Environmentally Assisted Cracking Research Articles

Elucidating the loading rate dependence of hydrogen environment-assisted cracking in a Ni-Cu superalloy

The effect of loading rate (dK/dt) on the hydrogen environment-assisted cracking (HEAC) behavior of Monel K-500 immersed in 0.6 M NaCl and polarized to −950 and −1200 mVSCE was assessed via a slow-rising stress intensity (K) testing framework at dK/dt ranging from 0.2 to 20 MPa√m/hr. Comparative testing on Monel K-500 in an inert environment (dry N2 with RH < 5%) at dK/dt ranging from 0.1 to 4 MPa√m/hr revealed a clear dK/dt-dependence of local plasticity effects on the direct current potential difference (dcPD) signal. A method for decoupling this “false” signal caused by local plasticity from the ‘true’ crack growth kinetics is presented. Analysis of the corrected data showed that the threshold stress intensity for the onset of HEAC (KTH) was nominally invariant with dK/dt. The HEAC growth rates (da/dt) were only mildly dependent on dK/dt under severe environmental conditions (−1200 mVSCE). However, for moderate conditions (−950 mVSCE), a strong influence of dK/dt on HEAC growth rates (da/dt) was observed. For this latter testing environment, two characteristic regimes of behavior were noted depending on the applied dK/dt: (1) a ‘plateau’ regime where da/dt is independent of dK/dt was observed for dK/dt > 8 MPa√m/hr and (2) a ‘linear’ regime where log(da/dt) linearly scales with log(dK/dt) was observed for dK/dt < 8 MPa√m/hr. These trends are interpreted using resistance curves calculated from the measured da/dt vs. KJ data. The results of this analysis demonstrate that diffusion-mediated crack growth is only observed for dK/dt > 10 MPa√m/hr at −950 mVSCE and all evaluated dK/dt at −1200 mVSCE, while mechanics-based factors govern the da/dt response for dK/dt < 10 MPa√m/hr at −950 mVSCE. The influence of dK/dt is considered in the context of current testing standards for evaluating HEAC susceptibility; specifically, rising K-based frameworks consistently yield conservative HEAC metrics.

Environmentally Assisted Cracking Behavior of 18Ni 250 Maraging Steel Welds Obtained with and Without Flux in Chloride Media

Gas tungsten arc welding is extensively used in the fabrication of 18Ni 250 maraging steel motor casings. In this work, the role of flux addition on the environmentally assisted cracking (EAC) and electrochemical corrosion behaviors of these welds in their peak-aged condition is presented. Studies were performed in order to bring out the potential influence of long-term exposure of maraging steel welds to neutral 0.1 M NaCl (mildly corrosive) and acidified 0.1 M NaCl (a relatively more corrosive) environments to understand the EAC behavior of the welds. The study showed that both the fluxed and unfluxed weld metals suffered 21% to 23% loss in ductility in neutral 0.1 M NaCl solution from that of obtained in air. The EAC of the welds in the neutral chloride solution was due to anodic dissolution, while in the acidified chloride solution it was found to be predominantly due to hydrogen embrittlement. The effect of flux in refining the weld microstructure was reflected in: (a) preventing intergranular mode of hydrogen embrittlement of the unfluxed welds and transforming the mode to transgranular cracking, (b) lowering a wide scatter in electrochemical corrosion rate, and (c) reducing the selective attack in the interdendritic regions of the unfluxed welds. Such a transition from intergranular to transgranular cracking might affect long-term service behavior of the welds to corrosion and EAC, though it is not reflected in the short-term tests.

Effect of cold working on crack growth rate of environmentally assisted cracking of 316L SS

Environmentally assisted cracking (EAC) is a main failure mode of key structural materials of nuclear power pressure vessels and pipelines in service. The key structural materials of nuclear power will undergo plastic deformation due to cold working during manufacturing, assembly and welding, and cold working will affect the EAC crack growth rate. In order to understand effect of cold working on crack growth rate of EAC, a combination of theory, experiment and finite element simulation is used. There are experiments to obtain the mechanical properties of 316L stainless steel at different degrees of cold working, and the creep characteristics of 316L stainless steel at low temperature and high stress were obtained by experiment. Based on experiment and theoretical analysis, creep rate of crack tip is used to replace strain rate of crack tip in Ford-Andresen model, and effect of cold working on crack growth rate of EAC is analyzed with hardening at crack tip and without hardening at crack tip. The results show that cold working accelerates crack growth rate of EAC, and the hardening of crack tip micro zone restrains crack growth rate of EAC, causing the “hysteresis” phenomenon in the EAC crack growth process. It is more reasonable to use creep rate of crack tip instead of strain rate of crack tip to calculate crack growth rate of EAC.

