A binary phosphating solution for galvanized steel and a preparation method and application thereof

Abstract

The application discloses a binary phosphating solution for galvanized steel materials and a preparation method and application thereof. The binary phosphating solution is prepared from the following components: 8-18 g / L of zinc dihydrogen phosphate, 2.0-5.6 g / L of zinc oxide, 10-15 mL / L of 50% aqueous manganese nitrate solution, 5-30 mL / L of 75% phosphoric acid, 0-5 mL / L of 65% nitric acid, 2-6 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, 1.5-3.5 g / L of polyepoxysuccinic acid, and the rest is deionized water; the wall material of the pH responsive microcapsules is hydroxypropyl methylcellulose phthalate, the core material is sodium chlorate, and 0.2-0.5 g of sodium chlorate is contained in each gram of the microcapsules; the galvanized steel treated by the phosphating solution has a phosphating film with a crystal size of less than or equal to 2 microns, an adhesion of 0 grade, a neutral salt spray test of 1000 hours, and a single-side rust corrosion width of less than or equal to 2 mm.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material surface treatment, and particularly relates to a binary phosphating solution for galvanized steel materials and a preparation method and application thereof. BACKGROUND

[0002] Before cold working, carbon steel materials need to be surface treated, and then can be subjected to subsequent processing processes such as drawing, cold heading, electrophoresis, etc. At present, the most important treatment method for surface treatment is phosphating treatment. Phosphating is to generate a dense phosphating film on the surface of the cleaned steel material. The phosphating film layer is crystalline and dense, the film layer is fine and has good adhesion, and forms a phosphonium soap film layer with strong adhesion and good lubricity in the subsequent lubrication process. In the subsequent processing process, the phosphonium soap film layer can ensure that the wire material is subjected to multiple drawing or cold heading processes, and at the same time protects the die from damage, thereby saving customers a great production cost.

[0003] Therefore, phosphating is a very important step in the metal cold working process. It is a typical local multiphase reaction and essentially belongs to an electrochemical reaction. When the metal is immersed in a solution containing phosphate, many micro-corrosion cells are formed on the surface, slight etching occurs, the acid concentration is reduced at the solution-metal interface, and the formed metal phosphate chemical conversion film is called a phosphating film.

[0004] The current phosphating solution generally refers to a phosphating system containing zinc ions (Zn 2+ ), manganese ions (Mn 2+ ) and nickel ions (Ni 2+ ). This is one of the most widely used phosphating systems in the industry, especially in fields with high performance requirements (such as automobiles and home appliances). For example, the zinc-manganese-nickel ternary phosphating solution disclosed in patent documents CN1749432A, CN115261841A, CN108977802A and CN105369238A.

[0005] The ternary phosphating solution has excellent corrosion resistance, high adhesion and impact resistance, and the film layer is uniform and dense. The phosphating film formed is a typical pseudo-conversion film layer, and the combination with the base metal is ionic bonding, the binding force is firm, and the overall protection effect (film weight, P ratio) is good. However, nickel is a heavy metal that is strictly controlled, and is limited in environmental regulations (such as REACH and RoHS). The nickel-containing wastewater and phosphating residue are classified as hazardous waste, and the treatment cost is high; the reaction is relatively violent, the amount of phosphating residue generated is more than that of some improved binary systems, more frequent tank emptying and cleaning are required, and the maintenance cost is increased; and the final coating product may face challenges in export or environmental certification.

[0006] The development trend of phosphating solution in the existing literature is: environmental protection: avoid using toxic accelerators such as nitrite, reduce the use of heavy metals (such as nickel), and reduce the sediment. Low temperature: reduce the phosphating temperature to save energy. Multifunctional: phosphating solution not only forms a phosphating film, but also may introduce other functions (such as improving corrosion resistance, compatibility with electrophoresis, etc.).

[0007] The wastewater treatment of binary phosphating solution is simple, and the phosphating residue can be treated as general industrial waste, which greatly reduces the cost of hazardous waste treatment, completely avoids all environmental, regulatory and health problems caused by nickel, greatly reduces the compliance risk and disposal cost. Moreover, the reaction of binary phosphating solution is relatively mild, the amount of phosphating residue is less, the crystalline particles are compact and not easy to adhere to the workpiece, the appearance is very perfect, the bath solution has a long service life and low maintenance frequency. Therefore, binary phosphating solution has become one of the development directions of phosphating solution.

