Stress Corrosion Cracking Of Alloy Research Articles

Corrosion and stress corrosion cracking resistances of the 17-4 precipitation hardened martensitic stainless steel additively manufactured using binder jet printing

The corrosion and stress corrosion cracking (SCC) properties of an additively manufactured (AM) 17-4 PH martensitic stainless steel that was produced using the binder jet printing (BJP) technique are compared with those of the conventionally manufactured (CM) steel, to critically examine the influence of porosity and subtle microstructural variations, introduced due to sintering and hot isostatic pressing (HIP), on them. Microstructures of the BJP and HIP specimens contain porosity, δ-ferrite, and MnS inclusions, while the CM ones do not contain either, but have NbC inclusions. All the corrosion properties of the BJP alloy, investigated using the immersion, cyclic potentiodynamic polarization, and galvanostatic (GL) tests, are inferior (compared to those of the CM alloy) due to the porosity in it, while the NbC inclusions present in the CM alloy adversely affected its corrosion characteristics. Surprisingly, the CM 17-4 PH specimens subjected to the SCC tests failed, while those of the BJP specimens did not (despite the relatively large porosity in it). Detailed analyses (including the X-ray computed tomography) show that crack bridging induced by the δ-ferrite phase, which has superior corrosion resistance compared to the martensite, is the reason for the BJP alloy's SCC resistance. HIP, which reduces both the porosity and δ-ferrite content, resulted in enhanced corrosion and SCC resistances that are even superior to those of the CM 17-4 PH. These results demonstrate, emphatically, that the subtle microstructural variations in the AM alloys can have profound effects on their properties, especially those related to structural integrity and reliability.

Effect of Ti on stress corrosion cracking behavior and mechanism of Monel K500 alloy in flowing seawater

Purpose The purpose of this paper is to investigate the effect Ti on stress corrosion cracking (SCC) and flow accelerated stress corrosion cracking (FA-SCC) behavior and mechanisms of Monel K500 alloy. Design/methodology/approach Monel K500 alloy with different Ti contents was designed. A metallurgical microscope (XJP-3C) and scanning electron microscopy (EV0 MA15 Zeiss) with an energy dispersive spectroscopy were used to analyze the microstructure of the Monel K500 alloy. In situ electrochemical tests were carried out in static and flowing seawater to study FA-SCC behavior. Findings The number of TiCN particles in the alloy increased as the increase of Ti content. The static corrosion and SCC of Monel K500 alloy are reduced as the content of Ti increases. Generally, the SCC of alloys was caused by the synergistic effect of the anodic dissolution at exposed metal matrix and the pit corrosion of metal matrix adjacent to TiCN particles, which was further accelerated by flowing. Originality/value The corrosion behavior and mechanism of Monel K500 alloy with different Ti contents in a complex flowing seawater environment are still unclear, which remain systematic study to insure the safe service of the alloy.

Investigating the susceptibility of Fe39Mn20Co20Cr15Si5Al1 high entropy alloy to stress corrosion cracking in a 3.5 wt.% NaCl environment

This study examines the stress corrosion cracking (SCC) behavior of the Fe39Mn20Co20Cr15Si5Al1 (at.%) high entropy alloy in a salt solution at room temperature. The research employs slow strain-rate tensile tests and advanced analytical techniques, such as electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), to understand the SCC behavior. Results show that the alloy has a high susceptibility to SCC. The TEM analysis supports a slip-step dissolution model for SCC. The EBSD results suggest that cracks form at boundaries between martensite and austenite phases and propagate through interphase and inter-variant boundaries. Reducing such boundaries may mitigate the alloy's SCC susceptibility.

A mechanistic study on dealloying-induced stress corrosion cracking of Alloy 800 in boiling caustic solutions

Alloy 800 (Fe-30Ni-20Cr) is used in steam generators of some nuclear power plants. In-service performance has been excellent, but stress corrosion cracking (SCC) can occur in certain off-chemistry environments. In this study, the SCC mechanism for Alloy 800 in boiling caustic solution is investigated. After exposure, a Ni-rich porous surface film is formed with a large density of cracks. Complementary electron microscopy techniques are applied to study the SCC mechanism at the nanoscale from chemical and mechanical perspectives. Results suggest SCC initiation occurs by transmission of a micro-crack from the Ni-rich layer into the substrate material via a cleavage mechanism.

