Stress Corrosion Cracking Initiation Research Articles

Stress-Corrosion-Cracking Sensitivity of the Sub-Zones in X80 Steel Welded Joints at Different Potentials

X80 steel plays a pivotal role in the development of oil and gas pipelines; however, its welded joints, particularly the heat-affected zone (HAZ), are susceptible to stress corrosion cracking (SCC) due to their complex microstructures. This study investigates the SCC initiation mechanisms of X80 steel welded joints under practical pipeline conditions with varying levels of cathodic protection. The SCC behaviors were analyzed through electrochemical measurements, hydrogen permeation tests, and interrupted slow strain rate tensile tests (SSRTs) conducted in a near-neutral pH environment under different potential conditions (OCP, −1.1 VSCE, −1.2 VSCE). These behaviors were influenced by microstructure type, grain size, martensite/austenite (M/A) constituents, and dislocation density. The sub-zones of the weld exhibited differing SCC resistance, with the fine-grain (FG) HAZ, base metal (zone), welded metal (WM) zone, and coarse-grain (CG) HAZ in descending order. In particular, the presence of coarse grains, low dislocation density, and extensive M/A islands collectively increased corrosion susceptibility and SCC sensitivity in the CGHAZ compared to other sub-zones. The SCC initiation mechanisms of the sub-zones within the X80-steel welded joint were primarily anodic dissolution (AD) under open-circuit potential (OCP) condition, shifting to either hydrogen-enhanced local plasticity (HELP) or hydrogen embrittlement (HE) mechanisms at −1.1 VSCE or −1.2 VSCE, respectively.

Quantification of SCC mechanisms in austenitic alloys under PWR primary water conditions

Abstract In order to achieve a full mechanistic understanding of stress corrosion cracking (SCC), the key operating mechanisms need to be identified but also quantified. In this study, we summarize and rationalize key findings from the last 15 years of high-resolution characterization of SCC in our group. A comprehensive characterization of a set of austenitic alloys with different Ni content and constant Cr level, tested under simulated pressurized water reactor (PWR) primary water conditions at various temperatures, has revealed evidence for at least two operating mechanisms: one diffusion-related and the other deformation-related. For their relevance to the nuclear industry, two additional alloys with increased Cr content were also studied (A800 and A690). Key precursors for SCC initiation and propagation are identified and their effect on alloy degradation discussed. A list of key materials’ properties that ensure low SCC susceptibility is proposed.

The effect of surface grinding on the stress corrosion cracking initiation of 316LN stainless steel in 600 °C supercritical CO2

The effect of surface grinding on the stress corrosion cracking initiation of 316LN stainless steel in 600 °C supercritical CO2 was studied through constant extension rate tensile test. The ground surface exhibited much higher oxidation resistance than the polished surface as the deformation layer can greatly enhance the diffusion of active elements and promote the formation of more protective oxide scale. Moreover, the intergranular cracking was effectively suppressed on the ground surface as the ingress of oxidant along the original grain boundary was inhibited.

Effect of laser shock peening on stress corrosion cracking of TC4/2A14 dissimilar metal friction stir welding joints

Stress corrosion cracking (SCC) of welded joints is a significant issue that affects the widespread application of multimetal welded structures in aerospace, transportation, and other fields. This paper investigates the effect of laser shock peening (LSP) on the SCC of TC4/2A14 friction stir welded (FSW) joints with dissimilar metals. The microstructural evolution, mechanical properties, and corrosion behavior with and without LSP treatment were investigated using optical microscopy, transmission electron microscopy, X-ray analysis, electrochemical testing, and slow strain rate tensile testing (SSRT). The results showed that the LSP significantly refined the grains and induced work-hardening on the near-surface of the material. After LSP, the compressive residual stresses on the surface of the titanium and aluminium sides were −817.6 MPa and −153.6 MPa for the joint, respectively. Corrosion resistance was also improved, with the self-corrosion current density on the Al side of the joints reduced by 77% and the SCC susceptibility index (Issrt) reduced from 0.140 to 0.121. The enhanced SCC resistance of the LSP-treated TC4/2A14 dissimilar metal FSW joints is primarily due to grain refinement, work-hardening effect, and high-level compressive residual stress. These factors not only enhance the material strength and improve the pitting resistance of the joints but also prevent the initiation and propagation of SCC.

