Binary phosphating solution for galvanized steel as well as preparation method and application of binary phosphating solution

Abstract

The invention discloses a binary phosphating solution for galvanized steel and a preparation method and application of the binary phosphating solution. The binary phosphating solution is prepared from 8-18 g / L of zinc dihydrogen phosphate or / and 2.0-5.6 g / L of zinc oxide, 10-15 mL / L of a 50% manganous nitrate aqueous solution, 5-30 mL / L of 75% phosphoric acid, 0-5 mL / L of 65% nitric acid, 2-6 g / L of pH response microcapsules, 0.5-1.0 g / L of sodium m-nitrobenzenesulfonate, 0.2-0.5 g / L of potassium fluotitanate, 1.5-3.5 g / L of polyepoxysuccinic acid and the balance deionized water. The wall material of the pH response microcapsule is hydroxypropyl methyl cellulose phthalate, the core material of the pH response microcapsule is sodium chlorate, and each gram of the microcapsule contains 0.2-0.5 g of sodium chlorate; according to the galvanized steel treated by the phosphating solution, the crystal size of a phosphating film is smaller than or equal to 2 microns, the adhesive force can reach grade 0, the neutral salt spray test can reach 1000 hours, and the unilateral corrosion width is smaller than or equal to 2 mm.

Description

Technical Field

[0001] This invention belongs to the field of metal material surface treatment technology, specifically relating to a binary phosphating solution for galvanized steel, its preparation method, and its application. Background Technology

[0002] Before cold working, carbon steel materials require surface treatment to undergo post-processing techniques such as drawing, cold heading, and electrophoresis. Currently, the most important surface treatment method is phosphating. Phosphating forms a dense phosphate film on the cleaned steel surface. This phosphate film is densely crystalline, fine-grained, and has good adhesion, forming a highly adhesive and lubricating saponified phosphate film that interacts well with subsequent lubrication processes. During subsequent processing, this saponified phosphate film ensures the wire can withstand multiple drawing or cold heading operations while protecting the dies from damage, significantly reducing production costs for customers.

[0003] Therefore, phosphating is a crucial step in the cold working of metals. It is a typical localized multiphase reaction, essentially an electrochemical reaction. When a metal is immersed in a solution containing phosphates, numerous micro-corrosion cells form on its surface, resulting in slight corrosion. At the solution-metal interface, the acid concentration decreases, and the resulting metal phosphate chemical conversion film is called a phosphating film.

[0004] Current phosphating solutions typically refer to solutions containing zinc ions (Zn). 2+ ), manganese ions (Mn) 2+ ) and nickel ions (Ni 2+ This is a phosphating system based on zinc, manganese, and nickel. It is one of the most widely used phosphating systems in industry, especially in fields with high performance requirements (such as automobiles and home appliances). For example, patent documents CN1749432A, CN115261841A, CN108977802A, and CN105369238A disclose zinc-manganese-nickel ternary phosphating solutions.

[0005] Ternary phosphating solutions exhibit excellent corrosion resistance, high adhesion and impact resistance, and a uniform and dense film. The resulting phosphating film is a typical pseudo-conversion film, exhibiting strong ionic bonding with the substrate metal and excellent overall protective performance (film weight, phosphorus ratio). However, nickel is a strictly controlled heavy metal, restricted by environmental regulations (such as REACH and RoHS). Nickel-containing wastewater and phosphating slag are classified as hazardous waste, resulting in high treatment costs. The reaction is also more vigorous, generating more phosphating slag than some improved binary systems, requiring more frequent tank emptying and cleaning, increasing maintenance costs. Furthermore, the final coated product may face challenges in exporting or obtaining environmental certifications.

[0006] Existing literature indicates the following development trends in phosphating solutions: Environmental friendliness: avoiding the use of toxic accelerators such as nitrites, reducing the use of heavy metals (such as nickel), and reducing sludge. Low-temperature operation: lowering the phosphating temperature to save energy. Multifunctionality: phosphating solutions may not only form a phosphating film but also incorporate other functions (such as improving corrosion resistance and compatibility with electrophoresis).

