A thio-group-based phosphorus-free pre-filming agent and its preparation method and application

Abstract

The present invention discloses a phosphorus-free pre-filming agent based on a thio group, which is composed of the following components in percentage by mass: 10-15% of S-carboxyethyl thiosuccinic acid, 2-6% of thiocyanate, 14-20% of zinc salt, 4-6% of acrylic acid-itaconic acid copolymer and 53-70% of softened water. The present invention also discloses a preparation method of the above-mentioned phosphorus-free pre-filming agent based on a thio group and a method for performing pre-filming treatment in a circulating cooling water system. The pre-filming agent of the present invention can effectively avoid the problem of eutrophication of water bodies easily caused by traditional phosphorus-containing pre-filming agents. The circulating water after pre-filming does not contain phosphorus and can be recycled. Therefore, the pre-filming agent of the present invention does not need to discharge sewage after the pre-filming is completed and can be directly transferred to normal operation, thereby saving water resources while saving the cost of cleaning the pre-filming.

Description

Technical Field

[0001] The present invention relates to a thio group-based phosphorus-free pre-filming agent, a preparation method of the phosphorus-free pre-filming agent, and finally a method for pre-filming the phosphorus-free pre-filming agent in a circulating cooling water system. Background Art

[0002] Before the circulating cooling water system is put into production for trial operation, during shutdown for maintenance, or after chemical cleaning of the system, by adding a pre-filming agent to the system, a complete corrosion-resistant protective film can be pre-formed on the metal (such as carbon steel / stainless steel) surface of the system pipes, valves and equipment (such as heat exchanger components). This is used to slow down the corrosion of system components by high alkalinity / hardness concentrated water during long-term operation, thereby extending the service life of the circulating cooling water system.

[0003] Currently, the vast majority of pre-filming agents on the market utilize the conventional polyphosphate + zinc salt formulation. For example, patent CN1715445A discloses a pre-filming agent for metal surfaces, and patent CN102398944A discloses a pre-filming agent for cooling water. In actual applications, while these methods offer good pre-filming effects and relatively high film stability, the phosphorus content in the tail water after pre-filming is high, and the phosphorus can decompose into orthophosphoric acid, which can easily cause eutrophication in surrounding water bodies, resulting in significant environmental pollution. Summary of the Invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a phosphorus-free pre-filming agent, so as not to cause the harm of phosphorus eutrophication to natural water bodies; another purpose of the present invention is to provide a method for preparing the above-mentioned phosphorus-free pre-filming agent and a method for pre-filming treatment thereof in a circulating cooling water system.

[0005] Technical solution: The thio group-based phosphorus-free pre-filming agent of the present invention is composed of the following components in percentage by mass: 10-15% of S-carboxyethyl thiosuccinic acid, 3-6% of thiocyanate, 15-20% of zinc salt, 4-6% of acrylic acid-itaconic acid copolymer and 55-65% of softened water.

[0006] Wherein, the S-carboxyethyl thiosuccinic acid is prepared by the following method, which specifically comprises the following steps:

[0007] (1) adding sodium hydrosulfide aqueous solution and sulfur powder to a three-necked flask, heating and dissolving, then adding acrylonitrile dropwise under heating conditions, and fully reacting to obtain mercaptoacrylonitrile; acidifying with concentrated hydrochloric acid (adjusting the pH to a strong acidic state), adding water and refluxing at a high temperature, cooling after reflux, adding zinc powder, and hydrolyzing to obtain a β-mercaptopropionic acid aqueous solution;

[0008]

[0009] (2) extracting the β-mercaptopropionic acid aqueous solution with ethyl acetate, repeatedly to obtain an organic phase; distilling the extractant from the organic phase under positive pressure to obtain a crude product; subjecting the crude product to vacuum distillation, collecting the high-temperature fraction under slightly positive pressure to obtain pure β-mercaptopropionic acid as a colorless, transparent liquid;

[0010] (3) Add maleic anhydride and distilled water to a three-necked flask, and stir at normal pressure in a constant temperature oil bath until the maleic anhydride is completely dissolved; add the above-mentioned refined β-mercaptopropionic acid to the three-necked flask, adjust the pH to weak acidity with glacial acetic acid, and add a small amount of granular resin as a catalyst, and heat to >100°C and reflux under stirring; after the reaction, filter while hot and cool to low temperature for recrystallization, filter again, and vacuum dry the obtained filter cake to obtain pure S-carboxyethyl thiosuccinic acid white powder;

[0011]

[0012] The thiocyanate is an inorganic compound containing thiocyanate, including potassium thiocyanate, sodium thiocyanate or ammonium thiocyanate. Sodium thiocyanate is preferred due to its high stability.

