hydrochloric acid and sodium chloride in water; based on the volume of the activation solution, the content of hydrochloric acid is 30g·L -1, the content of sodium chloride is 145g·L -1 ;

[0085] The working polarity of the activation treatment is anode, the working voltage is 13V, and the working time is 20s;

[0086] 4) using the highly sulfur-resistant and cyanide-free composite brush plating solution provided in this embodiment to perform brush plating on the surface of the copper workpiece after activation treatment to form a composite silver plating layer on the surface of the copper workpiece;

[0087] The working voltage of brush plating is 4V, and the relative movement speed between the brush plating pen and the workpiece is 18m / min. -1 , the current density is 3.1A·dm -2 , time is 8min;

[0088] The thickness of the composite silver-plated layer is 30 μm.

[0089] Comparative Example 1

[0090] This comparative example provides a highly sulfur-resistant cyanide-free composite silver-plating brush plating solution and a cyanide-free composite silver-plating brush plating method.

[0091] The only difference between the embodiment 1 and the embodiment 1 is that the high sulfur-resistant and cyanide-free composite silver-plating brush plating solution does not contain graphene and carbon nanotubes.

[0092] Comparative Example 2

[0093] This comparative example provides a highly sulfur-resistant cyanide-free composite silver-plating brush plating solution and a cyanide-free composite silver-plating brush plating method.

[0094] The only difference between it and Example 1 is that the highly sulfur-resistant and cyanide-free composite silver-plating brush plating solution does not contain ammonium acetate and NH3·H2O.

[0095] Experimental example

[0096] The hardness, friction coefficient and anti-sulfur performance of the composite silver-plated layers prepared in Example 1, Example 2, Example 3, Comparative Example 1 and Comparative Example 2 were tested respectively.

[0097] The anti-sulfur performance test method is: immerse the workpiece in a Na2S solution with a Na2S concentration of 1wt% for 48h and compare the surface color change.

[0098] Figure 1 This is a graph showing the test results of contact resistance, sulfur resistance and friction coefficient of the composite coating obtained in Example 1.

[0099] The test results are shown in Table 1. Figure 1 , Figure 2 , Figure 3 , Figure 4 shown.

[0100] Table 1

[0101] Hardness / HV Friction coefficient Anti-sulfur performance / 48h Example 1 132 0.19 No color change Example 2 125 0.18 No color change Example 3 135 0.17 No color change Comparative Example 1 105 0.48 Turn brown Comparative Example 2 98 0.56 Turn brown

[0102] According to the test results in Table 1, the composite coating obtained by the present invention has excellent sulfur resistance, small friction coefficient and low contact resistance.

[0103] The embodiments described above are for better explaining the present invention. For technical researchers in the field, it is not difficult to make various modifications to these embodiments without departing from the principles and spirit of the present invention. Therefore, the present invention is not limited to the embodiments herein. The improvements and changes made by those skilled in the art to the present invention according to the principles and spirit of the present invention should be within the protection scope of the present invention.

Claims

1. A highly sulfur-resistant and cyanide-free composite silver-plating brush plating solution, wherein: The raw material composition of the brush plating solution includes: Potassium iodide, nicotinic acid, 5,5-dimethylhydantoin, ammonium acetate, NH3·H2O, silver nitrate, potassium carbonate, graphene, carbon nanotubes, dispersant, solvent; Based on the volume of the brush plating solution, the content of potassium iodide is 100-300g·L -1 , 5,5-dimethylhydantoin content is 1-150g·L -1 , niacin content is 2-160g·L -1 , the content of ammonium acetate is 8-85g·L -1 、NH3·H2O content is 5-50g·L -1 ; The ratio of the total molar amount of potassium iodide, nicotinic acid and 5,5-dimethylhydantoin to the molar amount of silver nitrate is 5-20:

1.

2. The brush plating solution according to claim 1, wherein The molar ratio of potassium carbonate to silver nitrate is 1.5-3:1; Based on the volume of the brush plating solution, the graphene content is 1-3g·L -1 ; Based on the volume of the brush plating solution, the content of carbon nanotubes does not exceed 3g·L -1 .

3. The brush plating solution according to claim 1, wherein: The ratio of the total molar amount of ammonium acetate and NH3·H2O to the molar amount of silver nitrate is 3-8:

1.

4. The brush plating solution according to claim 1, wherein: The solvent is water; The dispersant includes at least one of sodium carboxymethylcellulose, N-acetyl-L-proline, sodium dodecyl sulfate and sodium dodecylbenzene sulfonate.

5. A cyanide-free composite silver-plating brush plating method, the method comprising: The highly sulfur-resistant and cyanide-free composite brush plating solution according to any one of claims 1 to 4 is used to perform brush plating on the surface of a workpiece to form a composite silver plating layer on the surface of the workpiece.

6. The method according to claim 5, wherein: The working voltage of brush plating is 2-5V, and the relative movement speed between the brush plating pen and the workpiece is 8-20m·min -1 , current density is 0.5-50A·dm -2 .

7. The method according to claim 5, wherein: The thickness of the composite silver-plated layer formed on the surface of the workpiece is 8-50 μm.

8. The method according to claim 5, wherein: The cyanide-free composite silver brush plating method also includes: Before brush plating the workpiece surface, the workpiece surface is treated to remove the oxide scale, corrosion marks and insoluble substances adhering to the workpiece surface. The surface treatment of the workpiece surface includes: grinding, electro-cleaning and activation treatment of the workpiece surface in sequence.

9. The method according to claim 8, wherein: The raw materials of the electro-cleaning liquid used in the electro-cleaning process include sodium hydroxide, sodium carbonate, sodium phosphate, sodium silicate and solvent; based on the volume of the electro-cleaning liquid, the content of sodium hydroxide is 10-45g·L -1 , the content of sodium carbonate is 11-18g·L -1 , the content of sodium phosphate is 25-35g·L -1 , the content of sodium silicate is 2-15g·L -1 ; Preferably, water is used; Preferably, the pH value of the electro-cleaning liquid is 10-13.5; Preferably, the working polarity of the electro-cleaning treatment is cathode, the working voltage is 8-13V, and the working time is 10-20s.

10. The method according to claim 8, wherein: The raw materials of the activation solution used in the activation treatment include hydrochloric acid, sodium chloride and solvent; based on the volume of the activation solution, the content of hydrochloric acid is 15-30g·L -1 , the content of sodium chloride is 110-145g·L -1 ; Preferably, the solvent is water; Preferably, the working polarity of the activation treatment is anode, the working voltage is 8-13V, and the working time is 10-20s.

11. A composite silver-plated layer, wherein: The composite silver-plated layer is prepared by the cyanide-free composite silver-plating brush plating method described in any one of claims 5 to 10.

12. Use of the composite silver-plated layer according to claim 11 in electrical contact materials.

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