ZIF-8 composite material, epoxy resin composite material and preparation method and application thereof

Abstract

The invention belongs to the technical field of metal coatings, and particularly relates to a ZIF-8 composite material, an epoxy resin composite material and a preparation method and application thereof. The ZIF-8 composite material comprises ZIF-8 and a corrosion inhibitor, the corrosion inhibitor is embedded in a ZIF-8 frame to form a pomegranate-shaped structure, the pomegranate-shaped structure comprises n layers, and n is an integer greater than or equal to 1. When the epoxy resin composite material based on the first ZIF-8 composite material is used as a magnesium alloy coating, the pomegranate-shaped multi-cavity structure is beneficial to improving the loading capacity of an 8HQ corrosion inhibitor, and meanwhile, the release efficiency of the corrosion inhibitor can be enhanced through the multi-stage pore channel structure of the first ZIF-8 composite material. And when the coating is damaged, the nano container can quickly release a large amount of 8HQ, and the 8HQ and Mg < 2 + > on the surface of the magnesium alloy generate chelation to form a compact and complete protective layer, so that the self-repairing anti-corrosion performance of the magnesium alloy epoxy coating is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal coatings, and specifically relates to ZIF-8 composite materials, epoxy resin composite materials, and their preparation methods and applications. Background Art

[0002] Magnesium alloys have attracted much attention in the industrial field due to their light weight and high strength. However, due to their active chemical properties, they are extremely prone to electrochemical corrosion in humid, high-temperature, or saline environments, resulting in the destruction of the surface oxide film and the initiation of local corrosion, which severely restricts their application in high-end equipment manufacturing. Currently, organic coating technology is the main means to improve the corrosion resistance of magnesium alloys, and epoxy resin coatings are widely used due to their good adhesion and chemical resistance. However, traditional epoxy coatings are prone to generating microcracks due to shrinkage stress during the curing process, and are easily affected by external factors such as ultraviolet rays and temperature fluctuations during long-term service, resulting in a decline in the protective performance and difficulty in meeting the long-term protection requirements under complex working conditions.

[0003] To improve the comprehensive performance of coatings, in recent years, researchers have tried to introduce nano-fillers or corrosion inhibitor modification technologies. For example, adding nano-oxides (such as SiO2, TiO2) can improve the densification of coatings, but problems such as poor dispersion and easy agglomeration often lead to insufficient coating uniformity; while corrosion inhibitors (such as 8-hydroxyquinoline) can inhibit corrosion reactions by coordinating with metal ions, but their direct doping is prone to premature release of corrosion inhibitors and it is difficult to achieve long-term protection. In addition, in the prior art, functional components are introduced into the coating matrix by physical mixing, which is prone to poor interfacial compatibility and weakens the mechanical strength and stability of the coating.

[0004] Although the design of multi-layer composite structures shows potential in the field of functional materials, such as constructing gradient or encapsulation systems through biomimetic strategies to synergistically enhance the physical barrier and chemical corrosion inhibition properties of coatings, its application in magnesium alloy surface coatings still faces challenges. The prior art is difficult to achieve the controllable loading and directional release of functional components, resulting in low self-healing efficiency and insufficient anti-permeability of coatings. In addition, the interfacial stability and long-term durability of coatings in dynamic corrosion environments still need to be broken through, and there is an urgent need to develop a new composite coating system with high-efficiency protection, self-healing ability, and environmental adaptability. Summary of the Invention

[0005] In the present invention, a corrosion inhibitor is in-situ introduced during the synthesis of ZIF-8, and a multi-level pomegranate-like structure ZIF-8 composite material (hereinafter also referred to as a nano-container) is successively constructed by using the homogeneous liquid phase epitaxial growth technology. Subsequently, a novel intelligent nano-container (8HQ@ZIF-8)3 / PDA is constructed on the surface of the nano-container through dopamine supramolecular self-assembly, and it is incorporated into epoxy resin to prepare a magnesium alloy epoxy composite coating.

[0006] To achieve the above object, the present invention may adopt the following technical solutions:

[0007] On the one hand, the present invention provides a first ZIF-8 composite material, which includes ZIF-8 and a corrosion inhibitor. The corrosion inhibitor is embedded in the ZIF-8 framework to form a pomegranate-like structure. The pomegranate-like structure has n layers, where n≥1 and is an integer. The corrosion inhibitor is selected from one or a combination of 8-hydroxyquinoline, methylbenzotriazole, benzotriazole, mercaptobenzothiazole, or zinc gluconate.

[0008] Preferably, in the above first ZIF-8 composite material, n is 1-3.

[0009] More preferably, in the above first ZIF-8 composite material,

[0010] When n is 1, the pomegranate-like structure has 1 layer, and the particle size of the first ZIF-8 composite material is 145 nm - 150 nm;

[0011] When n is 2, the pomegranate-like structure has 2 layers, and the particle size of the first ZIF-8 composite material is 460 nm - 465 nm;

[0012] When n is 3, the pomegranate-like structure has 2 layers, and the particle size of the first ZIF-8 composite material is 655 nm - 660 nm.

[0013] It should be noted that the connection between the layers in the present invention is as follows: After preparing the dispersion liquid of the first layer of 8HQ@ZIF-8 nanocontainers, centrifugation is carried out, and then 8HQ@ZIF-8 is dispersed in methanol again. The corrosion inhibitor 8HQ and the ZIF-8 raw materials (2-methylimidazole and zinc nitrate hexahydrate) are re-added. Using the homogeneous liquid phase epitaxy technology, the new ZIF-8 raw materials grow a new ZIF-8 framework on the surface of the first layer of 8HQ@ZIF-8 nanoparticles along the specified crystal plane orientation, and at the same time encapsulate the 8HQ corrosion inhibitor, resulting in an increase in the particle size of the 8HQ@ZIF-8 nanocontainers, forming the second layer of (8HQ@ZIF-8) 2 nanocontainers; the third layer and so on.

[0014] On the other hand, the present invention provides a preparation method of the first ZIF-8 composite material in the present invention.

[0015] When the pomegranate-like structure of the first ZIF-8 composite material has 1 layer, the preparation method of the first ZIF-8 composite material includes:

[0016] (1) Mix the 2-methylimidazole solution with the corrosion inhibitor to obtain a mixed solution;

[0017] (2) Mix the mixed solution with the zinc ion solution to obtain the first ZIF-8 composite material;

[0018] When the pomegranate-like structure of the first ZIF-8 composite material is n layers, where n≥2, the preparation method of the first ZIF-8 composite material includes:

[0019] (1) Mix the solution of the first ZIF-8 composite material with a pomegranate-like structure of n-1 layers with 2-methylimidazole, and then add an inhibitor to obtain a mixed solution;

[0020] (2) Mix the mixed solution with a zinc ion solution to obtain the first ZIF-8 composite material with a pomegranate-like structure of n layers.

[0021] On the other hand, the present invention also provides a second ZIF-8 composite material, which includes the first ZIF-8 composite material in the present invention and polydopamine, and the first ZIF-8 composite material is wrapped by polydopamine.

[0022] It should be noted that the purpose of coating the ZIF-8 composite material with polydopamine is mainly to utilize the possible cross-linking reaction between the amino groups in the polydopamine molecules and the epoxy resin, as well as the intermolecular interaction of polar molecules, to provide interfacial compatibility and dispersion stability with the epoxy resin.

[0023] On the other hand, the present invention also provides a preparation method of the second ZIF-8 composite material in the present invention, which includes: mixing and reacting the dispersion of the first ZIF-8 composite material with the dopamine hydrochloride solution to obtain the second ZIF-8 composite material.

[0024] On the other hand, the present invention also provides an epoxy resin composite material, which includes the first ZIF-8 composite material and / or the second ZIF-8 composite material in the present invention, a curing agent, and an epoxy resin.

[0025] Preferably, in the above epoxy resin composite material, the mass fraction of the first ZIF-8 composite material and / or the second ZIF-8 composite material is 1%-3%.

[0026] It should be noted that the above mass fraction refers to the mass ratio of the first ZIF-8 composite material and / or the second ZIF-8 composite material in the epoxy resin composite material.

[0027] On the other hand, the present invention also provides an application of the epoxy resin composite material in the present invention as a surface coating for magnesium alloy, and the magnesium alloy surface coating has self-healing anti-corrosion performance.

