【作者】 杨旭；

【导师】 潘复生； 张丁非；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2009， 博士

【摘要】 镁合金因具有一系列优异的性能而被广泛地应用于航空航天、汽车制造和电子工业等领域,被认为是21世纪最富有开发和应用潜力的“绿色工程材料”。但镁具有高的化学活性,极易被腐蚀,从而影响其表面形貌和力学性能,极大地限制了镁合金的推广应用。镁合金表面防护是在镁合金表面形成一个阻挡层,将腐蚀介质与基体隔开,阻挡腐蚀电流的形成,是一种有效的防腐方法。在冶金控制及新型合金开发取得决定性进展之前,通过各种表面处理技术来控制镁合金的腐蚀,是当前业界最重要的研究课题。本文针对镁合金表面防护技术所面临的环境污染、成本、前处理步骤复杂、结合力以及恶劣条件下的耐腐蚀性差等几个常见问题,通过选择环境友好型有机多功能团化合物,在水溶液中通过沉积作用制备表面具有反应性功能基的有机转化膜,着力简化前处理步骤,改善镁合金表面防护技术中的环境污染问题以及为提高防腐蚀性能而形成复合防护层时转化膜与基体及表面防护漆的结合力问题。为在水溶液中制备有机转化膜提供了必要理论参考,实验研究了镁合金在水溶液中的腐蚀行为。结果表明,在pH值小于11.5的溶液中,镁合金易被H+氧化而腐蚀;在pH值大于11.5的强碱性条件下镁合金能稳定存在,在一定的外加电位作用下可以氧化腐蚀镁合金,在碱性水溶液中的极化行为分三个区域,阴极极化区、钝化区、阳极极化区,在阳极极化区镁合金氧化腐蚀。镁合金氧化腐蚀受pH值影响,随着溶液的pH值升高,钝化区域变宽,氧化电位正移。在了解了镁合金在水溶液中腐蚀行为并借鉴镁合金其它表面防护技术及其它金属缓蚀技术的基础上,结合实验建立了镁合金表面有机缓蚀剂的选择方法。此方法以“结构上初筛(选择具有含氧、氮、硫或磷原子的多官能团有机物)—化学沉淀实验对腐蚀抑制潜能评估(能在酸性或碱性条件下形成难溶物)—电化学实验对腐蚀抑制作用测试(测量腐蚀电流密度的大小)—表面形貌的观察和表面成分分析”为选择程序,并筛选出了三种环境友好型的镁合金有机缓蚀剂:“对-硝基苯偶氮间苯二酚(PNBAR)”、“植酸(PA)”和“鞣酸(AT)”。提出了在含有PNBAR的碱性溶液中用电化学沉积的方法在镁合金表面制备PNBAR转化膜的技术。研究了溶液的酸碱性和外加工作电位对制备有机转化膜的影响。通过控制工作电位,使镁合金的表面氧化与有机物PNBAR的沉积同步进行,PNBAR通过共价键Mg-O-N实现在镁合金表面的沉积。在pH值为13、浓度为5mmol/L的植酸溶液中施加3.2V的工作电位进行30分钟的恒电位极化,制得了使镁合金自腐蚀电位正移110毫伏,结合力达到1级(按照GB/T 9286)的有机转化膜。结合植酸的分子结构特点,提出并研究了四种在镁合金表面制备植酸转化膜的方法:利用表面的水合羟基与植酸羟基脱水成膜;利用表面氧化物与植酸反应成膜;利用植酸氧化合金表面金属沉积成膜;碱性条件下电化学沉积成膜。研究表明,在离开溶液进行的羟基脱水成膜和氧化物反应成膜实验中,固相反应活性低,不能形成转化膜。在pH值为13、植酸浓度为0.5mg/ml的碱性溶液中,经过3.2V的工作电位进行30分钟的恒电位极化,得到植酸转化膜。在pH值为13的溶液中,植酸与镁离子形成难溶性的[Mg5(H2L)].22H2O化合物,生成氢氧化镁的反应成为植酸沉积的竞争反应,氢氧化镁与植酸一起沉积在镁合金表面成膜。植酸在pH值为5的酸性溶液中经过20分钟的浸泡成膜可以形成约4~5微米厚的转化膜,植酸转化膜致密、生长有序,比碱性条件下得到的植酸转化膜耐蚀性强,使镁合金自腐蚀电位正移185毫伏,结合力达到1级(按照GB/T 9286)。植酸在酸性条件下与镁离子形成可溶性的配合物,植酸转化膜的形成主要是三价铝离子与植酸根的沉积作用。最后,考察了镁合金材料的化学组成和加工状态对植酸在酸性条件下成膜的影响。酸性条件下所有的多价金属离子(镁离子除外)都可以与植酸根沉积成膜,多价金属离子与植酸形成的配合物溶解性越低,越有利于沉积成膜。一般来讲,金属离子与植酸形成的配合物溶解性顺序为:Mg2+>> Zn2+、Mn2+>> Zr2+、Al3+。对常见的镁合金,元素铝和锆由于含量和性能两方面的综合影响,对植酸成膜作用最显著,因此,AZ系和ZK系镁合金有利于制备植酸转化膜。同时,在酸性条件下,因易于提供金属离子,铸态比挤压态镁合金利于制备植酸转化膜。更多还原

