【作者】 张世艳；

【导师】 李青；

【作者基本信息】 西南大学 ， 无机化学， 2010， 硕士

【摘要】 镁及镁合金具有密度低,比强度、比刚度、比弹性模量高,铸造性能和切削加工性好等特点而被广泛应用于航空航天、汽车制造和电子工业等领域,被誉为21世纪的“时代金属”。然而,镁是一种非常活泼的金属,具有极高的化学和电化学活性,极易被腐蚀,即便是将镁合金置于空气中,它的表面都会形成一层很薄的氧化膜,使其应用范围受到极大限制。因此,采用表面防护技术来提高镁合金的耐腐蚀性能对于扩展镁合金的应用有着重要的意义。常见金属表面的保护手段中,电镀方法操作简单,所得金属镀层具有装饰性、耐腐蚀性、可焊性、导电性和耐摩擦性等优点。但镁合金难于直接实施电镀,因为其在空气和水中易迅速生成MgO和Mg(OH)2,阻碍镀层的生成；镁的电位低,常规镀液易对其腐蚀;其结构的多相性会导致被镀金属沉积不均匀；易发生电偶腐蚀。因此,本文在综述了镁合金腐蚀机理及近年来表面防护方法的应用基础上,针对镁合金的特点,采用了不同的前处理方法,对其实施多种电镀工艺研究。对已成功制备出的多种防护性涂镀层的耐腐蚀性能及其耐腐蚀机理做了相应的研究,论文的主要研究内容及取得的科研成果如下：首先以环保、低成本工艺技术为前提,研究了AZ91D镁合金无铬前处理及镀锌层对其耐蚀保护作用。通过锡酸盐转化处理在AZ91D镁合金表面形成转化层,并以此作为前处理底层,再进行电镀锌,得到了耐蚀性良好的锌镀层。所获得锌镀层经过扫描电镜、X射线衍射表征可知,镀层表面是由不规则的多边形晶簇组成,其尺寸大小约为1-2μm,镀层的厚度大约为10μm。在3.5 wt.% NaCl中性溶液中的电化学极化和电化学阻抗谱测试结果表明,锌镀层的自腐蚀电位为-1.23 V,极化曲线有明显的钝化区,阻值也明显高于AZ91D镁合金裸基体,证明锌镀层为AZ91D镁合金提供了优良的耐蚀性能。为了进一步提高AZ91D镁合金表面防护层的耐蚀性能,本研究设计了以常见化学镀镍层作为中间过渡层,采用溶胶凝胶技术修补化学镀镍层的缺陷以达到增强对AZ91D镁合金保护性能的目的。实验结合各种表征手段揭示：以Si溶胶封孔的复合涂层表面均一,能够大幅度提高基体的耐蚀性能,在3.5 wt%NaCl腐蚀性溶液中浸泡时间长达120小时。其失效过程分为两部分,首先是外层溶胶层逐渐失效,最后是侵蚀性离子穿过化学镀层并达到镁合金基体进而诱发腐蚀。针对化学镀镍层的固有缺陷,文章选取耐蚀性能好的镀层金属镍,在AZ91D镁合金化学镀镍层上实施电镀镍层来进一步提高镁合金的耐蚀能力。研究结果表明,此工艺可操作性强,工艺合理；获得的镍镀层均匀致密、光亮,与基体结合力良好,基本无缺陷。电化学测试结果显示,该镀层在3.5 wt% NaCl溶液中持续浸泡达72小时。其呈现出的耐腐蚀性能与所报道的AZ91D镁合金其它防护处理的相比大大增强。以提高AZ91D镁合金的耐腐蚀及机械性能为研究目标,本文研究了在AZ91D镁合金表面的复合镀技术。利用纳米微粒特有的小尺寸效应,量子尺寸效应等,将其复合到金属镀层中将产生许多优异的物理、化学性能。因此,本文通过将纳米TiO2添加在镍镀液中对AZ91D镁合金实施电镀得到了Ni-TiO2保护镀层。研究结果显示该镀层不仅改善了基体的耐蚀性能,同时也增加了其耐摩擦性能且其表面显微硬度达到445HV。在3.5 wt% NaCl溶液中浸泡时间能够达到96小时。更多还原

