Chromium-free passivation solution for tin coating of methanesulfonic acid system as well as preparation method and application of chromium-free passivation solution

Abstract

The invention belongs to the technical field of metal surface treatment, and particularly relates to a chromium-free passivation solution for a methanesulfonic acid system tin coating as well as a preparation method and application of the chromium-free passivation solution. The invention provides a chromium-free passivation solution for a methanesulfonic acid system tin coating. The chromium-free passivation solution comprises an antioxidant A agent and an antioxidant B agent, the antioxidant A agent comprises a film-forming agent, a pH regulator and pure water; the antioxidant B comprises an additive and diluted alcohol; the volume ratio of the antioxidant A agent to the antioxidant B agent is (5-9): (5-1). The chromium-free passivation solution for the methanesulfonic acid system tin coating can rapidly form a film on the tin surface, a compact chemical conversion film can be formed within 1-5 min, the tin surface is isolated from air and free of air oxidation, and the chromium-free passivation solution has excellent sulfur resistance and acid resistance. The chromium-free passivation solution for the methanesulfonic acid system tin coating does not contain chromium and is good in environmental protection property, and sewage treatment is easy.

Description

Technical Field

[0001] This invention belongs to the field of metal surface treatment technology, specifically relating to a chromium-free passivation solution for tin plating in a methanesulfonic acid system, its preparation method and application, and its preparation and application. Background Technology

[0002] Tin plating, due to its excellent solderability, corrosion resistance, and non-toxic properties, is widely used in electronic components, food packaging cans (such as aluminum cans and tin cans), automotive parts, and electrical materials. Traditional tin plating processes often use phenolsulfonic acid (PSA) as the base system for the plating solution. However, the PSA system poses significant environmental and health risks: residual phenolic substances in the system are biotoxic and persistent in the environment; volatile irritating gases are easily released during production; and wastewater treatment is complex and costly. With increasingly stringent global environmental regulations (such as the EU RoHS and REACH regulations restricting hazardous substances) and the widespread adoption of green manufacturing concepts, the PSA system, due to its inherent environmental drawbacks, is being gradually phased out and its use restricted in mainstream tin plating applications (especially food contact materials).

[0003] Although the MSA (Metal Sulfate Acid) tin plating process has matured, the post-plating passivation process still faces significant challenges. Compared to traditional tin plating systems, the MSA system offers advantages such as faster deposition rates, higher current efficiency, better solution stability, and better environmental friendliness. The resulting tin plating typically has finer grains, higher density, and lower internal stress. However, precisely because of the high surface activity, dense microstructure, and large surface energy of the MSA tin plating layer, it is highly susceptible to oxidation in air, placing higher demands on the uniformity, adhesion, and density of the passivation film. Furthermore, residual organic additives or trace metal impurities in the MSA system can interfere with the passivation film formation process, making it difficult for traditional passivation processes to build a stable, continuous protective film with excellent anti-discoloration capabilities on the MSA tin plating surface. To improve the oxidation resistance and subsequent soldering performance of the tin layer, tin-plated parts must undergo passivation treatment, but traditional passivating agents suffer from long-standing technical problems such as insufficient density, poor adhesion, and weak anti-discoloration capabilities. Currently, the industry commonly uses hexavalent chromium (Cr)-containing passivating agents. 6+ Chromate passivation solutions are commonly used. While these passivation solutions provide good protection, hexavalent chromium is an internationally recognized high-risk carcinogen and a strong environmental pollutant, and its use and emission are strictly restricted. Therefore, developing a dedicated chromium-free passivation technology for MSA tin plating layers not only needs to meet environmental protection requirements but also needs to solve the unique challenges of its interfacial reaction kinetics and film bonding mechanisms. Developing environmentally friendly chromium-free passivation solutions suitable for high-performance MSA tin plating layers has become an urgent industry need and a key focus of technological research. Summary of the Invention

