(Invited) Cryo-Based FIB/SEM Characterization of Salt Film on Metal

Abstract

Metal salt films play a critical role in high rate dissolution processes such as within an active pit.1-5 To better understand the film growth mechanism, the salt film structure and composition must be characterized with high resolution, which however, is challenging owing to the dynamic nature of the dissolving interface. In this work, a novel cryogenic specimen preparation approach to preserve the pit solution and salt film was introduced, allowing ex situ characterization of the native structure formed on the dissolving metal surface. The salt film formed on a 304 stainless steel one-dimensional artificial pit electrode was preserved by flash freezing in liquid nitrogen while under an external driving potential, processed using cryo-based focused ion beam, and investigated with cryo-based scanning electron microscopy cross sectional imaging. The salt film exhibits a porous structure, with many voids and gaps between salt crystallite clusters, forming channels to provide ionic connection between the metal/film and film/solution interfaces. A spatial separation of Cr and Fe was observed in the salt film using energy dispersive X-ray spectroscopy, while Ni was found to follow the distribution of Fe, indicating the existence of co-precipitated salt containing Fe2+ and Ni2+. Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584.Reference G. S. Frankel, T. Li, and J. R. Scully, Journal of the Electrochemical Society, 164 (4), C180-C181 (2017).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (9), C484-C491 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (11), C762-C770 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (6), C115-C124 (2019).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (11), C3341-C3354 (2019).