Theoretical analysis of the galvanic corrosion behavior of Mg-Ge binary alloy

A semi-empirical model based on the mixed potential theory and first principles calculation is developed in this study to analyze the galvanic corrosion of the Mg-Ge alloys composed of Mg and Mg₂Ge. The thermodynamic driving force of the Mg matrix dissolution is much stronger than that of the Mg₂Ge second phase, and Mg₂Ge will serve as the local cathode during the galvanic corrosion. The combination of the large anode equilibrium potential difference between Mg and Mg₂Ge, and the Schottky barrier across the interface indicates that the Mg₂Ge second phase can prevent the Mg grain from serving as the cathode and impede the electron transfer between the Mg grains. First principles calculations on the kinetics of hydrogen evolution reaction upon Mg₂Ge reveal that the rate-determining step is the hydrogen adsorption, which is extremely energetically unfavored but an inevitable intermediate state. The estimated exchange current density of the hydrogen evolution upon Mg₂Ge is about 3 orders of magnitude smaller than that on pure Mg. The depressed galvanic corrosion of the Mg-Ge alloys is the simultaneous result of the low hydrogen exchange current upon Mg₂Ge and preventing the Mg grains from serving as cathodes by the Mg₂Ge second phase. The calculated corrosion potentials of the Mg-Ge alloys in our model agree with the experimental values and our model can be used to guide the design of the corrosion-resistant Mg alloys.