[0028] The beneficial effects of the present invention include: when the epoxy resin composite material provided by the present invention is used as a magnesium alloy coating, the pomegranate-like multi-chamber structure helps to increase the loading amount of the inhibitor (such as 8HQ), and at the same time, its hierarchical pore structure can enhance the release efficiency of the inhibitor. When the coating is damaged, the nanocontainers can quickly release a large amount of inhibitor, and react with Mg on the surface of the magnesium alloy 2+Chelation occurs to form a dense and complete protective layer, thereby enhancing the self-healing anti-corrosion performance of the epoxy coating on the magnesium alloy. Description of the Drawings

[0029] Figure 1 is (8HQ@ZIF-8) 3 / PDA preparation process schematic diagram;

[0030] Figure 2 is the electron microscopy image and transmission electron microscopy image of the ZIF-8 composite; among them, (a) is the electron microscopy image of 8HQ@ZIF-8;

[0031] (b) (8HQ@ZIF-8) 2 electron microscopy image; (c) is the electron microscopy image of (8HQ@ZIF-8) 3 electron microscopy image; (d) to (f) are the electron microscopy images of (8HQ@ZIF-8) 3 / PDA at different magnifications; (g) is the transmission electron microscopy image of 8HQ@ZIF-8; (h) to (i) are the transmission electron microscopy images of (8HQ@ZIF-8) 3 at different magnifications;

[0032] Figure 3 is the Mapping diagram of C, N, O, Zn;

[0033] Figure 4 is the different characterization situations of ZIF-8 and its composites; among them, (a) is the FT-IR spectra of ZIF-8 and its composites (8HQ@ZIF-8, (8HQ@ZIF-8) 2 , (8HQ@ZIF-8) 3 and (8HQ@ZIF-8) 3 / PDA); (b) is the XRD patterns of ZIF-8 and its composites (8HQ@ZIF-8, (8HQ@ZIF-8) 2 , (8HQ@ZIF-8) 3 and (8HQ@ZIF-8) 3 / PDA); (c) is the fluorescence spectra of 8HQ@ZIF-8, (8HQ@ZIF-8) 2 , and (8HQ@ZIF-8) 3 ; (d) is the thermogravimetric curves of ZIF-8, 8HQ@ZIF-8, (8HQ@ZIF-8) 3 ;

[0034] Figure 5 is the evaluation of the anti-corrosion ability of ZIF-8 and its composites; among them, (a) is the magnesium alloy exposed to blank brine, 8HQ@ZIF-8, (8HQ@ZIF-8) 3, (8HQ@ZIF-8) 3 Bode / Phase diagram of the extract of / PDA; (b) shows the magnesium alloy exposed to blank saline, 8HQ@ZIF-8, (8HQ@ZIF-8) 3 , (8HQ@ZIF-8) 3 Nyquist diagram of the extract of / PDA;

[0035] (c) shows the magnesium alloy exposed to blank saline, 8HQ@ZIF-8, (8HQ@ZIF-8) 3 , (8HQ@ZIF-8) 3 |Z| of the extract of / PDA 0.01Hz and the bar graph of Rct; (d) shows the equivalent circuit diagram for the analysis of the electrochemical impedance spectroscopy (EIS) of ZIF-8 and its composites;

[0036] Figure 6 For ZIF-8 / EP, (8HQ@ZIF-8) 3 / EP, (8HQ@ZIF-8) 3 / PDA / EP water absorption curves;

[0037] Figure 7 shows the Bode diagram and Nyquist diagram during the immersion process of the epoxy resin coatings of ZIF-8 and its composites in 3.5 wt% NaCl solution; among them, (a1) and (a2) are the Bode diagram and Nyquist diagram of the pure epoxy resin coating (pure EP) respectively; (b1) and (b2) are the Bode diagram and Nyquist diagram of (8HQ@ZIF-8) 3 / EP respectively; (c1) and (c2) are the Bode diagram and Nyquist diagram of (8HQ@ZIF-8) 3 / PDA / EP respectively;

[0038] Figure 8 shows the anti-corrosion performance test of the epoxy resin coatings of ZIF-8 and its composites; among them, (a) shows the changes of |Z| 3 during the 50-day immersion period of pure EP, (8HQ@ZIF-8) 3 / EP, (8HQ@ZIF-8) 0.01Hz / PDA / EP in 3.5 wt% NaCl solution; (b) shows the changes of Rct during the 50-day immersion period of pure EP, (8HQ@ZIF-8) 3 / EP, (8HQ@ZIF-8) 3 / PDA / EP in 3.5 wt% NaCl solution; (c) shows the equivalent circuit diagram for the analysis of the electrochemical impedance spectroscopy (EIS) of the epoxy resin coatings of ZIF-8 and its composites;

[0039] Figure 9 Pictures of the epoxy resin coatings of ZIF-8 and its composites at different times in a salt spray test chamber; among them, (a) are pictures of pure EP at different times in the salt spray test chamber; (b) are pictures of (8HQ@ZIF-8) 3 / EP at different times in the salt spray test chamber; (c) are pictures of (8HQ@ZIF-8) 3 / PDA / EP at different times in the salt spray test chamber;

[0040] Figure 10 Pictures of the epoxy resin coatings of ZIF-8 and its composites with artificial scratches at different times in a salt spray test chamber; among them, (a) are pictures of pure EP at different times in the salt spray test chamber; (b) are pictures of (8HQ@ZIF-8) 3 / EP at different times in the salt spray test chamber; (c) are pictures of (8HQ@ZIF-8) 3 / PDA / EP at different times in the salt spray test chamber;

[0041] Figure 11 SEM micrographs of the scratched areas and the surrounding coatings of pure EP and (8HQ@ZIF-8) 3 / PDA / EP after 6 days of salt spray test; among them, (a1) and (a2) are SEM micrographs of the scratched area of pure EP respectively; (b1) and (b2) are SEM micrographs of the coatings around the scratched area of pure EP respectively; (c1) and (c2) are SEM micrographs of the scratched area of (8HQ@ZIF-8) 3 / PDA / EP respectively; (d1) and (d2) are SEM micrographs of the coatings around the scratched area of (8HQ@ZIF-8) 3 / PDA / EP respectively;

[0042] Figure 12 Schematic diagram of the anti-corrosion mechanism of (8HQ@ZIF-8) 3 / PDA / EP. Detailed implementation manners

[0043] The examples given are for better illustration of the present invention, but the content of the present invention is not limited only to the examples given. Therefore, those skilled in the art who make non-essential improvements and adjustments to the implementation manners according to the above-mentioned invention content still fall within the protection scope of the present invention.

[0044] The terms used in this document are only for describing specific embodiments and are not intended to limit the present disclosure. Unless otherwise clearly different in context, singular expressions include plural expressions. As used herein, it should be understood that terms such as "including", "having", "containing" are intended to indicate the existence of features, numbers, operations, components, parts, elements, materials, or combinations. Terms of the present invention are disclosed in the specification, and are not intended to exclude the possibility of the existence or addition of one or more other features, numbers, operations, components, parts, elements, materials, or combinations thereof. As used herein, depending on the circumstances, " / " can be interpreted as "and" or "or".

[0045] To better understand the present invention, the content of the present invention will be further clarified below with specific examples, but the content of the present invention is not limited to the following examples only.

[0046] In the following examples, zinc nitrate hexahydrate (Zn(NO3)2·6H2O, AR, 99%) was purchased from Chengdu Jinshan Chemical Reagent Co., Ltd., 2-methylimidazole (2-Methylimidazole, AR, 98%) was purchased from Adamas-beta, 8-hydroxyquinoline (8-HQ) was purchased from Aladdin; methanol and absolute ethanol were both purchased from Chengdu Jinshan Chemical Reagent Co., Ltd.; dopamine hydrochloride (3-Hydroxytyramine Hydrochloride, 99%+) was purchased from Adamas-beta, epoxy resin E51 and curing agent were both provided by Nanchang Chenfang Adhesive Products Co., Ltd.; sodium chloride (NaCl, AR, 99%) was purchased from Adamas-beta. All reagents were used as received samples.

[0047] I. Preparation of biomimetic pomegranate structure

[0048] Example 1

[0049] (I) Synthesis of 8HQ@ZIF-8

[0050] (1) Add 7.39 g of 2-methylimidazole to 100 mL of methanol. After stirring and dissolving, ultrasonicate (180 W, 40 KHz, the same below) for 10 min, then add 0.083 g of 8-HQ (8-hydroxyquinoline), and stir and ultrasonicate for 10 min to obtain a 2-methylimidazole solution;

[0051] (2) Add 2.86 g of Zn(NO3)2·6H2O to 100 mL of methanol solution. After stirring and dissolving, ultrasonicate for 10 min to obtain a Zn(NO3)2·6H2O methanol solution;

[0052] (3) Then quickly add the Zn(NO3)2·6H2O methanol solution to the 2-methylimidazole solution (containing 8HQ) in step (1), and then stir for 16 h and let it stand for 4 h to obtain 8HQ@ZIF-8;

[0053] (4) Centrifuge the synthesized 8HQ@ZIF-8 at 10000 for 20 min, wash it 3 times with methanol, and centrifuge.

