Accelerated Stress Corrosion Crack Initiation of Alloys 600 and 690 in Hydrogenated Supercritical Water
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INTRODUCTION
ACCELERATED testing is often used to study stress corrosion cracking on a laboratory scale timeline instead of directly replicating reactor conditions, which could require years for cracks to initiate. Strategies to accelerate testing are typically either based on making the material more susceptible (cold work, sensitizing, higher stress intensity) or by increasing the aggressiveness of the environment. Increasing the test temperature is a common method of accelerating the kinetics of temperature-activated processes, but care must be taken to ensure that there is not a change in the underlying mechanism being studied. Accelerated testing of stress corrosion crack initiation and growth of Alloy 600 and Alloy 690 in pressurized water reactor (PWR) is routinely conducted to reduce the time of the experiment on materials that are relatively resistant to SCC.[1–5] Cracking is often accelerated by increasing the temperature to 633 K (360 C), a limit imposed by the phase change to supercritical water at 647 K (374 C). Increasing the temperature further is appealing for highly resistant alloys such as Alloy 690, but only if the mechanism of stress corrosion crack initiation in supercritical water is the same as that in subcritical water. SCC of Alloy 600 was shown to be a temperature-activated TYLER MOSS and GARY S. WAS are with the University of Michigan, 2355 Bonisteel Blvd, Ann Arbor, MI 48109. Contact e-mail: [email protected]. Manuscript submitted June 21, 2015. Article published online January 30, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
process by Bandy and Van Rooyen,[6] who demonstrated that it followed an Arrhenius relationship between 563 K and 638 K (290 C and 365 C). Similarly, Economy et al.[7] increased the temperature further into the high-density steam phase between 605 K and 673 K (332 C and 400 C) and also observed an Arrhenius relationship for crack initiation of Alloy 600. In both the cases, a single activation energy was fit to the data over the entire temperature range, which was interpreted to mean that there was no change in the cracking mechanism. Furthermore, Economy observed that the cracking susceptibility for Alloy 600 exposed to 641 K (368 C) 19.3 MPa steam was nearly the same as experiments conducted in 641 K (368 C) 20.5 MPa water. Characterization of the stress corrosion cracking behavior of Alloy 690 is largely limited to crack growth rate testing of the alloy either in the solution-annealed or cold-worked condition.[8–12] Crack growth in pressurized water reactor primary water conditions is generally fairly slow (108 to 109 mm/s), but has consistently been observed in cold-worked compact tension specimens. Crack growth rate studies on compact tension specimens of Alloy 690 have revealed some cracking trends similar to those observed in Alloy 600.[9–11,13] Like Alloy 600, hydrogen has been shown to have an effect on the crack growth rate of Alloy 690. Peng et al.[11] observed a 2.5 times increase in the crack growth rate in proximity of the Ni/NiO bound
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