[0004] The purpose of this invention is to provide a chromium-free passivation solution for tin plating using a methanesulfonic acid system, its preparation method, and its application. The chromium-free passivation solution for tin plating using a methanesulfonic acid system provided by this invention can rapidly form a film on the surface of the tin plating layer, forming a dense chemical conversion film within 1-5 minutes. This isolates the tin surface from air, preventing oxidation, and provides excellent resistance to sulfur and acids. Furthermore, the chromium-free passivation solution for tin plating using a methanesulfonic acid system provided by this invention is chromium-free, environmentally friendly, and easy to treat wastewater.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A chromium-free passivation solution for tin plating in a methanesulfonic acid system comprises: Agent A and Agent B; Agent A includes a film-forming agent, a pH adjuster, and pure water; Agent B includes an additive and a dilute alcohol.

[0006] This invention addresses the rapidly developing tin plating technology based on methanesulfonic acid (MSA) systems by developing a chromium-free passivation solution that is highly efficient, stable, environmentally friendly, and adaptable to various processes. This is of great significance for promoting the green upgrading of the tin plating industry and meeting stringent environmental regulations and end-application requirements.

[0007] The film-forming agents (such as phytic acid, tannic acid, ammonium fluorozirconate, etc.) contained in Agent A of this invention, under the promotion of Agent B, can undergo coordination complexation reactions with the tin surface to form a dense, continuous, and chemically stable organic-inorganic hybrid passivation film. After this film covers the surface of the tin plating layer, it significantly reduces the number of exposed highly active tin atoms, thereby lowering the overall surface free energy and greatly reducing the possibility of oxidation in air. Furthermore, due to the strong coordination ability and rapid adsorption kinetics of specific additives in Agent B (such as 2-amino-2-thiazoline, sodium N,N-dimethyldithiocarbonylpropanesulfonate), they preferentially adsorb onto the active sites on the tin surface, not only accelerating the nucleation of the passivation film but also "shielding" sites that may be occupied by impurities, thus repelling the adsorption of organic contaminants (such as brightener and wetting agent residues from previous processes). The combined use of Agent A and Agent B further improves the uniformity, adhesion, and density of the passivation film, providing more comprehensive protection for the tin plating layer.

[0008] Preferably, the volume ratio of agent A to agent B is 5:9~1.

[0009] The chromium-free passivation solution for tin plating using a methanesulfonic acid system provided by this invention comprises antioxidant A and antioxidant B. Antioxidant A includes a film-forming agent, a pH adjuster, and pure water. Antioxidant B includes additives and dilute alcohol. The volume ratio of antioxidant A to antioxidant B is (5-9):(5-1). The chromium-free passivation solution for tin plating using a methanesulfonic acid system provided by this invention can rapidly form a film on the tin surface, creating a dense chemical conversion film within 1-5 minutes. This isolates the tin surface from air, preventing oxidation, and provides excellent resistance to sulfur and acids. Furthermore, the chromium-free passivation solution for tin plating using a methanesulfonic acid system provided by this invention is chromium-free, environmentally friendly, and easy to treat wastewater.

[0010] Preferably, the film-forming agent includes one or more of phytic acid, tannic acid, ammonium fluorozirconate, ammonium fluorotitanate, and potassium titanium oxalate; the pH adjuster includes one or more of methanesulfonic acid, phosphoric acid, and citric acid; the additive includes one or more of 2-amino-2-thiazoline and sodium N,N-dimethyldithiocarbonylpropanesulfonate; the dilute alcohol includes an alcohol and water mixed at a mass ratio of 5-50:100; the alcohol is selected from one or more of methanol, ethanol, propylene glycol, and n-propanol.