[0054] (II) (8HQ@ZIF-8) 2 、(8HQ@ZIF-8) 3 Synthesis

[0055] (1) (8HQ@ZIF-8) 2 Synthesis

[0056] 1) Dissolve 1.0 g of the centrifuged 8HQ@ZIF-8 in 100 mL of methanol and ultrasonicate for 10 min to obtain the 8HQ@ZIF-8 methanol solution; add 7.39 g of 2-methylimidazole to 100 mL of methanol, stir to dissolve and then ultrasonicate for 10 min to obtain the 2-methylimidazole methanol solution;

[0057] 2) Mix the 8HQ@ZIF-8 methanol solution and the 2-methylimidazole methanol solution evenly, then add 0.083 g of 8-HQ, stir and ultrasonicate for 10 min to obtain a mixed solution;

[0058] 3) Add 2.86 g of Zn(NO3)2·6H2O to 100 mL of methanol solution, stir to dissolve and then ultrasonicate for 10 min to obtain the Zn(NO3)2·6H2O methanol solution, then quickly add 100 mL of the Zn(NO3)2·6H2O methanol solution to the mixed solution in 2) above, and then stir for 10 h and let it stand for 4 h to obtain (8HQ@ZIF-8) 2 ;

[0059] 4) Centrifuge the (8HQ@ZIF-8) synthesized in 3) 2 at 12000 rpm for 20 min, wash it 3 times with methanol, and centrifuge.

[0060] (2) (8HQ@ZIF-8) 3 Synthesis

[0061] 1) Dissolve 1.0 g of the centrifuged (8HQ@ZIF-8) 2 in 100 mL of methanol and ultrasonicate for 10 min to obtain the (8HQ@ZIF-8) 2 methanol solution; add 7.39 g of 2-methylimidazole to 60 mL of methanol, stir to dissolve and then ultrasonicate for 10 min to obtain the 2-methylimidazole methanol solution;

[0062] 2) Mix the (8HQ@ZIF-8)2 The methanol solution and the 2-methylimidazole methanol solution were mixed evenly, and then 0.083 g of 8-HQ was added, and the mixture was stirred and sonicated for 10 min to obtain a mixed solution;

[0063] 3) 2.86 g of Zn(NO3) 2 ·6H2O was added to 60 mL of methanol solution, and after stirring and dissolving, it was sonicated for 10 min to obtain a Zn(NO3) 2 ·6H2O methanol solution. Then, 60 mL of the Zn(NO3) 2 ·6H2O methanol solution was quickly added to the mixed solution in 2) above, and then stirred for 6 h and left to stand for 4 h to obtain (8HQ@ZIF-8) 3 ;

[0064] 4) The synthesized (8HQ@ZIF-8) 3 was centrifuged at 14500 rpm for 20 min, washed with methanol 2-3 times, and then centrifuged.

[0065] (III) Preparation of the biomimetic pomegranate structure ((8HQ@ZIF-8) 3 / PDA)

[0066] The preparation process of the biomimetic pomegranate structure ((8HQ@ZIF-8) 3 / PDA) is as Figure 1 shown, and the specific steps are as follows:

[0067] (1) 1.0 g of (8HQ@ZIF-8) 3 Ns (the (8HQ@ZIF-8) 3 ) prepared above was added to 500 mL of anhydrous methanol, sonicated and dispersed for 30 min, and taken out and shaken well every 10 min to obtain a (8HQ@ZIF-8) 3 methanol solution;

[0068] (2) 0.1 g of hydrochloric acid dopamine was dissolved in 200 mL of Tris-HCl buffer (tris(hydroxymethyl)aminomethane hydrochloride) (PH = 8.5), and sonicated for 30 min to obtain a hydrochloric acid dopamine solution;

[0069] (3) The above hydrochloric acid dopamine solution was poured into the (8HQ@ZIF-8) 3 methanol solution, and stirred for 4 h to obtain a mixture;

[0070] (4) The mixture in (3) was centrifuged (8000 r / 5 min), washed 3 times with anhydrous methanol, and then vacuum dried at 60 °C for 12 hours to obtain the sample, which is (8HQ@ZIF-8) 3 / PDANs.

[0071] II. Characterization of ZIF-8 and its composites

[0072] (1) Electron microscope characterization

[0073] The field emission scanning electron microscope (Tescan Mira3 LMU) was used to observe the morphologies of ZIF-8 in Example 1 (prepared in the same way as 8HQ@ZIF-8 in Example 1, but without adding 8HQ inhibitor), 8HQ@ZIF-8, (8HQ@ZIF-8) 2 and (8HQ@ZIF-8) 3 / PDA to evaluate their particle size and morphological changes.

[0074] The multi-level structure growth mechanism of the 8HQ@ZIF-8 composite material can be divided into three main stages, which are as follows:

[0075] First, the first layer structure is the initial formation of 8HQ@ZIF-8 crystals as Figure 2 (a, g) shown. At this stage, the particles in 8HQ@ZIF-8 exhibit uniform small sizes, with an average particle diameter of about 146.8 nm, tightly packed and overall flat, with clear particle boundaries, presenting a cubic or approximately polyhedral morphology. At the same time, the tight packing between particles does not show obvious agglomeration phenomena, indicating that the introduction of 8-HQ in this layer does not cause significant interference to the nucleation and morphology of the crystals. This process mainly depends on the coordination between 8-hydroxyquinoline (8-HQ) molecules and zinc ions. The hydroxyl group in 8-HQ can form complexes with zinc ions, thus affecting the nucleation and growth of ZIF-8 crystals. At this stage, the addition of 8-HQ inhibits the excessive nucleation of crystals, promotes the increase in crystal particle size, and makes the crystal morphology tend to be stable.

[0076] As it enters the second layer structure (8HQ@ZIF-8) 2 (as Figure 2(b)), the particle size increases significantly, and the shape of some particles tends to be regular, showing an obvious cubic structure; with the increase of 8-HQ concentration, 8-HQ not only embeds into the ZIF-8 framework through coordination, but also stabilizes the crystal growth through intermolecular forces such as hydrogen bonds. This layer structure presents a uniform rhombic dodecahedron morphology, with the particle size increasing to about 461.5 nm, and 8-HQ molecules embed into the pores, further enhancing the physicochemical stability of ZIF-8. The increase in particle size indicates that the increase in 8-HQ concentration has an inhibitory effect on nucleation during crystal growth, enabling the particles to continue growing on the existing crystal nuclei, forming relatively large particles. At the same time, the packing between particles becomes relatively loose and the distribution becomes more dispersed. At this time, the addition of 8-HQ not only affects the lateral expansion of the crystal, but also promotes the longitudinal growth of the crystal through weak coordination or hydrogen bond interaction with the ZIF-8 surface. Although 8-HQ plays an inhibitory role in the nucleation process, it also provides additional reaction sites on the crystal surface, enabling Zn 2+ and 2-methylimidazole (2-MeIm) to continue to orient and aggregate on the crystal surface. With the formation of the second layer structure, the pore structure of the ZIF-8 crystal is gradually optimized, and the porosity and surface properties are improved, thereby enhancing the functionality and stability of the composite material.

[0077] After further increasing the sample level, the third layer structure (8HQ@ZIF-8) is entered 3 , the particle size continues to increase, and it significantly shows a massive or aggregated state. The surface of some particles becomes rough and obvious agglomeration phenomena exist. The average particle size reaches 658.8 nm (as shown in Figure 2 (c), 2(h) and 2(i)). This significant increase in particle size and the aggregation phenomenon between particles indicate the crystal growth acceleration effect caused by the further increase of 8-HQ concentration.

[0078] In addition, for the (8HQ@ZIF-8) 3 / PDA composite material ( Figure 2 (d) and Figure 2 (f)), SEM shows that a uniform nanoscale rough structure (thickness 20 - 50 nm) is formed on the particle surface after PDA coating, and the particle size increases to about 700 nm.