[0007] Binary phosphating solutions offer simple wastewater treatment, and the phosphating slag can be treated as general industrial waste, drastically reducing hazardous waste treatment costs. They completely avoid all environmental, regulatory, and health issues associated with nickel, significantly lowering compliance risks and disposal costs. Furthermore, the reaction in binary phosphating solutions is relatively mild, producing less phosphating slag, with tightly packed crystals that are less prone to adhering to workpieces, resulting in a perfect appearance, a long bath life, and low maintenance frequency. Therefore, binary phosphating solutions have become one of the development directions for phosphating solutions.

[0008] Binary phosphating solutions mainly refer to zinc-based solutions (Zn). 2+ Phosphating solutions, with their superior systems typically referring to zinc-calcium (Zn) solutions. 2+ -Ca 2+ ) series or zinc-manganese (Zn) 2+ -Mn 2+ Component phosphating solutions improve the performance of traditional zinc-based phosphating by introducing calcium or manganese ions. However, binary phosphating films are needle-like or plate-like crystals, and their porosity is usually higher than that of ternary phosphating films. As a result, their resistance to salt spray corrosion, especially their resistance to under-film corrosion diffusion, is usually not as good as that of ternary systems.

[0009] Patent document CN107326353A discloses a rapid phosphating agent for galvanized steel sheets used in color coating. It mainly consists of 5-12% zinc oxide, 12-20% phosphoric acid, 2-6% nitrates, 0.1-3% organic acids (citric acid, acrylic acid, tartaric acid, oxalic acid, and gluconic acid and their salts), 0.5-2% phytic acid and its salts, 0.1-2% inorganic fluorine complexes (fluorosilicic acid, fluoroboric acid, fluorophosphate and their salts), 0.1-1% composite accelerators (sodium chlorate, sodium molybdate, sodium nitrate, sodium nitrite, hydroxylamine sulfate, nitroguanidine, and sodium nitrobenzenesulfonate), 0.1-0.5% additives (0.2-0.8 mol / L ferrous sulfate solution and 0.2-0.8 mol / L EDTA), and water. It features short processing time, high film formation efficiency, and the ability to form a phosphating film on the surface of the galvanized substrate in 5-15 seconds, requiring no heating during room temperature processing. However, this type of phosphating solution uses composite accelerators such as sodium chlorate, sodium nitrite, and potassium permanganate. Chlorate ions will reduce the corrosion resistance of the wire after phosphating, and nitrite will release harmful gases during the production process, affecting the health of workshop workers. Moreover, excessive addition can easily lead to an increase in sludge after phosphating. Furthermore, this composite accelerator is fully released in the early stage of the reaction, resulting in fast film formation but coarse crystals, which can easily lead to excessive corrosion of the zinc plating layer (producing "white rust"), thereby affecting product quality and service life.

[0010] Patent document CN112391618A discloses an environmentally friendly phosphating solution containing graphene oxide and its preparation method, comprising 15-35 parts phosphoric acid, 5-15 parts zinc oxide, 5-16 parts nitric acid (68%), 6-15 parts calcium nitrate, 0.01-0.1 parts graphene oxide, 0.2-1 parts isopropanol, 0.01-0.1 parts cerium nitrate or lanthanum nitrate, 0.5-5 parts composite complexing agent, and the balance being water; the composite complexing agent is two or more of citric acid, tartaric acid, gluconic acid, sulfosalicylic acid, hydroxyethylidene diphosphonic acid (HEDP), o-phenanthroline, and phytic acid. This document avoids the use of composite accelerators such as sodium chlorate, sodium nitrite, and potassium permanganate, but it still cannot avoid the complete release of the composite accelerator in the initial stage of the reaction, thus failing to prevent the problem of excessive corrosion of the zinc plating layer. Summary of the Invention

[0011] The purpose of this invention is to provide a binary phosphating solution for galvanized steel, its preparation method, and its application, so as to solve the above-mentioned technical problems existing in the prior art, and to take into account environmental protection, corrosion resistance, and process economy.

