Mechanistic Studies on the Surface Reactions of Aluminum–Lithium Alloy Oxides

Aluminum–lithium (Al-Li) alloys have attracted great interests in aerospace, solid propellants, and explosives industries. However, the practical use of Al-Li remains challenging because of instability during storage. Poor corrosion resistance and passivation of the Al-Li alloys are ascribed to the surface cracking of the oxidation layer. Using a variety of ab initio quantum chemistry methods, the cracking mechanisms of Al/Li/O oxides induced by H2O, LiOH, and Li2O have been revealed theoretically by means of Al4O6 and Al8O12 cluster models. All six reactions are shown to be highly exergonic dissociative adsorption processes. In terms of the Gibbs free energy profiles, the adsorption energy and reactivity are in the order Li2O > LiOH > H2O, which is independent of sizes of clusters. However, cluster size does have an impact on the adsorption energies of H2O, LiOH, and Li2O. For the reactions of H2O, the energetic routes are dominated by proton transfer and followed by the O-Al bond cleavage to generate trench or protrusion structures. However, proton transfer is inhibited considerably by the O-Li interaction. As the Li atom migrates to form various Li-O coordinates along with the O-Al bond cleavage, the alumina clusters are cracked stepwisely through the interlayer O-Al bond association or displacement. The edge Al sites are always less reactive than the topmost surface Al. The Li atoms are prone to migrate from the edge to the surface as accompanied by the O-Al bond rearrangement. Present calculations provide a deep understanding of the oxidation behavior of the Al-Li alloys and present new insights towards increasing storage stability.