Photocurrent spectroscopy study of passive films on hafnium and hafnium-tungsten sputtered alloys

Anodic and air-formed films on sputtered Hf and W-Hf alloys of different composition have been investigated by Rutherford back scattering, photocurrent spectroscopy (PCS) and transmission electron microscopy (TEM) techniques. In alkaline solutions the PCS data suggest the formation on Hf metal of a duplex layer with anodic hafnia covered by an external layer of composition close to HfO(OH)2. This last compound is also present on Hf air-formed films. In acidic solutions the initial oxy-hydroxide film disappears at high anodising potentials (Vf>10 V). In the case of W-Hf alloys films of different composition and semiconducting behaviour are formed by air exposure or by anodising in different electrolytes. A PCS analysis of films grown on sputtered alloys is performed on the basis of previously proposed correlation between the band gap of anodic films and difference of electronegativity of their constituents