Cracking Of Austenitic Stainless Steel Research Articles

Towards understanding stress corrosion cracking of austenitic stainless steels exposed to realistic sea salt brines

Stress corrosion cracking behavior of stainless steel 304 L was investigated in full immersion, evaporated artificial sea salt brines (ASW) at 55 °C. It was observed that brines representative of thermodynamically stable brines at lower relative humidity (40% RH, MgCl2-dominant) had a faster crack growth rate than high relative humidity brines (76% RH, NaCl-dominant). Observed crack growth rates (da/dt) under constant stress intensity (K) conditions were determined to be independent of transitioning procedure (rising K or decreasing frequency) regardless of solutions investigated for the orientation presented. Further, positive strain rates had little to no impact on the observed da/dt. The observed behavior suggests an anodic dissolution enhanced hydrogen embrittlement mechanism for SS304L in concentrated ASW environments at 55 °C. Additional explorations further examined environmental influences on da/dt. Nitrate additions to 40% ASW at 55 °C solutions were shown to decrease measured da/dt and further additions stopped measurable crack growth. After sufficient nitrate had been added to fully stifle crack growth, a temperature increase to 75 °C induced cracking again, and a subsequent decrease to 55 °C once again stopped da/dt. These tests demonstrate the importance of ascertaining both brine-specific chemical and dynamic environmental influences on da/dt.

Stress Corrosion Cracking of Austenitic Stainless Steels in Environments Relevant to Spent Nuclear Fuel Storage

Localized corrosion and stress corrosion cracking (SCC) are potential degradation mechanisms for stainless steels (SS) during exposure to corrosive environments. When localized corrosion features are subject to stresses (either external or residual), a crack can nucleate and grow. One potential scenario under which chloride-induced SCC may pose a risk is the interim storage of spent nuclear fuels (SNF) in SS canisters. In the U.S., these SS canisters are located at interim spent fuel storage installations across the country. As canisters cool over time, dust deposition can lead to the formation of corrosive brines, which form by various salt compositions. Initial studies have shown significant influences of environment on corrosion severity and morphology, which may in turn influence crack nucleation and growth. Recent, compositional analysis of salts on the surface of interim SNF SS storage canisters has been carried out with representative brine mixing formulations developed for laboratory exposures. This presentation evaluates the corrosivity, specifically in-situ crack growth rates influenced by the electrochemical environment of the prepared brines as compared to saturated salts and other seawater surrogate brines. The crack growth rates, in-situ electrochemical measurements, as well as fractography will be discussed in terms of testing methodologies and potential risks for canister degradation. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. This document is SAND2023-02249A.

On the corrosion cracking of austenitic stainless steel in molten solar salt: Experiments and modeling

The mechanical behaviors of 304 and 316 L steels was investigated in air and in solar salt with different contents of chloride impurity under slow strain rate tensile (SSRT) tests using a novel experimental approach at 565 °C. Results show that the yield strength (YS) and ultimate tensile strength (UTS) are negligibly affected by the molten salt corrosion whereas the ductility decreases with increasing contents of chloride. Corrosion cracks initiate and propagate along the grain boundary due to its higher corrosion rate and incremental oxide rupture. Moreover, a damage model that captures the corrosion and chloride effects is proposed and validated. • SSRT tests were performed for stainless steels in molten solar salt. • The YS and UTS are negligibly affected by molten salt corrosion. • Ductility of stainless steels decreases with increasing chloride contents. • Cracks are prone to initiate and propagate along grain boundary. • A damage model that captures the corrosion and chloride effects are proposed.

Failure analysis of 316L stainless steel bellows serving in steam tube

Premature failure of bellows in pipeline transportation system was reported frequently. Burst failure occurred at 316L stainless steel bellows in a turbine after three years of service. Optical microscope, energy dispersive spectrometer, scanning electronic microscopy and numerical simulation were used to analysis the failure reason of the failed bellows. Through the investigation of the operating environment, analysis of the chemical compositions of bellows material and corrosion products, micromorphology and metallographic test of specimens with cracks, the cracking mode can be described as the stress corrosion cracking of austenitic stainless steel. The simulation analysis results further proved that there exists high level tensile stress zone on the inner surface. Based on the analysis results, failure causes can be drawn: under the combine action of working stress and corrosion medium (caustic Na and chloride), stress cracks came into being at the inner wall of the bellows. And the cracks extended under the continuous impact of high temperature steam, finally resulting in burst. Practical countermeasures were put forward to prevent similar failure of the stainless steel bellows as well.

