(A) study on the semiconducting properties of passive film formed on Fe-Cr alloys by photocurrent and capacitance measurements

Abstract

The semiconducting properties of the passive film formed on Fe-20Cr ferritic stainless steel pH 8.5 buffer solution were examined by photocurrent measurements and Mott-Schottky analysis. The passive film showed characteristics of highly defective n-type semiconductor, which was revealed by photocurrent and capacitance measurements; gradual increase in anodic photocurrent, a large capacitance hysteresis, a positive slope and frequency dependence of Mott-Schottky plots. The photocurrent spectrum for the passive films formed on Fe-20Cr was almost same in shape to that for the passive film on Fe except for the large difference in photocurrent intensity, which demonstrates that the passive film on Fe-20Cr is composed of Cr-substituted $γ-Fe_2O_3$ involving the d-d and p-d electron transitions. The large difference in photocurrent intensity for passive films on Fe and Fe-20Cr was due presumably to the fact that $Cr^{3+}& ion in the passive film on Fe-20Cr act as an effective recombination site of electron-hole pairs, which was con-firmed by the abrupt increase in photocurrent intensity at Cr-transpassive potentials and the decrease in photocurrent intensity with increasing Cr content of Fe-xCr (x = 0, 5, 10.5, 15, 20) alloys. Mott-Schottky plot for the passive film formed at Cr-transpassive potentials exhibited the second linear region induced by deep donors, which were associated with $Cr^{6+}$ ions present in the film. Polarization of thermal oxide at a Cr-transpassive potential also induced the second linear region in Mott-Schottky plots confirming that oxidation of $Cr^{3+}$ to $Cr^{6+}$ was the origin of deep donors in the film. Band gap energies of the passive film in pH 8.5 buffer solution were estimated to be ~3.0 eV, in-dependent of film formation potential and Cr content in the alloys. With increasing the film formation potential of Fe-20Cr from -100 to 900 $mV_SCE$, the flat band potential increased from -480 to -360 $mV_SCE$, the thickness of space charge...