Cyanide-free brass tin electroplating process

Abstract

The invention relates to a cyanide-free brass tin electroplating process which comprises the following steps: adding basic cupric carbonate, zinc oxide, sodium stannate, conducting salt, potassium hydroxide and potassium carbonate into a plating tank, and dissolving in deionized water; then adding a brightening agent, a complexing agent and a wetting agent, and adding deionized water to a set liquid level of the plating solution; the workpiece is put into the plating solution for electroplating, and the metal ion concentration, the current density and the temperature of the plating solution are controlled to be within the preset range; the brightener comprises the following components in percentage by mass: 0.5% of nicotinic acid, 1% of polyethyleneimine and 98.5% of deionized water; the wetting agent is an alkyl polyglucoside solution with the mass concentration of 0.5%. The active groups of nicotinic acid can slow down the reduction rate of metal ions, refine plating grains, improve the bright effect of the plating and reduce the porosity. The polyethyleneimine can optimize the crystal structure of the coating, disperse impurities to avoid the formation of rigid defect points, and improve the flexibility of the coating. The alkyl polyglucoside can reduce the surface tension of the plating solution, so that the workpiece is uniformly soaked by the plating solution, bubble retention is avoided, and the flatness of a plating layer is improved.

Description

Technical Field

[0001] This invention relates to the field of metal surface treatment technology, specifically a cyanide-free brass-tin electroplating process, which is particularly suitable for use as a substitute for gold in the electroplating of artificial jewelry and as an undercoat layer before gold plating. Background Technology

[0002] In the electroplating field, brass-tin plating is a commonly used coating that combines decorative and functional properties. It is widely used as a decorative and protective layer for everyday hardware, as well as a functional coating for electronics and machinery. It can also meet the needs of specific industries such as automotive interiors and printing rollers. Traditional brass-tin electroplating processes mostly rely on cyanide-containing plating solutions. Although these solutions can produce coatings with superior performance, cyanide is highly toxic, posing a serious threat to the health of operators and generating difficult-to-treat cyanide-containing wastewater and exhaust gases, causing severe environmental pollution. Furthermore, cyanide-containing processes operate at high temperatures and consume a lot of energy, which contradicts increasingly stringent environmental regulations and the demand for green production.

[0003] To address the environmental challenges of cyanide-containing processes, cyanide-free brass-tin electroplating processes have been gradually developed. Among these, the basic copper carbonate system has become an important research direction due to the readily available raw materials and relatively controllable costs. However, this system still faces significant technical bottlenecks in practical applications, particularly in terms of coating performance: the coating often suffers from insufficient gloss, poor flexibility, and low leveling, making it difficult to meet the requirements of artificial jewelry for delicate appearance and moldability; the coating also has high porosity and weak corrosion resistance, even inferior to traditional bright nickel plating, failing to provide long-term protection for the substrate. Summary of the Invention

[0004] The purpose of this application is to provide a cyanide-free brass-tin electroplating process to solve the technical problem of insufficient performance of cyanide-free brass-tin electroplating coatings based on the basic copper carbonate system in the prior art.

[0005] To achieve the above objectives, this application adopts the following technical solution: a cyanide-free brass-tin electroplating process, comprising the following steps: S1. Add basic copper carbonate, zinc oxide, sodium stannate, conductive salt, potassium hydroxide, and potassium carbonate to the plating tank and dissolve them in deionized water to obtain the basic plating solution. S2. Add brightener, complexing agent, and wetting agent to the base plating solution, and add deionized water to the set liquid level of the plating solution to obtain the plating solution required for electroplating. S3. Place the workpiece into the plating solution for electroplating, and control the metal ion concentration, current density and temperature of the plating solution within the predetermined range. The brightener comprises 0.5% nicotinic acid, 1% polyethyleneimine, and 98.5% deionized water by mass ratio; The wetting agent is a 0.5% (w / w) alkyl polysaccharide solution.

[0006] Optionally, the base plating solution is a mixture of 35-45 g / L basic copper carbonate, 3-5 g / L zinc oxide, 8-12 g / L sodium stannate, 110-130 g / L conductive salt, 18-22 g / L potassium hydroxide, and 8-12 g / L potassium carbonate.

[0007] Optionally, in the plating solution preparation step, the conductive salt comprises, by mass ratio, 1.5% copper sulfate, 2% zinc oxide, 15% potassium hydroxide, and 79.5% sodium chloride.

[0008] Optionally, the base plating solution is treated with activated carbon after preparation.

[0009] Optionally, the plating solution uses a base plating solution as a substrate and further comprises 0.4-0.6 ml / L of brightener, 9-11 ml / L of complexing agent, and 1.8-2.2 ml / L of wetting agent; The complexing agent is a 15% sodium citrate dihydrate solution.

[0010] Optionally, in step S3, the concentration of the metal ions is controlled in the following manner: The total copper content in the plating solution was controlled to be 10-18 g / L, the total tin content to be 4-8 g / L, the total zinc content to be 2-4 g / L, and the potassium ion concentration to be 10-25 g / L, respectively.

[0011] Optionally, in step S3, the electroplating temperature is controlled at 35℃-45℃, and the cathode current density is 0.25-3A / dm³. 2 The anode current density is 0.25-2 A / dm³. 2 .

[0012] Optionally, for every 3500 ampere-minutes of electroplating work, add 250 ml of supplement, 100 ml of brightener, 300 ml of complexing agent, 50 ml of wetting agent, 120 g of basic copper carbonate, 25 g of sodium stannate, 240 g of conductive salt and 60 g of potassium hydroxide to each liter of plating solution.

[0013] Optionally, the supplement is prepared as follows: Weigh out the conductive salt, potassium hydroxide, brightener, complexing agent or wetting agent, and add them to deionized water in sequence. Stir continuously until dissolved after each addition before adding the next component. The supplement includes 500 g / L of conductive salt, 12 g / L of potassium hydroxide, 150 ml / L of complexing agent, or 25 ml / L of wetting agent.

[0014] Optionally, when the substrate of the workpiece is nickel metal, a copper base plating layer is pre-plated first, and then a cyanide-free brass-tin electroplating process is performed.

[0015] Optionally, in the electroplating step, the anode is a brass plate, the stirring method is mechanical shaking, and the plating solution is circulated and filtered.

