Electrochemical Pourbaix diagrams of Mg-Zn alloys from first-principles calculations and experimental thermodynamic data.

Mg-Zn alloys have attracted much attention as biodegradable alloys owing to their superior mechanical properties and excellent biocompatibility. However, their corrosion/degradation behaviour has become a major issue for various biomedical applications. To understand their corrosion behaviours in aqueous environments, the first-principles informed Pourbaix diagrams, that is, electrochemical phase diagrams with respect to electrode potential and solution pH, were constructed for Mg-Zn alloys and compared with experimental observations. It was found that for Mg-rich alloys, the MgZn phase has a higher potential than the Mg matrix and may act as a cathode, resulting in galvanic corrosion, while for Zn-rich alloys, the phase Mg2Zn11 corrodes first. In Zn-rich alloys, Mg(OH)2 preferably precipitates under alkaline conditions, thus hindering the increase in pH and preventing the release of dissolved ZnO22- ions. In a Cl-containing solution, the soluble ZnCl2 eases the corrosion of the Zn matrix by decreasing the corrosion potential. These results are supported by various experimental observations; thus, they provide an in-depth understanding of the degradation behaviour of various Mg-Zn alloys as well as a feasible pathway in the design of biocompatible Mg-Zn alloys with first-principles informed Pourbaix diagrams.