Tannic acid / carbon nitride / polydimethylsiloxane composite coating and its preparation method and application

Abstract

The present invention discloses a tannic acid / carbon nitride / polydimethylsiloxane composite coating, a preparation method, and an application thereof. The composite coating is obtained by mixing a tannic acid-modified carbon nitride compound with polydimethylsiloxane. The composite coating, as a component of a photothermal self-repairing intelligent anti-corrosion composite coating, is applied to the surface of Q235 carbon steel and exhibits significant corrosion resistance. Even after immersion in a 3.5wt% NaCl solution for 28 days, its impedance remains at 8.31×10 9 Ω·cm ‑2 ; In addition, the TA-CN / PDMS composite coating exhibits obvious self-healing ability. The photothermal effect of TA stimulates the movement of PDMS chains, and repair can be achieved within 180 minutes of light irradiation.

Description

Technical Field

[0001] The present invention relates to a composite coating, in particular to a tannic acid / carbon nitride / polydimethylsiloxane composite coating, and also to a preparation method and application of the composite coating. Background Art

[0002] Carbon steel is an important metallic material used in various industries, such as marine equipment and offshore infrastructure. However, it is subject to significant corrosion due to a complex combination of factors, including increasing salinity levels of approximately 3.5% and abundant oxygen content, the presence of a large number of marine microorganisms and macroorganisms, and further affected by the interaction of ocean waves and solar radiation. Polydimethylsiloxane (PDMS) coatings are widely used in marine corrosion research due to their remarkable chemical inertness, electrical isolation, and strong adhesion. However, defects caused by micropores and solvent evaporation during the curing process prevent the coating from providing long-term protection.

[0003] Coatings are susceptible to physical damage in practical applications. Corrosive media can penetrate micropores or defects on the coating surface, weakening the coating's barrier function. Therefore, self-healing anti-corrosion coatings are intelligent coatings that can repair damaged areas and restore their anti-corrosion properties, either actively or passively. Based on the healing process, self-healing anti-corrosion coatings are categorized as autonomous or non-autonomous. Autonomous self-healing coatings achieve this by embedding film-forming substances or corrosion inhibitors, but the coatings cannot heal multiple times. In contrast, non-autonomous self-healing coatings primarily rely on external stimuli such as light, heat, pH, and electricity. In recent years, photothermally triggered self-healing coatings, which can be achieved through remote operation and precise positioning, have become a major research focus in self-healing coatings. Photothermally triggered self-healing coatings are an advanced coating technology with self-healing capabilities, but they also have potential drawbacks. These coatings rely on external light and heat sources to trigger the self-healing process, which can be limited by spectral and temperature conditions. This results in slow self-healing rates, limited to repairing surface damage, and poor stability and a short service life. Summary of the Invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a tannic acid / carbon nitride / polydimethylsiloxane composite coating that is corrosion-resistant and has self-healing ability. The second purpose is to provide a method for preparing the above-mentioned composite coating. The third purpose is to provide applications of the above-mentioned composite coating.

[0005] Technical solution: The tannic acid / carbon nitride / polydimethylsiloxane composite coating of the present invention is obtained by mixing a tannic acid-modified carbon nitride compound with polydimethylsiloxane; the mass ratio of the tannic acid-modified carbon nitride compound to polydimethylsiloxane is: 0.05-0.1:10.

[0006] Preferably, the tannic acid-modified carbon nitride composite is obtained by hydrothermal polymerization of flaky carbon nitride and tannic acid, and the mass ratio of tannic acid to carbon nitride is 0.1 to 0.5:1.

[0007] The preparation method of the composite coating comprises the following steps:

[0008] (1) calcining urea and subjecting it to thermal polycondensation to obtain flaky carbon nitride;

[0009] (2) dissolving the flaky carbon nitride and tannic acid in water, stirring, and subjecting the solution to a hydrothermal reaction to obtain a tannic acid-modified carbon nitride composite;

[0010] (3) The tannic acid-modified carbon nitride composite is dispersed in an organic solvent, ultrasonically treated to obtain a uniform dispersion, mixed with polydimethylsiloxane, and stirred to obtain a tannic acid / carbon nitride / polydimethylsiloxane composite coating.

[0011] Preferably, in step (1), the calcination is carried out at a heating rate of 3 to 5°C / min, a holding temperature of 500 to 550°C, and a holding time of 1.5 to 3 hours.

