The Passivity of Iron‐Chromium Alloys

Abstract

Chronopotentiometry and differential capacity measurements were used to study the passive films formed by anodic polarization of a series of Fe‐Cr alloys in dilute . Steady‐state conditions were established by electronic control of the electrode potential prior to making measurements.Cathodic reduction studies indicated the presence of at least three types of surface materials. One is nonprotective in the case of Fe and several of the lower Cr alloys, but achieves considerable stability to reduction or dissolution at an alloy composition approaching 12% Cr. Another material is essential for passivity of Fe and the alloys with Cr content less than 12% and plays an important role in the electrical properties of the films on these alloys. A third type of surface material appears as a by‐product of processes occurring in the transpassive region.In the composition range 2.70–9.02% Cr, capacity‐potential curves are quite similar and capacities follow a reciprocal relation with electrode potential over a range of potentials. Extrapolation of these data was used to obtain a relation between film thickness, dielectric constant, and charge equivalents involved in film reduction. Film dielectric constant values, calculated assuming the reduction of normal , varied from 29.1 to 87.5, depending on the reduction reaction assumed. At an alloy composition approaching 12% Cr, the passive film assumes optimum protective properties. Correspondingly, humps develop in the capacity‐potential curves, and the curves shift in the direction of higher capacity values with increasing Cr content.