[0012] To achieve the above objectives, the technical solution of the present invention is as follows: The present invention discloses a binary phosphating solution for galvanized steel, which is composed of the following components: zinc dihydrogen phosphate 8-18 g / L or / and zinc oxide 2.0-5.6 g / L, 50% manganese nitrate (Mn(NO3)2) aqueous solution 10-15 mL / L, 75% phosphoric acid 5-30 mL / L, 65% nitric acid 0-5 mL / L, pH-responsive microcapsules 2-6 g / L, sodium m-nitrobenzenesulfonate 0.5-1.0 g / L, potassium fluorotitanate (K2TiF6) 0.2-0.5 g / L, polyepoxysuccinic acid 1.5-3.5 g / L, and the balance being deionized water; the wall material of the pH-responsive microcapsules is hydroxypropyl methylcellulose phthalate, and the core material is sodium chlorate (NaClO3), with each gram of microcapsule containing 0.2-0.5 g of sodium chlorate.

[0013] Preferably, the binary phosphating solution for galvanized steel of the present invention is prepared from the following components: 13-18 g / L zinc dihydrogen phosphate, 10-15 mL / L of 50% manganese nitrate aqueous solution (Mn(NO3)2), 5-10 mL / L of 75% phosphoric acid, 3-5 mL / L of 65% nitric acid, 3-6 g / L of pH-responsive microcapsules, 0.5-1.0 g / L of sodium m-nitrobenzenesulfonate, 0.2-0.5 g / L of potassium fluorotitanate (K2TiF6), 1.0-3.5 g / L of polyepoxysuccinic acid, and the balance being deionized water.

[0014] Hydroxypropyl methylcellulose phthalate (HPMCP, CAS No.: 9050-31-1) is a common enteric coating material. The phthalate groups in its molecule exist in acidic form when pH < 5, and the polymer is insoluble in water; when pH > 5, the phthalate groups form salts, and the polymer dissolves. Therefore, the dissolution pH threshold of the pH-responsive microcapsules is 5.0 to 5.2, which perfectly matches the interfacial pH change during the phosphating process.

[0015] The binary phosphating solution has a pH of 3.0–3.5, a total acid content of 24–30 pt, a free acid content of 1.0–1.8 pt, and a TA / FA acid ratio of 15–25.

[0016] The polyepoxysuccinic acid (PESA, CAS No.: 51274-37-4) is a green and highly efficient stabilizer. The preferred molecular weight range is 2000–5000. PESA within this range exhibits both good chelating properties and excellent dispersion threshold effect, and can strongly complex Ca... 2+ / Mg 2+ Prevents scale buildup and aids in the complexation of Fe. 3+ This effectively prevents the precipitation and growth of phosphate sludge, keeps the bath clean, and extends the bath's lifespan. If the molecular weight of PESA is too low (<2000 Da), although it has high chelating ability, its critical threshold effect and lattice distortion effect are weak, resulting in insufficient ability to disperse and stabilize suspended particles. If the molecular weight is too high (>5000 Da), the molecular chains are too long, which may lead to decreased solubility in the acidic medium of the phosphating solution, and even the risk of precipitation. Simultaneously, excessively long chains may entangle the phosphating nuclei, affecting film formation.

[0017] Further, the preparation method of the pH-responsive microcapsules includes the following steps: S1, dissolving 30-50g of sodium chlorate in 100mL of deionized water at 20-40℃ to form a saturated solution, obtaining an inner aqueous phase; S2, dissolving 5.0-10.0g of hydroxypropyl methylcellulose phthalate (HPMCP) in 200mL of dichloromethane to form an oil phase solution; S3, under high-speed shear, slowly adding the inner aqueous phase to the oil phase solution, and adding 1.0-2.0g of Span 80. S4, continue shearing for 3-10 minutes until a milky white pre-emulsion is formed; S5, dissolve 0.6-1.8 g of polyvinyl alcohol in 300 mL of deionized water as the external aqueous phase; S6, pour the pre-emulsion into the external aqueous phase under medium-speed stirring, and continue stirring for 10-20 minutes to form a water / oil / water multiphase emulsion; S7, stir at medium-low speed for 4-10 hours to completely evaporate the dichloromethane in the multiphase emulsion and form microcapsules; S8, collect the microcapsule precipitate, wash, and dry to obtain the pH-responsive microcapsules.

