An organic composite protective coating for photoelectrochemical cathodic protection

Abstract

An organic composite protective coating for photoelectrochemical cathodic protection relates to the field of corrosion and protection of metal materials. The composite coating includes ultra-thin treatment of graphite phase carbon nitride, and a graphite phase carbon nitride coating and a silane coupling agent coating are sequentially prepared on the metal surface. The graphite phase carbon nitride and the silane coupling agent are organically combined, and the overall performance of the organic composite coating is significantly improved. The introduction of the silane coupling agent not only improves the photoelectrochemical cathodic protection performance of the oxidized graphite phase carbon nitride, but also the composed organic composite coating has a physical shielding effect. The preparation method of the organic composite coating is simple and mild, and it is easy to prepare on the metal surface, which is expected to achieve full-effect protection for the metal.

Description

Technical Field

[0001] The invention belongs to the technical field of metal corrosion and protection, and in particular relates to an organic composite protective coating for photoelectrochemical cathode protection. Background Art

[0002] Photoelectrochemical cathodic protection technology is a green, environmentally friendly, energy-saving and sustainable metal corrosion protection technology. It uses sunlight as energy and uses semiconductor photogenerated electrons to cathodically polarize metals to slow down or inhibit metal corrosion. Graphite carbon nitride is a two-dimensional layered non-metallic polymer semiconductor material with a band gap of about 2.7eV, visible light response, and a relatively negative conduction band potential (-1.3eV). It is an ideal photoelectrochemical cathodic protection material. Currently, most of the reported graphite carbon nitride is coupled to metals in the form of photoanodes. Although it has photocathodic protection properties under light, it cannot provide effective protection for metals under dark light.

[0003] Graphite carbon nitride is also used as a nanofiller in organic anti-corrosion coatings due to its high chemical stability, corrosion resistance, and low cost, to improve the barrier effect of organic coatings on corrosive media. However, due to the influence of the coating's adhesion to the substrate, the proportion of graphite carbon nitride powder filler in the coating is extremely low, and it is difficult to form an electron path with the metal, so it cannot provide photoelectrochemical cathodic protection for the metal.

[0004] From the perspective of practical protection, the direct coating method is more suitable for metal photoelectrochemical cathodic protection technology than the external coupled photoanode method. However, graphite carbon nitride materials have no adhesion to metal substrates, and in-situ thermal deposition of graphite carbon nitride materials on metal surfaces can easily cause metal surface defects, which is not conducive to metal protection. Therefore, efficient graphite carbon nitride coating materials for photoelectrochemical cathodic protection are still a challenge. Summary of the invention

[0005] The present invention aims to provide an organic composite protective coating for photoelectrochemical cathode protection. The coating material is composed of oxidized graphite phase carbon nitride and a silane coupling agent (KH550, etc.). The surface modification of the silane coupling agent is used to improve the photoelectrochemical cathode protection performance of the graphite phase carbon nitride; by compounding the graphite phase carbon nitride with the silane coupling agent, a dense oxide coating is formed on the metal surface, achieving the physical barrier effect of the coating on the metal in the absence of light, and improving the protective performance of the composite coating on the metal.

[0006] The above object of the present invention is achieved through the following technical solutions:

[0007] An organic composite protective coating for photoelectrochemical cathode protection includes ultra-thin treatment of graphite phase carbon nitride, and sequential preparation of graphite phase carbon nitride coating and silane coupling agent coating on the metal surface. The graphite phase carbon nitride and the silane coupling agent are organically combined, and the overall performance of the organic composite coating is significantly improved.

