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Role of Grain Boundary Cr5B3 Precipitates on Intergranular Attack in Alloy 600

  • Daniel K. SchreiberEmail author
  • Matthew J. Olszta
  • Karen Kruska
  • Stephen M. Bruemmer
Conference paper
Part of the The Minerals, Metals & Materials Series book series (MMMS)

Abstract

It is well established that thermally treated (TT) alloy 600 exhibits superior resistance to intergranular stress corrosion cracking (IGSCC) in pressurized water reactor (PWR) primary water environments than do solution annealed (SA) and mill annealed (MA) equivalents of the same material. This improved resistance is nominally ascribed to the prevalence of grain boundary (GB) Cr-carbide precipitates (M7C3 or M23C6). In this study, we perform high-resolution characterization by scanning transmission electron microscopy (STEM) and atom probe tomography (APT) on a heat of alloy 600 with a very low concentration of C (0.01 at.%) and a modest B concentration (46 appm). After SA+TT annealing, this heat exhibits IG Cr-boride precipitates (Cr5B3) in the absence of Cr carbides. Despite IG precipitation of Cr5B3, the pristine GB shows no measurable Cr depletion and B remains segregated at the GB. During intergranular attack (IGA) in PWR primary water at 360 °C, the Cr5B3 precipitates dissolve rapidly. The observed IGA is subtly different from what is typical for alloy 600, specifically with higher concentrations of Li and B within the corrosion oxides while the GB ahead exhibits less depletion of oxidizing species (e.g. Cr, Fe, Si and B) than other alloy 600 heats. Together these observations suggest that IG precipitation of Cr5B3 in the absence of Cr carbides has a neutral to slightly positive effect on the IG corrosion resistance of alloy 600 GBs in PWR primary water.

Keywords

Stress corrosion cracking Intergranular corrosion Ni alloys Thermal treatment Atom probe tomography Transmission electron microscopy 

Notes

Acknowledgements

Financial support from U.S. Department of Energy (DOE), Office of Nuclear Energy Light Water Sustainability (LWRS) Program is acknowledged for characterization of the un-exposed heat 31907. DKS acknowledges support from the U.S. DOE Office of Science, Basic Energy Sciences, Materials Science and Engineering Division for characterization of heat 31907 after exposure in the thermally treated condition and in preparing this manuscript. Remaining characterizations on other materials and conditions have been supported by EPRI. FIB and APT analyses were accomplished using EMSL, a U.S. DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for DOE under Contract No. DE-AC05-76RL01830.

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Copyright information

© The Minerals, Metals & Materials Society 2019

Authors and Affiliations

  • Daniel K. Schreiber
    • 1
    Email author
  • Matthew J. Olszta
    • 1
  • Karen Kruska
    • 1
  • Stephen M. Bruemmer
    • 1
  1. 1.Pacific Northwest National LaboratoryRichlandUSA

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