Journal
JOURNAL OF NUCLEAR MATERIALS
Volume 402, Issue 2-3, Pages 147-156Publisher
ELSEVIER
DOI: 10.1016/j.jnucmat.2010.05.010
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The oxide film formed on nickel-based alloys in pressurized water reactors (PWR) primary coolant conditions (325 degrees C, aqueous media) is very thin, in the range of 1-100 nm thick, depending on the surface state and on the corrosion test duration The nature and the structure of this scale have been investigated by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). TEM observations revealed an oxide layer divided in two parts. The internal layer was mainly composed of a continuous spinel layer, identified as a mixed iron and nickel chromite (Ni(1-x)FexCr2O4) Moreover. nodules of Cr2O3, with a size about 5 nm, were present at the interface between this simnel and the alloy No chromium depletion was observed in the alloy, at the alloy/oxide interface The external layer is composed of large crystallites corresponding to a spinel structure rich in iron (Ni(1-z)Fe(2+z)O4) resulting from precipitation phenomena SEM and TEM observations showed a link between the nucleation and/or the growth of crystallites of nickel ferrite and the crystallographic orientation of the substrate A link between the presence of surface defects and the nucleation of the crystallites was also underlined by SEM observations Partially hydrated nickel hydroxide, was also observed by TEM in the external scale. Based on these results. some considerations about the mechanism of formation of this oxide layer are discussed (C) 2010 Elsevier B V All rights reserved
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