In-situ analysis of SCC initiation on Alloy 600 surface in primary water environment using digital image correlation with electron backscatter diffraction

This study examined the stress corrosion cracking (SCC) phenomenon of Alloy 600 in a high-temperature primary water environment. The investigation involved the observation of local plastic deformation and SCC initiation on the specimen surface using an observation apparatus, comprising a laser confocal microscope (LCM) and an autoclave equipped with a diamond window view-port. Prior to conducting the SCC initiation test, the crystalline structure of the specimen, including grain boundary (GB) characteristics, Schmid factor, and trace inclination angle, underwent examination using an electron backscatter diffraction (EBSD) technique. The local strain distribution was subsequently analyzed using a digital image correlation (DIC) method, employing LCM images obtained during the SCC experiments. The ex-situ EBSD analysis indicated that SCC mainly initiated at random high-angle grain boundaries (HAGB) with trace inclination angles close to 90° relative to the tensile axis. DIC analysis further revealed that most GBs in the vicinity of the highly strained area cracked among random HAGBs with a high trace inclination angle to the tensile axis. This confirms that the local strain gradient at GBs plays an important role in SCC initiation, serving as a more suitable indicator of an additional stress component to effective normal stress acting on GBs compared to the Schmid factor mismatch.

Parametric study on stress distribution of thin disk specimen of rupture disk corrosion test influencing SCC initiation using finite element analysis

Rupture disk corrosion test (RDCT) method has been recently developed for real-time measurement of initiation of stress corrosion cracking (SCC) in a high-temperature water. This work presents a parametric study on the stress distribution of a thin disk specimen of RDCT to consider the fixture shape and friction using finite element analysis (FEA). The FEA results showed a dome-shaped deformation of the specimen. From the stress analysis as a function of friction coefficient, it was suggested that the maximum stress was applied around the dome-edge, which was invariant with change to the friction coefficient. This indicates that friction between the fixture and the specimen has little effect on stress distribution. On the other hand, the stress analysis as a function of a rounded-corner radius (Rc) revealed the location at which the maximum stress was applied shifted from the dome edge to the dome center as Rc increased. From SCC initiation tests using the RDCT apparatus in a primary water environment, it was found that SCC initiates at the dome edge when Rc is 0.5 mm, while SCC initiates near the disk center when Rc is 2.0 mm. This experimental result is in good agreement with the results of FEA.

Study of stress corrosion cracking susceptibility of irradiated ferrite-martensitic stainless steel 07Kh12NMFB in supercritical water. Part 2. Development of corrosion crack identification technique and analysis of autoclave test results

The studies of stress corrosion cracking have been carried out for the stainless ferritic-martensitic steel with chromium content of 12% irradiated to a damage neutron dose of ~12 dpa. This steel was chosen as a candidate material for the internals of supercritical water-cooled reactors (SWCR).The first part of the paper was devoted to autoclave testing of specially designed disk specimens under constant load in a supercritical water environment (at 450°C and 250 atm pressure). In this part of the article the developed technique for corrosion cracks identification is presented and analysis of autoclave tests results is performed.As a result of the experiments performed, the threshold stress values below which stress corrosion cracking initiation doesn’t occur. The values of threshold stresses are determined for the studied steel irradiated at temperatures of 390 and 550°C. Possible mechanisms of stress corrosion cracking of the studied steel are analyzed and directions for further research are proposed.

