Corrosion and Corrosion Inhibition Studies of Aerospace Aluminium Alloys at the Nanoscale using TEM Approaches

Abstract

For many decades, corrosion and corrosion inhibition of high-strength aluminium alloys have been studied indirectly and through traditional and separately performed electrochemical, spectroscopic and microscopic techniques. These approaches employed to date commonly lack sufficient lateral and time resolution to unravel early-stage events which is controlled at the nanoscopic levels at which microstructural heterogeneities actually steer local and dynamic electrochemical activities. Besides, techniques with appropriate resolution like transmission electron microscopy (TEM) have been applied to the field, but carried out ex-situ, normally providing no detailed on-site time-resolved information to investigate distinctive-but-consecutive stages of corrosion and corrosion inhibition phenomena. That is why theories of relevance are established through bridging and linking separately-obtained information and therefore are described in rather stochastic than deterministic terms. This is particularly the case for the legacy alloy AA2024-T3 which is prone to complicated forms of local corrosion resulting from extremely complex and heterogeneous local microstructures. Local corrosion in AA2024-T3 is site-specific where complicated local degradation events predominantly take place at surface intermetallic particles (IMPs) dispersed in the alloy matrix and eventually lead to pitting and intergranular forms of corrosion. Thus, the detailed understanding of space- and time-resolved local corrosion mechanisms of engineered microstructures is of pivotal importance to developing reliable and active protection strategies. However, despite the high demand for time- and space-resolved mechanistic information of local corrosion, it has not yet been possible to unambiguously define the morphological and micro-electrochemical characteristics during local corrosion and corrosion inhibition owing to extremely demanding experimental challenges. Nevertheless, this thesis put efforts into carrying out dedicated TEM experimental approaches including in-situ liquid-phase, quasi in-situ and ex-situ TEM to provide time-resolved and direct nanoscopic evidence of local corrosion and corrosion inhibition processes from early surface initiation to an advanced stage of propagation.