[0013] The zinc salt is an inorganic zinc compound, including zinc chloride, zinc sulfate heptahydrate, zinc nitrate or zinc gluconate. Zinc chloride is preferred due to its high solubility.

[0014] The ferrous salt is an inorganic compound containing ferrous ions, including ferrous chloride, ferrous sulfate heptahydrate or ferrous nitrate. Ferrous chloride is preferred due to its high relative iron content and stability.

[0015] The relative molecular mass of the acrylic acid-itaconic acid copolymer is between 3000 and 5000. Within this range, itaconic acid monomers and acrylic acid monomers are alternately connected in the polymer. The copolymer has a narrow molecular weight distribution and is approximately homogeneous in chemical composition. The preferred relative molecular mass is 4700.

[0016] The film-forming mechanism of the S-carboxyethyl thiosuccinic acid prepared by the present invention is mixed adsorption film formation. Compared with traditional passivation film formation and precipitation film formation, mixed adsorption film formation occurs in various areas of the metal surface (passivation film formation only occurs in the anode area of the metal surface, and precipitation film formation only occurs in the cathode area of the metal surface, and the pre-filming process has limitations), and the film is dense and uniform, and has strong adhesion to the base metal. S-carboxyethyl thiosuccinic acid is adsorbed on the metal surface to form a monomolecular film through the highly polar thio groups (R1-S-R2) on its molecules; especially when it (S-carboxyethyl thiosuccinic acid) is dissolved in water to provide an acidic atmosphere, the onium ions formed by its thio groups further enhance the adsorption effect with the metal surface (the onium ion is the reaction between the lone pair of electrons of S on the thio group and H in water).+ Forming positively charged coordination bond ions, and performing strong heteroelectric adsorption with the free negative charges on the metal surface), thereby improving the stability of the single molecule film. - The active anion effect of S-carboxyethyl thiosuccinic acid aqueous solution in the acidic atmosphere (under acidic conditions, SCN - The better the stability of the valence bond as the connecting "bridge", the more stable the multi-molecular layer of the film; the anionic SCN formed by NaSCN dissolving in water - The overall negative charge is present, and the acidic aqueous solution provides multiple positively charged H + Proton nucleus, because N and S atoms contain lone pairs of electrons in addition to the bonding electrons, according to the principle of opposite charges attract, SCN - The lone pair electrons of itself and the N and S atoms at its two ends preferentially adsorb H + , thus organically combining the S-carboxyethyl thiosuccinic acid molecules with each other to form a multi-molecular layer protective film), SCN - The S on it is negatively charged and the N and S contain multiple lone pairs of electrons, which can adsorb the onium ions on the S-carboxyethyl thiosuccinic acid. Through "bridging", the S-carboxyethyl thiosuccinic acid molecules are cross-linked and stacked to form a network cross-linked structure, thereby forming a cross-linked network multi-molecular adsorption film layer on the metal surface, which improves the film stability while further enhancing the impact resistance and corrosion resistance of the film.

