Pacific Northwest National Laboratory Investigation of Stress Corrosion Cracking in Nickel-Base Alloys (NUREG/CR-7103, Volume 1, PNNL-16613)

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Publication Information

Manuscript Completed: August 2011
Date Published: September 2011

Prepared by:
S. M. Bruemmer and M. B. Toloczko

Pacific Northwest National Laboratory
P.O. Box 999
Richland, WA 99352

E. G. Reichelt, NRC Project Manager

NRC Job Code N6007

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

The objective of this ongoing program is to conduct stress-corrosion cracking (SCC), crack-growth-rate (CGR) testing of nickel-base stainless alloys in high-temperature, light-water-reactor (LWR) environments. An emphasis is placed on structural alloys with higher Cr content, specifically alloy 690 and its weld metals alloy 152 and 52. Other relevant nickel- and iron-base alloys may also be in the test matrix including materials removed from LWR service. In order to accomplish this objective, autoclave systems in suitable load frames and the associated water supply, conditioning and pressurization subsystems were designed and constructed. These autoclave systems enable testing under simulated and/or accelerated pressurized-water reactor (PWR) and boiling-water reactor (BWR) conditions (e.g., increased temperature, more aggressive chemical environments, increased load range or load interaction effects) with quantitative in-situ measurement of crack extension and electrochemical corrosion potential.

Three CGR test systems have been assembled and capabilities qualified through experiments first on cold-worked (CW), 300-series, austenitic stainless steels in high-purity BWR environments followed by tests on CW alloy 600 and alloy 182 weld metal in both BWR and PWR environments as part of round-robin collaborations. The CGR tests on stainless steels evaluated stress intensity, cyclic loading, ECP, and sulfate additions on SCC propagation rates. These tests were also used to demonstrate accurate control of environmental/mechanical test conditions and reproducible direct-current, potential drop (dcpd) crack-length measurements with resolution down to micrometer dimensions.

Crack-growth responses of five CW 316SS samples were evaluated and all heats exhibited consistent intergranular stress corrosion crack propagation rates in oxidizing, high-purity BWR water. Unexpected high crack-growth rates at low ECPs in BWR hydrogen water chemistry (HWC) conditions were observed for one CW 316LSS heat. Two samples of this material were tested in different systems over a range of stress intensities and hydrogen levels showing propagation rates near 10^-7 mm/s. Only a small decrease (2–3X) was seen at HWC in comparison to BWR oxidizing conditions (2000 ppb O₂). The other two CW 316SS heats evaluated exhibited the more typical benefit when adding H₂ and removing O₂ from the water with CGRs decreased by a factor of 50–100 times. Round-robin CGR testing (organized by the International Cooperative Group on Environment-Assisted Cracking [ICG-EAC]) was performed on alloy 600 and alloy 182 materials. Experiments were conducted in BWR oxidizing water for both alloys and in simulated PWR primary water for the alloy 182 weld metal. Results demonstrated state-of-the-art capabilities for the Pacific Northwest National Laboratory (PNNL) systems and testing methodology through comparisons with other laboratories in the U.S., Europe, and Japan. Stable and reproducible crack extension could be measured at length changes below 1 µm enabling propagation rates to be documented below 10^-9 mm/s. In addition, the influence of H2 content on stress corrosion in PWR primary water was examined for alloy 182 weld metal documenting higher propagation rates at intermediate H2 concentrations where the specimen ECP crosses the Ni/NiO transition line at 325°C (617°F). Preliminary data for the first tests on alloy 152 weld metal and alloy 690 control rod drive mechanism (CRDM) materials are also described along with various characterization activities on the project materials.