A nano-modified corrosion resistant coating for ductile cast iron and a method for preparing the same

Abstract

This invention belongs to the field of coating technology, specifically relating to a nano-modified corrosion-resistant coating for ductile iron and its preparation method. The coating employs a two-layer structure: the bottom interface passivation layer is formed by the co-condensation of KH560 and diethylphosphorylethyltriethoxysilane to create a Si-O-Si crosslinked network, anchoring the substrate through Fe-O-Si covalent bonds and relying on phosphonate groups for adsorption in the cathode region of the graphite spheres, thus weakening micro-galvanic corrosion; the top protective layer uses epoxy resin as a matrix, doped with modified nano-SiO2 and phytic acid-modified cerium-pyromellitic acid coordination polymer, where the modified nano-SiO2 reacts with released Ce... 3+ The Ce-O-P crosslinked barrier network enhances the barrier effect. This coordination polymer releases phytic acid and Ce in response to a localized pH decrease induced by corrosion. 3+ Ce 3+ A passivation film is deposited in the cathode region to suppress oxygen reduction and phytic acid chelates Fe. 2+ / Fe 3+ Blocking corrosion channels inhibits anodic dissolution and improves the long-term corrosion resistance of the coating.

Description

Technical Field

[0001] This invention relates to the field of coating technology, and in particular to a nano-modified corrosion-resistant coating for ductile iron and its preparation method. Background Technology

[0002] Ductile iron pipes, with their excellent mechanical strength, pressure resistance, and structural stability, are widely used in municipal water supply and drainage, underground integrated pipe networks, industrial fluid transportation, and many other fields. During long-term service, the inner wall of these pipes is constantly in contact with water, corrosive ions, and environmental permeating media. Because ductile iron itself contains graphite spheres with good electrical conductivity that are evenly distributed in the matrix, it forms a typical micro-galvanic corrosion system with the surrounding ferrite. The graphite spheres act as cathodes and continuously undergo oxygen reduction reactions, while the ferrite acts as anodes and continuously undergoes metal dissolution. The micro-galvanic effect significantly accelerates the corrosion failure of the inner wall of the pipe. Therefore, corrosion-resistant coating technology for the inner wall of ductile iron pipes has always been a focus of industry attention and research. Currently, the commonly used internal wall protection methods in the industry mainly include cement mortar lining, single-component silane passivation film, and conventional epoxy anti-corrosion coating. Among them, cement mortar lining is inexpensive and easy to construct, but the material is brittle and has poor resistance to deformation. It is prone to cracking and falling off under fluid impact, pipeline vibration, or foundation settlement conditions, and cannot achieve long-term protection. Single silane passivation film can form a basic passivation structure on the cast iron surface, but its film thickness is thin, which can only achieve short-term interface protection and is difficult to prevent long-term penetration of corrosive media. The most widely used ordinary epoxy coating mainly relies on the density of the coating to achieve physical barrier. Some products will simply be compounded with conventional nanofillers or single corrosion inhibitors to optimize performance. However, most of these coatings only focus on single physical barrier or unidirectional corrosion inhibition, which is difficult to meet the long-term service requirements of complex corrosive environments. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the present invention aims to provide a nano-modified corrosion-resistant coating for ductile iron and its preparation method. This coating employs a two-layer structure: the bottom interfacial passivation layer is formed by the co-condensation of KH560 and diethylphosphorylethyltriethoxysilane to create a Si-O-Si crosslinked network, anchoring the substrate through Fe-O-Si covalent bonds and adsorbing onto the cathode region of the graphite spheres via phosphonate groups, thus mitigating microgalvanic corrosion; the top protective layer uses epoxy resin as a matrix, doped with modified nano-SiO2 and phytic acid-modified cerium-pyromellitic acid coordination polymer, where the modified nano-SiO2 reacts with released Ce... 3+ The Ce-OP cross-linked barrier network enhances the barrier effect. This coordination polymer releases phytic acid and Ce in response to a localized pH decrease induced by corrosion. 3+ Ce 3+ A passivation film is deposited in the cathode region to suppress oxygen reduction and phytic acid chelates Fe. 2+ / Fe 3+Blocking corrosion channels inhibits anodic dissolution and improves the long-term corrosion resistance of the coating.

[0004] To achieve the above objectives, the present invention employs the following technical solution:

[0005] In a first aspect, the present invention provides a nano-modified corrosion-resistant coating for ductile iron, the coating comprising an interface passivation layer and a protective layer coated on the interface passivation layer;

[0006] The interface passivation layer includes KH560 and diethylphosphorylethyltriethoxysilane;

[0007] The protective layer comprises a matrix, modified nano-SiO2, phytic acid-modified cerium-pyromellitic acid coordination polymer, and a curing agent;

[0008] The modified nano-SiO2 is obtained by grafting nano-SiO2 with diethylphosphorylethyltriethoxysilane;

[0009] The phytic acid-modified cerium-pyromellitic acid coordination polymer is obtained by synthesizing cerium-pyromellitic acid coordination polymer by solvothermal reaction of cerium nitrate hexahydrate and pyromellitic acid, followed by a coordination reaction with phytic acid.

[0010] The matrix is ​​E-51 epoxy resin; the curing agent is polyamide 650.

[0011] Further, the mass ratio of KH560, diethylphosphorylethyltriethoxysilane, matrix, modified nano-SiO2, phytic acid modified cerium-pyromellitic acid coordination polymer and curing agent is (4.2-8):(1.5-2.5):(25-35):(1.5-2.5):(0.8-1.5):(20-28).

[0012] Furthermore, the nano-SiO2 is Aerosil 200 type fumed silica with a specific surface area of ​​200 m². 2 / g; the epoxy value of the E-51 epoxy resin is 0.48-0.54eq / 100g; the amine value of the polyamide 650 is 180-220mgKOH / g.

[0013] Secondly, the present invention provides a method for preparing a nano-modified corrosion-resistant coating for ductile iron, comprising the following steps:

[0014] S1. Mix nano-SiO2 and anhydrous ethanol to obtain a nano-SiO2 suspension; mix diethylphosphorylethyltriethoxysilane and anhydrous ethanol, add deionized water dropwise to adjust the pH to obtain a silane hydrolysate, add all of it dropwise to the nano-SiO2 suspension, reflux the reaction, and then centrifuge, wash and dry to obtain modified nano-SiO2.

[0015] S2. Phytic acid is dissolved in deionized water to obtain a phytic acid solution; cerium nitrate hexahydrate is dissolved in deionized water to obtain a cerium nitrate aqueous solution; pyromellitic acid is dissolved in DMF, added to the cerium nitrate aqueous solution, mixed, reacted, washed, and dried, then ground, dispersed, and dried to obtain a cerium-pyromellitic acid coordination polymer; deionized water is added, the phytic acid solution is added dropwise, the pH is adjusted, a water bath reaction is performed, and after washing and vacuum drying, a phytic acid-modified cerium-pyromellitic acid coordination polymer is obtained.

[0016] S3. Sandblasting pretreatment is performed on the inner wall of the ductile iron pipe to obtain a pretreated ductile iron pipe; KH560 hydrolysate and phosphoryl ethyl silane hydrolysate are mixed to obtain an interface passivation solution; the interface passivation solution is coated onto the inner wall of the pretreated ductile iron pipe by centrifugal coating, and after curing and cleaning with anhydrous ethanol, an interface passivation layer is formed.

[0017] S4. Mix E-51 epoxy resin and anhydrous ethanol, add modified nano-SiO2 and phytic acid-modified cerium-pyromellitic acid coordination polymer, disperse, add curing agent, filter, and obtain nano-composite coating; use centrifugal coating to coat the nano-composite coating onto the surface of the interface passivation layer, cure, and obtain a nano-modified corrosion-resistant coating for ductile iron.

[0018] In one feasible implementation, in S1, the mass-to-volume ratio of nano-SiO2 to anhydrous ethanol is (6-10) g:(150-250) mL; the mass-to-volume ratio of diethylphosphorylethyltriethoxysilane, anhydrous ethanol, and deionized water is (3-5) g:(80-120) mL:(0.41-0.82) mL; the rate at which the deionized water is added is 0.1-0.2 mL / min; the step of adjusting the pH is as follows: adjusting the pH to 4.5-5.0 with glacial acetic acid; the rate at which the silane hydrolysate is added is 0.5-2.5 mL / min.