[0017] On the one hand, the pre-filming agent of the present invention uses zinc salt as a film-forming substance to form a Zn(OH)2 precipitated inorganic film in the cathode area of the metal surface; on the other hand, it uses S-carboxyethyl thiosuccinic acid as a film-forming substance to form an adsorption-type organic film on the metal surface. The precipitation film forms quickly, is porous and thick, and has poor adhesion; the adsorption film forms dense and uniform film, is thin and has strong adhesion stability. The two cooperate and intertwine to form an organic + inorganic composite film on the metal surface, thereby improving the film-forming effect and quality. The composite film on the metal surface forms quickly, is compact and dense, smooth and uniform, and has strong adhesion and corrosion resistance. At the same time, through the numerous -COO on the S-carboxyethyl thiosuccinic acid molecules and the long molecular chains of the acrylic acid-itaconic acid copolymer - The synergistic effect of functional groups and Zn in water 2+ , Ca 2+ The coordination and chelation effect of the isocations ensures that the film-forming substances in the cathode area of the metal surface will not precipitate in the form of alkaline metal salts (precipitate from the solution), but the metal ions are dissolved and dispersed in the solution, and a precipitated inorganic film is formed in the cathode area of the metal surface by co-precipitation, thereby promoting the stability of the pre-film layer on the metal surface.

[0018] The preparation method of the above-mentioned thio group-based phosphorus-free pre-filming agent comprises the following steps:

[0019] (1) Add softened water to beaker A and add the formulated amount of zinc salt while stirring until it is completely dissolved;

[0020] (2) Add softened water to beaker B and slowly add the formulated amount of S-carboxyethyl thiosuccinic acid solid powder while stirring until it is completely dissolved and the solution becomes clear and transparent;

[0021] (3) Transfer beaker B to a water bath at a temperature of 75-85°C. Add the formulated amount of acrylic acid-itaconic acid copolymer to the beaker under stirring and react in the water bath for 40-60 minutes. This step can form a synergistic effect between the -COOH functional groups in the S-carboxyethyl thiosuccinic acid and acrylic acid-itaconic acid copolymer molecules, thereby enhancing the dispersion effect of metal cations during the pre-filming process.

[0022] (4) Add the formulated amount of thiocyanate to beaker B under ultrasonic conditions to promote SCN through the resonance effect of ultrasound. - It bridges with the onium ion on S-carboxyethyl thiosuccinic acid and is used for the formation of multi-molecular layer adsorption film during subsequent pre-filming.

[0023] (5) While stirring, slowly add the solution in beaker A to beaker B dropwise through a dropper and stir for 5 to 6 hours to obtain a phosphorus-free pre-filming agent. If the addition acceleration is too fast, the zinc salt will easily precipitate from the solution in the form of a precipitate.

[0024] The method for pre-filming treatment of the above-mentioned phosphorus-free pre-filming agent in a circulating cooling water system is specifically as follows: the above-mentioned pre-filming agent is added to the pre-filming raw water of the circulating cooling water system (tap water is generally used as the raw water for pre-filming), the addition concentration of the pre-filming agent is 300-600ppm, the pH of the raw water is controlled to be 6.2-6.8 (adjusted to the required pH by using an acid-base regulator), the calcium hardness of the water (measured in CaCO3) is controlled to be >100ppm (soluble calcium salt is used to control the hardness of the water), the temperature of the pre-filming process is 5-40°C (i.e., the external ambient temperature), the linear velocity of the circulating water should be at least >0.5m / s, and the pre-filming time is 48-72h.

[0025] Before pre-coating, the circulating cooling water system requires pipeline degreasing, descaling, and rust removal to achieve a clean, shiny metal surface, thus preparing for pre-coating. After the circulating cleaning is complete, sewage must be drained and water replenished until the water quality meets pre-coating requirements (turbidity <20 NTU and total iron <5 ppm). Only then can pre-coating be performed.

[0026] After pre-coating, the surface of the carbon steel metal coupons inside the pipelines of the circulating cooling water system is evenly covered with a dense purple-black halo coating. According to the CuSO4 solution titration method of HG / T3778-2005 standard, the red dot color development time of the coated coupon is far greater than 10s.

