Anomalously low levels of anion incorporation into anodic oxide films on tungsten

The incorporation of electrolyte species into anodic oxide films formed on tungsten in H2SO4 and H3PO4 solutions, which is shown to be anomalously low compared with that into films formed on other valve metals, has been examined by secondary ion mass spectrometry and by transmission electron microscopy of ultramicrotomed sections. For films formed in relatively dilute, i.e. 0.1 M, solutions, levels of anion incorporation are so low that their distributions in the films could not be revealed. In contrast, for the film formed in more concentrated, i.e. 14.7 M, H3PO4 solution, the presence of PO43- anions in approximately the outer half of the film is disclosed by SIMS, although the anion incorporation is at a low level such that the duplex nature of the films is revealed only faintly by transmission electron microscopy. EDX analysis indicates an atomic ratio of phosphorus and tungsten in the PO43--doped layer of about 0.01. Further, electron-beam-induced crystallization of the amorphous anodic film to monoclinic WO3 indicated no significant differences in the susceptibilities of film layers to crystallization. (C) 1998 Elsevier Science Ltd. All rights reserved