(A) study on the semiconducting properties of passive films on stainless steels and Ni-based alloys by photocurrent and capacitance measurements

Abstract

The structure and defect properties of passive films formed on stainless steels and Ni-based alloys were investigated by photoelectrochemical technique and Mott-Schottky analysis. The passive film formed on Fe-20Cr-10Ni in pH 8.5 buffer solution was found to be mainly composed of Cr-substituted $\gamma -Fe_2O_3$ from similarities in photocurrent response for the passive films formed on the alloy and Fe-20Cr. On the other hand, the photocurrent spectra for the passive films of Fe-20Cr-15Ni-(0, 4)Mo alloys exhibited the spectral components associated with NiO and Mo oxide $(MoO_2 and/or MoO_3)$ in addition to that induced by Cr-substituted $\gamma -Fe_2O_3$. Mott-Schottky plots for the passive films formed on Fe-20Cr-(10, 15)Ni and Fe-20Cr-15Ni-4Mo confirmed that the passive films on those alloys have a base structure of Cr-substituted $\gamma -Fe_2O_3$ with variation of densities of shallow and deep donors depending on the Ni and Mo contents in the alloys. I suggest that the passive film formed on Fe-20Cr-(10, 15)Ni and Fe-20Cr-15Ni-4Mo alloys are composed of (Cr, Ni, Mo)-substituted $\gamma -Fe_2O_3$ when the concentrations of Ni and Mo are below critical values. However, NiO and Mo oxide $(MoO_2 and/or MoO_3)$ would be precipitated in the passive films when the concentrations of Ni and Mo exceed critical values. The base structure of (Cr, Ni)-substituted $\gamma -Fe_2O_3$ of the passive film formed on Fe-20Cr-15Ni was maintained when the film was formed either in buffer solution or in 0.1 M sulfuric acid solution with different pH (2.0, 4.0), as confirmed photocurrent and Mott-Schottky analyses. However, the photocurrent spectra suggested that formation of NiO in the passive film was prohibited in sulfuric acid solution presumably by preferential dissolution of Ni. The intensity of photocurrent, flat band potential, and donor density for the passive film were varied by solution pH and/or sulfate ion, due primarily to the Cr enrichment in the film as a res...