[0027] Beneficial effects: Compared with the prior art, the present invention has the following significant effects: (1) The pre-filming agent of the present invention can effectively avoid the problem of eutrophication of water bodies easily caused by traditional phosphorus-containing pre-filming agents. The water body after pre-filming does not contain phosphorus and can be recycled. Therefore, the pre-filming agent of the present invention can directly enter normal operation without sewage discharge after the pre-filming is completed, thereby saving water resources and saving the cost of cleaning the pre-filming; (2) The components of the pre-filming agent of the present invention have a good synergistic effect, forming an interwoven organic + inorganic composite film on the metal surface, and the composite film has the characteristics of rapid film formation, compactness, smoothness and uniformity, stable adhesion and strong corrosion resistance; (3) In actual application, the pre-filming effect of the pre-filming agent of the present invention is outstanding, which is much longer than the red dot color development time of 10s specified in the HG / T3778-2005 standard; the pre-filming agent of the present invention is suitable for various large / medium / small industrial circulating cooling water systems and has a wide spectrum of application. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is the appearance of the test piece obtained when the pH of the pre-filming water is 4.5 to 8.5;

[0029] Figure 2 This is the film-forming mechanism of the pre-filming agent of the present invention on the metal surface. DETAILED DESCRIPTION

[0030] Example 1

[0031] The phosphorus-free pre-filming agent of the present invention is composed of the following components in percentage by weight: 13% of S-carboxyethyl thiosuccinic acid, 17% of zinc chloride, 4% of sodium thiocyanate, 5% of acrylic acid-itaconic acid copolymer and 61% of softened water; wherein the relative molecular mass of the acrylic acid-itaconic acid copolymer is about 4700, the itaconic acid structural monomers and the acrylic acid structural monomers in the polymer are alternately connected, the copolymer is a composition with a narrow molecular weight distribution and a nearly homogeneous chemical composition, thereby achieving the effect of calcium in water. 2+ / Zn 2+ The best dispersion effect of metal ions.

[0032] Wherein, S-carboxyethyl thiosuccinic acid is prepared by the following method, which specifically comprises the following steps:

[0033] (1) Add 261.3 g of a 30% sodium hydrosulfide aqueous solution and 2.7 g of sulfur powder to a 1000 mL three-necked flask, heat to 30-35° C. to dissolve, and react for 1 h. Add 53 g of acrylonitrile dropwise at 45° C., react for 2 h, acidify with 254 mL of concentrated hydrochloric acid (the mass concentration of concentrated hydrochloric acid is 37%), add 81 g of water, reflux at 110-112° C. for 3 h, cool to 80-90° C., add 44 g of zinc powder, and adjust the pH to 1-2 with hydrochloric acid to obtain a β-mercaptopropionic acid aqueous solution;

[0034]

[0035] (2) extracting the β-mercaptopropionic acid aqueous solution with ethyl acetate, repeating this process three times, to obtain an organic phase; distilling the organic phase to remove the extractant at 0.095 MPa to obtain a crude product; subjecting the crude product to vacuum distillation, collecting the fraction at 100-102° C. at 665 Pa, to obtain a colorless, transparent β-mercaptopropionic acid liquid;

[0036] (3) 46 g of maleic anhydride and 500 mL of distilled water were added to a 1000 mL three-necked flask, and the system was heated to about 75° C. in a constant temperature oil bath under normal pressure with stirring to completely dissolve the maleic anhydride; the β-mercaptopropionic acid prepared above was then added to the system, the pH of the system was adjusted to 5 with glacial acetic acid, and 3 to 5 g of granular resin D101 was added as a catalyst. The system temperature was raised to 120° C. under stirring, and the mixture was refluxed at 120° C. for 2 h, then filtered while hot, and the filtrate was cooled to 4° C. for recrystallization. After filtering again, the filter cake was vacuum dried to obtain pure S-carboxyethyl thiosuccinic acid as a white powder;

[0037]

[0038] The preparation method of the present invention can significantly increase the product yield of the intermediate product β-mercaptopropionic acid, which can reach 87%, and can also reduce the generation of the intermediate by-product dicarboxyl sulfide. Finally, due to the increased yield of the intermediate product β-mercaptopropionic acid, the yield of the final product S-carboxyethyl thiosuccinic acid is naturally and positively promoted, and the yield of the final product S-carboxyethyl thiosuccinic acid can be greater than 80%.