[0016] The beneficial effects of the cyanide-free brass-tin electroplating process provided in this application are as follows: Compared with the prior art, the cyanide-free brass-tin electroplating process proposed in this application effectively solves the problems of insufficient gloss, poor flexibility, and high porosity of traditional basic copper carbonate systems by optimizing the plating solution formulation. The active groups of nicotinic acid in the brightener adsorb onto the active sites of the plating layer, slowing down the reduction rate of metal ions, refining the plating grains, and improving the brightening effect and reducing porosity of the plating layer. Polyethyleneimine in the brightener can adsorb onto the plating surface, promoting the oriented arrangement of grains, optimizing the crystal structure of the plating layer, and reducing defects; under external stress, it can disperse stress along the uniform crystal structure, avoiding local overload, thereby improving the flexibility of the plating layer. At the same time, the polyethyleneimine polymer chains can effectively disperse tiny impurities in the plating solution, preventing impurities from adhering and forming stains, and also preventing impurities from forming hard spots, voids, and other rigid defects in the plating layer, thus improving the flexibility of the plating layer. Alkyl polysaccharide wetting agents can reduce the surface tension of the plating solution, enhance the wettability of the workpiece surface, avoid pinholes in the coating caused by trapped air bubbles, reduce the porosity of the coating, make the plating solution wet the workpiece evenly, and improve the smoothness of the coating. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A flow chart of a cyanide-free brass-tin electroplating process is provided for an embodiment of this application. Detailed Implementation

[0019] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with embodiments of the present invention. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this application.

[0020] The basic copper carbonate system in cyanide-free brass-tin electroplating still suffers from problems in practical applications, such as insufficient gloss, poor flexibility, and low leveling, making it difficult to meet the requirements of artificial jewelry for delicate appearance and moldability. The plating has high porosity and weak corrosion resistance, even inferior to traditional bright nickel plating, failing to provide long-term protection for the substrate. Furthermore, some processes still rely on nickel plating to enhance adhesion or corrosion resistance, but nickel metal easily causes skin sensitization, limiting its application in jewelry where products come into direct contact with the skin. The plating thickness of this system is limited; thicker plating easily leads to gloss decay and cracking, further restricting its application in scenarios with high corrosion resistance requirements.

[0021] To address the aforementioned problems, this application provides a cyanide-free brass-tin electroplating process, comprising the following steps: S1. Add basic copper carbonate, zinc oxide, sodium stannate, conductive salt, potassium hydroxide, and potassium carbonate to the plating tank and dissolve them in deionized water to obtain the basic plating solution. S2. Add brightener, complexing agent, and wetting agent to the base plating solution, and add deionized water to the set liquid level of the plating solution to obtain the plating solution required for electroplating. S3. Place the workpiece into the plating solution for electroplating, and control the metal ion concentration, current density and temperature of the plating solution within the predetermined range. The brightener comprises 0.5% nicotinic acid, 1% polyethyleneimine, and 98.5% deionized water by mass ratio; The wetting agent is a 0.5% (w / w) alkyl polysaccharide solution.

[0022] It should be noted that step S1 involves preparing the basic plating solution by dissolving components such as basic copper carbonate, zinc oxide, sodium stannate, conductive salt, potassium hydroxide, and potassium carbonate in deionized water, providing a basic solution system containing metal ions such as copper, zinc, and tin for subsequent electroplating. Step S2 refines the plating solution formula based on the basic solution by adding specific proportions of brighteners, complexing agents, and wetting agents, and adjusting the liquid level to optimize the plating solution performance, ensuring solution stability, reducing plating defects, and improving the brightness and smoothness of the plating layer. Step S3 places the workpiece into the prepared plating solution. Under the action of current, the metal ions in the plating solution will deposit on the surface of the workpiece to form a plating layer. Controlling the metal ion concentration, current density, and temperature of the plating solution within a predetermined range is to ensure that the plating layer has uniform thickness, stable composition, good adhesion, and other properties that meet the requirements.

[0023] Basic copper carbonate serves as the core copper source. In an alkaline plating bath, it dissolves efficiently through the coordination of a complexing agent, forming a stable water-soluble copper-complexing agent complex. This complex, after cathodic reduction, allows copper to be uniformly deposited in elemental form, providing the core copper component for the brass-tin plating layer. The presence of copper not only forms the basis for the excellent conductivity and ductility of the plating layer but also works synergistically with zinc and tin to establish the framework of the dense structure of the plating layer, making it a key substance ensuring the basic mechanical properties of the plating layer. Zinc oxide in the alkaline system generates zincate ions (ZnO2). 2- ZnO2 2- Further, it forms a stable complex with the complexing agent, migrates directionally to the cathode surface under the action of an electric field, and transforms into elemental zinc through dissociation and reduction reactions, which is then deposited as an alloying element in the coating construction. The introduction of zinc directly affects the mechanical properties (such as hardness, wear resistance, and adhesion) and corrosion resistance of the coating, and is one of the key components for controlling the overall performance of the coating. Sodium stannate, as the source of tin in the coating composition, hydrolyzes and complexes in the alkaline plating solution to form stable stannate ions (SnO3). 2- The complexing agent will react with SnO3. 2- Further, a more stable complex is formed, which migrates directionally to the cathode surface under the drive of an electric field, and is converted into elemental tin through an electrochemical reduction reaction and deposited. The addition of tin can improve the hardness, wear resistance and weldability of the coating.

[0024] Among these, the ions dissociated from conductive salts can reduce the resistance of the plating solution, decrease energy loss during electroplating, and ensure uniform current distribution, preventing areas of the workpiece without plating or with uneven plating. Potassium hydroxide provides a strongly alkaline environment, ensuring the stable dissolution and dissociation of metal salts such as basic copper carbonate and sodium stannate into metal ions; it adjusts the pH of the plating solution to a suitable range, ensuring normal deposition of metal ions; and it enhances the conductivity of the plating solution, reducing energy loss. Potassium carbonate, as a weakly alkaline buffer, stabilizes the pH of the plating solution, preventing drastic fluctuations in alkalinity caused by reactions or operations; it helps improve the conductivity of the plating solution, optimizes current distribution, and also improves the microstructure of the plating layer, reducing defects.