[0018] Preferably, the polyvinyl alcohol is a partially hydrolyzed PVA with a degree of hydrolysis of 87% to 89%, a degree of polymerization of 500 to 1500, and a molecular weight of 30,000 to 70,000, such as PVA-1588, PVA-1288, PVA-1088, PVA-0588, etc.

[0019] Preferably, in step S6, the dichloromethane is continuously stirred at a low to medium speed in a fume hood or closed system to allow it to evaporate completely, and the temperature can be controlled at 25–35°C to accelerate solvent evaporation.

[0020] Preferably, in step S7, the microcapsules are collected by filtration or centrifugation, washed with deionized water, and dried by fluidized bed drying or vacuum freeze drying to obtain pH-responsive microcapsule powder.

[0021] The stirring speed for high-speed shearing is 10,000 to 15,000 rpm; the stirring speed for medium-speed stirring is 500 to 1,200 rpm; and the stirring speed for medium-low speed stirring is 300 to 500 rpm.

[0022] The present invention discloses a method for preparing a binary phosphating solution for galvanized steel, comprising the following steps: adding 50% to 70% of deionized water to a container; slowly adding zinc dihydrogen phosphate and / or zinc oxide under stirring until completely dissolved; adding manganese nitrate solution, phosphoric acid, and nitric acid; adding polyepoxysuccinic acid under continuous stirring until completely dissolved; adding potassium fluorotitanate and stirring thoroughly until completely dissolved; adding sodium m-nitrobenzenesulfonate and stirring until dissolved; slowly sprinkling in pH-responsive microcapsule powder and gently stirring to evenly disperse it in the phosphating solution; and adding the remaining deionized water and stirring until homogeneous.

[0023] The present invention discloses a binary phosphating solution for galvanized steel, used for spraying or immersion phosphating treatment of steel, zinc-containing and galvanized steel; the phosphating treatment process includes the following steps: D1, alkaline degreasing of the steel workpiece, followed by thorough water washing; surface conditioning, followed by thorough water washing; D2, spraying or immersing the cleaned steel workpiece in the binary phosphating solution, maintaining at a temperature of 30-45℃ for 3-6 minutes; D3, two-stage water washing, deionized water washing, followed by electrophoresis or drying.

[0024] The alkaline degreasing is performed in an alkaline solution at 40–60°C for 3–5 minutes; the water washing is a two-stage countercurrent rinsing. The surface conditioning requires selecting the appropriate surface treatment method and agent based on the actual needs of the galvanized steel workpiece. It is recommended to use a surface conditioning agent containing colloidal titanium salts, treated at a pH of 7.5–9.0 for 30–60 seconds.

[0025] Compared with the prior art, the positive effects of the present invention are: This invention utilizes the pH-responsive characteristics of microcapsules to achieve timed and targeted release of the accelerator. In the initial stage of phosphating (pH ≈ 3.0 of the bath solution), the microcapsules are stable, and the accelerator is released slowly, ensuring sufficient and uniform formation of phosphating film nuclei and avoiding impact on the active zinc layer. In the middle stage of phosphating (the pH of the micro-region rises to ≈ 5.0 at the metal / solution interface due to H+ consumption), the microcapsule wall material dissolves, releasing a large amount of accelerator, accelerating the growth and coverage of the film, and ensuring the formation of a complete and dense phosphating film in a short time. For workpieces with different zinc layer thicknesses and activities, the rate of pH increase at the interface varies. The phosphating solution of this invention can automatically match its reaction kinetics to achieve "adaptive" phosphating, resulting in a wider processing window, excellent process stability, insensitivity to pretreatment fluctuations and differences in the zinc plating layer, and improved product yield.

[0026] In addition, sodium nitrobenzenesulfonate, an auxiliary reducing accelerator, is added to the phosphating solution of this invention. This sodium nitrobenzenesulfonate can form a redox pair with sodium chlorate, stabilizing the phosphating rate and preventing Fe... 3+ Accumulation; Polyepoxysuccinic acid strongly complexes with Ca 2+ / Mg 2+ Prevents scale buildup and aids in the complexation of Fe. 3+ The amount of sludge is reduced by more than 50% compared to traditional phosphating solutions of the same type, significantly reducing the cost of waste residue treatment. Detailed Implementation

[0027] Those skilled in the art should recognize that this embodiment is only used to illustrate the present invention and is not intended to limit the present invention. Any changes or modifications to the embodiment within the scope of the present invention are within the scope of the claims of the present invention.