[0008] Binary phosphating solution mainly refers to zinc-based (Zn 2+ ) phosphating solution, and the advantage system usually refers to zinc-calcium (Zn 2+ -Ca 2+ ) or zinc-manganese (Zn 2+ -Mn 2+ ) phosphating solution. The performance of traditional zinc-based phosphating is improved by introducing calcium ions or manganese ions. However, the binary phosphating film is needle-shaped or sheet-shaped crystalline, and its porosity is usually higher than that of ternary phosphating film, which leads to its salt mist corrosion resistance, especially the ability to resist under-film corrosion, is usually not as good as that of ternary system.

[0009] Patent document CN107326353A discloses a rapid phosphating agent for galvanized steel sheet for color coating, which is mainly composed of zinc oxide 5-12%, phosphoric acid 12-20%, nitrate 2-6%, organic acid (citric acid, acrylic acid, tartaric acid, oxalic acid and gluconic acid and their salts) 0.1-3%, phytic acid and its salt 0.5-2%, inorganic fluorine complex (fluorosilicic acid, fluoroboric acid, fluorophosphoric acid and their salts) 0.1-2%, composite accelerator (sodium chlorate, sodium molybdate, sodium nitrate, sodium nitrite, hydroxylamine sulfate, nitroguanidine and sodium nitrobenzenesulfonate) 0.1-1%, additive (0.2-0.8 mol / L ferrous sulfate solution and 0.2-0.8 mol / L EDTA) 0.1-0.5% and water. It has the advantages of short treatment time, high efficiency of film formation, and can form a phosphating film on the surface of the galvanized substrate in 5-15 seconds. However, this type of phosphating solution uses composite accelerators such as sodium chlorate, sodium nitrite and potassium permanganate. Chlorate will reduce the corrosion resistance of the phosphated wire, nitrite will release harmful gases during production and affect the health of workers in the workshop, and excessive addition will also lead to an increase in phosphating residue. Moreover, this composite accelerator is fully released at the beginning of the reaction, the film forms quickly but the crystalline is coarse, which easily leads to excessive corrosion of the galvanized layer (producing "white rust"), thereby affecting the product quality and service life.

[0010] Patent document CN112391618A discloses an environmentally friendly phosphating solution containing graphene oxide and a preparation method thereof, which comprises 15-35 parts of phosphoric acid, 5-15 parts of zinc oxide, 5-16 parts of nitric acid (68%), 6-15 parts of calcium nitrate, 0.01-0.1 parts of graphene oxide, 0.2-1 parts of isopropyl alcohol, 0.01-0.1 parts of cerium nitrate or lanthanum nitrate, 0.5-5 parts of a composite complexing agent, and the balance is 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 still cannot avoid the comprehensive release of composite accelerators at the initial stage of the reaction, thus cannot avoid the problem of easy overcorrosion of the zinc plating layer. SUMMARY

[0011] The purpose of the present application is to provide a binary phosphating solution for zinc-plated steel and its preparation method and application, in order to solve the above technical problems existing in the prior art, and to balance environmental protection, corrosion resistance, and process economy.

[0012] In order to achieve the above-mentioned purpose, the technical solution of the present application is:

[0013] A binary phosphating solution for zinc-plated steel according to the present application is prepared from 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 is deionized water; the wall material of the pH-responsive microcapsules is hydroxypropyl methylcellulose phthalate, and the core material is sodium chlorate (NaClO3), containing 0.2-0.5 g of sodium chlorate per gram of microcapsules.

[0014] Preferably, a binary phosphating solution for zinc-plated steel according to the present application is prepared from the following components: zinc dihydrogen phosphate 13-18 g / L, 50% manganese nitrate aqueous solution (Mn(NO3)2) 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 (K2TiF6) 0.2-0.5 g / L, polyepoxysuccinic acid 1.0-3.5 g / L, and the balance is deionized water.

