Hot water corrosion behaviour of Zr–Cu–Al–Ni bulk metallic glass

Cylindrical Zr55Cu30Al10Ni5 (numbers indicate at.%) alloy specimens with 3 mm diameter were prepared by copper mould casting and characterised concerning their microstructure and thermal stability by X-ray diffraction, optical and scanning electron microscopy and by differential scanning calorimetry. The samples revealed an amorphous structure except for a few micro-crystalline inclusions. The hot water corrosion behaviour was studied in comparison to that at room temperature using an autoclave cell at 150 and 250°C, respectively. Applying potentiodynamic and potentiostatic polarisation measurements, the anodic behaviour of the samples was investigated in 0.1 m Na2SO4 solution and in 0.001 m NaCl solution (both with pH25°C=8) regarding oxide layer formation and pitting susceptibility. Furthermore, electrochemically treated sample surfaces were characterised by scanning electron microscopy and Auger electron spectroscopy. With increasing temperature the Zr55Cu30Al10Ni5 alloy forms thicker but more porous and, thus, more permeable oxide layers as compared to barrier-type films formed at 25°C. A change from a high field-controlled layer growth at 25°C to a diffusion-controlled oxide layer formation already at 150°C was detected. Also at elevated temperatures, the surface layer formed in aqueous electrolyte consists mainly of zirconium oxides. Under hydrothermal conditions pitting processes mainly initiated at the transition zone between crystalline and amorphous phase are strongly enhanced.