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

One of the fundamental challenges in understanding the early stages of corrosion pitting in metals protected with an oxide film is that there are relatively few techniques that can probe microstructure with sufficient resolution while maintaining a wet environment. Here, we demonstrate that microstructural changes in Al thin films caused by aqueous NaCl solutions of varying chloride concentrations can be directly observed using a liquid flow cell enclosed within a transmission electron microscope (TEM) holder. In the absence of chloride, Al thin films did not exhibit significant corrosion when immersed in de-ionized water for 2 days. However, introducing 0.01 M NaCl solutions led to extensive random formation of blisters over the sample surface, while 0.1 M NaCl solutions formed anomalous structures that were larger than the typical grain size. Immersion in 1.0 M NaCl solutions led to fractal corrosion consistent with previously reported studies of Al thin films using optical microscopy. These results show the potential of in situ liquid cell electron microscopy for probing the processes that take place before the onset of pitting and for correlating pit locations with the underlying microstructure of the material.