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How irradiation promotes intergranular stress corrosion crack initiation
Irradiation assisted stress corrosion cracking (IASCC) is a form of intergranular stress corrosion cracking that occurs in irradiated austenitic alloys. It requires an irradiated microstructure along with high temperature water and stress. The process is ubiquitous in that it occurs in a wide range of austenitic alloys and water chemistries, but only when the alloy is irradiated. Despite evidence of this degradation mode that dates back to the 1960s, the mechanism by which it occurs has remained elusive. Here, using high resolution electron backscattering detection to analyze local stress-strain states, high resolution transmission electron microscopy to identify grain boundary phases at crack tips, and decoupling the roles of stress and grain boundary oxidation, we are able to unfold the complexities of the phenomenon to reveal the mechanism by which IASCC occurs. The significance of the findings impacts the mechanical integrity of core components of both current and advanced nuclear reactor designs worldwide.
Plastic Deformation Processes Accompanying Stress Corrosion Crack Propagation in Irradiated Austenitic Steels
During stress corrosion crack propagation, stress values in the crack tip vicinity often exceed material yield stress limit. Plastic deformation processes may accompany and influence cracking. Here, stress corrosion crack propagation and deformation mechanisms were investigated using EBSD analysis. The investigated material was 304L Ti-enriched austenitic stainless steel irradiated to 10.4 dpa at 320°C in the BOR-60 fast reactor. Crack growth tests were conducted in a simulated Normal Water Chemistry (NWC) environment in the temperature range 288–320 °C using compact tension specimens. By analyzing crack trajectory and grain structure in the crack vicinity, it was established that grain orientation with respect to the acting stress direction was not a key factor controlling crack propagation. No crystallographic orientation susceptible to cracking was identified. Also, EBSD analysis revealed strong inhomogeneity in plastic strain distribution along the crack path. Most crack-adjacent grains remained virtually strain-free whereas few grains experienced strong plastic strain. These areas were presumed to be “plastic bridges” or “ductile ligaments.”
Effect of Grain Boundary and Crystallographic Orientation on the Stress Corrosion Behavior of an Al-Zn-Mg Alloy
The effect of grain boundary and crystallographic orientation on the stress corrosion behavior of an Al-4.5Zn-1.4Mg alloy was investigated by comparing the performance of constant load stress corrosion test (CLSCT) and tensile test of specimens parallel to the longitudinal and transverse directions of the extruded plate. The results revealed that the strength of the longitudinal and transverse specimens decreased by 9.68 and 18.13%, respectively, after 10-day CLSCT. The transverse specimens show poor resistance to stress corrosion cracking (SCC). The grain boundaries (GB) of the longitudinal section are less dense, and more of them are distributed along the extrusion direction compared with the cross section with selected areas of the same size. The SCC of all specimens starts from the side faces of the specimen, and it is dominated by intergranular cracking. But the corrosion of the transverse specimens was more serious. The cracks of all specimens tend to propagate along the GBs with misorientation of about 50°-60°, and the difference of Schmidt factor (SF) values of grains on both sides of the crack is significantly large. The crack of longitudinal specimens propagates relatively parallel to the tensile direction, while propagation of the crack in transverse specimens is more zigzag.
Corrosion-Resistant Coatings on Spent Nuclear Fuel Canisters to Mitigate and Repair Potential Stress Corrosion Cracking (FY22 Status)
This report summarizes the activities performed by Sandia National Laboratories in FY22 to identify and test coating materials for the prevention, mitigation, and/or repair of potential chloride-induced stress corrosion cracking in spent nuclear fuel dry storage canisters. This work continues efforts by Sandia National Laboratories that are summarized in previous reports in FY20 and FY21 on the same topic. The previous work detailed the specific coating properties desired for application and implementation to spent nuclear fuel canisters (FY20) and identified several potential coatings for evaluation (FY21). In FY22, Sandia National Laboratories, in collaboration with four industry partners through a Memorandum of Understanding, started evaluating the physical, mechanical, and corrosion-resistance properties of 6 different coating systems (11 total coating variants) to develop a baseline understanding of the viability of each coating type for use to prevent, mitigate, and/or repair potential stress corrosion on cracking on spent nuclear fuel canisters. This collaborative R&D program leverages the analytical and laboratory capabilities at Sandia National Laboratories and the material design and synthesis capabilities of the industry collaborators. The coating systems include organic (polyetherketoneketone, modified polyimide/polyurea, modified phenolic resin), organic/inorganic ceramic hybrids (silane-based polyurethane hybrid and a quasi-ceramic sol-gel polyurethane hybrid), and hybrid systems in conjuncture with a Zn-rich primer. These coatings were applied to stainless steel coupons (the same coupons were supplied to all vendors by SNL for direct comparison) and have undergone several physical, mechanical, and electrochemical tests. The results and implications of these tests are summarized in this report. These analyses will be used to identify the most effective coatings for potential use on spent nuclear fuel dry storage canisters, and also to identify specific needs for further optimization of coating technologies for their application on spent nuclear fuel canisters. In FY22, Sandia National Laboratories performed baseline testing and atmospheric exposure tests of the coating samples supplied by the vendors in accordance with the scope of work defined in the Memorandum of Understanding. In FY23, Sandia National Laboratories will continue evaluating coating performance with a focus on thermal and radiolytic stability.
