0 10 20 30 40 50 60 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -60 -40 -20 0 20 40 60 Measured Cr and O Content, wt% Measured Pb Content, wt% Distance from Particle Center, nm Matrix FEG-TEM EDS Analysis Pb Cr Matrix Cr 2 O 3 O (b) Figure 4: Intergranular attack of alloy 600 in high-temperature PWR secondary water: (a) former grain boundary Cr carbide converted to Cr oxide, (b) composition profile across attacked particle showing Pb enrichment at the metal-oxide (M/O) interface and (c) porous corrosion product oxides around grain boundary particles with Pb present at leading edge of attack. Cr7C3 carbides and Ni23B6 borides. The porous oxide structure in the IG attack zones was similar to the primary-water samples, but the carbides were converted to fine-grained Cr2O3 oxides and borides were completely removed. Solution impurities appear to have a strong influence on the stability of these particles. EDS analyses of the oxidized particles (Figure 4a) revealed that environmental impurities, notably Pb, had penetrated the structures and concentrated in the reaction layers along the metal-oxide interfaces (Figure 4b). Further observations at the leading edges of attack showed tunnel-like corrosion zones formed around the IG particles as presented in Figure 4(c). A penetrative porous oxide is seen to envelop a partially converted Cr7C3 precipitate and end adjacent to a Ni-rich particle (former boride). The corrosion product in these tunnels consisted of nanocrystalline Cr-Ni spinel containing high concentrations of Pb at the leading edge of attack. This is consistent with Pb promoting dissolution and/or impairing passivity to enhance boundary and precipitate degradation. DISCUSSION The application of high-resolution ATEM methods to buried corrosion interfaces in stress-corrosion cracked materials has revealed important details of the degradation processes and mechanisms. This advance comes from two developments: (1) improvements in cross-sectional sample preparation on cracked materials, and (2) the availability of FEG TEMs that allow structural, compositional and crystallographic analyses at resolutions down to atomic dimensions. Significant findings of this continuing research include the recognition that active-path corrosion of grain boundaries plays a major role in SCC of alloy 600 in a wide variety of steam generator environments. This IG attack produces a remarkably thin (<20 nm in width) band of porous, non-protective oxide that extends for many µm. It is clear that water (or steam) penetrates throughout corroded structures. No evidence of plastic deformation was found associated with the IG attack and it appears that plasticity is not required for grain boundary degradation to occur. However, it is likely to accelerate growth rates via a stress-assisted corrosion process. Solution impurities such as Pb are shown to concentrate along the narrow (few nm) reaction layers at buried interfaces. High impurity enrichments at the leading edge of attack suggest that Pb promotes metal (and precipitate) dissolution and/or impairs oxide formation. Secondary cracking was found in the austenitic stainless steel samples, but no evidence of significant IG attack. The formation of non-porous, protective oxides on crack walls and at the crack tip is more consistent with a classical SCC slip-oxidation mechanism. Crack-wall films are in general agreement with surface films reported on stainless steels after high-temperature water exposure. The presence of the FeO-structure inner film was unexpected and it appears to have a close structural relationship with the spinel phase that forms adjacent to this oxide. In general, non-porous protective films tend to form on exposed 316SS surfaces in these high-temperature water environments, while porous, non-
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