[0012] Preferably, in step (2), the mass ratio of tannic acid to carbon nitride is 0.1-0.5:20; and the hydrothermal reaction is carried out at a holding temperature of 70-90° C. for 24-36 hours.

[0013] Preferably, in step (3), the mass ratio of the tannic acid-modified carbon nitride composite to the organic solvent is 1:10 to 1:3, the ultrasonic time is 0.5 to 1 hour, and the stirring rate is 800 to 1200 r / min, and the stirring time is 0.5 to 2 hours.

[0014] Application of the above-mentioned tannic acid / carbon nitride / polydimethylsiloxane composite coating in anti-corrosion and photothermal self-healing coatings.

[0015] Preferably, the tannic acid / carbon nitride / polydimethylsiloxane composite coating is mixed with a hardener and evenly coated on the surface of the carbon steel substrate to form a composite coating with a thickness of 50 to 150 μm.

[0016] Preferably, the mass ratio of the composite coating to the hardener is 1:0.05-0.2.

[0017] Preferably, the coating is specifically performed by first performing low-speed rotary coating for 1 to 2 minutes at a rotation speed of 300 to 600 r / min, and then performing high-speed rotary coating for 3 to 5 minutes at a rotation speed of 1200 to 1500 r / min.

[0018] Principle of the invention: The present invention uses flaky carbon nitride as a carrier and tannic acid as a modifying material. The tannic acid-modified carbon nitride obtained by polymerization has a maze effect due to the two-dimensional flaky carbon nitride, which extends the path of corrosive ions into the coating, effectively reducing corrosion. Introducing tannic acid (TA) into g-C3N4 can further reduce the thickness of the nanosheets and improve the photothermal performance of the material. Thinner carbon nitride nanosheets usually have a larger surface area, which increases the contact area between the coating and the corrosive substance, provides more opportunities to react with harmful substances in the environment, and helps to improve the anti-corrosion performance of the nanosheets. At the same time, in the application scenario of the composite coating, when light is injected into the coating, the photogenerated electrons in TA-CN are excited, transition from VB to CB, and enter the substrate surface to form cathodic protection, further slowing down corrosion. When the coating surface is scratched, the TA-CN in the composite coating converts light into heat, promoting the recombination and breaking of intermolecular hydrogen bonds under the action of the photothermal effect, further stimulating the rearrangement of the polydimethylsiloxane molecular chain, filling the scratches, realizing the photothermal self-healing of the coating, and at the same time achieving the effect of promoting the anti-corrosion of the coating.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) Excellent corrosion resistance. The coating formed on the surface of the carbon steel substrate has an EIS impedance radius of 1.25×10 10 Ω·cm 2 , the corrosion current is 3.86×10 - 12 A.cm -2 Compared with other coatings, its EIS impedance is improved by 2 to 3 orders of magnitude; and after 60 days of corrosion immersion test, there is still no corrosion point on the coating surface, and the corrosion resistance is long-lasting and stable; (2) The photothermal self-healing ability is extremely strong. With the increase of irradiation time and temperature, the scratches are obviously healed and gradually reduced. When the irradiation time is extended to 180 minutes, the scratches are completely healed and the surface is restored to be smooth. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is the transmission electron microscope (TEM) in all embodiments and comparative examples;

[0021] Figure 2 X-ray diffraction analysis spectra (XRD) of CN of Comparative Example 1 and TA-CN-4 of Example 1;

[0022] Figure 3 Fourier transform infrared analysis spectra (FT-IR) of CN of Comparative Example 1 and TA-CN-4 of Example 1;

[0023] Figure 4 X-ray photoelectron spectroscopy (XPS) analysis of CN of Comparative Example 1 and TA-CN-4 of Example 1;

[0024] Figure 5 High-resolution XPS spectra of CN of Comparative Example 1 and TA-CN-4 of Example 1, Figure a is C1s, Figure b is N1s, and Figure c is O1s;

[0025] Figure 6 Characterization of the anti-corrosion chemical properties of the examples and comparative examples. Figure a is a schematic diagram of the photocatalytic anti-corrosion test, Figure b is the OCP value at different immersion times (1, 7, 14, 21, and 28 days), Figure c is the Tafel curve, and Figure d is the electrochemical parameters corresponding to the prepared coatings;

[0026] Figure 7 Nyquist plots, Bode plots, and phase angle diagrams of the PDMS in Comparative Example 1, the CN / PDMS in Comparative Example 2, and the TA-CN-4 / PDMS coating in Example 1 under 3.5 wt % sodium chloride solution conditions. Figures a, d, g, j, and m are Nyquist plots, Figures N, e, h, k, and n are Bode plots, and Figures c, f, i, l, and o are phase angle diagrams.

