Passivity behavior of melt-spun Mg-Y Alloys

Several Mg-Y binary ribbons with Y content up to ∼ 17.9 at.% were fabricated by melt-spinning. X-ray diffraction (XRD) revealed that the phase structure changes with increasing Y content from extended solid solution to partially amorphous, and then fully intermetallic Mg₂₄Y₅. Anodic potentiodynamic polarization performed in 0.01M NaCl electrolyte (pH = 12) revealed improved anodic passivity behavior compared to pure Mg for all the Mg-Y alloys. X-ray photoelectron spectroscopy (XPS) revealed that the improved passivity of Mg-Y was more related to the elemental oxidation state rather than the concentration of the surface components. To study the effect of Cl ^- ion on the passivity behavior, anodic potentiodynamic and potentiostatic polarization were performed on Mg-17.9at.% Y in alkaline (pH = 12) NaCl electrolytes containing Cl^- ion in the concentration range from 0.00 to 0.50 M. The passive films formed in 0.01 M NaCl electrolyte were similar to the native film, which were composed of MgO and Y₂O ₃. No CO₃^2- and Cl^- ions were incorporated into the passive film. The passivity was significantly degraded in the electrolytes containing higher Cl^- concentration (0.1 and 0.5 M). Detailed XPS revealed that the surface films under these conditions were composed of much hydrated species Mg(OH)₂ and YOOH and/or Y(OH) ₃ and CO₃^2- was incorporated into the surface film. The incorporation of Y₂O₃ in the passive film was given as the reason for the enhanced passivity properties of Mg-Y ribbons. The mechanism of Cl^- and CO₃^2- ions to the degradation of the passivity was discussed.