In Search of the True Temperature and Stress Intensity Factor Dependencies for PWSCC

This paper discusses the implications of the aqueous hydrogen level stress corrosion cracking (SCC) functionality on modeling the temperature dependency (thermal activation energy) of nickel alloy SCC. Prior testing has identified a significant effect of hydrogen level on the SCC of nickel-based alloys in high temperature, high purity water. A maximum in susceptibility occurs near the Ni/NiO phase transition. This functionality has been fundamentality characterized by the extent that the alloy’s electrochemical potential (EcP) deviates from the EcP of the Ni/NiO phase transition ( EcP = EcPNi/NiO EcP). The implication of this understanding to the determination of SCC thermal activation energies is that thermal activation energy tests need to be conducted at a constant EcP, not at a constant hydrogen level. Thermal activation energies are often biased high when determined from tests conducted at a constant hydrogen level. Recent testing and analyses show that the true (hydrogen level independent) thermal activation energy for Alloy 600 and X-750 is ~ 35 kcal/mol (150 kJ/mol). In order to better understand the dependence of the crack growth rate on the applied stress intensity factor, SCC growth rate tests were conducted on Alloy 600 as a function of the stress intensity factor and specimen size (0.6T to 2T compact tension specimens). Smaller specimens produced faster crack growth rates at high stress intensity factors (e.g., 66 MPa m) relative to larger specimens tested at the same stress intensity factor. These results suggest that Alloy 600 stress intensity factor growth rate modeling could be biased by a data set largely populated with “small” specimens (1T CT and less) at high stress intensity factors. Introduction The stress corrosion cracking (SCC) of nickel based alloys in high purity deaerated water has been extensively studied since 1959 when Couriou et al. [1] identified that Alloy 600 and other similar nickel based alloys are susceptible to SCC. Elevated temperatures and high stress intensity factors have been employed in laboratory studies to accelerate SCC such that experiments can be performed in relatively short time frames (e.g., months). Generally, an Arrhenius temperature dependency has been employed to describe the temperature functionality of nickel alloy SCC and a power law dependency has been employed to characterize the stress intensity factor functionality [2]. The testing in this study was conducted to investigate how the effects of aqueous hydrogen level [3] and specimen size influence the description of nickel alloy SCC temperature and stress intensity factor dependencies. The accurate description of the temperature and stress intensity factor dependencies of nickel alloy SCC is required to extrapolate laboratory accelerated test data to less aggressive conditions (i.e., reduced temperature and lower stress intensity factors). Experimental Procedure SCCGR Materials and Test Methods The nickel alloys investigated in this study were millannealed Alloy 600, and Alloy X-750 in the AH and HTH conditions. A single heat of material was used for each alloy tested. The compositions of these alloys were provided in Reference [4]. For the aqueous hydrogen level studies, Alloy 600 was fabricated into standard 1T (25.4 mm thick) compact tension (CT) specimens with 10% side grooves and Alloy X-750 was fabricated into standard 0.4T (10.2 Proceedings of the 12th International Conference on Environmental Degradation of Materials in Nuclear Power System – Water Reactors – Edited by T.R. Allen, P.J. King, and L. Nelson TMS (The Minerals, Metals & Materials Society), 2005