[0025] In this process, the nicotinic acid in the brightener ionizes in the alkaline plating bath to form nicotinic acid ions (C6H4NO). 2-The pyridine ring (containing a nitrogen atom with a lone pair electron) and the polar oxygen atom derived from the carboxyl group can adsorb onto the metal active sites in the high current density region of the workpiece, slowing down the reduction rate of the metal-complexing agent ligand in that region. This prevents metal atoms from accumulating too quickly, forcing these metals to grow in a finer grain morphology, reducing grain boundary gaps, lowering the coating porosity, and thus enhancing the reflectivity of the coating surface to light, resulting in a bright effect. Nicotinic acid, as a small molecule organic additive, requires strict dosage control at a low level (0.5% of the brightener). If the concentration is too high, it will be over-adsorbed on the cathode surface of the workpiece, strongly inhibiting the metal ion reduction reaction, leading to a slow coating deposition rate and insufficient thickness. If the concentration is too low, it cannot effectively refine the grains, making it difficult to achieve a bright effect. Polyethyleneimine can synergistically work with nicotinic acid to regulate the deposition uniformity in low current density regions, improving the overall brightness consistency of the coating. Polyethyleneimine's molecular chains contain multiple highly polar amino groups (-NH2), which can adsorb onto polar adsorption sites in low current density areas of the workpiece. The interchain gaps naturally formed by the unfolded polymer chains create deposition channels for metal ions, which then deposit in an orderly manner along these channels, resulting in more regular and uniformly sized crystal particles in the coating. When external stress is applied to the coating, it can be dispersed along the uniform crystal structure, avoiding localized stress overload and increasing the coating's flexibility. Simultaneously, the polyethyleneimine polymer chains effectively disperse tiny impurities in the plating solution, preventing impurities from adhering and forming spots, and also preventing impurities from forming rigid defects (such as hard spots or voids) in the coating. These defects become weak areas of flexibility under stress, easily leading to microcrack propagation. 1% polyethyleneimine in the brightener allows molecules to form a uniform and appropriately thick adsorption layer in low current density areas of the workpiece, avoiding both insufficient adsorption due to excessively low concentration and hindering metal ion deposition due to excessively high concentration. 98.5% deionized water, through a high-percentage aqueous phase environment, uniformly dissolves or suspends all components of the brightener.

[0026] The wetting agent is a 0.5% (w / w) solution of alkyl polyglycoside (APG). The hydrophobic chain (alkyl group) of the alkyl polyglycoside molecule faces the air, while the hydrophilic chain (polysaccharide) faces the plating solution, forming an oriented monomolecular film on the plating solution surface, significantly reducing the surface tension of the plating solution. This allows the plating solution to quickly wet difficult-to-wet areas such as tiny pores and grooves on the workpiece surface, avoiding the formation of air bubbles. The absence of air bubbles ensures uniform wetting, improving the smoothness of the coating. Simultaneously, good wettability promotes the uniform diffusion of metal ions across all areas of the workpiece surface, reducing coating thickness differences caused by insufficient local ion supply, ensuring uniform coverage of the coating across all areas of the workpiece, and further improving the leveling effect. Alkyl polyglycoside is a surfactant, and its concentration must be strictly controlled at a low level (0.5%). The critical micelle concentration of surfactants is usually low. Once the critical micelle concentration is exceeded, excess molecules will form micelles, which not only wastes the agent but also causes a large amount of foam to be generated in the plating solution. If the concentration is below 0.1%, it cannot effectively reduce the surface tension of the plating solution and the wettability is insufficient.

[0027] In another embodiment of this application, the base plating solution is a mixture of 35-45 g / L basic copper carbonate, 3-5 g / L zinc oxide, 8-12 g / L sodium stannate, 110-130 g / L conductive salt, 18-22 g / L potassium hydroxide, and 8-12 g / L potassium carbonate.

[0028] It should be noted that 35-45 g / L of basic copper carbonate can stably provide sufficient copper ions (Cu). 2+ To ensure the copper content in the coating is appropriate, a low concentration will lead to Cu... 2+ Insufficient supply results in low copper content in the plating, affecting the plating density; excessively high concentration may lead to Cu... 2+ Too rapid deposition results in a rough coating or uneven composition. A zinc oxide concentration of 3-5 g / L allows for precise control of ZnO2. 2- The concentration of zinc in the coating should be appropriate to ensure sufficient zinc incorporation. Excessive concentration leads to over-deposition of zinc, increasing the brittleness of the coating; insufficient concentration results in inadequate zinc content, hindering its ability to improve coating performance. 8-12 g / L sodium stannate provides an appropriate amount of SnO3. 2-To ensure a proper tin content in the plating solution, the concentration must be appropriate. Too low a concentration will result in insufficient tin content and decreased plating performance; too high a concentration will cause preferential tin deposition, disrupting the alloy balance of copper, zinc, and tin. A conductive salt concentration of 110-130 g / L significantly reduces the resistance of the plating solution, ensuring uniform current distribution on the workpiece surface. Insufficient concentration leads to poor conductivity, increased energy consumption, and uneven plating; excessive concentration may induce salt crystallization, compromising the stability of the plating solution. A potassium hydroxide concentration of 18-22 g / L maintains a strongly alkaline environment in the plating solution, ensuring the stable dissolution of basic copper carbonate, sodium stannate, and other metal salts, while providing suitable pH conditions for ion migration. Too low a concentration will cause metal salt precipitation and plating solution failure; too high a concentration may accelerate substrate corrosion or cause defects such as pinholes and roughness in the plating. Potassium carbonate at a concentration of 8-12 g / L acts as a weak alkaline buffer. This concentration can stabilize the pH of the plating solution (counteracting pH fluctuations during the reaction) and also help improve conductivity. Insufficient concentration results in weak buffering capacity and easy pH fluctuations. Excessive concentration may lead to an imbalance in the ionic strength of the plating solution, affecting the metal ion deposition rate.

[0029] In another embodiment of this application, the conductive salt comprises, by mass ratio, 1.5% copper sulfate, 2% zinc oxide, 15% potassium hydroxide, and 79.5% sodium chloride.