Mechanical criterion for nucleation of intergranular stress corrosion cracking in austenitic stainless steel

This study investigated mechanical criteria for the nucleation of intergranular stress corrosion cracking (IGSCC) in thermally-sensitized type 304 austenitic stainless steel. Constant load testing was conducted on a stainless-steel specimen in a tetrathionate solution, and the cracking behavior, including crack initiation, coalescence, and growth at grain boundaries (GBs) on the specimen surface, was observed in situ. Local normal strains along GBs were measured on the smooth surface via the digital image correlation (DIC) technique. Local normal stresses at GBs were estimated using the Schmid-modified grain boundary stress (SMGBS) model, which has been widely used for evaluating the stress state of GBs, considering the effect of the GB plane geometry and grain orientations of grains adjacent to the GBs. The relationship between IGSCC nucleation sites, strains, and stresses was discussed. As a result, there was a slight correlation between local normal stress obtained via the SMGBS model and local normal strain via DIC measurement. The use of experimentally measured local normal strain was found to be suitable to characterize IGSCC nucleation sites.

Analysis of the early stage of stress corrosion cracking in austenitic stainless steel by EBSD and XRD

This study examined nucleation behavior during the early stage of stress corrosion cracking (SCC) in sensitized austenitic stainless steel based on microstructural assessments, using X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD). The SCC experiment was conducted while applying a constant load corresponding to an initial strain of 2% in a tetrathionate solution. The crystal orientations on the smooth surface of a thermally sensitized specimen were determined via EBSD before and after testing, and diffraction spots were intermittently examined using XRD. Some grains having a high Schmid factor were found to be locally deformed near grain boundaries (GBs) due to constant tensile loading in the corrosive solution, such that intergranular stress corrosion cracks occurred at these GBs. The results of this work demonstrate that the passivation film on the metal first fractures due to localized deformation, after which newly created surfaces near GBs are exposed to the corrosive solution. Localized crack-like corrosion subsequently proceeds in chromium-depleted zones near GBs to generate intergranular SCC.

Study on reducing the risk of stress corrosion cracking of austenitic stainless steel hydraulically expanded joints

Stress corrosion cracking (SCC) is one of the main failure modes of tube-to-tubesheet expanded joints in tubular heat exchangers, and the residual tensile stress is an important factor in the development of these cracks. Many tubular heat exchangers in nuclear-power stations are made of 304L stainless steel, and SCC often occurs in the tube-to-tubesheet expanded joints during service. However, few studies on the failure analysis of these expanded joints of 304L stainless steel have been reported. In this paper, the residual stresses in the hydraulically expanded joints of a high-pressure heater used in one nuclear-power station were evaluated by the finite element method (FEM) to reveal the cause of crack formation. Meanwhile, the effects of the expansion pressure and yield strength on the residual stress of 304L tubes in the expanded joint were studied by the FEM and SCC tests. The FEM results showed that the residual stress of the tube in the expanded zone decreased with the increase in the expansion pressure. But the maximum residual stress, which was located at the transition zone between the expanded and unexpanded zone of the joint, was not affected by the expansion pressure. The yield strength of the tube had a significant influence on the residual stress in the hydraulically expanded joint. Reducing the yield strength of the tube could reduce the maximum residual tensile stress to reduce the risk of SCC. The FEM results were in good agreement with the SCC test results, which were performed following the Design and Construction Rules for Mechanical Components of PWR Nuclear Islands (RCC-M). This study provides a basis for the manufacturing process of austenitic stainless-steel tube-to-tubesheet hydraulically expanded joints.

