层状双金属氢氧化物 (LDH)，又名水滑石，是由带正电荷的主体层板和层间阴离子通过非共价键的相互作用组装而成的层状结构化合物，其化学组成可以表示为[M 2+1-xMx3+(OH)2] x+(A n-) x/n ·mH2O，其中M 2+和M 3+分别代表二价和三价金属离子 (一价的金属阳离子Li+也可以与Al3+形成LDH)[1]，A n-为层间阴离子。M 2+离子有Mg2+、Zn2+、Ni2+、Cu2+、Mn2+等，M 3+离子有Al3+、Cr3+、Fe3+、In3+等，层间阴离子有CO32-、NO3-、Cl-等。LDH具有层板化学组分可调控以及层间阴离子易交换等特点，可以作为催化剂、药物以及缓蚀剂的载体，因此多年来一直是大家的研究热点之一。在金属腐蚀与防护领域，由于LDH具有优异的屏蔽性能和离子交换性能，因此非常适合于在镁合金表面制备具有物理隔绝和自愈性能的LDH涂层。本文主要从镁合金表面缓蚀剂插层LDH涂层的制备和自愈性能评价两个方面近几年来的研究进展进行综述，并针对目前研究和应用中存在的问题进行了展望。
1 LDH在镁合金腐蚀防护方面的应用
1.1 缓蚀剂插层的LDH
LDH涂层对镁合金基体的保护机制一般认为是LDH层间的缓蚀剂离子与进入到涂层内部的氯离子发生了离子交换，降低了侵蚀性Cl-的浓度，同时释放出的缓蚀剂离子对基体也有保护作用，这两方面的综合作用使得LDH涂层对基体产生了很好的保护效果。另外，镁合金表面上形成的LDH形态一般呈现刀片状，有较多的孔洞；使用天冬氨酸、植酸等缓蚀剂插层后LDH的形态会发生变化，为花状或玫瑰花状，孔洞数目减少，因此对基体的保护作用加强。此外，制备LDH时的水热时间对形态也有影响。Chen等[22]在AZ31镁合金表面制备天冬氨酸插层的MgAl-LDH时，水热时间12 h时形似玫瑰花的LDH平铺在合金表面，耐蚀效果最好；15 h时，这种形态的LDH消失，耐蚀效果下降。
1.2 LDH的封闭处理
单一LDH涂层对基体的保护作用有限，因此许多学者使用低表面能物质对LDH 涂层进行修饰或使用SiO2等对LDH进行封闭以进一步提高对镁合金基体的保护效果，Chen等[18]采用植酸对AZ31镁合金表面上Mg-Al LDH处理后，能得到防蚀效果更好的涂层；Zhou等[23]在AZ91D镁合金表面首先原位制备了一层纳-微结构膜 (纳米棒状ZnO/微米LDH)，用硬脂酸封闭后，接触角高达165.6o，在3.5% (质量分数) NaCl溶液中腐蚀电流密度为2.6×10-5 A/cm2；Zhang等[24]在AZ31镁合金表面制备了用硬脂酸封闭的Mg(OH)2/Mg-Al LDH膜，接触角为153.5o，腐蚀电流密度低至3.4×10-10 A/cm2；Zhang等[25]用硬脂酸对Mg-Li-Ca合金表面的微弧氧化膜进行封闭，接触角为146.5o，腐蚀速率从MAO处理的4.23×10-5 A/cm2降到5.36×10-8 A/cm2。Ba等[26]在AZ91D镁合金基体上制备了Mg-Al LDH，然后在不同浓度的硝酸铈溶液中浸泡，同时施加交流电促进Ce离子的迁移，耐蚀性提高了10倍左右；Zeng等[27]使用聚乳酸对Zn-Al LDH进行封闭处理，腐蚀电流密度从6.78×10-8 A/cm2降到了1.20×10-8 A/cm2；Peng等[28]在AZ31镁合金微弧氧化膜层使用Mg-Al LDH进行了封闭处理，腐蚀速率下降了一个数量级；Zhang等[29]对AZ31镁合金实施了4步表面处理，如先微弧氧化，再在硝酸铈+双氧水溶液中浸泡形成化学转化膜，然后利用水热法制备Mg-Al LDH，最后在植酸溶液中浸泡，经过这一系列处理后，镁合金在3.5%NaCl溶液中的腐蚀电流密度降到5×10-8 A/cm2；后来简化了步骤[30]，先对AZ31镁合金阳极氧化，然后采用两步法在AZ31镁合金表面制备了掺杂钒酸根离子的Mg-Al-Ce LDH膜层，在3.5%NaCl溶液中浸泡1和14 d后的腐蚀电流密度分别为2.2×10-7和6.7×10-7 A/cm2，表现出长时间的保护效果。