Aerospace aluminum alloys (AA) are prone to corrosion, necessitating effective, environmentally friendly protective coatings. Hexavalent chromium-based coatings, while effective, are carcinogenic, prompting the search for sustainable alternatives. Lithium salts have emerged as a promising candidate, offering the advantage of leaching from organic coatings to damaged areas, forming a protective conversion layer upon contact with the aluminum substrate. This conversion layer typically exhibits a multi-layered structure with varying barrier properties, comprising columnar, porous, and dense sublayers. Understanding the formation mechanism of this multi-layered structure is crucial for optimizing the corrosion protection offered by lithium-leaching technology. While the structure and composition of the fully developed conversion layer are relatively well-understood, detailed knowledge of the formation sequence from initial nucleation to the final layered arrangement is lacking. This paper addresses this gap by utilizing in-situ liquid-phase transmission electron microscopy (LP-TEM) to directly observe nanoscopic morphological changes during the conversion process with high temporal and spatial resolution. The challenges in applying LP-TEM to this problem are addressed via careful specimen preparation, utilizing sandwiched thin specimens to control the oxidation direction and prevent rapid dissolution of the sample.

Numerous chromate-free conversion coating systems have been investigated, including those based on trivalent chromium, rare earth salts, zirconium/titanium systems, and transition metal oxyanions. Lithium salts have shown considerable promise, functioning both as conversion bath chemicals and as leachable inhibitors within organic coatings. Lithium salts, incorporated into organic coatings, leach into damaged areas (scribes) of the coating upon exposure to corrosive environments. The anionic part of the lithium salt (e.g., carbonate) establishes an alkaline environment (pH 9–11), causing instability in the aluminum oxide passive layer and initiating corrosion. Lithium ions stabilize the corrosion products, leading to the formation of the characteristic multi-layered conversion coating. Prior studies have characterized the morphology and composition of the fully formed conversion layer, showing a columnar outer layer, a porous intermediate layer, and a dense inner layer. The dense inner layer, with its low porosity, provides the greatest contribution to corrosion protection. However, the precise sequence of formation events and the mechanisms involved remained unclear.

The study employed liquid-phase scanning transmission electron microscopy (LP-STEM) to visualize the conversion process in real-time. A crucial aspect of the methodology was the preparation of sandwiched TEM specimens. AA2024-T3 lamellae, approximately 300-400 nm thick, were fabricated using a focused ion beam (FIB)/scanning electron microscope (SEM). These lamellae were then sandwiched between two 30 nm thick layers of tetraethyl orthosilicate (TEOS), an inert material, to control the oxidation direction and prevent rapid dissolution of the specimen. This approach ensures that the conversion process occurs in the plane of the specimen, allowing for cross-sectional observation. The sandwiched lamella was carefully transferred to a chip, with three sides masked with additional TEOS, leaving only one edge exposed to the electrolyte (0.01 M NaCl + 0.01 M Li2CO3, pH 10.6). The experiments were performed using a Cs-corrected Thermo-Fisher Titan 300 kV TEM in STEM mode, employing bright-field (BF) and high-angle annular dark-field (HAADF) detectors. A low beam current (0.1 nA) was used to minimize beam-induced artifacts. Ex-situ analyses, including SEM and TEM with energy-dispersive X-ray spectroscopy (EDS), were performed on bulk samples subjected to similar conversion treatments to complement the in-situ LP-STEM observations. This combination of in-situ and ex-situ analyses provides a comprehensive understanding of the conversion process.

In-situ LP-STEM observations revealed a dynamic conversion process. Initially, uniform dissolution of the alloy matrix was observed. After approximately 42 minutes, precipitation of the conversion layer began in the form of tiny needles at the solution/alloy interface, particularly at grain boundaries. These needles grew in length and number as the corrosion process progressed. A porous layer formed subsequently, followed by a dense, less porous layer closest to the alloy matrix. The kinetics of dissolution slowed down as the conversion layer developed. HAADF-STEM images showed that nanoscopic intermetallic particles (IMPs) also underwent local corrosion, with some completely dissolving while others left behind bright remnants. Ex-situ analyses on bulk samples confirmed the formation of a multi-layered conversion layer with columnar, porous, and dense sublayers. EDS analysis showed the presence of aluminum, oxygen, and copper in the conversion layer; lithium detection was hindered by the specimen thickness. The formation of the conversion layer was found to be dependent on the electrolyte volume; a larger volume delayed conversion layer formation, likely due to a need for greater substrate dissolution to achieve supersaturation of Al(OH)4− ions. The results illustrate the sequence of the conversion process: Initial dissolution of the alloy matrix, needle-like precipitation, formation of a porous layer, and ultimately a dense layer. The rate of the precipitation process increases over time.

The study provides a detailed mechanistic understanding of Li-based conversion layer formation. The key factor driving conversion layer formation is the supersaturation of Al(OH)4− ions at the alloy matrix/solution interface. This supersaturation is achieved through active dissolution of the alloy matrix, particularly at grain boundaries and IMPs. In the sandwiched specimens, the confinement between TEOS layers enhances local supersaturation, leading to faster conversion layer formation compared to bulk samples. The multi-layered structure arises from the dynamic interplay between dissolution and precipitation rates. The initial columnar morphology transitions to a porous layer and finally to a dense layer as the dissolution rate slows. The dense layer, with its superior barrier properties, ultimately limits further penetration of corrosion. The findings from the sandwiched thin specimens correlate well with observations from bulk samples, demonstrating the validity of the approach. The study underscores the critical role of supersaturation in the formation of stable and protective Li-incorporated corrosion products.

This study provides the first in-situ nanoscopic observations of Li-based conversion layer formation on AA2024-T3 using LP-TEM with a novel sandwiched specimen preparation method. The findings highlight the critical role of Al(OH)4− supersaturation and the dynamic interplay between dissolution and precipitation in the formation of the multi-layered structure. The methodology presented offers a valuable tool for understanding complex surface conversion processes and optimizing the performance of environmentally friendly corrosion protection technologies. Future studies could explore the influence of different lithium salts and solution chemistries on the conversion process. Investigating the role of lithium within different sublayers using advanced characterization techniques would provide further insights.

The LP-TEM experiments were conducted under conditions that differ from those encountered in typical bulk corrosion testing. Specifically, the limited sample size and the confinement of the electrolyte between TEOS layers may influence the local solution chemistry and potentially affect the kinetics of the conversion process. While ex-situ analyses on bulk samples support the findings, direct in-situ observation of lithium incorporation within the conversion layer was limited by the thickness of the sandwiched specimens. Future investigations should explore the long-term corrosion behavior of the conversion layer formed under various conditions.