[0008] The technical solution adopted by the present invention is: a method for preparing an organic composite protective coating for photoelectrochemical cathode protection, comprising the following steps:

[0009] (1) Protonation and oxidation treatment of graphite phase carbon nitride:

[0010] Melamine and thiourea are used as precursors, melamine and thiourea are ground and mixed uniformly in a mass ratio of (3-5):1, placed in a corundum boat with a cover in a muffle furnace, calcined at 500° C.-700° C. for 1-3 hours, cooled and ground to obtain graphite phase carbon nitride powder;

[0011] The graphite phase carbon nitride powder is added to a 0.1-0.2 mol / L hydrochloric acid solution for ultrasonic treatment, filtered, washed until neutral, and dried to obtain protonated graphite phase carbon nitride;

[0012] The protonated graphite phase carbon nitride powder is placed in concentrated nitric acid and stirred in a water bath at 80-90° C., the pH is adjusted to neutral, washed, and freeze-dried to obtain an oxide graphite phase carbon nitride powder;

[0013] (2) Preparation of graphite phase carbon nitride film:

[0014] Ultrasonic dispersion of the graphite oxide phase carbon nitride powder in acetone, and then adding elemental iodine to prepare a standby solution A;

[0015] The electrophoretic deposition method is adopted, with metal as the negative electrode, Pt as the positive electrode, and the standby liquid A as the electrolyte, to deposit the graphite phase carbon nitride on the metal surface, and then dry it in a vacuum drying oven at 50-60° C. to obtain a substrate with a graphite phase carbon nitride film deposited thereon;

[0016] (3) Compounding of silane coupling agent:

[0017] The substrate with the graphite phase carbon nitride film deposited thereon is immersed in a silane coupling agent mixed solution and heated in a water bath at 50-70°C for t hours, 1<t<4; and dried to obtain an organic composite protective coating. The amount of graphite phase carbon nitride powder: elemental iodine: acetone in the standby solution A is 4mg:5mg:10mL.

[0018] The volume ratio of the silane coupling agent to the ethanol aqueous solution in the silane coupling agent mixed solution is (5-8):1000, and the volume ratio of ethanol to water in the ethanol aqueous solution is 1:10.

[0019] The silane coupling agent is selected from KH550, KH560 and KH570, all of which are commercial products. The organic composite protective coating is used to protect the photoelectrochemical cathode.

[0020] The present invention develops an organic coating material composed of graphite phase carbon nitride and silane coupling agent for photocathode protection. The coating can be directly prepared on the metal surface without destroying the metal structure and surface composition. Under light excitation, the gC in the organic composite coating 3 N 4 The electrons in the valence band will jump to the conduction band, and the electrons will migrate to the metal surface, causing the metal to undergo cathodic polarization and inhibiting its electrochemical dissolution. Due to the introduction of the silane coupling agent, the electron-hole separation efficiency in the coating is improved, and the holes migrate to the coating surface and are consumed. Therefore, the surface modification of the silane coupling agent promotes the photoelectrochemical cathodic protection performance of graphite carbon nitride. In addition, in the absence of light, the graphite carbon nitride and the silane coupling agent are organically compounded to form a dense and firmly attached coating on the metal surface, preventing the corrosive medium from penetrating into the pores of the graphite carbon nitride, and providing physical protection for the metal.

[0021] Beneficial effects of the present invention:

[0022] 1. The graphite phase carbon nitride in the coating is protonated and oxidized, which is beneficial to promote the graphite phase carbon nitride to form ultra-thin nanosheets and increase the specific surface area of ​​the material; at the same time, after the oxidation treatment, the surface hydroxyl groups of the graphite phase carbon nitride increase, the hydrophilicity is enhanced, and the silane coupling agent solution is easy to enter the pores of the graphite phase carbon nitride, which is beneficial to the subsequent composite with the silane coupling agent.

[0023] 2. The silane coupling agent is introduced into the graphite phase carbon nitride in the coating to form an organic composite coating, and the composite coating exhibits excellent performance in photoelectrochemical cathodic protection. The surface of the graphite phase carbon nitride is modified with the help of a silane coupling agent to promote the separation of photogenerated electron-hole pairs in the composite coating and improve the photoelectrochemical cathodic protection capability of the graphite phase carbon nitride.

