Abstract
Lithium leaching coatings have recently been developed as eco-friendly active corrosion protection technology for aerospace aluminium alloys (AAs) by the formation of a conversion layer at coating defects. While general conversion layer formation characteristics were studied and reported before, here we study the local layer formation process with sub-micron resolution at and around intermetallic particles (IMPs) in AA2024-T3. Top- and cross-sectional-view morphological electron micrograph observations along with open circuit potential (OCP) measurements are performed, mimicking coating defect conditions upon lithium carbonate leaching from the coating matrix. The results revealed five stages of the conversion process in which the alloy matrix and different IMPs evolve morphologically, compositionally, and electrochemically. Besides, we found a correlation between the OCP response of the AA2024-T3 system and the morphological and compositional evolutions of the alloy matrix and IMPs at different stages of exposure. Passive layer and alloy matrix dissolution leading to surface Cu-enrichment and S-phase dealloying occur at early stages of exposure. They precede the formation of a columnar layer on the alloy, followed by the establishment of a dense-like layer at the final stage. Dealloying of Al2CuMg can assist the conversion process by providing local supersaturation. Through complementary experiments in a sodium carbonate solution and besides X-ray diffraction analysis, we found out that lithium plays a critical role in stabilising the corrosion product throughout the conversion process.
Highlights
In recent years, lithium salts have drawn high attention as hex avalent chromium-free corrosion inhibitors due to a passivating char acter for a variety of aluminium alloys (AAs) [1,2,3,4,5]
While our previous works provided a general lithium-based con version layer formation mechanism, here we unravelled the laterally resolved formation mechanisms of the layer at the matrix and intermetallic particles (IMPs) of AA2024-T3 in a condition mimicking coating defects when lithium carbonate is used as the leaching inhibitor
We found that AA2024-T3 does not acquire passivation in early exposure times up to 800 s (Stage I and II)
Summary
Lithium salts have drawn high attention as hex avalent chromium-free corrosion inhibitors due to a passivating char acter for a variety of aluminium alloys (AAs) [1,2,3,4,5]. Drewien et al confirmed the formation of Li-Al layered double hydroxide (Li-Al LDH) on AA1100 through a conversion layer process in a lithium carbonate-lithium hydroxide solution (pH 11–12) [8] Another major breakthrough was established many years later in 2010 when Visser and Hayes showed that lithium salts can be incorporated into organic coat ings to provide active corrosion protection [9]. In this scheme, incor porated lithium salts [10,11] are released with a long-range throwing power at damaged coating locations [12], establishing an alkaline environment of approximately pH 10 [13]. Thereafter, the studies were dedicated to characterising the generated layer in artificial scribes subjected to neutral salt spray (NSS) exposure [15,16]
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