Abstract
The composition of anodic oxide films formed on iron in neutral borate-boric-acid solutions with and without chloride ions was examined by sensitive ESCA, IMMA, electron diffraction and FT-IR spectroscopy. In general, the oxide thin film consisted of a two layer structure of “Fe3O4 or γ-Fe2O3” next to the metal and “γ-FeOOH or ferric boron hydroxo complexes such as Fe(OH)(BO2)2 or FeOHB4O7” at the oxide solution interface, with iron in its trivalent state. The outermost part of the film contained minor impurities or contaminations such as boron, calcium and silicon, all of which were in the oxide state. The thickness of the passive film was about 20–50 Å. The addition of the chloride ion to borate solutions increases the amount of γ-FeOOH present. Corrosion pits are preferentially nucleated at inclusions such as Si and Mn.
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