Bayesian approach for prediction of primary water stress corrosion cracking in Alloy 690 steam generator tubing

Alloy 690 tubing has been shown to be highly resistant to primary water stress corrosion cracking (PWSCC). Nevertheless, predicting the failure by PWSCC in Alloy 690 SG tubes is indispensable. In this work, a Bayesian-based statistical approach is proposed to predict the occurrence of failure by PWSCC in Alloy 690 SG tubing. The prior distributions of the model parameters are developed based on the prior knowledge or information regarding the parameters. Since Alloy 690 is a replacement for Alloy 600, the parameter distributions of Alloy 600 tubing are used to gain prior information about the parameters of Alloy 690 tubing. In addition to estimating the model parameters, analysis of tubing reliability is also performed. Since no PWSCC has been observed in Alloy 690 tubing, only right-censored free-failure life of the tubing are available. Apparently the inference is sensitive to the choice of prior distribution when only right-censored data exist. Thus, one must be careful in choosing the prior distributions for the model parameters. It is found that the use of non-informative prior distribution yields unsatisfactory results, and strongly informative prior distribution will greatly influence the inference, especially when it is considerably optimistic relative to the observed data.

Insights into the roles of intergranular carbides in the initiation of intergranular stress corrosion cracking of alloy 690 in simulated PWR primary water

The effect of intergranular carbides on the stress corrosion crack (SCC) initiation of alloy 690 in high-temperature hydrogenated water was systematically studied. Irrespective of the aging temperature, the resistance to SCC initiation continuously increases with increasing carbide coverage. The understanding of carbide effects was further advanced. Mechanically, the carbide mitigates the breach of surface oxide film over the grain boundary by suppressing the local apparent plastic strain. The carbide also greatly diminishes grain boundary migration by retarding the diffusion of Cr. More importantly, it decomposes when exposed to the environment and serves as Cr source for forming protective oxide film.

On the role of intergranular nanocavities in long-term stress corrosion cracking of Alloy 690

The life extension of nuclear power plants beyond 60-years relies on the integrity of the steam generator tubes during the extended life. Alloy 690 steam generator tubes have been generally believed to be resistant to stress corrosion cracking (SCC) failures until the recent unexpected observation of intergranular cavities during long-term SCC testing. Here, a multi-technique characterisation approach has been used to study the behaviour of 30% cold worked samples exposed to long-term SCC testing at constant load. This has provided quantitative results on the effect of intergranular cavities and oxidation ahead of the crack tip. SCC mechanisms of creep cavity formation and associated oxidation through the cavity networks are discussed and the required stress to failure calculated, providing a quantitative measure of their weakening effect and a reliable way of assessing and predicting this important degradation problem.

Predicting Stress Corrosion Crack Tip Electrochemical Conditions and the Influence of Exposure Environment

Metallic structures are commonly exposed to marine atmospheric conditions in real world conditions, characterized by thin water layers (WL) allowing for corrosion to occur. For austenitic stainless steels in such atmospheric environments, pitting and stress corrosion cracking (SCC) are potential degradation mechanisms. The rate and extent of corrosion and SCC on the alloy surface is dictated by the combination of environmental, physicochemical, and geometric variables, which include relative humidity, temperature, electrolyte conductivity, electrolyte film thickness, in addition to the electrochemical kinetics on the alloy surface. Due to the large potential variation in environmental conditions, modeling can serve as an important tool to evaluate environmental effects on the corrosion and SCC of alloys and galvanic couples.In this study, Finite Element Modeling approaches are used to evaluate chemical and electrochemical conditions present in a crack exposed to marine, thin film environments. A full reactive transport model, incorporating metal hydrolysis and salt formation, is built to evaluate SCC crack tip conditions. The influence of WL thickness, stress intensity level, crack length, and specimen geometry are evaluated. Specifically, crack tip pH and equilibrium potential among other variables will be compared between the various atmospheric factors both spatially and temporally. The modeling results will be compared to atmospheric SCC experiments and help give insight into critical experimental and testing phenomena. Acknowledgements Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This document is SAND2021-5051 A.

Correlation between Grain Boundary Migration and Stress Corrosion Cracking of Alloy 600 in Hydrogenated Steam

Solution-annealed Alloy 600 samples were tested under active load conditions in a superheated low pressure H2-steam environment over a range of oxidising potentials relevant to Primary Water Stress Corrosion Cracking (PWSCC). Increased SCC susceptibility was observed for potentials the more reducing than the Ni/NiO transition where a deeper Preferential Intergranular Oxidation (PIO) occurred. Advanced microstructural analysis clearly showed that SCC initiated along the grain boundaries that exhibited Diffusion-Induced Grain Boundary Migration (DIGM), as well as enrichments in Al and Ti. Rather than the result of a single dominant mechanism, the initiation of SCC in Ni-base alloys involves the synergistic interactions between DIGM and the formation of Al- and Ti-enriched oxides, promoting PIO that can fracture with applied stress.