[0015] Hydroxypropyl methylcellulose phthalate (HPMCP, CAS No. 9050-31-1) is a common enteric coating material, the phthalate group in its molecule exists in acid form at pH <5, the polymer is insoluble in water; when pH >5, the phthalate group forms a salt, the polymer dissolves; therefore the dissolution pH threshold of the pH-responsive microcapsule is 5.0-5.2, perfectly matching the interface pH change of the phosphating process.

[0016] The pH of the binary phosphating solution is 3.0-3.5, the total acid is controlled at 24-30 pt, the free acid is controlled at 1.0-1.8 pt, and the TA / FA acid ratio is 15-25.

[0017] The polyepoxysuccinic acid (PESA, CAS No. 51274-37-4) is a green and efficient stabilizer, and the preferred molecular weight range is 2000-5000. PESA in this range has good chelating effect and excellent dispersion threshold effect, can strongly complex Ca 2+ / Mg 2+ Prevents scale and helps complex Fe 3+ , can effectively prevent the precipitation and growth of phosphate sludge, keep the bath clean, and prolong the bath life. 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, and it is insufficient to disperse and stabilize suspended particles. If the molecular weight is too high (>5000 Da), the molecular chain is too long, which may lead to a decrease in solubility in the acidic medium of the phosphating solution, and even the risk of precipitation. At the same time, the long chain may entangle the phosphating crystal nucleus, affecting film formation.

[0018] Further, the preparation method of the pH-responsive microcapsule comprises the following steps: S1, dissolving 30-50 g of sodium chlorate in 100 mL of deionized water at 20-40°C to form a saturated solution, to obtain an inner aqueous phase; S2, dissolving 5.0-10.0 g of hydroxypropyl methylcellulose phthalate (HPMCP) in 200 mL of dichloromethane to form an oil phase solution; S3, slowly adding the inner aqueous phase to the oil phase solution under high-speed shearing, and adding 1.0-2.0 g of Span 80, continuing to shear for 3-10 min, to form a milky-white primary emulsion; S4, dissolving 0.6-1.8 g of polyvinyl alcohol in 300 mL of deionized water as an outer aqueous phase; S5, pouring the primary emulsion into the outer aqueous phase under moderate stirring, and continuing to stir for 10-20 min to form a water / oil / water multiple emulsion; S6, stirring the multiple emulsion at a low speed for 4-10 h to completely volatilize the dichloromethane in the multiple emulsion to form microcapsules; S7, collecting the microcapsule precipitate, washing, and drying to obtain the pH-responsive microcapsule.

[0019] Preferably, the polyvinyl alcohol is partially alcoholysis PVA, alcoholysis degree 87%~89%, polymerization degree 500~1500, molecular weight 30000~70000, such as PVA-1588, PVA-1288, PVA-1088, PVA-0588, etc.

[0020] Preferably, in S6, the dichloromethane is completely volatilized by continuously stirring at low or medium speed in a fume hood or a closed system, and the temperature can be controlled at 25~35℃ to accelerate the solvent volatilization.

[0021] Preferably, in 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 the pH-responsive microcapsule powder.

[0022] The stirring speed of the high-speed stirring is 10000~15000 rpm; the stirring speed of the medium-speed stirring is 500~1200 rpm, and the stirring speed of the low or medium-speed stirring is 300~500 rpm.

[0023] The preparation method of the binary phosphating solution for galvanized steel material comprises the following steps: adding 50%~70% of deionized water in a container; slowly adding zinc phosphate or / and 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 until completely dissolved; adding sodium m-nitrobenzenesulfonate, and stirring until dissolved; slowly adding the pH-responsive microcapsule powder, and slightly stirring to uniformly disperse the powder in the phosphating solution; and adding the remaining deionized water, and stirring until uniform.

[0024] The binary phosphating solution for galvanized steel material is used for spray or immersion phosphating treatment of steel, zinc-containing and galvanized steel materials; the phosphating treatment process comprises the following steps: D1, performing alkaline degreasing on the steel workpiece, and then sufficiently washing with water; performing surface adjustment, and then sufficiently washing with water; D2, spraying or immersing the cleaned steel workpiece in the binary phosphating solution, and keeping at a temperature of 30~45℃ for 3~6 min; and D3, two-stage water washing, deionized water washing, and then electrophoresis or drying.

