Institutional Repository of Key Laboratory of Marine Environmental Corrosion and Bio-fouling, IOCAS
|Place of Conferral||中国科学院海洋研究所|
|Keyword||水滑石 缓蚀剂 涂层 自修复 电化学|
近年来，随着新材料新技术的发展，腐蚀防护技术逐渐趋向于功能复合化的方向发展，而微纳米材料在其中扮演了非常重要的角色。水滑石（Layered double hydroxides，LDH）作为二维材料家族的一员，集众多优点于一身，特别是灵活的组成和固有的阴离子交换性能，使其在高氯的海洋环境下具有先天的防护优势，因此能够在众多的微纳米材料中脱颖而出。但是在腐蚀防护领域，人们对LDH的合成以及结构特征研究甚少，特别是与无机或有机缓蚀剂的结合，传统的制备方法不仅耗时耗力，而且缓释剂的插层效率低下，极大的限制了其缓蚀性能。此外，LDH作为粘土类材料，其与有机涂层基体之间存在不相容性，导致在涂层中的分散性较差。因此，本论文围绕LDH的合成、缓蚀剂的负载以及其与有机涂层基体之间的界面相互作用三个方面，结合先进和全面的微观表征技术，开展深入系统的研究，致力于构建一套集绿色、高效、功能化于一体的缓蚀−涂层防护体系。具体的研究内容如下：
1.在LDH的可控合成与生长机制方面，结合LDH的合成反应机理，探讨了反应气氛、生长温度、时间、pH以及层间阴离子对LDH的形貌、结构和组成的影响。结果表明，CO2对LDH的层间阴离子具有很大的影响，溶液和空气中的CO2都可以参与LDH的合成反应生成CO32− 并插层到LDH层间，且其与LDH层板的结合优先级明显的高于NO3−。生长温度和时间主要影响LDH的晶粒尺寸，特别是增加温度（65 ℃ ~ 110 ℃）可以使LDH的尺寸成数倍增长，即从几百纳米增加到几微米。pH（OH− 浓度）对LDH的形成具有非常重要的作用，在适当的pH（10 ± 0.5），可以获得典型的片状的LDH；而pH过低或过高时，产物主要为不规则的金属氧化物，并呈现出明显的聚集和堆叠。此外，对于小的无机阴离子缓蚀剂如NO2− 和MoO42−，可以直接通过共沉淀法合成，并且获得的产物均为单相结构。
2.在提高缓蚀剂的负载率方面，利用LDH的记忆效应，创新性的通过剥离−重构法合成了高负载量有机缓蚀剂（5−甲基−2−巯基−1,3,4−噻二唑，MTT）的LDH。结果表明，重构后的产物不仅能够恢复LDH典型的层状结构，而且还可以获得纯净的MTT− 插层的LDH。而传统的阴离子交换法并不能实现完全的交换取代，获得的LDH−MTT− 为多相结构。此外，通过剥离-重构制备的LDH，其缓蚀剂的负载量相比于离子交换法提高了约3倍，并且其对Cl− 还具有快速的响应释放性能。电化学结果表明，剥离−重构法获得的LDH−MTT− 在3.5 wt.% NaCl溶液中对碳钢具有优异的防腐蚀性能，最大缓蚀效率约为94%，相比于离子交换法制备的LDH−MTT− 提高了30%以上。
首先，创新性的引入石墨纤维（GF）来提高LDH与涂层基体的相互作用：通过一步水热法在氧化石墨纤维（OGF）表面原位生长LDH纳米片阵列，然后将其添加到有机涂层聚乙烯醇缩丁醛（PVB）中制备高分散性复合涂层。结果表明，LDH在涂层基体中存在明显的团聚现象，会引入较多的缺陷；而OGF/LDH在PVB基体中具有较好的分散性，呈单根交错分布。腐蚀实验表明LDH的加入虽然可以提高PVB涂层的屏障性能，但由于缺陷的存在，更易发生点蚀；相反，OGF/LDH的加入能够显著的提高PVB涂层的防腐蚀性能，在浸泡30天后，阻抗值变化较小，依然维持在较高的水平（108 ~ 109 Ω cm2）。
最后，在前面研究的基础上，制备了集优异的屏障和自修复性能于一体的LDH基功能化仿生复合涂层：首先通过共沉淀法直接合成了插层无机缓蚀剂（钼酸根）的LDH，并成功的对其进行了表面硅烷化改性，然后结合LB和旋转涂膜法制备了类珍珠层结构的PVB/LDH复合涂层。结果表明，该复合涂层（含有3.4 wt.% 的LDH纳米片）具有十分优异的屏障性能，在浸泡50天后，其阻抗值仍然维持在较高的水平（1010 ~ 1011 Ω cm2），相比于纯PVB涂层提高了5个数量级。此外，当腐蚀介质（Cl−）渗透进涂层到达碳钢表面时，涂层中的LDH能够响应的释放缓蚀剂MoO42−，并参与到碳钢的腐蚀反应中，生成致密的钝化层（Fe2(MoO4)3和FeMoO4）覆盖在暴露的碳钢表面，进而有效的抑制腐蚀介质的进一步侵蚀，即表现出较好的自修复性能。
In recent years, the corrosion protection technology with multifunctionality has garnered considerable attention with the emerging of new materials and technology, in which the micro/nano materials have play a very important role. Layered double hydroxides (LDH) is a class of 2D intercalation anionic clay, combines numerous advantages, especially the intrinsic anion exchange capacity, which endow it with natural advantage for corrosion protection in