Hao等[31]通过在LDH沉积SiO2溶胶，提高了复合膜的阻隔性能；Zhang等[32]和Yan等[33]在LDH上使用石墨烯进行封闭处理，也提高了耐蚀性。
1.3 含Ce的LDH
稀土元素Ce对镁/铝合金的腐蚀具有较好的抑制效果，因此有学者在镁合金表面上制备了一层由Mg2+或Zn2+、Al3+和掺杂Ce的三元LDH涂层，Wu等[30]在AZ31镁合金表面制备了Mg-Al-Ce LDH涂层，在3.5%NaCl溶液中，腐蚀电流密度为3.6×10-7 A/cm2；Zhang等[34]在铝合金表面原位生长出了Zn-Al-Ce LDHs，耐蚀效果也较好。本课题组使用Mg2+与Al3+、Ce3+合成了三元LDHs，腐蚀电流密度为5.12×10-8 A/cm2，防护效果显著好于前人工作[35]；在此基础上，又在镁合金表面制备了Zn-Ce LDH涂层，表明在pH≈11及Zn/Ce摩尔比等于3时，能够形成一层致密、附着力好的涂层，在3.5%NaCl溶液中腐蚀电流密度为3.3×10-8 A/cm2，这个电流密度是目前除了水蒸气制备法外已报道的最低的腐蚀电流密度之一[36]。如能将缓蚀剂进行插层，有可能进一步提高其保护效果。
2 LDH涂层自愈性能的评价
缓蚀剂插层的LDH涂层备受关注的另一个重要原因是其具有自愈效果。当涂层有缺陷或者遭到划伤破坏后，腐蚀溶液中的Cl-等侵蚀性离子进入LDH涂层内，与LDH层间的缓蚀性阴离子发生交换作用，层间阴离子释放出来，起到自愈效果。自愈机理根据层间插入的离子的不同大概可以分为以下两种：(1) 钝化作用：如LDH层间的六价铬酸盐离子，被交换出来后，在缺陷处形成一层Cr(OH)3/Cr2O3钝化膜，从而起到修复作用[37]。钒酸盐的作用与此类似。由于铬酸盐和钒酸盐有毒，因此有学者使用毒性较低的三价铬[38]；(2) 螯合作用：层间插入的钼酸盐[39]、磷酸盐[40]、四苯基卟啉[41]、8-羟基喹啉等可以和Mg2+螯合，生成沉淀物，封闭腐蚀通道，起到自愈功能；然而LDH中若多种缓蚀剂同时存在时的自愈机理还未被深入研究。
评价镁合金表面防护涂层的自愈性能，主要用到了以下方法：(1) 人为划伤后的涂层，在腐蚀溶液中浸泡一段时间后取出，用光学显微镜观察划痕处的涂层修复情况，结合扫描电镜观察和元素分析，可进行机理的研究。Zhao等[42]使用spin-spray layer-by-layer (SSLBL) 层层组装技术在镁合金表面制备了多层含有CeO2和SiO2的涂层，划伤的涂层在3.5%NaCl溶液中浸泡72 h后，没有明显的破坏；(2) 电化学技术能得到更详细的涂层自愈信息，常规的有动电位极化 (PDP) 和电化学阻抗谱 (EIS)，Zeng等[39]使用动电位扫描研究表明表面有一层LDH-MO42-涂层的镁合金，在阳极极化曲线上出现了多个钝化区，作者认为这是由于该涂层具有自愈能力引起的。Gnedenkov等[43]先对镁合金试样进行阳极极化，然后再用EIS评价涂层的自愈能力。