A novel test technique for the mechanistic study of initiation of environmentally assisted cracking on a Ni–30Cr–10Fe alloy in simulated pressurized water reactor primary water

Recurring step straining was developed to investigate the environmentally assisted cracking (EAC) initiation of a Ni-30Cr-10Fe alloy (Alloy 690) in simulated pressurized water reactor (PWR) primary water. Thanks to this technique, the roles of corrosion and straining in the EAC initiation of Alloy 690 was decoupled and typical microstructures in stress corrosion cracking, i.e. diffusion induced grain boundary migration and oxidation into the migration zone, were reproduced. Digital image correlation using indigenous surface oxide particles as markers was successfully incorporated into this technique for high-resolution in-plane strain measurement. The preliminary results show that intergranular crack initiation is correlated with the high local strain in both adjacent grains, indicating that slip transfer across the grain boundary promotes crack initiation. Recurring step straining is desirable for the mechanistic study of EAC as it makes the separate roles of chemical and mechanical processes adjustable and traceable.

Bayesian model updating for the corrosion fatigue crack growth rate of Ni-base alloy X-750

Nickel base Alloy X-750, which is used as fastener parts in light-water reactor (LWR), has experienced many failures by environmentally assisted cracking (EAC). In order to improve the reliability of passive components for nuclear power plants (NPP's), it is necessary to study the failure mechanism and to predict crack growth behavior by developing a probabilistic failure model.In this study, The Bayesian inference was employed to reduce the uncertainties contained in EAC modeling parameters that have been established from experiments with Alloy X-750. Corrosion fatigue crack growth rate model (FCGR) was developed by fitting into Paris’ Law of measured data from the several fatigue tests conducted either in constant load or constant ΔK mode.These parameters characterizing the corrosion fatigue crack growth behavior of X-750 were successfully updated to reduce the uncertainty in the model by using the Bayesian inference method. It is demonstrated that probabilistic failure models for passive components can be developed by updating a laboratory model with field-inspection data, when crack growth rates (CGRs) are low and multiple inspections can be made prior to the component failure.

Environmentally assisted cracking behavior of U-bend specimens of Mg–RE alloys in chloride containing basic solution

Abstract Environmentally assisted cracking (EAC) behavior of two Mg-rare earth (RE) alloys such as Mg–Zn–Gd–Nd–Zr (EV31A) and Mg–Y–Nd (WE43C) alloys was investigated by using U-bend specimens. Open circuit potentials (OCP) of the U-bend specimens were monitored during the EAC tests in 0.1 M NaOH solution with different chloride concentrations at room temperature. EV31A (as-received, and peak aged) and WE43C (peak aged) specimens failed by SCC in 80 ppm chloride containing 0.1 M NaOH solution at OCP. When the EAC initiation occurred, the OCP decreased continuously. Irregular fluctuations of the OCP were observed in the absence of EAC. The OCP versus time profile could be used for monitoring EAC failure of the Mg–RE alloy components in real life service. Applied potentials did not cause cracking of the EV31A alloy in 80 ppm Cl− containing 0.1 M NaOH. Accelerated cracking was observed on the WE43C alloy in peak-aged condition under the applied potentials in the transpassive region when compared to that of OCP condition. Overaging decreased the susceptibility to cracking.

Environmental cracking performance of new generation thick plate 7000-T7x series alloys in humid air

The hydrogen environmentally-assisted cracking (HEAC) behaviour of new generation (new-gen) Al-Zn-Mg-Cu thick-plate alloys (AA7449, AA7085, and AA7037), under accelerated humid warm-air exposure, has been compared to the aerospace industry benchmark alloy AA7050 in over-aged T7x type tempers. Constant load time-to-failure and DCB crack growth tests have been performed at a relative humidity of 85 %, at 70 °C. It has been found that in constant load tests the failure time of the new generation alloys can be reduced by at least an order of magnitude and in some cases cracking was observed in only a few days at stress levels of 85 % of yield. Analysis of the failed fractured specimens has confirmed that the fracture behaviour is predominantly intergranular, and highly localised to high angle grain boundaries. The progression from surface initiation to self-propagating cracks occurred much more rapidly in the new higher Zn content alloys, but with less prior surface chemical attack. In DCB v-K tests stage II crack growth rates were also found to be significantly higher (by a factor of 6–20 times) in the new-gen alloys and the K1HEAC threshold stress intensity was less than half that for AA7050-T7651. Under long crack growth conditions, propagation thus required a higher mechanical driving force and crack growth stagnated more readily in the AA7050-T7651 benchmark material, compared to in the new generation alloys. The results have been discussed in the context of the potential influences of differences in the alloy’s chemistries and microstructures.

