Atomic-Scale Coordination Effects on the Initial Stage of Passive Film Formation in Fe-Cr Alloys

Abstract

The self-healing passivation behavior Fe-Cr alloys serve as a model for the development of corrosion resistant alloys and coatings. Both first principles-based calculations [1,2], neutron scattering [3] and Mössbauer spectroscopy [4] studies demonstrated that in FepCr1-p alloys, the atomic-scale distribution of Cr atoms in the BCC lattice depends on both composition and thermal history. These studies showed that there is an inversion in the Warren Cowley short range order (SRO) parameter [5] that occurs for 1-p ≅ 0.11 At. Cr compositions, less than this value, the alloys show a tendency toward ordering while at higher Cr compositions the alloys show a tendency for clustering. In terms of a simple regular solution model, this behavior is determined by the interaction parameter, ΔE = [EFe-Cr - 1/2(EFe-Fe + ECr-Cr)], where the E’s are nearest neighbor bond energies taken to be negative. Then for ΔE < 0 the system tends to short-range order, while for ΔE > 0, clustering behavior is expected. In order to study these atomic scale coordination effects on passivation, FepCr1-p alloys were heat-treated according to the protocols used in the neutron scattering and Mössbauer spectroscopy studies. The behaviors of these alloys were compared to that of their random solid solution counterparts. Linear sweep voltammetry and chronoamperometry were used to characterize the initial stage of passivation in these two sets of alloys. Our results reveal some subtle yet significant differences in their passivation behaviors [6]. These results are interpreted within the context of the percolation conjecture suggested by Sieradzki and Newman [7]. Klaver, T. P. C., Drautz, R. & Finnis, M. W. “Magnetism and Thermodynamics of Defect-Free Fe-Cr Alloys", Rev. B. 74, 94435 (2006).Olsson, P., Abrikosov, I. A., Wallenius, J. “Electronic Origin Of The Anomalous Stability of Fe-Rich Bcc Fe-Cr Alloys", Rev. B. 73, 104416 (2006).Mirebeau, I., Parette, G. “Neutron Study of The Short Range Order Inversion in Fe1− x Crx" Rev. B. 82, 104203 (2010).Dubiel, S. M., Cieslak, J.“ Short-Range Order in Iron-Rich Fe-Cr Alloys as Revealed by Mössbauer Spectroscopy", Rev. B. 83, 180202 (2011).Cowley, J.M. “Short-Range Order and Long-Range Order Parameters, Rev., 138, A1384 – 1389 (1965).Liu, M. Aiello, A., Xie, Y. Sieradzki, K., “The Effect of Short-Range Order on Passivation of Fe-Cr Alloys”, Electrochem. Soc., 165, C830-C834 (2018).Sieradzki, K., Newman, R. C.“ A. Percolation Model for Passivation in Stainless Steels", Electrochem. Soc. 133, pp. 1979–1980 (1986).