Preparation method and application of crystalline KZnP3O9 corrosion inhibitor

Abstract

The invention relates to a preparation method and application of a crystalline KZnP3O9 corrosion inhibitor, the basic construction unit of the crystal structure of the crystalline KZnP3O9 corrosion inhibitor is a (PO3) infinity chain, and the chain is formed by connecting PO4 tetrahedrons; the (PO3) infinity chain extends in a three-dimensional space by sharing oxygen atoms to form a long-range ordered three-dimensional network structure based on metaphosphate radical (PO3 <->). The corrosion inhibitor designed by the invention has ultrahigh electrochemical impedance, excellent stability and longer service life, and the protection level of carbon steel in a severe environment is improved.

Description

Technical Field

[0001] This invention relates to the field of anti-corrosion materials technology, specifically to a method for preparing and applying a crystalline KZnP3O9 corrosion inhibitor. Background Technology

[0002] Carbon steel is one of the most widely used structural materials in industry, indispensable in critical facilities such as buildings, pipelines, vehicles, and ships. However, carbon steel is prone to severe corrosion in acidic environments (such as pickling and petrochemical processes), causing not only huge economic losses but also threatening public safety and health. Adding corrosion inhibitors is an economical and efficient corrosion prevention strategy.

[0003] Among numerous corrosion inhibitors, inorganic phosphates have been widely studied due to their low cost and environmental friendliness. Traditionally, given the generally high glass-forming ability of phosphate systems, research and application as corrosion inhibitors in acidic environments have mainly focused on their glassy form. These glassy phosphates are typically prepared via a one-step direct chemical synthesis method. However, products prepared by such methods have significant drawbacks: on the one hand, the raw materials may volatilize or undergo side reactions during the high-temperature reaction, leading to inaccurate chemical composition and unstable performance of the final product; on the other hand, the product is essentially a metastable substance directly generated by a solid-state reaction, lacking clear glass transition characteristics and not being a "true glass" obtained through melt quenching. This may result in insufficient long-term stability as a corrosion inhibitor.

[0004] More importantly, although glassy phosphates have been widely used, their corrosion protection performance in strong acid environments still faces limitations. Existing glassy phosphate corrosion inhibitors generally have low electrochemical impedance values, and their corrosion inhibition efficiency and durability are insufficient to meet increasingly stringent industrial demands. Summary of the Invention

[0005] The present invention aims to provide a crystalline KZnP3O9, its preparation method and application, which, as a corrosion inhibitor, has higher electrochemical impedance, better stability and longer life, thereby improving the protection level of carbon steel in harsh environments.

[0006] To solve the above technical problems, the specific solution adopted in this invention is: a crystalline KZnP3O9, the basic building block of which is (PO3). ∞ A chain, formed by the connection of PO4 tetrahedra; the (PO3) ∞ The chain extends in three-dimensional space by sharing oxygen atoms, forming a long-range ordered chain based on metaphosphate (PO3). - A three-dimensional mesh structure.

[0007] Preferably, its crystal structure belongs to the hexagonal crystal system, and its space group is P. c2 (188).

[0008] Preferably, its cell parameters are: a = [6.6262] Å, b = [6.6262] Å, c = [9.7700] Å, α = β = 90°, γ = 120°.

[0009] A method for preparing crystalline KZnP3O9 includes the following steps: S1. Weigh the raw materials according to the molar ratio of NH4H2PO4, K2CO3 and ZnO of (5.5~6.5):1:(1.8~2.2), mix and grind them evenly; S2. The mixture is subjected to a high-temperature solid-phase reaction in an oxygen-containing atmosphere, the reaction comprising first heating the material to 285 ~ 315 ℃ and holding it at that temperature for 8 ~ 15 hours, and then heating it to 485 ~ 515 ℃ and holding it at that temperature for 20 ~ 40 hours; S3. The material after the reaction in step S2 is cooled from 485 to 515°C to 385 to 415°C at a rate of 0.01 to 0.5°C / min.

[0010] Preferably, the heating rate to 285~315℃ is 0.5~2℃ / min; the heating rate to 485~515℃ is 0.5~2℃ / min; and the cooling rate in step S3 is 0.03~0.1℃ / min.