由于PDP和EIS 测量的是整个电极/电解液界面的平均响应信号，不能获得局部电化学信息，因此微区电化学测试技术如扫描振动电极技术 (SVET)、局部电化学阻抗技术 (LEIS) 、扫描电化学显微镜 (SECM)、扫描离子选择电极技术 (SIET)、扫描开尔文探针 (SKP) 等在涂层自愈性能方面也有较多的应用，Zhang等[29]使用SVET研究了多层膜的自愈能力；Jamali等[44]使用SECM证明镁合金表面稀土元素Pr的转化膜有自修复作用；Calado等[45]使用了EIS、SVET和LEIS等多种技术研究了负载纳米CeO2颗粒的硅烷膜涂层对镁合金基体的保护作用，表明CeO2的加入能降低阴极反应活性，且能主动对涂层缺陷进行修复。Ding等[46]采用划伤、SVET技术研究了镁合金表面LDH涂层及超疏水涂层的自愈性能，结果表明，LDH经超疏水处理后，具有更好、更快的自愈能力。
以上这些微区电化学仪器有助于深入了解膜层的自愈机理，但是这些仪器通常较为昂贵，且重现性往往受多种因素的影响，因此如能辅助其他一些能够进行电化学原位监测手段，将更有助于深入了解涂层的自愈过程和机理。
涂层的破坏和自愈过程中，会伴随着开路电位波动等电化学参数的变化，这些信号比较容易获取，而且具有原位、快速和直观等特点，如碳钢和不锈钢的亚稳态点蚀发生和发展过程中，开路电位的变化特点与钝化膜的破坏与修复有着密切关系。我们在最近研究镁合金LDH涂层在3.5%NaCl溶液中腐蚀失效行为时，也观察到类似的现象。除了开路电位外，本课题组[47]还研究表明单频电化学阻抗谱 (EIS) 是原位监测涂层破坏与自愈过程的一种很好的方法。单频电化学阻抗谱是在低频下 (如0.1 Hz或1 Hz) 测试得到的涂层阻抗，它可以代表涂层的防护性能。Anjum等[47]在AZ31镁合金基体上，分别制备了Mg-Al LDH涂层和插入8-羟基喹啉 (8Q) 的Mg-Al LDH涂层，将涂层人为划伤后，在3.5%NaCl溶液中测试了低频阻抗随浸泡时间的变化，Mg-Al LDH涂层浸泡10 h后失去了自修复作用，而且涂层的阻抗也不高；Mg-Al-8HQ LDH涂层在浸泡220 h后，仍有很好的自修复作用。因此，这类无损、原位、实时监测技术将有助于深入了解LDH涂层在腐蚀溶液中的失效与自愈过程。
3 展望
虽然对LDH的研究较多，但还存在LDH的形成机制和对基体的保护机理不甚清楚等问题，具体有以下几个方面：
对镁合金表面LDH的形成机制、保护机理的研究不够深入，对涂层失效过程中的涂层结构的变化了解甚少，各种因素的影响规律也不是很清楚，因此，需要在基础方面进行更多的研究工作。
不同的制备工艺、不同的主体，都可能影响缓蚀剂在LDH层间插层量的多少以及释放速率，这方面的研究报道较少。
镁合金LDH涂层在腐蚀环境中的耐久性一直是其应用中的短板，LDH在腐蚀溶液中长期浸泡后，有可能破坏其原来的层状结构，或者当腐蚀性离子吸附达到饱和后不再吸附，失去了其离子交换性能和防护性能，因此如何提高耐久性应是今后重点的研究内容。
合成LDH时用到的金属盐，多为硝酸盐，因硝酸盐一般为危化品，使用受到限制，而氯盐十分便宜，比如氯化镁，我国盐湖中含有丰富的氯化镁资源，因此今后应研究使用氯盐合成LDH的工艺和效果，以期为LDH大规模应用奠定基础。
总之，缓蚀剂插层的LDH在镁合金防护中有着十分广阔的应用前景。通过今后的深入研究，将有助于提高镁合金腐蚀防护水平以及自愈涂层的开发。
参考文献
Self-healing Li-Al layered double hydroxide conversion coating modified with aspartic acid for 6N01 Al alloy
[J].