[0025] The alkaline degreasing is performed in an alkaline solution at 40~60℃ for 3~5 min; the water washing is two-stage countercurrent rinsing; and the surface adjustment needs to select a corresponding surface treatment method and treatment agent according to the actual needs of the galvanized steel workpiece, and a surface conditioner containing colloidal titanium salt is recommended to be used for treatment at a pH value of 7.5~9.0 for 30~60 s.

[0026] Compared with the prior art, the positive effects of the present application are:

[0027] The application utilizes the pH response characteristics of the microcapsule to realize the timed and fixed-point release of the accelerator. In the initial stage of phosphating (the bulk pH of the bath solution is about 3.0), the microcapsule is stable, and the accelerator is slowly released to ensure the sufficient and uniform formation of the phosphating film crystal nucleus and avoid the impact on the active zinc layer. In the middle stage of phosphating (the micro-area pH rises to about 5.0 due to the consumption of H+ at the metal / solution interface), the microcapsule wall material dissolves, a large amount of accelerator is released, and the growth and coverage of the film layer are accelerated to ensure the formation of a complete and dense phosphating film in a short time. For workpieces with different zinc layer thicknesses and activities, the interface pH rises at different speeds, and the phosphating solution of the application can automatically match the reaction kinetics to realize "self-adaptive" phosphating, has a wider processing window, excellent process stability, is not sensitive to pretreatment fluctuations and zinc plating layer differences, and improves the product yield.

[0028] In addition, the phosphating solution of the application also adds the auxiliary reducing accelerator sodium nitrobenzenesulfonate, which can form an oxidation-reduction pair with sodium chlorate to stabilize the phosphating speed, prevent Fe 3+ accumulation; the strong polyepoxysuccinic acid complex Ca 2+ / Mg 2+ prevents scale, and assists in complexing Fe 3+ , and the amount of sediment is reduced by more than 50% than that of traditional phosphating solutions of the same type, significantly reducing the cost of waste residue treatment. DETAILED DESCRIPTION

[0029] A person of ordinary skill in the art should recognize that the embodiments are only used to illustrate the application and are not used as a limitation on the application, and as long as the embodiments are changed and modified within the scope of the application, the changes and modifications are within the scope of the claims of the application.

[0030] The preparation method of the pH-responsive microcapsule used in examples 1-6 and comparative examples 3-4 includes the following steps:

[0031] S1, at 30℃, 100mL of deionized water is taken, 40g of sodium chlorate is added, stirred until completely dissolved to form a saturated solution, and an inner water phase (W1) is obtained, if there are undissolved particles after supersaturation, only the supernatant is taken when used;

[0032] S2, 8.0g of hydroxypropyl methylcellulose phthalate (HPMCP, Guangzhou Yuanda New Materials Co., Ltd.) is dissolved in 200mL of dichloromethane to form an oil phase solution (O);

[0033] S3, under high-speed shearing (10000rpm), the inner water phase (W1) is slowly added to the oil phase solution (O), and 1.2g of Span 80 is added, and shearing is continued for 5min until a uniform milky white primary emulsion (W1 / O) is formed;

[0034] S4, 1.5 g of polyvinyl alcohol (PVA-1088, Shanghai Yingjia Industry) was dissolved in 300 mL of deionized water as the external aqueous phase (W2);

[0035] S5, the colostrum (W1 / O) was poured into the external aqueous phase (W2) under moderate stirring, and stirring was continued for 15 min to form a water / oil / water multiple emulsion;

[0036] S6, in a fume hood, low-speed stirring was maintained for 8 h to completely volatilize dichloromethane in the multiple emulsion, HPMCP was insoluble in water, thereby precipitating and solidifying around sodium chlorate solution droplets to form microcapsules;

[0037] S7, the microcapsules were collected by filtration or centrifugation, washed with deionized water, and dried by vacuum freeze-drying to obtain about 12.7 g of pH-responsive microcapsule powder.

