First Principles, Explicit Interface Studies of Oxygen Vacancy and Chloride in Alumina Films for Corrosion Applications

Abstract

Pitting corrosion is a much-studied and technologically relevant subject. However, the fundamental mechanisms responsible for the breakdown of the passivating oxide layer are still subjects of debate. Chloride anions are known to accelerate corrosion; relevant hypotheses include Cl insertion into positively charged oxygen vacancies in the oxide film, and Cl adsorption on passivating oxide surfaces, substituting for surface hydroxyl groups. In this work, we conduct large-scale first principles modeling of explicit metal/Al 2 O 3 interfaces to investigate the energetics and electronic structures associated with these hypotheses. The explicit interface models allow electron transfer that mimics electrochemical events, and the establishment of the relation between atomic structures at different interfaces and the electronic band alignment. For multiple model interfaces, we find that doubly charged oxygen vacancies, which are key ingredients of the point defect model (PDM) often used to analyze corrosion data, can only occur in the presence of a potential gradient that raises the voltage. Cl − insertion into oxide films can be energetically favorable in some oxygen vacancy sites, depending on the voltage. We also discuss the challenges associated with explicit DFT modeling of these complex interfaces.

Model Study of Penetration of Cl− Ions from Solution into Organic Self-Assembled-Monolayer on Metal Substrate: Trends and Modeling Aspects

“…The electric potentials φ 11 and φ 12 involve infinite lattice sums and therefore the dependence of the interaction energies, Eqs. [10] and [13], on the interlayer distance is not trivially obvious. If one is only interested in the main trend then these two equations can be simplified by approximating the lattice sums by integrals (for the corresponding derivation, see Appendix).…”

“…This implies that approximating Eqs. [10] and [13] of the parallel plate capacitor * (Fig. 2), where the electric potential and potential energy within a given dielectric-i are linear with the slope proportional to σ/ε i .…”

“…Electrostatic interaction energy for two infinite parallel layers of point charges near the interface between two dielectrics are given by Eqs. [10] and [13] that are defined in terms of electric potentials φ 11 and φ 12 , i.e. : q + φ 12 (0, 0, z 0 ) + q − φ 12 (0, 0, z)…”

“…While the fundamental mechanisms responsible for pitting corrosion that involve localized passive film breakdown remain debated, [9][10][11] the mechanism of chloride adsorption and penetration into the oxide film on Al is supported by experimental evidence. 12 Moreover, even mechanisms that do not attribute to adsorption and penetration of chlorides the seminal cause of passivation breakdown, such as the point defect model (PDM) or the film breakdown mechanism (FBM), include, at some point, either the penetration of Cl − into the oxide film (PDM)-an aspect recently studied computationally using DFT 13 -or the adsorption of chlorides at places where the passive film is weakened (FBM). Here also, this aspect has been recently investigated using DFT, showing a clear trend between a chloride susceptibility index and the repassivation potential.…”

Anion-Assisted Delivery of Multivalent Cations to Inert Electrodes

“…Moreover, the inhomogeneous polarization of the graphene electrode induced by the hard Mg dication remains small even at its shortest distance to the electrode (see SI Figure S4). More rigorous calculations would imply the simulations at a constant electrode potential , to account for the effect of the induced charge on graphene and its localization. However, given the size of the unit cell, we expect no significant changes if one to use more advanced treatments of the Coulomb interactions (see SI for discussion).…”