[0011] Preferably, the temperature is heated to 300°C at a rate of 1°C / min and held for 12 hours; then heated to 500°C at a rate of 1°C / min and held for 30 hours; subsequently cooled from 500°C to 400°C at a rate of 0.05°C / min.

[0012] Preferably, after step S3, step S4 is further included: cooling the material from 385 ~ 415 °C to room temperature.

[0013] Application of crystalline KZnP3O9 as a corrosion inhibitor in the preparation of metal corrosion-resistant compositions.

[0014] Preferably, the metal is carbon steel, and the corrosion inhibitor is used to inhibit the corrosion of the carbon steel in an acidic aqueous solution environment.

[0015] Preferably, the acidic aqueous solution is a hydrochloric acid solution with a concentration of 0.5 mol / L to 2 mol / L; the concentration of the crystalline KZnP3O9 corrosion inhibitor added to the solution is 50 ppm to 200 ppm.

[0016] Compared with the prior art, the technical solution provided by the present invention has the following significant advantages: For the first time, high-quality crystalline KZnP3O9 was successfully synthesized, and its unique crystal structure was revealed. This invention overcomes the challenge of the easy glass transition of this phosphate system. Through optimized high-temperature solid-state reaction and ultra-slow programmed cooling process, stoichiometrically accurate and structurally well-defined KZnP3O9 crystals were obtained for the first time. Its structure is based on (PO3). ∞ The chain is the basic unit, connected by oxygen bridges to form long-range ordered, continuous and dense metaphosphate (PO3) groups. - The three-dimensional mesh structure provides a structural foundation for understanding its performance.

[0017] This invention provides the best-performing inorganic phosphate corrosion inhibitor to date: For the first time, crystalline KZnP3O9 is used as a corrosion inhibitor. Experiments have shown that, at a concentration of 100 ppm, its corrosion protection performance for Q235 carbon steel in 1 M hydrochloric acid solution far surpasses that of all known inorganic phosphate systems. Its electrochemical impedance can reach a maximum of 525 Ω·cm. 2 The corrosion inhibition efficiency reached as high as 95%, which is the highest value reported to date. This breaks the traditional understanding that "glassy phosphates are superior corrosion inhibitors".

[0018] The mechanism behind the superior performance of crystalline corrosion inhibitors was elucidated: comparative studies revealed that crystalline KZnP3O9 can induce the formation of a more uniform, dense, and less rough passivation film on the surface of carbon steel. Molecular dynamics simulations confirmed that the metaphosphate ions (PO3) in its structure... - It exhibits the strongest adsorption energy on iron substrates, thereby achieving long-lasting and efficient corrosion protection by forming a stable three-dimensional iron-metaphosphate network structure. Compared to its glassy state (which mainly forms isolated or short-chain phosphate groups, resulting in a loose and cracked film), the crystalline structure is the fundamental reason for its high performance. Attached Figure Description

[0019] Figure 1 X-ray diffraction patterns of KZnP3O9 in crystalline and glassy states. Figure 2 Electrochemical tests and Raman spectroscopy results of 100 ppm concentration of KZnP3O9 glassy and crystalline corrosion inhibitors on the corrosion protection of carbon steel Q235 in 1 mol / L hydrochloric acid; Figure 3 Scanning images of the corrosion protection effect of 100 ppm KZnP3O9 glassy (53h) and crystalline (65h) corrosion inhibitors on carbon steel Q235 in 1 mol / L hydrochloric acid. (a) Scanning images of pure Q235 surface at different scales. (b) Scanning images of KZnP3O9 crystalline surface at different scales. (c) Scanning images of KZnP3O9 glassy surface at different scales; Figure 4(a) Maximum electrochemical impedance of different inorganic phosphate corrosion inhibitors and KZnP3O9 in crystalline and glassy states; (b) Maximum corrosion inhibition efficiency of different inorganic phosphate corrosion inhibitors and KZnP3O9 in crystalline and glassy states; (c) Changes in the atomic ratios of Fe, P, and O in the crystalline and glassy states of Q235 and KZnP3O9; (d) Changes in surface roughness Sa and line roughness Ra in the crystalline and glassy states of Q235 and KZnP3O9; (e) Evolution of adsorption energies of Fe on the substrate with iron ions and different phosphate groups over time; (f) Pair distribution functions of Fe on the substrate with different types of phosphate groups and Fe ions. Figure 5 Atomic schematic diagrams are provided to illustrate the atomic and ion distribution on the KZnP3O9 crystal (a) and glass (b) corrosion inhibitor substrate Q235, passivation film, and solvent surface. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the crystalline KZnP3O9, its preparation method, and corrosion-inhibiting applications provided by this invention will be described in detail below with reference to the accompanying drawings and specific experimental data. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of this invention.