Effect of intergranular carbides on the cracking behavior of cold worked alloy 690 in subcritical and supercritical water

The effect of intergranular (IG) carbides on the cracking behavior of cold worked Alloy 690 was studied in both subcritical and supercritical water (SCW) environments. The crack growth rate (CGR) was lower when IG carbides were removed by prior solution annealed (SA) treatment, indicating a detrimental effect of IG carbides. The presence of IG carbides enhances the local strain accumulation at the grain boundary due to the lattice mismatch, thus promotes the crack tip strain rate and increases the CGR. The detrimental effect of IG carbides is more significant in subcritical water than in SCW, which can be attributed to the different controlling parameters of cracking between these two environments. The widely recognized beneficial effect of IG carbides on stress corrosion cracking in Alloy 600 is detrimental in cold worked Alloy 690, which might be related to the different degrees of grain boundary oxidation that contributed to IG cracking.

The roles of thermal mechanical treatment and δ phase in the stress corrosion cracking of alloy 718 in primary water

Eleven variants of the precipitation-hardened alloy 718 were tested for stress corrosion cracking (SCC) resistance in a simulated pressurized water reactor primary water environment. The optimized grade had the lowest SCC susceptibility, while more traditional thermal-mechanical treatment exhibited the highest. The high susceptibility was attributed primarily to δ phase formation at grain boundaries (GBs), with localized deformation playing a secondary role. Intergranular cracks initiated most often at GBs decorated with either the high temperature δ phase, which aligned with respect to the slip planes intersecting the GB, or the low temperature δ phase, which is a film on the GB.

Magnetite-accelerated stress corrosion cracking of alloy 600 in water containing 100 ppm lead oxide at 315 °C

The purpose of this work is to verify the effect of magnetite coupled with Alloy 600 on stress corrosion cracking (SCC) behavior in water containing 100 ppm PbO at 315 °C. SCC tests were conducted using U-bend specimens partially covered with magnetite, simulating a condition wherein magnetite was deposited on steam generator tubes. The maximum crack depths of the Alloy 600 specimens coupled with magnetite were 20–68% greater than those of the uncoupled specimens for a given test time. The coupling of magnetite to Alloy 600 led to the fast growth of oxide films, resulting in the formation of relatively Cr-depleted inner oxides with defects. The corrosion potential of Alloy 600 was 90 mV lower than that of magnetite at 80 °C, indicating that the corrosion of Alloy 600 could be stimulated by galvanic coupling with magnetite. The accelerated stress corrosion and altered film properties were discussed from the viewpoint of a galvanic coupling effect.

Mechanistic understanding of the temperature dependence of crack growth rate in alloy 600 and 316 stainless steel through high-resolution characterization

Stress corrosion cracking of Alloy 600 (A600) and 316 stainless steel (316SS) exposed to simulated pressurized water reactor primary water at temperatures of 320–360 °C has been investigated and compared. Intergranular oxidation developed ahead of all crack tips prepared from these two alloys. High-resolution characterization reveals that there are two main rate-controlling mechanisms contributing to crack propagation: a diffusion-based and a mechanical deformation-based. The different temperature dependence of crack growth rate (CGR) in A600 and 316SS can be explained after confirming that the two rate-controlling mechanisms exhibit different “weights” in the two alloys. The diffusion-based mechanism plays a dominant role in accelerating the CGR of A600, while the mechanical deformation-based mechanism is responsible for the observed CGR decrease of 316SS at higher temperatures.

Mechanistic insights into lead-accelerated stress corrosion cracking of Alloy 600

The purpose of this work is to provide insights into the lead-accelerated stress corrosion cracking mechanisms of Alloy 600 from the perspective of the electrochemical role of lead. Lead-species were electrochemically reduced at the oxide/matrix interface, which increased the dissolution rate of the alloy elements (preferentially Cr) from and oxidation of the matrix. These electrochemical processes resulted in the fast formation of defective thick oxides with a relatively lower Cr content, which provide paths for the migration of lead-species toward the interface. Finally, the mechanistic model of fast cracking by lead contamination is described.

