电场对铝表面低温氧化膜原位生长调控机制的研究

The properties of the oxide film on metal surface are quite different from those of its bulk materials. If we have better understanding of the atomic mechanism of growth and the microstructure, we can optimize the physicochemical properties of metal surface scientifically, such as corrosion resistance and electrical conductivity, by controlling the microstructure of the film. However, the deep understanding has been greatly restricted due to the self-limited growth (the limiting thickness is only several nanometers) at low temperatures...To solve the problem, from the view of the inner driving force of formation of film—Mott potential, the monocrystalline aluminum as the research object, new ideas are proposed by setting up an electric field (EF) on both sides of the film. First, The initial Mott potential can be enhanced by accelerating the reaction rate of the film/substrate interface; second the adsorption energy of oxygen on film can be increased by adjusting the surface composition. By applying multi-field coupling holder in Environmental Transmission Electron Microscopy (E-TEM), we aim to reveal atomic mechanisms of the growth and develop a new method for the controlling of performance of oxide film under EF (by varying parameters, including intensity, direction, loading duration, etc.) via: observing the in-situ growth of the film, quantizing the diffusion energy barrier of ions and Mott potential in the film and establishing the relationships among EF parameters, microstructures and film properties (corrosion resistance properties and electrical properties). Hopefully, we can rich the theory of the oxidation of metals at low temperatures, and establish a stable basis for further experiments for its application.

金属表面天然形成的氧化物膜与其氧化物块体材料有着迥异的性能，研究膜层的原子生长机制和微观结构，可以科学地调控膜层结构来优化金属表面的物化性能（如耐蚀性和导电性）。但低温氧化膜厚度自限（极限厚度仅为几个纳米）的特点，大大限制了目前对膜层生长机制和性能的深入研究。针对这一难点，本项目以铝单晶为研究对象,从膜层生长的内在驱动力——Mott电势出发，提出通过在氧化膜两侧建立外电场，以增大初始Mott电势和提高膜层表面氧分子吸附能为途径，增加膜层厚度的新思路。借助力电耦合的环境透射电镜测试系统，通过外电场调控（电场强度、方向、加载时间等）原位观察膜层生长，量化离子扩散能垒和Mott电势，建立膜层微观结构、性能（耐蚀性和电特性）与电场参数之间的理论关系，阐明Mott电势对膜层生长的影响机制，发展一种通过电场调控氧化膜性能的新方法。本项目的顺利实施有望丰富金属低温氧化理论，并为其应用奠定坚实的实验基础。

由于点蚀对金属结构材料的破坏性极大，点蚀的腐蚀机理一直是金属腐蚀领域最为关注的基本问题之一。关于点蚀发展的决速步骤——钝化膜破裂、亚稳态点蚀的发展、点蚀的稳定化，一直以来没有定论。一部分研究人员认为钝化膜破裂是点蚀的起点也是决速步骤，另一部分则认为点蚀的稳定化是评价合金耐蚀性能的关键。还有一部分研究人员提出，在弱腐蚀环境中前者是决速步骤，而在强腐蚀性环境中后者是决定因素。我们通过一种新型低温等离子体技术，在多种金属和合金表面原位生长出一层10纳米左右的金属氧化物薄膜，该薄膜能否显著降低金属和合金在硼酸缓冲溶液中的阳极电流密度。对于316L不锈钢，这一新的表面处理方式不但能将合金在0.6M氯化钠溶液中的阳极电流密度降低2个数量级，还能将点蚀电位提升400mV。与此同时，我们在对处理后的合金进行长期耐蚀性能评价时发现，由于该膜层在微观结构、化学成分、动力学生长(电场驱动)等方面与自然钝化膜非常接近，因此在长期腐蚀过程经历与自然钝化膜一样的上述三步骤，只是亚稳态阶段的自修复性能更弱。通过与基体（自然钝化膜覆盖的合金）对比，采用先进的统计学软件分析浸泡过程中（14周）点蚀的数量与尺寸的发展，发现316L合金优异的点蚀性能源于其自然钝化膜的自修复性，也即亚稳态点蚀的发展。因此，这一新方法的提出不但为金属防护提出一种不同于传统合金化和表面镀膜的新策略，还为揭示合金点蚀机理提供了新的观点与科学依据。