[0019] In one feasible implementation, in step S1, the reflux reaction temperature is 60-80℃, and the reflux reaction time is 3-6h; the centrifugal washing step is as follows: centrifuge at 6000-10000rpm for 10-20min to collect the precipitate, wash with anhydrous ethanol 2-4 times, each time using 40-60mL of anhydrous ethanol, sonicate at 150-250W for 3-8min, and then centrifuge at 6000-10000rpm for 8-15min; the drying is vacuum drying, and the drying conditions are: temperature 70-90℃, vacuum degree ≤50-150Pa, and time 8-16h.

[0020] Using fumed silica as a matrix modification material, this nano-silica possesses characteristics such as large specific surface area, extremely small particle size, and stable structure, naturally exhibiting excellent potential for physical filling and media barrier properties. However, the high activity and strong polarity of the hydroxyl groups on the surface of the original nano-silica make it prone to agglomeration in organic epoxy resin systems, forming pore defects within the coating and thus reducing its anti-corrosion performance. Therefore, diethylphosphonylethyltriethoxysilane is introduced as a surface modifier. This silane molecule contains both hydrolyzable siloxane groups and functional phosphonate groups. After hydrolysis, it generates highly active silanol groups, which can undergo dehydration condensation reactions with a large number of hydroxyl groups on the surface of nano-silica, grafting onto the particle surface in a stable covalent bond form. This significantly reduces the surface polarity of the nanoparticles, inhibits agglomeration defects, and greatly improves the compatibility and dispersion uniformity of nano-silica in organic resins.

[0021] The modified nano-silica can, on the one hand, construct tortuous penetration paths within the coating by means of uniformly dispersed nanoparticles, significantly extending the penetration channels of corrosive media such as water and chloride ions, and greatly improving the physical barrier properties and overall density of the coating; on the other hand, it successfully introduces phosphonate functional groups into the particle surface. These active groups can form stable Ce-OP coordination bonds with cerium ions released by the system during the service of the coating, crosslinking the discretely distributed nanoparticles into a barrier network, further strengthening the overall structure of the coating, effectively inhibiting the penetration of corrosive media along the interface, and avoiding interlayer debonding problems.

[0022] In one feasible implementation, in step S2, the mass ratio of phytic acid, cerium nitrate hexahydrate, and trimellitic acid is (0.6-1.0):(4.0-5.0):(1.8-2.5); the mass-to-volume ratio of phytic acid to deionized water is (0.6-1.0) g:(8-15) mL; the mass-to-volume ratio of cerium nitrate hexahydrate to deionized water is (4.0-5.0) g:(40-60) mL; the mass-to-volume ratio of trimellitic acid to DMF is (1.8-2.5) g:(40-60) mL; the reaction temperature is 75-95℃, and the reaction time is 8-15 minutes. The washing and drying steps are as follows: wash with DMF 2-4 times, 20-40 mL each time, then wash with anhydrous methanol 2-4 times, 20-40 mL each time. After each washing, centrifuge at 5000-8000 rpm for 8-15 min, and dry in a vacuum drying oven at 50-70℃ for 6-12 h. The grinding conditions are: rotation speed 150-250 rpm, time 25-50 min. The dispersion steps are as follows: disperse in 20-40 mL of anhydrous ethanol, sonicate at 150-250 W for 3-8 min, and collect by centrifugation at 5000-8000 rpm for 8-15 min.

[0023] In one feasible implementation, in step S2, the drying temperature is 50-70℃, and the drying time is 3-6h; the mass-to-volume ratio of the cerium-pyromellitic acid coordination polymer to deionized water is (1.5-3.0) g : (30-50) mL; the phytic acid solution is added at a rate of 0.3-1.2 mL / min; the pH adjustment step is to adjust the pH of the system to 3.0-4.0 with glacial acetic acid; the water bath reaction temperature is 35-50℃, and the water bath reaction time is 4-6h; the washing step is to collect the precipitate by centrifugation at 5000-8000 rpm for 8-15 min, and wash it with deionized water 3-5 times, 20-40 mL each time, until the pH of the supernatant is 5.0-5.5; the vacuum drying temperature is 50-70℃, and the vacuum drying time is 8-16h; the median particle size of the cerium-pyromellitic acid coordination polymer is 3-5 μm.

[0024] A solvothermal coordination reaction was conducted between cerium nitrate hexahydrate and trimellitic acid. Cerium nitrate dissociates into trivalent cerium ions in the solvent system. These ions possess excellent metal passivation and corrosion inhibition properties. Trimeric acid, as a polycarboxyl organic ligand, allows multiple carboxyl groups in its molecule to coordinate with the trivalent cerium ions, gradually constructing a cerium-trimeric acid coordination polymer with a network-like framework. The coordination framework of this polymer is sensitive to acid and alkaline environments; the coordination interaction between carboxyl groups and cerium ions is easily interfered with by hydrogen ions, naturally exhibiting pH-responsive structural characteristics. Simultaneously, the porous framework can stably support active substances, laying the structural foundation for subsequent functional modification and controlled release of components.

[0025] Based on this, phytic acid is introduced for surface functional modification. Phytic acid molecules contain a large number of phosphate active groups and have excellent metal ion coordination and chelation abilities. In the reaction system, the phosphate groups of phytic acid will undergo coordination exchange reactions with the exposed trivalent cerium active sites on the surface and in the pores of the cerium-pyromellitic acid coordination polymer, thereby stably grafting onto the polymer backbone. Due to the distribution of coordination sites and the binding form, phytic acid will form two binding modes: when the local distribution of cerium ion sites on the polymer surface is relatively sparse, a single phytic acid molecule can only rely on one phosphate group to form a coordination bond with a single trivalent cerium ion, and the other phosphate groups do not participate in the bonding, thus forming a single-node coordination structure. This structure relies on only a single coordination bond, resulting in weak overall force and low stability. In the region where cerium ion sites are dense, a single phytic acid molecule can use two or more phosphate groups to bind to multiple adjacent trivalent cerium ions, forming a multi-node cross-linked coordination structure. This structure relies on the synergistic effect of multiple coordination bonds, resulting in high binding strength and high structural stability. The two binding modes also determine the differentiated release pattern of phytic acid.

[0026] When the complete coating is in service, the coordination polymer skeleton structure remains stable. Trivalent cerium ions act as metal nodes in the coordination skeleton to maintain its integrity, while phytic acid is anchored on the exposed cerium ion active sites on the skeleton surface and within the pores. Once the coating develops scratches, cracks, or other defects, external corrosive media can penetrate and cause electrochemical corrosion in the ductile iron. The pH of the local microenvironment in the defect area decreases, and acidic protons attack the coordination bonds between cerium ions and pyromellitic acid ligands, continuously weakening the coordination forces between carboxyl groups and trivalent cerium ions. This causes the coordination polymer skeleton to gradually dissociate, thereby simultaneously releasing trivalent cerium ions and phytic acid. The single-node coordinated phytic acid is expected to be released immediately upon slight dissociation of the framework, chelating with ferrous and ferric ions generated during corrosion to form a chemically stable, water-insoluble phytate-iron solid complex. This complex effectively fills the interior of corrosion defects, quickly sealing the medium penetration channels, inhibiting the rapid spread of initial corrosion, and forming a chemical inhibition film in the anodic region, effectively suppressing the anodic dissolution reaction of ferrite. The multi-node cross-linked coordinated phytic acid has a more stable bond and is expected to be released slowly with continuous dissociation of the framework, providing long-term replenishment of active components and achieving a long-term protective effect. Simultaneously released trivalent cerium ions migrate to the cathode region of the graphite spheres. In the cathode region, the oxygen reduction reaction generates hydroxide ions, forming a localized alkaline microenvironment. Hydrolysis and deposition generate a Ce(OH)3 / CeO2 composite inorganic passivation film, effectively blocking the oxygen reduction active sites on the surface of the graphite spheres and inhibiting the cathode reaction. At the same time, the free trivalent cerium ions can also act as coordination bridging points, forming Ce-OP coordination bonds with the phosphonate groups on the surface of modified nano-silica, further strengthening the internal network of the coating. This phytic acid-modified cerium-pyromellitic acid coordination polymer, with its skeletal structure, dual-morphology phytic acid coordination mode, and multi-component synergistic effect, has become the core functional carrier for coatings to achieve pH-responsive self-healing and synergistic inhibition of micro-galvanic corrosion by anode and cathode.