[0011] Tin metal has fundamental differences in electronic structure, surface oxidation behavior and coordination characteristics compared to other common electroplating metals. Tin is an amphoteric metal, and its natural oxide layer is mainly composed of SnO and SnO2. It is also prone to uneven oxidation and grain boundary corrosion under weakly acidic to neutral conditions. At the same time, the surface of tin plating layer in the methanesulfonic acid system has a large number of residual organic brightener molecules, which makes it difficult for traditional corrosion inhibitors to effectively penetrate and anchor to active sites.

[0012] This invention is the first to apply 2-amino-2-thiazoline and sodium N,N-dimethyldithiocarbonylpropanesulfonate to the chromium-free passivation process of tin plating in a methanesulfonic acid system. It was found that the two can not only be efficiently adsorbed on the tin surface, but also have a significant synergistic effect with film-forming agents (such as ammonium fluorotitanate, phytic acid, etc.) to form a dense, continuous composite passivation film with both organic and inorganic properties, thereby achieving an antioxidant performance improvement far exceeding expectations.

[0013] Specifically: First, the 2-amino-2-thiazoline molecule contains nitrogen and sulfur dual heteroatoms and a cyclic conjugated structure, exhibiting a very strong coordination ability for tin. Experiments show that this compound can preferentially adsorb onto grain boundaries and defect regions of tin plating, filling micropores, and interacting with Sn through S and N atoms. 2+ / Sn 4+ It forms stable five- or six-membered chelate rings, significantly inhibiting the initiation of localized corrosion. However, on copper or iron substrates, this substance mainly exhibits physical adsorption, resulting in significantly poorer film uniformity and adhesion.

[0014] Secondly, sodium N,N-dimethyldithiocarbonylpropane sulfonate is a sulfur-containing anionic surfactant that combines a hydrophilic sulfonic acid group with a hydrophobic thiocarbonyl group structure. In the formulation of this invention, it not only improves the wettability of the passivation solution, but more importantly, its thiol group (–C(=S)S–) can chemically bond with the tin surface to form a poorly soluble tin sulfide (SnS) micro-deposit layer, further enhancing the film density. It is worth noting that in other metal systems (such as zinc and nickel), such compounds are prone to initiating displacement reactions or causing the coating to blacken, making them unstable for use; however, in the tin methanesulfonate system, due to the relatively positive potential and moderate reactivity of tin itself, controllable film formation can be achieved without affecting the appearance.

[0015] Third, and more importantly, when 2-amino-2-thiazoline and sodium N,N-dimethyldithiocarbonylpropanesulfonate coexist, they exhibit a significant synergistic enhancement effect on the tin surface: the former provides strong adsorption sites and guides ordered film formation, while the latter fills the voids and strengthens the hydrophobic barrier. Salt spray test results show that when either additive is used alone, the corrosion resistance time of the tin-plated plate is approximately 24–36 hours; however, when both are combined, the corrosion resistance time can reach over 72 hours, which cannot be explained by simple superposition. XPS analysis confirms the presence of various chemical bonds such as Sn–N, Sn–S, and Ti–O–Sn in the composite film, indicating the formation of a multi-level cross-linked network structure.

[0016] Furthermore, this invention employs an A / B component packaging system (film-forming agent + pH adjuster / additive + dilute alcohol), effectively solving the problem of sulfur-containing additives easily decomposing, polymerizing, or deactivating during long-term storage in aqueous phase, ensuring product storage stability for more than 6 months. In contrast, if all components are mixed, turbidity and precipitation occur within a week.

[0017] In summary, this invention does not simply apply known corrosion inhibitors to new scenarios. Rather, based on a deep understanding of the chemical properties of tin metal surfaces, it creatively screens and optimizes specific additive combinations suitable for the methylsulfonic acid tin plating system, solving long-standing technical problems such as insufficient density, poor adhesion, and weak anti-discoloration ability of traditional chromium-free passivation films in this system.

[0018] Preferably, in Agent A, the mass-to-volume ratio of the film-forming agent to pure water is 1~10 g:1L; and the mass-to-volume ratio of the additive to the dilute alcohol is 0.1~5 g:1L.