Combined effects of lead and magnetite on the stress corrosion cracking of alloy 600 in simulated PWR secondary water

The combined effects of lead and magnetite on the stress corrosion cracking of Alloy 600 were investigated in simulated PWR secondary water at 315 °C. Lead induced fast stress corrosion cracking of Alloy 600, and this cracking was accelerated when the alloy was in contact with magnetite. The coupling with magnetite caused the rapid growth of the oxide layer on Alloy 600, resulting in the formation of a defective oxide layer. Based on these results, the effect of magnetite as an additional contributor to the stress corrosion cracking of Alloy 600 is discussed.

An atom probe tomography study of Pb-caustic SCC in alloy 800

Stress corrosion cracking (SCC) of Alloy 800 was induced in 330 °C Pb-caustic solution (pH330°C 9.5). These laboratory tests simulated extreme crevice environments in nuclear plants, but do not occur under normal operating conditions. Atom probe tomography (APT) was used to examine crack tips, a powerful but scarcely used technique for this application. APT revealed sub-nanometre scale crack chemistry: successive Ni-rich, de-alloyed regions and segregated Fe- and Cr-rich oxides. Also, APT allowed for quantification of impurities, such as Pb which segregated to oxide-metal interfaces (<4 at.%). Discussion highlights new mechanistic insights from APT, complementing prior transmission electron microscopy (TEM) analysis.

Effect of serrated grain boundary on stress corrosion cracking of Alloy 600

Abstract The effect of a serrated grain boundary on stress corrosion cracking (SCC) of Alloy 600 was investigated in terms of improvement of SCC resistance. Serrated grain boundaries and straight grain boundaries were obtained by controlled heat treatment. SCC cracks preferentially initiated and grew at grain boundaries normal to the tensile loading axis. Resolved tensile stress normal to the grain boundary was lower in serrated grain boundaries compared to straight grain boundaries. The specimen with serrated grain boundaries showed higher SCC resistance than that with straight grain boundaries due to a lower resolved tensile stress normal to the grain boundary.

Customization of the coupled environment fracture model for predicting stress corrosion cracking in Alloy 600 in PWR environment

The coupled environment fracture model (CEFM) has been modified and calibrated to predict crack growth rate (CGR) in Alloy 600 under typical PWR primary coolant conditions. The customized CEFM provides quantitative predictions of the effects of stress intensity factor, hydrogen concentration, yield strength, and temperature on CGR in Alloy 600 in PWR primary coolant environments, as well as explaining the dominating mechanism of stress corrosion cracking (SCC) in this alloy. The importance of the mechanical properties, such as yield strength and stress intensity factor, as identified previously from Artificial Neural Network (ANN) analysis of CGR data in the literature, has been confirmed theoretically. The success in explaining the intergranular stress corrosion cracking (IGSCC) of Alloy 600, argues that the CEFM, which was originally developed to describe IGSCC in sensitized stainless steels, is equally applicable for describing IGSCC in Alloy 600.

Theoretical aspects of stress corrosion cracking of Alloy 22

Theoretical aspects of the stress corrosion cracking of Alloy 22 in contact with saturated NaCl solution are explored in terms of the Coupled Environment Fracture Model (CEFM), which was calibrated upon available experimental crack growth rate data. Crack growth rate (CGR) was then predicted as a function of stress intensity, electrochemical potential, solution conductivity, temperature, and electrochemical crack length (ECL). From the dependence of the CGR on the ECL and the evolution of a semi-elliptical surface crack in a planar surface under constant loading conditions it is predicted that penetration through the 2.5-cm thick Alloy 22 corrosion resistant layer of the waste package (WP) could occur 32,000 years after nucleation. Accordingly, the crack must nucleate within the first 968,000 years of storage. However, we predict that the Alloy 22 corrosion resistant layer will not be penetrated by SCC within the 10,000-year Intermediate Performance Period, even if a crack nucleates immediately upon placement of the WP in the repository.

A mechanistic study of SCC in Alloy 600 through high-resolution characterization

High-resolution characterization was used to understand the mechanisms controlling stress corrosion cracking (SCC) in Alloy 600 exposed to simulated PWR primary water conditions. Three potential active crack tips obtained from different types of grain boundaries were studied and compared. The results suggest that the dominant mechanism controlling SCC propagation is intergranular internal oxidation. The applied stress, pre-existent residual strain, the accumulation of defects around the crack tip, the formation of a Fe-Cr-depleted zone, and a porous intergranular oxide are acknowledged as necessary precursors to SCC. Based on the results obtained in this study, a model of SCC propagation is proposed.