[0027] In one feasible implementation, in step S3, the specifications of the ductile iron pipe are: nominal diameter DN200, wall thickness grade K9, and pipe section length 300mm; the sandblasting pretreatment steps are: using 60-100 mesh white corundum sand as the sandblasting medium, controlling the sandblasting pressure to 0.3-0.6MPa and the sandblaster travel speed to 80-200mm / min, so that the inner wall reaches a near-white cleanliness grade Sa2.5, and after sandblasting, using 0.2-0.6MPa compressed air to blow away residual abrasive; the preparation steps of the KH560 hydrolysate are: adding 4.2-8.0g of KH560 to a mixture of 40-60mL of anhydrous ethanol and 0.46-1.22mL of deionized water, adjusting the pH to 4.5-5.0 with glacial acetic acid, and magnetically stirring at 250-450rpm for 45-75min.

[0028] In one feasible implementation, the preparation step of the phosphoryl ethyl silane hydrolysate in step S3 is as follows: 1.5-2.5g of diethylphosphoryl ethyl triethoxysilane is added to a mixture of 15-30mL anhydrous ethanol and 0.21-0.41mL deionized water; the pH is adjusted to 4.5-5.0 with glacial acetic acid; and the mixture is magnetically stirred at 250-450rpm for 30-60min. The centrifugal coating step is as follows: the pretreated ductile iron tube is vertically fixed on a rotating support. Inject 5-15 mL of interface passivation solution from the top of the tube segment using a syringe at a rate of 0.5-3.0 mL / s, then rotate at 15-60 rpm for 1-3 min; the curing temperature is 80-100℃, and the curing time is 45-90 min; the ethanol cleaning step is as follows: inject 2-8 mL of anhydrous ethanol into the inner wall of the tube at a rate of 0.3-1.5 mL / s, rotate the tube segment at 8-25 rpm for 3-15 s, and then dry with cold air; the thickness of the interface passivation layer is 5-8 μm.

[0029] The pretreated ductile iron substrate surface possesses moderate roughness and abundant hydroxyl active sites, providing a foundation for the adsorption and anchoring of the silane passivation layer. KH560 epoxy silane and diethylphosphorylethyltriethoxysilane undergo a co-condensation reaction after hydrolysis to generate active silanol groups, constructing a continuous, dense, and stable Si-O-Si crosslinking network. KH560 primarily serves as an interface anchor and interlayer transition agent. Its hydrolyzed silanol groups can form high-strength Fe-O-Si covalent bonds with the hydroxyl groups on the ductile iron metal surface, achieving chemical anchoring between the passivation layer and the metal substrate. This effectively solves the problems of easy coating desorption and weak interfacial adhesion. Simultaneously, its terminal epoxy groups can participate in the curing and crosslinking reaction of the top epoxy resin system, strengthening the interlayer adhesion of the double-layer coating.

[0030] Diethylphosphonoethyltriethoxysilane primarily serves as a cathode passivation and corrosion inhibitor. Its phosphonate active groups can form chemical adsorption on the graphite sphere surface, supplementing and covering areas of the graphite spheres where silanol groups cannot be effectively bonded, thus forming a stable cathode passivation film. This effectively inhibits the oxygen reduction cathodic reaction on the graphite sphere surface, fundamentally weakening the driving force of micro-galvanic corrosion in ductile iron. This interface passivation layer, as the core structure of the bottom layer of the double-layer coating, combines interface anchoring, interlayer transition, and cathode corrosion inhibition functions, providing a stable adhesion substrate for the top protective layer while simultaneously inhibiting micro-galvanic corrosion of the substrate at its source, ensuring the structural and performance stability of the coating during long-term service.

[0031] In one feasible implementation, in step S4, the mass-to-volume ratio of the E-51 epoxy resin and anhydrous ethanol is (25-35):(5-8); the dispersion conditions are: power 200-400W, ice-water bath temperature control ≤35℃, time 15-30min; the filtration medium is a 200-250 mesh stainless steel filter screen; the curing steps are: holding at 50-70℃ for 0.5-1.5h, raising the temperature to 70-90℃ and holding for 0.5-1.5h, finally raising the temperature to 100-130℃ and holding for 1.5-3h, and then cooling to room temperature in the furnace; the thickness of the nano-modified corrosion-resistant coating is 90-120μm.

[0032] In one feasible implementation, the centrifugal coating step in S4 is as follows: vertically fix the ductile iron tube with the interface passivation layer coated on the inner wall onto the rotating support, inject 30-38 mL of nano-composite coating from the top of the tube section with a syringe, rotate at 200-350 rpm for 3-5 min, horizontally fix the tube section onto the rotating support and rotate at a low speed of 20-60 rpm, and keep it in the rotating state for surface drying at room temperature for 0.5-3 h.

[0033] Using E-51 epoxy resin as the film-forming matrix for the top protective layer, combined with polyamide 650 as the curing agent, the two can undergo a highly efficient cross-linking and curing reaction to form a dense, stable, and mechanically excellent polymer network structure, constituting the basic framework of the top protective layer and endowing the coating with excellent film-forming properties, integrity, and basic resistance to media penetration. Two functional fillers, modified nano-silica and phytic acid-modified cerium-pyromellitic acid coordination polymer, are introduced into the resin matrix. Uniform dispersion treatment eliminates filler agglomeration defects, ensuring that the functional components are evenly distributed throughout the overall coating system.

[0034] Modified nano-silica primarily functions as a physical filler, labyrinth barrier, and structural enhancer, filling the micropores created during resin curing to further improve coating density and extend the penetration path of corrosive media. Simultaneously, Ce-OP coordination crosslinking constructs a coordination barrier network, strengthening the overall structural strength and stability of the coating. Phytic acid-modified cerium-based coordination polymers endow the coating with pH-responsive self-healing capabilities, actively releasing active components under corrosive conditions to achieve self-repair of defects and dynamic corrosion inhibition. Centrifugal coating leverages centrifugal force to achieve uniform and smooth spreading of the coating on the inner wall of the pipe, effectively avoiding defects such as uneven thickness, sagging, and missed areas caused by manual coating. Combined with gradient curing technology, the internal stress of coating curing is slowly released, effectively suppressing forming defects such as cracking, craters, and pinholes, further improving coating integrity and density.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] This scheme uses the inner wall of ductile iron pipe as a substrate. By preparing modified nano-SiO2 and phytic acid-modified cerium-pyromellitic acid coordination polymer, a two-layer coating structure of interface passivation layer and protective layer is adopted to obtain a nano-modified corrosion-resistant coating for ductile iron, achieving long-term corrosion protection. With the interface passivation layer as the bottom layer, diethylphosphonoethyltriethoxysilane forms a passivation layer in the cathode region of the graphite sphere due to the chemical adsorption of its phosphonate groups. This inhibits the oxygen reduction reaction of the graphite sphere cathode, cuts off the electron consumption pathway on the cathode side in microgalvanic corrosion, and causes the anolyte iron to dissolve without electrochemical driving force. Simultaneously, KH560 forms Fe-O-Si covalent bonds with the substrate surface for chemical anchoring and co-condenses with diethylphosphonoethyltriethoxysilane to form a Si-O-Si crosslinked network, chemically fixing the phosphonate groups to the crosslinked network and ensuring its stability during long-term service.

[0037] With a protective layer as the top layer and epoxy resin as the continuous phase matrix, phytic acid-modified cerium-pyromellitic acid coordination polymers are used. 3+ As a framework metal node, when corrosion induces a local pH decrease, protons attack the carboxyl groups of the pyromellitic acid ligands, weakening the coordination bonds between cerium and the ligands, causing partial dissolution of the framework and release of Ce. 3+ And phytic acid. Released Ce 3+ A Ce(OH)3 / CeO2 passivation film was deposited in the alkaline microenvironment of the graphite sphere cathode region to suppress the oxygen reduction reaction (cathode inhibition), and phytic acid chelated Fe. 2+ / Fe 3+ An insoluble phytic acid-iron film is formed to inhibit anodic iron dissolution (anodic inhibition), and the two, together with the interface passivation layer, constitute a cathodic-anodic synergistic inhibition. Modified nano-SiO2, by grafting diethylphosphonylethyltriethoxysilane, extends the penetration path of the corrosive medium due to the uniform dispersion of the nano-SiO2 core in the epoxy resin system. The grafted phosphonate groups can react with the released Ce... 3+ Ce-OP coordination bonds are formed at the interfaces between adjacent particles, crosslinking the dispersed modified nano-SiO2 particles into a barrier network, thereby enhancing the bulk barrier performance; at the same time, Ce is released. 3+ Coordination with phosphonate groups chemically adsorbed on the surface of graphite spheres increases anchoring stability and further enhances the cathode passivation effect. Furthermore, phytic acid release exhibits a dual-mode characteristic: coordination only with a single Ce group... 3+ Released immediately upon dissolution of the framework at the node, chelating Fe 2+ / Fe 3+ The formation of insoluble phytic acid-iron complex blocks corrosion channels to quickly respond to initial corrosion; the multi-node cross-linked phytic acid requires more nodes to dissolve successively to gradually release it, thus providing long-term protection and significantly improving the long-term corrosion resistance of the inner wall of ductile iron pipe in corrosive environments. Attached Figure Description

[0038] Figure 1This invention provides a flowchart for the preparation of a nano-modified corrosion-resistant coating for ductile iron. Detailed Implementation

[0039] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the application will be further described in detail below with reference to embodiments. However, this should not be construed as limiting the scope of this application to the following examples. All other embodiments obtained by those skilled in the art without creative effort without departing from the above-described methodological spirit of this application are within the scope of protection of this application.