[0030] It should be noted that 1.5% copper sulfate, as a secondary copper ion supplement source, can assist the basic copper carbonate in the base plating bath in providing Cu. 2+ Meanwhile, its dissociated Cu 2+ Sulfate ions enhance the ionic strength of the plating bath and improve conductivity. The low proportion of copper sulfate is to avoid excessive copper ions leading to a high copper content in the plating layer, which would disrupt the alloy balance of copper, zinc, and tin. 2% zinc oxide serves as a secondary zinc ion source, assisting the zinc oxide in the base plating bath in providing ZnO2. 2- This further enhances the conductivity of the plating solution. The proportion is still controlled at a low level to prevent excessive zinc ion deposition that could cause coating brittleness. 15% potassium hydroxide dissociates into a large number of potassium and hydroxide ions, which significantly improves the conductivity of the plating solution while maintaining its strong alkaline environment, promoting the dissolution of salts such as copper sulfate and zinc oxide, and neutralizing any acidic substances that may be generated during electroplating, thus stabilizing the pH. 79.5% sodium chloride is the core component of the conductive agent; the dissociated sodium and chloride ions are highly conductive and stable, significantly reducing the resistance of the plating solution and ensuring uniform current distribution on the workpiece surface. Simultaneously, chloride ions do not react adversely with other metal ions in the plating solution, and sodium ions do not interfere with the coating composition, maximizing conductivity without interfering with the alloy ratio.

[0031] In another embodiment of this application, activated carbon treatment is performed after the base plating solution is prepared.

[0032] It should be noted that activated carbon treatment is performed after the base plating solution is prepared. The porous structure and surface properties of activated carbon adsorb and remove various impurities from the base plating solution to ensure the quality of subsequent electroplating. Activated carbon can adsorb organic residues carried by the raw materials and organic pollutants introduced from the environment, preventing these impurities from hindering metal ion deposition and causing defects such as pinholes, pitting, and dark spots in the plating layer. Simultaneously, the removal of impurities reduces side reactions such as hydrolysis and oxidation within the plating solution, avoiding turbidity and fluctuations in metal ion concentration, improving the stability of the plating solution system, ensuring controllable subsequent electroplating processes, and ultimately obtaining a high-quality brass-tin plating layer with uniform thickness and consistent composition.

[0033] In another embodiment of this application, the plating solution uses a base plating solution as a substrate and further comprises 0.4-0.6 ml / L of brightener, 9-11 ml / L of complexing agent, and 1.8-2.2 ml / L of wetting agent; The complexing agent is a 15% sodium citrate dihydrate solution.

[0034] It should be noted that a brightener concentration of 0.4-0.6 ml / L can prevent excessive adsorption of brightener components onto the workpiece surface. If the brightener concentration is too high, it will inhibit the reduction rate of metal ions, resulting in slower coating deposition and insufficient thickness. If the brightener concentration is too low, it will not effectively refine the coating grains, resulting in a weak brightening effect.

[0035] Complexing agents can slowly dissociate and release free metal ions, keeping the concentration of free ions in the plating bath consistently low and stable. This prevents excessively rapid deposition and rough coatings caused by locally high metal ion concentrations, ensuring a uniform and controllable deposition process. A concentration of 9-11 ml / L ensures a balance between the complexing ability of the complexing agent and the ion release rate: too low a concentration results in insufficient complexation, leading to easy precipitation of metal ions and plating bath failure; too high a concentration results in over-complexation, leading to excessively low free ion concentrations, significantly reducing the coating deposition rate, and even preventing the formation of a continuous coating. The carboxyl group (-COO) in sodium citrate... - It can be used with Cu 2+ ZnO2 2- SnO3 2- Forms stable soluble complexes (e.g., [Cu(C6H5O7)]) - This prevents these metal ions from forming precipitates such as copper hydroxide and zinc hydroxide in the alkaline plating solution, maintaining the clarity and stability of the plating solution; a concentration of 15% provides sufficient citrate ions to ensure the interaction of Cu in the plating solution. 2+ ZnO2 2- SnO3 2- They combine completely to form a stable, soluble chelate.

[0036] A wetting agent concentration of 1.8-2.2 ml / L can effectively reduce the surface tension of the plating solution, ensuring that the solution can quickly wet the workpiece surface. If the wetting agent concentration is too high, the plating solution is prone to generating a large number of bubbles, which, when adhering to the workpiece surface, will also cause defects. If the wetting agent concentration is too low, the wettability will be insufficient, and bubbles will remain.

[0037] In another embodiment of this application, in step S3, the concentration of the metal ions is controlled in the following manner: The total copper content in the plating solution was controlled to be 10-18 g / L, the total tin content to be 4-8 g / L, the total zinc content to be 2-4 g / L, and the potassium ion concentration to be 10-25 g / L, respectively.

[0038] It should be noted that in alkaline plating solutions, copper, zinc, and tin exist as stable metal-complexing complexes. The concentrations mentioned above refer to the total content of the corresponding metal elements in the plating solution. This concentration is detected through a process of "breaking the complex to release free metal ions - converting to a suitable titration state - titration." This total content directly determines the amount of metal reduction and deposition during electroplating, thus affecting the composition ratio and overall performance of the coating. The final color of the coating is primarily determined by the ratio of copper, zinc, and tin. When the copper content is dominant (e.g., 50%-70%) and the zinc and tin contents are low, the warm yellow hue of copper is retained as a base, while the silvery-white of zinc and the bright white of tin neutralize the original purplish-red hue of copper, resulting in a soft and uniform warm golden-yellow coating. During electroplating, the reduction of metal ions is positively correlated with the ion concentration in the plating solution. Therefore, when the total copper content in the plating solution is within the reasonable range of 10-18 g / L, Cu... 2+ The reduction probability of copper ions is significantly higher than that of zinc and tin ions, thus forming a dominant proportion in the coating and providing a stable base for the warm golden yellow hue. If the total copper content is less than 10 g / L, it is easy to cause insufficient copper content in the coating, and the warm yellow will be pale or even lean towards the silvery white of zinc and tin; if it is higher than 18 g / L, the copper ions may be reduced too quickly, resulting in excessive local deposition on the workpiece, which will affect the uniformity of the coating.

[0039] Setting the concentrations of tin ions (4-8 g / L) and zinc ions (2-4 g / L) at lower levels prevents the silvery-white tones of tin and zinc from masking the warmth of copper, thus neutralizing the purplish-red hue of copper and resulting in a soft, warm golden-yellow finish rather than the purplish-red of pure copper. Zn 2+ Sn 4+ Setting the concentration can prevent insufficient reinforcement of coating performance due to too low a concentration, or damage to the color balance of the coating due to too high a concentration.

