【作者】 高丽丽；

【导师】 张密林；

【作者基本信息】 哈尔滨工程大学 ， 材料学， 2008， 博士

【摘要】 镁锂合金是目前最轻的金属结构材料，具有超轻高强的特点，并且具有韧性好、抗弯强度大、容易变形加工等优点，是宇航、兵器、汽车、电子等领域中优选的一类结构材料。但是，镁锂合金的耐蚀性较差，使它的应用受至很大的限制。本文研究了NaCl溶液的浓度和pH值对镁锂合金腐蚀行为的影响，通过腐蚀速率、腐蚀形貌、腐蚀产物以及电化学分析对合金在不同体系中的腐蚀行为进行了研究。在NaCl溶液中，由于Cl^-的特殊作用，镁锂合金所发生的是有选择的点蚀，并向基体内部及四周逐渐扩大，形成较深的蚀坑，在碱性溶液中所生成的腐蚀产物最多，在酸性溶液中形成的蚀坑最大。镁锂合金在碱性NaCl溶液中的腐蚀产物为Mg（OH）₂、Al₂O₃和Li₂O₂，呈现晶态结构特征，而在中性和酸性溶液中腐蚀产物的主要成分为Mg（OH）₂和Al₂O₃。随着Cl^-浓度的升高，合金的平均腐蚀速率增大，腐蚀电流增大，腐蚀越严重；碱性增强时，合金的点蚀电位正移，表面膜钝化作用增强，减缓了腐蚀的进行；酸性增强时，合金的腐蚀电流增大，线性极化电阻减小，加快了腐蚀的进行。本文通过正交试验法对镁锂合金稀土铈盐和植酸两种化学转化工艺的实验参数进行了优化，铈盐转化膜最佳工艺为：Ce（NO₃）₃浓度为0．05mol/L；转化温度为35℃；转化时间为10min。植酸转化膜最佳工艺为：转化液pH为6；植酸浓度20g/L；成膜时间10min；成膜温度35℃。电化学测试和析氢实验结果表明铈盐转化膜和植酸转化膜能明显提高镁锂合金的耐腐蚀能力，处理后合金硬度也有较大幅度的提高。利用SEM观察了转化膜的表面形貌，铈盐转化膜为灰白色均匀薄膜，由针状物堆积而成，膜层厚度约12μm。植酸转化膜呈现深灰色，表面有白色花絮状物质沉积，膜层中存在微小裂纹。采用EDS、XRD、XPS及FT-IR分析了膜层的主要成分，铈盐转化膜主要为Ce（Ⅲ）和Ce（Ⅳ）的氧化物和氢氧化物的混合物，其中CeO₂呈晶态结构。植酸转化膜主要含Mg、O、P、Al和C，深灰色区域主要为镁的螯合物，白色区域主要为铝的螯合物。结合OCP曲线和膜层的组成，分别探讨了铈盐和植酸转化膜的成膜过程。采用溶胶-凝胶法合成纳米SiO₂前驱体，用γ-环氧丙氧基三甲氧基硅烷进行原位改性，XRD和FT-IR分析证实改性成功。利用纳米SiO₂前驱体制备环氧/纳米SiO₂杂化涂层，SEM分析表明杂化涂层呈明显的两相结构，无机相以纳米尺度均匀地分散于有机相中。杂化涂层的硬度和耐腐蚀性明显优于环氧涂层。电化学极化曲线和交流阻抗的分析表明当SiO₂质量分数为3%时，镁锂合金腐蚀速率最低，防腐效果最佳。在镁锂合金铈盐和植酸转化膜上分别涂覆环氧/纳米SiO₂杂化涂层，进行复合防护处理。析氢实验和盐雾腐蚀实验表明复合涂层相对于单层涂层有明显的防腐优势，其腐蚀速率明显降低，其中植酸-环氧/纳米SiO₂复合涂层的防腐效果更好。并通过交流阻抗图，用扩散理论分析了植酸-环氧/纳米SiO₂复合涂层的防腐蚀过程及防腐蚀优势。复合涂层的硬度和附着力明显优于环氧涂层和单纯的杂化涂层，有利于提高对镁锂合金的防护效果。更多还原

