The corrosion inhibiting effect of lithium carbonate has been studied over the pH range relevant for atmospheric corrosion conditions at the polymer/metal interface. It is demonstrated that lithium carbonate offers optimum corrosion protection under moderately alkaline conditions attributed to the formation of a protective layer identified as aluminum-lithium layered double hydroxide. It is proposed that an active-to-passive transition occurs in the presence of lithium carbonate under moderately alkaline conditions where anodic dissolution of the aluminum matrix is an important step prior to the formation of the protective layer. After prolonged exposure, the protective layer is uniformly formed on the surface of AA2024-T3 and lithium containing AA2198-T8 covering both the matrix and active intermetallic particles thus hindering further corrosion reactions.@en

It has been demonstrated previously that lithium salts containing organic coatings offer effective active corrosion protection on AA2024-T3 alloys when exposed to neutral salt spray test for 168 hours [1, 2]. In the presence of a coating defect the lithium salts leach from the organic coating to the exposed metal substrate increasing the local pH to moderate alkaline conditions followed by the formation of a protective layer within the defect site [2, 3]. The lithium accumulation at the defect site and hence the formation of the protective layer is influenced by the inhibitor loading and solubility in the coating as well as the coating defect size. Three different surface compositions of the lithium protective layer have been recently identified as lithium-based layered double hydroxide (Li-LDH), lithium mixed pseudo-boehmite (Li-PB) and pseudo-boehmite (PB) each depending on the lithium leaching rate and the coating defect size [4]. For small defect sizes a high lithium concentration is hypothesized to result in the generation of Li-LDH. Furthermore, Li-PB is identified within moderate defect sizes and moderate inhibitor concentration and PB is generated within large defect sizes where low lithium concentrations are expected. On this basis, this paper describes the lithium active corrosion protection on aerospace aluminium alloys as a function of the inhibitor concentration. In doing so, commercial AA2198-T8 and AA2024-T3 aluminium alloys are immersed for 24 hours in lithium salt containing aqueous solutions at systematically varied lithium salts concentration from 10-6 M to 10-1 M. The alloys are then transferred to 10-1 M aqueous NaCl solution at near neutral pH conditions and either LPR measurements are performed as a function of time over 100 hours or anodic polarization is performed. The surface composition of the alloys after 24 hours immersion in lithium containing aqueous solution is also characterized. The surface composition and the electrochemical characteristics of each lithium passive layer generated is determined as a function of the lithium concentration. The surface composition of the passive layer ranges from PB to Li-PB and Li-LDH with increasing inhibitor concentration. The polarization resistance obtained as a function of time is correlated to the corrosion rate of each generated passive layer thus demonstrating the corrosion protection performance of Li-LDH, Li-PB and PB. The generation and corrosion protection performance of each passive layer is also demonstrated under relatively thin electrolyte layers mimicking the exposure of a coating defect and the leaching of lithium salt from the coating matrix under atmospheric conditions. @en