镁合金表面新型微弧氧化涂层制备及性能表征研究

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	镁合金表面新型微弧氧化涂层制备及性能表征研究
	刘锋
学位类型	博士
导师	韩恩厚
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	镁合金微弧氧化氧化锆耐腐蚀性自封孔 Magnesium Alloys Plasma Electrolytic Oxidation Zirconium Oxides Corrosion Resistance Self-sealed Micro Pores
摘要	"由于具有低密度、高比强度、高比刚度和良好的电磁屏蔽性能等机械和物理性能，镁合金在航空、航天、汽车和电子等工业领域有广阔的应用前景。但较差的耐腐蚀性能在一定程度上限制了这种材料的应用范围。微弧氧化技术可以赋予镁合金良好的耐腐蚀性能和表面机械性能，被认为是一种理想镁合金防护技术。本文从提高膜层化学稳定性和改善膜层结构入手，开发出了两种新的镁合金微弧氧化处理液体系，并制备出了具有较高耐腐蚀性能的膜层。在深入分析微弧氧化膜形成过程的基础上，设计了一种表面微孔呈自封闭状态的镁合金微弧氧化膜的制备溶液。膜层主要由Mg2Zr5O12和t-ZrO2组成，而不是传统镁合金微弧氧化膜的主要成分氧化镁；膜层表面微孔中的填充物主要为氟化镁。由于具有较高的致密性以及良好的化学稳定，膜层具有优异的耐腐蚀性能。电解液中，较高浓度的氟锆酸钾和磷酸二氢铵的加入有益于提高溶液的钝化能力以及改善膜层的致密性；膜层生长促进剂的添加提高了氧化膜的生长速度，对膜层表面微孔的状态有着重要的影响；溶液稳定剂的加入提高了溶液的稳定性，有益于膜层生长速度的提高。考察了处理工艺对氧化膜性能的影响，确定了最佳工艺条件：氧化电压400 V，频率500 Hz，占空比50 %，处理时间20 min。为了使镁合金表面氧化锆涂层的制备工艺更适合于工业化生产，本文从提高处理液的钝化能力入手，开发出了一种可以采用恒电流电源模式工作的处理液体系。研究了处理溶液中添加剂的浓度对膜层性能的影响。提高氟锆酸钾浓度有利于处理液钝化能力的提高、膜层厚度的增加以及膜层组织均匀性的改善；磷酸二氢铵的加入有益于提高火花放电强度以及增加氧化物在放电通道内的沉积量，提高磷酸二氢按浓度有益于膜层的致密性提高以及膜层厚度逐渐增加；膜层生长促进剂的加入会显著改善溶液的钝化能力并提高火花放电的强度，膜层的生长速度随着膜层生长促进剂浓度的增加而逐渐提高；溶液稳定剂的加入有利于提高溶液的稳定性；有机胺的加入将大幅提高溶液的钝化能力。为了获得耐腐蚀性能更好的涂层，采用了一种先恒电流后恒电压的氧化处理工艺，最优工艺参数为：氧化电流密度为20 mA∙cm-2，氧化电压为420 V，频率为500 Hz，占空比为50 %，处理时间为10 min~30 min。在此工艺条件下获得的氧化膜与基体、车身胶有良好的结合力，膜层与基体的结合强度可达到40 MPa以上，膜层与车身胶的结合强度大于17.5 MPa。由于膜层中氧化锆的大量存在，膜层具有较高的硬度和耐腐蚀性能。膜层表层平均硬度值可达到792.2 Hv，氧化处理的AM30样品可耐1000 h以上的盐雾实验。研究了膜层表面自封闭微孔的形成过程。电解液中，膜层生长促进剂的加入有利于提高膜层中氟化镁的含量，氟锆酸钾的添加有利于降低放电通道内气体的压力并增加氧化物在放电通道内的沉积量。电解液中氟化镁与膜层生长促进剂的同时存在，促进了膜层表面自封闭微孔的形成。在另一种电解液体系中，膜层表面微孔呈开放状态，其特征与处理液中有机胺的浓度密切相关。同时，在此溶液中，火花放电促进了锆的氧化物在基体表面的沉积，氧化电压的提高有利于膜层致密性的改善。这两种涂层氯化钠溶液中的腐蚀破坏过程大致相同。在浸泡过程中，膜层的初期腐蚀破坏主要发生在膜层表面微孔及裂纹附近区域。随着浸泡时间的延长，腐蚀介质进一步向膜层内部渗透。由于膜层中氧化镁的水解，膜层组织逐渐受到破坏。膜层中氧化锆的大量存在一定程度的抑制了膜层的腐蚀破坏速度。"
其他摘要	". Magnesium alloys have wide application prospect in the industrial fields of aerospace, aviation, automobile, electronic, etc., due to their excellent mechanical and physical properties, such as low density, high strength/weight ratios, high stiffness, excellent electromagnetic shielding performance. However, the application scope of these materials has been limitted due to their poor corrosion resistance. The technology of PEO (Plasma Electrolytic Oxidation) can provide Mg alloys with high corrosion resistance and excellent superficial mechanical properties. Hence, it is considered as an ideal surface treatment technology for Mg alloys. This paper developed two novel electrolytic solutions from the views of increasing the coatings’ chemical stability and improving the coatings’ structure. PEO coatings with high corrosion were obtained in these two electrolytic solutions. Based on the deeply analysis of the formation process of PEO coating, an electrolytic solution in which a PEO coating with self-sealed superficial pores could be produced was developed. The PEO coating obtained in this solution is mainly composed of Mg2Zr5O12 and t-ZrO2, rather than MgO which is the mainly composition of traditional PEO coatings on Mg alloys; the self-sealed superficial pores in the coating are mainly filled with the compound of MgF2. The coating has superior corrosion resistance due to its compact structure and high chemical stability. High concentration of K2ZrF6 and the addition of NH4H2PO4 are beneficial for increasing the passivation effect of the base solution on Mg alloys and improving the compactness of the coating; the addition of film growth promoter enhances the growth rate of the coating, and this additive has great influence on the properties of superficial pores; the addition of solution are beneficial for increasing the stability of the electrolytic solution and increasing the growth rate of the coating. The effects of processing parameters on the properties of coatings were investigated, and the optimum parameters are listed as follows: applied voltage is 400 V, the frequency is 500 Hz, duty