Stress Corrosion Cracking of High-Strength Ni-Base Alloys in Pressurized Water Reactor Primary Water Containing KOH vs. LiOH

The stress corrosion cracking (SCC) behavior of high-strength Ni-base Alloys X-750 and 718 has been investigated to assess the effect of replacing LiOH with KOH as the pH moderator in pressurized water reactor (PWR) primary circuit coolant. The SCC initiation behavior of X-750 was evaluated by multitensile specimen constant load test in either LiOH- or KOH-containing PWR primary water. The SCC growth behavior of X-750 and Alloy 718 was evaluated on compact tension specimens with on-the-fly changes between LiOH- and KOH-containing water in three different PWR primary water chemistries. Direct current potential drop technique was used for in situ monitoring of crack initiation time and crack extension in SCC initiation and propagation tests, respectively. The results suggest no significant effect of KOH vs. LiOH on the SCC initiation or propagation of the tested materials.

Stress Measurement of Stainless Steel Piping Welds by Complementary Use of High-Energy Synchrotron X-rays and Neutrons

Probabilistic fracture mechanics (PFM) is increasingly recognized as a viable approach for evaluating the structural integrity of nuclear components, such as piping, primarily affected by stress corrosion cracking (SCC). PFM analysis requires several input parameters, among which welding residual stress is critically important due to its significant influence on SCC initiation and propagation. Recently, a novel technique involving a double-exposure method (DEM) utilizing synchrotron X-rays was introduced as an effective means for measuring welding residual stress with high spatial resolution. In this paper, we applied DEM to assess the residual stress of a plate specimen, which was extracted from a welded pipe through electrical discharge machining. Consequently, detailed stress maps under a plane stress state were generated. Additionally, the residual stress distributions in the welded pipe under a triaxial stress state were evaluated using neutron diffraction. Based on these findings, we proposed a methodology to acquire detailed stress maps of welded pipes by combining high-energy synchrotron X-rays and neutron diffraction.

Correlation of Microstructural Events with Stress Corrosion Cracking Initiation Behaviour in Additively Manufactured Ni-Based Alloy 718: Microcapillary Electrochemical Technique Implementation

During the past decade, metal additive manufacturing has gained tremendous attention for the production of substitutive components for oil, gas, and nuclear industries as a consequence of the possibility of fabricating complex full-density components with fewer manufacturing processes compared to conventional methods. Laser powder bed fusion (L-PBF) is one of the most precise additive manufacturing techniques based on layer-by-layer production of the final component with a high-power laser beam. Most investigations to dat focus on comparing the mechanical properties of additively manufactured components with conventional counterparts. Process parameter optimization has been extensively studied for obtaining almost full-density components with comparable and in some cases even better mechanical performance compared to conventionally manufactured components. However, the corrosion and more specifically stress corrosion cracking (SCC) behaviour of L-PBF and in general, additively manufactured components has received less attention. Since the major concern in critical industries is to reduce the risk of SCC occurrence, the mechanistic study of the effect of various micro and submicron structural alterations on SCC risk assessment is of paramount importance.It is well recognized in the corrosion community that the most critical stage of SCC is related to the incubation/initiation stage. From the information gathered by various authors in the past decades, having a prolonged incubation/initiation stage of SCC could significantly increase the lifetime of components, which results from the balance between the formation and destruction reactions occurring in the passive layer-environment interface. Thus, it is essentially always the case that by retarding the initiation of cracks, we significantly increase the life cycle. Until now, traditional electrochemical techniques are not able to detect the SCC initiation stage as a consequence of the large surface area in contact with the solution and the shortcomings of the used electrochemical techniques. Thus, during the past decades, extensive research has been done on the crack propagation stage by utilizing various techniques and statistical models for the prediction of crack growth rate. This study reports for the first time the correlation of electrochemical polarization behaviour with the susceptibility of the material to SCC initiation. Such an approach fulfils the knowledge gap covering the initiation mechanism by proposing the microcapillary method as the method of choice for mechanistic analysis of initiation susceptibility.In the current investigation, various electrochemical techniques combined with high-resolution microstructural analysis were implemented to elucidate the importance of submicron characteristics of L-PBF fabricated components on SCC initiation risk assessment. L-PBF fabricated Ni-Fe-Cr based alloy 718 with a slight variation of laser power in the range of achieving full density was used for elucidating the effect of slight alterations in micro and nanostructure in corrosion and SCC initiation behaviour. It was found that even with a slight variation of L-PBF process parameters such as laser power, various alterations occurred in micron and submicron scale namely, a slight decrease in hardness, an increase in subgrain width, porosity, dislocation density, the density of nanovoids adjacent to subgrain boundaries, and presence of carbide particles. Such alterations consequently lead to significant electrochemical behaviour and SCC initiation susceptibility. Furthermore, the underlying mechanisms responsible for the observed variations in SCC initiation susceptibility had been explained as a consequence of alterations in subgrain width, dislocation density, intrinsic nano cavities density adjacent to subgrain boundaries, and the presence of nanoscale carbide particles adjacent to subgrain boundaries. From the obtained results, the microcapillary technique proved to be a very useful and powerful tool for assessing SCC initiation susceptibility for additive manufactured components with submicron scale structures.

