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

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.