Laterally-resolved formation mechanism of a lithium-based conversion layer at the matrix and intermetallic particles in aerospace aluminium alloys

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.