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 layer increased from 1.2 to 1.9 nm, and the donor density decreases from $5×10^20$ to $1×10^20 cm^{-3}.$
It was confirmed from the photocurrent spectra obtained for passive film on Fe-20Cr in pH 4.0 and 11.0 buffer solution that the photocurrent was generated by the two types of electron transition. In pH 11.0 solution, the contribution of the d-d transition to the photocurrent spectrum was predominant while in pH 4.0 solution only the photocurrent generated by the p-d transition appeared. Band gap energies of the d-d and the p-d transition were measured to be about 2.9 and 3.1 eV, respectively.
Photocurrent spectra and Mott-Schottky plots measured in pH 8.5 buffer solution for the thermal oxide on Fe-20Cr demonstrated that polarization of the thermal oxide in the solution generates localized state and donors, which was revealed by the gradual increase in photocurrent with incident photon energy and the increase in donor density, respectively.
The passive film formed in pH 8.5 solution with EDTA still exhibited similar semiconducting properties to those of the passive film formed in the solution without EDTA, although in the solution with EDTA, the maximum Cr concentration in the film was exposed to the solution. The result demonstrates that the semiconducting properties of the passive film on Fe-20Cr are controlled by $γ-Fe_2O_3$ even though the film is enriched in Cr. Addition of EDTA to pH 8.5 buffer solution increased the flat band potential and the donor density with the reduced thickness of space charge layer. These results could be attributed to the formation of more defective film due to the dissolution of iron as revealed by polarization re-sponses and XPS analysis. Based on these results, an electronic structure model for passive film on Fe-20Cr was proposed; the passive film formed on Fe-20Cr consists of Cr-substituted $γ-Fe_2O_3$ involving the d-d and p-d electron transitions. $Cr^{3+}$ ion present in the film reduces the photocurrent intensity by acting as a recombination site of the electron-hole pairs, and oxygen vacancy and $Cr^{6+}$ ion act as a shallow and deep donor, respectively.
