Atmospheric Rust Formation Process on Fe–Cr and Fe–Ni Alloys under Wet/Dry Cycles Observed by Synchrotron Radiation X-ray Diffraction

Abstract

In situ observation of initial process of rust formation on Fe–5 mass%Cr and Fe–9 mass%Ni alloys using SPring-8 synchrotron radiation was carried out under wet/dry cycling with 3.5% Na2SO4 and NaCl solution films. It was shown that the rust constituents detected during the quite initial wet/dry cycles were Fe(OH)2 and Fe(OH)3 independent of alloying elements. Most characteristic X-ray diffraction peaks obtained under the Na2SO4 solution film are indexed as those from goethite, α-FeOOH, both in the Fe–Cr and Fe–Ni alloys. In the case of the Fe–Cr alloy, coexistence of Cr3+ ion in the Na2SO4 film weakened the α-FeOOH diffraction peaks probably due to formation of ultrafine goethite crystals. Akaganéite, β-FeOOH, appears only under the NaCl solution film. Especially, the Fe–Ni alloy forms a considerable amount of akaganéite, which can be interpreted in terms of formation of the monoclinic akaganéite.