[0040] The singular forms “for,” “or,” “a,” “any,” and “described” used in this application are intended to include the plural forms unless the context clearly indicates otherwise. Furthermore, the terms “first” and “second” are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0041] The polyamide 650 used in the examples and comparative examples of this application has an amine value of 180-220 mgKOH / g.

[0042] Example 1

[0043] like Figure 1 As shown, a method for preparing a nano-modified corrosion-resistant coating for ductile iron includes the following steps:

[0044] S1, 8g of material with a specific surface area of ​​200m² 2 / g of nano-SiO2 (Aerosil 200 type fumed silica) and 200mL of anhydrous ethanol were ultrasonically dispersed for 30min at 400W power, 40kHz frequency, and ice-water bath temperature control ≤30℃ to obtain a nano-SiO2 suspension. 4g of diethylphosphonylethyltriethoxysilane was added to 100mL of anhydrous ethanol, and 0.61mL of deionized water was added dropwise at 0.15mL / min under magnetic stirring at 350rpm. The pH was adjusted to 4.8 with glacial acetic acid, and the mixture was hydrolyzed at 350rpm for 45min at room temperature to obtain silane hydrolysate. The silane hydrolysate was added dropwise to the nano-SiO2 suspension at a rate of 1.5 mL / min. After the addition was complete, the temperature was raised to 70 °C and refluxed for 4.5 h. After the reaction was completed, the mixture was cooled to room temperature, and the precipitate was collected by centrifugation at 8000 rpm for 15 min. The precipitate was washed three times with anhydrous ethanol, and each time it was sonicated at 200 W for 5 min with 50 mL of anhydrous ethanol, followed by centrifugation at 8000 rpm for 10 min. The precipitate was then vacuum dried at 80 °C and a vacuum degree ≤100 Pa for 12 h to obtain modified nano-SiO2.

[0045] S2. Dissolve 0.8g phytic acid in 12mL deionized water to obtain a phytic acid solution; dissolve 4.5g Ce(NO3)3·6H2O in 50mL deionized water to obtain a cerium nitrate aqueous solution; dissolve 2.2g trimesic acid in 50mL DMF, add the cerium nitrate aqueous solution, and stir magnetically at 225rpm for 6min. Transfer to a reaction vessel and react at 85℃ in an oven for 12h. After natural cooling to room temperature, collect the precipitate, wash three times with 30mL DMF each time, and three times with 30mL anhydrous methanol each time. After each washing, centrifuge at 6500rpm for 10min, and dry in a vacuum drying oven at 60℃ for 9h. Grind using a planetary ball mill (zirconia grinding balls, ball-to-material ratio 5:1) at 200rpm for 35min. After grinding, disperse the powder in 30mL anhydrous ethanol, sonicate at 200W for 5min, and collect by centrifugation at 6500rpm for 10min. The cerium-pyromellitic acid coordination polymer powder with a median particle size of 4 μm was obtained by vacuum drying at ℃ for 4.5 h. 2.0 g of cerium-pyromellitic acid coordination polymer powder was dispersed in 40 mL of deionized water and ultrasonically dispersed at 200 W for 15 min. Phytic acid solution was added dropwise at 0.75 mL / min, and the pH of the system was adjusted to 3.5 with glacial acetic acid. The reaction was carried out in a constant temperature water bath at 42 ℃ with stirring at 225 rpm for 5 h. After the reaction was completed, the precipitate was collected by centrifugation at 6500 rpm for 10 min. The precipitate was washed 4 times with 30 mL of deionized water each time until the pH of the supernatant was 5.2. The precipitate was dried in a vacuum drying oven at 60 ℃ for 12 h to obtain phytic acid modified cerium-pyromellitic acid coordination polymer.

[0046] S3. The inner wall of a ductile iron pipe specimen with a nominal diameter of DN200, wall thickness grade K9, and pipe section length of 300mm was sandblasted using a pipe inner wall sandblasting device. The sandblasting medium was 80-mesh white corundum abrasive. The sandblasting pressure was controlled at 0.45MPa and the sandblasting device travel speed at 140mm / min, so that the inner wall reached a cleanliness level of Sa2.5, close to white. After sandblasting, residual abrasive was blown away with 0.4MPa compressed air to obtain a pre-treated ductile iron pipe; 6g KH560 was added to a mixture of 50 mL anhydrous ethanol and 1.03 mL deionized water, and the pH was adjusted to 4.8 with glacial acetic acid. The mixture was then hydrolyzed at room temperature with magnetic stirring at 350 rpm for 60 min to obtain KH560 hydrolysate. 2.0 g of diethylphosphorylethyltriethoxysilane was added to a mixture of 20 mL anhydrous ethanol and 0.3 mL deionized water, and the pH was adjusted to 4.8 with glacial acetic acid. The mixture was then hydrolyzed at room temperature with magnetic stirring at 350 rpm for 45 min to obtain phosphorylethylsilane hydrolysate. The KH560 hydrolysate and phosphorylethylsilane hydrolysate were mixed and magnetically stirred at 350 rpm for 15 min to obtain an interface passivation solution. The inner wall was pretreated... The ductile iron pipe is vertically fixed on a rotating support (ensuring coaxial rotation with an eccentricity of <1%). 8 mL of interface passivation solution is injected from the top of the pipe section at 1.5 mL / s using a syringe. Then, the pipe is rotated at 40 rpm for 2 minutes to ensure the passivation solution evenly covers the inner wall of the pipe. After rotation, the pipe section is inverted to drain excess passivation solution. It is then allowed to air dry at room temperature for 10 minutes and placed in a 90℃ oven for 70 minutes to cure. After curing, 5 mL of anhydrous ethanol is injected into the inner wall of the pipe at 0.9 mL / s using a syringe. The pipe section is rotated at 15 rpm for 10 seconds to ensure the ethanol flows evenly through the inner wall. Then, the pipe is inverted to drain the ethanol and dried with cold air to obtain a ductile iron pipe with a 6 μm interface passivation layer coated on the inner wall.

[0047] S4. Add 6.5g of anhydrous ethanol as a diluent to 30g of E-51 epoxy resin with an epoxy value of 0.52eq / 100g. Add 2.0g of modified nano-SiO2 in three portions, stirring for 10min after each addition. Add 1.0g of phytic acid-modified cerium-pyromellitic acid coordination polymer in two portions, stirring for 10min after each addition. Continue stirring for 20min. Ultrasonically disperse for 20min under 300W power and ice-water bath temperature control ≤35℃. Add 24g of polyamide 650 and mechanically stir at 400rpm for 5min. After standing for 10min to defoam, filter through a 220-mesh stainless steel filter to obtain the nano-composite coating. The ductile iron pipe with the interface passivation layer coated on the wall was vertically fixed on a rotating support (ensuring coaxial rotation with an eccentricity of <1%). 34 mL of nano-composite coating was injected from the top of the pipe section using a syringe. Then, the pipe was rotated at 280 rpm for 4 min to allow the coating to evenly cover the inner wall of the pipe under centrifugal force. After rotation, the pipe section was inverted to remove excess coating. The pipe section was then horizontally fixed on the rotating support and rotated at a low speed of 40 rpm. The pipe section was kept in a rotating state and allowed to dry at room temperature for 2 h. Then, the pipe section was placed horizontally in an oven for gradient curing: the pipe section was heated to 60℃ for 1 h, heated to 80℃ for 1 h, and finally heated to 115℃ for 2 h. The pipe section was then cooled to room temperature with the oven. The dry film thickness was controlled at 100 μm to obtain a nano-modified corrosion-resistant coating for ductile iron.

