ABSTRACT

The effect of loading, corrosion potential, water purity, and heat treatment condition on stress corrosion crack growth rate behavior of alloy X750 (AH and HTH and annealed + 20% cold work) and alloy 718 was investigated in 288 C water. These studies provide quantitative data on the behavior of alloy X750 and alloy 718 in welldefined water chemistry using modern experimental techniques, and provide some insight into the role of yield strength on other iron and nickel base materials whose yield strength is varied by cold work or irradiation.

INTRODUCTION

Stress corrosion cracking (SCC) has occurred in most structural materials in boiling water reactors (BWRs) and pressurized water reactors (PWRs). While the crack growth behavior of various grades of stainless steel, alloy 600, alloy 182 weld metal, and low alloy and carbon steels have been extensively investigated [1-4], there are only limited SCC growth rate data in high temperature, pure water on alloy X750 and alloy 718 [1].

There are strong and compelling similarities between BWR and PWR environments [2?7], as in both cases the crack tip is deaerated and at low potential, and with the increasing adoption of NobleChem in BWRs, even the surface corrosion potential is low. BWRs and PWRs differ primarily in: temperature (274 C, to up to 338 C in PWRs), H2 fugacity (10 ? 150 ppb vs. 3000 ppb in PWRs) and coolant additives that shift the pH at temperature from 5.6 to 6.8 ? 7.4 in PWRs. Temperature often has the most pronounced effect on SCC. The best estimates ? which must account for the Ni/NiO phase transition by maintaining a fixed potential difference relative to Ni/NiO line ? support an activation energy of 134 kJ/mole (32 kcal/mole) for Ni alloys in deaerated water [8,9]. Since most structural materials in a BWR are exposed to 274 C water (after feed water mixing), the PWR primary is up to 50 C hotter (65 C in the PWR pressurizer). This translates to an increase in growth rate relative to 274 C, to 2.1X at 288 C (BWR core outlet, PWR core inlet), to 11.3X at 323 C (PWR core outlet), to 21.9X at 338 C (PWR pressurizer), to 54.9X at 360 C (accelerated testing). H2 fugacity also has a pronounced effect on growth rate, even though it changes the corrosion potential at 300 C only by 17.1 mV for a 2X change in H2 (56.9 mV for a 10X change). In the vicinity of the Ni/NiO transition, a peak in growth rate is observed that is about 3X higher (relative to > 50 mV away) for Alloy 600 (low yield strength) and about 7X higher for Ni weld metals (higher yield strength) [8,9]. Temperature and H2 fugacity (which changes with temperature) affect the proximity of the corrosion potential to the Ni/NiO [8,9]. By contrast, pH has a less significant role on crack growth in the near-neutral regime (e.g., 5.6 = pure water, to 6.8 ? 7.4 for PWR primary).

The evaluation of alloy X750 also contributes to an improved understanding of the role of yield strength in SCC, which is important because a broad range of yield strengths exist in structural materials because of cold work, weld shrinkage strain, irradiation hardening, precipitation hardening, etc. Prior work on the effects of yield strength by cold work [10-14], weld shrinkage strains [10-14] and irradiation [6,15] in austenitic 304/304L/316L stainless steels has shown that 20% reduction in thickness (resulting in about 552 MPa (80 ksi) yield strength) can produce large enhancements in crack growth rate (Figure 1). At high potential, these rates are equivalent to those observed on sensitized stainless steel, while at low potential the rates are roughly an order of magnitude higher (Figures 1 ? 3). Similar effects of yield strength were observ

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