[0079] In addition, Mapping analysis ( Figure 3 ) shows that the distribution of C, N, O, and Zn elements extends to the entire coating layer. This significant increase in particle size and the aggregation phenomenon between particles indicate the crystal growth acceleration effect caused by the further increase of 8-HQ concentration.

[0080] Comprehensive analysis shows that from 8HQ@ZIF-8 to (8HQ@ZIF-8) 3, the particle size has experienced a development process from uniform small particles to larger blocky or cluster structures. This process not only reflects the regulatory effect of 8-HQ as an encapsulant on the crystal morphology and growth of ZIF-8, but also reveals its important influence on crystal properties. Although the increase in particle size may trigger aggregation between particles, the crystals still maintain a good polyhedral morphology, indicating that 8-HQ mainly affects the crystal structure of ZIF-8 by inhibiting nucleation and promoting crystal growth. In addition, the particle distribution gradually changes from a dense distribution in the first layer to an aggregation phenomenon in the third layer, which may be closely related to the concentration of 8-HQ, reaction time and other conditions in the experiment. In summary, the introduction of 8-HQ not only promotes the growth of ZIF-8 crystals, but also inhibits the nucleation process, resulting in the growth of particle size and changes in distribution.

[0081] (2) Characterization by infrared spectrometer

[0082] To evaluate the structure and properties of the prepared materials, a Nicolet 6700 Fourier transform infrared spectrometer was used, and the test wavelength range was 4000 cm -1 -400 cm -1 , and the functional group characteristics of 8HQ@ZIF-8, (8HQ@ZIF-8) 2 and (8HQ@ZIF-8) 3 / PDA were analyzed by FTIR respectively.

[0083] The results are as Figure 4 shown in (a). The results show that there are obvious changes in the chemical composition and structure of composite materials with different hierarchical structures (such as 8HQ@ZIF-8, 8HQ@ZIF-8 / PDA, etc.), further proving the effective construction of these materials.

[0084] Among them, the characteristic peak at 3744 cm-1 corresponds to the vibration of the hydroxyl (-OH) functional group in the 8HQ molecule. This absorption peak can be confirmed in 8HQ@ZIF-8, indicating that 8HQ is successfully encapsulated in the pores of ZIF-8 and no significant chemical changes have occurred, demonstrating the stability and effectiveness of the aggregation of 8HQ in the material.

[0085] In addition, the peak at 3135 cm-1 is usually associated with the C-H stretching vibration in aromatic compounds. This peak is seen in 8HQ@ZIF-8 and other composite materials, indicating that the 8HQ molecule maintains its aromatic structure. The characteristic peak at 2928 cm-1 is usually associated with the -CH2 and -CH3 groups in hydrocarbon compounds. Its presence in all samples indicates the presence of long-chain hydrocarbons or functional groups in the molecule, which shows that the organic part of the material plays an important role in the formation of the composite material.

[0086] In addition, the Zn-O vibration peak at 760 cm-1 indicates that the metal-ligand structure of ZIF-8 remains stable. This peak was observed in all samples, supporting the retention of the ZIF-8 structure in the composite material and indicating that it was not significantly affected by the synthesis process.

[0087] In addition, the Zn-N vibration peak at 421 cm-1 indicates the metal coordination between zinc and the ligand. Its presence indicates the integrity of ZIF-8 and shows the stability in the material system.

[0088] In summary, FTIR spectral analysis indicates that 8HQ is effectively embedded in the structure of ZIF-8, and after coating with polydopamine (PDA), the basic metal-ligand structure remains. The changes and presence of these characteristic peaks illustrate the formation of hierarchical structure composites and the interactions between components. In particular, the addition of PDA may enhance the corrosion resistance and stability of the material by introducing additional functional groups. In summary, FTIR spectral analysis indicates that 8HQ is effectively embedded in the structure of ZIF-8, and after coating with polydopamine (PDA), the basic metal-ligand structure remains. The changes and presence of these characteristic peaks illustrate the formation of hierarchical structure composites and the interactions between components. In particular, the addition of PDA may enhance the corrosion resistance and stability of the material by introducing additional functional groups.

[0089] (III) X-ray Characterization

[0090] Using a DX-2700BH type X-ray diffractometer with a scanning range of 5° to 60°, the crystal structures of ZIF-8, 8HQ@ZIF-8, (8HQ@ZIF-8) 2 and (8HQ@ZIF-8) 3 / PDA in Example 1 were analyzed respectively.

[0091] The analysis results are as Figure 4 shown in (b), and the results show that:

[0092] First, in the XRD spectrum of the first-layer structure 8HQ@ZIF-8, characteristic peaks similar to those of pure ZIF-8 were observed at positions 2θ = 7.3°, 10.4°, 12.7°, 14.7°, 16.5° and 18.1°. These peaks correspond to the (011), (002), (112), (022), (013) and (222) crystal planes of ZIF-8 respectively, indicating that the introduction of 8HQ molecules did not significantly damage the crystal structure of ZIF-8. The 8-HQ molecules mainly exist in a dispersed manner in the pores of ZIF-8. The peak shape and peak position in the spectrum are consistent with the ZIF-8 standard spectrum reported in the literature Figure 1It shows that the framework structure of ZIF-8 remains intact and has a relatively high crystallinity.

[0093] Secondly, in the XRD pattern of the second-layer structure (8HQ@ZIF-8) 2 , XRD features similar to those of the first layer were still observed, and the positions of all the main diffraction peaks were consistent with those of ZIF-8. This indicates that in the second-layer structure, the 8-HQ molecules did not cause significant effects on the crystal structure of ZIF-8. Although the loading amount of 8-HQ increased slightly, no obvious new peaks or peak position shifts appeared in the XRD pattern, further proving the uniform dispersion of 8-HQ in the ZIF-8 framework and that it did not affect the crystallinity and crystal structure of ZIF-8.

[0094] Furthermore, for the third-layer structure (8HQ@ZIF-8) 3 , the XRD pattern continued to exhibit characteristics very similar to those of pure ZIF-8, indicating that at this level, the crystal structure of ZIF-8 remained stable. Although the loading amount of 8-HQ increased further, the diffraction peaks were still clear and there were no obvious peak position shifts. This shows that despite multiple loadings of 8-HQ molecules, the framework structure of ZIF-8 could still maintain stability to a certain extent, and the crystallinity of ZIF-8 did not decrease significantly.

[0095] In addition, in the XRD pattern of the (8HQ@ZIF-8) 3 / PDA composite material, it was basically consistent with the patterns of pure ZIF-8 and (8HQ@ZIF-8) 3 , showing the characteristic diffraction peaks of ZIF-8, indicating that the three-dimensional framework structure of ZIF-8 remained stable after PDA modification. However, after PDA modification, the intensities of some diffraction peaks decreased, especially the characteristic peaks related to the (220) and (310) crystal planes. This might be due to the covering effect of PDA, which limited the detection depth of XRD. The introduction of PDA did not change the basic framework structure of ZIF-8, but through the comparison of XRD patterns, it could be speculated that PDA modification might lead to the formation of a thin film on the surface of ZIF-8, thus hindering the further transmission of X-rays to a certain extent.

[0096] From these XRD analysis results, it can be concluded that the crystal structure of ZIF-8 remains stable in composite materials at different levels. Whether 8-HQ molecules are introduced or PDA modification is carried out, the basic framework structure of ZIF-8 does not change significantly. The introduction of 8-HQ does not significantly change the crystallinity of ZIF-8, and the characteristic peaks in the XRD pattern are still clear, indicating that the dispersion of 8-HQ in the ZIF-8 framework is uniform. The influence of PDA modification on ZIF-8 is small. Although the introduction of PDA results in the weakening of the intensity of some diffraction peaks, the overall structure remains intact, and the framework of ZIF-8 does not disintegrate or change significantly. Especially (8HQ@ZIF-8) 3 / PDA composite materials exhibit high structural stability, which indicates that PDA modification can effectively improve the stability of ZIF-8 in composite materials while maintaining high crystallinity. These series of XRD analysis results prove that when synthesizing the multi-level structure materials of ZIF-8 encapsulated with 8-hydroxyquinoline based on polydopamine biomimetic pomegranate, the ZIF-8 frameworks at each level can maintain good crystallinity and structural integrity, verifying the potential of multi-level structure design in improving the performance protection of magnesium alloys.

[0097] (IV) Fluorescence spectroscopy analysis

[0098] The ZIF-8, 8HQ@ZIF-8, (8HQ@ZIF-8) 2 and (8HQ@ZIF-8) 3 / PDA in Example 1 were respectively analyzed for fluorescence intensity by a fluorescence spectrophotometer F-380 (Tianjin Gangdong Technology Co., Ltd.).

