Microgalvanic corrosion mechanism of the rare-earth phase in Mg binary alloys through first-principles calculation

In this study, we examined the influence mechanism of heavy rare-earth elements Gd and Y on the corrosion behavior of magnesium alloys by combining experimental characterization with the first-principles calculation. We investigated the changes in the work function and surface energies of the second rare-earth phase and matrix phase in binary rare-earth magnesium alloys, Mg-3.5%Gd and Mg-3.5%Y. In addition, we focused on determining the electronic properties of the second rare-earth phase and matrix phase, the correlation between the interface energy of both phases, and the corrosion behavior of the magnesium alloys. The experimental results show Mg3Gd and Mg24Y5 as the second phases of Mg-3.5%Gd and Mg-3.5%Y, respectively; these have different roles in the galvanic corrosion of the alloys. That is, Mg3Gd is the cathode phase, which promotes the corrosion of the surrounding matrix, while Mg24Y5 dissolves itself as the anode phase in the corrosion process. The results of the first-principles calculation show that the work function of the second phase in the alloy is higher than that of the Mg matrix phase doped with rare earth. Moreover, the work function of each crystal plane of the Mg3Gd phase is generally higher than that of Mg24Y5. This implies that the corrosion rate of Mg-3.5%Y is faster than that of Mg-3.5%Gd. The corrosion law obtained through an electrochemical test and a scanning Kelvin probe force microscope test is consistent with the work function calculation results. The calculation results of interface energy show that the interface between Mg3Gd and the matrix is more stable than that between Mg24Y5 and the matrix. In addition, the galvanic corrosion between Mg24Y5 and the matrix is stronger. This result reaffirms the higher corrosion rate of Mg-3.5%Y over that of Mg-3.5%Gd.