the marine environment that rich in chloride. Therefor, LDH has became an ideal nanomaterial for smart anticorrosion technology. However, in the field of corrosion protection, there is little research on the synthesis and structural characteristics of LDH, particularly in combination with inorganic or organic corrosion inhibitors. The traditional anion exchange method is time-consuming and labor-intensive, and the intercalation of corrosion inhibitors in LDH is also inefficient. In addition, the intrinsic incompatibility between nanoclay and organic matrix leads to poor dispersion and weak interfacial interactions. Herein, this paper focuses on the synthesis of LDH, the loading of corrosion inhibitor and the interface interaction between LDH and organic coating matrix, combined with advanced and comprehensive characterization technology, to conducts in-depth and systematic research. The aim is to build a high performances of inhibition-coating protection system with green, efficient and functional properties. The main contributions are as follows:
1. In terms of the controllable synthesis and growth mechanism of LDH, the effects of reaction atmosphere, growth temperature and time, pH as well as the interlayer anions on the morphology, structure and composition of LDH were discussed according to the reaction mechanism of LDH. The results show that CO2 has a great influence on the interlayer anions of LDH. CO2 from solution and air can able to participates in the formation of LDH through generating CO32− and intercalated into the interlayer of LDH. Besides, the binding priority with LDH of CO32− is significantly higher than that of NO3−. The growth temperature and time mainly affect the grain size of LDH. The size of LDH would increase several times (from several hundred nanometers to several microns) when increase of temperature (65 ℃ ~ 110 ℃). pH (the concentration of OH−) plays a very important role in the formation of LDH. In a certain pH range (10 ± 0.5), a typical lamellar LDH can be obtained. However, the products are mainly composed of irregular metal oxides and exhibit obvious aggregation and stacking when the pH is too low or too high. In addition, the small inorganic anionic inhibitors such as NO2− and MoO42− can be directly intercalated into the interlayer of LDH via the coprecipitation method, and the obtained products are all single-phase structure.