Biomedical coatings on magnesium alloys-A review
[J].
This review comprehensively covers research carried out in the field of degradable coatings on Mg and Mg alloys for biomedical applications. Several coating methods are discussed, which can be divided, based on the specific processing techniques used, into conversion and deposition coatings. The literature review revealed that in most cases coatings increase the corrosion resistance of Mg and Mg alloys. The critical factors determining coating performance, such as corrosion rate, surface chemistry, adhesion and coating morphology, are identified and discussed. The analysis of the literature showed that many studies have focused on calcium phosphate coatings produced either using conversion or deposition methods which were developed for orthopaedic applications. However, the control of phases and the formation of cracks still appear unsatisfactory. More research and development is needed in the case of biodegradable organic based coatings to generate reproducible and relevant data. In addition to biocompatibility, the mechanical properties of the coatings are also relevant, and the development of appropriate methods to study the corrosion process in detail and in the long term remains an important area of research.Copyright © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
The corrosion properties of phosphate coating on AZ31 magnesium alloy: the effect of sodium dodecyl sulfate (SDS) as an eco-friendly accelerating agent
[J].
Electrochemical deposition and characterization of layered double hydroxide film on magnesium alloys
[J].
镁合金表面层状双氢氧化物的电化学沉积和表征
[J].
Layered double hydroxide coatings on magnesium alloys: A review
[J].Layered double hydroxides (LDHs) as a class of anionic clays have extensive applications due to their unique structures. Nowadays, the emphasis is laid on the development of LDH coatings for corrosion resistance and medical applications. Thus, this review highlights synthetic methods of LDH coatings and LDH-based composite coatings on magnesium alloys. Special attention is focused on self-healing, biocompatible and self-cleaning LDH-based composite coatings on magnesium alloys.
Characterization of Mg, Al-hydrotalcite conversion film on Mg alloy and Cl- and CO3 2- anion-exchangeability of the film in a corrosive environment
[J].
Electrochemical deposition and characterization of Zn-Al layered double hydroxides (LDHs) films on magnesium alloy
[J].
Fabrication and characterization of Mg-M layered double hydroxide films on anodized magnesium alloy AZ31
[J].
Fabrication and characterization of PO4 3- intercalated Zn-Al-layered double hydroxide nanocontainer
[J].
A novel approach to fabricate protective layered double hydroxide films on the surface of anodized Mg-Al alloy
[J].
Synthesis, characterization and anticorrosion performance of molybdate pillared hydrotalcite/in situ created ZnO composite as pigment for Mg-Li alloy protection
[J].
Anticorrosion organic coating with layered double hydroxide loaded with corrosion inhibitor of tungstate
[J].
Duplex coating combining layered double hydroxide and 8-quinolinol layers on Mg alloy for corrosion protection
[J].
Chemical modification of hydrotalcite coating for enhanced corrosion resistance
[J].
In-situ intercalation of 8-hydroxyquinoline in Mg-Al LDH coating to improve the corrosion resistance of AZ31
[J].
Comparison of corrosion resistance of MgAl-LDH and ZnAl-LDH films intercalated with organic anions ASP on AZ31 Mg alloys
[J].
PPA-containing layered double hydroxide (LDH) films for corrosion protection of a magnesium alloy
[J].
Modifications of the hydrotalcite film on AZ31 Mg alloy by phytic acid: the effects on morphology, composition and corrosion resistance
[J].
Enhancement of corrosion resistance of AZ31 Mg alloys by one-step in situ synthesis of ZnAl-LDH films intercalated with organic anions (ASP, La)
[J].
Development of a thiophene derivative modified LDH coating for Mg alloy corrosion protection
[J].