[0048] Example 2

[0049] A method for preparing a nano-modified corrosion-resistant coating for ductile iron includes the following steps:

[0050] S1, 6g of material with a specific surface area of ​​200m² 2 / g of nano-SiO2 (Aerosil 200 type fumed silica) and 150mL of anhydrous ethanol were ultrasonically dispersed for 20min at 300W power, 35kHz frequency, and ice-water bath temperature control ≤30℃ to obtain a nano-SiO2 suspension. 3g of diethylphosphonylethyltriethoxysilane was added to 80mL of anhydrous ethanol, and 0.41mL of deionized water was added dropwise at 0.1mL / min under magnetic stirring at 250rpm. The pH was adjusted to 4.5 with glacial acetic acid, and the mixture was hydrolyzed at 250rpm for 30min at room temperature to obtain silane water. Hydrolysis solution: The silane hydrolysate was added dropwise to the nano-SiO2 suspension at a rate of 0.5 mL / min. After the addition was complete, the temperature was raised to 60℃ and refluxed for 3 h. After the reaction was completed, the mixture was cooled to room temperature, and the precipitate was collected by centrifugation at 6000 rpm for 10 min. The precipitate was washed twice with anhydrous ethanol. Each time, 40 mL of anhydrous ethanol was used to sonicate the mixture at 150 W for 3 min, followed by centrifugation at 6000 rpm for 8 min. The precipitate was then vacuum dried at 70℃ and a vacuum degree ≤150 Pa for 8 h to obtain modified nano-SiO2.

[0051] S2. Dissolve 0.6g phytic acid in 8mL deionized water to obtain a phytic acid solution; dissolve 4.0g Ce(NO3)3·6H2O in 40mL deionized water to obtain a cerium nitrate aqueous solution; dissolve 1.8g trimesic acid in 40mL DMF, add the cerium nitrate aqueous solution, and stir magnetically at 150rpm for 3min. Transfer to a reaction vessel and react at a constant temperature of 75℃ for 8h. After natural cooling to room temperature, collect the precipitate, wash twice with 20mL DMF each time, and twice with 20mL anhydrous methanol each time. After each washing, centrifuge at 5000rpm for 8min, dry in a vacuum drying oven at 50℃ for 6h, and grind using a planetary ball mill (zirconia grinding balls, ball-to-material ratio 3:1) at 150rpm for 25min. After grinding, disperse the powder in 20mL anhydrous ethanol, sonicate at 150W for 3min, and collect by centrifugation at 5000rpm for 8min. Cerium-pyromellitic acid coordination polymer powder with a median particle size of 3 μm was obtained by vacuum drying at 0℃ for 3 h. 1.5 g of cerium-pyromellitic acid coordination polymer powder was dispersed in 30 mL of deionized water and ultrasonically dispersed at 150 W for 10 min. Phytic acid solution was added dropwise at 0.3 mL / min, and the pH of the system was adjusted to 3.0 with glacial acetic acid. The mixture was stirred at 150 rpm in a constant temperature water bath at 35℃ for 4 h. After the reaction was completed, the precipitate was collected by centrifugation at 5000 rpm for 8 min. The precipitate was washed three times with 20 mL of deionized water each time until the pH of the supernatant was 5.0. The mixture was then dried in a vacuum drying oven at 50℃ for 8 h to obtain phytic acid modified cerium-pyromellitic acid coordination polymer.

[0052] S3. The inner wall of a ductile iron pipe specimen with a nominal diameter of DN200, wall thickness grade K9, and pipe section length of 300mm was sandblasted using a pipe inner wall sandblasting device. The sandblasting medium was 60-mesh white corundum abrasive. The sandblasting pressure was controlled at 0.3MPa and the sandblasting device travel speed at 80mm / min, so that the inner wall reached a cleanliness level of Sa2.5, close to white. After sandblasting, residual abrasive was blown away with 0.2MPa compressed air to obtain a pre-treated ductile iron pipe; 4.2g KH560 was added to a mixture of 40 mL anhydrous ethanol and 0.46 mL deionized water, and the pH was adjusted to 4.5 with glacial acetic acid. The mixture was then hydrolyzed at room temperature with magnetic stirring at 250 rpm for 45 min to obtain KH560 hydrolysate. 1.5 g of diethylphosphorylethyltriethoxysilane was added to a mixture of 15 mL anhydrous ethanol and 0.21 mL deionized water, and the pH was adjusted to 4.5 with glacial acetic acid. The mixture was then hydrolyzed at room temperature with magnetic stirring at 250 rpm for 30 min to obtain phosphorylethylsilane hydrolysate. The KH560 hydrolysate and phosphorylethylsilane hydrolysate were mixed and magnetically stirred at 250 rpm for 10 min to obtain an interface passivation solution. The inner wall... The pretreated ductile iron pipe is vertically fixed on a rotating support (ensuring coaxial rotation with an eccentricity of <1%). 7 mL of interface passivation solution is injected from the top of the pipe section at 0.5 mL / s using a syringe. Then, the pipe is rotated at 15 rpm for 1 min to ensure the passivation solution evenly covers the inner wall of the pipe. After rotation, the pipe section is inverted to drain excess passivation solution. It is then allowed to air dry at room temperature for 5 min and placed in an 80℃ oven for curing for 45 min. After curing, 2 mL of anhydrous ethanol is injected into the inner wall of the pipe at 0.3 mL / s using a syringe. The pipe section is rotated at 8 rpm for 3 s to ensure the ethanol flows evenly through the inner wall. Then, the pipe is inverted to drain the ethanol and dried with cold air to obtain a ductile iron pipe with a 5 μm interface passivation layer coated on the inner wall.

[0053] S4. Add 5.0g of anhydrous ethanol as a diluent to 25g of E-51 epoxy resin with an epoxy value of 0.48eq / 100g. Add 1.5g of modified nano-SiO2 in two portions, stirring for 5min after each addition. Add 0.8g of phytic acid-modified cerium-pyromellitic acid coordination polymer in one addition, stirring for 5min. Continue stirring for 15min. Ultrasonically disperse for 15min under 200W power and ice-water bath temperature control ≤35℃. Add 20g of polyamide 650, mechanically stir at 250rpm for 3min, let stand to defoam for 5min, and then filter through a 200-mesh stainless steel filter to obtain a nano-composite coating. Coat the inner wall with an interface passivation layer. The ductile iron pipe was vertically fixed on a rotating support (ensuring coaxial rotation with an eccentricity of <1%). 30 mL of nano-composite coating was injected from the top of the pipe section using a syringe. Then, the pipe was rotated at 200 rpm for 3 minutes to allow the coating to evenly cover the inner wall of the pipe under centrifugal force. After rotation, the pipe section was inverted to remove excess coating. The pipe section was then horizontally fixed on the rotating support and rotated at a low speed of 20 rpm. The pipe section was kept in a rotating state and allowed to dry at room temperature for 0.5 h. Then, it was placed horizontally in an oven for gradient curing: the pipe section was kept at 50℃ for 0.5 h, then heated to 70℃ for 0.5 h, and finally heated to 100℃ for 1.5 h. The pipe section was then cooled to room temperature in the oven. The dry film thickness was controlled at 90 μm to obtain a nano-modified corrosion-resistant coating for ductile iron.

[0054] Example 3

[0055] A method for preparing a nano-modified corrosion-resistant coating for ductile iron includes the following steps:

[0056] S1, 10g of material with a specific surface area of ​​200m² 2 / g of nano-SiO2 (Aerosil 200 type fumed silica) and 250mL of anhydrous ethanol were ultrasonically dispersed for 40min at a power of 500W, a frequency of 45kHz, and an ice-water bath temperature control ≤30℃ to obtain a nano-SiO2 suspension. 5g of diethylphosphonylethyltriethoxysilane was added to 120mL of anhydrous ethanol, and 0.82mL of deionized water was added dropwise at 0.2mL / min under magnetic stirring at 450rpm. The pH was adjusted to 5.0 with glacial acetic acid, and the mixture was hydrolyzed at room temperature with stirring at 450rpm for 60min to obtain silane hydrolysate. The silane hydrolysate was added dropwise to the nano-SiO2 suspension at a rate of 2.5 mL / min. After the addition was complete, the temperature was raised to 80 °C and refluxed for 6 h. After the reaction was completed, the mixture was cooled to room temperature, and the precipitate was collected by centrifugation at 10,000 rpm for 20 min. The precipitate was washed four times with anhydrous ethanol, and each time it was sonicated at 250 W for 8 min with 60 mL of anhydrous ethanol, followed by centrifugation at 10,000 rpm for 15 min. The precipitate was then vacuum dried at 90 °C and a vacuum degree ≤50 Pa for 16 h to obtain modified nano-SiO2.