Irradiation-assisted stress corrosion cracking of Type 347 and Type 316 steels irradiated in commercial pressurized water reactors

Investigations of Type 347 and Type 316 austenitic stainless steel baffle-former bolt failures at 4-loop downflow pressurized water reactors (PWRs) in the US have identified significant amounts of transgranular cracking on the fracture surfaces along with intergranular cracking and ductile rupture. The expected failure mode for baffle-former bolts is irradiation-assisted stress corrosion cracking (IASCC), which typically results in an intergranular cracking mode. The presence of transgranular cracking raised the question of whether IASCC was responsible for the failures, or if a fatigue component was contributory. Detailed fractography and metallography of bolts with and without cracks confirmed that an environmentally-assisted cracking mechanism was responsible. Reviews of the literature identified numerous cases where stress corrosion cracks (including irradiation-assisted stress corrosion cracks) in austenitic stainless steels transitioned from intergranular to transgranular, typically as a more severe stress state developed. The potential cracking mechanisms are discussed. This allowed the conclusion that IASCC was responsible for the degradation observed in the baffle-former bolts, though a component of alternating loading cannot be excluded. The downflow plant configuration results in increased stress on neighboring intact bolts after bolt failure, resulting in the development of ‘clusters’ of failed bolts, which grew as additional bolts failed. This cluster growth contributed to the severe stress state imposed on the bolts.

Multiscale Assessment of Deformation Induced by Hydrogen Environment-Assisted Cracking in a Peak-Aged Ni-Cu Superalloy

A multiscale approach leveraging electron backscatter diffraction (S-EBSD), high resolution EBSD (HR-EBSD), and transmission electron microscopy (TEM) was employed to assess the deformation induced proximate to the crack path by hydrogen environment-assisted cracking (HEAC) in peak-aged Monel K-500. Kernel average misorientation (KAM) results calculated from S-EBSD indicate that the deformation pertinent to HEAC is localized to within 25 μm of the crack path. Geometrically necessary dislocation (GND) density maps calculated from HR-EBSD confirm this localization. The evaluation of the deformation distribution in three separate grains along the crack path using complementary HR-EBSD and TEM suggest a qualitative similarity in dislocation density between the two techniques, though HR-EBSD is unable to spatially resolve the fine dislocation structures observed via TEM. However, non-negligible differences in dislocation patterning were observed in the three evaluated grains, highlighting the importance of a multiscale approach for characterizing deformation to understand the governing microstructural and mechanical factors.

Nanometre to micrometre length-scale techniques for characterising environmentally-assisted cracking: An appraisal.

The appraisal is strongly focussed on challenges associated with the nuclear sector, however these are representative of what is generally encountered by a range of engineering applications. Ensuring structural integrity of key nuclear plant components is essential for both safe and economic operation. Structural integrity assessments require knowledge of the mechanical and physical properties of materials, together with an understanding of mechanisms that can limit the overall operating life. With improved mechanistic understanding comes the ability to develop predictive models of the service life of components. Such models often require parameters which can be provided only by characterisation of processes occurring in situ over a range of scales, with the sub-micrometre-scale being particularly important, but also challenging. This appraisal reviews the techniques currently available to characterise microstructural features at the nanometre to micrometre length-scale that can be used to elucidate mechanisms that lead to the early stages of environmentally-assisted crack formation and subsequent growth. Following an appraisal of the techniques and their application, there is a short discussion and consideration for future opportunities.

Application of the Theory of the Critical Distances based methodology for the analysis of Environmentally Assisted Cracking processes in biomaterials

The complex interaction between physiological stresses and corrosive human fluids can lead to the premature failure of metallic biomaterials due to the development of Environmental Assisted Cracking (EAC) processes. In this paper, the EAC phenomenon is analysed through a Theory of Critical Distances based methodology, which has been validated in other materials and aggressive environments, and the apparent crack propagation threshold in notched conditions is estimated. Notch-like defects, which are frequently found in aggressive environments, may present higher values of crack propagation thresholds than those exhibited in cracked components. The knowledge of this higher material performance makes it possible to address the problem avoiding oversizing or unnecessary replacements in biomaterials, which leads to an improvement in the quality of life of the people carrying these materials.In this study, the susceptibility of AZ31 magnesium alloy to EAC and the evolution of the apparent crack propagation threshold have been analysed. The aggressive environment used was Simulated Body Fluid (SBF). The main conclusion is that the Theory of Critical Distances predicts the behaviour of this biomaterial in notched conditions and subjected to the aggressive environment being studied.

