A First Principles Investigation of Corrosion Chemistry of Common Elemental Impurities in Mg-Al Alloys

In Mg-Al alloys, experiments show that Volta potential changes with Al concentration in eutectic α-phase and the potential is nobler around the cathodic precipitate β-phase (Mg₁₇Al₁₂). Other elemental impurities in the anodic matrix can also change corrosion potential. Density functional theory computations are used to investigate the effects of surface impurities and the cathodic precipitate in Mg alloys. The fundamental aspects of water adsorption and dissociation reaction are linked to corrosion rates. The changes in the surface energy were calculated after alloying as an indication of the energetic tendency for surface atom dissolution. Finally, as a means to measure the role of electrode potential on reaction thermodynamics and kinetics, the changes in the stability of water, hydrogen, and hydroxyl as a function of potential on a standard hydrogen electrode were calculated and, thus, the equilibrium potentials of adsorption for these species were evaluated. This provides an estimate or trend of hydrogen evolution reactions resulting from the presence of some common impurities.