[0057] S2. Dissolve 1.0 g of phytic acid in 15 mL of deionized water to obtain a phytic acid solution; dissolve 5.0 g of Ce(NO3)3·6H2O in 60 mL of deionized water to obtain a cerium nitrate aqueous solution; dissolve 2.5 g of trimesic acid in 60 mL of DMF, add the cerium nitrate aqueous solution, and stir magnetically at 300 rpm for 8 min. Transfer to a reaction vessel and react at a constant temperature of 95℃ for 16 h. After natural cooling to room temperature, collect the precipitate, wash it four times with 40 mL of DMF each time, and then wash it four times with 40 mL of anhydrous methanol each time. After each washing, centrifuge at 8000 rpm for 15 min, dry in a vacuum drying oven at 70℃ for 12 h, and grind it using a planetary ball mill (zirconia grinding balls, ball-to-material ratio 6:1) at 250 rpm for 50 min. After grinding, separate the powder... The cerium-pyromellitic acid coordination polymer with a median particle size of 5 μm was obtained by sonicating in 40 mL of anhydrous ethanol at 250 W for 8 min, centrifuging at 8000 rpm for 15 min, and vacuum drying at 70 °C for 6 h. 3.0 g of the cerium-pyromellitic acid coordination polymer was dispersed in 50 mL of deionized water and sonicated at 250 W for 20 min. Phytic acid solution was added dropwise at 1.2 mL / min, and the pH of the system was adjusted to 4.0 with glacial acetic acid. The reaction was carried out in a constant temperature water bath at 50 °C with stirring at 300 rpm for 6 h. After the reaction, the precipitate was collected by centrifugation at 8000 rpm for 15 min, washed 5 times with 40 mL of deionized water each time, until the pH of the supernatant was 5.5. The precipitate was then dried in a vacuum drying oven at 70 °C for 16 h to obtain the phytic acid-modified cerium-pyromellitic acid coordination polymer.

[0058] S3. The inner wall of a ductile iron pipe specimen with a nominal diameter of DN200, wall thickness grade K9, and pipe section length of 300mm was sandblasted using a pipe inner wall sandblasting device. The sandblasting medium was 100-mesh white corundum abrasive. The sandblasting pressure was controlled at 0.6MPa and the sandblasting device travel speed at 200mm / min, so that the inner wall reached a cleanliness level of Sa2.5, close to white. After sandblasting, residual abrasive was blown away with 0.6MPa compressed air to obtain a pretreated ductile iron pipe; 8g KH560 was added to a mixture of 60 mL anhydrous ethanol and 1.22 mL deionized water, and the pH was adjusted to 5.0 with glacial acetic acid. The mixture was then hydrolyzed at room temperature with magnetic stirring at 450 rpm for 75 min to obtain KH560 hydrolysate. 2.5 g of diethylphosphorylethyltriethoxysilane was added to a mixture of 30 mL anhydrous ethanol and 0.41 mL deionized water, and the pH was adjusted to 5.0 with glacial acetic acid. The mixture was then hydrolyzed at room temperature with magnetic stirring at 450 rpm for 60 min to obtain phosphorylethylsilane hydrolysate. The KH560 hydrolysate and phosphorylethylsilane hydrolysate were mixed and magnetically stirred at 450 rpm for 20 min to obtain an interface passivation solution. The inner wall was pretreated... The ductile iron pipe is vertically fixed on a rotating support (ensuring coaxial rotation with an eccentricity of <1%). 9 mL of interface passivation solution is injected from the top of the pipe section at 3.0 mL / s using a syringe. The pipe is then rotated at 60 rpm for 3 minutes to ensure the passivation solution evenly covers the inner wall. After rotation, the pipe section is inverted to drain excess passivation solution. It is then allowed to air dry at room temperature for 15 minutes and placed in a 100℃ oven for 90 minutes to cure. After curing, 8 mL of anhydrous ethanol is injected into the inner wall of the pipe at 1.5 mL / s using a syringe. The pipe section is rotated at 25 rpm for 15 seconds to ensure the ethanol flows evenly across the inner wall. The pipe is then inverted to drain the ethanol and dried with cold air to obtain a ductile iron pipe with an 8 μm interface passivation layer coated on the inner wall.

[0059] S4. Add 8.0g of anhydrous ethanol as a diluent to 35g of E-51 epoxy resin with an epoxy value of 0.54eq / 100g. Add 2.5g of modified nano-SiO2 in four portions, stirring for 15min after each addition. Add 1.5g of phytic acid-modified cerium-pyromellitic acid coordination polymer in three portions, stirring for 15min after each addition. Continue stirring for 30min. Ultrasonically disperse for 30min under 400W power and ice-water bath temperature control ≤35℃. Add 28g of polyamide 650 and mechanically stir at 550rpm for 8min. After standing for 15min to defoam, filter through a 250-mesh stainless steel filter to obtain a nano-composite coating. Apply the coating to the inner wall. The ductile iron pipe with the passivation layer was vertically fixed on a rotating support (ensuring coaxial rotation with an eccentricity of <1%). 38 mL of nano-composite coating was injected from the top of the pipe section using a syringe. Then, the pipe was rotated at 350 rpm for 5 minutes to allow the coating to evenly cover the inner wall of the pipe under centrifugal force. After rotation, the pipe section was inverted to remove excess coating. The pipe section was then horizontally fixed on the rotating support and rotated at a low speed of 60 rpm. The pipe section was kept in a rotating state and allowed to dry at room temperature for 3 hours. Then, the pipe section was placed horizontally in an oven for gradient curing: it was heated at 70℃ for 1.5 hours, then heated to 90℃ for 1.5 hours, and finally heated to 130℃ for 3 hours. The pipe section was then cooled to room temperature with the oven. The dry film thickness was controlled at approximately 120 μm, resulting in a nano-modified corrosion-resistant coating for ductile iron.

[0060] Comparative Example 1

[0061] A method for preparing a nano-modified corrosion-resistant coating for ductile iron differs from Example 1 in that the interface passivation layer is not applied in step S3; instead, the pretreated ductile iron pipe is directly coated with the protective layer. The remaining steps and parameters are the same.

[0062] Comparative Example 2

[0063] A method for preparing a nano-modified corrosion-resistant coating for ductile iron differs from Example 1 in that unmodified nano-SiO2 is used instead of modified nano-SiO2 in step S4, while the remaining steps and parameters are the same.

[0064] Comparative Example 3

[0065] A method for preparing a nano-modified corrosion-resistant coating for ductile iron differs from Example 1 in that only cerium-pyromellitic acid coordination polymer is prepared in step S2, without the phytic acid post-modification step. That is, the cerium-pyromellitic acid coordination polymer replaces the phytic acid modification of the cerium-pyromellitic acid coordination polymer, while the remaining steps and parameters are the same.

[0066] Comparative Example 4

[0067] A method for preparing a nano-modified corrosion-resistant coating for ductile iron is disclosed. The implementation steps and parameters differ from those in Example 1 except that diethylphosphorylethyltriethoxysilane is not added in step S3, while the remaining steps and parameters are the same.

[0068] Performance testing:

[0069] Pull-off adhesion test: A coating adhesion pull-off tester, a 20mm diameter steel standard test column, and high-strength structural adhesive were used. Test points were marked on the smooth inner wall of the nano-modified corrosion-resistant coating samples for ductile iron prepared in Examples 1-3 and Comparative Examples 1-4, respectively, and surface dust and impurities were removed. The test column was made of 45# steel, the loading rate was 5mm / min, the test environment temperature was 23℃, and the relative humidity was 50%. Each group was tested in parallel five times. The structural adhesive was evenly applied to the end face of the test column and vertically pasted to the marked points. The column was allowed to stand at room temperature for 24 hours to ensure complete curing. The pipe section with the test column clamped was fixed to the tester, and a uniform upward load was applied. The maximum destructive load during coating peeling was recorded, and the adhesion value (MPa) was calculated. The failure mode (cohesive failure / interlayer peeling / interfacial detachment) was also recorded. Outliers were removed, and the arithmetic mean was taken.

