ANOMALOUSLY LOW-LEVELS OF ANION INCORPORATION INTO ANODIC OXIDE-FILMSON TUNGSTEN

The incorporation of electrolyte species into anodic oxide films forme d on tungsten in H2SO4 and H3PO4 solutions, which is shown to be anoma lously low compared with that into films formed on other valve metals, has been examined by secondary ion mass spectrometry and by transmiss ion electron microscopy of ultramicrotomed sections. For films formed in relatively dilute, i.e. 0.1 M, solutions, levels of anion incorpora tion are so low that their distributions in the films could not be rev ealed. 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 incor poration is at a low level such that the duplex nature of the films is revealed only faintly by transmission electron microscopy. EDX analys is indicates an atomic ratio of phosphorus and tungsten in the PO43--d oped layer of about 0.01. Further, electron-beam-induced crystallizati on of the amorphous anodic film to monoclinic WO3 indicated no signifi cant differences in the susceptibilities of film layers to crystalliza tion. (C) 1998 Elsevier Science Ltd. All rights reserved