[0028] The method for preparing the pH-responsive microcapsules used in Examples 1-6 and Comparative Examples 3-4 includes the following steps: S1, at 30℃, take 100mL of deionized water, add 40g of sodium chlorate, stir until completely dissolved to form a saturated solution, and obtain the inner aqueous phase (W1). If there are undissolved particles after supersaturation, only the supernatant should be taken when using it. S2, Dissolve 8.0g of hydroxypropyl methylcellulose phthalate (HPMCP, Guangzhou Yuanda New Materials Co., Ltd.) in 200mL of dichloromethane to form an oil phase solution (O). S3, under high-speed shearing (10000rpm), the internal aqueous phase (W1) is slowly added to the oil phase solution (O), and 1.2g of Span 80 is added. Shearing continues for 5min until a uniform milky white primary emulsion (W1 / O) is formed. S4, dissolve 1.5g of polyvinyl alcohol (PVA-1088, Shanghai Yingjia Industrial) in 300mL of deionized water as the external aqueous phase (W2). S5, the colostrum (W1 / O) is poured into the external aqueous phase (W2) under medium speed stirring, and stirring is continued for 15 minutes to form a water / oil / water multiphase emulsion; S6. In a fume hood, maintain low-to-medium speed stirring for 8 hours to allow the dichloromethane in the multiphase emulsion to completely evaporate. HPMCP is insoluble in water, thus precipitating and solidifying around the sodium chlorate solution droplets to form microcapsules. S7. Collect the microcapsules by filtration or centrifugation, wash with deionized water, and freeze-dry under vacuum to obtain approximately 12.7 g of pH-responsive microcapsule powder.

[0029] Microscopic measurements showed that the microcapsule size was approximately 5–20 μm, and ultraviolet spectrophotometry determined that each gram of microcapsule contained approximately 0.37 g of sodium chlorate (NaClO3).

[0030] Examples 1-6 and Comparative Examples 1-4 The binary phosphating solutions for galvanized steel in Examples 1-6 and Comparative Examples 2-4 are listed in Table 1. Their preparation methods include the following steps: adding deionized water (2 / 3 of the total volume) to a container; slowly adding zinc dihydrogen phosphate and / or zinc oxide while stirring, stirring until completely dissolved; sequentially adding manganese nitrate solution, phosphoric acid, and nitric acid; adding polyepoxysuccinic acid (molecular weight approximately 3800, Shandong Changyao New Materials Co., Ltd.) while continuously stirring, stirring until completely dissolved; adding potassium fluorotitanate, stirring thoroughly until completely dissolved; adding sodium m-nitrobenzenesulfonate, stirring until dissolved; slowly sprinkling in pH-responsive microcapsule powder, gently stirring to evenly disperse it in the phosphating solution; adding the remaining deionized water, stirring until homogeneous, thus obtaining the binary phosphating solution.

[0031] Comparative Examples 1-4 are comparative examples of Example 2, with the following differences: Comparative Example 1 does not add pH-responsive microcapsules, but instead adds 2.5 g / L sodium chlorate; Comparative Example 2 does not add pH-responsive microcapsules; Comparative Example 3 does not add sodium m-nitrobenzenesulfonate; Comparative Example 4 does not add polyepoxysuccinic acid; the rest are the same as Example 2.

[0032] Table 1. Proportions of binary phosphating solutions for galvanized steel in Examples 1-6 and Comparative Examples 2-4 Test Experiment Example The galvanized steel sheet samples were phosphated using the binary phosphating solutions of Examples 1-6 and Comparative Examples 1-4. The application method included the following steps: D1, the galvanized steel sheet samples were degreased with an alkaline degreasing agent (10 min at 50°C), followed by a two-stage countercurrent rinsing with water; the surface was then adjusted with PL-303B surface conditioner, followed by a two-stage countercurrent rinsing with water; D2, the cleaned galvanized steel sheet samples were immersed in the binary phosphating solution at 35°C for 5 min; D3, the samples were rinsed thoroughly with water using a two-stage countercurrent rinsing, followed by two deionized water rinsings, and then dried before cathodic electrophoretic coating.