[0019] Generally, in Agent A, when the film-forming agent is phytic acid, the mass-to-volume ratio of phytic acid to pure water is 3-10 g:1 L; when the film-forming agent is tannic acid, the mass-to-volume ratio of tannic acid to pure water is 2-8 g:1 L; when the film-forming agent is ammonium fluorozirconate, the mass-to-volume ratio of ammonium fluorozirconate to pure water is 1-10 g:1 L; when the film-forming agent is ammonium fluorotitanate, the mass-to-volume ratio of ammonium fluorotitanate to pure water is 2-10 g:1 L; and when the film-forming agent is potassium titanium oxalate, the mass-to-volume ratio of potassium titanium oxalate to pure water is 3-8 g:1 L.

[0020] Preferably, the additive comprises a mixture of 2-amino-2-thiazoline and sodium N,N-dimethyldithiocarbonylpropanesulfonate in a mass ratio of 9~7:1~3.

[0021] Preferably, the pH of agent A ranges from 0 to 4.

[0022] Generally, in Agent A, when the pH adjuster is methanesulfonic acid, the mass-to-volume ratio of methanesulfonic acid to pure water is 0.1~10g:1L; when the pH adjuster is phosphoric acid, the mass-to-volume ratio of phosphoric acid to pure water is 0.1~10g:1L; and when the pH adjuster is citric acid, the mass-to-volume ratio of citric acid to pure water is 5~20g:1L.

[0023] A method for preparing the above-mentioned chromium-free passivation solution for tin plating in a methanesulfonic acid system includes the following steps: S1. Dissolve the film-forming agent and the pH adjuster in pure water to obtain agent A; S2. Dissolve the additive in the dilute alcohol to obtain agent B; S3. Mix the A agent and the B agent to obtain the chromium-free passivation solution for tin plating in the methanesulfonic acid system.

[0024] An application of the above-mentioned chromium-free passivation solution for tin plating in a methanesulfonic acid system, used for passivation of the tin plating layer.

[0025] Preferably, it includes the following steps: A. Immerse the tin plating layer to be treated with antioxidant properties in the chromium-free passivation solution used for tin plating layers in the methanesulfonic acid system; B. After the soaking is completed, the product is dried to obtain a passivated tin plating layer.

[0026] Preferably, in step A, the soaking time is 1 to 5 minutes; the soaking temperature is 20 to 60°C; and in step B, the drying temperature is 70 to 80°C.

[0027] Immerse the tin plating layer to be treated for oxidation in a chromium-free passivation solution for tin plating using a methanesulfonic acid system. Soak at 20-60℃ for 1-5 minutes. No water rinsing is required. Dry at 70-80℃ to obtain a passivated tin plating layer with excellent oxidation resistance.

[0028] Compared with the prior art, implementing the present invention has the following beneficial effects: The film-forming agent in antioxidant A of this invention can form an organic thin film on the tin surface, while the additive in antioxidant B has a good synergistic effect with the film-forming agent, enabling the composite corrosion inhibitor to quickly form an adsorption film and an oxide film adhering to the tin surface. 2-Amino-2-thiazoline contains electron-rich nitrogen atoms (amino and heterocyclic nitrogen) and sulfur atoms (heterocyclic sulfur), while sodium N,N-dimethyldithiocarbonylpropane sulfonate contains a strongly complexing dithiocarboxyl group (-C(=S)S-) and a hydrophilic sulfonic acid group (-SO3-). The dithiocarboxyl group is a very effective metal chelating group that can strongly adsorb onto the tin surface, forming a chemisorption layer. This layer can isolate the tin surface from the air, thereby hindering electrochemical charge transfer, inhibiting anodic and cathodic polarization processes, thus improving corrosion resistance, and possessing excellent sulfur and acid resistance. The chromium-free passivation solution for tin plating using the methanesulfonic acid system provided by this invention is chromium-free, low in cost, environmentally friendly, and easy to treat wastewater. Detailed Implementation