Modelling Hydrogen Induced Stress Corrosion Cracking in Austenitic Stainless Steel

ABSTRACTA model has been developed which simulates the deformation of single crystal austenitic stainless steels and captures the effects of hydrogen on stress corrosion cracking. The model is based on the crystal plasticity theory which relates critical resolved shear stress to plastic strain and the strength of the crystal. We propose an analytical representation of hydrogen interactions with the material microstructure during deformation and simulate the effects hydrogen will have on void growth prior to fracture. Changes in the mechanical properties of the crystal prior to fracture are governed by the interaction of hydrogen atoms and ensembles of dislocations as the crystal plastically deforms and is based on the hydrogen enhanced localised plasticity (HELP) mechanism. The effects of hydrogen on void growth are considered by analysing the effect of hydrogen on the mechanical property of material bounding an embedded void. The model presented has been implemented numerically using the User Material (UMAT) subroutine in the finite element software (ABAQUS) and has been validated by comparing simulated results with experimental data. Influencing parameters have been varied to understand their effect and test sensitivities.

Finite element simulation of stress corrosion cracking in austenitic stainless steel using modified Lemaitre damage model

The material failure due to stress corrosion cracking (SCC) is widely observed phenomena in many of the industrial applications. Many-a-times it causes drastic loss to the organisation. In the present work, the study is performed to evaluate the stress corrosion cracking (SCC) behaviour of SS304. The Lemaitre damage model is widely employed to evaluate the failure or damage of the engineering materials. In the present work, the Lemaitre damage model is modified by incorporating the effect of temperature and Chloride concentration to analyse SCC by recording the initiation of cracks over a period of time. The finite element analysis is performed over a rectangular specimen of SS304 by applying tensile loading under different testing temperatures and Chloride concentrations. It is observed that with the increase in temperature and Chloride concentration the number of cracks initiated increases and the time required for crack initiation decreases. Further, it is found that the effect of temperature is prominent for the initiation of crack during SCC in comparison to the Chloride concentration within the testing range.

Investigation on nucleation of intergranular stress corrosion cracking in austenitic stainless steel by in situ strain measurement

This study evaluates the nucleation criterion for intergranular stress corrosion cracking (SCC) on a smooth surface of type 304 austenitic stainless steel from a mechanical viewpoint. SCC testing was conducted under a constant load, and microscopic strains on a smooth surface were measured in situ by the digital image correlation technique. The initiation period and site of intergranular cracks were estimated from the change in the microscopic strain distribution near grain boundaries (GBs), and the crack initiation was evaluated using strains at GBs just before cracking. As a result, the initiation of an intergranular crack could be characterized by evaluating only the maximum normal strain along the GB. The criterion for crack initiation was determined to be a maximum normal strain >0.01 from the viewpoint of local strain.

Severity of corrosion under insulation (CUI) to structures and strategies to detect it

Abstract This review paper considers the research conducted in the past 30 years in the field of corrosion under insulation (CUI). CUI is a significant challenge in many industries for reasons of high cost and risk because it proceeds undetected under insulation. CUI is an unavoidable problem which may potentially cause personnel injury or even fatalities. Presently, CUI counts for as much as 40 to 60% of pipeline-repairing cost. The use of coating under the insulation has been shown very efficient in the mitigation of CUI. Moisture and ion penetration, particularly chlorides, through the insulation and coating may produce stress corrosion cracking of austenitic stainless steels. The process of applying insulating materials on metal surfaces and the precautions that need to be taken are explained. Beside visual inspection, techniques such as neutron backscatter, capacitive imaging and pulsed eddy current can be used to detect the presence of CUI. The strength and limitations of the available commercial detection techniques are evaluated. A comprehensive list of industrial standards dealing with CUI is provided.

Elucidating Nucleation Stages of Transgranular Stress Corrosion Cracking in Austenitic Stainless Steel by In Situ Electrochemical and Optical Methods

The pitting and environmentally assisted cracking resistance of austenitic stainless steels (SS) is challenged in several industrial applications particularly those involving hot chloride-concentrated streams. Directional drilling used in the oil and gas exploration is one of these applications. Indeed, high strength CrMn-SS commonly used in drilling technology have a high tendency to fail by stress corrosion cracking (SCC) preceded by localized corrosion once subjected to highly chloride-concentrated drilling fluids at elevated temperatures. A comprehensive understanding regarding the mechanisms governing the transition from pitting into SCC is not currently available, though. Therefore, mechanistic aspects such as the effect of loading conditions on pit nucleation and repassivation as well as the synergistic effect between pit stabilization and the nucleation of a stress corrosion crack are of great practical significance. To investigate this an electrochemical-, optical- and mechanical- monitored SCC test was conducted on a CrMn-SS in an alkaline brine at elevated temperature. The transition from metastable to stable pitting and subsequently to SCC in this system was documented in-situ for the first time. Results supported H.S. Isaacs postulates regarding the interpretation of electrochemical signals and demonstrated that loading conditions affect pit nucleation and repassivation leading to a higher susceptibility of the material to pitting, which preceded SCC.