【Abstract】 Magnesium alloys have been widely used in aerospace, automotive and electronic manufacturing industry and other fields because of a series of excellent performances. It is considered to be the most potential for the development and application of "green engineering materials" in the 21st century. However, magnesium has a high chemical reactivity, it is susceptible to corrosion which can cause severe pitting in the metal resulting in a decreased mechanical stability and unattractive appearance. All these have greatly limited the application of magnesium alloy. The surface protection technology of magnesium alloy is to form a barrier between matrix and corrosion medium for blocking the formation of corrosion current, therefore it is an effective anti-corrosion method. Until the metallurgical control and the development of new alloys have been made decisive progress, the adoption of a variety of surface treatment technology to control corrosion of magnesium alloy is the most important question to investigate. As to the surface protection technology for magnesium alloys, there are some questions need to face, such as environmental pollution, cost, complexity of the pre-treatment steps, the poor adhesive ability, as well as the poor corrosion resistance, etc. In the present work, we attempted to improve two of the above mentioned problems, i.e., environmental pollution and the poor adhesive ability, by choosing environmentally friendly organic compounds with multi-functional group, as well as deposition in the preparation of the surface organic conversion coating with a reactive functional group in aqueous solution.In order to provide the necessary information for the preparation of organic conversion coating in aqueous solutions, the corrosion of magnesium alloys in aqueous solution was investigated. The results showed that when the pH value was less than 11.5, magnesium alloy is susceptible to oxidation by H + and corrosion; when the pH value was more than 11.5, magnesium alloy is stable. The polarization behavior caused by working potential is divided into three regions, cathode polarization region, passive region, anode polarization region. In the region of anode polarization, magnesium alloy can be eroded. Oxidation corrosion of magnesium alloy is affected by the pH value: with the solution pH value increases, passivation region become wider, and the the region of anode polarization shifted positively.The method of choosing organic corrosion inhibitor for magnesium alloy was established on the basis of previous reports concerning other surface protection technologies of magnesium alloy and other metal corrosion inhibition technologies. The way of "concerns on structure (choice of the multi-functional group organic compounds with oxygen, nitrogen, sulfur or phosphorus atoms)—chemical precipitation experiment (in acidic or alkaline conditions, the formation of insoluble materials)—electrochemical corrosion experiments to test the anti-corrosion property—surface morphology observition and the surface composition analyzation" is the selection process and three types of environment-friendly organic corrosion inhibitors for magnesium alloy, p-nitro-benzene-azo-resorcinol (PNBAR), phytic acid (PA) and acidum tannicum (AT), were successfully selected.Technology for preparing PNBAR conversion coating on magnesium alloy surface by electrochemical deposition method in alkaline solution was established. The pH value and working potential were taken into account in the process of preparing organic conversion coating. Magnesium alloy surface oxidation and the deposition of PNBAR are synchronous by controlling the working potential. The PNBAR conversion coating was prepared through polarizing 30 minutes with constant potential (3.2V) in the PNBAR solution (5mmol/L) of pH 13. PNBAR bonds to the surface of magnesium alloy through covalent bond Mg-ON, the corrosion potential was shifted positively about 110 mV by conversion coating when compared with control, and its adhesive ability reached to Grade one(in accordance with GB / T 9286).Four methods of preparation phytic acid conversion coating on magnesium alloy surface were proposed according to the characteristics of the molecular structure of phytic acid: 1) esterification by phytic acid with surface hydrated magnesium ion, 2) the reaction between surface oxide and phytic acid, 3) deposition of phytic acid with the surface metal in acidic solution or 4) deposition in alkaline phytic acid solution. The conversion coating could not be formed by dehydration and oxide reaction because of low activity of solid-phase reaction. In 0.5mg/ml phytic acid solution with pH 13, phytic acid conversion coating was formed by polarizing for 30 minutes at 3.2V potential. In solution with pH 13, phytic acid and magnesium ions form insoluble of [Mg5(H2L)].22H2O compound, generation of magnesium hydroxide become the competitive reaction to phytic acid deposition, so phytic acid deposited together with magnesium hydroxide. In 0.5mg/ml acidic phytic acid solution with pH 5, an orderly generation and compact phytic acid conversion coating formed after 20 minutes of immersion. The conversion coating has the thickness of 4μm~5μm, and has better corrosion resistance than the coating formed under alkaline condition, the corrosion potential was shifted positively about 185 mV, and the adhesive ability also reached to Grade one(in accordance with GB / T 9286).The influence of chemical composition and state of magnesium alloy on forming process and properties of phytic acid conversion coating in the acidic condition was studied. In the acidic condition, phytic acid can be deposited on magnesium alloy surface through forming insolubility compounds with multivalent elemental ions (except magnesium ion) of magnesium alloy and formed conversion coating. For the common magnesium alloy, it is notable that the elements of aluminum and zirconium play a vital role in the process of phytic acid conversion coating formation because of their concentration and performance in the alloys.更多还原