cycle is 50 % and the duration time is 20 min. stabilizer In order to make the preparation process of zirconium oxides coating on Mg alloys suitable for industrial application, a novel electrolytic solution in which a PEO coating could be produced by constant current model was developed from the view of increasing the passivation effect of the solution. The effects of concentrations of various additives on the properties of coatings were investigated. Increasing the concentration of K2ZrF6 is beneficial for improving the passivation effect of the solution, enhancing the growth rate of the coating and improving the structure uniformity of the coating; the addition of NH4H2PO4 is beneficial for increasing the intensity of discharge and increasing the deposition amountNH4H2PO4 is beneficial for improving the compactness of the coating as well as enhancing the growth rate of the coating; the addition of film growth promoter can remarkable improve the passivation effect of the solution and increase the intensity of discharge, and the growth rate of the coating increases with increasing the concentration of film growth promoter; the addition of solution stabilizer is beneficial for improving the stability of electrolytic solution; the addition of organic amine can remarkable improve the passivation effect of the solution on Mg alloys. An oxidation method of constant current first and constant voltage second was adopted in order to obtain a PEO coating with higher corrosion resistance. The optimum parameters are listed as follows: the current density is 20 mA∙cm-2, the applied voltage is 420 V, the plus frequency is 500 Hz, the duty ratio is 50 % and the duration time of the constant voltage oxidation is 10 min~30 min. It has high bonding strength between the PEO coating and substrate as well as car body adhesive. The bonding strength between the PEO coating and substrate is higher than 40 MPa, and that between the PEO coating and the car body adhesive is higher than 17.5 MPa. Due to the existence of zirconium oxides in great quantity, the coating has high hardness and superior corrosion resistance. Average hardness of the surface layer of the coatings is up to 792.2 Hv; coated AM30 Mg alloy sample can afford 1000 h salt spray test. of oxides in discharge channels, increasing the concentration of The formation process of the self-sealed superficial pores in the coating was investigated. The addition of film growth promoter increases the content of MgF2 in the coating, and addition of K2ZrF6 is beneficial for decreasing the gas pressure of the plasma and increasing the content of coating compounds deposited in the discharge channels. The addition of K2ZrF6 together with film growth promoter promotes the formation of the self-sealed superficial pores. The superficial pores in the coating obtained in the other electrolytic solution exhibits open characteristics, which are greatly influenced by the concentration of organic amine in the solution. Discharge promotes the deposition of zirconium oxides on the coating. Meanwhile, increasing the applied voltage is beneficial for improving the compactness of the coating. The deterioration processes of these two coatings in NaCl solution are similar. The initial corrosion damage of the PEO coating occurs at the regions in the vicinity of superficial pores and cracks in the outer layer of the coating. And then, corrosive medium further penetrates the PEO coating by diffusion with the increase of immersion time. The structure of the coating is destroyed with the decomposition of MgO in the coating. Due to the existence of zirconium oxides in great quantity, the deterioration rate of the coating is restrained to some extent.
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64458
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	刘锋. 镁合金表面新型微弧氧化涂层制备及性能表征研究[D]. 北京. 中国科学院金属研究所,2012.