[0039] The preparation method of the phosphorus-free pre-filming agent of the present invention comprises the following steps:

[0040] (1) Add 30.5 g of softened water to beaker A and add 17 g of zinc chloride solid powder while stirring at 300 rpm until completely dissolved;

[0041] (2) Add 30.5 g of softened water to beaker B, and slowly add 13 g of S-carboxyethyl thiosuccinic acid solid powder while stirring at 350 rpm until it is completely dissolved and the solution becomes clear;

[0042] (3) Beaker B was transferred to a water bath at 81°C. 5 g of acrylic acid-itaconic acid copolymer was added thereto while stirring at 330 rpm. The mixture was reacted in a water bath for 45 min. This step allowed the -COOH functional groups in the S-carboxyethyl thiosuccinic acid and acrylic acid-itaconic acid copolymer molecules to form a synergistic effect, thereby enhancing the dispersion effect during the pre-filming process.

[0043] (4) Turn on the ultrasonic device and continue to add 4g of sodium thiocyanate to beaker B under ultrasonic conditions to promote SCN through the resonance effect of ultrasound. - Form a bridge with the Onium ion on the S-carboxyethylthiosuccinic acid molecule;

[0044] (5) At a rotation speed of 500 rpm, the solution in beaker A was slowly added dropwise to beaker B through a dropper, and stirring was continued at a rotation speed of 200 rpm for 5.5 h to obtain the phosphorus-free pre-filming agent product of the present invention.

[0045] Example 2

[0046] The method for pre-filming the phosphorus-free pre-filming agent prepared in Example 1 in a circulating cooling water system is specifically as follows:

[0047] The large-scale circulating cooling water system of an electronics factory uses the membrane recycled water from the front-end process as the raw water for pre-membrane use. The water quality is shown in Table 1:

[0048] Table 1

[0049]

[0050] The circulating cooling water system uses a functional cleaning agent to clean its pipes, heat exchangers, and circulating pump devices before pre-coating. During the cleaning process, the system is replenished with water and replaced according to the water concentration. After 3.5 to 4.5 hours of circulating cleaning, the water turbidity is detected to be 16.7 NTU and the total iron in the water is 3.87 ppm. The system is then drained to prepare for the pre-coating process.

[0051] The pre-filming agent of Example 1 was slowly added to the raw water for pre-filming. The concentration of the pre-filming agent was 450 ppm. The pH of the pre-filming water was controlled at 6.2-6.8. The calcium hardness of the water (calculated as CaCO3) was controlled at 200 ppm. The pre-filming temperature was 31.5°C (the actual ambient temperature on the day of pre-filming). The circulation flow rate of the circulation pump was 1.85 m / s. The pre-filming time was 60 h.

[0052] After the pre-coating process, observation of the Type II standard carbon steel coupons within the circulation piping revealed a dense, uniform, purple-black halo coating covering the previously shiny metal surface. Testing the pre-coated coupons using the CuSO₄ solution titration method according to HG / T3778-2005 revealed a red dot development time of 43 seconds, far exceeding the pre-coating standard.

[0053] Comparative Example 1

[0054] The test used tap water from an electronics factory's circulating cooling water system as the test water sample (raw water for pre-filming) and a 20# carbon steel coupon as the test object. Formula A consisted of zinc salt, Formula B consisted of zinc salt + acrylic acid-itaconic acid copolymer, and Formula C consisted of zinc salt + acrylic acid-itaconic acid copolymer + S-carboxyethyl thiosuccinic acid. Formula D was the pre-filming agent formula of Example 1 of the present invention. The four pre-filming agent formulations were compared under the same dosage concentration (450 ppm), pH (pH = 6.2-6.8), and calcium hardness (200 ppm, calculated as CaCO3). The results showed that the four formulations exhibited film-forming properties, film thickness, red dot discoloration time, and post-pre-filming water quality. Four parallel experiments (1, 2, 3, and 4) were conducted for each formulation under identical experimental conditions. The tests were conducted in accordance with the HG / T3778-2005 "Technical Rules for Chemical Cleaning and Pre-filming of Cooling Water Systems," with a pre-filming cycle of 60 hours.