[0029] To make the technical solution of the present invention easier to understand, the present invention will be further described in detail below with reference to specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments are not intended to limit the present invention. Modifications or substitutions made to the methods, steps, or conditions of the present invention without departing from the spirit and substance of the present invention are all within the scope of the present invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0030] Example 1 Prepare a chromium-free passivation solution according to the following steps: S1. Preparation of antioxidant A: Weigh 6.0 g of phytic acid, dissolve it in deionized water and bring the volume to 1 L. Then add 5.0 g of methanesulfonic acid and stir well to obtain antioxidant A.

[0031] S2. Preparation of antioxidant agent B: Weigh 3.0 g of 2-amino-2-thiazoline, dissolve it in ethanol (concentration of 95%, ethanol to water mass ratio of 95:100), and make up to 1 L. Stir well to obtain antioxidant agent B.

[0032] S3. Mixed use: Antioxidant A and antioxidant B are mixed at a volume ratio of 7:3 to obtain a chromium-free passivation solution. The tin plating layer to be treated with anti-oxidation is placed in the above-mentioned chromium-free passivation solution, immersed at 25°C for 2 minutes, without water washing, and dried at 80°C to obtain the tin plating layer after anti-oxidation treatment.

[0033] Example 2 Prepare a chromium-free passivation solution according to the following steps: S1. Preparation of antioxidant A: Weigh 5.0 g of tannic acid and 6.0 g of ammonium fluorozirconate, dissolve them in deionized water and bring the volume to 1 L. Then add 8.0 g of phosphoric acid, adjust the pH to 2.8, and stir well to obtain agent A.

[0034] S2. Preparation of antioxidant agent B: Weigh 4.0 g of sodium N,N-dimethyldithiocarbonylpropanesulfonate, dissolve it in n-propanol (n-propanol:water = 40:100) and bring the volume to 1 L. Stir to dissolve and obtain agent B.

[0035] S3. Mixed use: Agent A and Agent B are mixed at a volume ratio of 8:2 to obtain the final passivation solution.

[0036] The tin plating layer to be treated with anti-oxidation is placed in the above-mentioned chromium-free passivation solution, immersed at 25°C for 2 minutes, without water washing, and dried at 80°C to obtain the tin plating layer after anti-oxidation treatment.

[0037] Example 3 Prepare a chromium-free passivation solution according to the following steps: S1. Preparation of antioxidant A: Weigh out 5.0 g of potassium titanium oxalate and 4.0 g of ammonium fluorotitanate, dissolve them in deionized water, add 15.0 g of citric acid, and bring the volume to 1 L. Stir well until the pH is approximately 3.2 to obtain reagent A.

[0038] S2. Preparation of antioxidant agent B: Weigh 2.0 g of 2-amino-2-thiazoline and 2.0 g of sodium N,N-dimethyldithiocarbonylpropanesulfonate (composite additive), dissolve them in propylene glycol aqueous solution (propylene glycol:water = 30:100), make up to 1 L, stir evenly, and obtain agent B.

[0039] S3. Mixed use: Mix agent A and agent B at a volume ratio of 9:1 to obtain the passivation solution.

[0040] The tin plating layer to be treated with anti-oxidation is placed in the above-mentioned chromium-free passivation solution, immersed at 30°C for 3 minutes, without water washing, and dried at 80°C to obtain the tin plating layer after anti-oxidation treatment.

[0041] Example 4 Prepare a chromium-free passivation solution according to the following steps: S1. Preparation of antioxidant A: Take 3.0 g of phytic acid and 2.0 g of tannic acid, dissolve them in deionized water, add 0.5 g of methanesulfonic acid, and make up to 1 L to obtain agent A.

