XPS investigations on the formation of passive layers on Fe/Si alloys at pH 5 and 9

Abstract

Binary Fe/Si alloys (2-25 at.% Si) were passivated potentiostatically in an alkaline (pH 9.0) and an acidic (pH 5.0) electrolyte. The specimens were sputter-cleaned and passivated under a protecting purified argon atmosphere and finally transferred into the ultrahigh vacuum of an ESCA spectrometer without contact with the laboratory atmosphere. X-ray photoelectron spectroscopy provides qualitative and quantitative information about the chemical composition and structure of the passive layers. Systematic variation of the passivation time in the range of milliseconds to hours at a low and a high potential yields insight into the mechanism of the formation of protecting passive layers on the examined Fe/Si alloys. The growth of the passive layers starts with the oxidation of Si, followed by the formation of Fe(II) which finally oxidizes to Fe(III). Depending on the Si content of the alloys and the pH, a protecting SiO 2 layer or an Fe(III) oxide layer may form. Small amounts of Si (2-10 at. %) prevent the formation of a stable passive layer in acidic solutions by iron silicate formation, so that none of the cations may form a continuous protecting passive film. An Si content of >20 at.% clearly improves the passivation behaviour again.