One-pot scalable in situ growth of highly corrosion-resistant MgAl-LDH/MBT composite coating on magnesium alloy under mild conditions
[J].Current corrosion-resistant layered double hydroxide (LDH) coating on Mg alloy is usually in situ grown in autoclave by hydrothermal methods under high temperature and high-pressure conditions, which is unfavorable for industrial application. We report that an inhibitor (2-mercaptobenzothiazole, MBT) incorporated composite (MgAl-LDH/MBT) coating can be in situ deposited on bare AZ31 Mg alloy surface with the assistance of a chelating agent (ethylenediaminetetraacetic acid) under a relatively low temperature (95 °C) and ambient pressure by a one-pot method. The successful formation of LDH/MBT composite coating is confirmed by a series of characterizations, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS). The corrosion resistance of the composite coating is evaluated by means of hydrogen evolution measurement, electrochemical impedance spectroscopy (EIS), Tafel polarization curves, and neutral salt spray test. The tests show that the LDH/MBT composite coating has a very low corrosion current density (1.73 10-8 A cm-2), an extremely high charge transfer resistance (2.336 MΩ cm2), and does not show any corrosion pits even after 15 d of exposure to a NaCl solution or 7 d of exposure to salt fog environment, manifesting the good and robust corrosion protection. Lastly, the deposition and corrosion protection mechanisms of the MgAl-LDH/MBT composite coating are also discussed and proposed based on the EDS characterization of the coating after long-time exposure.
Corrosion resistance of a self-healing rose-like MgAl-LDH coating intercalated with aspartic acid on AZ31 Mg alloy
[J].
Insitu grown superhydrophobic Zn-Al layered double hydroxides films on magnesium alloy to improve corrosion properties
[J].
Corrosion resistance of the superhydrophobic Mg(OH)2/Mg-Al layered double hydroxide coatings on magnesium alloys
[J].
Corrosion resistance of a superhydrophobic surface on micro-arc oxidation coated Mg-Li-Ca alloy
[J].
Cerium-based modification treatment of Mg-Al hydrotalcite film on AZ91D Mg alloy assisted with alternating electric field
[J].
Corrosion resistance of Zn-Al layered double hydroxide/poly (lactic acid) composite coating on magnesium alloy AZ31
[J].
Sealing the pores of PEO coating with Mg-Al layered double hydroxide: enhanced corrosion resistance, cytocompatibility and drug delivery ability
[J].In recent years, magnesium (Mg) alloys show a promising application in clinic as degradable biomaterials. Nevertheless, the poor corrosion resistance of Mg alloys is the main obstacle to their clinical application. Here we successfully seal the pores of plasma electrolytic oxidation (PEO) coating on AZ31 with Mg-Al layered double hydroxide (LDH) via hydrothermal treatment. PEO/LDH composite coating possess a two layer structure, an inner layer made up of PEO coating (similar to 5 mu m) and an outer layer of Mg-Al LDH (similar to 2 mu m). Electrochemical and hydrogen evolution tests suggest preferable corrosion resistance of the PEO/LDH coating. Cytotoxicity, cell adhesion, live/dead staining and proliferation data of rat bone marrow stem cells (rBMSCs) demonstrate that PEO/LDH coating remarkably enhance the cytocompatibility of the substrate, indicating a potential application in orthopedic surgeries. In addition, hemolysis rate (HR) test shows that the HR value of PEO/LDH coating is 1.10 +/- 0.47%, fulfilling the request of clinical application. More importantly, the structure of Mg-Al LDH on the top of PEO coating shows excellent drug delivery ability.
Active corrosion protection by a smart coating based on a MgAl-layered double hydroxide on a cerium-modified plasma electrolytic oxidation coating on Mg alloy AZ31
[J].
Doublely-doped Mg-Al-Ce-V2O7 4- LDH composite film on magnesium alloy AZ31 for anticorrosion
[J].
Fabrication and anticorrosion properties of composite films of silica/layered double hydroxide
[J].
LDHs/graphene film on aluminum alloys for active protection
[J].