[0040] Furthermore, a potassium ion concentration of 10-25 g / L can regulate ionic strength and inhibit the hydrolysis of low-concentration metal ions. Large amounts of K... +It can form an ionic atmosphere around the metal and complexing agent coordination complex through electrostatic interactions, reducing the activity of the dissociated metal ions and decreasing their interaction with OH groups. - The probability of collision and combination. After the activity of these ions decreases, the hydrolysis equilibrium (e.g., Cu) is disrupted. 2+ When potassium ions react with water to form Cu(OH)2, they move in the reverse direction, thus inhibiting the formation of hydroxide precipitates. When the potassium ion concentration is below 10 g / L, the effect of inhibiting hydrolysis weakens, and the plating solution is prone to turbidity due to the hydrolysis of metal ions; when it is above 25 g / L, the ions may compete for adsorption sites on the workpiece surface, interfering with the normal reduction and deposition of metal on the workpiece surface.

[0041] Among them, Cu 2+ The concentration detection method is as follows: Take 2 ml of plating solution, add 50 ml of water and 1 g of ammonium persulfate, stir the mixture thoroughly, and then heat it to 45°C on an electric furnace. At this point, the mixture turns green. Add 10 ml of pH=10 buffer solution (such as ammonia-ammonium chloride buffer), shake well, and the mixture turns blue. Add 5 drops of 0.1% PAN indicator, and titrate the mixture with 0.1N EDTA standard solution. When the last drop of EDTA standard solution is added, the color of the mixture suddenly changes from dark blue to a stable bright green, and does not return to blue within 30 seconds of standing, indicating that the titration endpoint has been reached. Record the volume of EDTA standard solution consumed.

[0042] Cu 2+ Concentration (g / L) = 3.184 × titration volume (mL) Among them, Sn 4+ The concentration detection method is as follows: Take 5 ml of plating solution, add 50 ml of water, 20 ml of concentrated hydrochloric acid and 4 g of iron powder, heat until the mixture boils and then cool to room temperature, add 3-5 g of sodium hydrochloride and 5 ml of 1% starch indicator, and titrate with 0.1 N iodine solution. When the last drop of 0.1 N iodine solution is added, the mixture turns blue-black and does not turn back to blue within 30 seconds of standing, which means the titration endpoint has been reached. Record the volume of 0.1 N iodine solution consumed.

[0043] Sn 4+ Concentration (g / L) = 1.18 × titration volume (mL) Among them, Zn 2+ The concentration detection method is as follows: Take 5 ml of plating solution, add 50 ml of water, 0.5 g of ascorbic acid, 10 ml of pH=10 buffer solution, add an appropriate amount of EBT indicator (enough to make the mixture turn light blue), add 10 ml of 10% methanol solution, and titrate the mixture with 0.1 N EDTA standard solution. When the last drop of EDTA standard solution is added, the mixture changes from red to blue and does not turn back to red within 30 seconds, which means the titration endpoint has been reached. Record the volume of 0.1 N EDTA standard solution consumed.

[0044] Zn 2+ Concentration (g / L) = 1.308 × titration volume (mL) In another embodiment of this application, in step S3, the electroplating temperature is controlled to be 35°C-45°C, and the cathode current density is 0.25-3 A / dm³. 2 The anode current density is 0.25-2 A / dm³. 2 .

[0045] It should be noted that an electroplating temperature between 35℃ and 45℃ can balance ion migration rate and plating solution stability. Excessively high electroplating temperatures will accelerate the decomposition of additives (such as brighteners) and damage the plating solution composition; excessively low temperatures will slow metal ion diffusion, reduce the deposition rate, and easily lead to uneven coating thickness.

[0046] Furthermore, the cathode current density determines the reduction rate of metal ions on the workpiece (cathode) surface. (0.25-3 A / dm³) 2 Within this range, the electron transfer rate matches the metal ion migration rate, allowing metal ions to be orderly reduced to metal atoms and accumulate into a dense coating. Too low a cathode current density results in slow deposition and low efficiency; too high a density leads to increased hydrogen evolution at the cathode (hydrogen adhering and forming pinholes), or insufficient replenishment of metal ions, resulting in a rough coating and scorching. An anode current density of 0.25-2 A / dm² is used to adjust the anode reaction efficiency according to the anode type (soluble or inert) to ensure a stable metal ion concentration in the plating solution. For soluble anodes, this current density promotes stable dissolution of the anode metal; for inert anodes, it maintains normal conductivity and reaction (e.g., 4OH⁻). - -4e - = O 2 (↑+2H2O), avoid excessively high concentrations that could lead to anode oxidation and the formation of a passivation film, or excessively low concentrations that could result in a weak anode reaction.

[0047] In another embodiment of this application, every 3500 ampere-minutes of electroplating operation, 250 ml of supplement, 100 ml of brightener, 300 ml of complexing agent, 50 ml of wetting agent, 120 g of basic copper carbonate, 25 g of sodium stannate, 240 g of conductive salt and 60 g of potassium hydroxide are added to each liter of plating solution.

[0048] It should be noted that during the electroplating process, the deposition of metal ions, the loss of additives, and the consumption of basic components are all proportional to the total amount of electricity (ampere-minutes) passing through the plating solution. If the electroplating continues for 3500 ampere-minutes, the concentration of metal ions, basic components, and additives will be insufficient, and if they are not replenished in time, the quality of the coating will decline.

[0049] Metal ion replenishment: Cu in the plating bath 2+ As the coating layer continues to decrease, 120 g / L of dissolved basic copper carbonate can be precisely replenished to maintain Cu. 2+The process concentration is set to avoid insufficient copper content in the plating layer, which would result in a pale color; 25g / L sodium stannate is used to replenish the tin consumed during deposition, maintaining the set concentration of tin in the process and ensuring the hardness and wear resistance of the plating layer.

[0050] Additive Replenishment: As a comprehensive functional additive, the replenisher is consumed during the continuous electroplating process. Replenishing with 250ml / L of replenisher maintains the activation effect on the workpiece surface, ensuring initial coating adhesion. Brighteners are fixed on the coating surface or shallow layer during coating growth; excessive consumption leads to coarse coating grains and loss of luster. A replenishment of 100ml / L of brightener maintains the brightener concentration set in the process, ensuring a uniform and bright coating. Complexing agents continuously complex with metal ions. Those complexing agents adsorbed on the coating surface will leave the plating solution along with the workpiece and coating, gradually decreasing the complexing agent concentration. A replenishment of 300ml / L maintains the complexing agent concentration set in the process, ensuring the adhesion of Cu... 2+ Zn 2+ Sn 4+ The stable complexation avoids the hydrolysis and precipitation of metal ions; due to the continuous loss of the wetting agent when reducing the surface tension of the plating solution, insufficient concentration will lead to residual bubbles on the workpiece surface. A replenishment of 50ml / L can maintain the wetting agent concentration set in the process, ensure the wettability of the plating solution on the workpiece, and eliminate pinhole defects.