[0021] I. Preparation and Structural Confirmation of Crystalline KZnP3O9 This invention first solves the technical problem that the KZnP3O9 system is difficult to crystallize due to its extremely high glass-forming ability, and successfully prepares high-quality crystalline materials through an original synthesis process.

[0022] Example 1: Preparation of crystalline KZnP3O9 polycrystalline powder This embodiment provides a preferred method for preparing crystalline KZnP3O9, and the specific steps are as follows: Raw material preparation: High-purity raw materials were selected, including NH4H2PO4 (99.99%), K2CO3 (99.98%), and ZnO (99.99%). NH4H2PO4, K2CO3, and ZnO were accurately weighed according to the stoichiometric ratio of the target product KZnP3O9, with a molar ratio of 6:1:2. This ratio is crucial for obtaining a pure-phase product.

[0023] Mixing and grinding: Place the weighed raw materials in an agate mortar and grind and mix thoroughly for more than 30 minutes until a uniform and fine mixed powder is obtained.

[0024] High-temperature solid-state reaction: The mixed powder was transferred to a clean graphite crucible and placed in a temperature-controlled muffle furnace for reaction under an air atmosphere. A staged heating and ultra-long holding period strategy was employed to promote crystallization. The first stage (low-temperature decomposition and pretreatment): heating from room temperature to 300 °C at a rate of 1 °C / min, and holding at this temperature for 12 hours. This stage aims to fully decompose NH4H2PO3, remove NH3 and H2O, avoid violent volatilization at high temperatures, and initiate the initial solid-phase reaction.

[0025] The second stage (high-temperature synthesis and crystallization): The temperature was then increased to 500 °C at a rate of 1 °C / min, and held at this temperature for an extended period of 30 hours. This stage ensures sufficient reaction between the carbonate and the oxide / phosphate intermediates to complete the formation of the KZnP3O9 crystalline phase.

[0026] Key slow cooling process: After the reaction is complete, the core step of this invention—ultra-slow programmed cooling—is implemented. The muffle furnace is slowly cooled from 500 °C to 400 °C at an extremely slow rate of 0.05 °C / min. The core purpose of this "slow cooling" process is to maximize the suppression of the glass transition tendency of the melt or supercooled liquid, providing sufficient kinetic conditions for crystal nucleus growth, thereby obtaining a crystalline product with high crystallinity and a single phase. After cooling to 400 °C, the power is turned off, and the sample is allowed to cool naturally to room temperature with the furnace, yielding white blocky crystalline KZnP3O9, which is then ground and ready for use.

[0027] Example 2: Preparation of glassy KZnP3O9 (as a performance comparison sample) For comparative studies, the crystalline KZnP3O9 polycrystalline sample obtained in Example 1 was used as a precursor. It was placed in a crucible and heated to above its melting point (about 650°C) and held for 1 hour to completely melt it. The molten liquid was then quickly poured onto a metal plate and quenched to room temperature to obtain a glassy KZnP3O9 sample.

[0028] Structural characterization results: Powder X-ray diffraction analysis: The product of Example 1 was tested, and its diffraction pattern (see patent drawings) showed that all diffraction peaks were sharp and perfectly matched the simulated pattern, confirming the successful synthesis of pure-phase, well-crystallized crystalline KZnP3O9. The sample pattern of Example 2 showed only one broadened "bun peak", confirming its amorphous characteristics.

