The semiconducting properties of the passive film formed on Ni and austenitic stainless steels containing Ni and/or Mo in pH 8.5 buffer solution were examined. Physical and electronic structures of passive films were identified by characteristics of photocurrent spectra such as position of peaks, bandgap energies, and flat band potentials. Effects of alloying elements (Ni, Mo) on the defect structure of the passive film were examined by Mott-Schottky analysis.
The photocurrent spectrum for the passive film formed on Ni showed two peaks which are presumably induced by NiO and $Ni(OH)_{2}$. Bandgap energy of NiO was about 5.0 eV for both passive film and thermally grown oxide, and that of $Ni(OH)_{2}$ was about 3.0 eV. Mott-Schottky plot of the passive film and thermally grown oxide of Ni exhibited a linear region with a negative slope, which indicate that passive film on Ni has p-type semiconductivity for both of NiO and $Ni(OH)_{2}$. Density of acceptor of passive film and thermal oxide on Ni was $8.8 × 10^{20} cm^{-3}$ and $9.6 × 10^{19} cm^{-3}$, respectively. A model of electronic structure composed of p-p heterojunction was proposed, and photocurrent and Mott-Schottky behavior of the passive film on Ni could be explained by this model.
For the passive film formed on Fe-20Cr-xNi (x = 10, 15), the photocurrent spectra were almost same in shape to those for the passive film on Fe and Fe-20Cr below 5 eV of incident photon energy, which demonstrate Cr-substituted $γ-Fe_{2}O_{3}\$ involving d-d and p-d electron transitions. But, another photocurrent peak induced by NiO was observed in the higher photon energy region. Bandgap energies of d-d and p-d transitions of $γ-Fe_{2}O_{3}$ were 2.9∼3.2 eV, 3.2∼3.6 eV, respectively. And that of NiO was 4.2∼4.9 eV and increased with increase in Ni content. Intensity of photocurrent was varied with polarization time. It may be related to the growth of thickness and the progress of crystallization of the passive film. Mott-Schottky plots for the passive film formed on Fe-20Cr-xNi (x = 10, 15) at Cr-transpassive potential were identical with that of Fe-20Cr, which exhibited two linear regions with positive slopes. The density of shallow donor (oxygen vacancy) was not dependent on Ni content, but deep donor density ($Cr^{6+}$) was increased with increasing Ni content. It means that some of $Ni^{2+}$ is incorporated in Cr-substituted $γ-Fe_{2}O_{3}$ structure substituting $Fe^{3+}$ instead of forming NiO, so that gives an increase $Cr^{3+}$ to maintain charge balance.
Mo in passive film on Fe-20Cr-15Ni-4Mo induced another photocurrent peak at 5.1 eV in addition to three photocurrent peaks due to d-d and p-d transitions of $γ-Fe_{2}O_{3}$ and NiO. Bandgap energies of d-d and p-d transitions of $γ-Fe_{2}O_{3}$ and NiO were similar to those for the passive film on Fe-20Cr-xNi (x = 10, 15); 2.8∼3.2 eV, 3.7∼3.8 eV, and 4.7 eV, respectively. The source of electron transition which induced the photocurrent peak at 5.1 eV might be $MoO_{2}$ and/or $MoO_{3}$, although it was not clarified in this study. Bandgap energy corresponding to this photocurrent peak was 4.0∼4.2 eV, which was smaller than that measured for passive film formed on pure Mo in pH 2.0 buffer solution (4.8 eV). Mott-Schottky plot for passive film on Fe-20Cr-15Ni-4Mo showed identical shape with that of Fe-20Cr and Fe-20Cr-15Ni. But, it was found that density of deep donor ($Cr^{6+}$) decreased by addition of Mo in comparison with that of Fe-20Cr-15Ni.
From these results, it can be concluded that base structure of passive film formed on austenitic stainless steels in pH 8.5 buffer solution is Cr-substituted $γ-Fe_{2}O_{3}$ involving d-d and p-d electron transitions. Ni and Mo form their own oxides (NiO, $MoO_2$ or $MoO_3$) in the passive film, and also are incorporated in $γ-Fe_{2}O_{3}$. Bandgap energies of NiO and Mo oxide in the passive film on alloys were smaller than those in passive films on pure metals by 0.1∼0.8 eV. Such behavior is considered to be associated with increase in degree of disorder of oxides of Ni and Mo due to small amount of them in the passive film. Incorporated $Ni^{2+}$ promote transpassive oxidation of $Cr^{3+}$ while $Mo^{4+}$ and $Mo^{6+}$ suppress it, resulting in increase and decrease of concentration of $Cr^{6+}$, respectively.
