Real Time Nanoscale Observations of Localized Corrosion in Metallic Films

技术概述：本项目采用一种新颖的原位透射电子显微镜（TEM）方法实时研究液固界面的腐蚀反应。利用密封在电子透明膜之间的微流体室，研究了发生在水与纯铝或铝合金界面的纳米级腐蚀过程。该计划的重点是在钝化膜坑起始的基本问题。这种原位方法解决了一个长期存在的问题，即在电子显微镜下检查钝化膜，在钝化和腐蚀过程中，真空通过保持表面水化来影响膜的结构和性能。这种原位技术可以观察钝化膜的形成，也可以实时引入诱导坑的氯化物溶液。通过在开始腐蚀之前合成不同结晶度和厚度的钝化膜，可以区分由膜的内在表面结构和化学性质主导的成核事件和由金属的结构和化学性质控制的成核事件。还将研究Al-Li和Al-Cu合金膜，以了解合金元素与钝化膜的生长和稳定性以及随后的点蚀反应的相互作用。虽然在微米尺度上已知合金元素会影响点蚀行为，但尚不清楚这是由于纳米水平上的电化学相互作用，还是由于这些元素也被纳入钝化膜中，从而影响了膜对局部击穿的固有电阻。原位透射电镜方法辅以x射线光电子能谱和俄歇电子能谱，利用直接连接在仪器上的沉积和反应室。非技术总结：腐蚀对国家基础设施和制成品的影响是巨大的（估计占国民生产总值的5-10%），对国民经济造成了巨大的消耗，对基础设施和运输安全产生了重大影响。美国国家科学院（National Academies of Science and Engineering）最近发表的一份研究报告确定了一些重要的研究领域，以减轻对国家资源的腐蚀流失。其中确定的问题是局部腐蚀，如点蚀，因为它的发生往往是不可预测的。本项目采用一种新颖的原位透射电子显微镜（TEM）方法，在纳米尺度上实时研究液固界面的腐蚀反应。从这个研究项目中获得的基础知识可能有助于理解并随后减轻局部腐蚀反应。虽然该计划主要解决铝和铝合金膜的被动膜形成和局部击穿问题，但从该计划中获得的信息将适用于其他金属和合金系统的局部腐蚀技术问题。现场实时检测方法的优点在于，它允许在实际腐蚀事件的典型环境条件下观察纳米尺度的现象。除了推进对一种重要腐蚀现象的科学认识外，该项目还通过培养博士级研究生，为国家的知识基础做出了贡献。在前面提到的NAS/NAE对国家腐蚀研究需求的研究中，我们清楚地认识到缺乏足够的训练有素的腐蚀科学家队伍。该计划的另一个主要方面是，它使用本科实验室研究人员，使他们接触先进的研究方法，并有机会参与理解和可能减轻一个重要的技术问题。

TECHNICAL SUMMARY:This project employs a novel in-situ transmission electron microscopy (TEM) method to study corrosion reactions at liquid-solid interfaces in real-time. Using a micro-fluidic chamber sealed between electron transparent membranes, nanoscale corrosion processes that occur at interfaces between water and pure aluminum or aluminum alloys are studied. The program focuses on the fundamental issue of pit initiation in passivating films. This in-situ method addresses a long standing problem of examining passive films in electron microscopes where vacuum is known to affect the structure and properties of the films by maintaining hydrated surfaces throughout the characterization of the passivation and corrosion processes. This in-situ technique allows the observation of the formation of passive films and also allows the introduction of pit inducing chloride solutions in real time. By synthesizing passivating films of varying crystallinity and thickness prior to initiating corrosion, differentiation between nucleation events dominated by the intrinsic surface structure and chemistry of the films, and those controlled by the structure and chemistry of the metal can be accomplished. Alloy films of Al-Li and Al-Cu, will also be studied to understand the interaction of the alloying elements with the growth and stability of the passivating films and the subsequent pitting reactions. While it is known at the micrometer scale that alloying elements affect pitting behavior, it is not known if that is due to electrochemical interactions at the nanometer level or if the elements are also incorporated into the passive films, thus affecting the intrinsic resistance of the films to localized breakdown. The in-situ TEM methods are supplemented by x-ray photoelectron spectroscopy and Auger electron spectroscopy, utilizing deposition and reaction chambers directly attached to the instruments.NON-TECHNICAL SUMMARY:The impact of corrosion upon the national infrastructure and upon manufactured goods is enormous (estimated at 5-10% of GNP), creating a substantial drain on the national economy and having major impacts upon infrastructure and transportation safety. A recent study published by the National Academies of Science and Engineering identified a number of important research areas that should be pursued to alleviate the drain of corrosion on the nation's resources. Among those areas identified is the problem of localized corrosion such as pitting, since its occurrence is often unpredictable. This project employs a novel in-situ transmission electron microscopy (TEM) method to study corrosion reactions at liquid-solid interfaces at the nano-scale in real-time. The fundamental knowledge derived under this research program may help to understand, and subsequently to mitigate, localized corrosion reactions. While this program primarily addresses passive film formation and local breakdown on aluminum and aluminum alloy films, the information obtained from the program will be applicable to the technological problem of localized corrosion in other metal and alloy systems. The advantage of an in-situ, real time examination method is that it allows the observation of nanometer-scale phenomena under environmental conditions typical of actual corrosion events. In addition to advancing scientific knowledge of an important corrosion phenomenon, this project contributes to the nation's intellectual base by educating graduate students at the Ph.D. level. In the aforementioned NAS/NAE study of the nation's needs in corrosion research, the absence of an adequate cadre of trained corrosion scientists was clearly identified. Another major aspect of the program is that it uses undergraduate laboratory researchers, exposing them to advanced research methods and the opportunity to be involved in the understanding and possible mitigation of an important technical problem.