【作者】 扈俊颖；

【导师】 李青；

【作者基本信息】 西南大学 ， 无机化学， 2009， 硕士

【摘要】 镁合金因具有密度低,比强度、比刚度、比弹性模量高且易于回收以及很好的铸造、切削加工性能等特点,被誉为“21世纪绿色工程材料”。然而,镁合金的化学稳定性差,在使用环境中容易发生氧化,在湿热条件下会发生严重的电化学腐蚀,其应用范围受到极大限制。镁合金的的耐蚀性已成为国内外关注的焦点,因为它直接关系到镁合金的发展及应用。因此研究镁合金的表面防护具有十分重要理论价值和实际意义。论文在综述了镁合金腐蚀机理及近年来镁合金表面防护技术的基础上,采用动电位极化、电化学阻抗谱等电化学测试方法;并辅以傅立叶红外、扫描电镜等材料表征技术,研究了AZ91D镁合金表面的几种防护技术及相关机理探讨。论文的主要研究内容如下:1、研究氧化硅膜层对AZ91D镁合金的耐蚀保护作用。利用有机醇盐水解法开发了可以直接涂覆在镁合金表面的SiO₂溶胶,采用浸渍提拉法在AZ91D镁合金表面制备了氧化硅膜层。以正硅酸乙酯和乙烯基三乙氧基硅烷为前驱体,乙醇为溶剂,采用醋酸作水解反应催化剂、氨水作缩聚反应催化剂的两步法制备了均匀、透明的SiO₂（TV）溶胶。同时以正硅酸乙酯为前驱体制备SiO₂（T）溶胶作为对比。采用傅立叶红外、扫描电镜表征膜层的结构和表面形貌。通过动电位极化、电化学阻抗等电化学测试方法研究氧化硅膜层在3.5 wt.%NaCl溶液中的腐蚀电化学行为。结果表明,SiO₂（TV）溶胶得到的氧化硅膜层耐蚀性能大大优于SiO₂（T）溶胶得到的氧化硅膜层。2、研究高聚物涂层对AZ91D镁合金的耐蚀保护作用。硅烷上所带官能团的化学结构对溶胶制备过程中的水解缩聚反应会产生很大影响,从而会进一步影响到溶胶的均匀性和相应膜层对AZ91D镁合金的耐蚀性能。论文通过正硅酸乙酯（TEOS）、乙烯基三乙氧基硅烷（VTEO）、γ-氨丙基三乙氧基硅烷（APTS）和γ-（2,3环氧丙氧）丙基三甲氧基硅烷（GPTMS）四种硅烷前驱物之间的相互组合,制备了几种不同组分的硅溶胶。采用旋转涂膜法在AZ91D镁合金表面制备了相应的高聚物涂层。利用傅立叶红外、扫描电镜、接触角测试以及JB-5C技术对不同组分的高聚物涂层的结构和性质进行表征与检测。研究表明硅烷中吸电子性质的官能团使硅溶胶中Si-O-Si反对称伸缩振动发生红移,而给电子性质的官能团使硅溶胶中的Si-O-Si反对称伸缩振动发生蓝移;涂层的平均厚度为2μm。通过动电位极化、电化学阻抗等电化学测试方法对这些高聚物涂层的耐蚀性能进行了研究。结果表明硅烷前驱体中较小的空间位阻可使硅烷的水解缩聚反应进行得较为完全,相应涂层的耐蚀性能较好。3、论文在充分考虑环境友好、工艺成本低廉和操作工艺简单的前提下,在AZ91D镁合金表面制备了钼酸盐转化膜。讨论了转化液组成、成膜温度、成膜时间等实验参数对转化膜层形貌和性能的影响。通过扫描电镜、动电位极化、电化学阻抗等测试手段确定了钼酸盐转化体系的实验参数。结果表明在pH值等于5、（NH₄）₆Mo₇O₂₄·6H₂O浓度为7.3 g/L的钼酸盐转化液中反应30 min,可得到耐蚀性能较好的钼酸盐转化膜层。在优化的钼转化膜层基础上,结合溶胶凝胶技术,得到了耐蚀性能更优的钼/硅复合膜层。4、镁合金在氢氟酸溶液中可以得到一层薄的钝化膜层,但该钝化膜层表面不致密,有孔洞等缺陷,因此限制了镁合金耐蚀能力的进一步提高。为此,论文研究了钝化膜层与溶胶复合技术对镁合金的保护作用。采用动电位极化和电化学阻抗技术研究了钝化膜与硅溶胶以及钝化膜与镧溶胶对AZ91D镁合金的保护作用。结果表明,由于镧溶胶的不均匀性,F/La复合膜层对AZ91D镁合金的耐蚀保护能力有限,而F/Si复合膜层对AZ91D镁合金提供了优异的保护。更多还原

