ASSESSING THE CORROSION-RESISTANCE OF NONEQUILIBRIUM MAGNESIUM-YTTRIUM ALLOYS

Magnesium-yttrium (Mg-Y) alloys containing 7 at% to 26 at% solute were fabricated using magnetron cosputter deposition. X-ray diffraction (XRD) revealed that no second phases were present in any of the alloys and that all but two of the alloys (Mg-7% Y and Mg-14% Y) were nanocrystalline and/or amorphous. Anodic potentiodynamic polarization performed in 0.1 M sodium chloride (NaCl) at both near-neutral and high pH values showed improved anodic polarization behavior for most of the alloys when compared to either pure Mg or one of the most corrosion-resistant commercial Mg alloys (alloy WE43 [UNS M18430]). In the high-pH solution, it was not uncommon for breakdown potentials (E(b)) of the Mg-Y alloys to be 2,000 mV higher than values for alloy WE43. Long-term galvanic and potentiostatic testing was performed on selected alloys to verify anodic polarization behavior Potentiostatic testing also was performed in a pH 12 buffer solution. When defects did not affect corrosion behavior, low current density values were obtained, with one specimen exhibiting a current density of 0.9 mu A/cm(2) (5.8 mu A/in.(2)) at a potential > 300 mV(SCE). X-ray photoelectron spectroscopy (XPS) conducted on Mg-22% Y revealed Y was enriched significantly in the passive film once the alloy was immersed in solution. This presence of oxidized Y in the passive film was believed to be responsible for the exceptional corrosion resistance of these alloys.