Insight at the atomic scale of corrosion inhibition: DFT study of 8-hydroxyquinoline on oxidized aluminum surfaces

8-Hydroxyquinoline (8-HQ) is a promising organic molecule for the corrosion protection of aluminum and its alloys in the replacement of chromate salts. On the aluminum surface, the presence of an oxide layer naturally formed can influence the inhibition efficiency which depends on molecule-surface interactions. In the present study, we performed quantum chemical calculations on native 8-HQ, tautomer and 8-Q (deprotonated, H-abstracted or radical) molecules, adsorbed on an oxidized aluminum surface (γ-Al₂O₃(111)/Al(111)). All species have the ability to interact strongly with the oxidized aluminum surface and can form stable and dense organic films. The bonding strength of different species of 8-HQ on oxidized aluminum surfaces is more favorable for 8-Q and tautomer species than for the native 8-HQ molecule. On the surface, the native 8-HQ molecule is physisorbed, forming H-bonds, in contrast to the tautomer and 8-Q species that show the predominance of chemisorption modes, involving both H-bonds and covalent bonds at the molecule/substrate interface. The dispersion energy significantly contributes to the adsorption mechanism and increases with increasing molecular surface coverage, due to attractive molecule-molecule interactions. Regardless of surface coverage and considered reaction mechanisms, the 8-Q species is able to enhance the stability of all aluminum sites, and thus to slow down the anodic reaction. In contrast, the native molecule and the tautomeric form have no significant effect or even weakened the stability of aluminum surface atoms.