Mechanistic Insights on Environmental Degradation From Nanometer-Scale Crack-Tip Measurements
Mechanisms controlling environmental degradation and cracking in light-water-reactor (LWR) systems have been investigated by analytical transmission electron microscopy (ATEM) of cracks and crack tips. The current work focuses on intergranular stress corrosion cracking (IGSCC) of 300-series, austenitic stainless steels in high-temperature LWR environments. Comparisons are made between cold-worked 304SS containing stress-corrosion cracks produced in a simulated boiling-water-reactor (BWR) environment during crack-growth tests, and a 304SS core component with cracks produced during 26-year BWR service. Similar corrosion products consisting of duplex-layered spinel oxides were found along the walls of open cracks in the service and laboratory test samples. These oxide films consisted of oriented Cr-rich spinel up to approximately 30 nm thick along the metal crack walls and large-grained Fe-rich spinel at the crack centers. Cracks in the service sample were generally more filled with oxide, perhaps reflecting the much longer times available for corrosion to occur after the crack passage. Crack tips in the BWR top guide sample exhibited unique and unexpected structures with oxide-filled cracks less than 10 nm wide ending in finger-like attack and locally “dealloyed” zones of Fe/Cr-depleted, Ni-rich metal. Alloy compositions measured at numerous crack tips were 40 wt percent Fe, 4 wt percent Cr and 55 wt percent Ni immediately ahead of the degradation front versus approximately 70 wt percent Fe, 19 wt percent Cr and 9 wt percent Ni in the bulk 304SS. Laboratory samples with cracks grown over much shorted times (approximately 1.5 months) did not show the distinctive crack tip structures or strong Ni enrichment in the metal ahead of the crack tips as for the service sample. This suggests that although selective oxidation processes occur during degradation, significant composition differences may only develop after crack propagation has slowed or stopped. Additional nanometer-scale measurements elucidating corrosion processes occurring during crack advance are presented to provide insights into mechanisms controlling IGSCC.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 965615
- Report Number(s):
- PNNL-SA-42453; KC0201020; TRN: US200919%%649
- Resource Relation:
- Conference: 11th International Conference on Fracture, March 20-25, Turin, Italy
- Country of Publication:
- United States
- Language:
- English
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Insights into Stress Corrosion Cracking Mechanisms from High-Resolution Measurements of Crack-Tip Structures and Compositions
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