Studies of Corrosion of Al Thin Films using Liquid Cell Transmission Electron Microscopy
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Studies of Corrosion of Al Thin Films using Liquid Cell Transmission Electron Microscopy See Wee Chee1, Frances M. Ross2, David Duquette1 and Robert Hull1 1 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A. 2 IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, U.S.A.
ABSTRACT A fundamental understanding of the processes that occur during early stages of corrosion is traditionally limited by the dearth of techniques that probe the liquid-solid interface with both high spatial resolution and microstructural detail such as grain size and orientation. Here, we demonstrate that with a microfluidic liquid flow cell holder, we can track the progress of corrosion in situ in Al thin films with transmission electron microscopy (TEM). To mitigate the loss of resolution caused by imaging through liquid, we developed a method in which the liquid is temporarily de-wetted from the entire windowed area by switching the liquid stream from pure water to a mixture of ethanol and water. In the de-wetted region, we then collected images of the film microstructure with high spatial resolution over regular intervals while maintaining a low electron flux over the imaged area to minimize beam-induced effects. For as-deposited films, we find that the corrosion progresses in a fractal manner, consistent with reported behavior for films studied in water with low iron and chloride concentrations. For films that were subjected to rapid thermal annealing, we observe a higher density of pitting events, which we attribute to defects created by thermal stress in the oxide film. Furthermore, we observe that the pits can form at multiple locations in a single grain and are not confined to grain boundaries.
INTRODUCTION At the basic level, most modern engineering metal alloys are protected from environmental corrosion by forming a passivating layer, usually in the form of the metal oxide. Pitting is a particularly insidious form of corrosion that attacks metals and alloys, especially those protected by passive films. Pit initiation is generally believed to be preceded by the localized breakdown of the protective oxide, but why the oxide fails at certain sites and how that location relates to the underlying microstructure of the metal or alloy remains poorly understood [1]. One reason is that relatively few characterization techniques can probe the corrosion of materials at both the microstructural level and in the aqueous environment. At present, the most important techniques for in situ imaging of the liquid-solid interface include synchrotron radiation and variants of scanning probe microscopy. In this paper, we will present our early results using in situ liquid cell transmission electron microscopy (LC-TEM) to study the relationship between pitting corrosion and microstructure. Transmission electron microscopy (TEM) is the pre-eminent tool for studying the microstructure of materials due to its high spatial resolution and analytical capabilities. However, to look at materials immer
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