Advanced 3D characterisation of iodine induced stress corrosion cracks in zirconium alloys
Highlights: • Macroscopic crack direction is affected by metallurgical state. • High resolution XCT reveals complex crack morphology. • 3D-EBSD used to fully characterise attacked grain boundaries. • Evidence of iodine segregation ahead of a crack tip. - Abstract: In this study, a range of advanced techniques have been used to characterise iodine-induced stress corrosion cracks, and investigate their interaction with microstructure in zirconium alloys used as cladding material in nuclear power reactors. C-ring samples, machined from cladding tubes in a recrystallised and cold worked condition, were subjected to compressive stress while submerged in an iodine-ethanol solution to simulate stress corrosion cracks arising from pellet cladding interaction. The complex morphology of these stress corrosion cracks was imaged in 3D using X-ray computed tomography (XCT), and subsequent analysis using both 2D and 3D electron backscatter diffraction (EBSD) allowed for the interaction of these cracks with microstructure to be investigated. Intergranular cracking modes were observed to dominate in the recrystallised microstructure, resulting in cracking in the radial direction with variations on the order of the grain size. Crack propagation in the cold worked microstructure was observed to be correlated with the direction of the elongated grains, resulting in a more tangential crack path with less local variation and a microstructure potentially more susceptible to iodine-induced stress corrosion cracking. 3D-EBSD using serial sectioning allows for the determination of the full five-parameter grain boundary description, and opens up the possibility of a statistical analysis of susceptible boundaries to SCC. Nanoscale secondary ion mass spectrometry (NanoSIMS) was also used to map the chemical distribution in the crack tip region and provides possible evidence of iodine segregation ahead of the crack tip. The results provide new insight into crack propagation mechanisms in recrystallised and cold worked microstructures, and pave the way for a more mechanistic understanding of crack propagation through advanced characterisation of iodine-induced stress corrosion cracks in zirconium alloys.
Nondestructive Evaluation of Stress Corrosion Cracking in a Welded Steel Plate Using Guided Ultrasonic Waves
Stress corrosion cracking (SCC) had occurred in early-generation high-level nuclear waste tanks constructed by welding carbon steel. This paper describes an ultrasonic inspection system and its fundamental ability to detect and quantify the length of SCC on thick welded steel plates. The finite element method (FEM) was applied to simulate the welding process to estimate the welding residual stress field. Growth of stress corrosion cracks is driven by crack stress intensities exceeding the subcritical cracking threshold intensity. The subject plate was experimentally inspected with ultrasonic nondestructive evaluation (NDE) techniques to characterize the extent of SCC. The NDE system uses a piezoelectric transducer to generate guided waves in the thick steel plate, and a scanning laser Doppler vibrometer (SLDV) to measure multidimensional time–space wavefield data over a user-defined scanning area in the plate surface. The measured wavefield data can show wave interactions in a localized area in the plate due to the presence of the discontinuities of the SCC. To generate an inspection image that can precisely show the crack’s location and/or the dimension, the wavefield data are further processed to generate inspection image that maps the entire sample plate so the crack can be clearly identified in the plate while its length can be readily estimated. In conclusion, the ultrasonic test results for crack length agree well with the visually estimated length and are close to that predicted by the FEM for cracks in the weld residual stress field.