[0099] The results are as Figure 4 (c) shown. The results show that the significant presence of the fluorescence emission peak at 547 nm represents the effective loading of 8-hydroxyquinoline (8-HQ) in these three metal-organic framework (ZIF) materials and maintains its fluorescence characteristics. This peak is usually attributed to the π-π transition process in 8-HQ molecules, that is, the electronic transition between the excited state and the ground state of the benzene ring. With the increase in the loading amount of 8-HQ, the enhancement of fluorescence intensity can be explained by different mechanisms. Firstly, more 8-HQ molecules provide abundant fluorescence active sites, resulting in the enhancement of the overall fluorescence signal. Secondly, the increase in the loading amount may trigger intermolecular interactions, leading to the enhancement of energy transfer and the improvement of fluorescence quantum yield. In addition, a higher loading amount may cause a molecular aggregation effect, in which the interaction between 8-HQ molecules may promote the efficient emission of fluorescence to some extent.

[0100] (V) Thermogravimetric analysis characterization

[0101] The NETZSCH STA 449C thermogravimetric analyzer was used to analyze the thermal stability of ZIF-8, 8HQ@ZIF-8, (8HQ@ZIF-8) 2 and (8HQ@ZIF-8) 3 / PDA in Example 1, with a heating rate of 10 °C / min and a temperature range of 28 °C to 600 °C.

[0102] The results are as Figure 4 (d) shown (in the figure, Zn(8HQ) refers to the chelation product of metal zinc ions and the corrosion inhibitor 8HQ, representing the product after the reaction of 8HQ with zinc nitrate hexahydrate (i.e., zinc ions)). The results show that the loading amounts of 8-HQ in 8HQ@ZIF-8 and (8HQ@ZIF-8) 3 are 4.8% and 6.5% respectively. This result indicates that with the multi-layered design of the ZIF-8 structure, the loading capacity of 8-HQ is significantly improved, which provides an important material basis for the development of high-performance anti-corrosion coatings. From the thermogravimetric analysis results, it can be seen that the loading amount of (8HQ@ZIF-8) 3 is about 35% higher than that of the single-layer 8HQ@ZIF-8. This phenomenon may be related to the larger specific surface area and more pores provided by the multi-layer structure. The multi-layer ZIF-8 structure not only increases the loading sites of 8-HQ but also optimizes the distribution of 8-HQ through its unique pore structure, thereby improving the loading efficiency. In addition, the multi-layer structure may further promote the effective binding of 8-HQ to the ZIF-8 pores by reducing the steric hindrance between 8-HQ molecules. In addition, although the loading amount of 8-HQ increases in the multi-layer structure, its loading efficiency does not increase linearly with the increase in the concentration of 8-HQ. This phenomenon may be related to the saturation effect of 8-HQ in the ZIF-8 pores. When the concentration of 8-HQ exceeds a certain threshold, additional 8-HQ molecules may not be able to effectively enter the pores but exist in the solution in a free state or even form aggregates, resulting in a decrease in the loading efficiency. In addition, the competition and steric hindrance between 8-HQ molecules at high concentrations may further limit their binding ability to ZIF-8. Therefore, although the loading amount of (8HQ@ZIF-8) 3 is higher than that of the single-layer structure, it is still limited by the pore capacity of the material itself and the intermolecular interactions.

[0103] III. Corrosion Resistance Testing of ZIF-8 and Its Composites

[0104] (1) 10 mg of each nanocomposite (8HQ@ZIF-8, (8HQ@ZIF-8) 2 and (8HQ@ZIF-8) 3(The / PDA powder) was separately dissolved in 100 mL of 0.5 wt% NaCl solution to obtain a mixture. The mixture was stirred with a mechanical stirrer at 100 rpm for 24 hours, and then centrifuged (10,000 rpm / 10 min) and filtered to obtain the extract;

[0105] At the same time, 0.5 wt% NaCl solution was used as a control.

[0106] (2) The magnesium alloy sample with an exposed area of 1 cm 2 (In the experiment, AZ31 magnesium alloy with dimensions of 28 mm × 18 mm × 1.8 mm was selected as the substrate, and its elemental composition and content were 2.25 wt.% A1, 0.95 wt.% Zn, 0.21 wt.% Mn, and 6.59 wt.% Mg.) was immersed in the extract, and its corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS). Among them, in the polarization curve test, the same corrosion conditions and electrode system as those in the EIS test were adopted. The electrochemical cell used consisted of the following electrodes: working electrode (sample), saturated calomel reference electrode (SCE), its electrolyte was 3 M KCl solution, and the auxiliary electrode was a platinum sheet with an active surface area of 4 cm 2 . The EIS data was fitted by an equivalent circuit (EEC) using ZsimpWin V.3.20 software. The test was carried out from the negative value (-900 mV) to the positive value (+900 mV) of the open circuit potential (OCP) at a scanning rate of 1 mV / s. All electrochemical measurements were repeated three times to ensure the repeatability of the data.

[0107] The results are shown as follows:

[0108] The EIS data was fitted by an equivalent circuit model ( Figure 5 (d)), and the obtained parameters are listed in Table 1.

[0109] Table 1 Electrochemical change parameter values of magnesium alloy in 0.5% NaCl solution containing blank, 8HQ@ZIF-8 / PDA, (8HQ@ZIF-8) 3 / PDA extract

[0110]

[0111] It can be seen from the EIS results ( Figure 5 (a)) that the low-frequency impedance value (|Z| 0.01Hz ) of the magnesium alloy in the blank saline solution is 2264 Ω·cm 2 , while in the 8HQ@ZIF-8, (8HQ@ZIF-8) 3 and (8HQ@ZIF-8) 3 / PDA extract, the low-frequency impedance values are respectively increased to 3611 Ω·cm 2, 11569 Ω·cm 2 and 27820 Ω·cm 2 . This significant growth trend indicates that with the multi - layerization of the ZIF - 8 structure and the introduction of PDA modification, the anti - corrosion performance of the coating has been significantly enhanced. Especially for the (8HQ@ZIF - 8) 3 / PDA extract, the |Z| 0.01Hz value reaches 12.3 times that of the blank brine, fully demonstrating its excellent corrosion inhibition ability.

[0112] In addition, by further analyzing the change of charge transfer resistance (Rct) ( Figure 5 (c)), a similar trend can be found. The Rct value of the magnesium alloy in the blank brine is 2171 Ω·cm 2 , while in the 8HQ@ZIF - 8, (8HQ@ZIF - 8) 3 and (8HQ@ZIF - 8) 3 / PDA extract, the Rct values increase to 3398 Ω·cm 2 , 9849 Ω·cm 2 and 26310 Ω·cm 2 respectively. The significant increase in the Rct value indicates that the release of 8 - HQ and PDA - Zn complexes effectively inhibits the electrochemical reaction on the surface of the magnesium alloy. Especially for the (8HQ@ZIF - 8) 3 / PDA extract, the Rct value reaches 12.1 times that of the blank brine, further verifying its excellent anti - corrosion performance. This phenomenon may be closely related to the formation of PDA–Zn complexes and their adsorption behavior on the surface of the magnesium alloy. PDA can not only form stable complexes with Zn 2+ through its abundant amino and phenolic hydroxyl groups, but also chelate with Mg 2+ through its oligomeric structure, thus forming a protective film in the anodic region and inhibiting the anodic reaction. At the same time, the adsorption of Zn 2+ in the cathodic region and the Zn(OH)2 compound formed by its reaction with OH - can effectively reduce the cathodic reaction rate, thus achieving dual inhibition of the corrosion process.

[0113] In addition, from the Bode diagram and Nyquist diagram ( Figure 5 (a) and Figure 5 (b)), it can be observed that as the content of 8 - HQ and PDA–Zn complexes in the extract increases, the position of the maximum phase angle gradually moves towards the low - frequency region, indicating that a denser and more stable protective film is formed on the surface of the magnesium alloy. This protective film can not only physically block the penetration of corrosive media, but also inhibit the electrochemical reaction through chemical action, thus significantly improving the long - term anti - corrosion performance of the coating.

[0114] It is worth noting that (8HQ@ZIF-8) 3 The multi-layer structure design significantly improves the loading efficiency of 8-HQ and provides a barrier for the diffusion of corrosive media through its porous properties, further delaying the corrosion process. Compared with the single-layer 8HQ@ZIF-8, the |Z| 3 and Rct values of (8HQ@ZIF-8) 0.01Hz are increased by 3.2 times and 2.9 times respectively, which fully demonstrates the advantages of the multi-layer structure in enhancing the anti-corrosion performance. The introduction of PDA further optimizes the interfacial compatibility and mechanical stability of the coating, making its application in complex environments more promising.