[0024] 3. The graphite phase carbon nitride in the coating is deposited by electrophoresis and the surface is modified with a silane coupling agent. The preparation method is simple and mild, does not affect the metal surface structure, and the organic composite coating is dense and firmly attached to the metal surface. The physical shielding effect of the coating on the corrosive medium can inhibit the corrosion of the metal in the absence of light.

[0025] 4. The organic composite coating composed of graphite phase carbon nitride and silane coupling agent used for photoelectrochemical cathodic protection in the coating has not been reported yet. This coating shows excellent performance in both photoelectrochemical cathodic protection and physical protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a SEM image of graphite oxide phase carbon nitride powder prepared in Example 2 of the present invention.

[0027] Figure 2 This is a TEM image of graphite oxide phase carbon nitride powder prepared in Example 2 of the present invention.

[0028] Figure 3 Solid UV-visible diffuse reflection absorption spectra of the graphite oxide phase carbon nitride coating and the graphite oxide phase carbon nitride / silane coupling agent composite coating prepared in Example 2 of the present invention.

[0029] Figure 4 The photocurrent-time curves are of the organic composite coatings based on FTO conductive glass prepared in Examples 1 to 3 of the present invention, and the coatings prepared in Comparative Examples 1 and 2 after being connected to 304 stainless steel via a salt bridge.

[0030] Figure 5 The open circuit voltage-time curves of the organic composite coatings based on FTO conductive glass prepared in Examples 1 to 3 of the present invention, the coatings prepared in Comparative Examples 1 and 2, and connected to 304 stainless steel via a salt bridge.

[0031] Figure 6 Polarization curves of the organic composite coating prepared with 304 stainless steel as the substrate in Example 2 of the present invention, and the coating prepared with 304 stainless steel as the substrate in Comparative Example 1 and Comparative Example 2. DETAILED DESCRIPTION

[0032] The specific implementation modes of the present invention are further described below in conjunction with examples. It should be noted that the specific implementation modes described herein are only for illustrating and explaining the present invention and are not limited to the present invention.

[0033] Example 1

[0034] (1) Protonation and oxidation treatment of graphite phase carbon nitride:

[0035] Weigh 10g of melamine and thiourea mixed powder, put it into a corundum boat and place it in a muffle furnace, heat it to 570℃ at 2℃ / min for 2h, and grind it into powder with a mortar after cooling to obtain graphite phase carbon nitride. Melamine and thiourea mixed powders are ground and mixed evenly in a mass ratio of 4:1. Weigh 2g of the above powder and put it into a hydrochloric acid solution for ultrasonic treatment for 4h, stir it for 6h, filter it, wash it with water until neutral, and dry it in a vacuum drying oven at 80℃ for 12h to obtain protonized graphite phase carbon nitride. Weigh 0.2g of protonized graphite phase carbon nitride and transfer it to a 50mL round-bottom flask and add 20mL of concentrated nitric acid. Heat it in a water bath at 85℃ for 6h under strong magnetic stirring, let it stand at room temperature for sedimentation, remove the supernatant, adjust the pH to neutral with 1mol / L NaOH solution; wash it with water 3 times and ethanol once, and freeze-dry it to obtain an oxidized graphite phase carbon nitride sample.

[0036] (2) Preparation of graphite phase carbon nitride coating:

[0037] Use metal or FTO conductive glass as the negative electrode, Pt as the positive electrode, and an acetone solution containing graphite oxide phase carbon nitride as the electrolyte. The electrolyte consists of 12 mg of graphite oxide phase carbon nitride, 15 mg of elemental iodine and 30 mL of acetone. After stirring the graphite oxide phase carbon nitride for 3 hours and dispersing it in acetone, add elemental iodine and ultrasonicate for 1 hour.

[0038] The electrophoretic deposition method was used. The distance between the negative electrode and the positive electrode was 20 mm. The polished surface of 304 stainless steel or the conductive surface of FTO should face the Pt sheet. The parameters of electrophoretic deposition were as follows: the electrophoretic deposition voltage was 25 V, the electrophoresis time was 5 min, and the area of ​​the Pt sheet was 1 cm*1 cm.

