Studies on the Characteristics of Fe-Si Alloys in Potentiostatic Polarization

Abstract

The anodic behavior of Fe-Si alloys containing 2∼22% Si in deaerated H2SO4 solution (20°C) was studied by the potentiostatic method. The effects of the Si content on the corrosion potential Ecorr., passivation potential Ep, passivation current density Ip and passive current density ik were determined. The effects of chloride ion on the passivity of some high Si alloys were also studied. The results are summarized as follows.(1) The effect of Si addition on the corrosion potential of iron is not so remarkable: it shifts slightly in negative direction when Si content is 2∼6%, but the shift is in positive direction when Si content is 12∼16%.(2) Low Si alloys (2∼8% Si) exhibit polarization behaviors similar to that exhibited by iron except that their passive current densities are considerably higher than that of iron.(3) High Si alloys especially containing over 15% Si have an excellent passivating ability, i.e. more negative value of Ep and very low values of Ip and ik. Also these alloys are characterized by the large difference between the potential of passivation and that of the complete passivation. From this behavior of passivation, it is suggested that silicon contributes greatly to passivation in case of high Si-Fe alloys containing over 12% Si and the mechanism of passivation of these alloys differs from those of iron and low Si alloys.(4) During the active dissolution of high Si alloys such as 15% Si-Fe, the preferential dissolution of iron occurs, and the corrosion current decreases remarkably with time until it drops as low as ik value, but the potential of specimen shifts only slightly in the positive direction, showing that the passivity is not attained.(5) The flade potential of 15% Si alloy shows the value of +0.29V (S.C.E.) as a clear arest on the potential decay curve. It is more negative than Eps (+0.38V), but far more positive than Ep (About-0.2V).(6) The effect of chloride ion in case of high Si alloys, appears as the continuous increase of passive current density with increasing chloride ion concentration without being accompanied by pitting corrosion within the scope of our experiments.