Understanding the Mg aqueous corrosion and the Mg/water interface through first-principles simulation

Mg, as the lightest metal among engineering metals, has been applied in various industries thanks to its good mechanical property. Nevertheless, further applications of Mg and its alloys are limited by the poor aqueous corrosion resistance of Mg. The very first thing of unravelling the mechanism of Mg aqueous corrosion is to investigate the property of the Mg (0001)/water interface at a molecular level. We use second-generation Car-Parrinello molecular dynamics (MD) to explore the interface structural information, combined with static density functional theory calculations to probe the atomic interactions and the potential of zero charge of the Mg (0001) surface. Several corrosion phenomena of Mg, such as the negative difference effect (NDE), could be attributed to the corrosion product formed during the aqueous corrosion. , which forms underneath the metal surface in a humid environment, could also play an important role in subsequent corrosion reactions. However, the formation mechanism of this hydride is still unclear on an atomistic scale. Therefore, we apply the density functional theory (DFT) to study hydrogen adsorption at different adsorption sites on the clean/hydroxylated/oxidized Mg (0001) surface. The stable structure of the subsurface Mg hydride-like layer is determined, and we demonstrate the connection between the energetically favourable H adsorption with the electron localisation function (ELF) of the surface. To determine the stability of the Mg surface in different adsorption states (with various adsorbates at several coverage), we construct the surface Mg Pourbaix (E-pH) diagram based on a semiconductor-defect-inspired approach. The steadiest surface phase in vacuum/implicit water at different pH and voltage is illustrated in the diagram, which also provides a rational mechanism for explaining the NDE of Mg in the view of thermodynamics.