[0039] Carbon nitride in the form of graphite oxide is deposited on the metal surface or the conductive surface of FTO glass, and the obtained coating material is dried in a vacuum drying oven at 50°C for 4 hours.

[0040] The metal is 304 stainless steel with a size of 10mm*25mm*1.5mm; the size of FTO conductive glass is 10mm*25mm*2mm; stainless steel 304 needs to be polished with 400 mesh, 800 mesh, and 1500 mesh sandpaper before use until there are no obvious scratches on the surface.

[0041] Before use, FTO conductive glass needs to be ultrasonically cleaned with acetone and ethanol for 3 times each. The surface is clean and free of impurities, and then blown dry with nitrogen for later use.

[0042] (3) Compounding of silane coupling agent:

[0043] The substrate with graphite phase carbon nitride material was immersed in a mixed solution containing a silane coupling agent and heated in a water bath at 60°C for 1 hour, and then placed in a vacuum drying oven at 80°C for 4 hours.

[0044] The silane coupling agent mixture was prepared by adding 25 mL of ethanol and water mixture (volume ratio 1:10) into a 50 mL round-bottom flask, stirring and mixing, adding 0.15 mL of silane coupling agent (KH550), and ultrasonically treating for 30 minutes.

[0045] The substrate sheet is suspended above the silane coupling agent mixed liquid, and the carbon nitride portion with the graphite phase is vertically inserted into the KH550 mixed liquid.

[0046] Example 2

[0047] (1) Protonation and oxidation treatment of graphite phase carbon nitride:

[0048] Weigh 10g of melamine and thiourea mixed powder, put it into a corundum boat and place it in a muffle furnace, heat it to 570℃ at 2℃ / min for 2h, and grind it into powder with a mortar after cooling to obtain graphite phase carbon nitride. Melamine and thiourea mixed powders are ground and mixed evenly in a mass ratio of 4:1. Weigh 2g of the above powder and put it into a hydrochloric acid solution for ultrasonic treatment for 4h, stir it for 6h, filter it, wash it with water until neutral, and dry it in a vacuum drying oven at 80℃ for 12h to obtain protonized graphite phase carbon nitride. Weigh 0.2g of protonized graphite phase carbon nitride and transfer it to a 50mL round-bottom flask and add 20mL of concentrated nitric acid. Heat it in a water bath at 85℃ for 6h under strong magnetic stirring, let it stand at room temperature for sedimentation, remove the supernatant, adjust the pH to neutral with 1mol / L NaOH solution; wash it with water 3 times and ethanol once, and freeze-dry it to obtain an oxidized graphite phase carbon nitride sample.

[0049] (2) Preparation of graphite phase carbon nitride coating:

[0050] Use metal or FTO conductive glass as the negative electrode, Pt as the positive electrode, and an acetone solution containing graphite oxide phase carbon nitride as the electrolyte. The electrolyte consists of 12 mg of graphite oxide phase carbon nitride, 15 mg of elemental iodine and 30 mL of acetone. After stirring the graphite oxide phase carbon nitride for 3 hours and dispersing it in acetone, add elemental iodine and ultrasonicate for 1 hour.

[0051] The electrophoretic deposition method was used. The distance between the negative electrode and the positive electrode was 20 mm. The polished surface of 304 stainless steel or the conductive surface of FTO should face the Pt sheet. The parameters of electrophoretic deposition were as follows: the electrophoretic deposition voltage was 25 V, the electrophoresis time was 5 min, and the area of ​​the Pt sheet was 1 cm*1 cm.

[0052] Carbon nitride in the form of graphite oxide is deposited on the metal surface or the conductive surface of FTO glass, and the obtained coating material is dried in a vacuum drying oven at 50°C for 4 hours.

[0053] The metal is 304 stainless steel with a size of 10mm*25mm*1.5mm; the size of FTO conductive glass is 10mm*25mm*2mm; stainless steel 304 needs to be polished with 400 mesh, 800 mesh, and 1500 mesh sandpaper before use until there are no obvious scratches on the surface.

