A final important crack characteristic illustrated in Figures 1 and 2(b) is the crack-tip locations that are often adjacent to, and not centered on, the grain boundary. Crack propagation along a path several nm off the boundary was clearly evident at about one-third of the crack tips examined. The only measured material feature in these mill-annealed 316SS samples that may play a role is the grain boundary composition. Significant Mo and Cr enrichment is present at grain boundaries as shown in Figure 2(c). This non-equilibrium segregation has been seen in many mill-annealed stainless steel heats (often with B enrichment) and is most likely produced during initial processing before service [11]. The profile presented was obtained ahead of a crack tip that was centered off the boundary interface and it reveals the maximum segregation adjacent to the boundary. It is possible that the altered local composition influences dissolution and passivation, thereby SCC propagation. Intergranular Attack and SCC in Alloy 600 Recent work [3-7] on cracked alloy 600, pressurized-water-reactor (PWR), steam-generator tubes has shown a remarkable tendency for IG attack along with SCC. TEM examinations of samples cracked in various high-temperature water environments has revealed narrow oxidized zones, 5-to 20-nm in width and up to tens of µm in length, along nearly every intersected grain boundary in the wakes of cracks. An example of these IG corrosion zones is shown in Figure 3 for mill-annealed alloy 600 after long-term exposure to PWR primary water at 330°C. TEM images show no visible cracks in the corrosion zones and no significant deformation of the surrounding metal matrix. Fresnel contrast imaging in Figure 3(a) shows that these structures are highly porous on a scale of 1-2 nm. Electron diffraction analyses revealed that the corrosion product consisted of nanocrystalline oxides (NiO, Cr2O3 and/or spinel depending on the particular environment). Lattice imaging identified individual crystallites with sizes down to a few nm as demonstrated in Figure 3(b). The fine porosity at the tip of a corrosion zone is illustrated in Figure 3(c). A complex network of nm-size voids and tunnels is present in the narrow oxide layer along an inclined grain boundary, as well as voids along the grain boundary plane ahead of the tip. The oxide at the attack tips was Cr2O3, even though the predominant oxide some distance (>100 nm) behind the tip was NiO (Cr and Fe levels similar to the matrix). Significant Cr enrichment at the oxidized tip was measured by high-resolution EELS, but the adjacent matrix was not depleted. The observation of pores in the metal ahead of the oxidation front suggests that vacancies may be injected during the corrosion process. Due to the narrow dimensions of the corroded IG zones in alloy 600, these structures have been undetectable by other examination methods including optical metallography, SEM or secondary-ion mass spectroscopy. Exposure to secondary water or steam environments can produce similar IG attack and aggressive degradation of precipitates in alloy 600 samples [6,7]. An example of this behavior is shown in Figure 4 for once-through steam generator tubing removed from the high-superheat (steam) region. Alloy 600 tubing is put into service in the stress-relieved and sensitized condition with closely spaced Figure 3: Intergranular attack of alloy 600 in high-temperature PWR primary water: (a) Fresnel contrast images showing narrow porous oxidized zone, (b) lattice images identifying crystallites of Cr2O3 and NiO in the corroded zone and (c) leading edge of attack with oxidized material ending within grain boundary plane and presence of isolated pores ahead of corroded region.
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