【Abstract】 Mg-Li alloy is one of the lightest metal structural materials,which should bewidely used in automotive,electronic,weapon and aeronautical industries owingto its excellent properties,such as high strength/weight ratio,high impacttoughness,bending strength and deforming machining easily.However,it is thepoor corrosion resistance that limits its extensive applications.In this thesis,the effects of concentration and pH value of corrosive solutionson corrosion behavior of Mg-Li alloy were studied.The corrosion behavior wasdescribed through corrosion rate,morphology of the corroded surface,corrosionproducts and electrochemical analysis.In NaCl solution,the main corrosionmechanism was the selective pitting due to the special characteristics of chlorideion.The large corrosion pits were formed because the corrosion pits expandedtowards the matrix inner and enlarged all around.The corrosion products of thealloy in alkaline NaCl solution were the most,and the size of the corrosion pits inacidic NaCl solution was the largest.The main components of the corrosionproducts in alkaline solution were Mg（OH）₂,Al₂O₃ and Li₂O₂,which were allcrystals.But in acidic and neuter solution the products were Mg（OH）2 and Al₂O₃.The corrosion of Mg-Li alloy developed more seriously with the increase in Cl^-concentration,because the corrosion rate and corrosion current increased.Thecorrosion potential gradually increased with the increase of alkalescence,whichindicated that the passivation of the surface film was improved and the corrosionvelocity was restrained provisionally.When the acidity increased,the corrosioncurrent gradually increased and the linear resistance decreased,this means that thecorrosion reaction accelerated.The technical parameters of conversion coatings on the Mg-Li alloy wereoptimized by orthogonal experiments.The optimum processing parameters of Ce-based conversion coating were confirmed as follows:solution concentration is0.05 mol/L,treating temperature is 35℃and treating time is 10 rain.Theoptimum process parameters ofphytic acid conversion coating were confirmed asfollows:pH value of the solution is 6,solution concentration is 20 g/L,treatingtime and treating temperature is 10 min and 35℃,respectively.The corrosionresistance ability was evaluated by hydrogen evolution experiment andelectrochemical analysis.The results showed that the conversion coatingsimproved the corrosion resistance of the alloy.Analysis by SEM indicated that theCe-based conversion coating was a grey uniform coating with a fiber-likemorphology,its thickness was 12μm.The phytic acid conversion coating was adark grey coating with white flower-like deposit and some micro cracks.Thecomponents of coatings were investigated by EDS,XRD,XPS and FT-IR.Themajor component of the Ce-based conversion coating was a mixture of oxide andhydroxide of Ce（Ⅲ） and Ce （Ⅳ）.The phytic acid conversion coating mainlyconsisted of Mg,Al,O,P and C,the dark grey area was mainly magnesic chelatecompounds and white area was mainly aluminous chelate compounds.Theforming processes of the conversion coatings were discussed through the OCPevolution and components of coatings.Nano-SiO₂ precursor was synthesized through sol-gel technique,then it wasmodifed in situ byγ-epoxypropoxytrimethoxysilane.The modified nano-SiO₂precursor was characterized by XRD and FT-IR.The epoxy/nano-SiO₂ hybridcoating was prepared by using the modified nano-SiO2 precursor.The analysis bySEM showed that coating was two-phase structure,and nano-scale SiO₂ particleswere dispersed in epoxy matrix homogeneously.The corrosion resistance andhardness of hybrid coating were better than epoxy coating.The analyses ofelectrochemical potentiodynamic polarization and impedance spectroscopyindicated that the protection was best when mass fraction of nano-SiO₂ was 3%. The epoxy/nano-SiO₂ hybrid coating was prepared on the conversioncoatings of Mg-Li alloy,this is called composite coatings or composite protectiontechnique.The results of hydrogen evolution and salt spray corrosion experimentsindicated that the composite coatings had better corrosion resistance than themono-coatings.The corrosion resistance of phytic acid-epoxy/nano-SiO₂composite coating was the best.The protection process and predominance ofphytic acid- epoxy/nano-SiO₂ composite coating was analyzed by diffusion theorythrough electrochemical impedance spectroscopy.The hardness and adhesion ofcomposite coatings were better than epoxy coating and hybrid coating,which canprovide more effective protection for Mg-Li alloy.更多还原