A mechanistic understanding of corrosion andoxidation processes is crucial to ensure long-termresistance to stress corrosion cracking of Ni-basedstructural alloys in PWR primary and secondarysystems. Aqueous corrosion of complex Ni-basedalloys under PWR water conditions as well asgaseous oxidation of low-solute Ni binary alloys athigh temperatures (>800 °C) have been studiedextensively. However, a significant knowledge gapexists between response of Ni-based alloys inaqueous and gaseous environmental conditions.Selective oxidation at different temperatures wasinvestigated in a series of gaseous Rhines Packexperiments for a high-purity Ni-5Cr binary alloyand a commercial Ni-18Cr-9Fe Alloy 600. Focusedion beam milling enabled the extraction of sitespecificspecimens containing high-energy grainboundaries. Analytical transmission electronmicroscopy was employed to analyze themicrostructure and chemical composition of theresulting localized oxidation. The oxidationmechanism in the gaseous Rhines Pack atmosphereat 420 °C included the formation of a protective Croxidecap above grain boundaries in all cases. Athigher temperatures (600 and 800 °C), penetrativeintergranular and transgranular internal oxidationwas observed. Direct comparisons are made tocorrosion response for these same materials insimulated PWR primary water at 320-360 °C. Theseresults reveal a temperature dependence for gaseousoxidation and indicate mechanistic differencesbetween aqueous and gaseous degradation in Ni-Cralloys. These results imply that selectiveintergranular corrosion in PWR water is governed byprocesses beyond mere oxidation. Consideration ofcomplex corrosion/dissolution processes at surfacesand in open cracks is essential for the betterunderstanding of stress corrosion crackingmechanisms in PWR primary water.
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