Pilgrim Station Steam Separator Lifting Lug Material Evaluation and Analysis

A review is presented of models that have been developed by the authors for estimating crack growth rate in sensitized Type 304 SS in Boiling Water Reactor (BWR) heat transport circuit (HTC) structural components, with emphasis on the electrochemistry of the process and on determinism (i.e., models whose prediction are constrained explicitly by the natural laws). Intergranular Stress Corrosion Cracking (IGSCC) under normal BWR operating conditions (T = 288°C, pure water) is primarily an electrochemical process that occurs at potentials that are more positive than a critical value of EIGSCC = - 0.23 Vshe. However, the crack growth rate (CGR) at E > EIGSCC is also a function of potential, conductivity, degree of sensitization of the steel, flow velocity, mechanical load, and crack length. The dominance of electrochemical, solution, and hydrodynamic factors in controlling CGR has led to the development of various techniques for mitigating IGSCC in sensitized Type 304 SS by modifying the environment, such that the corrosion potential (ECP) is displaced to a value that is more negative than EIGSCC. However, even in those regions of the HTC where the corrosion potential cannot be displaced sufficiently in the negative direction to satisfy the condition ECP < EIGSCC, considerable benefit is obtained because of the roughly exponential dependence of CGR on potential. The most important parameters in affecting electrochemical control over the ECP and CGR in an operating reactor are the kinetic parameters (exchange current densities and Tafel constants) for the redox reactions involving the principal radiolysis products of water (O2, H2, and H2O2), and feed water composition and conductivity. The kinetic parameters for the redox reactions essemially determine the charge transfer impedance of the steel surface, which is known to be instrumental in determining the magnitude of the coupling current that flows through the solution from the crack to the external surface and hence in determining the CGR. The exchange current densities, in particular, are amenable to control by catalysis or inhibition of the redox reactions occurring on the surfaces external to the crack enclave, with the result that surface modification techniques have proven to be highly effective in mitigating and controlling IGSCC in reactor heat transport circuits.

Slow strain rate tests (SSRTs) on differently shaped tensile specimens machined from two heats of Alloy 600 tubing were conducted at 350 C in deaerated aqueous solutions containing small amounts of lithium hydroxide and boric acid at different partial pressures of hydrogen, using a recirculating autoclave system. In addition, split half tube specimens cut from the same two heats of Alloy 600 were subjected to reverse U-bend testing in pure water and steam at 363 and 400 C, respectively, using two static autoclaves. In both types of tests, intergranular stress corrosion cracking (IGSCC) was found to occur predominantly at sites in the alloy where a complex state of stress and strain existed. During the SSRTs, the strained portions of specimens absorbed much more hydrogen than the unstrained portions. Still greater contents of hydrogen were found at sites close to the fracture zone. The IGSCC susceptibility and the crack growth rate both increased with increasing partial pressure of hydrogen; however, absorption of hydrogen by the alloy also occurred in absence of hydrogen gas in the system. Results of this study suggest that hydrogen plays an essential role in the IGSCC mechanism.

Pressure vessel nozzle repair

This paper describes the intergranular and transgranular stress corrosion cracking of Type 304 stainless steel in high purity water. Numerous authors have previously reported on various aspects of this stress corrosion cracking (SCC) problem(1-6). Temperatures of 150°, 200°, 250°, and 290°C and dissolved oxygen concentrations of 5 ppb to 32 ppm have been investigated. Intergranular stress corrosion cracking (IGSCC) was observed at temperatures above 150°C and oxygen concentrations approximately in excess of 100 ppb. Transgranular stress corrosion cracking (TGSCC) was observed at all test temperatures and oxygen concentrations approximately less than 100 ppb.

Fracture mechanics investigation regarding the effects of cracks on the structural integrity of a BWR-core shroud

Large-diameter Type 304 stainless steel pipe weld heat-affected zone (HAZ) was investigated to determine the rate at which low temperature sensitization (LTS) can occur in weld HAZ at nuclear reactor operating temperatures and to determine the effects of LTS on the initiation and propagation of intergranular stress corrosion cracks (IGSCC). The level of sensitization was determined with the electrochemical potentiokinetic reactivation (EPR) test, and IGSCC susceptibility was determined with constant extension rate tests (CERT) and actively loaded compact tension (CT) tests. Substructural changes and carbide compositions were analyzed by electron microscopy. Weld HAZ was found to be susceptible to IGSCC in the as-welded condition for tests conducted in 8-ppm-oxygen, high-purity water at 288 °C. For low oxygen environments (i.e., 288 °C/0.2 ppm O2 or 180 °C/1.0 ppm O2), IGSCC susceptibility was detected only in weld HAZ that had been sensitized at temperatures from 385 °C to 500 °C. Lower temperature heat treatments did not produce IGSCC. The microscopy studies indicate that the lack of IGSCC susceptibility from LTS heat treatments below 385 °C is a result of the low chromium-to-iron ratio in the carbide particles formed at grain boundaries. Without chromium enrichment of carbides, no chromium depleted zone is produced to enhance IGSCC susceptibility.