【Abstract】 Magnesium alloys, which are considered as time metal materials of 21 century, have unique characteristics of low density, the best strength-to-weight ratio of the commonly die-cast metals, generally a better machinability, and often a higher production rate, thus they are of great value for applications in theerospace, automotive industries and functional materials. However, its insufficient corrosion resistance as an actually usable metal for its relatively low standard potential and high reactive nature makes it easily oxidized in air, which strongly limited its applications. Thus, it is of great significance to make great efforts to increase the corrosion resistance of the magnesium alloys.In the conventional surface treatments, the electrochemical deposition is a promising method for the protection of magnesium alloys due to many pre-dominant performances such as decorative appearance, corrosion resistance, solderability, electrical conductivity, high microhardness and anti-wearing. While it is well known that magnesium alloys are "difficult to plate metal" for the oxide film with low electrical conductivity and bonding ability which is always present on the light metal surface, prevents the adhesion of the metallic coating. In addition, magnesium alloys are susceptible to be corroded in the deposition solution not containing chromate and fluoride. Furthermore, the coating has to be pore-free due to the potential difference between magnesium alloys substrate and metal coating which increases the difficultly of deposition. Therefore, based on the summary of corrosion mechanism and the currently surface protection technologies of magnesium alloy, several pretreatments were adopt to investigate the corrosion resistance technologies on AZ91D magnesium alloys, the main contents as follows.Firstly, considerning environment-friendly, low cost technology, a stannate chemical conversion process followed by an activation procedure was employed as the pre-treatment process for AZ91D magnesium alloy substrate. Zn was electroplated onto the pre-treated AZ91D magnesium alloy surface from pyrophosphate bath to improve the corrosion resistance and the solderability. The surface morphologies of conversion coating and zinc coating were examined with scanning electron microscope (SEM). The phase composition of conversion coating was investigated by X-ray diffraction (XRD).The zinc coating on top is even and filled with distorted polygonal which is 1-2μm in diameter and the coating thickness is about 10μm. The electrochemical corrosion behavior of the coatings in neutral 3.5 wt.% NaCl solution was investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The experimental results showed that the corrosion resistance of the AZ91D substrate was improved by the zinc coating. To improve the corrosion resistance of AZ91D magnesium alloy, a protectivemultilayer coating, with electroless Ni coating as bottomlayer and sol-gel SiO2 film as top layer, was successfully prepared on AZ91D magnesium alloy by a combination of electroless and sol-gel techniques. The experiment tests showed that, surface of the composite coating was compact, and its corrosion resistance was also further improved compared with AZ91D magnesium substrate and electroless Ni coating. In the long-term immersion test, it was maintained in the corrosive electrolyte for 120 hours. Two steps were involved in the dregradation process, the top sol film failed and followed by the inner layer in long period of immersion in corrosive electrolyte.Due to the inhere limitation of the electroless Ni coating, a protectivemultilayer coating, with electroless Ni coating as bottomlayer and electrodeposited Ni coating as top layer, was successfully prepared on AZ91D magnesium alloy by a combination of electroless and electrodeposition techniques. The test results showed that, the process was exercisable and reasonable, the surface morphologies was uniformly and compact. The experimental results obtained by electrochemical behaviors showed that, the immersion test in neutral 3.5 wt.% NaCl solution last for 72 hours. The corrosion resistance of AZ91D magnesium substate was improved by the metal coating compared with other technologies.Electrodeposition of nanoparticles within metal matrix produces composite coatings with the attractive properties such as high wear resistance, corrosion resistance and electrocatalysis. Thus, TiO2 nanoparticles were codeposited with Ni matrix to obtain a Ni-TiO2 composite coating. A protectivemultilayer coating with electroless Ni coating as bottomlayer and electrodeposited Ni-TiO2 composite coating as top layer was successfully prepared on AZ91D magnesium alloy. The results showed that Ni-TiO2 composite coating could afford better corrosion and mechanical protection for the AZ91D magnesium alloy compared with single electroless Ni coating. The micro-hardness of the Ni-TiO2 composite coating improved more than 5 times than that of the AZ91D magnesium alloy. The immersion test of Ni-TiO2 composite coating in neutral 3.5 wt.% NaCl solution last for 96 hours.更多还原