[0070] Neutral salt spray test: The outer walls and nozzles of the nano-modified corrosion-resistant coating samples for ductile iron prepared in Examples 1-3 and Comparative Examples 1-4 were completely sealed with corrosion-resistant sealing tape, leaving only the inner wall coating as the test surface. The corrosive medium was a 5.0% sodium chloride aqueous solution, pH controlled at 6.7, chamber temperature 35℃, saturation tank temperature 47℃, continuous spray mode, and salt spray deposition rate 1.5 mL / (80 cm²·h). The sealed samples were evenly placed on the sample rack, with the inner wall test surface at a 25° angle to the vertical direction; the equipment was turned on for continuous spraying, and inspections were conducted at fixed intervals to observe whether defects such as bulging, peeling, pinholes, and rust spots appeared on the inner wall coating. The cumulative test time (h) for the first obvious corrosion defect of each group of samples was recorded.

[0071] Overall electrochemical impedance spectroscopy (EIS) testing: An electrochemical workstation and a three-electrode testing system were used. Regularly shaped coating samples of the nano-modified corrosion-resistant coatings for ductile iron prepared in Examples 1-3 and Comparative Examples 1-4 were cut from the inner wall of the pipe section to be tested and used as working electrodes. After rinsing with deionized water and drying with cold air, the samples were allowed to stand for 10 minutes. The test electrolyte was a 3.5% sodium chloride aqueous solution at 23°C. The three-electrode system consisted of the coating sample as the working electrode, a platinum sheet as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode. The AC perturbation potential amplitude was 10 mV, and the test frequency range was 10 mV. -2 Hz-10 6Hz, after the open-circuit potential stabilizes for 300s, the test is repeated 3 times for each group. The three electrodes are sequentially inserted into the electrolytic cell and completely submerged, with a fixed electrode spacing and no contact between them; after the open-circuit potential stabilizes, impedance data is collected, and the low-frequency region (10 Hz) is extracted. -2 The impedance modulus (Ω·cm²) is used as an evaluation index to reflect the overall barrier ability of the coating against corrosive media.

[0072] Electrochemical impedance spectroscopy (EIS) tracking test of the scratched area: An electrochemical workstation, a standard cross-cutting apparatus, and a three-electrode testing system were used. Standard linear scratches with a width of 0.3 mm and a length of 10 mm were uniformly prepared on the inner wall of the nano-modified corrosion-resistant coatings for ductile iron prepared in Examples 1-3 and Comparative Examples 1-4, penetrating the coating to the ductile iron substrate. The scratched pipe section was completely immersed in a 3.5% sodium chloride aqueous solution. The immersion temperature was 23℃, and tests were conducted at three time points: 0 h, 24 h, 72 h, and 168 h of immersion. The electrolyte, three-electrode system, perturbation potential, and frequency range were consistent with the overall EIS test, and each group was tested in parallel three times. After the set time, the pipe section was removed, and the scratched area was designated as the working electrode test area. A three-electrode system was constructed and immersed in the electrolyte. Impedance data were collected after the open circuit potential stabilized. The self-healing capability was evaluated by the change in impedance modulus of the scratched area at different immersion times; a more significant increase in impedance modulus indicated a better self-healing effect.

[0073] Table 1. Performance test results of the nano-modified corrosion-resistant coatings for ductile iron prepared in Examples 1-3 and Comparative Examples 1-4.

[0074]

[0075] Table 2. Results of electrochemical impedance tracing tests on the nano-modified corrosion-resistant coatings for ductile iron prepared in Examples 1-3 and Comparative Examples 1-4 in the scratched areas.

[0076]

[0077] As shown in Tables 1 and 2, the adhesion, time to first appearance of corrosion defects in salt spray test, and low-frequency impedance modulus of the nano-modified corrosion-resistant coatings for ductile iron prepared in Examples 1-3 are generally higher than those in Comparative Examples 1-4. Furthermore, the impedance modulus of the coatings prepared for different immersion times is higher than that in Comparative Examples 1-4. This indicates that the interfacial bonding, long-term corrosion resistance, and damage self-healing performance of the nano-modified corrosion-resistant coatings for ductile iron prepared in Examples 1-3 are superior to those in Comparative Examples 1-4.

[0078] Comparative Example 1, by eliminating the interface passivation layer coating process and relying solely on the protective coating for protection, resulted in a significant deterioration in overall protective performance. Due to the absence of the cross-linked network and covalent anchoring structure formed by the co-condensation of KH560 and diethylphosphorylethyltriethoxysilane, the interfacial bonding ability between the coating and the metal substrate was significantly reduced, and the interfacial bonding stability was greatly weakened, making the coating prone to interfacial delamination failure. Simultaneously, the system lacked the specific passivation effect of phosphonate groups on the cathode region of the ductile iron graphite spheres, failing to inhibit the cathode oxygen reduction reaction and cut off the electron transport pathway for micro-galvanic corrosion. The inherent micro-galvanic corrosion of the ductile iron substrate continued spontaneously, significantly reducing the overall medium barrier capacity and long-term aging resistance of the coating. Furthermore, without passivation buffer protection at the bottom layer, corrosive media could directly erode the metal substrate after penetrating the surface coating, losing the stable interfacial basis for self-healing reactions. Corrosion in the damaged area continued to expand, significantly reducing self-healing efficiency and making effective repair difficult. This fully demonstrates that the interface passivation layer is the core structural foundation for ensuring the interfacial bonding strength of the coating, inhibiting micro-galvanic corrosion of the substrate, and supporting the long-term stable operation of the top-layer protective system.

[0079] Comparative Example 2, which replaced silane-grafted modified nano-SiO2 with unmodified nano-SiO2, showed a significant decrease in coating density and overall structural stability. Unmodified nano-silica exhibits poor compatibility with the epoxy resin matrix, readily agglomerating and creating numerous micropores and defects within the coating. This drastically reduces the coating's physical barrier properties, significantly shortens the penetration path of corrosive media, and significantly diminishes overall resistance to media penetration and long-term corrosion resistance. Furthermore, the unmodified particles lack grafted phosphonate active groups on their surface, preventing the formation of a Ce-OP coordination crosslinking network with released cerium ions. This means the unmodified particles cannot construct a dense and continuous barrier system to improve coating integrity, nor can they form auxiliary coordination and sealing structures in damaged areas. Consequently, the coating's structural strength decreases slightly, and the interface damage pattern shifts from simple cohesive damage to mixed damage. Moreover, there is no auxiliary repair benefit after coating damage, and self-healing stability is significantly weakened. This demonstrates that silane modification can simultaneously achieve multiple effects, including uniform dispersion of nanoparticles, improved coating density, and enhanced coordination crosslinking.

[0080] Comparative Example 3 used a cerium-pyromellitic acid coordination polymer without phytic acid modification. While the coating's basic corrosion resistance slightly decreased, its core self-healing performance suffered a precipitous decline. The cerium-based coordination polymer could still release cerium ions upon corrosion triggering, forming a cerium-based passivation film in the anodic region of the substrate. Combined with the physical barrier effect of the interface passivation layer and modified nano-SiO2, the coating's basic interface bonding performance, conventional barrier performance, and salt spray aging resistance remained relatively good. However, due to the lack of a dual-mode phytic acid release mechanism, it could neither rapidly release free phytic acid to chelate iron ions and form insoluble complexes in the early stages of corrosion, thus failing to quickly block corrosion channels and inhibit initial corrosion, nor could it rely on multi-node cross-linked phytic acid for long-term sustained-release protection. The passivation effect of cerium ions alone was insufficient to continuously repair coating damage defects, resulting in a significant weakening of the coating's impedance recovery ability after damage. The scratched areas could not be completely closed and repaired, and corrosion defects persisted. This fully demonstrates that phytic acid modification is the core key to achieving rapid response and efficient long-term self-healing performance in this coating.

[0081] In Comparative Example 4, the interface passivation system lacked diethylphosphorylethyltriethoxysilane and relied solely on KH560 to construct the interface layer, resulting in a significant weakening of the interface passivation capability and interlayer synergistic protection performance. The lack of phosphonate functional groups in the interfacial silane crosslinking network significantly reduces the chemical anchoring strength between the coating and the substrate. Simultaneously, the loss of interlayer coordination sites makes the coating prone to interfacial peeling and debonding, resulting in a significant decrease in interfacial bonding stability. More critically, the system loses the passivation groups that can specifically adsorb onto the graphite sphere cathode region, failing to inhibit the oxygen reduction reaction at the graphite sphere cathode and thus unable to block the core driving pathway of micro-galvanic corrosion in ductile iron. This leads to continuous and accumulating micro-corrosion reactions in the substrate, causing a sustained deterioration in the overall dielectric barrier capacity and long-term aging resistance of the coating. Furthermore, the absence of the coordination anchoring enhancement mechanism between interfacial phosphonate groups and cerium ions significantly reduces cathode passivation stability. After coating damage, there is no synergistic repair effect, and corrosion continues to spread without effective inhibition. This confirms that diethylphosphonyl ethyltriethoxysilane is the core component for achieving cathode passivation, improving the interfacial crosslinking structure, and enhancing the synergistic protective performance of the double-layer coating.