Correlation between susceptibility to environment-assisted cracking of super-elastic TiNi alloy and the states and their amount of hydrogen in it

Susceptibility to environment-assisted cracking (EAC) of super-elastic TiNi alloy has been correlated with the states and their amount of hydrogen in the material. The TiNi alloy specimen was immersed in sulfate solutions at a given pH, and a giving cathodic potential was applied to it for 13 ks to absorb hydrogen into it. The specimen was subjected to X-ray diffraction (XRD) test to detect hydride, and to thermal gas desorption spectroscopy (TDS) test to determine the state and its amount of hydrogen in it. In the case that a potential of -0.5 VAg/AgCl was applied to the specimen in the solution of pH 3, the XRD detected no hydride on it. Whereas, application of -1.5 VAg/AgCl was induced formation of hydride. The TDS indicated that the specimen treated at -0.5 VAg/AgCl involved hydrogen desorbing around 810 K, and the one at -1.5 VAg/AgCl involved hydrogen desorbing around 500 as well as 810 K. Therefore, it was suggested that the hydrogen desorbing at lower and higher temperatures obtained by the TDS were of hydride state and solid-solution state, respectively. The specimens treated under various conditions were analyzed by the same way as mentioned before, and an amount of the hydrogen in each state as well as a cathodic charge density applied to the specimen were obtained to be correlated. A logarithm of the amount of the hydrogen in the each state increased almost linearly with an increase in a logarithm of a charge density. Susceptibility to the EAC of the material suddenly increased around a charge density of 0.025 MC·m-2, and the charge density was considered to induce the amounts of hydrogens in hydride and solid-solution state of 10 and 100 mass ppm, respectively.

Smart assessment of environment assisted cracking of grade 92 material using different test solutions

The Environment Assisted Cracking (EAC) of grade P92 was investigated by employing a new testing method. The main aim is to understand the mechanism of the EAC. The specimen was subjected to a static load of approximately 90% of the material’s yield strength. The experimental work was carried out under the various corrosive environments such as NaOH, KOH, HCl, CuSO4, and CH4N2S test solutions until it gets fractured, which takes a duration of one week (168 h). These specimens were subjected to the Scanning Electron Microscope (SEM) analysis for obtaining its microstructural characterization. Further, the obtained microstructural images can be utilized to understand the failure mechanism. It can be witnessed that the majority of failures appear in the region around the coarse-grained HAZ. Subsequently, the results indicate that the passive path corrosion mechanism seems to be the preferred one as P92 welds are vulnerable to EAC.

Crack Propagation Behaviour in Thin Aluminium Alloy Sheet Repaired with Adhesively Bonded CFRP Patch

For reducing the crack tip stress intensity factor, adhesively bonded composite patches are employed for crack repair in thin structure. Experimental work was carried out to study environmental assisted cracking behaviour of crack repaired with single-sided bonded patch in aluminium alloy of 7xxx series in peak-aged condition. The observations were also made on crack in this alloy without any patch repair. The specimen developed for conducting the experiments simulated plane stress condition of thin structure. The patched and un-patched tensile test specimens were loaded simultaneously at constant load in the presence of 3.5% weight NaCl solution. The experimental set-up simulated the saline or corrosive environment. Aggressive effect of corrosion attack on crack due to anodic dissolution along with hydrogen embrittlement leading to faster crack growth in patched specimen was revealed compared to un-patched specimen. The present study is the improvement over similar researches carried out earlier in terms of design of test specimen under plane stress condition so as to obtain all the three regions of crack growth.

Environment-assisted corrosion damage of steel bridges: a conceptual framework for structural integrity

Abstract A framework is presented in this paper to assess the structural integrity of steel bridges for environment-assisted corrosion damage. Forms of corrosion of steel bridges and their effects, possible locations of occurrence, and most appropriate inspection techniques are first studied and the lessons learned from past steel bridge failures are discussed. A review of the corrosion damage of steel bridges, including causes, effects, and control measures, is presented along with guidelines for the recognition and investigation of environment-assisted cracking (EAC) of steel bridges. Experimental approaches are proposed to investigate the EAC of structural steel. The proposed approach is used to investigate the EAC susceptibility of cathodically protected ST52 steel. In addition, the feasibility of the proposed approach is tested. Hence, the conceptual framework is proposed. The framework aims to support the inspection and maintenance practices of steel bridges.

