Pitting corrosion of zirconium-based bulk glass-matrix composites

The corrosion behaviour of two bulk glass-matrix composite alloys formed by copper mould casting was analysed: (i) Zr66.4Nb6.4Cu10.5Ni8.7Al8.0 with precipitated bcc dendrite phase and (ii) Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 with quasicrystalline phase, respectively. The electrochemical behaviour was tested on the cross-sectional areas of the composite rod samples in 0.5 M sulphuric acid solution with up to 0.5 M sodium chloride addition by potentiodynamic polarization measurements. The composition of anodically formed passive films was studied by means of Auger electron spectroscopy (AES). The morphology of local damages generated by a chloride-induced pitting process was examined with scanning electron microscopy. In result of those studies, a pitting mechanism is described. For both composites in chloride media, a pronounced selective dissolution of the glassy matrix phase occurs, whereas the crystalline phases are not attacked. Niobium and titanium components are beneficial in inhibiting the pitting initiation due to their participation in the passive film formation. The dissolution of the glassy matrix phase is explained by the principal higher chloride reactivity of metastable phases compared to that of stable crystalline phases and by the detected enrichment of copper in these phase regions. The chloride attack progressing along the matrix phase causes the formation of deep channels in the bulk composite material leading to an excavation and, finally, detachment of the bcc dendrites or quasicrystals, respectively. The re-passivation ability is very poor.