An iron-based surface phosphorylated carbon nitride coating and its preparation method

Abstract

An iron-based surface phosphorylated carbon nitride coating and its preparation method disclosed by the present invention belong to the technical field of coating anti-corrosion. First, the iron-based surface is pretreated; then, the pretreated iron-based surface is phosphorylated with a phosphorylation solution and dried; the phosphorylation solution is composed of phosphoric acid, water, absolute ethanol, and potassium ferrocyanide; finally, a layer of melamine is laid on the phosphorylated iron-based surface for roasting, and after cooling, a phosphorylated carbon nitride coating is obtained. The preparation of the present invention has mild conditions, simple process, cheap raw materials, and good repeatability, solves the problem of poor adhesion between the coating and the metal matrix, and the obtained coating has strong corrosion resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of coating anti-corrosion, and relates to an iron-based surface phosphorylated carbon nitride coating and a preparation method thereof. Background Art

[0002] Metal corrosion is the damage of metal materials under the action of surrounding media, showing a rusty form. Metal materials are an important part of modern production and life and are widely used in various fields such as aerospace, marine ships, transportation, and people's daily lives. However, metal corrosion has caused serious economic losses and safety hazards to the national economy and people's daily lives. According to a large amount of data, the amount of metal scrapped globally due to corrosion is about 100 million tons per year, accounting for about 30% of the annual output. Moreover, with the advancement of industrialization, the corrosion problem is becoming more and more serious. In 1975, the economic loss caused by metal corrosion in the United States was 70 billion US dollars, accounting for 4.2% of the national economic gross product of that year. The huge harm of metal corrosion is not only reflected in economic losses, but also in environmental pollution, serious waste of resources and energy, hindering the development of new technologies, promoting the depletion of natural resources, and having many safety hazards, which has attracted wide attention from people. Therefore, effective protection of metals is of great significance.

[0003] At present, the protection methods of metal substrates include surface modification, covering a protective layer, electrochemical methods, and adding corrosion inhibitors, etc. Compared with other methods, the substrate coating is the simplest, most economical, and most practical surface treatment method to protect metals from corrosion. The anti-corrosion mechanism of the substrate coating is to form a protective coating on the surface of the metal substrate to prevent water and corrosion particles from contacting the surface of the metal substrate. Among many catalysts, g-C3N4, as a visible light-responsive catalyst with a band gap of 2.7 eV, has a reduction potential of -1.2 eV for photo-generated electrons and is commonly used in reduction reactions such as water splitting to produce H2 and CO2 reduction; the oxidation potential of photo-generated holes is +1.5 eV and is commonly used in oxidation reactions such as water oxidation and organic pollutant degradation. Analyzing from the perspective of band gap and redox reactions, g-C3N4 is an ideal coating that can be used for photocatalytic anti-corrosion. In addition, characteristics such as good photothermal stability and chemical stability, being simple to obtain, low cost, consisting only of biocompatible C and N elements, and not causing secondary pollution to the environment can ensure its good and stable photocatalytic performance during the service life.

[0004] However, g-C3N4 also has its inevitable defects. Especially when used as an anti-corrosion coating, the characteristics of its two-dimensional layered structure cause it to tend to lie flat on the surface of the metal matrix. Firstly, it is difficult for the catalyst coating to form an effective contact interface on the steel surface to ensure the further migration of photo-generated electrons to the surface of the metal matrix. Secondly, the adhesion between the g-C3N4 coating and the metal matrix is poor and it is easy to fall off. The Schottky barrier formed by charge accumulation limits the secondary migration of photo-generated electrons to the steel surface. So far, there is still no good solution to solve the problems of poor adhesion between the carbon nitride coating and the metal matrix and low photo-generated electron transfer efficiency. Summary of the Invention

[0005] In order to solve the above problems, the purpose of the present invention is to provide an iron-based surface phosphorylated carbon nitride coating and its preparation method, which have mild preparation conditions, simple process, cheap raw materials, good repeatability, solve the problems of poor adhesion between the g-C3N4 coating and the metal matrix and low photo-generated electron transfer efficiency, and the prepared coating has strong corrosion resistance.

[0006] The present invention is realized through the following technical solutions:

[0007] The present invention discloses an iron-based surface phosphorylated carbon nitride coating, including:

[0008] S1: Pretreat the iron-based surface;

[0009] S2: Perform phosphorylation treatment on the iron-based surface pretreated in S1 with a phosphorylation solution, and air dry; the phosphorylation solution is composed of phosphoric acid, water, absolute ethanol and potassium ferrocyanide;

[0010] S3: Lay a layer of melamine on the iron-based surface after phosphorylation treatment in S2 and bake it, and obtain a phosphorylated carbon nitride coating after cooling.

