Investigations on the electrochemical behaviour of Zr-based bulk metallic glasses

Bulk amorphous ZrAlCuNi alloy samples can be prepared by slow cooling from the melt due to their low critical cooling rate for amorphization. The passivation behaviour of the amorphous Zr55Al10Cu30Ni5 alloy in a 0.1 M Na2SO4 (pH=8) electrolyte is characterized in comparison to its crystalline alloy counterpart and to zirconium. Potentiodynamic and potentiostatic polarization measurements reveal that the alloys form strong protective surface layers by anodization, which is, in general, quite similar to the behaviour of zirconium. The barrier effect of surface layers grown on the alloys is slightly lower than that of films on zirconium. From AES investigations, it is obvious that all alloying elements participate in the formation of anodic films, which also show a gradient of composition along the growth direction. In 0.001–0.1 M NaCl electrolytes, pitting occurs on bulk amorphous ZrAlCuNi samples due to the existence of micrometer-sized crystalline inclusions. The oxygen-induced formation of such crystalline phases during slow cooling is studied in detail by characterizing samples of different oxygen content with X-ray diffraction, optical and scanning electron microscopy and differential scanning calorimetry.