Electrochemical properties of bulk glass-forming Zr-based alloys

tive high potential for applications. Bulk amorphous Zr-based alloys exhibit interesting mechanical properties, i.e. extremely high yield strength combined with a relatively low Young’s modulus at ambient temperatures as well as Newtonian flow in the supercooled liquid state [3]. However, applications of these materials require also high stability in various environments in order to ensure an acceptable lifetime. In general, the electrochemical reactivity of amorphous alloys in comparison to that of crystalline materials must be considered in terms of structure, chemical composition and homogeneity [4]. For bulk amorphous Zr-Al-Cu-Ni alloys an improved anodic passivation behaviour in various electrolytes at room temperature as compared to that of the multiphase crystalline alloy was detected. Under hot water conditions up to 250°C more porous and, thus, more permeable oxide layers form. In chloride solutions, bulk amorphous alloy samples are susceptible to pitting caused by the existence of microcrystalline inclusions formed during the preparation due to impurities in the melt [5]. These results show that further optimisation of alloy composition and microstructure is necessary to attain high corrosion resistance. This is subject of current investigations. Furthermore, amorphous Zr-Al-Cu-Ni alloys are of special interest regarding their interaction with hydrogen which is mainly related to the combination of early and late transition metals (ETMLTM). The first step of an electrolytical hydrogen charging process (schematically illustrated in Fig. 1) is the cathodic reduction of protons or water molecules at the metal surface to form adsorbed hydrogen atoms. These atoms can recombine to form molecular hydrogen or they are absorbed into the bulk metal. In amorphous ETM-LTM alloys, absorbed hydrogen occupies tetrahedral interstitial site types (n) at different energy levels (En) which exist at a certain probability in the chemically random structure [6]. Theoretically, amorphous alloys have a high number of coordinatively unsaturated surface atoms. Therefore, a high activity for electron transfer reactions is expected. For melt-spun amorphous Zr-based alloys from capacity measurements (Fig. 2) a significant increase of the active surface area after pre-etching in low concentrated hydrofluoric acid, which obviously removes air-formed surface oxides, can be concluded. Compared to the corresponding crystalline alloy the cathodic hydrogen reduction was found to generally take place at lower overpotentials on the amorphous alloy surface. For two glassy Zr-Al-Cu-Ni alloys the electrochemical hydrogenation and the effect of hydrogen on their thermal Highlights 2000