Photoelectrochemical analysis of the effects of pH and sulfate ions on the structure and the composition of the passive film formed on Fe-20Cr-15Ni alloy

The effects of pH and sulfate ions on the structure and compositions of the passive film formed on Fe-20Cr- 15Ni were examined using a photoelectrochemical technique and Mott-Schottky analysis. The photocurrent spectra for the passive film formed on Fe-20Cr-15Ni in the buffer solutions are composed of two spectral components, one of which is generated from Cr-substituted γ-Fe₂O₃ and the other of which is generated from NiO. However, the passive film formed in sulfate solutions showed only the photocurrent spectrum of Cr-substituted γ-Fe₂O₃, suggesting that the formation of NiO in the passive film is suppressed because of a severe selective dissolution of Ni in the presence of sulfate ions. Mott-Schottky plots confirmed that the base structure of the passive film on Fe-20Cr-15Ni is n-type (Cr, Ni)-substituted γ-Fe₂O₃ regardless of solution pH and sulfate ions. The photocurrent intensity, flat band potential, and donor density for the passive film varied depending on the solution pH or the presence of sulfate ions in the solution, due primarily to the Cr enrichment in the film caused by the preferential dissolution of Fe and Ni that is more appreciable in highly acidic solutions containing sulfate ions.