[0054] Before use, FTO conductive glass needs to be ultrasonically cleaned with acetone and ethanol for 3 times each. The surface is clean and free of impurities, and then blown dry with nitrogen for later use.

[0055] (3) Compounding of silane coupling agent:

[0056] The substrate with graphite phase carbon nitride material was immersed in a mixed solution containing a silane coupling agent and heated in a water bath at 60°C for 2 hours, and then placed in a vacuum drying oven at 80°C for 4 hours.

[0057] The silane coupling agent mixture was prepared by adding 25 mL of ethanol and water mixture (volume ratio 1:10) into a 50 mL round-bottom flask, stirring and mixing, adding 0.15 mL of silane coupling agent (KH550), and ultrasonically treating for 30 minutes.

[0058] The substrate sheet is suspended above the silane coupling agent mixed liquid, and the carbon nitride portion with the graphite phase is vertically inserted into the KH550 mixed liquid.

[0059] Example 3

[0060] (1) Protonation and oxidation treatment of graphite phase carbon nitride:

[0061] Weigh 10g of melamine and thiourea mixed powder, put it into a corundum boat and place it in a muffle furnace, heat it to 570℃ at 2℃ / min for 2h, and grind it into powder with a mortar after cooling to obtain graphite phase carbon nitride. Melamine and thiourea mixed powders are ground and mixed evenly in a mass ratio of 4:1. Weigh 2g of the above powder and put it into a hydrochloric acid solution for ultrasonic treatment for 4h, stir it for 6h, filter it, wash it with water until neutral, and dry it in a vacuum drying oven at 80℃ for 12h to obtain protonized graphite phase carbon nitride. Weigh 0.2g of protonized graphite phase carbon nitride and transfer it to a 50mL round-bottom flask and add 20mL of concentrated nitric acid. Heat it in a water bath at 85℃ for 6h under strong magnetic stirring, let it stand at room temperature for sedimentation, remove the supernatant, adjust the pH to neutral with 1mol / L NaOH solution; wash it with water 3 times and ethanol once, and freeze-dry it to obtain an oxidized graphite phase carbon nitride sample.

[0062] (2) Preparation of graphite phase carbon nitride coating:

[0063] Use metal or FTO conductive glass as the negative electrode, Pt as the positive electrode, and an acetone solution containing graphite oxide phase carbon nitride as the electrolyte. The electrolyte consists of 12 mg of graphite oxide phase carbon nitride, 15 mg of elemental iodine and 30 mL of acetone. After stirring the graphite oxide phase carbon nitride for 3 hours and dispersing it in acetone, add elemental iodine and ultrasonicate for 1 hour.

[0064] The electrophoretic deposition method was used. The distance between the negative electrode and the positive electrode was 20 mm. The polished surface of 304 stainless steel or the conductive surface of FTO should face the Pt sheet. The parameters of electrophoretic deposition were as follows: the electrophoretic deposition voltage was 25 V, the electrophoresis time was 5 min, and the area of ​​the Pt sheet was 1 cm*1 cm.

[0065] Carbon nitride in the form of graphite oxide is deposited on the metal surface or the conductive surface of FTO glass, and the obtained coating material is dried in a vacuum drying oven at 50°C for 4 hours.

[0066] The metal is 304 stainless steel with a size of 10mm*25mm*1.5mm; the size of FTO conductive glass is 10mm*25mm*2mm; stainless steel 304 needs to be polished with 400 mesh, 800 mesh, and 1500 mesh sandpaper before use until there are no obvious scratches on the surface.

[0067] Before use, FTO conductive glass needs to be ultrasonically cleaned with acetone and ethanol for 3 times each. The surface is clean and free of impurities, and then blown dry with nitrogen for later use.

[0068] (3) Compounding of silane coupling agent:

[0069] The substrate with graphite phase carbon nitride material was immersed in a mixed solution containing a silane coupling agent and heated in a water bath at 60°C for 4 hours, and then placed in a vacuum drying oven at 80°C for 4 hours.

