Abstract
Lithium salts have been proposed as promising environmentally friendly alternatives to carcinogenic hexavalent chromium-based inhibitors for the corrosion protection of aerospace aluminium alloys (AAs). Incorporated into organic coatings, lithium salts are released at damaged locations to establish a conversion layer in which distinct sublayers have different barrier characteristics. Thus, detailed knowledge on the sequence of formation events from the early stages of nucleation towards the final multi-layered arrangement is essential for developing and optimising lithium-leaching technology for protective coatings. Here, liquid-phase-transmission electron microscopy (LP-TEM) is employed to observe nanoscopic morphological evolutions in situ during the lithium-based conversion process of AA2024-T3. Thanks to dedicated preparation of delicate sandwiched TEM specimens allowing us to explore the events cross-sectionally, we provide real-time direct mechanistic information on the conversion process from the initiation to an advanced growth stage. In parallel, we perform supplementary ex situ SEM and TEM investigations to support and validate the LP-TEM findings. The unprecedented experimental approach developed and executed in this study provides an inspiring base for studying also other complicated surface conversion processes in situ and at the nanoscopic scale.
Highlights
Aerospace aluminium alloys (AAs) are highly susceptible to local corrosion attack[1], demanding reliable, efficient and active protection during their service life[2]
The porouslike layer becomes darker in contrast as it gets closer to the alloy matrix
It should be noted that here we merely report on the morphological evolutions; the sublayers might undergo structural and compositional modifications at different formation stages, but the current technique is unable to reveal them at that time itself
Summary
Aerospace aluminium alloys (AAs) are highly susceptible to local corrosion attack[1], demanding reliable, efficient and active protection during their service life[2]. The highly complex microstructure of AAs, leading to enhanced mechanical properties[3], brings along complicated corrosion mechanisms that make effective inhibition challenging[4,5,6,7,8,9]. Reliable active protection has been fulfilled by the application of hexavalent chromate-based conversion and organic coatings[10,11,12,13]. Several chromate-free conversion systems that are potentially worthy of consideration for industrial applications have been introduced, thanks to dedicated and extensive research[15]
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