[0055] The tap water quality of the above system is shown in Table 2:

[0056] Table 2

[0057]

[0058] The above formulas have a summary of the pre-filming effects on the hanging sheets and related data as shown in Table 3:

[0059] Table 3

[0060]

[0061] It can be seen from Table 3 that since the A formula does not introduce a group with dispersing function, the Zn 2+ , Ca 2+ The cations cannot effectively form a film in the cathode region of the metal surface. Under the influence of the alkalinity of the water, they instead form alkaline metal salt precipitates, causing turbidity in the water. Formulation B adds acrylic acid-itaconic acid copolymer as a high-efficiency dispersant component, which utilizes its own coordination and chelation effect to improve the film formation probability on the metal surface. However, because the zinc salt in the formula is a cathode precipitation-type film-forming substance, the film formation is uneven and porous and loose on the surface of the coupon. Formulation C, based on Formulation B, introduces S-carboxyethyl thiosuccinic acid. Through its unique onium ion adsorption film-forming mechanism and the synergistic effect with zinc salt precipitation film formation and acrylic acid-itaconic acid copolymer, a dense and uniform blue-purple halo mixed film is formed on the surface of the coupon. The pre-filming agent formulation of Example 1 of the present invention introduces NaSCN on the basis of Formulation C. Through its active anion effect + bridging mechanism, the film thickness of the mixed adsorption film is increased, and the film appearance color becomes darker, thereby enhancing the impact and corrosion resistance of the film.

[0062] It can also be clearly seen from Table 3 that at the same dosage concentration, the film thickness on the coupon surface and the time for the red dot to change color after titration are in the order of the pre-filming agent formulation of Example 1 of the present invention > formulation C > formulation B > formulation A, indicating that the metal surface pre-filming formed using the formulation of the present invention has the best effect, the highest stability, and the strongest impact and corrosion resistance (the longer the red dot change color after titration of the CuSO4 solution, the stronger the adhesion of the coupon protective film to the metal substrate and the ability of the protective film to resist electrochemical corrosion of the solution, and the more stable the film formation).

[0063] Comparative Example 2

[0064] The method for pre-filming the phosphorus-free pre-filming agent prepared in Example 1 in a circulating cooling water system is specifically as follows:

[0065] The large-scale circulating cooling water system of an electronics factory uses the membrane recycled water from the front-end process as the raw water for pre-membrane. The water quality is as follows:

[0066]

[0067] The circulating cooling water system uses a functional cleaning agent to clean its pipes, heat exchangers, and circulating pump devices before pre-coating. During the cleaning process, the system is replenished with water and replaced according to the water concentration. After 4 to 5 hours of circulating cleaning, the water turbidity is detected to be 18 NTU and the total iron in the water is 2.54 ppm. The system is then drained to prepare for the pre-coating process.

[0068] The pre-filming agent of Example 1 is slowly added to the raw water for pre-filming. The concentration of the pre-filming agent is 450ppm. The pH of the pre-filming water is controlled at 6.2-6.8. The calcium hardness of the water (calculated as CaCO3) is 100, 200 and 400ppm respectively (soluble calcium salt is used to adjust and control the hardness of the pre-filming water). The pre-filming temperature is 28°C, the circulation flow rate of the circulating pump is 2.5m / s, and the pre-filming time is 60h. Under each different calcium hardness working condition, four groups of parallel experiments (1, 2, 3, 4) with exactly the same experimental conditions are carried out. The pre-filming effect and related data of each group of experiments are summarized as shown in Table 4:

[0069] Table 4

[0070]

[0071]

[0072] As can be seen from Table 4, when the calcium hardness concentration is controlled too low (100ppm), although a film can still be formed on the surface of the metal coupon, the film formation will be uneven and the metal surface will not be fully covered. When the calcium hardness concentration is controlled too high (400ppm), the film formed on the metal surface presents a blue halo coating, and the tail water after pre-filming appears light milky white and turbid, with the highest turbidity. This improvement indicates that some metal salts have been precipitated in the solution during pre-filming. Comparison of the color change time of the red dots of the pre-filming: ①200ppm calcium hardness condition>②400ppm calcium hardness condition>③100ppm calcium hardness condition. In summary, the pre-filming effect is best, the color change time is longest, and the impact on the pre-filming tail water is minimal when the calcium hardness (CaCO3) of the pre-filming water body is 200ppm.

