Studies of Corrosion of Al Thin Films using Liquid Cell Transmission Electron Microscopy

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.

Metastable Structures in Al Thin Films Before the Onset of Corrosion Pitting as Observed using Liquid Cell Transmission Electron Microscopy

“…Figure 3 shows a series of micrographs taken from Al films exposed to 0.01 M NaCl for a total of about 15 h. Numerous, randomly dispersed blisters, a few hundred nanometers in size, were found over the entire surface of the film. These features are considerably larger and different in contrast from the pits that were observed in Al films from our earlier study (Chee et al, 2013). We believe these features to be blisters because they show up clearly in lighter contrast even when the cell is filled with liquid (Fig.…”

“…It should be noted here that corrosion experiments are very sensitive to the purity of the water used. Our earlier experiments (Chee et al, 2013) showed that Al thin films will corrode in single distilled water. For the experiments reported here, we purchased Milli-Q (Millipore Corporation) water from a commercial vendor and opened bottles were refrigerated immediately after dispensing.…”

“…Images were collected using a Gatan DualVision 300W camera (1,300 × 1,024 pixels). The experimental technique has been described in detail elsewhere (Chee et al, 2012(Chee et al, , 2013. A schematic of the assembled microfluidic cell is given in Figure 1a.…”

Characterization of Nanomaterials Using In Situ Liquid-Cell Transmission Electron Microscopy: A Review

“…There have been many similar reports on various metals, − ,− oxides, − and alloys − ,− in recent years. Dynamic imaging observation of corroded nanoscale microstructure features in liquid-cell TEM has exciting application prospects.…”

Locally Condensed Water as a Solution for In Situ Wet Corrosion Electron Microscopy

“…Thus for practical applications, one needs, on the one hand, a relatively thick liquid layer (the controlled flow and a good supply of chemical species to prevent depletion) and, on the other hand, an extremely thin layer to allow high-resolution TEM. Removing the electrolyte (e.g., water) from the NR either by evacuating the liquid using a vacuum pump or using the segmented flow of water/water–ethanol in the liquid stream would be options to increase the resolution in in situ corrosion studies in TEM (Chee et al, 2013).…”