[0051] Basic component replenishment: The conductive salt concentration can decrease due to small amounts of waste liquid loss or crystallization, leading to a decline in the conductivity of the plating solution. Replenishing with 240 g / L of conductive salt maintains the concentration set in the process, ensuring uniform current distribution and preventing uneven coating thickness. During electroplating, the plating solution may experience side reactions (such as the hydrolysis of metal ions to generate H₂). + Consume OH - This leads to a decrease in alkalinity. Adding 60g / L potassium hydroxide can maintain the alkaline environment of the plating bath (to match the function of the complexing agent) and prevent the metal ion complexation balance from being disrupted by pH fluctuations.

[0052] In another embodiment of this application, the supplement is prepared as follows: Weigh out the conductive salt, potassium hydroxide, brightener, complexing agent or wetting agent, and add them to deionized water in sequence. Stir continuously until dissolved after each addition before adding the next component. The supplement includes 500 g / L conductive salt, 12 g / L potassium hydroxide, 50 ml / L brightener, 150 ml / L complexing agent or 25 ml / L wetting agent.

[0053] It should be noted that the conductive salt is added first because its high solubility provides a stable solution system for subsequent components. The high concentration of conductive salt (500 g / L) in the supplement allows for effective compensation of the conductive salt loss in the plating bath even when added in small amounts (e.g., a few hundred milliliters at a time). This avoids the need for large additions due to low supplement concentration, reducing interference with the overall volume of the plating bath and the concentration of other components.

[0054] Next, potassium hydroxide is added. The strong alkaline potassium hydroxide quickly adjusts the solution pH to the required range, creating a suitable environment for the complexing agent to complex with metal ions after being added to the plating bath. Simultaneously, stirring prevents potassium hydroxide from crystallizing or reacting adversely with other components due to excessively high local concentrations during subsequent additions. The 12 g / L concentration of potassium hydroxide in the supplement ensures that each addition of supplementary agent carries the OH- ions into the plating bath. - The amount is kept within a fine-tuning range to avoid a sudden increase in the pH of the plating solution due to excessively high potassium hydroxide concentration in the supplement, which could disrupt the complexation balance or cause metal ion precipitation with just a small amount of addition.

[0055] Finally, brighteners, complexing agents, or wetting agents are added to prevent premature contact with unstable solution environments and protect the activity of these functional additives. The effective concentration of brighteners in the plating bath is usually low; the 50ml / L concentration of brightener in the supplement is designed to precisely compensate for brightener loss in the plating bath with a small amount, avoiding the need for large additions due to low concentrations, which could lead to an instantaneous excess of brightener in the plating bath and affect coating quality. The high concentration of complexing agents (150ml / L) allows for rapid compensation for complexing agent loss in the plating bath with a small amount of supplement. The 25ml / L concentration of wetting agents is higher than their critical micelle concentration, effectively reducing the surface tension of the solution to improve the wettability of the plating bath on the workpiece, while avoiding foam interference or affecting coating adhesion due to excessive concentration.

[0056] In another embodiment of this application, when the substrate of the workpiece is nickel metal, a copper base plating layer is pre-plated first, and then a cyanide-free brass-tin electroplating process is performed.

[0057] It should be noted that a dense oxide layer easily forms on the surface of nickel metal. Direct cyanide-free brass-tin plating can easily lead to insufficient adhesion of the plating layer due to the blockage of the oxide layer. However, the pre-plated copper layer can form a good metallic bond with the nickel substrate workpiece. At the same time, the chemical activity of copper can effectively remove the trace oxide layer on the nickel surface, providing a clean and firmly bonded adhesion base for the subsequent brass-tin plating. Furthermore, the copper base plating has excellent ductility, which can fill in any minor scratches, dents, or other defects that may exist on the surface of the nickel-based workpiece, making the surface of the workpiece smoother. This avoids problems such as uneven thickness, pinholes, or poor local deposition in the subsequent cyanide-free brass-tin plating due to the unevenness of the nickel-based workpiece. At the same time, the electrochemical properties of copper and cyanide-free brass-tin plating are more similar, which can reduce the potential difference between the two and reduce the risk of stress concentration or corrosion caused by excessive potential difference during the electroplating process. In addition, the high conductivity of copper ensures that the current is evenly distributed on the workpiece surface during the subsequent cyanide-free brass-tin electroplating, further ensuring the crystal refinement, uniform appearance, and overall performance stability of the brass-tin plating, ultimately achieving high-quality deposition of cyanide-free brass-tin plating on the nickel-based workpiece.

[0058] In another embodiment of this application, in the electroplating step, the anode is a brass plate, the stirring method is mechanical shaking, and the plating solution is circulated and filtered.

[0059] It should be noted that the dissolution of the anolyte brass plate continuously replenishes the copper and zinc ions required for the cyanide-free brass-tin plating solution, maintaining a stable metal ion concentration and reducing the introduction of external impurities, thus ensuring uniform plating composition. The mechanical agitation method breaks up the plating solution diffusion layer on the electrode surface, accelerating the transport of metal ions to the workpiece surface. This prevents uneven plating thickness caused by excessively low local metal ion concentrations and reduces bubble adhesion to the workpiece surface, minimizing defects such as pinholes and pitting. Circulating and filtering the plating solution continuously removes metal debris, anolyte sludge, and organic impurities, maintaining solution cleanliness, preventing impurities from embedding in the plating layer and affecting its appearance and adhesion, and extending the service life of the plating solution.

[0060] Cyanide-free brass-tin plating produces coatings with high gloss, good flexibility, and excellent leveling effect. Due to its strong corrosion resistance, cyanide-free brass-tin plating is particularly suitable as a substitute for gold in the electroplating of artificial jewelry, or as a base coat before gold plating. In the electroplating of artificial jewelry, nickel is a traditionally used base metal, but cyanide-free brass-tin plating can completely replace nickel as a base coat. Furthermore, cyanide-free brass-tin plating has low porosity, providing superior corrosion resistance compared to bright nickel plating, and avoiding skin irritation caused by nickel. In addition, to achieve even better corrosion resistance, the maximum thickness of cyanide-free brass-tin plating can be increased to 10μm while still maintaining its original gloss. The cyanide-free brass-tin plating process can be directly applied to copper, bronze, or zinc workpieces.

