DFT Modeling of Corrosion Inhibition by Organic Molecules: Carboxylates as Inhibitors of Aluminum Corrosion

Abstract

In order to model the adsorption of organic corrosion inhibitors on ultrathin oxide layers on aluminum, the formation of a self-assembled layer of gallic acid, the smallest tannin molecule, at the surface of an Al2O3 ultrathin layer supported by Al(111) was studied using periodic density functional theory including dispersion forces (DFT-D). A dense self-assembled layer (SAM) was formed with a density of 3.6 molecules/nm2, adsorbed in a monodentate way by ligand exchange, with an adsorption energy of 2.27 eV/molecule. The electronic analysis shows that the oxide levels are significantly stabilized by the covalent bonding with the COOH moiety of the molecule. The permittivity of the surface layers falls from 109 for the ultrathin Al2O3 film to 4 for the joint oxide and organic film. The permittivity of the surface constrained gallic acid SAM is 1.4, a value slightly lower than that of the free hexagonal compact, SAM (2.4). The organic layer forms an efficient barrier against electron transfer to dioxygen, thus providing good cathodic inhibition. The potential drop at the edge of the oxide and organic layer is found to be around 2 V. Studies of benzoates derivatives (benzoic acid and nitrobenzoic acid) show that the potential drop and the electronic work function difference (with respect to the nonfunctionalized oxide film) induced by the organic SAM increase linearly with the charge of the adsorbed molecules. The reported results give indications for the rational design of corrosion inhibitors.