[0027] Figure 8 The corrosion optical photographs of the coatings of the examples and comparative examples are shown;

[0028] Figure 9 Analytical diagrams of the photothermal effects of the PDMS coating in comparative example 1, CN / PDMS in comparative example 2, and TA-CN-4 / PDMS in example 1; Figure a is a schematic diagram, Figure b is a temperature change curve under AM1.5G irradiation, and Figure c is an infrared thermal imaging photograph;

[0029] Figure 10 The scanning electron microscope images of the scratches and the healing of the coatings of the examples and comparative examples after 3 hours;

[0030] Figure 11 Characterization of the photoelectrochemical properties of Example 1 TA-CN-4 and Comparative Example 2 CN; where a is the UV-visible diffuse reflectance spectrum, b is the estimated band gap value, c is the VB-XPS spectrum, d is the VB-XPS energy level, e is the prepared PL spectrum, and f is the transient photocurrent response of the TA-CN-4 sample. DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0032] Example 1

[0033] (1) Synthesis of carbon nitride by thermal polymerization: 10 g of urea was placed in a 50 mL crucible in a muffle furnace and heated from room temperature to 500°C at a heating rate of 3°C / min, and then maintained at a constant temperature for 2 hours. The resulting sample was ground in an agate mortar and collected for later use. The sample was labeled CN.

[0034] (2) Preparation of tannic acid-modified carbon nitride composites: 0.4 g of tannic acid (TA) and 1 g of CN were dissolved in 50 mL of deionized water and stirred for 10 min. The solution was then transferred to an autoclave and reacted at 80°C for 24 h. After cooling to room temperature, the resulting composites were washed three times with deionized water and ethanol by centrifugation and dried at 60°C to obtain the desired products, which were labeled as TA-CN-4.

[0035] (3) Preparation of tannic acid / carbon nitride / polydimethylsiloxane composite coating: 0.05 g of tannic acid-modified carbon nitride complex was dispersed in an organic solvent, ultrasonicated to obtain a uniform dispersion, mixed with 10 g of polydimethylsiloxane, and stirred with a magnetic stirrer at 800 r / min for half an hour to obtain a tannic acid / carbon nitride / polydimethylsiloxane composite coating.

[0036] 1g of tannic acid / carbon nitride / polydimethylsiloxane composite coating was mixed with 0.1g of hardener and applied to a carbon steel surface using a spin coater. The coating was dried at room temperature for 24 hours and then cured at 60°C for 2 hours. The resulting coating sample, labeled TA-CN-4 / PDMS, had a thickness of 67μm.

[0037] Example 2

[0038] Compared with Example 1, the content of tannic acid was changed:

[0039] (1) Synthesis of carbon nitride by thermal polymerization: 10 g of urea was placed in a 50 mL crucible in a muffle furnace and heated from room temperature to 500°C at a heating rate of 3°C / min, then maintained at this temperature for 2 h. The resulting sample was ground in an agate mortar and collected for later use. The sample was labeled CN.

[0040] (2) Preparation of tannic acid-modified carbon nitride composites: 0.1 g of tannic acid (TA) and 1 g of CN were dissolved in 50 mL of deionized water and stirred for 10 min. The solution was then transferred to an autoclave and reacted at 80°C for 24 h. After cooling to room temperature, the resulting composites were washed three times with deionized water and ethanol by centrifugation and dried at 60°C to obtain the desired products, which were labeled as TA-CN-1.

[0041] (3) Preparation of tannic acid / carbon nitride / polydimethylsiloxane composite coating: 0.05 g of tannic acid-modified carbon nitride complex was dispersed in an organic solvent, ultrasonicated to obtain a uniform dispersion, mixed with 10 g of polydimethylsiloxane, and stirred with a magnetic stirrer at 800 r / min for half an hour to obtain a tannic acid / carbon nitride / polydimethylsiloxane composite coating.

[0042] 1g of tannic acid / carbon nitride / polydimethylsiloxane composite coating was mixed with 0.1g of hardener and applied to a carbon steel surface using a spin coater. The coating was dried at room temperature for 24 hours and then cured at 60°C for 2 hours. The resulting coating sample, labeled TA-CN-1 / PDMS, had a thickness of 65μm.

