Insights into Stress Corrosion Cracking Mechanisms from High-Resolution Measurements of Crack-Tip Structures and Compositions
Recent results are presented demonstrating the application of cross-sectional analytical transmission electron microscopy (ATEM) to corrosion and cracking in high-temperature, light-water-reactor (LWR) environments. Structural, compositional and crystallographic characterizations of crack-tip oxide films and interfaces at near-atomic resolutions reveal evidence for unexpected local environments, corrosion reactions and local changes in the alloy metallurgy. Information obtained by high-resolution imaging and analysis indicates the corrosion processes that occur during crack advance, and provides insights into the mechanisms controlling environmental degradation. Examples of intergranular stress-corrosion cracking (IGSCC) in Ni- and Fe-base stainless alloys are reviewed to illustrate the value of this approach. Comparisons are made between crack characteristics found in components removed from long-term LWR service and those in materials tested under well-controlled laboratory conditions. Key insights into crack corrosion environments and advance mechanisms are established for Ni-base alloy 600 in steam-generator, secondary-water environments. Solution impurities such as Pb are often found in high concentrations at leading-edge reaction zones within porous, corrosion-product films. The presence of nanometer-wide, deeply attacked grain boundaries off the main SCC cracks (without evidence for plastic deformation) is believed to indicate a major role of active-path IG corrosion in the SCC process. Similar characteristics are identified for alloy 600 cracking in primary water. This suggests that Pb may only accelerate the IG corrosion process and not alter the basic degradation mechanism. Quite different IGSCC crack and crack-tip characteristics have been discovered for Fe-base stainless steels in LWR environments. More classic SCC crack and crack-tip structures have been seen in non-sensitized materials with wall oxide films extending to the tips. Cracked components after long-term service exposure show distinct differences in the crack-tip region microchemistry versus shorter-term laboratory SCC samples. Compositional changes in the metal ahead of many crack tips suggest that selective dissolution/oxidation occurs at the leading edge of degradation. Results for specific samples are used to demonstrate the ability of cross-sectional ATEM to reveal new details of crack-tip structures that cannot be detected by other methods.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 948788
- Report Number(s):
- PNNL-SA-42149; KC0201020; TRN: US200907%%245
- Resource Relation:
- Conference: Environment-Induced Cracking of Materials: International Conference on Environment-Induced Cracking of Metals, 2:95-106
- Country of Publication:
- United States
- Language:
- English
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