[0011] Preferably, in S1, the pretreatment is to polish the iron-based surface first and then perform ultrasonic treatment.

[0012] Further preferably, the ultrasonic treatment uses an organic solvent, and the ultrasonic treatment time is not less than 1 h.

[0013] Preferably, in S2, the phosphorylation treatment is to dip a cotton ball in the phosphorylation solution and then evenly apply it on the iron-based surface.

[0014] Preferably, in S2, the components of the phosphorylation solution are in mass concentration, including 45 wt% phosphoric acid, 35 wt% water, 15 wt% absolute ethanol and 5 wt% potassium ferrocyanide; the concentration of phosphoric acid is 85%.

[0015] Preferably, in S3, the thickness of melamine on the iron-based surface is 1-3 mm.

[0016] Preferably, in S3, the roasting is carried out by heating from room temperature to 400 - 600 °C within 2 - 3 h and holding for 2 - 3 h.

[0017] Preferably, in S3, the roasting is carried out in a constant-temperature heating device, and the temperature deviation of the constant-temperature heating device is ±5 °C.

[0018] The present invention discloses an iron-based surface phosphorylated carbon nitride coating prepared by the above preparation method.

[0019] Preferably, the thickness of the coating is 0.1 - 0.2 mm.

[0020] Compared with the prior art, the present invention has the following beneficial technical effects:

[0021] In the preparation method of the iron-based surface phosphorylated carbon nitride coating disclosed by the present invention, in the phosphorylation reaction, metal ions in the solution react with phosphates to generate phosphates and monohydrogen phosphates, which accumulate on the metal surface to form phosphide grains. The phosphide grains diffuse and grow around and chemically combine with the metal surface to form a phosphide film. The interfacial bonding ability between the phosphorylated coating and the metal matrix is the result of both physical and chemical effects. Among them, the physical effect is attributed to the mechanical locking and van der Waals forces between the coating and the metal matrix, while the binding reaction between the iron ions dissolved at the interface and the phosphates in the coating creates the chemical effect, resulting in a continuous dense structure at the interface. Therefore, the phosphate coating has good interfacial bonding ability with the metal matrix. The phosphorylated coating has the following advantages: a) The phosphide film is firmly bonded to the metal matrix; b) The corrosion current under the paint film can be reduced; c) It has anti-chemical corrosion ability; d) It improves the ability to resist the diffusion of oxygen and water. The phosphate layer firmly bonds g-C3N4 to the iron-based surface. Under light illumination, g-C3N4 excites photo-generated electrons, which are transferred to the metal matrix through the phosphate layer. Since the reduction potential of g-C3N4 is more negative than that of Fe 3+ is more negative, the photo-generated electrons can replace the free electrons on the iron sheet to undergo oxidation reactions with H2O and CO2 in the air, and the free electrons on the iron sheet are not lost, achieving the anti-corrosion effect. Theoretically, as long as light illumination is ensured, g-C3N4 will continuously generate excited electrons, which are transported to the metal surface through the phosphorylated coating. The synergistic effect of the phosphorylated coating and g-C3N4 improves the anti-corrosion performance of the metal matrix; the phosphorylated coating solves the problems of poor adhesion between carbon nitride and the metal matrix and low photo-generated electron transport efficiency.

[0022] Furthermore, in the pretreatment of the iron-based surface, polishing is to avoid the influence of residual coating materials on the experimental results; ultrasonic treatment is to remove impurities so as not to affect the subsequent phosphorylation treatment.

[0023] Even further, using an organic solvent can remove organic debris on the iron-based surface.

[0024] Furthermore, the thickness of melamine on the iron-based surface is 1-3 mm. If the thickness is too high, the g-C3N4 coating will be too thick and prone to peeling off as a whole.

[0025] Furthermore, the concentration of phosphoric acid used in the phosphorylation solution is greater than 85%. If the concentration is too low, the growth of the phosphoric acid layer will be insufficient and the coating will be prone to peeling off.

