Preferential Intergranular Oxidation and Stress Corrosion Cracking of Alloy X-750 in CO-CO2 Gas Mixtures
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Authors
Yaedu, Adriano Eidi
Date
2025-01-30
Type
thesis
Language
eng
Keyword
Alloy X-750 , Internal oxidation , PWSCC , CANDU spacer , Preferential intergranular oxidation
Alternative Title
Abstract
The mechanical integrity of garter spring spacers used in CANada Deuterium Uranium (CANDU®) reactors are important to the integrity of pressure tubes that house fuel bundles. Brittle intergranular fracture was observed on Alloy X-750 ex-service spacers, instigating materials research to identify the responsible mechanisms. Alloy X-750 is a precipitation-hardened variant of Alloy 600 (72Ni-16Cr-9Fe) with the addition of Al and Ti, resulting in the formation of ’ precipitates, providing high strength. Alloy 600 and X-750 are susceptible to primary water stress corrosion cracking (PWSCC) in both pressurized water reactors (PWR) and boiling water reactors (BWR). Currently, preferential intergranular oxidation (PIO) is the most accepted mechanism for PWSCC, where the primary water environment (~300 °C hydrogenated water) enables selective oxidation of alloying elements, notably Cr, while Ni remains metallic. While spacers normally reside in a dry oxidizing mixture of CO2 and O2 within the annular gas system (AGS), extreme off-chemistry reducing conditions are being considered, which could promote PIO and embrittlement. In this project, PIO of Alloy X-750 was studied in 480 °C CO-CO2 reducing environments, in scenarios well away from the AGS but useful for establishing boundary conditions. Aged and annealed samples of Alloy X-750 were exposed to reducing CO-CO2 mixtures with the oxygen partial pressure ranging from 500× to 100,000× below the NiO oxygen dissociation pressure. Internal and intergranular oxidation were observed under all conditions, similar to Alloy 600 when exposed to 480 °C H2-steam in reducing thermodynamic conditions. Further nanoscale analysis of the samples tested at 500× and 5000× revealed that Al and Ti oxides lead PIO, while intergranular carbides hindered intergranular oxidation. Pre-notched samples tested in the 500× condition at 480 °C and with a stress intensity of 29.1 MPa×m0.5 resulted in fracture with characteristics analogous to PWSCC and internal oxidation: intergranular cracking driven by PIO with Ni nodules covering the fracture surface. In conclusion, Alloy X-750 spacers can undergo PIO in CO-CO2 gaseous environments via a PWSCC-like mechanism; however, this is conditional on the absence of O2 and maintaining a reducing environment for an extended period of time, both of which are unlikely in the AGS.
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ProQuest PhD and Master's Theses International Dissemination Agreement
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This publication is made available by the authority of the copyright owner solely for the purpose of private study and research and may not be copied or reproduced except as permitted by the copyright laws without written authority from the copyright owner.