[0061] The present invention will be further illustrated below through specific embodiments: Example 1 S1. Add basic copper carbonate, zinc oxide, sodium stannate, conductive salt, potassium hydroxide, and potassium carbonate to the plating tank and dissolve them in deionized water to obtain the basic plating solution. After the basic plating solution is prepared, treat it with activated carbon until the solution is clear. The basic plating solution contains 38 g / L basic copper carbonate, 3 g / L zinc oxide, 8 g / L sodium stannate, 110 g / L conductive salt, 18 g / L potassium hydroxide, and 8 g / L potassium carbonate. The conductive salt comprises, by mass ratio, 1.5% copper sulfate, 2% zinc oxide, 15% potassium hydroxide, and 79.5% sodium chloride.

[0062] S2. Add a brightener, a complexing agent, and a wetting agent to the base plating solution, and add deionized water to the set plating solution level to obtain the plating solution required for electroplating. The plating solution contains a base plating solution, 0.4 ml / L of brightener, 9 ml / L of complexing agent, and 1.8 ml / L of wetting agent. The brightener comprises 0.5% nicotinic acid, 1% polyethyleneimine, and 98.5% deionized water by mass ratio. The wetting agent is a 0.5% alkyl polysaccharide glycoside solution. The complexing agent is a 15% sodium citrate dihydrate solution.

[0063] S3. Place the workpiece into the plating solution for electroplating, and control the Cu concentration in the plating solution accordingly. 2+ Concentration 10g / L, Sn 4+ Concentration 4g / L, Zn 2 + Concentration 2g / L and K + Concentration 10 g / L, electroplating temperature controlled at 35℃, cathode current density 2 A / dm³ 2 The anode current density is 0.8 A / dm³. 2 The electroplating time is 16 minutes.

[0064] Example 2 S1. The base plating solution contains 40 g / L basic copper carbonate, 4 g / L zinc oxide, 10 g / L sodium stannate, 120 g / L conductive salt, 20 g / L potassium hydroxide and 10 g / L potassium carbonate.

[0065] S2. The plating solution contains a base plating solution, 0.5 ml / L brightener, 10 ml / L complexing agent, and 2 ml / L wetting agent.

[0066] S3. Immerse the workpiece in the plating solution for electroplating. Control the concentrations of copper ions (14 g / L), tin ions (4.5 g / L), zinc ions (3 g / L), and potassium ions (20 g / L) in the plating solution. Control the electroplating temperature at 40℃ and the cathode current density at 2 A / dm³. 2 The anode current density is 1 A / dm³ 2 The electroplating time is 16 minutes.

[0067] Example 3 S1. The base plating solution contains 42 g / L basic copper carbonate, 5 g / L zinc oxide, 12 g / L sodium stannate, 130 g / L conductive salt, 22 g / L potassium hydroxide and 12 g / L potassium carbonate.

[0068] S2, the plating solution contains 0.6 ml / L of brightener, 11 ml / L of complexing agent, and 2.2 ml / L of wetting agent.

[0069] S3. Immerse the workpiece in the plating solution for electroplating. Control the concentrations of copper ions (16 g / L), tin ions (5 g / L), zinc ions (4 g / L), and potassium ions (25 g / L) in the plating solution. Control the electroplating temperature at 45℃ and the cathode current density at 2 A / dm³. 2 The anode current density is 1.5 A / dm³. 2 The electroplating time is 16 minutes.

[0070] Comparative Example: The electroplating nickel process in this comparative example differs from the cyanide-free brass-tin electroplating process in Example 1 in that the brightener consists of 5% nicotinic acid and 95% deionized water by mass, and the wetting agent consists of a 0.4% sodium dodecylbenzenesulfonate solution by mass.

[0071] Experimental sample preparation: A uniform 100mm×50mm×0.3mm brass sheet (H62) was used, and pre-treated by degreasing, pickling, and activation (to ensure that the surface was free of oxide layer and impurities). Four groups of coating samples were prepared according to the process parameters of Examples 1-3 and the comparative example, and three parallel samples were prepared for each group. By adjusting the electroplating time, the coating thickness of all samples was made to be 7-9μm (measured with an eddy current thickness gauge, and the average value of three points was taken).

[0072] (a) Flexibility test Test principle: The coating’s resistance to cracking when the substrate is deformed is evaluated by bending test. The better the flexibility, the less likely the coating is to crack or peel off.

[0073] Experimental steps: Gently sand the edges of the sample with sandpaper (to avoid edge burrs affecting the test); using a cylindrical shaft bending tester, select a cylindrical shaft with a diameter of 3mm (simulating moderate bending), fix the sample on the tester with the coating facing outward, and bend it 180° around the cylindrical shaft at a speed of 5mm / min to form a U-shape; observe the coating surface of the bent part with an optical microscope (100x magnification), and record whether cracks appear and the number of cracks; replace with a cylindrical shaft with a diameter of 1mm (simulating severe bending), repeat the above bending steps, and record the results.

[0074] Grading criteria: Grade 1 (no visible defects), Grade 2 (minor cracking without peeling), Grade 3 (cracked area >10% or localized peeling) Table 1. Data related to coating flexibility test Results Analysis: The example, containing polyethyleneimine (a brightener component), exhibited stronger adhesion between the coating and the substrate, and significantly better flexibility than the comparative example (which only contained nicotinic acid and lacked toughening components). Polyethyleneimine promotes grain orientation and optimizes the coating's crystal structure. Under external stress, it can disperse stress along the uniform crystal structure, avoiding localized overload and reducing crack formation. In contrast, while nicotinic acid in the comparative example promoted coating crystal refinement, it failed to provide an effective stress buffering mechanism, leading to significant cracking and peeling of the coating during severe bending due to stress release.

[0075] (ii) Gloss test Test principle: The reflectivity of the coating surface to light at a specific angle is measured by a gloss meter. The higher the reflectivity, the better the gloss.

[0076] Experimental procedure: Wipe the surface of each sample with anhydrous ethanol (to remove oil and fingerprints) and let it air dry naturally; use a 20° angle gloss meter to randomly select 5 test points on the sample surface (avoiding the 10mm edge area); calibrate the instrument with a standard plate before each measurement, measure each point 3 times, take the average value as the gloss value of that point, calculate the standard deviation to determine the gloss uniformity, and the unit is gloss unit (GU).