Irradiation Assisted Stress Corrosion Cracking Susceptibility of Alloy X-750 exposed to BWR environments
The effect of irradiation on stress corrosion cracking susceptibility and fracture toughness of alloy X-750 has been investigated. The material has been irradiated at a target temperature of 288°C in the Advanced Test Reactor at Idaho National Laboratory to a fluence of approximately 1.93x1020 n/cm2 (E > 1 MeV). Stress corrosion cracking crack growth rates were determined in both unirradiated and irradiated materials in normal water chemistry and hydrogen water chemistry environments. Although the effect of irradiation on tensile properties and fracture toughness was observed, there was no significant effect of irradiation observed on the propagation rate of stress corrosion cracks.
In situ synchrotron X-ray tomography of 304 stainless steels undergoing chlorine-induced stress corrosion cracking
To understand the mechanisms behind chlorine-induced stress corrosion cracking (CISCC) in 304 stainless steel, synchrotron high energy x-rays were used to perform in-situ x-ray microtomography on a pre-cracked sample under load in a corrosive simulated marine environment. The tomography scans showed the crack morphology evolving into a branching crack around the surface of the sample. Finite element analysis and stress intensity analysis are conducted to explain the observed crack branching.
Analysis of stress corrosion cracking in alloy 718 following commercial reactor exposure
Alloy 718 is generally considered a highly corrosion-resistant material but can still be susceptible to stress corrosion cracking (SCC). The combination of factors leading to SCC susceptibility in the alloy is not always clear enough. In this paper, alloy 718 leaf spring (LS) materials that suffered stress corrosion damage during two 24-month cycles in pressurized water reactor service, operated to >45 MWd/mtU burn-up, was investigated. Compared to archival samples fabricated through the same processing conditions, little microstructural and property changes occurred in the material with in-service irradiation, contrary to high dose rate laboratory-based experiments reported in literature. Though the lack of delta phase formation along grain boundaries would suggest a more SCC resistant microstructure, grain boundary cracking in the material was extensive. Crack propagation routes were explored through focused ion beam milling of specimens near the crack tip for transmission electron microscopy as well as in polished plan view and cross-sectional samples for electron backscatter diffraction analysis. It has been shown in this study that cracks propagated mainly along random high-angle grain boundaries, with the material around cracks displaying a high local density of dislocations. The slip lines were produced through the local deformation of the leaf spring material above their yield strength. Also, the cause for local SCC appears to be related to oxidation of both slip lines and grain boundaries, which under the high in-service stresses resulted in crack development in the material.
Precursor Evolution and Stress Corrosion Cracking Initiation of Cold-Worked Alloy 690 in Simulated Pressurized Water Reactor Primary Water
Stress corrosion crack initiation of two thermally-treated, cold-worked (CW) alloy 690 (UNS N06690) materials was investigated in 360oC simulated PWR primary water using constant load tensile (CLT) tests and blunt notch compact tension (BNCT) tests equipped with direct current potential drop (DCPD) for in-situ detection of cracking. SCC initiation was not detected by DCPD for either the 21% or 31%CW CLT specimens loaded at their yield stress after ~9,220 hours, however intergranular (IG) precursor damage and isolated surface cracks were observed on the specimens. The two 31%CW BNCT specimens loaded at moderate stress intensity after several cyclic loading ramps showed DCPD-indicated crack initiation after 10,400 hours of exposure at constant stress intensity, which was resulted from significant growth of IG cracks. The 21%CW BNCT specimens only exhibited isolated small IG surface cracks and showed no apparent DCPD change throughout the test. Post-test cross-section examinations revealed many grain boundary (GB) nano-cavities in the bulk of all the CLT and BNCT specimens particularly for the 31%CW materials. Cavities were also found along GBs extending to the surface suggesting an important role in crack nucleation. This paper provides an overview of the evolution of GB cavities and discusses their effects on crack initiation in CW alloy 690.
Nonlinear resonant ultrasound spectroscopy of stress corrosion cracking in stainless steel rods
Stainless steel containers are susceptible to stress corrosion cracking (SCC) under certain stress and corrosion conditions. Nonlinear ultrasonic techniques are very sensitive to the early presence of damage, more so than linear techniques. Nonlinear Resonant Ultrasound Spectroscopy (NRUS) is used here to measure a nonlinear shift in the resonance frequency due to a cumulative amount of SCC. Steel rods are immersed in a heated magnesium chloride solution and removed after different exposure times. NRUS measurements are conducted using the fundamental longitudinal mode of vibration. Rods exposed longer generally have a larger resonance frequency shift and are therefore more nonlinear. Thus NRUS might offer a means of detecting a cumulative SCC damage in a sample.
