For decades, corrosion and corrosion inhibition of high-strength aluminium alloys have been studied indirectly and separately using conventional electrochemical, spectroscopic, and microscopic approaches. However, early-stage processes are regulated at the nanoscopic levels, where microstructural heterogeneities influence local and dynamic electrochemical activities, and most current techniques lack adequate lateral and time resolution to untangle them. As an example, local corrosion in AA2024-T3 initiates with nanoscopic site-specific local degradation events that predominantly take place at surface intermetallic particles (IMPs) dispersed in the alloy matrix, eventually leading to pitting and intergranular forms of corrosion. Furthermore, the corrosion inhibition mechanisms in such a critical alloy are controlled by dynamic processes of heterogeneity/inhibitor interactions at the nanoscale. Since a complete knowledge of the sequential steps of corrosion initiation is critical for crafting effective inhibition strategies, this necessitates high resolution techniques as well as the facilities to record nanoscopic events in real time.

Transmission electron microscopy (TEM) is a technique for obtaining atomic/nanoscopic microstructural and chemical characteristics of materials, and it has extensively been applied to corrosion science. Apart from ex-situ and quasi-in-situ TEM studies, new technology improvements have enabled in-situ monitoring of morphological and even compositional changes in materials as a result of contact with liquid environments, an approach known as liquid phase transmission electron microscopy (LP-TEM) [1]. In spite of certain implementation challenges coming along, this has paved the way for a detailed spatially and time resolved understanding of corrosion and inhibition phenomena.

As pioneering attempts, we've combined ex-situ TEM, quasi-in-situ TEM, and in-situ liquid phase TEM approaches in several recent works to investigate local corrosion (inhibition) events in legacy AA2024-T3 [2-7]. This has given us additional insight into what happens from the onset of local corrosion to more advanced stages of in-depth propagation. We showed that intermetallic compounds' intrinsic electrochemical stability is a primary determinant of the kinetics of local IMP-induced corrosion. In essence, local corrosion initiation is triggered by preceding dealloying, which happens to both traditionally categorized cathodic and anodic IMPs [2-3]. Furthermore, the investigations provide mechanistic details about the early stages of dealloying for various IMPs affecting the local solution chemistry [7]. Besides, inhibitor/IMP interactions rely on such local activities steering inhibition mechanisms in emerging green inhibitor systems such as cerium and lithium-based technologies. In conclusion, detailed processes explaining local corrosion and corrosion inhibition of aerospace AAs are described at the nano- and microscale.

[1] A. Kosari, H. Zandbergen, F. Tichelaar, P. Visser, H. Terryn, A. Mol, Application of In Situ Liquid Cell Transmission Electron Microscopy in Corrosion Studies: A Critical Review of Challenges and Achievements, CORROSION, 76 (2020) 4-17.

[2] A. Kosari, H. Zandbergen, F. Tichelaar, P. Visser, P. Taheri, H. Terryn, J.M.C. Mol, In-situ nanoscopic observations of dealloying-driven local corrosion from surface initiation to in-depth propagation, Corrosion Science, Volume 177, December 202, Page 108912.

[3] A. Kosari, F. Tichelaar, P. Visser, H. Zandbergen, H. Terryn, A. Mol, Dealloying-driven local corrosion by intermetallic constituent particles and dispersoids in aerospace aluminium alloys, Corrosion Science, 177 (2020) 108947.

[4] A. Kosari, F. Tichelaar, P. Visser, P. Taheri, H. Zandbergen, H. Terryn, J.M.C. Mol, Nanoscopic and in-situ cross-sectional observations of Li-based conversion coating formation using Liquid- Phase TEM, npj Materials Degradation, 5:40 (2021).

[5] A. Kosari, F. Tichelaar, P. Visser, H. Zandbergen, H. Terryn, J.M.C. Mol, Laterally-resolved formation mechanism of a lithium-based conversion layer at the matrix and intermetallic particles in aerospace aluminium alloys, Corrosion Science, 190 (2021) 109651.

[6] A. Kosari, P. Visser, F. Tichelaar, S. Eswara, J-N. Audinot, T.Wirtz, H. Zandbergen, H. Terryn, J.M.C. Mol, Cross-sectional characterization of the conversion layer formed on AA2024-T3 by a lithium-leaching coating, Applied Surface Science, 512 (2020) 145665.

[7] A. Kosari, M. Ahmadi, F. Tichelaar, P. Visser, Y. Gonzalez-Garcia, H. Zandbergen, H. Terryn, J.M.C. Mol, Editors’ Choice—Dealloying-Driven Cerium Precipitation on Intermetallic Particles in Aerospace Aluminium Alloys, Journal of The Electrochemical Society, 168 (2021) 041505.