[0077] Table 2. Data related to coating gloss test Results Analysis: The examples, due to the addition of polyethyleneimine to the brightener (enhancing adsorption uniformity), exhibited superior gloss and uniformity compared to the comparative example (which only contained nicotinic acid, resulting in a singular brightening effect). This is because the amino groups on the polyethyleneimine molecular chain can form stable coordination bonds with metal ions on the coating surface, leading to more uniform adsorption of the brightener and reducing gloss fluctuations caused by local adsorption differences. In contrast, the nicotinic acid in the comparative example primarily enhances gloss through a single leveling effect, lacking the ability to regulate adsorption uniformity. This results in more diffuse reflection when the coating surface reflects light, leading to lower overall gloss and poorer uniformity. Data shows that the average gloss of the example group exceeded 90 GU, and the standard deviation of gloss uniformity was less than 1.1 GU, indicating excellent reflective properties and good consistency of the coating surface. The average gloss of the comparative example group was only 77.0 GU, with a standard deviation of 1.6 GU, further validating the significant advantage of adding polyethyleneimine in improving the gloss quality of the coating.

[0078] (III) Leveling test Test principle: By electroplating on a substrate with standard scratches, the ability of the coating to fill surface defects is evaluated. The better the filling, the less noticeable the scratches.

[0079] Experimental Procedure: During substrate pretreatment, standard scratches (approximately 5 μm depth, 10 μm width, and 20 mm length) were created on the surface using a diamond engraving tool. After electroplating according to the process, a sample was taken and the scratched area was scanned using a white light interferometer (scanning range 30 μm × 30 μm). The plating thickness (H1) at the scratch and the plating thickness (H2) of the surrounding normal area were measured, and the fill degree was calculated. Fill percentage (%) = (H1 / H2) × 100% (the closer the value is to 100%, the better the fill percentage).

[0080] Table 3. Data related to coating leveling test Results Analysis: Table 3 shows that the leveling percentages of Examples 1 to 3 reached 95.0%, 95.6%, and 96.3%, respectively, all at a high level. This indicates that the combination of alkyl polysaccharide wetting agent and brightener effectively promotes uniform deposition of the coating in the scratch area during electroplating. In contrast, the leveling percentage of the comparative group was only 63.8%, far lower than that of the Examples, further verifying that when sodium dodecylbenzenesulfonate is used as a wetting agent, its insufficient wettability and tendency to generate bubbles lead to poor coverage of the coating at the scratch area. The Examples, using alkyl polysaccharide as a wetting agent (high wettability inhibits bubble generation), exhibited better wettability of the plating solution on the scratches, and with the synergistic effect of the brightener, the leveling percentage was significantly better than that of the comparative group.

[0081] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A cyanide-free copper-tin electroplating process characterized in that, The method comprises the following steps: S1, adding basic copper carbonate, zinc oxide, sodium stannate, conductive salt, potassium hydroxide and potassium carbonate into a plating tank to dissolve in deionized water to obtain a basic plating solution; S2, adding brightener, complexing agent and wetting agent into the basic plating solution, adding deionized water to the set plating solution level to obtain a plating solution required for electroplating; S3, placing a workpiece into the plating solution to perform electroplating, and controlling the metal ion concentration, current density and temperature of the plating solution within a predetermined range; The brightener comprises 0.5% nicotinic acid, 1% polyethyleneimine and 98.5% deionized water by mass ratio; The wetting agent is an alkyl polyglycoside solution with a mass concentration of 0.5%.

2. A cyanide-free copper-tin electroplating process as claimed in claim 1, wherein, The basic plating solution is a mixed solution of 35-45 g / L basic copper carbonate, 3-5 g / L zinc oxide, 8-12 g / L sodium stannate, 110-130 g / L conductive salt, 18-22 g / L potassium hydroxide and 8-12 g / L potassium carbonate.

3. A cyanide-free copper-tin electroplating process as claimed in claim 2, wherein, The conductive salt comprises 1.5% copper sulfate, 2% zinc oxide, 15% potassium hydroxide and 79.5% sodium chloride by mass ratio.

4. A cyanide-free copper-tin electroplating process according to any one of claims 1 to 3, characterized in that: The basic plating solution is treated with activated carbon after preparation.

5. A cyanide-free copper-tin electroplating process as claimed in claim 1, wherein, The plating solution further comprises 0.4-0.6 ml / L brightener, 9-11 ml / L complexing agent and 1.8-2.2 ml / L wetting agent based on the basic plating solution; The complexing agent is a sodium citrate dihydrate solution with a mass concentration of 15%.

6. A cyanide-free copper-tin electroplating process as claimed in claim 1, wherein, In step S3, the metal ion concentration is controlled in the following manner: The total content of copper element in the plating solution is controlled to be 10-18 g / L, the total content of tin element is controlled to be 4-8 g / L, the total content of zinc element is controlled to be 2-4 g / L, and the potassium ion concentration is controlled to be 10-25 g / L.

7. A cyanide-free copper-tin electroplating process as claimed in claim 1, wherein, In step S3, the plating temperature is controlled to be 35-45°C, the cathode current density is 0.25-3 A / dm 2 , and the anode current density is 0.25-2 A / dm 2 .

8. A cyanide-free copper-tin electroplating process as claimed in claim 1, wherein, 250 ml of replenisher, 100 ml of brightener, 300 ml of complexing agent, 50 ml of wetting agent, 120 g of basic copper carbonate, 25 g of sodium stannate, 240 g of conductive salt and 60 g of potassium hydroxide are added to each liter of plating solution per 3500 ampere minutes of electroplating work.

9. A cyanide-free copper-tin electroplating process as claimed in claim 8, wherein, The replenisher is prepared in the following manner: The conductive salt, potassium hydroxide, brightener, complexing agent or wetting agent are weighed and added into deionized water in sequence, and each component is continuously stirred until dissolved before adding the next component; The replenisher comprises 500 g / L conductive salt, 12 g / L potassium hydroxide, 150 ml / L complexing agent or 25 ml / L wetting agent.

10. A cyanide-free copper-tin electroplating process as claimed in claim 1, wherein, When the substrate of the workpiece is nickel metal, a copper bottom plating layer is first pre-plated, and then the cyanide-free brass tin electroplating process is performed.

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