One-step in situ synthesis of reduced graphene oxide/Zn-Al layered double hydroxide film for enhanced corrosion protection of magnesium alloys
[J].As an effective and environmentally friendly material for corrosion prevention, layered double hydroxide (LDH) films have usually been degraded due to their inherent microporous structure. In this study, graphene derivatives were employed to enhance the corrosion resistance of LDH films. After ultrasonic treatment of a reaction solution mixture containing graphene oxide (GO) powder, a reduced graphene oxide/zinc-aluminum LDH (RGO/Zn-Al LDH) film was in situ synthesized on a magnesium alloy substrate by a one-step facile hydrothermal crystallization process. The characterization results demonstrated that the LDH nanosheets grew on both the GO surface and the magnesium substrate, and thus the agglomeration of graphene was effectively prevented. Furthermore, the GO plates were simultaneously reduced into RGO, which has better corrosion resistance. The as-prepared samples were individually assessed by potentiodynamic polarization measurements, and the RGO/Zn-Al LDH film showed good corrosion resistance with a lower corrosion current density (0.546 μA/cm) than that of the bare substrate (33.2 μA/cm) and Zn-Al LDH film (4.33 μA/cm). The penetration resistance of the Zn-Al LDH film to a corrosive environment was significantly improved through the organic combination with graphene oxide, and this method provides a simple and facile approach to effectively enhance the corrosion protection performance of LDH materials.
Double-doped LDH films on aluminum alloys for active protection
[J].
The effect of cerium cation on the microstructure and anti-corrosion performance of LDH conversion coatings on AZ31 magnesium alloy
[J].
Influence of pH value and Zn/Ce cations ratio on the microstructures and corrosion resistance of LDH coating on AZ31
[J].
Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3
[J].
A smart superhydrophobic coating on AZ31B magnesium alloy with self-healing effect
[J].
Corrosion of molybdate intercalated hydrotalcite coating on AZ31 Mg alloy
[J].
A pH-sensitive self-healing coating for biodegradable magnesium implants
[J].Self-healing coatings have attracted attention on surface modification of magnesium alloys, as it can recover the barrier ability of the coatings from corrosion attack. Nevertheless, previous works on this aspect are not suitable for biomedical magnesium alloys owing to the lack of biocompatibility. In this study, we fabricated a self-healing coating on biomedical Mg-1Ca alloy by compositing silk fibroin and KPO. PO ions act as corrosion inhibitor, while K ions help to regulate the secondary structures of silk fibroin. The scratch test, scanning vibrating electrode technique (SVET), and electrochemical impedance spectroscopy (EIS) provide comprehensive results, confirming the pH-sensitive self-healing capacity of the composite coating. Moreover, cells' (MC3T3-E1) multiple responses including spreading, adhesion, proliferation, and differentiation illustrate the preferable biocompatibility as well as the osteogenic activity of the coating. These primary findings might open new opportunities in the exploration of self-healing coatings on biomedical magnesium alloys. STATEMENT OF SIGNIFICANCE: Biomedical magnesium alloys surface modifications have been studied for years, which however the biomedical self-healing coatings were rarely involved. In this work, silk fibroin and phosphate (KPO) were composited to fabricate coating on biomedical magnesium alloys. The coating not only owned the self-healing ability with pH sensitivity, but also endowed the substrate preferable corrosion resistance as well as osteogenic activity. This work gives a new insight into surface modification for biomedical Mg alloys.Copyright © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Research on the inhibition mechanism of tetraphenylporphyrin on AZ91D magnesium alloy
[J].
Corrosion resistance of a self-healing multilayer film based on SiO2 and CeO2 nanoparticles layer-by-layer assembly on Mg alloys
[J].
Protective properties of inhibitor-containing composite coatings on a Mg alloy
[J].
Self-healing characteristic of praseodymium conversion coating on AZNd Mg alloy studied by scanning electrochemical microscopy
[J].
Self-healing ceria-modified coating for corrosion protection of AZ31 magnesium alloy
[J].
Superhydrophobic composite coating with active corrosion resistance for AZ31B magnesium alloy protection
[J].
Influence of the 8-quinolinol concentration and solution pH on the interfacial properties of self-healing hydrotalcite coating applied to AZ31 magnesium alloy
[J].