[0043] Example 3

[0044] Compared with Example 1, the content of tannic acid was changed:

[0045] (1) Synthesis of carbon nitride by thermal polymerization: 10 g of urea was placed in a 50 mL crucible in a muffle furnace and heated from room temperature to 500°C at a heating rate of 3°C / min, and then maintained at a constant temperature for 2 hours. The resulting sample was ground in an agate mortar and collected for later use. The sample was labeled CN.

[0046] (2) Preparation of tannic acid-modified carbon nitride composites: 0.2 g of tannic acid (TA) and 1 g of CN were dissolved in 50 mL of deionized water and stirred for 10 min. The solution was then transferred to an autoclave and reacted at 80°C for 24 h. After cooling to room temperature, the resulting composites were washed three times with deionized water and ethanol by centrifugation and dried at 60°C to obtain the desired products, which were labeled as TA-CN-2.

[0047] (3) Preparation of tannic acid / carbon nitride / polydimethylsiloxane composite coating: 0.05 g of tannic acid-modified carbon nitride complex was dispersed in an organic solvent, ultrasonicated to obtain a uniform dispersion, mixed with 10 g of polydimethylsiloxane, and stirred with a magnetic stirrer at 800 r / min for half an hour to obtain a tannic acid / carbon nitride / polydimethylsiloxane composite coating.

[0048] 1g of tannic acid / carbon nitride / polydimethylsiloxane composite coating was mixed with 0.1g of hardener and applied to a carbon steel surface using a spin coater. The coating was dried at room temperature for 24 hours and then cured at 60°C for 2 hours. The resulting coating sample, labeled TA-CN-2 / PDMS, had a thickness of 66μm.

[0049] Example 4

[0050] Compared with Example 1, the content of tannic acid was changed:

[0051] (1) Synthesis of carbon nitride by thermal polymerization: 10 g of urea was placed in a 50 mL crucible in a muffle furnace and heated from room temperature to 500°C at a heating rate of 3°C / min, and then maintained at a constant temperature for 2 hours. The resulting sample was ground in an agate mortar and collected for later use. The sample was labeled CN.

[0052] (2) Preparation of tannic acid-modified carbon nitride composites: 0.3 g of tannic acid (TA) and 1 g of CN were dissolved in 50 mL of deionized water and stirred for 10 min. The solution was then transferred to an autoclave and reacted at 80°C for 24 h. After cooling to room temperature, the resulting composites were washed three times with deionized water and ethanol by centrifugation and dried at 60°C to obtain the desired products, which were labeled as TA-CN-3.

[0053] (3) Preparation of tannic acid / carbon nitride / polydimethylsiloxane composite coating: 0.05 g of tannic acid-modified carbon nitride complex was dispersed in an organic solvent, ultrasonicated to obtain a uniform dispersion, mixed with 10 g of polydimethylsiloxane, and stirred with a magnetic stirrer at 800 r / min for half an hour to obtain a tannic acid / carbon nitride / polydimethylsiloxane composite coating.

[0054] 1g of tannic acid / carbon nitride / polydimethylsiloxane composite coating was mixed with 0.1g of hardener and applied to a carbon steel surface using a spin coater. The coating was dried at room temperature for 24 hours and then cured at 60°C for 2 hours. The resulting coating sample, labeled TA-CN-3 / PDMS, had a thickness of 67μm.

[0055] Example 5

[0056] Compared with Example 1, the content of tannic acid was changed:

[0057] (1) Synthesis of carbon nitride by thermal polymerization: 10 g of urea was placed in a 50 mL crucible in a muffle furnace and heated from room temperature to 500°C at a heating rate of 3°C / min, and then maintained at a constant temperature for 2 hours. The resulting sample was ground in an agate mortar and collected for later use. The sample was labeled CN.

[0058] (2) Preparation of tannic acid-modified carbon nitride composites: 0.5 g of tannic acid (TA) and 1 g of CN were dissolved in 50 mL of deionized water and stirred for 10 min. The solution was then transferred to an autoclave and reacted at 80°C for 24 h. After cooling to room temperature, the resulting composites were washed three times with deionized water and ethanol by centrifugation and dried at 60°C to obtain the desired products, which were labeled as TA-CN-5.

[0059] (3) Preparation of tannic acid / carbon nitride / polydimethylsiloxane composite coating: 0.05 g of tannic acid-modified carbon nitride complex was dispersed in an organic solvent, ultrasonicated to obtain a uniform dispersion, mixed with 10 g of polydimethylsiloxane, and stirred with a magnetic stirrer at 800 r / min for half an hour to obtain a tannic acid / carbon nitride / polydimethylsiloxane composite coating.