【Abstract】 Magnesium alloys,which are considered as green engineering materials of 21 century,have unique characteristics of low density,high strength-to-weight ratio,high rigidity and elastic modulus,good recyclability,excellent castability and cutting properties.Unfortunately,magnesium and its alloys are highly susceptible to corrosion,particularly in salt-spray conditions.This has limited its use in the automotive and aerospace industries,where exposure to harsh service conditions is unavoidable.As relation to the development span of the magnesium and its alloy,the corrosion resistance of the magnesium alloy has become the focus of the worldwide.Thus it is of great significance to investigate the surface modification technology of magnesium alloy.Based on the summary of corrosion mechanism and the currently surface modification technologies of magnesium alloy,several protection technology on AZ91D magnesium alloy and its relation mechanism were investigated using electrochemical impedance spectroscopy, potentiodynamie polarization test,fourier transform infrared spectrum as well as scanning electron microscope in this paper.The major research efforts of the present study are as follows:1.The SiO₂ film was studied for AZ91D magnesium alloy.Silicon dioxide sols which can be directly applied on the surface of AZ91D magnesium alloy to improve its corrosion resistance were prepared by hydrolyzing organic alkoxide,the dipping-costing method was employed to prepare the SiO₂ coating.In this study,tetraethyl orthosilicate（TEOS） and triethoxyvinylsilane（VTEO） were employed as the precursors to prepare SiO₂（TV） sol,as a comparison,the other SiO₂（T） sol prepared using only tetraethyl orthosilicate（TEOS） as precursor was also investigated.Fourier transform infrared（FT-IR） spectrum and scanning electron microscope（SEM） were performed to analyze the structure and the surface morphology of the SiO₂（TV） sol and SiO₂（T） sol.The corrosion resistance of the AZ91D magnesium alloy coated by silicon dioxide coatings was examined using electrochemical impedance spectroscopy（EIS） and potentiodynamic polarization tests in 3.5 wt.%NaCl aqueous solution.The results indicate that the corrosion resistance of the substrate is greatly improved by the silicon dioxide coatings obtained from SiO₂（TV） sol. 2.Polymer coating based silicon was explored.To investigate the effect of functional groups in the silicon polymer coating on the corrosion resistance of AZ91D magnesium alloy,various polymer coatings were prepared from the precursor’s intercombination among tetraethyl orthosilicate（TEOS）,triethoxyvinylsilane（VTEO）,3-glycidoxypropyltrimethoxysilane（GPTMS） and 3-aminopropyltriethoxysilane（APTS） via sol-gel method.The surface morphology,the hydrophobic property and the thickness of these coatings were characterized by scanning electron microscope（SEM）,contact angle goniometer and JB-5C system,respectively.The result shows that the coating thickness was about 2μm.The Fourier transform infrared（FT-IR） spectra were used to analyze the structure differences among the various gels.It demonstrated that electron with drawing group made the v_as（Si-O-Si） shift to shorter wave number while electron donating group made the peak shift to higher wave number.Electrochemical impedance spectroscopy（EIS） and potentiodynamic polarization tests were employed to confirm the corrosion resistance ability of these sol-gel films.The results showed that the smaller steric hindrance of the functional groups on the silica precursor made the hydrolysis and condensation reaction easily and the relevant film had a better corrosion resistance.3.Considering the environmental friendly and cost-effective,molybdate conversion coatings have been successfully prepared on magnesium alloy to improve its corrosion resistance.For molybdate conversion treatment,various conditions including the pH of the molybdate baths, immersion time and bath temperature were investigated using electrochemical measurements.The corrosion resistance of the AZ91D magnesium alloy was improved to some extent by the conversion coating with the optimal conversion parameters(7.3 g/L（NH₄）₆Mo₇O₂₄·6H₂O solution with pH 5 for 30 min at 30℃).In order to get a further improvement of corrosion protection for AZ91D magnesium alloy,three layers of silicon sol-gel coatings were successfully deposited on the molybdate conversion coating pre-applied to AZ91D alloy.4.Take use of the passivation of magnesium and its alloy in contact with the HF solution,the composite technology-passivation layer and sol-gel film-was used to improve the corrosion resistance of magnesium alloy.The electrochemical impedance spectroscopy "（EIS） and potentiodynamic polarization tests in 3.5 wt.%NaCl aqueous solution were used to test the corrosion resistance of passivation layer and silica sol（F/Si）,as well as passivation layer and lanthanum sol（F/La） composite film.The results showed that the F/La film had limited protection effect because of its non- uniformity.However,the F/Si film could offer better corrosion resistance to magnesium alloy.更多还原