【作者】 钟显康；

【导师】 李青；

【作者基本信息】 西南大学 ， 无机化学， 2010， 硕士

【摘要】 镁合金具有密度低,比强度、比弹性模量高,导热性能良好且易于回收等特点,还具有很好的铸造性能和切削加工性能,被誉为"21世纪绿色工程材料”。然而,镁合金的化学性质活泼,在使用环境中容易发生严重的腐蚀,大大限制了镁合金的实际应用。因此镁合金的表面防护技术研究具有十分重要的价值和意义。本文在综述了镁合金腐蚀机理和近年来国内外防腐研究进展的基础上,通过溶胶-凝胶技术在AZ91D镁合金表面制备了具有良好的耐蚀性能的氧化铈、氧化锆溶胶-凝胶膜层以及Si-Ce复合膜层。同时,针对溶胶-凝胶膜层在腐蚀环境中容易遭到破坏,而膜层本身缺乏自修复功能的缺点,本文初步研究了硝酸锌对Si溶胶-凝胶膜层在腐蚀介质中的修复作用及其修复机理。利用电化学阻抗谱(EIS)、动电位极化、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)、能谱分析(EDS)、傅立叶红外(FT-IR)、紫外可见(UV-Visible)以及粘度测试等多种检测手段,对膜层的耐蚀性能、表面形貌和膜层的表面化学组成进行了测试和表征。因镁合金表面化学活性太高,pH较低的溶胶会对镁合金基体造成腐蚀,故该类溶胶不适宜直接涂覆在镁合金表面,本文在制备氧化铈膜层前对镁合金基体预先进行氟化处理。以六水合硝酸铈为前驱体,以火棉胶为粘度调节剂,在氟化的镁合金表面成功制备了氧化铈薄膜。电化学实验和表面分析测试结果表明：氧化铈膜层能明显提高镁合金在氯离子介质中的耐蚀性能,膜层的主要成分为：氧化铈,氟化镁,氧化镁。其中,氧化铈在膜层的耐蚀性能中起着关键作用。同时,本研究也在氟化的镁合金表面制备了氧化锆膜层。首次以五水合硝酸锆为前驱体,以乙酰丙酮为络合剂,在氟化的镁合金表面制备了一种新型、耐蚀性能良好的氧化锆膜层。采用此法制备的氧化锆膜层具有操作简便、成本低、环保等优点。膜层的表征结果表明：氧化锆膜层在3.5 wt.%的NaCl溶液中具有良好的耐蚀性能,硝酸锆和乙酰丙酮发生了络合反应,在溶胶结构中形成了具有双齿配体的络合物。本文还首次讨论了镁合金表面Si-Ce复合溶胶-凝胶膜层中Ce含量对膜层的微观结构、表面形貌和耐蚀性能的影响。利用溶胶-凝胶技术,以γ-缩水甘油醚基丙基三甲氧基硅烷(GPTMS)和乙烯基三乙氧基硅烷(VETO)为前驱体,以六水合硝酸铈作为添加剂,在AZ91D镁合金表面制备了Si-Ce复合溶胶-凝胶膜层。实验结果表明:硅溶胶中加入适量的硝酸铈，会使膜层中的凝胶块消失,有利于提高膜层的耐蚀性能,但加入过量的硝酸铈会破坏膜层表面的完整性,大大降低膜层的耐蚀能力。实验结表明：0.01 M的硝酸铈为制备复合溶的最佳浓度，此时膜层的耐蚀性能最优。由于溶胶-凝胶膜层在腐蚀介质中容易遭到破坏，而本身又缺乏自修复功能,严重限制了溶胶-凝胶技术在镁合金防护方面的应用,为促进溶胶-凝胶技术在镁合金防腐方向的实际应用,本文研究了硝酸锌在氯离子介质中对Si溶胶-凝胶膜层的修复行为。实验结果表明,一定浓度下的硝酸锌对镁合金表面的si溶胶-凝胶膜层具有明显的修复作用。其修复作用可归因于Zn2+体系中的OH形成了难溶的氧化物或氢氧化物,填补了膜层的缺陷,从而抑制了膜层的进一步腐蚀或溶解,同时,硝酸根在这个体系中也可能起到修复/缓蚀作用。更多还原