[0073] Comparative Example 3

[0074] The large-scale circulating cooling water system of an electronics factory uses the membrane recycled water from the front-end process as the raw water for pre-membrane. The water quality is as follows:

[0075]

[0076] The circulating cooling water system uses a functional cleaning agent to clean its pipes, heat exchangers, and circulating pump devices before pre-coating. During the cleaning process, the system is replenished with water and replaced according to the water concentration. After 4 to 5 hours of circulating cleaning, the water turbidity is detected to be 18.4 NTU and the total iron in the water is 4.09 ppm. The system is then drained to prepare for the pre-coating process.

[0077] Phosphorus-free pre-filming agents (Formula 1 and Formulation 2) were adjusted according to the compatibility ratios shown in Table 5 and compared with the pre-filming agent formulation of Example 1 of the present invention. Each formulation was added to the circulating water system at a concentration of 450 ppm, a pH controlled between 6.2 and 6.8, a water calcium hardness (calculated as CaCO3) controlled at 200 ppm, a pre-filming temperature of 29°C, a circulating pump flow rate of 2.35 m / s, and a pre-filming time of 60 h. Four parallel experiments (1, 2, 3, and 4) were conducted under identical experimental conditions for each formulation.

[0078] The above formulas have a summary of the pre-filming effects on the coupons and related data as shown in Table 5:

[0079] Table 5

[0080]

[0081] As can be seen from Table 5, adjusting the proportions of the components in the formulation of the present invention is not conducive to film formation on the surface of the metal coupon, resulting in thinner film formation, uneven pre-filming on the metal surface, and shortened red dot discoloration time, thereby significantly reducing the ability of the coupon protective film to resist electrochemical corrosion of the solution and the film formation stability.

[0082] Comparative Example 4

[0083] Comparative Example 4 is compared with Example 2. The test conditions are exactly the same. The only difference is that the pH value of the pre-filming water is adjusted to 4.5, 5.5, 6.5, 7.5 and 8.5 respectively. Different pre-filming effects are obtained on the metal workpieces. Figure 1 shown. Figure 1 The six specimens from left to right are specimen No. 1 without pre-filming (sample No. 0, original specimen), specimen No. 2 with a pH of 4.5, specimen No. 3 with a pH of 5.5, specimen No. 4 with a pH of 6.5, specimen No. 5 with a pH of 7.5, and specimen No. 6 with a pH of 8.5. Figure 1 It can be seen that at pH 4.5 and pH 5.5, the metal surface corrodes and turns black. At pH 7.5 and pH 8.5, the metal surface is too alkaline, and the active ingredients of the pre-filming agent, zinc and iron, precipitate out of the solution, preventing the formation of a film on the metal surface. As a result, the metal surface exhibits a metallic luster and no pre-film is formed. Therefore, a pH of 6.5 is the best pre-filming effect, and only then can a complete iridescent coating be formed on the metal surface.

[0084] Figure 2 The film-forming mechanism of the pre-filming agent of the present invention forming a multilayer molecular adsorption film on the metal surface. In the figure, A is the molecular structure of acrylic acid-itaconic acid copolymer, B is the molecular structure of S-carboxyethyl thiosuccinic acid, C is the molecular structure of sodium thiocyanate, and ① is the onium ion structure (the pink double dots in the figure are the lone pair electrons on the S atom, H + is the hydrogen ion in the solution under acidic atmosphere), ② is -COO - The coordination and chelation effect of functional groups and metal cations (M n+ Represents Zn 2+ / Ca 2+ plasma), ③ is the active anion effect and bridging synergy of SCN- (the pink double dots in the figure are the lone pairs of electrons on the S and N atoms, H + is the hydrogen ion in the solution under acidic atmosphere).