Low carbon martensitic stainless steels, called supermartensitic stainless steels (SMSS), have widely been applied for flowlines transporting corrosive oil and gas. However, intergranular stress corrosion cracking (IGSCC) in girth welds of the steel has become a major concern recently. A number of research works have been conducted to clarify the mechanism and to establish the measures. The most likely mechanism of IGSCC is chromium depletion on grain boundary because it is observed in lean grade SMSS, which contains no molybdenum. However, neither depletion nor carbide of chromium has been detected so far in 2-3% Mo-added high grade SMSS, although the steel suffers IGSCC under some conditions. In order to clarify the mechanism in high grade steel, effects of chemical components on IGSCC resistance were investigated Simulated HAZ specimens with various conditions of thermal cycles were tested in hot acid environments by U-bend SCC test. The test results showed that reduction of carbon and addition of titanium are effective to improve the resistance to IGSCC. Both would inhibit to form chromium carbide that causes to chromium depletion. Therefore, it suggests that the mechanism of IGSCC in high grade SMSS is chromium depletion, as in lean grade.

Radiation hardening and radiation-induced chromium (Cr) depletion were related to intergranular stress corrosion cracking (IGSCC) response among various stainless steels (SS). Available data on neutron-irradiated materials were analyzed and correlations developed between fluence, yield strength, grain-boundary Cr concentration, and cracking susceptibility in high-temperature water environments. Large heat-to-heat differences in the critical fluence (0.2 neutrons/cm2 to 2.5 x 1021 neutrons/cm2) for IGSCC were documented. Variability often was consistent with yield strength differences among irradiated materials. IGSCC correlated better to yield strength than to fluence for most heats, suggesting a possible role for radiation-induced hardening (and microstructure) on cracking. However, isolated heats revealed a wide range of yield strengths (450 MPa to 800 MPa) necessary to promote IGSCC which could not be explained by strength effects alone. Grain-boundary Cr depletion qualitatively explained differences in IGSCC susceptibility for irradiated SS. Examination of measured Cr content vs SCC showed that all materials showing IG cracking had some grain-boundary depletion (> 2%). Grain-boundary Cr concentrations for cracking (< ~ 16 wt%) were in good agreement with results from similar SCC tests on unirradiated type 304 SS (UNS S30400) with controlled depletion profiles. Heats that prompted variability in the yield strength correlation were accounted for by differences in their interfacial Cr contents. Thus, certain SS apparently were more resistant to cracking, even though they had significant radiation-induced Cr depletion. Cr depletion was believed to be required for SCC of irradiated SS, but susceptibility in this study was modified by other microchemical and microstructural components.

Radiation hardening and radiation-induced chromium depletion are related to intergranular stress corrosion cracking (IGSCC) response among various stainless steels. Available data on neutron-irradiated materials have been analyzed and correlations developed between fluence, yield strength, grain boundary chromium concentration and cracking susceptibility in high-temperature water environments. Large heat-to-heat differences in the critical fluence (0.2 to 2.5 n/cm2) for IGSCC are documented. In many cases, this variability is consistent with yield strength differences among irradiated materials. IGSCC correlated better to yield strength than to fluence for most heats suggesting a possible role of the radiation-induced hardening (and microstructure) on cracking. However, isolated heats reveal a wide range of yield strengths from 450 to 800 MPa necessary to promote IGSCC which cannot be understood by strength effects alone. Grain boundary chromium depletion is found to qualitatively explain differences in IGSCC susceptibility for irradiated stainless steels. Examination of measured chromium contents versus SCC shows that all materials showing IG cracking have some grain boundary depletion (≥2%). Grain boundary chromium concentrations for cracking (below ~16 wt%) are in good agreement with similar SCC tests on unirradiated 304 SS with controlled depletion profiles. Heats that prompt variability in the yield strength correlation, are accounted for by differences in their interfacial chromium contents. Certain stainless steels are more resistant to cracking even though they have significant radiation-induced chromium depletion. It is proposed that chromium depletion is required for IASCC, but observed susceptibility is modified by other microchemical and microstructural components.

