(Invited) In-Situ Liquid Phase TEM Studies of Corrosion Initiation in Aluminium Alloys
Authors: Kosari Ali , Zandbergen Henny , Tichelaar Frans , Visser Peter , Taheri Peyman , Terryn Herman Albert , Mol Arjan 
Published: December 2020

Aluminium alloy 2024-T3 (AA2024-T3), widely applied in aerospace industries, may suffer from severe localized corrosion as a result of its relatively complex and heterogeneous microstructure. The corrosion initiation in such a critical alloy is governed by dynamic processes that take place at the nanoscale. A thorough understanding of the sequential stages of corrosion initiation is of pivotal importance to developing efficient inhibition strategies and demands high resolution techniques along with the ability of real-time recording of nanoscopic events.

However, to date, it has not yet been possible to unambiguously show the morphological and electrochemical characteristics during local corrosion of AA2024-T3. In particular, local corrosion initiation linked to nano-/microstructural heterogeneities, intermetallic particles (IMPs), take place relatively fast and at the nanoscopic scale, rendering the study of the early stages of corrosion experimentally virtually impossible until now. Herein, we show, for the first time, the dynamic evolution of site-specific local corrosion of AAs from early surface initiation to depth propagation at the nanoscale using a dedicated in-situ liquid phase-transmission electron microscopy (LP-TEM) experimental facility.

Transmission electron microscopy (TEM), capable of revealing microstructural and compositional variations in alloys at atomic and nanoscopic scale, has been applied widely in corrosion studies but mostly ex-situ and quasi in-situ. These approaches are not straightforward as artefacts and contaminations might be introduced into the system during the ex-situ and quasi in-situ experiments, including dehydration of surfaces and therefore may not fully represent the real conditions. With recent technological advances, thanks to the development of dedicated microelectronic mechanical systems (MEMS), it is now possible to fabricate components like nanoreactors (NRs) to study complicated corroding systems at the nanoscale in-situ. In fact, NRs employed in TEM studies enable to monitor morphological and compositional evolutions in materials in-situ as a result of interaction with aggressive environments like a gas or liquid.

In this study, we put efforts into providing direct evidence for the nanoscopic role of intermetallics in AA2024-T3 localized corrosion through quasi in-situ TEM and in-situ LP-TEM approaches. Quasi in-situ studies were implemented by intermittently exposing the Argon ion-milled thin samples to 0.01 M NaCl solution. For in-situ LP-TEM, the real TEM specimens (lamellae) were first fabricated out of regions of interest with a FEI Helios focused ion beam. Then, the lamellae were transferred successfully to home-made NRs using an easy-lift technique. The NRs, equipped with an electrochemical set-up, were specially designed for corrosion studies. A Cs-corrected FEI Titan TEM was employed to perform real-time studies in scanning TEM (STEM) mode. Although a good resolution is still a major challenge for in-situ LP-TEM, the results revealed that intermetallic phases regardless of their types are active and prone sites to localized corrosion attack themselves and the observations finally show and elucidate initial stages of IMP-induced pitting corrosion and copper redistribution mechanisms in AA2024-T3.