Abstract
Surface films on iron formed in methanol solutions containing various amounts of water with or without 0.1 kmol/m3 LiClO4 have been analyzed by in-situ ellipsometry and ex-situ XPS. It was found that surface films having refractive indices n2 and absorption coefficients k2 in the range of 1.8∼2.2 and 0.25∼0.3, respectively, are formed on iron at potentials of the passive state. The values of n2 and k2 for passive films on iron obtained in the methanol are smaller than those of passive films formed in aqueous solutions. The refractive index n2 of the surface film on iron in the methanol became larger with increasing water or dissolved oxygen content. In the deaerated methanol with the water content less than 0.07%, a surface film having a refractive index of 1.7 was formed on iron, and XPS spectrum of this film showed a spectrum which was similar to that obtained from iron methoxide Fe(OCH3)3 powder. By the addition of water or oxygen into methanol and also by anodic polarization, the surface film changed from an iron methoxide to an iron hydroxide or oxide. The above mentioned increase of refractive index is considered to be due to the oxidation from a ferrous methoxide to a ferric hydroxide or oxide. Therefore, the passivation of iron in methanol is thought to be brought about by the oxidation of the ferrous methoxide by water or dissolved oxygen.