[0033] The adhesion, resistance to neutral salt spray, and impact resistance of the phosphated film on the passivated galvanized steel sheet samples were measured, and the results are listed in Table 2.

[0034] Neutral Salt Spray Test (NSS): According to GB / T10125-2021, there are 6 samples for each type of workpiece. The electrophoretically coated cross-cut samples are placed at 30° to the vertical direction in a salt spray test chamber at 35°. At a pressure chamber temperature of 45°, a 5wt% sodium chloride saline solution with a pH of 7.0 is sprayed at an air pressure of 70 kPa. After spraying the samples for 500h, 720h and 1000h, 2 samples are taken out each time, washed with water and dried, and the surface of the sample is observed for corrosion. The width of the rust at the cross-cut on the workpiece surface is also observed.

[0035] Adhesion test: After electrophoresis, the workpiece is marked with a series of overlapping and intersecting circles on the paint film adhesion tester, and the paint film peeling is evaluated according to the national standard GB / T9286-2021.

[0036] Impact resistance test: According to the national standard GB / T1732-2020, a 1kg weight is dropped from a height of 50cm along the cylinder, and the distance from the impact point to the groove is greater than 2mm. The paint film peeling at the impact point is observed.

[0037] Table 2 shows the performance of the passivation films in Examples 1-6 and Comparative Examples 1-4. As shown in Table 2, the galvanized steel plate samples treated with the binary phosphating solution in Examples 1 to 5 of the present invention form a uniform and dense blue-gray to dark gray microcrystalline phosphating film with a crystal size ≤2μm, which has excellent compatibility with cathodic electrophoresis. After phosphating, the adhesion of the paint film can reach level 0, the neutral salt spray test can reach 1000h, and the single-sided rust width at the scratched part of the paint film is ≤2mm.

[0038] Compared to Example 3, Comparative Example 1 did not add pH-responsive microcapsules, but instead added 2.5 g / L of sodium chlorate. Due to the complete release of sodium chlorate in the initial stage of the reaction, the zinc coating's resistance to neutral salt spray corrosion could only reach 500 h. Comparative Example 2 did not add pH-responsive microcapsules, and without sodium chlorate accelerator, the adhesion and impact resistance of the phosphating film decreased, and the resistance to neutral salt spray corrosion decreased sharply. Comparative Example 3 did not add sodium nitrobenzenesulfonate, an auxiliary reducing accelerator, so it could not form a redox pair with sodium chlorate, resulting in an increase in sludge and a decrease in resistance to neutral salt spray. Comparative Example 4 did not add polyepoxysuccinic acid, resulting in a sharp increase in sludge and a decrease in resistance to neutral salt spray.

Claims

1. A binary phosphating solution for galvanized steel, characterized in that, It is made from the following components: zinc dihydrogen phosphate 8-18 g / L or / and zinc oxide 2.0-5.6 g / L, 50% manganese nitrate aqueous solution 10-15 mL / L, 75% phosphoric acid 5-30 mL / L, 65% nitric acid 0-5 mL / L, pH-responsive microcapsules 2-6 g / L, sodium m-nitrobenzenesulfonate 0.5-1.0 g / L, potassium fluorotitanate 0.2-0.5 g / L, polyepoxysuccinic acid 1.5-3.5 g / L, and the balance being deionized water; the wall material of the pH-responsive microcapsules is hydroxypropyl methylcellulose phthalate, the core material is sodium chlorate, and each gram of microcapsule contains 0.2-0.5 g of sodium chlorate.