[0115] In summary, the (8HQ@ZIF-8) 3 / PDA extract shows significant advantages in the corrosion inhibition of magnesium alloys. Its excellent anti-corrosion performance is mainly attributed to the high loading efficiency of the multi-layer ZIF-8 structure, the synergistic effect of the PDA-Zn complex, and the formation of a protective film. This study not only provides new ideas for the development of high-efficiency intelligent anti-corrosion coatings but also provides important experimental evidence for understanding the mechanism of action of 8-HQ and PDA-Zn complexes in corrosion inhibition. In the future, by further optimizing the multi-layer structure of ZIF-8 and the PDA modification strategy, it is expected to achieve more efficient anti-corrosion protection of magnesium alloys and promote their wide application in the aerospace, automotive industries, etc.

[0116] IV. Preparation of Composite Epoxy Coating

[0117] Example 2

[0118] (1) 1-3 wt% (8HQ@ZIF-8) 3 / PDA intelligent nanocontainers (i.e., nanoparticles) are completely mixed in E51 epoxy resin (from Nanchang Chenfang Adhesive Products Co., Ltd.) under stirring, and then ultrasonicated for 0.5-1 h to obtain a mixture;

[0119] (2) A curing agent (modified cycloaliphatic amine, from Nanchang Chenfang Adhesive Products Co., Ltd.) is added to the above mixture (mass ratio: E51 epoxy resin / modified cycloaliphatic amine curing agent = 2:1 - 4:1).

[0120] (3) Before preparing the coating, the magnesium alloy specimens are wet polished successively with sandpapers of 150, 240, and 1200 grit, and the oil stains on the surface of the magnesium alloy are removed with acetone and ethanol. The pure epoxy coating, (8HQ@ZIF-8) 3 / EP are prepared by the same method. The obtained uniform resin is evenly coated on the surface of AZ31 magnesium alloy with a wire bar coater. The coating samples are obtained by drying at room temperature for three days. The dry thickness of all coatings is about 80 μm.

[0121] V. Water Absorption Test of Composite Epoxy Coating

[0122] In this test, the effects of ZIF-8, 8HQ@ZIF-8, (8HQ@ZIF-8) 2 and (8HQ@ZIF-8) 3 / PDA in Example 1 on the water absorption of epoxy resin coatings were tested, and the changes in the water absorption rates of pure epoxy coatings, (8HQ@ZIF-8) 3 epoxy coatings and (8HQ@ZIF-8) 3 / PDA epoxy coatings were mainly analyzed; specifically as follows: By introducing ZIF-8, 8HQ@ZIF-8, (8HQ@ZIF-8) 2 and (8HQ@ZIF-8) 3 / PDA in Example 1 into epoxy resin respectively (according to Example 2), and then their roles in regulating the coating structure, molecular arrangement and water molecule penetration were tested, so as to deeply understand their effects on water absorption performance.

[0123] The results are as Figure 6 shown, and the results show that:

[0124] First of all, the pure epoxy resin coating showed obvious water absorption characteristics during the immersion process. This is mainly attributed to the pore structure formed after the curing of epoxy resin and the internal stress difference. These irregular voids and stresses enable water molecules to quickly penetrate into the coating interior, resulting in a continuous increase in the water absorption rate.

[0125] In addition, it was measured that after 34 days, the water absorption rate of the pure epoxy resin coating reached 1.59047%, which further verified its water absorption characteristics. When the (8HQ@ZIF-8) 3 nano-container was added to the epoxy resin, the water absorption rate of the coating decreased significantly. As a metal-organic framework material with a pore structure, the filling of ZIF-8 in the epoxy resin not only made up for the original pores and internal stress differences, but also significantly enhanced the density of the coating.

[0126] In addition, after 34 days of immersion, the water absorption rate of the coating containing (8HQ@ZIF-8) 3 dropped to 1.55507%, indicating that these nano-containers effectively reduced the penetration of water molecules. Although the PDA layer enhanced the hydrophilicity of the coating, which might lead to the penetration of local water molecules, the overall water absorption performance still remained at a low level, which was consistent with the data of the water absorption rate of 1.50012% of the (8HQ@ZIF-8) 3 / PDA composite coating, indicating that this multi-level nano-container still maintained good water tightness during long-term immersion.

[0127] In addition, from a mechanistic perspective, the water absorption of pure epoxy resin coatings is closely related to the pores and internal stress during the curing process. Water molecules can rapidly penetrate and be adsorbed in these irregular structures. And (8HQ@ZIF-8) 3 The addition significantly slows down the water molecule penetration rate by filling pores, reducing the internal stress difference, and improving the regularity of molecular arrangement. Although the introduction of PDA enhances the surface hydrophilicity of the coating, the designed structure still acts as an effective barrier layer, restricting the excessive penetration of water molecules.

[0128] In summary, the analysis of the water absorption of different coatings reveals the importance of the design of nano - containers (ZIF - 8 and its composites) in improving the water absorption of epoxy resin coatings. Through data analysis after 34 days, the application of (8HQ@ZIF - 8) 3 / PDA nano - containers significantly improves the structural compactness of the coating and reduces the water molecule permeability. At appropriate addition amounts, the water absorption of the coating is effectively inhibited. This finding provides an important theoretical basis for the design of intelligent coatings, indicating that by regulating the filling amount of nano - containers and designing multi - layer structures, the water tightness of the coating can be optimized and more excellent performance can be exerted in practical applications such as the protection of magnesium alloys.

[0129] VI. Self - healing and corrosion resistance tests of composite epoxy coatings

[0130] (I) Fitting test of EIS data

[0131] To evaluate the corrosion and self - healing mechanisms of magnesium alloy epoxy composite coatings in the long term, the electrochemical behavior of coating samples containing different types of nanoparticles and cross - scratches was studied using a Gamry electrochemical workstation (Gamry Interface, 1010E) in 3.5 wt% NaCl solution. The electrochemical cell includes a working electrode (sample), a saturated calomel reference electrode (SCE), and an auxiliary electrode (platinum electrode). The impedance spectra were collected logarithmically in the range from 100 MHz to 10 mHz, with five points per decade. EIS measurements were performed after the samples were exposed to the test environment for 5 days. The EIS data was fitted using an equivalent circuit (EEC) model and the ZsimpWin V.3.20 software. The morphology and element distribution of the scratches after immersion were also studied using SEM - EDS equipment.

[0132] Among them, the Bode plots and Nyquist plots of pure EP, (b) (8HQ@ZIF - 8) 3 / EP, (c) (8HQ@ZIF - 8) 3 / PDA / EP during immersion in 3.5 wt% NaCl solution are as Figure 7 shown.

[0133] In addition, the equivalent circuit model shown in Figure 8 (c) is used to fit the EIS data. In the model, the solution impedance (Rs) characterizes the resistance characteristics of the electrolyte itself; the coating capacitance (Qc) and the coating resistance (Rc) jointly reflect the ability of the coating to impede charge migration and the interfacial polarization behavior; the magnesium alloy oxide layer capacitance (Qo) and resistance (Ro) describe the dielectric response and ion transport resistance of the oxide film on the substrate surface; the charge transfer resistance (Rct) and the double-layer capacitance (Qdl) respectively correspond to the electrochemical reaction resistance and the interfacial charge distribution characteristics at the metal / electrolyte interface. Based on the differences in the penetration degree of the corrosive medium at different NaCl immersion stages, the equivalent circuit model adopts a dynamic adjustment strategy: at the initial stage of immersion, a double time-constant model (reflecting the synergistic protection effect of the coating and the oxide layer) is selected, and as the immersion time prolongs, the accumulation of corrosion products and the expansion of coating microdefects lead to the complication of the electrochemical response. At this time, a third time constant is introduced to characterize the Warburg diffusion process caused by the penetration of the corrosive medium to the metal interface. This change is directly related to the progressive attenuation of the protection ability of the coating / oxide layer, that is, the corrosive medium penetrates into the interior of the coating through micropores, accelerating the anodic dissolution of the magnesium alloy substrate (Mg → Mg 2+ + 2e - ) and the cathodic hydrogen evolution reaction (2H2O + 2e - → H2↑ + 2OH - ), thereby reducing the activation energy barrier of interfacial charge transfer. This phenomenon reveals the dynamic evolution law of electrochemistry kinetics during the coating failure process and provides a key quantitative basis for evaluating the long-term effectiveness of the anti-corrosion system. The obtained parameters are shown in Table 2.

