Oxidation versus salt-film formation: Competitive adsorption on a series of metals from first-principles

Abstract Chloride ion is known to accelerate corrosion in numerous metallic systems, although it is not well understood why. Using first-principles methods, such as density functional theory, it is possible to directly compute the energy of adsorption for chloride, and, using a thermodynamic framework based on statistical mechanics, to predict the environmental conditions under which it may be anticipated that chloride could displace other ions (such as hydroxide, incipient oxide, or other potentially corrosion inhibiting species) from the bare metal and/or oxide surface. By reviewing the literature, as well as performing some original density functional theory calculations, the authors present in this work the dependence of the surface coverage for H 2 O, OH, O and Cl on the electrochemical potential, pH and chloride concentration for a number of metallic systems, including nickel, iron, magnesium and aluminum, to predict the fundamental surface processes that may related to the role of chloride in potentially interfering with processes such as repassivation and metal dissolution. It is found that under some conditions certain metals can possess zones of mutual stability where both chloride and hydroxide may be coadsorbed, whereas other metals appear to have intrinsic resistance to chloride adsorption. Variations among the available first-principles adsorption energies obtained from different sources indicate that uncertainty analysis is necessary moving forwards. An analogous approach could be taken to consider the interaction of chloride with clean and defective oxide and/or hydroxylated surfaces.