2. The binary phosphating solution for galvanized steel according to claim 1, characterized in that, It is made from the following components: zinc dihydrogen phosphate 13-18 g / L, 50% manganese nitrate aqueous solution 10-15 mL / L, 75% phosphoric acid 5-10 mL / L, 65% nitric acid 3-5 mL / L, pH-responsive microcapsules 3-6 g / L, sodium m-nitrobenzenesulfonate 0.5-1.0 g / L, potassium fluorotitanate 0.2-0.5 g / L, polyepoxysuccinic acid 1.0-3.5 g / L, and the balance being deionized water.

3. The binary phosphating solution for galvanized steel according to claim 1 or 2, characterized in that, The pH-responsive microcapsules have a solubility pH threshold of 5.0–5.2; the polyepoxysuccinic acid has a molecular weight range of 2000–5000.

4. The binary phosphating solution for galvanized steel according to claim 1 or 2, characterized in that, The binary phosphating solution has a pH of 3.0–3.5, a total acid content of 24–30 pt, a free acid content of 1.0–1.8 pt, and a TA / FA acid ratio of 15–25.

5. The binary phosphating solution for galvanized steel according to claim 1 or 2, characterized in that, The method for preparing the pH-responsive microcapsules includes the following steps: S1, dissolving 30-50g of sodium chlorate in 100mL of deionized water at 20-40℃ to form a saturated solution, obtaining an inner aqueous phase; S2, dissolving 5.0-10.0g of hydroxypropyl methylcellulose phthalate in 200mL of dichloromethane to form an oil phase solution; S3, under high-speed shearing, slowly adding the inner aqueous phase to the oil phase solution, and adding 1.0-2.0g of Span 80, continuing shearing for 3... S4, dissolve 0.6-1.8 g of polyvinyl alcohol in 300 mL of deionized water as the external aqueous phase; S5, pour the initial emulsion into the external aqueous phase under medium-speed stirring, and continue stirring for 10-20 min to form a water / oil / water multiphase emulsion; S6, stir at medium-low speed for 4-10 h to completely evaporate the dichloromethane in the multiphase emulsion and form microcapsules; S7, collect the microcapsule precipitate, wash, and dry to obtain the pH-responsive microcapsules.

6. The binary phosphating solution for galvanized steel according to claim 5, characterized in that, The polyvinyl alcohol is a partially hydrolyzed PVA with a degree of hydrolysis of 87% to 89%, a degree of polymerization of 500 to 1500, and a molecular weight of 30,000 to 70,000.

7. The binary phosphating solution for galvanized steel according to claim 5, characterized in that, In step S6, the dichloromethane is continuously stirred at a low to medium speed in a fume hood or closed system to ensure complete volatilization, with the temperature controlled at 25–35°C.

8. The binary phosphating solution for galvanized steel according to claim 5, characterized in that, In step S7, microcapsules are collected by filtration or centrifugation, washed with deionized water, and dried by fluidized bed drying or vacuum freeze drying to obtain pH-responsive microcapsule powder.

9. The method for preparing a binary phosphating solution for galvanized steel according to claim 1 or 2, characterized in that, The steps include: adding 50% to 70% of the total water volume of deionized water to a container; slowly adding zinc dihydrogen phosphate and / or zinc oxide while stirring, and stirring until completely dissolved; adding manganese nitrate solution, phosphoric acid, and nitric acid; and adding polyepoxysuccinic acid while stirring continuously until completely dissolved. Add potassium fluorotitanate and stir thoroughly until completely dissolved; add sodium m-nitrobenzenesulfonate and stir until dissolved; slowly sprinkle in pH-responsive microcapsule powder and stir gently to disperse it evenly in the phosphating solution; add the remaining deionized water and stir until homogeneous.

10. The application of the binary phosphating solution for galvanized steel according to claim 1 or 2, characterized in that, Used for spraying or immersion phosphating treatment of steel, zinc-containing and galvanized steel.

11. The application of the binary phosphating solution for galvanized steel according to claim 10, characterized in that, The phosphating process includes the following steps: D1, alkaline degreasing of the steel workpiece, followed by thorough water washing; surface conditioning, followed by thorough water washing; D2, spraying or immersing the cleaned steel workpiece in the binary phosphating solution, maintaining it at a temperature of 30-45℃ for 3-6 minutes; D3, two-stage water washing, deionized water washing, followed by electrophoresis or drying.

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