Stress corrosion cracking failure of a SS 316L high pressure heater tube

Abstract A service failure by environmentally induced cracking in austenitic stainless steel 316 L is presented in this work. Extensive research has been carried out on stress corrosion cracking of different materials and environment combinations by various established experimental procedures. The list of alloy/environment combination that causes stress corrosion cracking is continuously increasing. Stress corrosion cracking is a great concern in corrosion resistant alloys exposed to aggressive environments. Stainless steel 316 L material used as high pressure water heater tube failed by stress corrosion cracking was evaluated by various metallurgical characterisation techniques. Results and apparent causes leading to failure were explained in detail.

Effect of Applied Stress on Pitting Corrosion Behavior of Type 304 Stainless Steel in Chloride Environment

Stainless steels are widely used in structural applications due to their superior corrosion resistance. However, stainless steels sometimes suffer from pitting corrosion and stress corrosion cracking in chloride environments. It was reported that applied stress promoted pitting corrosion and changed the morphology of pitting corrosion 1. Moreover, it is known that stress corrosion cracking is often initiated at pits in stress environment 2. Elucidating the effect of applied stress on pitting corrosion behavior is important to make clear the initiation mechanisms of the stress corrosion cracking of stainless steels. As yet, however, the effect of applied stress on pitting corrosion behavior remains unclear. A commercial AISI 304 (18Cr-8Ni) austenitic stainless steel sheet was used in this study. The chemical composition is shown in mass percentages: 0.06% C, 0.39% Si, 1.11% Mn, 0.029% P, 0.0026% S, 8.02% Ni, 18.0% Cr, 0.13% Mo, 0.22% Cu, 0.002% Ti, 0.002% Al, 0.038% M, 0.003% O, Fe bal. The 0.2% proof stress of the steel was 222 MPa. This steel was heat-treated at 1373 K for 30 min and then quenched in water as solution treatment. Sensitization was conducted by annealing the steel at 923 K for 2 h and then quenching in water after the solution treatment. After these heat-treatments, the surfaces of specimens were successively polished with a diamond paste down to 1 μm. Potentiodynamic anodic polarization measurements were carried out in 0.01 M, 0.1 M, 1M, 4 M MgCl2 solution to clarify the effect of chloride concentration on pitting corrosion. All the potentials reported in this study refer to an Ag/AgCl (3.33 M KCl) electrode. The electrode areas were ca. 10 mm ×10 mm. The potential scan rate as was set at 23 mV / min. Figure 1 shows the polarization curves of the solution-treated steel under no applied stress, and the morphology of pitting corrosion after polarization. As shown in Fig. 1a, the pitting potential decreased with chloride concentration. In 0.01 and 0.1 M MgCl2 solutions, a small pit with a lacy metal cover was initiated. On the other hand, a large shallow pit was generated in 4 M MgCl2. To analyze the effect of applied stress on the pitting corrosion behavior and the morphology of pitting corrosion, micro-electrochemical measurements were utilized. As a result of preliminary experiments of solution-treated steels, pitting corrosion always occurred in 4 M MgCl2 solution when the electrode area was 1 mm × 1 mm. Therefore, experiments were conducted at the condition that the electrode area was 1 mm × 1 mm. The effect of applied stress on the pitting corrosion behavior was analyzed in 4 M MgCl2 at 298 K. During potentiodynamic anodic polarization, the potential scan was stopped after pit initiation. After that, the specimen was left at open circuit potential for 2 days under applied stress. The applied stress was 80% of 0.2% proof stress. Figure 2 shows the change of the corrosion potential with time. It was observed that the corrosion potential increased with time. From the surface observation, no crack occurred in this study. It was suggested that the effect of temperature is also the important factor affecting the initiation of the pit-induced stress corrosion cracking of austenitic stainless steels. References N. Shimahashi, I. Muto, Y. Sugawara, N. Hara, J. Electrochem. Soc., 161 (2014), C494-C500H. Masuda, Corros. Sci., 49 (2007), 120-129 Figure 1