[0038] The particle size of the microcapsules was about 5-20 μm measured by microscopy, and about 0.37 g of sodium chlorate (NaClO3) was contained in each gram of microcapsules measured by ultraviolet spectrophotometry.

[0039] Examples 1-6 and Comparative Examples 1-4

[0040] The zinc-plated steel binary phosphating solution of Examples 1-6 and Comparative Examples 2-4 is listed in Table 1, and the preparation method comprises the following steps: adding 2 / 3 of the total amount of deionized water in a container; slowly adding zinc dihydrogen phosphate or / and zinc oxide under stirring until completely dissolved; sequentially adding manganese nitrate solution, phosphoric acid and nitric acid; adding polyepoxysuccinic acid (molecular weight about 3800, Shandong Changyao New Material Co., Ltd.) under continuous stirring until completely dissolved; adding potassium fluorotitanate, and fully stirring to completely dissolve; adding sodium m-nitrobenzenesulfonate, and stirring to dissolve; slowly sprinkling the pH-responsive microcapsule powder, and slightly stirring to uniformly disperse it in the phosphating solution; adding the remaining deionized water, and stirring uniformly to obtain the binary phosphating solution.

[0041] Comparative Examples 1-4 are comparative examples of Example 2, the difference being that in Comparative Example 1, no pH-responsive microcapsule is added, but 2.5 g / L of sodium chlorate is added; in Comparative Example 2, no pH-responsive microcapsule is added; in Comparative Example 3, no sodium m-nitrobenzenesulfonate is added; in Comparative Example 4, no polyepoxysuccinic acid is added; and the rest is the same as Example 2.

[0042] Table 1: Binary phosphating solution for zinc-plated steel of Examples 1-6 and Comparative Examples 2-4

[0043]

[0044] Test experimental examples

[0045] The galvanized steel plate sample was phosphated with the binary phosphating solution of Examples 1-6 and Comparative Examples 1-4, and the application method comprises the following steps: D1, the galvanized steel plate sample was degreased with an alkaline degreasing agent (10 min at 50℃), and then washed with water by two-stage countercurrent rinsing; the surface was adjusted with PL-303B surface conditioner, and then washed with water by two-stage countercurrent rinsing; D2, the cleaned galvanized steel plate sample was immersed in the binary phosphating solution, and soaked for 5 min at 35℃; D3, the sample was washed with water by two-stage countercurrent rinsing, and then washed twice with deionized water, and then dried for cathodic electrophoretic coating.

[0046] The adhesion, neutral salt spray resistance and impact resistance of the phosphating film of the passivated galvanized steel plate sample were measured, and the results are shown in Table 2.

[0047] Neutral salt spray test (NSS): according to GB / T10125-2021, 6 samples of each sample were prepared, the cathodically electrophoresed sample was placed at 30° to the vertical direction in a 35℃ salt spray test chamber, and the sample was sprayed with 5wt% sodium chloride solution with pH of 7.0 at 45℃ pressure barrel temperature and air pressure of 70kPa; after spraying the sample for 500h, 720h and 1000h, 2 samples were taken out respectively, washed with water and dried, and then the corrosion on the surface of the sample was observed, and the width of the corrosion on the scratch of the sample was observed.

[0048] Adhesion test: a series of overlapping circles were drawn on the electrophoresed sample on the paint film adhesion tester, and the paint film peeling was evaluated according to GB / T9286-2021.

[0049] Impact test: according to GB / T1732-2020, a 1kg weight was dropped from a height of 50cm along the cylinder, the distance between the impact head and the groove was greater than 2mm, and the paint film peeling at the impact site was observed.

[0050] Table 2 Properties of passivation film of Examples 1-6 and Comparative Examples 1-4

[0051]

[0052] As shown in Table 2, the galvanized steel plate sample treated with the binary phosphating solution of Examples 1-5 forms a uniform and dense blue-gray to dark gray microcrystalline phosphating film with a crystal size of ≤2μm, and has excellent compatibility with cathodic electrophoresis; after electrophoresis after phosphating, the paint film adhesion can reach 0 level, the neutral salt spray test can reach 1000h, and the single-side corrosion width at the paint film scratch is ≤2mm.