[0134] Table 2 Variation of parameter values of pure EP, (8HQ@ZIF-8) 3 / EP, (8HQ@ZIF-8) 3 / PDA / E coatings with immersion time in 3.5% NaCl solution

[0135]

[0136]

[0137] It can be seen from Table 2 above:

[0138] At the initial stage of immersion (the 5th day), the charge transfer resistance (Rct) and the low-frequency impedance value (|Z| 3 / PDA coating reach 1.02×10 0.01 Ω·cm 10 and 1.34×10 2 Ω·cm 10 respectively, which are higher than those of the pure epoxy coating (8.67×10 2 ), and 7Ω·cm 2 ) two orders of magnitude higher Figure 8 (a) and Figure 8 (b)). This phenomenon can be attributed to the unique design of the multi-layer ZIF-8 structure: its nested pore channels not only extend the diffusion path of corrosive media (such as Cl - and H2O) through the "labyrinth effect", but also provide more loading sites for 8-HQ molecules, thus significantly enhancing the physical barrier performance. At the same time, the introduction of PDA forms strong chemical bonds (such as Mg-O bonds) with the magnesium alloy surface through amino and phenolic hydroxyl groups, enhancing the interfacial adhesion between the coating and the substrate and inhibiting the interfacial peeling caused by microdefects. In addition, the oligomeric structure of PDA can dynamically respond to local environmental changes. For example, when the pH drops at the initial stage of corrosion, it triggers the release of 8-HQ and Zn 2+ in the ZIF-8 pore channels: 8-HQ forms a chelate ((8-HQ)-Mg) with Mg 2+ to repair the microcracks in the anodic region, while the Zn 2+ and OH - generated Zn(OH)2 precipitate inhibits the cathodic reaction. This dual corrosion inhibition mechanism effectively blocks the progress of the corrosion electrochemical reaction.

[0139] After long-term immersion (50 days), the |Z| 3 and R 0.01Hz values of the (8HQ@ZIF-8) ct / PDA coating still remain at 6.36×10 7 Ω·cm 2 and 5.87×10 7 Ω·cm 2 ( Figure 8 ), significantly higher than 1.49×10 5 Ω·cm 2 and 31180 Ω·cm 2This difference and the relatively slow downward trend reveal the importance of the dynamic protection mechanism of the coating: the barrier effect of the coating during the penetration of the corrosive medium is delayed compared to the pure epoxy coating. Due to the physical barrier effect of ZIF-8 and the adhesion enhancement of PDA, the coating can maintain good protective performance during long-term immersion. ZIF-8 itself has excellent chemical stability and can hinder the intrusion of corrosive media. As an efficient interfacial adhesive, PDA can improve the bonding force between the coating and the substrate and reduce the peeling and falling off of the coating. The multi-layer structure of ZIF-8 avoids pore blockage caused by molecular aggregation under high loading through hierarchical pore design, thus maintaining the continuous release ability of the corrosion inhibitor during long-term immersion. The chemical stability and self-healing behavior of PDA further delay the performance degradation of the coating. For example, the phase angle only slightly decreases from 86.2° to 82.8° within 50 days, indicating that the protective film still maintains a high integrity( Figure 7 ). In contrast, due to the lack of a dynamic protection mechanism, the phase angle of the pure epoxy coating drops sharply from 80.8° to 42.4°, and at the same time, the low-frequency impedance value decreases by five orders of magnitude, reflecting the severe local corrosion of the magnesium alloy caused by the penetration of the corrosive medium through the coating (such as the anodic dissolution reaction of Mg→Mg 2+ +2e - ).

[0140] (II) Salt spray test

[0141] Salt spray tests were carried out using a YWX / Q-150 instrument (Beijing Yashilin Testing Equipment Co., Ltd.) in a 5.0 wt% NaCl solution (pH value 6.55–7.0) according to Chinese standard GB / T 10125-2012. The exposed area of the scratched sample was 1.5 cm 2 , and the exposed area of the intact sample was 1 cm 2 .

[0142] Through the salt spray test (SST), the performances of the pure epoxy coating, (8HQ@ZIF-8) 3 / EP coating and (8HQ@ZIF-8) 3 / PDA / EP coating were evaluated, and their performances at different exposure times were compared. The experimental results are as follows( Figure 9 ):

[0143] After 20 days of salt spray exposure, significant corrosion signs appeared on the pure epoxy coating. Obvious corrosion products appeared on its surface, and the corrosion extended to the surface of the magnesium substrate, exposing obvious corrosion channels, indicating the limitations of the protective effect of this coating. In contrast, the epoxy composite coating containing (8HQ@ZIF-8) 3 showed excellent corrosion resistance, and almost no obvious corrosion phenomenon appeared on the surface.

[0144] After 35 days of salt spray exposure, (8HQ@ZIF-8) 3 / PDA / EP composite coating showed more excellent performance. The corrosion products of this coating were significantly less than those of (8HQ@ZIF-8) 3 / EP coating, and the spread of corrosion was relatively slight. The excellent performance of this composite coating can be attributed to the self-healing property of PDA and the synergistic effect of (8HQ@ZIF-8) 3 . During the corrosion process, the presence of PDA enabled the magnesium ions to undergo a complexation reaction with PDA, forming stable PDA–Mg complexes, which not only provided additional protection in the local corrosion area but also effectively restrained the further expansion of corrosion. In addition, the introduction of (8HQ@ZIF-8) 3 also enhanced the overall stability of the coating. When corrosion occurred, the organic molecules in (8HQ@ZIF-8) 3 were released through pH change, diffused to the damaged area, formed complexes with magnesium ions, further repaired the structure of the coating, and promoted the exertion of the self-healing function.

[0145] In addition, long-term corrosion evaluations of different types of coatings were also carried out through salt spray tests (SST) ( Figure 10 ), especially the analysis of the scratched pure epoxy coating, (8HQ@ZIF-8) 3 epoxy coating, and (8HQ@ZIF-8) 3 / PDA / EP ( Figure 11 ). Through this experiment, the corrosion resistance of different coatings under damaged conditions can be more intuitively observed, especially the performance of the self-healing property, barrier property, and durability of the coatings when the coatings are scratched or damaged by external mechanical forces. In contrast, obvious accumulation of corrosion products was shown in the pure epoxy coating after 35 days, the corrosion phenomenon spread from the edge inward, and by 50 days, most of the coating had detached from the substrate. This phenomenon reflects that the protective performance of the pure epoxy coating is relatively limited, and its corrosion resistance and durability are significantly lower than those of the composite coating. Generally speaking, the (8HQ@ZIF-8) 3 / PDA / EP composite coating showed significantly better performance than the pure epoxy coating and (8HQ@ZIF-8) 3 / EP coating in the salt spray test. Its excellent corrosion resistance, low accumulation of corrosion products, and superior self-healing property indicate that this composite coating has a significant improvement in protective performance and durability, especially in the protection application of magnesium matrix, with great potential.

[0146] In addition, after salt spray treatment, especially in the scratched area, the corrosion reaction of the pure epoxy coating starts rapidly, with a large amount of corrosion products appearing on the surface, and the corrosion spreads rapidly at the coating edges. The scratch damages the integrity of the coating, allowing corrosive media (such as sodium chloride solution) to quickly penetrate to the surface of the magnesium alloy substrate, thus accelerating the corrosion process. As time goes by, obvious peeling and delamination occur at the coating edges, indicating a serious lack of anti-corrosion protection ability. This phenomenon is mainly attributed to the relatively high permeability of the epoxy coating. Especially when there are scratch defects, the penetration rate of the corrosive media increases significantly, leading to a gradual decrease in the adhesion between the coating and the substrate, and ultimately resulting in the failure of the coating.

[0147] Among them, in the (8HQ@ZIF-8) containing ZIF-8 3 epoxy coating, a certain degree of corrosion also appears on the coating surface in the scratched area ( Figure 11 ), but compared with the pure epoxy coating, the corrosion propagation rate is significantly slowed down. The introduction of ZIF-8 particles improves the barrier function of the coating to a certain extent. Its characteristic of having a microporous structure enhances the protection effect of the coating and can reduce the penetration of corrosive media. Although corrosion occurs in the scratched area, the presence of ZIF-8 particles can effectively slow down the further expansion of corrosion and limit the formation of corrosion products. This indicates that ZIF-8 not only has a physical barrier effect but may also play a corrosion inhibition role to a certain extent through the interaction between its surface and the corrosive media. However, over time, corrosion will eventually gradually penetrate into the coating substrate, especially in the scratched area, which also exposes the deficiency of the ZIF-8 coating in terms of long-term durability.

