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 Na2SO 4 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. © 2005 The Japan Institute of Metals.