Electrochemical potential dependence of SCC initiation in X60 pipeline steel in near-neutral pH environment

Stress corrosion crack (SCC) in pipelines directly affects the long-term safety service and has a major impact on the continuity of oil and gas delivery. The near-neutral pH SCC propagation process has been widely reported in the past decades, while the crack initiation mechanism is rarely studied in-depth. Here, the crack initiation behavior of ×60 pipeline steel under different electrochemical potential levels was investigated by interrupted slow strain rate tensile testing (SSRT) and microstructural characterization. The experimental results show that the SCC initiation is dominated by hydrogen-facilitated anodic dissolution (AD) under the open circuit potential (OCP) environment, while it is by the hydrogen embrittlement (HE) under −1.1 and −1.2 VSCE cathodic potentials. All the SCC cracks in ×60 pipeline steel show transgranular features, pathing through the grains in different crystallographic orientations, which showed dependence on the electrochemical potential of the steel: initiation via slip-enhanced dissolution of {110}//ND or {112}//ND texture under OCP environment, but mostly by quasi-cleavage cracking of {100}//ND texture under −1.1 and −1.2 VSCE cathodic potentials.

Combining turning with slide burnishing to improve surface integrity and stress corrosion resistance

The surface characteristics play a pivotal role in stress corrosion cracking (SCC) resistance and fatigue life. Strengthening and modifying the material surface properties during manufacturing is a promising approach to improving the component service performance. Therefore, this study proposes a new surface finishing and strengthening mode that combines turning with slide burnishing (TSB) with a common turning tool. The core idea is that the cutting edge of the tool removes the material surface layer as a chip while the tool corner with a large radius performs a slide burnishing process on the manufactured surface. The machining-induced surface integrity in terms of residual stress, surface roughness, micro-hardness and microstructure changes was examined, and the SCC susceptibility was tested in a strong corrosion environment. The results showed that the surfaces processed by TSB suffered lower biaxial residual stresses, higher micro-hardness and deeper hardened layer when compared with the traditional turned surfaces. Besides, it was observed the TSB processed surfaces were less susceptible to SCC initiation which could be attributed to the lower residual stress introduced by TSB. Additionally, the multi-pass TSB treated surfaces showed significant low SCC susceptibility. Consequently, this work suggests that the surface integrity and the service performance like SCC resistance can be improved by TSB.

Corrosion damage repair of 7075-T6 aluminum alloy by ultrasonic nanocrystal surface modification

This study explored the corrosion resistance of 7075-T6 aluminum alloy by immersing it in a 3.5% NaCl solution to induce corrosion damage, and subsequently repairing the sample surface using ultrasonic nanocrystal surface modification (UNSM). UNSM was found to effectively repair the defects on the surface of the pre-corroded sample and improve its stress corrosion crack (SCC) resistance. The improvement in corrosion resistance and SCC resistance of sample was mainly due to the compressive residual stress and grain refinement caused by UNSM, which not only improved the pitting resistance but also effectively inhibited the initiation and extension of stress corrosion cracks.