2. In order to improve the loading capacity of LDH to organic corrosion inhibitor, a new method combining delamination and reconstruction was applied to obtain the LDH with high loading of the organic corrosion inhibitor − 5-Methyl-1,3,4-thiadiazole-2-thiol (MTT) according to the memory effect of LDH. The results show that the reconstructed LDH exhibits typical layered structure, and the interlayer anions is composed of pure MTT−. However, the traditional anion exchange method can not achieve complete exchange substitution, and the obtained LDH is polyphase structure. Besides, the corrosion inhibitor loading capacity of the reconstructed LDH improved is increased by about 3 times compared with the product prepared by anion-exchange, and the LDH also has a rapid Cl−–responsive release performance. The electrochemical results demonstrate that the LDH−MTT− obtained by the new strategy shows excellent corrosion inhibition for mild steel in 3.5 wt% NaCl solution. And the maximum corrosion inhibition efficiency is about 94%, which is more than 30% higher than that of LDH−MTT− prepared by ion exchange method.
3. In order to improve the interaction and dispersion between LDH and coating matrix, three efficient methods were proposed:
Firstly, graphite fiber (GF) was used for the first time to improve the interaction between LDH and coating matrix: the highly ordered layered double hydroxides (LDHs) nanosheet arrays was in-situ grown on oxidized graphite fiber (OGF) via a facile one-step hydrothermal approach, and then it was incorporated into the polyvinyl butyral (PVB) to prepare highly dispersed nanocomposite coating. The results show that the LDH filled PVB coating presented low-dispersed and defective features due to the agglomeration of LDH, while the OGF/LDH were distributed uniformly within PVB coating, and present interlaced distribution features. The corrosion experiments indicate that although the addition of LDH in PVB coating can improve the barrier performance, it is more prone to pitting corrosion due to the presence of defects; on the contrary, the PVB/OGF/LDH coating exhibits excellent corrosion resistance, the impedance value changes slightly and remains at a high level (108 ~ 109 Ω cm2) after 30 days’ immersion in 3.5 wt.% NaCl solution.
Secondly, a bioinspired multilayer hybrid film based on LDH monolayer nanosheets (LDH-NS) was fabricated: a large amount of LDH-NS with uniform features were directly synthesized via the bottom-up strategy, and the as-prepared nanosheets was used in a hybrid film with nacre-like structure via one-step coassembly process. The results show that LDH-NS has ultra-thin structure (0.71 nm) and high surface activity, and exhibits good interface interaction (hydrogen bond) with PVB molecules. Besides, the LDH-NS was well aligned and stacked along the substrate surface under flow and gravity induction. The corrosion experiments reveal that the multilayer PVB/LDH-NS composite film can effectively inhibit the diffusion of corrosive species and improve the corrosion resistance.
Finally, on basis of the above research, we have developed a nacre–inspired layered hybrid coating based on LDH nanoplatelets and PVB combining excellent barrier and self−healing performances: LDH intercalated with inorganic inhibitor (molybdate) was directly synthesized by coprecipitation method, and the obtained LDH was modified by silane successfully. Then the PVB/LDH composite coating with pearl-like structure was fabricated by combining LB and spin coating method. The results show that the composite coating (containing 3.4 wt.% LDH nanoplatelets) exhibits excellent barrier performance, the impedance value maintains at a high level (1010 ~ 1011 Ω cm2) after immersion of 50 days, which is 5 orders of magnitude higher than that of pure PVB coating. In addition, when corrosion species (Cl−) penetrate into coating and reach to the surface of mild steel, the intercalated inhibitor MoO42– will be released responsively from the incorporated LDH and react with corrosion products of mild steel to form a dense passivation layer (Fe2(MoO4)3 and FeMoO4) to cover the exposed surface, which could effectively prevent the aggressive medium from further erosion. That is to say, the coating has good self-healing performance.
|MOST Discipline Catalogue||理学::海洋科学|
|Table of Contents|
第1章 绪论 1
|刘昂. 水滑石基功能化缓蚀-涂层防护体系构建和机制研究[D]. 中国科学院海洋研究所. 中国科学院大学,2020.|
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