Nucleation behavior of intergranular stress corrosion cracking in austenitic stainless steel

Nucleation behavior of intergranular stress corrosion cracking in austenitic stainless steel

外面応力腐食割れ(ESCC)の感受性の評価

There are three elements, namely environment, material, and stress, as susceptibility conditions of damage patterns of the equipment. Susceptibility of damage can be mainly evaluated by temperature parameter. Damage patterns of the equipment become apparent in low temperature region, normal temperature region and high temperature region along with aging degradation, and these damage patterns cause leakage during operation. In particular, damage appears prominently under low temperature environment. Among them, it is difficult to extract susceptibility to external stress corrosion cracking External stress corrosion cracking of austenitic stainless steel is generated when grain boundary Cr depletion layer (sensitization) of weld heat affected zone (HAZ) occurs due to rainwater retention and sea salt particles. However, sensitization does not occur in all of HAZ. Occurrence of sensitization differs in every part. Therefore, this paper describes in detail findings obtained from survey results on the ESCC in a chemical plant.

Effects of Environment Factors on Stress Corrosion Cracking of Austenitic Stainless Steels in Atmospheric Environments

Effects of Environment Factors on Stress Corrosion Cracking of Austenitic Stainless Steels in Atmospheric Environments

Monitoring of chloride stress corrosion cracking of austenitic stainless steel: identification of the phases of the corrosion process and use of a modified accelerated test

A novel chloride stress corrosion cracking CSCC test was carried out on AISI 304L: the imposition of an anodic potential to shorten the test duration time. A stress is applied to promote cracks. Acoustic emission (AE) methods were used to characterize and monitor the phenomena. The degradation of the material was characterized by coupling of acoustic emission and electrochemical measurements. The evolution of current density and applied load was monitored to make links between the AE results and the various stages of CSCC. The fracture faces and the corrosion were observed by optical microscopy and SEM.

Stress Corrsion Cracking of Austenitic Stainless Steels in High Temperature Water and Alternative Stainless Steel

In order to clarify the effect of SFE on SCC resistance of austenitic stainless steels and to develop the alternative material of Type 316LN stainless steel for BWR application, the effect of chemical composition and heat treatment on SFE value and SCCGR in oxygenated high temperature water were studied. The correlation factors between SFE values for 54 heats of materials and their chemical compositions for nickel, molybdenum, chromium, manganese, nitrogen, silicon and carbon were obtained. From these correlation factors, original formulae for SFE values calculation of austenitic stainless steels in the SHTWC, SHTFC and AGG conditions were established. The maximum crack length, average crack length and cracked area of the IGSCC for 33 heats were evaluated as IGSCC resistance in oxygenated high temperature water. The IGSCC resistance of strain hardened non-sensitized austenitic stainless steels in oxygenated high temperature water increases with increasing of nickel contents and SFE values. From this study, it is suggested that the SFE value is a key parameter for the IGSCC resistance of non-sensitized strain hardened austenitic stainless steels. As an alternative material of Type 316LN stainless steel, increased SFE value material, which is high nickel, high chromium, low silicon and low nitrogen material, is recommendable.

Chloride-Induced Stress Corrosion Cracking of Austenitic Stainless Steel Used for Dry Storage of Spent Nuclear Fuel

Some dry cask storage system (DCSS) designs utilize austenitic stainless steel canisters surrounded by concrete shielding structures to store spent nuclear fuel (SNF) at Independent Spent Fuel Storage Installations (ISFSIs). ISFSIs that are located where chlorides can be deposited on the canisters may have the potential for initiating chloride-induced stress corrosion cracking (SCC). Susceptibility to chloride-induced SCC depends on the environmental conditions at the canister surface, including the following key parameters: temperature; relative humidity (RH); areal density, composition and aqueous concentration of deposited salts; and the stress state of the canister, particularly in the weld and the weld heat affected zone (HAZ). National and international literature was reviewed to understand threshold values and uncertainties associated with these key parameters. Current models have uncertainties associated with salt deposition rate, local humidity and aqueous chemistry on the canister surface, salt composition and water volume, gamma radiolysis, and laboratory test methods.