[0042] S2. Preparation of antioxidant agent B: Take 0.5 g of 2-amino-2-thiazoline, dissolve it in an aqueous ethanol solution (ethanol:water = 20:100), and make up to 1 L to obtain agent B.

[0043] S3. Mixed use: Agent A and Agent B are mixed at a volume ratio of 5:5 (i.e., 1:1).

[0044] The tin plating layer to be treated with anti-oxidation is placed in the above-mentioned chromium-free passivation solution, immersed at 30°C for 3 minutes, without water washing, and dried at 75°C to obtain the tin plating layer after anti-oxidation treatment.

[0045] Comparative Example 1 The passivation solution was prepared according to the method in Example 3, except that when preparing antioxidant A, potassium titanium oxalate, ammonium fluorotitanate, and citric acid were not added, and only deionized water was used.

[0046] The remaining steps are the same: Weigh 2.0 g of 2-amino-2-thiazoline and 2.0 g of sodium N,N-dimethyldithiocarbonylpropanesulfonate (composite additive), dissolve them in propylene glycol aqueous solution (propylene glycol:water = 30:100), make up to 1 L, stir evenly to obtain agent B; agent A and agent B are mixed at a volume ratio of 9:1.

[0047] The processing technology is the same as in Example 3.

[0048] Comparative Example 2 The passivation solution was prepared according to the method in Example 3, except that potassium titanium oxalate was not added when preparing antioxidant A. 9.0 g of ammonium fluorotitanate was weighed, dissolved in deionized water, 15.0 g of citric acid was added, and the volume was adjusted to 1 L. The mixture was stirred evenly to obtain agent A.

[0049] The remaining steps are the same: Weigh 2.0 g of 2-amino-2-thiazoline and 2.0 g of sodium N,N-dimethyldithiocarbonylpropanesulfonate (composite additive), dissolve them in propylene glycol aqueous solution (propylene glycol:water = 30:100), make up to 1 L, stir evenly to obtain agent B; agent A and agent B are mixed at a volume ratio of 9:1.

[0050] The processing technology is the same as in Example 3.

[0051] Comparative Example 3 The passivation solution was prepared according to the method in Example 3, except that: when preparing antioxidant A, ammonium fluorotitanate was not added. 9.0 g of potassium titanium oxalate was weighed, dissolved in deionized water, 15.0 g of citric acid was added, and the volume was adjusted to 1 L. The mixture was stirred evenly to obtain agent A.

[0052] The remaining steps are the same: Weigh 2.0 g of 2-amino-2-thiazoline and 2.0 g of sodium N,N-dimethyldithiocarbonylpropanesulfonate (composite additive), dissolve them in propylene glycol aqueous solution (propylene glycol:water = 30:100), make up to 1 L, stir evenly to obtain agent B; agent A and agent B are mixed at a volume ratio of 9:1.

[0053] The processing technology is the same as in Example 3.

[0054] Comparative Example 4 The passivation solution was prepared according to the method in Example 3, except that citric acid was not added when preparing antioxidant A. 5.0 g of potassium titanium oxalate and 4.0 g of ammonium fluorotitanate were weighed, dissolved in deionized water, and the volume was adjusted to 1 L. The solution was stirred evenly to obtain agent A.

[0055] The remaining steps are the same: Weigh 2.0 g of 2-amino-2-thiazoline and 2.0 g of sodium N,N-dimethyldithiocarbonylpropanesulfonate (composite additive), dissolve them in propylene glycol aqueous solution (propylene glycol:water = 30:100), make up to 1 L, stir evenly to obtain agent B; agent A and agent B are mixed at a volume ratio of 9:1.

[0056] The processing technology is the same as in Example 3.

[0057] Comparative Example 5 The passivation solution was prepared according to the method in Example 3, except that: when preparing antioxidant agent B, 2-amino-2-thiazoline and sodium N,N-dimethyldithiocarbonylpropanesulfonate were not added, and only an aqueous ethanol solution (95:100) was used as agent B.