Failure of copper wires in a railway Motor Operation Contact

Many engineering alloys can fail by environmentally assisted cracking (EAC). The risk of failure is dependent on the specific combination of material, the environment and the stress. Identification of the stresses (applied and residual) can be difficult because of the retrospective nature of failure investigation and the commonly complex character of stresses in real engineering applications. A routine inspection of a Motor operation contact installed aside a railway line in the British countryside found that the copper wire contact had failed. Standard failure investigation techniques combined with fractography were applied to identify an intergranular failure mode. Experimental observations combined with a review of the relevant literature allowed the root cause to be assigned to environmental assisted cracking in the presence of gaseous ammonia in the local environment.

The effect of isothermal heat treatment on hydrogen environment-assisted cracking susceptibility in Monel K-500

The effect of isothermal heat treatment on hydrogen environment-assisted cracking (HEAC) susceptibility in a single heat of Monel K-500 was evaluated via slow-rising stress intensity (K) testing while immersed in 0.6 M NaCl solution and exposed to applied potentials ranging from −1000 to −1200 mVSCE (vs. saturated calomel electrode). Four heat treatments, corresponding to the non-aged, under-aged, peak-aged, and over-aged conditions, conducted at a constant aging temperature of 923 K, were examined in this study. Crack growth kinetics and fractography demonstrate that alloys heat-treated to the under-aged and peak-aged conditions exhibit increased susceptibility to HEAC relative to alloys treated to the non-aged and over-aged conditions, respectively. Microstructural variables that are expected to vary under this isothermal heat treatment protocol are grain size, grain boundary character, grain boundary impurity concentration, yield strength, strain hardening, and precipitate morphology. A detailed assessment of grain size, grain boundary character, and grain boundary impurity concentration evolution with aging time suggests these variables do not explain the observed variation in HEAC susceptibility of Monel K-500. Differences in the crack tip hydrostatic stress field induced by differences in yield strength and strain hardening were directionally consistent with the observed HEAC susceptibility, but do not capture the enhanced susceptibility of the under-aged heat treatment relative to the peak-aged condition. However, transmission electron microscopy of each heat treatment revealed the presence of planar slip in the under-aged and peak-aged specimens, as compared to wavy slip in the non-aged and over-aged specimens, suggesting that the propensity for strain localization correlates well with HEAC susceptibility.

Effect of the Surface Grinding on the Environmentally Assisted Crack Initiation of 316 L Steel in Simulated Pressurized Water Reactor Water

This paper presents results on the effect of a surface treatment on the environmentally assisted corrosion cracking in a pressurized water reactor chemistry. Slow strain rate testing of 316 L austenitic steel with selected rates was performed at pressurized water reactor (PWR) simulated water at 350 °C and in air at 300 °C. Detailed prior and post-testing characterization of two types of surfaces including roughness, hardness, and microstructural analysis was made. Transgranular cleavage-like environmentally assisted cracking (EAC) initiation and growth were observed under PWR conditions. The effect of two surface finishes on the cracking initiation was observed: (i) first crack initiates from the polished surface in a vicinity of the necking area rather than from the ground surface and (ii) then the deeper crack develops in the minimal diameter from the polished surface side than from the ground one.

Probability prediction of crack growth rate of environmentally assisted cracks of nickel-based alloys based on Latin hypercube sampling

Environmentally assisted cracking (EAC) of nickel-based alloys is one of the most significant potential safety hazards in the primary circuit of nuclear power plants. Considering the uncertainties of load and material properties of base metal and the oxide film, Latin hypercube sampling method is applied to predict the EAC crack growth rate combining the film slip-dissolution/oxidation model with the elastic-plastic finite element method. Meanwhile, to improve the efficiency of numerical analysis, MATLAB is employed in the secondary development for ABAQUS. With the help of existing experimental EAC data, the probability prediction of the growth rate at the tip of EAC in one-inch compact tension specimen is simulated and analyzed in this study. The results show that the randomness of material properties and load markedly affect the uncertainty of strain and EAC growth rate. Among the variables, The randomness of the load has the greatest influence on the uncertainty of the crack growth rate of EAC.

Environmental degradation of fracture resistance in high-temperature water environments of low-alloy reactor pressure vessel steels with high sulphur or phosphorus contents

The fracture behaviour of low-alloy reactor pressure vessel steels with various sulphur and phosphorus contents, different environmentally-assisted cracking (EAC) and temper embrittlement (TE) susceptibilities was evaluated by elastic plastic fracture mechanics tests in air and various simulated light water reactors environments. A moderate but clear reduction of fracture initiation resistance occurred in a) high-sulphur steel with high EAC susceptibility in oxygenated high-temperature water with aggressive occluded crevice environment and preceding EAC growth, and in b) high-phosphorus steel with high TE susceptibility, where the reduction of fracture resistance was most pronounced in hydrogenated high-temperature water.