Characterization of Stress-Corrosion-Cracking in Plutonium-Bearing Storage Containers
Introduction • The Integrated Surveillance Program, under the Department of Energy, is responsible for the periodic surveillance of storage containers containing plutonium-bearing materials. • The container-packages are designed to isolate the materials for 50 to 100 years with minimal surveillance. • During monitoring, corrosion-pitting and stress-corrosion-cracking (SCC) has been identified in some of the inner-cans near the weld regions. • SCC through wall penetration would result in an undesired increased risk of leakage. • An investigation is in progress to identify and characterize corrosion events to determine the prevalence of corrosion features and the likelihood of a throughwall breach. • Here, we discuss the characterization efforts carried out by Scanning Electron Microscopy (SEM), including 3D-tomography by Focused Ion Beam (FIB) methods.
BOUNDING SURFACE FLAW CONFIGURATION SUSCEPTIBLE TO STRESS CORROSION CRACKING UNDER WELDING RESIDUAL STRESS IN A MULTIPLE-PURPOSE CANISTER
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Characterization of Stress Corrosion Cracking Initiation Precursors in Cold-Worked Alloy 690 Using Advanced High-Resolution Microscopy
Although highly cold-worked UNS N06690 has been shown to be susceptible to stress corrosion crack growth in pressurized water reactor primary water, it is unclear whether cold-work promotes stress corrosion crack initiation under constant load test conditions. To evaluate the stress corrosion cracking initiation response, constant load tensile and blunt notch compact tension testing were performed on 21% and 31% cold-worked samples from two thermally treated UNS N06690 materials in 360 °C simulated pressurized water reactor primary water. High-resolution examinations were conducted mid- and post-test to record the evolution of grain boundary precursor damage and intergranular crack nucleation by focused ion beam milling combined with scanning electron microscopy and by transmission electron microscopy. The surface and sub-surface morphology of intergranular cavities, local corrosion and shallow cracks were documented. Nanometer-sized cavities were observed associated with grain boundary carbides in all specimens. The 31% cold-work specimens consistently exhibited shallow intergranular cracks with oxidized crack flanks and a high density of cavities ahead of the oxide front. Three-dimensional distribution of carbides and cavities ahead of the cracked grain boundaries was analyzed to gain mechanistic insights into the processes that lead to crack initiation in highly cold-worked UNS N06690.
Assessment of Stress Corrosion Cracking Resistance of Activated Tungsten Inert Gas-Welded Duplex Stainless Steel Joints
The stress corrosion cracking behavior of duplex stainless steel (DSS) weld joint largely depends on the ferrite-austenite phase microstructure balance. This phase balance is decided by the welding process used, heat input, welding conditions and the weld metal chemistry. In this investigation, the influence of activated tungsten inert gas (ATIG) and tungsten inert gas (TIG) welding processes on the stress corrosion cracking (SCC) resistance of DSS joints was evaluated and compared. Boiling magnesium chloride (45 wt.%) environment maintained at 155 °C was used. The microstructure and ferrite content of different weld zones are correlated with the outcome of sustained load, SCC test. Irrespective of the welding processes used, SCC resistance of weld joints was inferior to that of the base metal. However, ATIG weld joint exhibited superior resistance to SCC than the TIG weld joint. The crack initiation and final failure were in the weld metal for the ATIG weld joint; they were in the heat-affected zone for the TIG weld joint.
Effect of dissolved oxygen level on stress corrosion cracking susceptibility of structural steels
Stress corrosion cracking (SCC) is a potential risk in structural steels used for heat exchanger and circulation loop in LWR. Generally, SCC susceptibility of structural steels increases with increasing the level of dissolved oxygen (DO) in circulating water, but DO tolerance below which no SCC risk is present could differ by the type of steel. To investigate the DO effect on SCC susceptibility, T23, T24 and T91 steels under static load were exposed to stagnant and circulating water at 200°C with varying DO levels. The results indicated that among the tested steels, the SCC susceptibility was highest in T91 but lowest in T23 which did not exhibit crack initiation with 100 ppb DO. T24 showed no cracking with 50 ppb DO but cracked with 100 ppb DO, implying intermediate SCC susceptibility in between T23 and T91. Based on the cracking results and hardness data, SCC susceptibility in the tested steels is considered driven by corrosion which was initiated/affected by DO in the water.