[0026] The phosphorylated carbon nitride coating on the iron-based surface prepared by the above preparation method disclosed in the present invention has good anti-corrosion performance by itself due to the dense phosphorylated passivation layer on the metal matrix surface. Moreover, this protective layer is directly passivated on the metal matrix surface, and the contact strength with the metal matrix is much greater than that of paint coatings. It not only helps to improve the carrier migration efficiency but also can enhance the adhesion strength between g-C3N4 and the metal matrix. It has guiding significance for both the preparation process and the use method when graphitic carbon nitride is used as a catalyst, and provides a certain theoretical basis and experimental foundation for improving the catalytic performance of photocatalytic technology in practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a diagram of the metal matrix after pretreatment according to the present invention;

[0028] Figure 2 It is a diagram of the metal matrix after phosphorylation according to the present invention;

[0029] Figure 3 It is a diagram of the phosphorylated carbon nitride on the metal matrix after calcination in a muffle furnace in Example 1;

[0030] Figure 4 It is a diagram of the phosphorylated carbon nitride on the metal matrix after calcination in a muffle furnace in Example 2;

[0031] Figure 5 It is a diagram of the phosphorylated carbon nitride on the metal matrix after calcination in a muffle furnace in Example 3;

[0032] Figure 6 It is a mechanism diagram of the phosphorylation of the metal matrix;

[0033] Figure 7 It is a result diagram of the scratch test of the phosphated coating;

[0034] Figure 8 It is a result diagram of the scratch test after calcination of the phosphorylated carbon nitride coating;

[0035] Figure 9(a) Macrophotograph (a-1), digital microscopic images (a-2, a-3), and digital microscopic 3D image (a-4) of bare steel after 20 days in air; (b) Macrophotograph (b-1), digital microscopic images (b-2, b-3), and digital microscopic 3D image (b-4) of g-C3N4 layer after 20 days in air; (c) SEM images (c-1 and c-2), scanning electron microscopy diffraction pattern, and elemental composition of bare steel after 20 days in air; (d) SEM images (d-1 and d-2), scanning electron microscopy diffraction pattern, and elemental composition of bare steel after 20 days in air;

[0036] Figure 10 It is a corrosion prevention mechanism diagram of a metal matrix phosphorylated carbon nitride coating. Detailed implementation manners

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0038] Example 1

[0039] Take an iron sheet, polish the surface of the iron sheet, and then immerse the iron sheet in absolute ethanol for ultrasonic treatment for 1 h; Prepare a phosphorylating solution. By mass concentration, the phosphorylating solution is composed of 45 wt% phosphoric acid, 35 wt% water, 15 wt% absolute ethanol, and 5 wt% potassium ferrocyanide; Dip a cotton ball in the phosphorylating solution and apply it to the surface of the iron sheet, and dry it in a ventilated place; Lay a layer of melamine on the surface of the acidified iron sheet. The dosage of melamine per unit area of the iron sheet surface is 0.05 g / cm 2 , and the laying thickness is 1 mm; After covering the part where melamine is laid, heat it from room temperature to 450 °C within 3 h in a muffle furnace and keep it at this temperature for 3 h for roasting. The function of covering is to prevent the melamine powder from flying away during roasting; After cooling, a phosphorylated carbon nitride coating is obtained, as Figure 3 shown.

[0040] Example 2

[0041] Take an iron sheet, polish the surface of the iron sheet, and then immerse the iron sheet in absolute ethanol for ultrasonic treatment for 1 h; Prepare a phosphorylating solution. By mass concentration, the phosphorylating solution is composed of 45 wt% phosphoric acid, 35 wt% water, 15 wt% absolute ethanol, and 5 wt% potassium ferrocyanide; Dip a cotton ball in the phosphorylating solution and apply it to the surface of the iron sheet, and dry it in a ventilated place; Lay a layer of melamine on the surface of the acidified iron sheet. The dosage of melamine per unit area of the iron sheet surface is 0.05 g / cm 2 , and the laying thickness is 1 mm; After covering the part where melamine is laid, heat it from room temperature to 500 °C within 3 h in a muffle furnace and keep it at this temperature for 3 h for roasting; After cooling, a phosphorylated carbon nitride coating is obtained, as Figure 4 shown.

