Abstract
To understand the chloride (Cl)-induced initiation mechanism of localized corrosion of Aluminum (Al) alloys, we apply density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations to investigate the interactions between Cl and hydroxylated α–Al2O3 surfaces, mainly (0001) orientation, under aqueous electrochemical conditions. Hydroxylated alumina surfaces thermodynamically stable in aqueous environments are constructed based on DFT calculations for both the single-crystal and bicrystal configurations. AIMD simulations suggest a Cl anion can only be stabilized on these surfaces by substituting a surface hydroxyl (OH) group. This substitution is thermodynamically favorable at sites on surface terminations of grain boundaries (GBs) in bicrystal configurations but not favorable at sites on single-crystal surfaces. Electronic structure analyses show that the different adsorption behaviors originate from the higher sensitivity of the Al–OH bond strength to the local coordination than its counterpart of the Al–Cl bond. The adsorbed Cl significantly increases the thermodynamic driving force for Al cation dissolution from alumina surfaces into the aqueous electrolyte, which can initiate localized corrosion.
Highlights
Metallic elements, such as Al, Cr, Ni, Cu, Ti, etc., are known to spontaneously develop passive oxide layers under exposure to ambient or aqueous conditions
We propose models to evaluate the relative sensitivity of single crystal and grain boundary surface sites on passive oxides
Based on the simple analyses of the bond-counting model and the confirmation of our density functional theory (DFT)/ab initio molecular dynamics (AIMD) simulations, the results suggest that the adsorption of Cl anion on α–Al2O3 surfaces saturated by H from the aqueous electrolyte can be so weak that thermal fluctuations at room temperature can make the Cl detached from the surfaces in a short time
Summary
Metallic elements, such as Al, Cr, Ni, Cu, Ti, etc., are known to spontaneously develop passive oxide layers under exposure to ambient or aqueous conditions. Bouzoubaa et al applied DFT calculations to study the adsorption and absorption of Cl ion on a hydroxylated NiO(111) terrace surface and surface steps[37,38] Another critical issue is to understand the atomic/electronic mechanisms related to the effects of bulk defects, such as the surface terminations of grain boundaries, which are most susceptible to the Cl attack[4,14,15]. Any electrochemical reaction involving H2O molecules on the stable surface, such as the adsorption or desorption of either H+ or OH− species, should be endothermic within the H2O stability region (electrode potential between 0 and 1.23 V with respect to the standard hydrogen electrode (SHE) at the standard condition)[39] These stable surfaces, including the hydroxylated (0001) surfaces of both single-crystal α–Al2O3 and its bicrystal containing Σ3ð1010Þ grain boundaries (GBs), are presented in the first subsection of the Results section and illustrated by Fig. 1.
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