ELECTROCHEMICAL-BEHAVIOR OF FERROSILICIDES (FEXSI) IN NEUTRAL AND ALKALINE AQUEOUS-ELECTROLYTES .3. THE NATURE OF PASSIVE FILMS ON FEXSI ELECTRODES

The nature of passive films on oxidized ferrosilicide (Fe(x)-Si) alloy electrodes has been studied as a function of alloy composition [0-30 mass percent (m/o) Si] in deoxygenated borate and phthalate buffers using photocurrent spectroscopy, impedance, and x-ray photoelectron spectroscopy (XPS). Passivation was shown to be due to a silica containing Fe(III) oxide film, exhibiting n-type semiconducting properties with a bandgap of 1.9-2.02 eV, for 0-15.8 m/o Si ferrosilicon alloy compositions. The photoelectrochemical and XPS results showed clear evidence of overgrowth by silica, thus explaining the superior corrosion resistance of ferrosilicides compared with iron. No boron species were found to be incorporated in the oxide film.