【Abstract】 Magnesium alloys which are compared as the 21st Green Engineering Material possess a unique set of properties such as light weight, high strength-to-weight ratio, high thermal conductivity, dimensional stability and damping characteristics, good electromagnetic shielding and machinability as well as recyclability. However, magnesium alloys have a number of undesirable properties including high chemical reactivity and poor corrosion resistance that have hindered its widespread use in many applications. Therefore, it is of great significance to study the surface treatment technology of magnesium alloy.Based on the review of corrosion mechanism of magnesium alloys and the current study advancement in the whole world, ceria film, zirconia coating, silane organic coating and Si-Ce hybrid coating were directly or indirectly prepared on the magnesium alloy surface via sol-gel method. Meanwhile, the primary research of healing for Si sol-gel coating was also carried out due to the lack of self-healing functionality. Electrochemistry impedance spectroscopy (EIS), potentiodynamic polarization tests, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared (FT-IR) spectrum analysis, UV-Visible spectrum analysis and viscosity measurements were employed to character the properties of the prepared coatings including the corrosion resistance, surface morphology and composition.It is not possible to apply the coating directly on the magnesium alloy surface for some sol-gel coating because the high chemical activities may cause the poor adhesion and subsequent corrosion of substrate. Therefore, a fluoride coating as an interlayer has been used before preparing the ceria film on the magnesium alloy. The cerium film was prepared on the fluorinated magnesium alloy via sol-gel method employing cerium nitrate hexahydrate as the precursor and celloidin as the additive. The tests results showed that the main composition of the film was ceria, magnesium fluoride and magnesium oxide, especially, ceria played a key role in corrosion resistance. The results of electrochemical measurements in 3.5 wt% NaCl solution demonstrated that the corrosion resistance of AZ91D magnesium alloy was greatly improved by the ceria film.Meanwhile, a novel anti-corrosion zirconia coating was prepared via the sol-gel method for fluorinated AZ91D magnesium alloy using zirconium nitrate hydrate as a precursor modified with acetylacetone (AcAc). Zirconium nitrate reacted with AcAc forming a net structure in the coating. This coating possessed many excellent properties such as simple, inexpensive and environmental friendly. The electrochemical measurements results demonstrated that zinconia coating could obviously improve the corrosion resistance of magnesium alloy AZ91D.For the first time, this work investigated the effect of cerium concentration on microstructure, morphology and corrosion resistance of cerium-silica hybrid coatings on AZ91D magnesium alloy. The cerium-silica hybrid coatings were prepared via sol-gel method employing Vinyltriethoxysilane (VETO) and y-glycidoxypropyltrimethoxysilane (GPTMS) as precursors, cerium nitrate hexahydrate as addititive. The results showed that small amount of cerium nitrate could accelerate the hydrolysis of the hybrid sol which was good to increase the corrosion resistance of the coating; however, a large amount of cerium nitrate could destroy the structure of the coating, leading to poor corrosion resistance of zirconia coating. The optimal concentration of cerium nitrate for increasing the corrosion resistance was 0.01 M.As soon as even small defects appear in the sol-gel coating system and the corrosive agents penetrate to the metal surface the coating system can not stop the corrosion process. The development of an environmental friendly approach which can stop the development of corrosion process and heal the corroded areas becomes an important issue for many industries where an adequate corrosion protection is needed. Present work demonstrated the possibility to use zinc nitrate as healing agent to repair the organic silane coatings in NaCl solution. The tests results demonstrated that introduction of zinc nitrate to the electrolyte could stop the development of corrosion process of the coating system and a remarkable recovery on corrosion resistance could be obtained. This effect may be attributed to the formation of zinc oxide/hydroxide on the defective areas, hindering the corrosion activities. At the same time, the nitrate ions might also play an important role in this healing process.更多还原