Claims

1. A phosphorus-free pre-filming agent based on a thio group, characterized in that: The invention is composed of the following components in percentage by mass: 10-15% of S-carboxyethyl thiosuccinic acid, 3-6% of thiocyanate, 15-20% of zinc salt, 4-6% of acrylic acid-itaconic acid copolymer and 55-65% of softened water.

2. The thio group-based phosphorus-free pre-filming agent according to claim 1, characterized in that The S-carboxyethyl thiosuccinic acid is prepared by the following method, which specifically comprises the following steps: (1) Mixing sodium hydrosulfide aqueous solution and sulfur powder, heating and dissolving, then adding acrylonitrile dropwise under heating conditions, and fully reacting to obtain mercaptoacrylonitrile; acidifying with concentrated hydrochloric acid, adding water and reflux at high temperature, cooling after reflux and adding zinc powder, hydrolyzing to obtain β-mercaptopropionic acid aqueous solution; (2) Extracting the β-mercaptopropionic acid aqueous solution with ethyl acetate, repeatedly, to obtain an organic phase; distilling the extractant from the organic phase under positive pressure to obtain a crude product; subjecting the crude product to vacuum distillation, collecting the high-temperature fraction under slightly positive pressure, to obtain a colorless, transparent liquid of β-mercaptopropionic acid; (3) Maleic anhydride and distilled water are mixed and stirred at normal pressure in a constant temperature oil bath until the maleic anhydride is completely dissolved; β-mercaptopropionic acid is added thereto, the pH is adjusted to weak acidity with glacial acetic acid, and granular resin is added as a catalyst. The temperature is raised to >100°C and refluxed under stirring; after the reaction, the mixture is filtered while hot and cooled to a low temperature for recrystallization. After filtering again, the filter cake is vacuum dried to obtain S-carboxyethyl thiosuccinic acid.

3. The thio group-based phosphorus-free pre-filming agent according to claim 2, characterized in that: In step (1), reflux at 110-112°C for 3-3.5 hours, then cool to 80-90°C and add zinc powder.

4. The thio group-based phosphorus-free pre-filming agent according to claim 2, characterized in that: In step (2), the fraction at 100-102° C. is collected at 665 Pa to obtain β-mercaptopropionic acid as a colorless transparent liquid.

5. The thio group-based phosphorus-free pre-filming agent according to claim 1, characterized in that: The thiocyanate is one of potassium thiocyanate, sodium thiocyanate or ammonium thiocyanate.

6. The thio group-based phosphorus-free pre-filming agent according to claim 1, characterized in that: The zinc salt is one of zinc chloride, zinc sulfate heptahydrate, zinc nitrate or zinc gluconate.

7. The thio group-based phosphorus-free pre-filming agent according to claim 1, characterized in that: The relative molecular mass of the acrylic acid-itaconic acid copolymer is 3000-5000.

8. The method for preparing the thio group-based phosphorus-free pre-filming agent according to claim 1, characterized in that: The steps include: (1) Add softened water to beaker A and add the formulated amount of zinc salt while stirring until it is completely dissolved; (2) Add softened water to beaker B and slowly add the formulated amount of S-carboxyethyl thiosuccinic acid solid powder while stirring until it is completely dissolved and the solution becomes clear and transparent; (3) Transfer beaker B to a water bath at 75-85°C. Add the formulated amount of acrylic acid-itaconic acid copolymer to the beaker under stirring and allow to react in the water bath for 40-60 minutes. (4) Add the formulated amount of thiocyanate to beaker B under ultrasonic conditions; While stirring, slowly add the solution in beaker A to beaker B dropwise through a dropper and stir for 5 to 6 hours to obtain a phosphorus-free pre-filming agent.

9. The method for pre-filming the phosphorus-free pre-filming agent according to claim 1 in a circulating cooling water system, characterized in that: Specifically: when pre-filming is carried out in the circulating cooling water system, the above-mentioned pre-filming agent is added to the pre-filming raw water of the circulating cooling water system. The concentration of the pre-filming agent is 300~600ppm, the pH is 6.2~6.8, the calcium hardness of the water is >100ppm, the pre-filming temperature is 5~40℃, the circulating water streamline velocity should be at least >0.5m / s, and the pre-filming time is 48~72h.

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