Competitive Chloride Chemisorption Disrupts Hydrogen Bonding Networks: DFT, Crystallography, Thermodynamics, and Morphological Consequences

Abstract

The consequence for aqueous corrosion of chloride ions in an aqueous environment on the surface structure and thermodynamics of hydroxylated magnesia (001) and alpha-alumina (001) and (100) is analyzed using density functional methods. It is shown that there is competitive chemisorption between hydroxide and chloride, with the chloride disrupting the hydrogen bonding network on the surface. There is a significant crystallographic dependence, as well as dependencies upon the environment in terms of the pH and chloride molarity. An analysis of the results in terms of existing, competing models in the literature for the effect of chloride indicates that rather than the existing models being competitors, most are correct but incomplete. Rather than the different models being viewed as competitors, or each being rate determining for some specific set of conditions, the majority are simultaneously correct. Conventional oxide surface science extrapolation of the results yields qualitative conclusions for the effects of, for instance, alloy dopants which are consistent with existing experimental data. The analysis also indicates the existence of a number of new phenomena in corrosion, for instance local galvanic couples due to the work-function change with chloride chemisorption, as well as thermodynamic dewetting of the oxide film.

Influence of Chloride on Nanoscale Electrochemical Passivation Processes

“…Because of this competition, oxide formation and coalescence are very inefficient in the aqueous case, as seen in Figure . Moreover, given that Cl – lowers the surface energy of oxides on Ni–Cr alloys, , it stands to reason that the nucleation behavior of oxides is modified and the critical nuclei radii differ in the presence of chloride. In nanocrystal precipitation reactions, the nucleation of terraces on islands is governed by an Arrhenius expression where activation energy is a function of the surface energy of the new step edge created squared.…”

Future Frontiers in Corrosion Science and Engineering, Part II: Managing the Many Stages of Corrosion

“…The difficulty in distilling complex corrosion behavior into key descriptors of corrosion susceptibility that engineers can use to guide materials selection, such as a pitting resistance equivalency number applied to corrosion resistant alloys (PRE N ) [3][4][5][6] or the chloride-to-sulfate mass ratio in the context of lead pipe (CSMR), [7][8] occurs because of the complexity of corrosion and the many mechanisms, triggers, drivers, and dependencies that control corrosion processes. [9][10] In some cases, there are no simple descriptors or there are too many exceptions to make due with simple correlations (e.g., the "Copson curve" [12][13] ) reliably. [14][15][16] Fortunately, it is no longer necessary to rely on empirical parameters as descriptors of corrosion.…”

Co-Adsorption of H2O, OH, and Cl on Aluminum and Intermetallic Surfaces and Its Effects on the Work Function Studied by DFT Calculations

“…In all, this implies that different interactions (e.g., electron transfer and dipole formation) between the metallic surface and the aqueous ad-layer can result in changes of the galvanic effect. As has been recently calculated by L. Marks [18], Cl adsorption can increase the work function of Al 2 O 3 , which promotes heterogeneous oxidation in areas where Cl adsorbs, compared to areas where there is no Cl adsorption.…”

“…This can be explained by the strong electronegative character of halogen atoms, particularly Cl, which facilitates the formation of a dipole moment pointing from the halogen species to the metal surface [15]. A recent paper by Marks [18] reported that the galvanic effect between different sites on the oxide might be caused by a Cl adsorption-induced work function increase on several oxide surfaces, including Al 2 O 3 .…”