Real Time Nanoscale Observations of Localized Corrosion in Metallic Films

技术总结：本项目采用一种新的原位透射电子显微镜（TEM）方法来实时研究液-固界面的腐蚀反应。 使用电子透明膜之间密封的微流体室，纳米级腐蚀过程中发生的水和纯铝或铝合金之间的界面进行了研究。该计划的重点是在钝化膜的坑启动的基本问题。这种原位方法解决了在电子显微镜中检查钝化膜的长期存在的问题，其中已知真空通过在钝化和腐蚀过程的整个表征过程中保持水合表面来影响膜的结构和性质。该原位技术允许观察钝化膜的形成，并且还允许在真实的时间内引入坑诱导氯化物溶液。通过在开始腐蚀之前合成不同结晶度和厚度的钝化膜，可以实现由膜的固有表面结构和化学性质主导的成核事件与由金属的结构和化学性质控制的成核事件之间的差异。 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影响点蚀行为，但尚不清楚这是否是由于纳米级的电化学相互作用，或者元素是否也掺入钝化膜中，从而影响膜对局部击穿的固有电阻。原位TEM方法由X射线光电子能谱和俄歇电子能谱补充，利用直接连接到仪器的沉积和反应室。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GNP的5-10%），对国民经济造成巨大的消耗，对基础设施和运输安全产生重大影响。 美国国家科学与工程院最近发表的一项研究确定了一些重要的研究领域，这些领域应该被用来减轻对国家资源的腐蚀。 在这些领域确定的是局部腐蚀的问题，如点蚀，因为它的发生往往是不可预测的。该项目采用一种新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。 根据这项研究计划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上的钝化膜形成和局部击穿，但从该计划中获得的信息将适用于其他金属和合金系统中的局部腐蚀技术问题。 原位、真实的时间检查方法的优点是，它允许在实际腐蚀事件典型的环境条件下观察纳米级现象。除了提高对重要腐蚀现象的科学认识外，该项目还通过培养博士研究生，为国家的知识基础做出贡献。水平 在上述NAS/NAE对国家腐蚀研究需求的研究中，明确指出缺乏足够的训练有素的腐蚀科学家骨干。 该计划的另一个主要方面是，它使用本科实验室研究人员，使他们接触到先进的研究方法，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.