Composition and Structure of Anodic Oxide Film on Iron in Neutral Phosphate Solution

The composition and structure of anodically-formed iron oxide film in nearly neutral phosphate solution was examined by means of ESCA, electron diffraction and FT-IR spectroscopy. In the active region and in deaerated phosphate solution at pH 6.0 or lower, ferrous phosphate –Fe3(PO4)2·8H2O– was formed. In the presence of air, a small amount of ferric phosphate was found. On the other hand, in the passive region at high pH, for example at pH 9.1, the iron was covered with a multiple oxide film. The structure of the film was found to consist of an inner “Fe3O4” layer and an outer “γ-Fe2O3, FePO4·nH2O or γ-FeOOH” layer, the outermost part of the film containing a small quantity of reaction products or contaminations such as Na+, Ca2+, PO3- 4. The most passive films were estimated to be 30–50 Å.