Ultrasonic detection and sizing of intergranular stress corrosion cracks (IGSCC) in nuclear reactor cooling systems is a difficult practical problem due to the complicated geometry of these defects and to the variety of other reflectors (e. g., welds) which produce competing ultrasonic indications. The use of models of scattering from such defects can help in improving physical insight into ultrasonic scattering from IGSCC’s and may ultimately be of use in defining inspection protocols and signal processing algorithms which can lead to improved inspection reliability and discrimination between IGSCC’s and other geometrical reflectors. This paper will discuss the application of a model of ultrasonic scattering from a simple Y-shaped crack based upon the Kirchhoff approximation. In this model, the ultrasonic beam is approximated by a Gaussian profile which includes the effects of diffraction and allows calculation of the full ultrasonic radiation pattern which may be used, for example, to simulate a scanned inspection. Included in this paper will be a brief description of the model and a presentation of simulated IGSCC results. Comparisons of the model to experimental measurements will then be addressed followed by an application to the problem of IGSCC sizing based upon the dB-drop and PAT (pulse-arrival-time) techniques.

Stress corrosion cracking growth rates of domestic forged nuclear grade 316L stainless steel (SS) were successfully measured in high temperature and high pressure water at various temperatures and under various loading mode. A direct current potential drop (DCPD) technique was used to monitor the crack growth throughout the test. The crack growth rate decreases with the increasing dissolved hydrogen content and the decreasing dissolved oxygen content. The crack growth rate under trapezoidal loading mode is bigger than that under constant loading. The fracture surface has typical intergranular stress corrosion cracking (IGSCC) characteristics.

A physicomechanical model for occurrence of fractures of austenitic steels upon intergranular stress corrosion cracking (IGSCC) is proposed on the basis of an analysis of experimental data on the susceptibility of irradiated austenitic steels to IGSCC during contact with the coolant of the primary circuit of a water-moderated water-cooled power reactor (VVER). A durability evaluation method for units of vessel internals by the criterion of the initialization of IGSCC is developed that allows for calculations under a timevarying stress-strain state of the unit and a damaging radiation dose.

The intergranular stress corrosion cracking (IGSCC) in heat affected zone (HAZ) of weldable super martensitic stainless steels has been reported. The Cr-carbide precipitation surrounded by Cr-depletion zone was discussed as the cause of IGSCC for Ti-free martensitic stainless steels. The Cr-depletion along grain boundary in the vicinity of the surface oxide layer on HAZ formed by welding was also reported and the initiation mechanism was discussed based on the Cr-depletion of the surface area. The improvement of SCC resistance by Post Weld Heat Treatment (PWHT) has been recognized. PWHT can be effective to reduce the IGSCC susceptibility because of the healing for Cr-depletion (sensitized zone and surface area). It has been also proposed that the effect of PWHT on the propagation of IGSCC can be explained by the enrichment of molybdenum at the grain boundary, which has the positive effect for enhancing the corrosion resistance, and/or by the elimination of the harmful effect of phosphorus segregation to be drained from the grain boundary to the molybdenum enriched phase. In this study, the molybdenum precipitation behavior along grain boundary by PWHT of super martensitic stainless steels is determined in detail. The quantitative analysis by the extracted residue is carried out with the varied combination of temperatures and durations of PWHT. The relation between the IGSCC susceptibility and the molybdenum precipitation along grain boundary is discussed.

The effect of pH on the intergranular stress corrosion cracking (IGSCC) susceptibility of sensitized AISI 304 SS in deaerated solutions at 250°C, where , was investigated using potentiostatic slow strain rate tests. A critical potential above which IGSCC occurs has been determined as a function of pH. It was found that depends on pH and decreases abruptly to values below the corrosion potential of the alloy in deaerated solutions at pH's lower than the pH of pure water . No critical potential was identified in the presence of indicating that IGSCC, enhanced by additions, occurs even at high cathodic overpotentials. The combined effect of pH and potential on the IGSCC behavior is discussed in terms of a previously advanced model involving anodic dissolution of the chromium‐depleted grain boundaries, in which the solubility of corrosion products modified by local acidification within the cracks seems to play a substantial role.

Over the last few years, an intergranular stress-corrosion cracking (IGSCC) mechanism has been observed for carbon steel pipelines and piping, particularly in in-situ thermal upstream production. All the cracks manifested on the bare (uncoated) outside diameter (OD) surface of the steel, and all were associated with wet mineral wool insulation. Common features include an operating temperature between 70°C and 160°C, intergranular cracking morphology, and exposure to wet mineral wool insulation with vintage post-2003. Recently, a similar IGSCC was observed in pressure vessel grade A516 Gr 70N steel. The potential of having this mechanism manifest in pressure vessels is of some concern, as the span of susceptible equipment goes beyond upstream production assets but to the larger oil and gas industry that includes upgrading, refining, terminals and distribution, and perhaps extending to other industries where the conditions listed above exist. This paper summarizes the investigation of the external IGSCC found on two carbon steel pressure vessels, with the intent of increasing the awareness of the industry with this new damage mechanism.