Abstract
Growth and dissolution of anodic oxide films formed on pure molybdenum in deaerated boric-borate solutions of pH 6.48, 8.42 and 10.43 proceed simultaneously at anodic potentials higher than 0.43V (HESS). The composition of anodic oxide films formed on molybdenum in the transpassive region was analyzed by Auger electron spectroscopy (AES) and X-ray photo electron spectroscopy (XPS) combined with argon ion sputter-etching. The uppermost layer (about 1nm) of the anodic oxide films consists of hexavalent Mo (VI) ions, O2- and OH- ions, whereas low valent Mo (V), Mo (IV) and Mo (III) ions and O2- ions exist in the inner layer (20-1000nm) whose composition is uniform in depth. The average valency of molybdenum in the inner layer increases from +3.4 to +5 as the solution pH increases from 6.48 to 10.43. The ratio of film dissolution current density iD to film growth current density iO also increases with increasing pH of solution. It seems that the composition of inner layer is determined by kinetic parameters of film growth and dissolution.