[0060] 1g of tannic acid / carbon nitride / polydimethylsiloxane composite coating was mixed with 0.1g of hardener and applied to a carbon steel surface using a spin coater. The coating was dried at room temperature for 24 hours and then cured at 60°C for 2 hours. The resulting coating sample, labeled TA-CN-5 / PDMS, had a thickness of 67μm.

[0061] Comparative Example 1

[0062] Compared with Example 1, pure phase PDMS was prepared:

[0063] 1g of polydimethylsiloxane and 0.1g of hardener were stirred for half an hour, then coated on a carbon steel surface using a coater. The mixture was dried at room temperature for 24 hours and cured at 60°C for 2 hours. The resulting coating sample, labeled PDMS, had a thickness of 59 μm.

[0064] Comparative Example 2

[0065] Compared with Example 1, the carbon nitride-polydimethylsiloxane coating was prepared:

[0066] (1) Synthesis of carbon nitride by thermal polymerization: 10 g of urea was placed in a 50 mL crucible in a muffle furnace and heated from room temperature to 500°C at a heating rate of 3°C / min, then maintained at this temperature for 2 h. The resulting sample was ground in an agate mortar and collected for later use. The sample was labeled CN.

[0067] (2) Preparation of carbon nitride-polydimethylsiloxane composite coating: 0.05 g of carbon nitride was dispersed in an organic solvent, ultrasonically obtained to obtain a uniform dispersion, mixed with 10 g of polydimethylsiloxane, and stirred with a magnetic stirrer at 800 r / min for half an hour to obtain a carbon nitride-polydimethylsiloxane composite coating.

[0068] 1g of carbon nitride-polydimethylsiloxane composite coating was mixed with 0.1g of hardener and applied to a carbon steel surface using a spin coater. The coating was dried at room temperature for 24 hours and then cured at 60°C for 2 hours. The resulting coating sample was labeled CN / PDMS and had a thickness of 65μm.

[0069] like Figure 1As shown, transmission electron microscopy (TEM) was performed on the embodiments and comparative examples. It can be seen from the transmission electron microscopy (TEM) that all samples have a flaky structure, which can produce a maze effect in the coating, hindering the penetration of the corrosive medium to a greater extent and delaying the occurrence of the metal-coating interface corrosion process. When the mass ratio of tannic acid to carbon nitride is 0.1 to 0.5:5, the flaky structure of CN is not destroyed. Among them, TA-CN-4 shows a thinner thickness of about 12nm compared to other comparative examples and embodiments. This thinner coating may be easier to penetrate into surface micro-unevenness or tiny cracks, thereby better protecting the substrate. This can improve the anti-corrosion performance of the coating because the coating can cover more potential corrosion points.

[0070] like Figure 2-5 As shown, the material-related characterization of the tannic acid / carbon nitride composite was performed on the examples and comparative examples, indicating that the tannic acid carbon nitride was successfully synthesized via hydrothermal method.

[0071] like Figure 6 As shown, an electrochemical workstation and a standard electrode three-electrode system, including a reference electrode (RE) and a working electrode (WE), were used to study the anti-corrosion performance of the embodiments and comparative examples under light irradiation. The results show that two-dimensional CN nanosheets were added to PDMS to form a CN / PDMS composite coating, which was then coated to form a coating, forming a maze effect in the middle layer, which enhanced the corrosion resistance of the coating. After the introduction of TA-CN, the corrosion resistance of all prepared composite coatings was higher than that of the PDMS coating and the CN / PDMS coating. It is worth noting that TA-CN-4 / PDMS exhibits the best corrosion resistance due to the hydrogen bond between TA and CN, which enhances the compaction of the coating, hinders the penetration of corrosive ions, and improves the corrosion resistance of the coating.

[0072] like Figure 7 As shown, to evaluate the long-term corrosion resistance of all examples and comparative examples in 3.5wt% sodium chloride solution, electrochemical impedance spectroscopy (EIS) measurements were performed at different immersion times, namely 1, 7, 14, 21, and 28 days. The results showed an overall downward trend with increasing immersion time, indicating that the coating was gradually attacked by the corrosive medium during the immersion process, and the coating resistance gradually decreased. The Rp value of the TA-CN / PDMS composite coating decreased and then increased. This is because TA converts the stainless steel into an Fe-TA complex, which prevents the penetration of the corrosive medium. At the same time, the TA replacement treatment improves the adhesion between the substrate and the coating, enhancing the corrosion resistance of the coating.