Effects of Acetic Acid on the Initiation of Pitting Corrosion and Stress Corrosion Cracking on Low-Alloy Steel in pH Buffer Solutions

Applying organic chemicals as an oxygen scavenger or a corrosion inhibitor to the water treatment of steam-water circuits of power plants possibly results in the generation of organic acids, such as acetic acid. This necessitates assessing the effects of the generated organic acids on the corrosion of the steam-water circuits prior to applying the organic chemicals. In this research, the effects of acetic acid on pitting corrosion and stress corrosion cracking (SCC) on low-pressure turbine materials of low-alloy steel were examined by electrochemical measurements and U-bend tests. Buffer solutions of various pHs from 4.5 to 10 were used as test solutions to investigate the effects of acidification by acetic acid on corrosion. Electrochemical measurements indicated that the steel was passivated at pHs above 8 and was the most susceptible to pitting corrosion at pH 8. In the U-bend test, SCC was initiated readily at a pH of around 8. Because SCC occurred at sites of pitting corrosion, pitting corrosion was suggested to be a precursor of SCC. Acetate ions did not trigger pitting corrosion or SCC on the steel. Rather, the addition of acetate ions to a Cl−-containing solution mitigated the initiation of pitting corrosion, resulting in the inhibition of SCC initiation.

Stress corrosion cracking behavior of AISI 316L stainless steel treated by surface mechanical grinding

The effects of surface mechanical grinding treatment (SMGT) on stress corrosion cracking (SCC) behavior of machined 316L stainless steel surface were investigated by comparing to those of turned and milled surfaces. The surface stress state, roughness, microstructure, and microhardness induced by the different surface machining processes were tested and analyzed. The SCC tests on the turned, milled and SMGT samples were conducted in boiling MgCl2 solution at 155 ℃, and the crack morphologies were analyzed in detail to establish the relationship between the surface integrities and the resulting SCC behavior. The findings indicated that the SMGT altered the SCC initiation behavior and improved SCC resistance of the turned and milled surfaces of 316L stainless steel, and the surface stress state, microstructure refinement and surface strengthening induced by SMGT contributed to the improvement of the SCC resistance.

Effect of Thermo-Mechanical Processing on Initiation and Propagation of Stress Corrosion Cracking in 304L Austenitic Stainless Steel

Despite the high corrosion resistance of austenitic stainless steels (SSs), a significant reduction of stress corrosion cracking (SCC) resistance has been reported in cases of high residual stress and metastable microstructural features. In this study, the effect of thermo-mechanical processing (TMP) on the initiation and propagation of SCC in 304L SS was studied. To better understand the SCC mechanisms, three TMPs conditions—welded, solution annealed at 1050 °C for tens of seconds, and straightened—were used. The research focused on analyzing the initial microstructure, residual stress, and hardness along the depth direction to assess SCC resistance and establish correlations with the observed SCC modes. Experimental results demonstrated that transgranular SCC was observed in regions exhibiting elevated residual stress induced by welding and straightening processes. Furthermore, the presence of strain-induced martensite transformation and slip bands formed during plastic deformation were identified as additional factors contributing to the susceptibility of SCC. The study findings highlighted that the magnitude and distribution of residual stresses, in conjunction with microstructural evolution, could be varied depending on the specific TMP condition, leading to different SCC susceptibilities, cracking modes, and directions.

Physics-based corrosion reliability analysis of miter gates using multi-scale simulations and adaptive surrogate modeling

Stress corrosion cracking (SCC) initiation is usually simulated at the mesoscale, and these computations are usually expensive. This is made more computationally challenging or impossible when such simulations are coupled with a macroscale structural model required for reliability analysis, due to the sources of uncertainty from both scales. This paper tackles this computational barrier to perform physics-based corrosion reliability analysis of large structures using mesoscale simulations via a novel, adaptive surrogate modeling framework. A global surrogate model of the structure is first constructed from a finite element (FE) mechanical model to propagate various sources of input uncertainty at the macroscale to the local stress responses. After that, a mesoscale surrogate model is constructed from phase-field (PF) simulations to predict the failure probability of a given location by accounting for uncertainty in both the macroscale and mesoscale models. In order to guarantee the accuracy of the mesoscale surrogate model and reduce the number of PF simulations, an adaptive surrogate modeling method is proposed using importance sampling (IS) and active learning to refine iteratively the surrogate model in critical regions. Corrosion reliability analysis of a miter gate structure is adopted to demonstrate the efficacy of the proposed method. The result shows that the proposed framework can efficiently and accurately generate a failure probability map for a large structure like a miter gate based on computationally expensive mesoscale PF simulations. In addition, the proposed method is more accurate and converges faster than existing surrogate model-based reliability analysis algorithms.