[0070] The silane coupling agent mixture was prepared by adding 25 mL of ethanol and water mixture (volume ratio 1:10) into a 50 mL round-bottom flask, stirring and mixing, adding 0.15 mL of silane coupling agent (KH550), and ultrasonically treating for 30 minutes.

[0071] The substrate sheet is suspended above the silane coupling agent mixed liquid, and the carbon nitride portion with the graphite phase is vertically inserted into the KH550 mixed liquid.

[0072] In Examples 1-3, the different silane coupling dosages are recorded as 15SCA-1 / O-CN, 15SCA-2 / O-CN, and 15SCA-4 / O-CN, respectively.

[0073] Comparative Example 1

[0074] Referring to step (1) and step (2) in Example 2, a silane-free graphite-phase carbon nitride coating material was prepared.

[0075] (1) Protonation and oxidation treatment of graphite phase carbon nitride:

[0076] Weigh 10g of melamine and thiourea mixed powder, put it into a corundum boat and place it in a muffle furnace, heat it to 570℃ at 2℃ / min for 2h, and grind it into powder with a mortar after cooling to obtain graphite phase carbon nitride. Melamine and thiourea mixed powders are ground and mixed evenly in a mass ratio of 4:1. Weigh 2g of the above powder and put it into a hydrochloric acid solution for ultrasonic treatment for 4h, stir it for 6h, filter it, wash it with water until neutral, and dry it in a vacuum drying oven at 80℃ for 12h to obtain protonized graphite phase carbon nitride. Weigh 0.2g of protonized graphite phase carbon nitride and transfer it to a 50mL round-bottom flask and add 20mL of concentrated nitric acid. Heat it in a water bath at 85℃ for 6h under strong magnetic stirring, let it stand at room temperature for sedimentation, remove the supernatant, adjust the pH to neutral with 1mol / L NaOH solution; wash it with water 3 times and ethanol once, and freeze-dry it to obtain an oxidized graphite phase carbon nitride sample.

[0077] (2) Preparation of graphite phase carbon nitride coating:

[0078] The metal was used as the negative electrode, Pt as the positive electrode, and an acetone solution containing graphite oxide phase carbon nitride was used as the electrolyte. The electrolyte consisted of 12 mg graphite oxide phase carbon nitride, 15 mg elemental iodine and 30 mL acetone. After the graphite oxide phase carbon nitride was dispersed in acetone by stirring for 3 hours, elemental iodine was added and ultrasonicated for 1 hour.

[0079] The electrophoretic deposition method was used. The distance between the negative electrode and the positive electrode was 20 mm. The polished surface of 304 stainless steel or the conductive surface of FTO should face the Pt sheet. The parameters of electrophoretic deposition were as follows: the electrophoretic deposition voltage was 25 V, the electrophoresis time was 5 min, and the area of ​​the Pt sheet was 1 cm*1 cm.

[0080] Carbon nitride in the form of graphite oxide is deposited on the metal surface or the conductive surface of FTO glass, and the obtained coating material is dried in a vacuum drying oven at 50°C for 4 hours.

[0081] The metal is 304 stainless steel with a size of 10mm*25mm*1.5mm; the size of FTO conductive glass is 10mm*25mm*2mm; stainless steel 304 needs to be polished with 400 mesh, 800 mesh, and 1500 mesh sandpaper before use until there are no obvious scratches on the surface.

[0082] Before use, FTO conductive glass needs to be ultrasonically cleaned with acetone and ethanol for 3 times each. The surface is clean and free of impurities, and then blown dry with nitrogen for later use.

[0083] Comparative Example 2

[0084] Referring to step (3) of the method of Example 2, a silane coating material free of graphite phase carbon nitride was prepared.

[0085] Preparation of silane coupling agent:

[0086] The substrate was immersed in a mixed solution containing a silane coupling agent and heated in a 60°C water bath for 2 h, and then placed in a vacuum drying oven at 80°C for 4 h.

