Corrosion behavior of NiCrFe Alloy 600 in high temperature, hydrogenated water

The corrosion behavior of Alloy 600 (UNS N06600) is investigated in hydrogenated water at 260 degrees C. The corrosion kinetics are observed to be parabolic, the parabolic rate constant being determined by chemical descaling to be 0.055 mg dm(-2) h(-1/2). A combination of scanning and transmission electron microscopy, supplemented by energy dispersive X-ray spectroscopy and grazing incidence X-ray diffraction, are used to identify the oxide phases present (i.e., spinel) and to characterize their morphology and thickness. Two oxide layers are identified: an outer, ferrite-rich layer and an inner, chromite-rich layer. X-ray photoelectron spectroscopy with argon ion milling and target factor analysis is applied to determine spinel stoichiometry; the inner layer is (Ni0.7Fe0.3)(Fe0.3Cr0.7)(2)O-4, while the outer layer is (Ni0.9Fe0.1)(Fe0.85Cr0.15)(2)O-4. The distribution of trivalent iron and chromium cations in the inner and outer oxide layers is essentially the same as that found previously in stainless steel corrosion oxides, thus confirming their invariant nature as solvi in the immiscible spinel binary Fe3O4-FeCr2O4 (or NiFe2O4-NiCr2O4). Although oxidation occurred non-selectively, excess quantities of nickel(II) oxide were not found. Instead, the excess nickel was accounted for as recrystallized nickel metal in the inner layer, as additional nickel ferrite in the outer layer, formed by pickup of iron ions from the aqueous phase, and by selective release to the aqueous phase. (C) 2005 Elsevier Ltd. All rights reserved.