Passivity behavior of melt-spun Mg-Y alloys

Several Mg-Y binary ribbons with Y content up to similar to17.9 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 0.01 M 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 potentiodynantic and potentiostatic polarization were performed on Mg-17.9 at.% 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 Y2O3. No CO32- 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 CO32- 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 CO32- ions to the degradation of the passivity was discussed. (C) 2003 Elsevier Ltd. All rights reserved.