[0087] The silane coupling agent mixture was prepared by adding 25 mL of ethanol and water mixture (volume ratio 1:10) into a 50 mL round-bottom flask, stirring and mixing, adding 0.15 mL of silane coupling agent (KH550), and ultrasonically treating for 30 minutes.

[0088] The substrate sheet is suspended above the silane coupling agent mixture and partially inserted vertically into the KH550 mixture.

[0089] The substrate is 304 stainless steel with a size of 10mm*25mm*1.5mm; the stainless steel 304 needs to be polished with 400 mesh, 800 mesh, and 1500 mesh sandpaper before use until there are no obvious scratches on the surface. The organic composite coatings prepared in Examples 1 to 3 and the coating prepared in Comparative Example 1 were tested for photoelectrochemical cathodic protection performance.

[0090] In order to eliminate the interference caused by stainless steel corrosion in the test, the coating is based on FTO conductive glass during the photoelectrochemical cathodic protection test, which can more directly obtain the performance of the coating. The FTO conductive glass and the metal are connected with copper wires and regarded as a conductive whole.

[0091] The photocathode protection test device includes two electrolytic cells, a corrosion cell and a photocell, which are connected by a salt bridge. The electrolyte in the corrosion cell is 3.5wt% NaCl solution, and the electrolyte in the photocell is 0.25mol / L Na 2 S and 0.35 mol / L Na 2 SO 3 Solution. The protected metal 304 stainless steel was used as the working electrode, silver chloride was used as the reference electrode, and platinum sheet was used as the counter electrode in the corrosion cell. The prepared coating sample FTO conductive glass was placed in the photocell, and the 304 stainless steel and FTO conductive glass were connected with wires. The light source used was a 300W Xe lamp with an AM1.5 filter. The changes in the photocurrent-time curve and the photovoltage-time curve under illumination were measured using a CHI 660E electrochemical workstation, where on means illumination and off means turning off the light source, i.e., no illumination.

[0092] The photoelectrochemical cathodic protection coating performance test results are as follows:

[0093] Figure 5 It can be seen that when 304 stainless steel is coupled with graphite nitride oxide in comparative example 1 under simulated sunlight, the potential of 304 stainless steel shifts negatively, and the cathode protection potential drops to about -0.27V at this time, indicating that graphite nitride oxide has photogenerated electrons migrated to the stainless steel surface under light, playing a photocathodic protection role for the metal. After compounding the silane coupling agent, the potential of 304 stainless steel shifts negatively more significantly, and the organic composite coating prepared in Example 2 shows the best photocathodic protection ability, and its potential shifts negatively by about -0.20V.

[0094] Figure 4 It can be seen that under simulated sunlight irradiation, the materials prepared in Examples 1 to 3 and Comparative Example 1 can provide cathodic protection current for 304 stainless steel. The composite coating prepared in Example 2 provides the largest cathodic protection current for 304 stainless steel, which is about 5 times that of the single oxide graphite phase carbon nitride prepared in Comparative Example 1.

[0095] from Figure 4 and5 It can also be seen that the photocathode protection performance of the composite coatings prepared in Examples 1 and 3 is lower than that of Example 2. The low photocathode protection performance of Example 1 is due to the small amount of silane coupling agent. As the amount of surface-modified silane coupling agent increases, the photocathode protection efficiency is significantly improved. The silane coupling agent has no obvious effect on the light absorption of the composite coating ( Figure 3 ), the reason for the improved efficiency of the composite coating may be that the addition of silane coupling agent promotes the separation of photogenerated carriers. The low photocathode protection performance of Example 3 is due to the excessive amount of silane coupling agent, which affects the consumption of photogenerated holes and reduces the separation of photogenerated electron-hole pairs. Therefore, Example 2 shows the best photoelectrochemical cathodic protection performance.

[0096] The physical barrier performance test was performed on the organic composite coating prepared with 304 stainless steel as the substrate in Example 2 and the coating prepared with 304 stainless steel as the substrate in Comparative Example 2.

[0097] The coating is prepared based on 304 stainless steel.

