Electrochemical Behavior of Ferrosilicides (FexSi) in Neutral and Alkaline Aqueous Electrolytes II. FexSi Electrochemical Kinetics and Corrosion Behavior

The corrosion and passivation behavior of ferrosilicide (FexSi) alloys has been studied in borate and phthalate buffers, as a function of alloy composition 30-30 mass percent (m/o) Si) using voltammetry and a (scraped) rotating ring-disk electrode (RRDE). In the previous paper, preferential oxidation of the silicon component of the alloy was predicted thermodynamically to produce an (amorphous) SiO2 surface phase at potentials more negative than required for oxidation of iron. At higher potentials the formation of Fe2SiO4 from elemental iron and Si02/Si(IV) species was predicted, though kinetic experiments reported here showed no evidence for its existence, indicating that the separate oxidation of the iron and silicon components was preferred. For 0-15.8 m/o Si ferrosilicon alloy compositions, passivation was shown to be due to a thin silica-rich Fe(III) oxide film, characterization of which is reported in the subsequent paper. The presence of this film did not significantly affect the rate of hydrogen evolution, but it did greatly decrease the rate of anodic dissolution and open-circuit corrosion. Oxygen reduction occurred under transport control at low mass transport rates and involved 2 rather than 4 mol e- per mol 02; at higher transport rates/oxygen concentrations, the reaction occurred under mixed control, probably due to the presence of the oxide film, even at -0.9V. A Pt ring -Fe3Si disk electrode was used to detect Fe(II) species either directly at high potentials by oxidation of Fe2+/FeOH+ ions, or indirectly at low potentials by monitoring the hydrogen evolution current changes resulting from local modulation of the pH due to the disk electrode reaction.