[0058] Agent A was prepared in the same way as in Example 3 (6.0 g phytic acid, 5.0 g methanesulfonic acid, and deionized water to a final volume of 1 L).

[0059] Agent A and Agent B are mixed at a volume ratio of 9:1 (Agent B is a blank solvent).

[0060] The processing technology is the same as in Example 3.

[0061] Example 1 The workpieces treated in the above examples and comparative examples were subjected to NSS salt spray tests, and the results are shown in Table 1.

[0062] Table 1

[0063] The test data above show that in Examples 1-4 using the present invention, the time for white rust to appear in NSS salt spray was over 40 hours, while in Comparative Examples 1-5, the time for white rust to appear dropped sharply to below 10 hours. This demonstrates that the present invention can significantly improve the rust prevention performance of the treated workpiece. In particular, Example 3, which used the composite additive, significantly extended the time for white rust to appear to 53 hours, and should be considered the optimal example.

[0064] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A chromium-free passivation solution for tin plating in a methanesulfonic acid system, characterized in that, include: Agent A and Agent B; Agent A includes film-forming agent, pH adjuster, and pure water; Agent B includes additives and dilute alcohol.

2. The chromium-free passivation solution for tin plating in a methanesulfonic acid system as described in claim 1, characterized in that, The volume ratio of agent A to agent B is 5:9~1.

3. The chromium-free passivation solution for tin plating in a methanesulfonic acid system as described in claim 1, characterized in that, The film-forming agent includes one or more of phytic acid, tannic acid, ammonium fluorozirconate, ammonium fluorotitanate, and potassium titanium oxalate; the pH adjuster includes one or more of methanesulfonic acid, phosphoric acid, and citric acid; the additive includes one or more of 2-amino-2-thiazoline and sodium N,N-dimethyldithiocarbonylpropanesulfonate; the dilute alcohol includes an alcohol and water mixed at a mass ratio of 5-50:100; the alcohol is selected from one or more of methanol, ethanol, propylene glycol, and n-propanol.

4. The chromium-free passivation solution for tin plating in a methanesulfonic acid system as described in claim 1, characterized in that, In Agent A, the mass-to-volume ratio of the film-forming agent to pure water is 1~10 g:1L; the mass-to-volume ratio of the additive to the dilute alcohol is 0.1~5 g:1L.

5. The chromium-free passivation solution for tin plating in a methanesulfonic acid system as described in claim 1, characterized in that, The additives comprise 2-amino-2-thiazoline and sodium N,N-dimethyldithiocarbonylpropanesulfonate in a mass ratio of 9-7:1-3.

6. The chromium-free passivation solution for tin plating in a methanesulfonic acid system as described in claim 1, characterized in that, The pH range of agent A is 0 to 4.

7. A method for preparing a chromium-free passivation solution for tin plating in a methanesulfonic acid system as described in claim 1, characterized in that, Includes the following steps: S1. Dissolve the film-forming agent and the pH adjuster in pure water to obtain agent A; S2. Dissolve the additive in the dilute alcohol to obtain agent B; S3. Mix the A agent and the B agent to obtain the chromium-free passivation solution for tin plating in the methanesulfonic acid system.

8. The application of the chromium-free passivation solution as described in claim 1 for tin plating in a methanesulfonic acid system, characterized in that, Used for passivation of tin plating.

9. The application as described in claim 8, characterized in that, Includes the following steps: A. Immerse the tin plating layer to be subjected to antioxidant treatment in the chromium-free passivation solution used for tin plating layers in the methanesulfonic acid system; B. After the soaking is completed, the product is dried to obtain a passivated tin plating layer.

10. The application as described in claim 9, characterized in that, In step A, the soaking time is 1 to 5 minutes; the soaking temperature is 20 to 60°C; in step B, the drying temperature is 70 to 80°C.

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