[0029] Crystal Structure Analysis: Millimeter-scale single crystals of KZnP3O9 were grown using a flux method, and their precise structure was determined by single-crystal X-ray diffraction. The results show that the KZnP3O9 crystal belongs to the hexagonal crystal system with space group P. c2 (188). Its most distinctive feature is its structure, which is formed by the connection of PO4 tetrahedra (PO3). ∞Chains, serving as basic structural units, extend infinitely in three-dimensional space by sharing oxygen atoms, constructing a long-range ordered, continuous, and dense three-dimensional network framework of metaphosphate (PO3⁻). This unique structure is fundamental to understanding its subsequent properties.

[0030] Raman spectroscopy analysis: Raman spectroscopy (see patent drawings) provides local structural information. Crystalline KZnP3O9 at 649 cm⁻ 1 The presence of significant characteristic peaks at these locations, attributed to the symmetric stretching vibrations of the POP bridging bonds, provides direct evidence for its three-dimensional metaphosphate network. At >1000 cm⁻ 1 In the high-frequency region (corresponding to terminal groups such as PO2), the crystalline sample showed no obvious vibrational peaks, indicating a highly polymerized structure and a lack of isolated phosphate units. In contrast, the glassy sample showed vibrational peaks at 1182 and 1224 cm⁻¹. 1 The presence of obvious peaks indicates that it contains a large number of short-chain structures such as orthophosphate or pyrophosphate, and its structural order is much lower than that of the crystalline state.

[0031] II. Performance Verification of Crystalline KZnP3O9 as a Corrosion Inhibitor KZnP3O9 powders from Examples 1 (crystalline) and 2 (glassy) were added to a 1 mol / L hydrochloric acid solution at a concentration of 100 ppm. The anti-corrosion performance of the system was tested using Q235 carbon steel as the research object.

[0032] 1. Electrochemical performance testing: Electrochemical impedance spectroscopy (EIS) testing: Figure 2 A and Figure 2 B shows the impedance spectra after immersion for different times. The capacitive arc radius of crystalline KZnP3O9 increases significantly with time, indicating that the protective film continuously thickens and strengthens. Through fitting, its charge transfer resistance reaches a peak of approximately 525 Ω·cm at 45 hours. 2 It remained stable for 69 hours, demonstrating excellent durability. In contrast, the impedance of the glassy KZnP3O9 increased slowly and remained at a consistently low level (maximum approximately 83 Ω·cm). 2 ).

[0033] Comprehensive performance comparison: The crystalline KZnP3O9 of this invention is compared with various inorganic phosphate corrosion inhibitors in the literature (see Table 1 and...). Figure 4 a, 4b), its 525 Ω·cm 2 Its impedance value and corrosion inhibition efficiency of 95% are both record high, establishing its top performance position in this type of material.

[0034] 2. Corrosion morphology and surface film analysis: Scanning electron microscope observation: Figure 3The surface of pure Q235 carbon steel is relatively smooth. Figure 3 a). After adding the crystalline corrosion inhibitor, the surface is covered with a relatively uniform layer of deposit, which appears as dense granules under high magnification. Figure 3 b). Surfaces with added glassy corrosion inhibitors showed obvious cracks and streaks (b). Figure 3 c) Poor membrane integrity.

[0035] III. In-depth explanation of the mechanism of efficient corrosion inhibition 2. Mechanistic Explanation Model: Based on all the above experimental and simulation data, a corrosion inhibition mechanism model for crystalline KZnP3O9 is proposed. Figure 5 ): Strong anchoring: The high adsorption energy metaphosphate (PO3⁻) released by crystalline KZnP3O9 in acidic media can be rapidly and firmly chemically adsorbed onto the active sites on the carbon steel surface.

[0036] In-situ construction of a three-dimensional network protective film: Metaphosphate ions adsorbed on the surface coordinate with Fe ions dissolved from the matrix. Because they originate from a three-dimensional network crystal, these metaphosphate ions tend to extend and cross-link in situ at the interface, forming a dense and continuous three-dimensional iron-metaphosphate network film similar to a crystal structure. This film effectively blocks corrosive media, resulting in high impedance.

[0037] The fundamental difference from the glassy state: After dissolving, glassy KZnP3O9 mainly produces orthophosphate / pyrophosphate ions with low adsorption energy. These ions form isolated, non-crosslinked phosphate particles with Fe ions, which accumulate into a loose, porous, and cracked film (corresponding to high surface roughness and cracks), resulting in poor protective performance.