Detrimental Effect of Surface Mechanical Grinding on Stress Corrosion Behavior of Monel 400 Alloy in Hydrofluoric Acid Vapor

Surface finish or integrity is always an important problem threatening the safety service of components. In this study, the stress corrosion cracking (SCC) and crack initiation behavior of Monel 400 alloy with different surface working treatments (mechanical grinding TD specimen [the grinding direction is perpendicular to the loading direction] and PD specimen [the grinding direction is parallel to the loading direction]) and electrolytic polishing [EP specimen]) in hydrofluoric acid (HF) vapor were compared by conducting slow strain rate test (SSRT) and constant loading test (CLT). Although SSRT can evaluate the enhancement effect of mechanical grinding on SCC susceptibility, it cannot quantify its contribution to the crack initiation stage. The CLT shows the cracking initiation rate of different specimens in order of: EP specimen < TD specimen < PD specimen. For mechanical grinding specimens, local corrosion and cracks are inclined to nucleate at the groove shoulder zone rather groove bottom, which is mainly ascribed to the accumulation of higher corrosion degradation and higher defects at the groove shoulder. As the shoulder edge on the PD specimen is less constrained, it is easier to induce cracking under the synergistic action of stress concentration and preferential corrosion dissolution. The shoulder edge of the TD specimen is not a stress concentration position under tensile loading, and the microcracks formed by grinding are parallel to the loading direction, which is not conducive to the development of microcracks. Therefore, the crack initiation sensitivity of the TD specimen is lower than that of the PD specimen.

Role of Zn injection on mitigating stress corrosion cracking initiation of Alloy 182 weld metal in simulated light water reactor environment

The stress corrosion cracking (SCC) behaviour of nickel-based weld metal Alloy 182 was studied in simulated light water reactor environments with or without zinc (Zn) addition. Accelerated SCC initiation tests with flat tapered tensile specimens, as well as SCC initiation and short crack growth tests with compact tension specimens revealed that Zn injection is able to effectively mitigate SCC. Even the lowest tested Zn concentration of 15 ppb seems to be enough to achieve a significant mitigation effect on SCC initiation and short crack growth. This beneficial effect may be related to the modified surface oxide film by Zn incorporation.

Stress Corrosion Cracking of 316L Stainless Steel Additively Manufactured with Sinter-Based Material Extrusion.

This study investigates the stress corrosion cracking (SCC) behavior of type 316L stainless steel (SS316L) produced with sinter-based material extrusion additive manufacturing (AM). Sinter-based material extrusion AM produces SS316L with microstructures and mechanical properties comparable to its wrought counterpart in the annealed condition. However, despite extensive research on SCC of SS316L, little is known about the SCC of sinter-based AM SS316L. This study focuses on the influence of sintered microstructures on SCC initiation and crack-branching susceptibility. Custom-made C-rings were exposed to different stress levels in acidic chloride solutions at various temperatures. Solution-annealed (SA) and cold-drawn (CD) wrought SS316L were also tested to understand the SCC behavior of SS316L better. Results showed that sinter-based AM SS316L was more susceptible to SCC initiation than SA wrought SS316L but more resistant than CD wrought SS316L, as determined by the crack initiation time. Sinter-based AM SS316L showed a noticeably lower tendency for crack-branching than both wrought SS316L counterparts. The investigation was supported by comprehensive pre- and post-test microanalysis using light optical microscopy, scanning electron microscopy, electron backscatter diffraction, and micro-computed tomography.