[0073] like Figure 8As shown, in order to verify the corrosion resistance of the examples and comparative examples in practical applications, a salt spray test was performed. The TA-CN / PDMS composite coating has superior corrosion resistance to the blank PDMS coating and the CN / PDMS coating.

[0074] like Figure 9 As shown, the photothermal performance of all embodiments and comparative examples was tested, and the results showed that the heating rate of the TA-CN / PDMS composite coating was very fast. With the increase of TA content, the temperature rise of the composite coating within the same irradiation time was higher, which indicates that due to the existence of hydrogen bonds between TA and CN, the photothermal effect of the TA-CN / PDMS composite coating was significantly improved.

[0075] like Figure 10 As shown, in order to verify the photothermal self-healing performance of all embodiments and comparative examples, the scratch coating was irradiated with a 300W xenon lamp at room temperature, and the changes in the surface morphology of the scratch coating were recorded using a scanning electron microscope (SEM). The results showed that after introducing TA-CN into the blank PDMS coating, the prepared composite coating self-healed to varying degrees within 180 minutes. The photothermal effect of TA stimulates the movement of PDMS chains. It can be seen that from TA-CN-2 / PDMS to TA-CN-5 / PDMS coatings, the irradiation time gradually decreases with the increase of temperature. When the irradiation time is extended to 180 minutes, the scratches are completely healed and the surface is restored to be smooth and flat.

[0076] like Figure 11 Therefore, the optical absorption properties of all examples and comparative examples were evaluated using UV-visible absorption spectroscopy and photochemical photoelectrochemical tests. The results showed that the TA-modified CN material had better light absorption and photogenerated charge separation efficiency, indicating that the cathode of carbon steel can generate more photogenerated charges, promoting its good corrosion resistance.

Claims

1. A tannic acid / carbon nitride / polydimethylsiloxane composite coating, characterized in that: The composite coating is obtained by mixing a tannic acid-modified carbon nitride compound with polydimethylsiloxane; the mass ratio of the tannic acid-modified carbon nitride compound to the polydimethylsiloxane is 0.05-0.1:10; the tannic acid-modified carbon nitride compound is obtained by hydrothermal polymerization of flaky carbon nitride and tannic acid, and the mass ratio of the tannic acid to the carbon nitride is 0.1-0.5:1; the preparation method of the composite coating comprises the following steps: (1) calcining urea to obtain flake carbon nitride; (2) dissolving the flaky carbon nitride and tannic acid in water, stirring, and subjecting the solution to a hydrothermal reaction to obtain a tannic acid-modified carbon nitride composite; (3) The tannic acid-modified carbon nitride composite is dispersed in an organic solvent, ultrasonically treated to obtain a uniform dispersion, mixed with polydimethylsiloxane, and stirred to obtain a tannic acid / carbon nitride / polydimethylsiloxane composite coating.

2. The composite coating according to claim 1, characterized in that In step (1), the calcination has a heating rate of 3-5 °C / min, a holding temperature of 500-550 °C, and a holding time of 1.5-3 h.

3. The composite coating according to claim 1, characterized in that In step (2), the hydrothermal reaction is carried out at a temperature of 70-90°C and a time of 24-36 hours.

4. The composite coating according to claim 1, characterized in that In step (3), the mass ratio of the tannic acid-modified carbon nitride composite to the organic solvent is 1:10-1:3, the ultrasonic time is 0.5-1 h; the stirring rate is 800-1200 r / min, and the stirring time is 0.5-2 h.

5. Use of the tannic acid / carbon nitride / polydimethylsiloxane composite coating according to claim 1 in anti-corrosion and photothermal self-healing coatings.

6. The use according to claim 5, characterized in that The tannic acid / carbon nitride / polydimethylsiloxane composite coating is mixed with a hardener and evenly coated on the surface of a carbon steel substrate to form a composite coating with a thickness of 50 to 150 μm.

7. The use according to claim 6, characterized in that The mass ratio of the composite coating to the hardener is 1:0.05~0.

2.

8. The use according to claim 6, characterized in that The coating is specifically performed by first performing low-speed rotary coating for 1 to 2 minutes at a rotation speed of 300 to 600 r / min, and then performing high-speed rotary coating for 3 to 5 minutes at a rotation speed of 1200 to 1500 r / min.

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