[0053] Compared with Example 3, no pH-responsive microcapsules were added in Comparative Example 1, but 2.5 g / L of sodium chlorate was added; due to the overall release of sodium chlorate at the initial stage of the reaction, the neutral salt spray corrosion resistance of the zinc plating layer can only reach 500 h; no pH-responsive microcapsules were added in Comparative Example 2, and there was no sodium chlorate accelerator, so the adhesion and impact resistance of the phosphating film decreased, and the neutral salt spray corrosion resistance sharply decreased; no sodium nitrobenzenesulfonate, an auxiliary reducing promoter, was added in Comparative Example 3, which cannot form an oxidation-reduction pair with sodium chlorate, so the sediment amount increased, and the neutral salt spray resistance also decreased; no polyepoxysuccinic acid was added in Comparative Example 4, so the sediment amount sharply increased, and the neutral salt spray resistance also decreased.

Claims

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

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

3. The binary phosphating solution for a galvanized steel material according to claim 1 or 2, characterized by The pH threshold value of the pH-responsive microcapsule is 5.0-5.2; the molecular weight of the polyepoxysuccinic acid ranges from 2000 to 5000.

4. The binary phosphating solution for a galvanized steel material according to claim 1 or 2, characterized by The pH value of the binary phosphating solution is 3.0-3.5, the total acid is controlled at 24-30 pt, the free acid is controlled at 1.0-1.8 pt, and the TA / FA acid ratio is 15-25.

5. The binary phosphating solution for a galvanized steel material according to claim 1 or 2, characterized by The preparation method of the pH-responsive microcapsule comprises the following steps: S1, dissolving 30-50 g of sodium chlorate in 100 mL of deionized water at 20-40℃ to form a saturated solution, obtaining an inner water phase; S2, dissolving 5.0-10.0 g of hydroxypropyl methylcellulose phthalate in 200 mL of dichloromethane to form an oil phase solution; S3, slowly adding the inner water phase into the oil phase solution under high-speed shearing, and adding 1.0-2.0 g of Span 80, continuing to shear for 3-10 min, until a milky white primary emulsion is formed; S4, dissolving 0.6-1.8 g of polyvinyl alcohol in 300 mL of deionized water as an outer water phase; S5, pouring the primary emulsion into the outer water phase under moderate stirring, and continuing to stir for 10-20 min to form a water / oil / water multiple emulsion; S6, stirring the multiple emulsion at a low speed for 4-10 h to completely volatilize the dichloromethane in the multiple emulsion, forming microcapsules; S7, collecting the microcapsule precipitate, washing, and drying to obtain the pH-responsive microcapsule.

6. The binary phosphating solution for a galvanized steel material according to claim 5, characterized by The polyvinyl alcohol is a partially alcoholized PVA with an alcoholysis degree of 87%-89%, a polymerization degree of 500-1500, and a molecular weight of 30000-70000.

7. The binary phosphating solution for galvanized steel materials according to claim 5, characterized by In S6, the dichloromethane is completely volatilized by continuously stirring at a low speed in a fume hood or a closed system, and the temperature is controlled at 25-35℃.

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

9. The method of preparing a binary phosphating solution for galvanized steel according to claim 1 or 2, characterized in that, The process comprises the following steps: adding 50-70% of deionized water in the total amount of water in the container; slowly adding zinc dihydrogen phosphate or / and 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, stirring until completely dissolved; adding sodium m-nitrobenzenesulfonate, stirring until dissolved; slowly adding pH-responsive microcapsule powder, stirring slightly to make it evenly dispersed in the phosphating solution; adding the remaining deionized water, stirring until uniform.

10. The use of a binary phosphating solution for a galvanized steel material according to claim 1 or 2, characterized in that, A spray or immersion phosphating process for zinc-containing and galvanized steel materials.

11. The use of a binary phosphating solution for galvanized steel according to claim 10, characterized in that, The phosphating process comprises 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 immersion of the clean steel workpiece in the binary phosphating solution at a temperature of 30-45°C for 3-6 min; D3, two-stage water washing with deionized water, followed by electrophoresis or drying.

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