[0042] Example 3

[0043] Take an iron sheet, polish the surface of the iron sheet, and then immerse the iron sheet in absolute ethanol for ultrasonic treatment for 1 h; prepare a phosphorylation solution. By mass concentration, the phosphorylation solution consists of 45 wt% phosphoric acid, 35 wt% water, 15 wt% absolute ethanol, and 5 wt% potassium ferrocyanide; dip a cotton ball in the phosphorylation solution and apply it to the surface of the iron sheet, and dry it in a ventilated place; lay a layer of melamine on the surface of the acidified iron sheet, and the dosage of melamine per unit area of the iron sheet surface is 0.05 g / cm 2 , and the laying thickness is 1 mm; after covering the part where melamine is laid, heat it from room temperature to 550 °C within 3 h in a muffle furnace and keep it warm for 3 h for roasting; after cooling, a phosphorylated carbon nitride coating is obtained, as Figure 5 shown.

[0044] Example 4

[0045] Take an iron sheet, polish the surface of the iron sheet, and then immerse the iron sheet in absolute ethanol for ultrasonic treatment for 1.5 h; prepare a phosphorylation solution. By mass concentration, the phosphorylation solution consists of 45 wt% phosphoric acid, 35 wt% water, 15 wt% absolute ethanol, and 5 wt% potassium ferrocyanide; dip a cotton ball in the phosphorylation solution and apply it to the surface of the iron sheet, and dry it in a ventilated place; lay a layer of melamine on the surface of the acidified iron sheet, and the dosage of melamine per unit area of the iron sheet surface is 0.05 g / cm 2 , and the laying thickness is 3 mm; after covering the part where melamine is laid, heat it from room temperature to 600 °C within 2 h in a muffle furnace and keep it warm for 2 h for roasting; after cooling, a phosphorylated carbon nitride coating is obtained, and the measured coating thickness is 0.15 mm.

[0046] The mechanism of metal matrix phosphorylation is as Figure 6 shown.

[0047] The reaction involves the following three processes.

[0048] 1) Potassium ferrocyanide reacts with phosphoric acid to form a white precipitate of ferrocyanic acid:

[0049] 4H3PO4 + 3K4Fe(CN)6 → 3H4Fe(CN)6 + 4K3PO4

[0050] 2) Ferrocyanic acid reacts with rust to form iron hexacyanoferrate (Prussian blue):

[0051] 4Fe(OH)3 + 3H4Fe(CN)6 → Fe4[Fe(CN)6]3 + 12H2O

[0052] 3) The excess phosphoric acid reacts with iron rust to form iron phosphate:

[0053] H3PO4+Fe(OH)3→FePO4+3H2O

[0054] Prussian blue adheres to the steel surface and becomes a pigment with strong covering power. Iron phosphate covers the steel surface, which is equivalent to forming a passivation film, exerting a certain hindrance to corrosion.

[0055] Figure 7 It is a result graph of the coating scratch test. a, b, c, and d are parallel control experiments. It can be seen from the test results that no coating peeling phenomenon occurs at the incision edge of the coating, and the adhesion ISO grade is 0. Research shows that the interfacial bonding ability between the coating and the metal matrix is the result of both physical and chemical effects. Among them, the physical effect is attributed to the mechanical interlocking and van der Waals forces between the coating and the metal matrix, while the binding reaction between the iron ions dissolved at the interface and the phosphate in the coating creates the chemical effect, making a continuous dense structure formed at the interfacial bonding. Therefore, the phosphate coating has good interfacial bonding ability with the metal matrix. Figure 8 It is a result graph of the scratch test of the phosphorylated carbon nitride coating. a, b, c, and d are parallel control experiments. Comparing Figure 7 and Figure 8 , it can be seen that loading carbon nitride is beneficial to improving the coating adhesion ability. It can be seen from the test results that no coating peeling phenomenon occurs at the incision edge of the coating, and the adhesion ISO grade is 0. The surface roughness of the coating is relatively low, and it is less likely to form stress concentration, thus improving the fatigue performance and corrosion resistance of the coating.

[0056] Such as Figure 9As shown, the corrosion of metals can generally be divided into electrochemical corrosion and chemical corrosion, and electrochemical corrosion is the most common. Therefore, in the anti-corrosion of metals, the excited electrons of graphitic carbon nitride can be used to prevent the occurrence of electrochemistry on iron sheets. Take the bare steel that has been polished and exposed to natural light for 20 days, and the iron sheet coated with g-C3N4 at 550 °C prepared by the solid-phase method. Under the microscope, the corrosion situation of the bare steel is as shown in a. After one month of light exposure, there are many rust spots on the surface of the bare steel. Not only does its digital microscopic image show that the corrosion depth reaches 5 μm, but its 3D stereogram (a-4) also shows that large-area corrosion has occurred on the entire surface of the carbon steel; the corrosion performance of the steel coated with the g-C3N4 layer is shown in Figure b, with fewer spots and less corrosion. There are only a few rust spots in the digital microscopic image, and there are no obvious rust spots in the 3D stereogram (b-4). Take the bare steel and the steel coated with the g-C3N4 layer for scanning electron microscopy tests. The SEM image of the bare steel also shows that due to the generation of corrosion rust spots, the porous structure of the bare steel has been damaged to a certain extent, as shown in Figure c; while the steel with the g-C3N4 layer still maintains the normal structure of the steel. The Fe / O ratios of the two are also significantly different. It is known that the higher the Fe / O ratio, the lower the corrosion degree of iron. The Fe / O ratio of the bare steel in Figure c is 0.96, while the Fe / O ratio of the steel with the g-C3N4 layer is 21.49. The results show that g-C3N4 has good corrosion resistance.