[0082] Comparative Examples 1-4 lacked an interface passivation layer, silane-grafted modified nano-silica, phytic acid functional components, and diethylphosphorylethyltriethoxysilane, respectively. This resulted in poor interfacial bonding stability of the coating, easy interlayer desorption of the double-layer coating, weakened coating density and medium barrier ability, inability to effectively suppress micro-galvanic corrosion of the ductile iron matrix, and failure of corrosion-triggered self-healing mechanism. All of these factors contributed to varying degrees of deterioration in interfacial bonding, long-term corrosion resistance, and damage self-healing performance.

[0083] The above results demonstrate and describe the basic principles and main features of this application, as well as its advantages.

[0084] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A nano-modified corrosion-resistant coating for ductile iron, characterized in that, The coating includes an interface passivation layer and a protective layer coated on the interface passivation layer; The interface passivation layer includes KH560 and diethylphosphorylethyltriethoxysilane; The protective layer comprises a matrix, modified nano-SiO2, phytic acid-modified cerium-pyromellitic acid coordination polymer, and a curing agent; The modified nano-SiO2 is obtained by grafting nano-SiO2 with diethylphosphorylethyltriethoxysilane; The phytic acid-modified cerium-pyromellitic acid coordination polymer is obtained by synthesizing cerium-pyromellitic acid coordination polymer by solvothermal reaction of cerium nitrate hexahydrate and pyromellitic acid, followed by a coordination reaction with phytic acid. The matrix is ​​E-51 epoxy resin; the curing agent is polyamide 650.

2. The nano-modified corrosion-resistant coating for ductile iron according to claim 1, characterized in that, The mass ratio of KH560, diethylphosphorylethyltriethoxysilane, matrix, modified nano-SiO2, phytic acid-modified cerium-pyromellitic acid coordination polymer and curing agent is (4.2-8):(1.5-2.5):(25-35):(1.5-2.5):(0.8-1.5):(20-28).

3. A method for preparing a nano-modified corrosion-resistant coating for ductile iron as described in claim 1 or 2, characterized in that, Includes the following steps: S1. Mix nano-SiO2 and anhydrous ethanol to obtain a nano-SiO2 suspension; mix diethylphosphorylethyltriethoxysilane and anhydrous ethanol, add deionized water dropwise to adjust the pH to obtain a silane hydrolysate, add all of it dropwise to the nano-SiO2 suspension, reflux the reaction, and then centrifuge, wash and dry to obtain modified nano-SiO2. S2. Phytic acid is dissolved in deionized water to obtain a phytic acid solution; cerium nitrate hexahydrate is dissolved in deionized water to obtain a cerium nitrate aqueous solution; pyromellitic acid is dissolved in DMF, added to the cerium nitrate aqueous solution, mixed, reacted, washed, and dried, then ground, dispersed, and dried to obtain a cerium-pyromellitic acid coordination polymer; deionized water is added, the phytic acid solution is added dropwise, the pH is adjusted, a water bath reaction is performed, and after washing and vacuum drying, a phytic acid-modified cerium-pyromellitic acid coordination polymer is obtained. S3. Sandblasting pretreatment is performed on the inner wall of the ductile iron pipe to obtain a pretreated ductile iron pipe; KH560 hydrolysate and phosphoryl ethyl silane hydrolysate are mixed to obtain an interface passivation solution; the interface passivation solution is coated onto the inner wall of the pretreated ductile iron pipe by centrifugal coating, and after curing and cleaning with anhydrous ethanol, an interface passivation layer is formed. S4. Mix E-51 epoxy resin and anhydrous ethanol, add modified nano-SiO2 and phytic acid-modified cerium-pyromellitic acid coordination polymer, disperse, add curing agent, filter, and obtain nano-composite coating; use centrifugal coating to coat the nano-composite coating onto the surface of the interface passivation layer, cure, and obtain a nano-modified corrosion-resistant coating for ductile iron.

4. The method for preparing a nano-modified corrosion-resistant coating for ductile iron according to claim 3, characterized in that, In step S1, the mass-to-volume ratio of nano-SiO2 to anhydrous ethanol is (6-10) g: (150-250) mL; the mass-to-volume ratio of diethylphosphorylethyltriethoxysilane, anhydrous ethanol, and deionized water is (3-5) g: (80-120) mL: (0.41-0.82) mL; the pH is adjusted to 4.5-5.0; the reflux reaction temperature is 60-80℃; and the reflux reaction time is 3-6 h.

5. The method for preparing a nano-modified corrosion-resistant coating for ductile iron according to claim 3, characterized in that, In step S2, the mass ratio of phytic acid, cerium nitrate hexahydrate, and trimesic acid is (0.6-1.0):(4.0-5.0):(1.8-2.5); the mass-volume ratio of phytic acid to deionized water is (0.6-1.0) g:(8-15) mL; the mass-volume ratio of cerium nitrate hexahydrate to deionized water is (4.0-5.0) g:(40-60) mL; the mass-volume ratio of trimesic acid to DMF is (1.8-2.5) g:(40-60) mL; the reaction temperature is 75-95℃, and the reaction time is 8-16 h.

6. The method for preparing a nano-modified corrosion-resistant coating for ductile iron according to claim 3, characterized in that, In step S2, the grinding conditions are: rotation speed 150-250 rpm, time 25-50 min; the dispersion medium is 20-40 mL of anhydrous ethanol; the mass-volume ratio of the cerium-trimethylammonium phosphate coordination polymer to deionized water is (1.5-3.0) g : (30-50) mL; the pH is adjusted to 3.0-4.0; the water bath reaction temperature is 35-50℃, the water bath reaction time is 4-6 h; and the washing is performed until the pH of the supernatant is 5.0-5.

5.

7. The method for preparing a nano-modified corrosion-resistant coating for ductile iron according to claim 3, characterized in that, In step S3, the preparation steps of the KH560 hydrolysate are as follows: add 4.2-8.0g of KH560 to a mixture of 40-60mL of anhydrous ethanol and 0.46-1.22mL of deionized water, and adjust the pH to 4.5-5.0; the preparation steps of the phosphoryl ethyl silane hydrolysate are as follows: add 1.5-2.5g of diethylphosphoryl ethyl triethoxysilane to a mixture of 15-30mL of anhydrous ethanol and 0.21-0.41mL of deionized water, and adjust the pH to 4.5-5.

0.

8. The method for preparing a nano-modified corrosion-resistant coating for ductile iron according to claim 3, characterized in that, In step S3, the centrifugal coating step is as follows: the pretreated ductile iron pipe is vertically fixed on a rotating support, and 5-15 mL of interface passivation liquid is injected from the top of the pipe section at 0.5-3.0 mL / s using a syringe, and then rotated at 15-60 rpm for 1-3 min; the curing temperature is 80-100℃, and the curing time is 45-90 min.

9. The method for preparing a nano-modified corrosion-resistant coating for ductile iron according to claim 3, characterized in that, In step S4, the mass ratio of E-51 epoxy resin to anhydrous ethanol is (25-35):(5-8); the centrifugal coating step is as follows: the ductile iron tube with the interface passivation layer coated on the inner wall is vertically fixed on the rotating support, 30-38 mL of nano-composite coating is injected from the top of the tube section with a syringe, and the tube section is rotated at 200-350 rpm for 3-5 min. The tube section is then horizontally fixed on the rotating support and rotated at a low speed of 20-60 rpm. The tube is kept in a rotating state and allowed to dry at room temperature for 0.5-3 h.

10. The method for preparing a nano-modified corrosion-resistant coating for ductile iron according to claim 3, characterized in that, In step S4, the curing step is as follows: holding at 50-70℃ for 0.5-1.5h, raising the temperature to 70-90℃ and holding for 0.5-1.5h, and finally raising the temperature to 100-130℃ and holding for 1.5-3h, and then cooling to room temperature in the furnace; the thickness of the nano-modified corrosion-resistant coating is 90-120μm.

Images