[0038] IV. Conclusion This invention, through an innovative method of "high-temperature solid-state reaction combined with ultra-slow programmed cooling," has for the first time successfully synthesized a unique (PO3) compound. ∞ Crystalline KZnP3O9 with a three-dimensional network structure. Systematic and rigorous experiments have demonstrated the material's corrosion inhibition performance on carbon steel in acidic environments (resistance 525 Ω·cm). 2 With an efficiency of 95%, it far surpasses its glassy state and all known inorganic phosphate systems. Its superior performance stems from the high adsorption energy of metaphosphate ions imparted by its crystal structure and its ability to form a dense three-dimensional network protective film in situ on the metal surface. This invention not only provides a novel corrosion inhibitor product with top-performance characteristics and its reliable preparation method, but also offers entirely new principles and solutions for designing next-generation high-performance anti-corrosion materials through a complete "structure-preparation-performance-mechanism" chain.

Claims

1. A crystalline KZnP3O9, characterized in that: Its basic building block of crystal structure is (PO3). ∞ A chain, formed by the connection of PO4 tetrahedra; the (PO3) ∞ The chain extends in three-dimensional space by sharing oxygen atoms, forming a long-range ordered chain based on metaphosphate (PO3). - A three-dimensional mesh structure.

2. The crystalline KZnP3O9 as described in claim 1, characterized in that: Its crystal structure belongs to the hexagonal crystal system, and its space group is P. c2 (188).

3. The crystalline KZnP3O9 as described in claim 2, characterized in that: Its unit cell parameters are: a = [6.6262] Å, b = [6.6262] Å, c = [9.7700] Å, α = β = 90° γ = 120°.

4. A method for preparing crystalline KZnP3O9, characterized in that: Includes the following steps: S1. Weigh the raw materials according to the molar ratio of NH4H2PO4, K2CO3 and ZnO of (5.5~6.5):1:(1.8~2.2), mix and grind them evenly; S2. The mixture is subjected to a high-temperature solid-phase reaction in an oxygen-containing atmosphere, the reaction comprising first heating the material to 285~315 ℃ and holding it at that temperature for 8~15 hours, and then heating it to 485~515 ℃ and holding it at that temperature for 20~40 hours; S3. The material after the reaction in step S2 is cooled from 485 to 515 °C to 385 to 415 °C at a rate of 0.01 to 0.5 °C / min.

5. The method for preparing crystalline KZnP3O9 as described in claim 4, characterized in that: The heating rate to 285~315℃ is 0.5~2℃ / min; the heating rate to 485~515℃ is 0.5~2℃ / min; the cooling rate in step S3 is 0.03~0.1℃ / min.

6. The method for preparing crystalline KZnP3O9 as described in claim 5, characterized in that: Heat to 300 °C at a rate of 1 °C / min and hold for 12 hours; then heat to 500 °C at a rate of 1 °C / min and hold for 30 hours; subsequently cool from 500 °C to 400 °C at a rate of 0.05 °C / min.

7. The method for preparing crystalline KZnP3O9 as described in claim 4, characterized in that: After step S3, step S4 is also included: cooling the material from 385 ~ 415 ℃ to room temperature.

8. The application of the crystalline KZnP3O9 as a corrosion inhibitor in the preparation of metal corrosion-resistant compositions as described in claim 1.

9. The application of crystalline KZnP3O9 as a corrosion inhibitor as described in claim 8 in the preparation of metal corrosion-resistant compositions, characterized in that: The metal is carbon steel, and the corrosion inhibitor is used to inhibit the corrosion of the carbon steel in an acidic aqueous solution environment.

10. The application of crystalline KZnP3O9 as a corrosion inhibitor as described in claim 9 in the preparation of metal corrosion-resistant compositions, characterized in that: The acidic aqueous solution is a hydrochloric acid solution with a concentration of 0.5 mol / L to 2 mol / L; the concentration of the crystalline KZnP3O9 corrosion inhibitor added to the solution is 50 ppm to 200 ppm.

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