[0148] Among them, for the (8HQ@ZIF-8) 3 / PDA epoxy coating, especially in the case of scratches ( Figure 11 ), it shows significantly better corrosion resistance than the previous two. The addition of PDA significantly improves the self-healing ability of the coating. When the coating is scratched or mechanically damaged, the (8HQ@ZIF-8) 3 / PDA microcontainers can release 8-hydroxyquinoline (8-HQ) in the local defect area and react with metal ions on the surface of the substrate metal to form a new protective film, thereby repairing the coating defects and restoring its protective function. This self-healing mechanism significantly improves the long-term anti-corrosion ability of the coating in the case of scratch damage. Especially on the 35th day of the salt spray test, the volume of corrosion products in the scratched area of the (8HQ@ZIF-8) 3 / PDA coating is significantly smaller than that of other specimens, and no large-scale peeling and corrosion expansion occur on the coating surface, showing its excellent durability and protection.

[0149] As a biomimetic material, the unique chemical structure of PDA can not only provide additional adhesion, but also its dopamine groups can enhance the bonding force between the coating and the magnesium alloy substrate through hydrogen bonding with the metal substrate, thereby improving the durability of the coating. In addition, the hydrophilic and self-assembly characteristics of PDA enable it to form a more stable interface between the coating and the substrate, further enhancing the anti-permeability and anti-corrosion ability of the coating. When local damage occurs to the coating, the PDA layer can also effectively promote the release of 8HQ, form a protective complex, and restore the function of the coating.

[0150] In addition, from a mechanistic perspective ( Figure 12 ), this is because ZIF-8, as a microstructural filler, plays a powerful physical barrier role in the coating, slowing down the penetration of corrosive media. At the same time, the introduction of PDA further improves the interfacial adhesion and self-healing ability of the coating. Especially the (8HQ@ZIF-8) 3 / PDA epoxy coating, through the action of microcontainers, can effectively release corrosion inhibitors and self-healing molecules, form a new protective layer in the damaged area, reduce the damage and corrosion propagation of the coating, and significantly extend the corrosion-resistant life of the coating. The anti-corrosion mechanism of this coating mainly relies on triple functions: physical barrier effect, release of corrosion inhibitor molecules, and chemical coordination self-healing. At the microscopic level, ZIF-8, as a metal-organic framework (MOF) material, has a highly ordered framework structure, extremely high specific surface area and porosity, which enables the coating to effectively prevent the penetration of corrosive media such as water, oxygen, and chloride ions to the surface of the magnesium substrate. After the ZIF-8 particles are uniformly dispersed in the coating, the number of micropores and pores is significantly reduced, thereby enhancing the physical barrier effect of the coating, reducing the intrusion of corrosive substances, and improving the anti-corrosion ability. However, although the physical barrier effect of ZIF-8 is significant, cracks or microdefects may appear in the coating after long-term exposure to the corrosive environment, and the corrosive media penetrate to the surface of the magnesium substrate through these defects. At this time, the PDA layer exhibits its self-healing characteristics. The PDA layer is formed on the surface of ZIF-8 particles through a self-polymerization reaction and contains abundant nitrogen and oxygen functional groups, especially phenolic hydroxyl groups (–OH). When the coating is damaged, these phenolic hydroxyl groups can undergo a coordination reaction with magnesium ions (Mg 2+ ) to form a stable PDA–Mg complex. This complex not only improves the mechanical strength of the coating but also further enhances its corrosion-resistant performance, preventing the further penetration of corrosive media. In addition, the metal ions Zn 2+ in ZIF-8 also play an important role in the corrosion process. When the coating is locally corroded, Zn 2+ ions will be released as the PDA is peeled off. Zn 2+ reacts with hydrated hydroxide ions (OH-) to form Zn(OH)2 deposits, and these deposits form a protective film in the cathodic region, effectively preventing the further expansion of corrosion. Zn2+ Ions not only have a corrosion inhibition effect but also can enhance the water resistance and durability of the coating by forming Zn(OH)2 deposits. As an organic corrosion inhibitor, 8HQ forms a stable 8HQ-Mg complex by complexing with metal ions (such as Mg 2+ ) on the surface of the magnesium alloy, significantly reducing the corrosion rate. The corrosion inhibition effect of 8HQ is particularly significant in the area of local coating damage, and it can continuously release corrosion inhibitor substances to further protect the magnesium substrate from corrosion. Therefore, the 8HQ@ZIF-8 / PDA coating provides effective anti-corrosion protection for the surface of the magnesium alloy through the synergistic effects of physical barrier, chemical coordination self-healing, and release of corrosion inhibition molecules.

[0151] In summary, the corrosion resistance of the coating is not only affected by its structure and materials but is also closely related to multiple factors such as the self-healing ability of the coating, physical barrier effect, and interfacial bonding force. On the magnesium alloy substrate, the (8HQ@ZIF-8) 3 / PDA composite coating exhibits excellent corrosion resistance, especially in terms of the corrosion resistance after scratch damage. Its self-healing mechanism and enhanced interfacial bonding force provide additional protection for the coating. Through this multifunctional coating design, the long-term stability and corrosion resistance of the magnesium alloy can be effectively improved, providing a new direction for the development and application of future coating materials.

[0152] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. The first ZIF-8 composite material, characterized in that, The first ZIF-8 composite material comprises ZIF-8 and a corrosion inhibitor. The corrosion inhibitor is embedded in the ZIF-8 framework to form a pomegranate-like structure. The pomegranate-like structure has n layers, where n ≥ 1 and is an integer. The corrosion inhibitor is selected from one or a combination of 8-hydroxyquinoline, methylbenzotriazole, benzotriazole, mercaptobenzothiazole, or zinc gluconate.

2. The first ZIF-8 composite material according to claim 1, characterized in that, n is 1 - 3.

3. The first ZIF-8 composite material according to claim 2, wherein When n is 1, the pomegranate-like structure has 1 layer, and the particle size of the first ZIF-8 composite material is 145 nm - 150 nm; When n is 2, the pomegranate-like structure has 2 layers, and the particle size of the first ZIF-8 composite material is 460 nm - 465 nm; When n is 3, the pomegranate-like structure has 2 layers, and the particle size of the first ZIF-8 composite material is 655 nm - 660 nm.

4. The preparation method of the first ZIF-8 composite material according to any one of claims 1 to 3, characterized in that, The pomegranate-like structure of the first ZIF-8 composite material has 1 layer. The preparation method of the first ZIF-8 composite material comprises: (1) Mix a 2-methylimidazole solution with the corrosion inhibitor to obtain a mixed solution; (2) Mix the mixed solution with a zinc ion solution to obtain the first ZIF-8 composite material.

5. The preparation method of the first ZIF-8 composite material according to any one of claims 1 to 3, characterized in that, The pomegranate-like structure of the first ZIF-8 composite material has n layers, where n ≥ 2. The preparation method of the first ZIF-8 composite material comprises: (1) Mix a solution of the first ZIF-8 composite material with a pomegranate-like structure of n - 1 layers with 2-methylimidazole, and then add the corrosion inhibitor to obtain a mixed solution; (2) Mix the mixed solution with a zinc ion solution to obtain the first ZIF-8 composite material with a pomegranate-like structure of n layers.

6. The second ZIF-8 composite material, characterized in that, The second ZIF-8 composite material comprises the first ZIF-8 composite material according to any one of claims 1 to 3 and polydopamine, and the first ZIF-8 composite material is encapsulated by polydopamine.

7. The preparation method of the second ZIF-8 composite material according to claim 6, characterized in that, Comprises: Mix a dispersion of the first ZIF-8 composite material with a dopamine hydrochloride solution and react to obtain the second ZIF-8 composite material.

8. Epoxy resin composite material, characterized in that, Comprises the first ZIF-8 composite material according to any one of claims 1 to 3 and / or the second ZIF-8 composite material according to claim 6, a curing agent, and epoxy resin.

9. The epoxy resin composite material according to claim 8, wherein, The mass fraction of the first ZIF-8 composite material and / or the second ZIF-8 composite material is 1% - 3%.

10. The application of the epoxy resin composite material according to claim 9 as a surface coating for magnesium alloy, and the magnesium alloy surface coating has self-healing anti-corrosion performance.

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