Formation of Li-Al LDH conversion layer on AA2024 alloy for corrosion protection

AA2024 aluminium alloy is widely used in the aerospace industry. However, it is known to be highly susceptible to localised corrosion, which is related to its complex microstructure, mainly the presence of numerous Cu-rich intermetallic particles. One of the effective solutions to increase their corrosion resistance relies on the formation of layered double hydroxide (LDH) based conversion coatings (CC). This investigation aims to understand how the conditions of in situ Li-Al LDH-CO32-/OH– CC growth affect their further protective ability. For that purpose, concentration of reactants (0.05–0.15 M Li2CO3), pH of the electrolyte (10–12), temperature (30–50 °C) and treatment time (15 min-30 h) were systematically varied. Additionally to that, NH4OH was applied as a chelating agent for the synthesis of Li-Al LDH-CO32-/OH– as well as for the pH control. The obtained protective coatings were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES), Raman spectroscopy, salt spray test (SST) and electrochemical impedance spectroscopy (EIS). Among all varied parameters, involvement of chelating agent was found to be the most efficient for the formation of a coating with the highest level of corrosion protection. This is attributed to the formation of soluble complexes between NH4OH and Cu, resulting in dissolution of the intermetallics. It also prevents further redeposition of Cu species and consequently, the formation of new highly active Cu cathodes on the AA2024 alloy surface.