Designing ultra-high corrosion resistance Mg alloys by passivating cathodic activity and promoting RE oxide films

Many studies on improving the corrosion resistance of Mg alloys have been focusing on identifying certain elements which can reduce the corrosion rate of Mg alloys. However, without quantitative analysis of individual contributions of alloying elements and their combined effects, the corrosion resistance alloy design is always performed by trial-and-error process. In this study, we have quantified the effects of binary, ternary and quaternary additions of Y, Ce, and Mn to Mg alloys and found that their electrochemical performances are not only a function of simple dissolution but also their existing states by microstructure quantification before and after immersion tests. Using first-principles calculations, we have found that the beneficial effects of Rare Earth (RE) elements, such as Ce, and Y, can only be effective when they form secondary phases and act as the anode to form oxide films in front of Mg matrix; while dissolution of Mn can actually passivate the cathodic activity and improve the work function of the matrix (from 3.66 eV to 3.80 eV). Measurements of as-cast Mg-2Y-0.5Ce-0.5Mn alloy exhibit the best corrosion resistance (0.395 mm/y), due to both reduction of galvanic corrosion sites and protection of RE-containing oxides. Therefore, ultra-high corrosion-resistant Mg alloys have been successfully developed by passivating cathodic activity and promoting RE oxide films, further illuminating a new route for stainless Mg alloy development.