[0098] The physical barrier performance test was carried out by using 304 stainless steel as the working electrode, platinum sheet as the counter electrode, saturated AgCl / Ag as the reference electrode, and 3.5wt% NaCl solution as the electrolyte. The polarization curves of Example 2, Comparative Example 1 and Comparative Example 2 with 304 stainless steel as the substrate were measured using a CHI 660E electrochemical workstation.

[0099] The test results of the physical barrier performance of the coating without light are as follows:

[0100] Figure 6 It can be seen that the self-corrosion potential of 304 stainless steel of the organic composite coating prepared in Example 2 is about -0.18V, which is significantly higher than that of Comparative Example 1 and Comparative Example 2; at the same time, the corrosion current of 304 stainless steel of the organic composite coating prepared in Example 2 is also the smallest. This shows that the physical barrier capacity of the three coatings is as follows: organic composite coating in Example 2 > silane coupling agent in Comparative Example 2 > graphite nitride oxide in Comparative Example 1. The structure of ultra-thin graphite nitride oxide nanomaterials is loose and porous ( Figure 1 and Figure 2 ), the physical barrier ability is poor, but the loose porous structure is conducive to the infiltration of the silane coupling agent into the pores. The graphite oxide phase carbon nitride and the silane coupling agent form a dense organic composite coating, which shows excellent physical barrier ability compared with comparison 1 and comparison 2, and has a protective effect on the metal.

Claims

1. A method for preparing an organic composite protective coating for photoelectrochemical cathodic protection, characterized in that: The following steps are involved: (1) Protonation and oxidation treatment of graphite carbon nitride: Melamine and thiourea are used as precursors, melamine and thiourea are ground and mixed evenly in a mass ratio of (3-5):1, placed in a corundum boat with a cover in a muffle furnace, calcined at 500°C-700°C for 1-3h, cooled and ground to obtain graphite phase carbon nitride powder; The graphite phase carbon nitride powder is added to a 0.1-0.2 mol / L hydrochloric acid solution for ultrasonic treatment, filtered, washed until neutral, and dried to obtain protonated graphite phase carbon nitride; The protonated graphite phase carbon nitride powder is placed in concentrated nitric acid and stirred in a water bath at 80-90° C., the pH is adjusted to neutral, washed, and freeze-dried to obtain an oxide graphite phase carbon nitride powder; (2) Preparation of graphite phase carbon nitride film: Ultrasonic dispersion of the graphite oxide phase carbon nitride powder in acetone, and then adding elemental iodine to prepare a standby solution A; The electrophoretic deposition method is adopted, with metal as the negative electrode, Pt as the positive electrode, and the standby liquid A as the electrolyte, to deposit the graphite phase carbon nitride on the metal surface, and then dry it in a vacuum drying oven at 50-60° C. to obtain a substrate with a graphite phase carbon nitride film deposited thereon; (3) Compounding of silane coupling agent: The substrate deposited with the graphite phase carbon nitride film is immersed in a silane coupling agent mixed solution and heated in a water bath at 50-70° C. for t hours, 1<t<4; and dried to obtain an organic composite protective coating.

2. The method for preparing an organic composite protective coating for photoelectrochemical cathodic protection according to claim 1, characterized in that: The ratio of graphite oxide phase carbon nitride powder: elemental iodine: acetone in the standby liquid A is 4 mg: 5 mg: 10 mL.

3. The method for preparing an organic composite protective coating for photoelectrochemical cathodic protection according to claim 1, characterized in that: The volume ratio of the silane coupling agent to the ethanol aqueous solution in the silane coupling agent mixed solution is (5-8):1000, and the volume ratio of ethanol to water in the ethanol aqueous solution is 1:

10.

4. The method for preparing an organic composite protective coating for photoelectrochemical cathodic protection according to claim 3, characterized in that: The silane coupling agent is selected from KH550, KH560 and KH570.

5. The organic composite protective coating prepared by the preparation method according to claim 1 is used for photoelectrochemical cathodic protection of metals.

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