First Principles Modeling of Cathodic Reaction Thermodynamics in Dilute Magnesium Alloys

The effect of dilute alloying additions on the cathodic reaction thermodynamics occurring on the magnesium basal (0001) surface was studied using a first principles approach. The stability of alloying additions on the Mg(0001) surface was considered as a function of surface energy and shown to be a function of implicit solvation. The work function of the alloyed surfaces was also largely affected by the use of an implicit solvent, with alloyed surfaces becoming increasingly noble and the pure magnesium surface becoming less noble after solvation. The cathodic reaction was considered as a three part sequence: (1) water dissociation, (2) hydrogen diffusion, and (3) hydrogen evolution. The water dissociation reaction became endothermic for Ge, In, Sb, and Sn dilute alloying, whereas Ca, Sc, Y, Ti, and Zr enhanced the already favorable water dissociation reaction. Two mechanisms of preventing hydrogen evolution were considered, with early period elements (Ca, Sc, Ti, Y, and Zr) preventing hydrogen evolution from the surface by binding the adsorbed hydrogen, whereas late period elements (Al, As, Cd, Ga, Ge, In, Si, Sn, Sb, and Zn) prevented local hydrogen recombination by repelling adsorbed hydrogen.