The Effect of Impurity Concentration and Cr Content on the Passive Oxide Films in Ferritic Stainless Steels

Abstract

The properties of passive oxides on three steels consisting of 11%Cr, 18%Cr, and 30%Cr-2%Mo were studied in acidic sulfate solution by the Mott-Schottky relation for semiconducting properties, potential modulation reflectance (PMR) spectroscopy for light absorption properties, X-ray photoelectron spectroscopy (XPS) for composition estimation, and Fe3+/ Fe2+ redox current measurement for properties of electron transfer. The influence of the impurity concentration in the steels was also studied. The donor density in n-type semiconducting passive oxides as estimated from the Mott-Schottky plot decreased with a higher Cr content and with a lower impurity concentration. Cr enrichment occurred in the passive oxide and, for example, for 30Cr-2Mo steels, the ratio of Cr ions to total metallic ions in the passive oxide was approximately 70 wt%. The light absorption edge of the passive oxide measured by PMR was approximately 2.4 eV, irrespective of the Cr content and impurity concentration. The redox current on the passive oxide was much inhibited compared with that on the Pt electrode. The redox current decreased in a similar manner as the donor density. The inhibition of the redox current was assumed to be due to the space charge layer in the n-type semiconducting passive oxide.