[0057] The anti-corrosion mechanism of the metal matrix phosphorylated carbon nitride coating is as Figure 10 shown. When g-C3N4 receives sunlight, g-C3N4 will excite photogenerated electrons. The photogenerated electrons have a reduction potential and are more negative than the reduction potential of Fe. The iron oxidized by oxygen and moisture in the air becomes rust, that is, Fe2O3. Therefore, the photogenerated electrons of g-C3N4 can reduce Fe 3+ to Fe. The principle is as follows: The phosphate layer firmly bonds g-C3N4 to the iron sheet. Under light illumination, g-C3N4 excites photogenerated electrons, which are transferred to the iron sheet through the phosphate layer. Since the reduction potential of g-C3N4 is more negative than that of Fe 3+ more negative, the photogenerated electrons can replace the free electrons on the iron sheet to undergo an oxidation reaction with water and CO2 in the air, and the free electrons on the iron sheet are not consumed, achieving the anti-corrosion effect.

[0058] It should be noted that the above description is only one of the embodiments of the present invention. Equivalent changes made according to the system described in the present invention are all included in the protection scope of the present invention. Those skilled in the technical field to which the present invention belongs can make similar substitutions for the specific examples described as long as they do not deviate from the structure of the present invention or exceed the scope defined by the claims of the present invention, and all belong to the protection scope of the present invention.

Claims

1. A preparation method of an iron-based surface phosphorylated carbon nitride coating, characterized in that, Including: S1: Pretreat the iron-based surface; S2: Perform phosphorylation treatment on the iron-based surface pretreated in S1 with a phosphorylation solution, and air dry it; the phosphorylation solution is composed of phosphoric acid, water, absolute ethanol and potassium ferrocyanide; S3: Lay a layer of melamine on the iron-based surface after phosphorylation treatment in S2 and bake it. After cooling, a phosphorylated carbon nitride coating is obtained. The carbon nitride in the phosphorylated carbon nitride coating is g-C3N4; the baking is carried out by heating from room temperature to 400-600 °C within 2-3 h and keeping warm for 2-3 h. The baking is carried out in a constant temperature heating device, and the temperature deviation of the constant temperature heating device is ±5 °C.

2. The preparation method of the iron-based surface phosphorylated carbon nitride coating according to claim 1, characterized in that, In S1, the pretreatment is to polish the iron-based surface first and then perform ultrasonic treatment.

3. The preparation method of the iron-based surface phosphorylated carbon nitride coating according to claim 2, wherein, The ultrasonic treatment uses an organic solvent, and the ultrasonic treatment time is not less than 1 h.

4. The preparation method of the iron-based surface phosphorylated carbon nitride coating according to claim 1, wherein, In S2, the phosphorylation treatment is to dip a cotton ball in the phosphorylation solution and then evenly apply it on the iron-based surface.

5. The preparation method of the iron-based surface phosphorylated carbon nitride coating according to claim 1, characterized in that, In S2, the components of the phosphorylation solution are in mass concentration, including 45 wt% of phosphoric acid, 35 wt% of water, 15 wt% of absolute ethanol and 5 wt% of potassium ferrocyanide; the concentration of phosphoric acid is greater than 85%.

6. The preparation method of the iron-based surface phosphorylated carbon nitride coating according to claim 1, wherein In S3, the thickness of melamine on the iron-based surface is 1-3 mm.

7. The phosphorylated carbon nitride coating on the iron-based surface prepared by the preparation method according to any one of claims 1 to 6.

8. The iron-based surface phosphorylated carbon nitride coating according to claim 7, wherein, The thickness of the coating is 0.1-0.2 mm.

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