Environmentally Assisted Cracking Research Articles

Mitigating environmental assisted cracking in heterogeneous welds by laser peening without coating

In this work Laser Peening without Coating (LPwC) was applied underwater on heterogeneous weld joint made of P265GH and X6CrNiTi18-10 steel tubes to prevent Environmental Assisted Cracking on the weld/P265GH steel fusion boundary. Special laser head was designed to precisely deliver 200 mJ green laser beam on the centrally located weld interface on the inner surface of a pipe 76 mm wide and 420 mm long. Residual stress analysis revealed that the LPwC treatment led to generation of compressive residual stresses in the critical fusion boundary area with the magnitude of −168 MPa despite the absence of a protective layer in the peening process. The depth of compressive stresses reached up to 0.8 mm. Plastic deformation induced by LPwC led to grain refinement in the microstructure and improved hardness by 16 %. Samples sectioned from the treated pipe were subjected to accelerated corrosion-mechanical testing which showed more than 2.5x increase in cycles to failure after LPwC. Fractography analysis showed shorter length and decreased number of corrosion cracks after LPwC as well as formation of protective oxide layer on top of the treated surface. 3-point bend testing further showed more than 8x increase in corrosion fatigue life.

A phase field finite element study and evaluation of sulfide stress cracking in DCB specimen testing

The challenges presented by sour environments rich in hydrogen sulfide (H2S) underscore the necessity for a comprehensive understanding of material behavior under such conditions. The cracking susceptibility of metals and alloys used for subsurface equipment in downhole oil and gas exploration operations is particularly concerning. The NACE Double Cantilever Beam (DCB) test has emerged as a widely used quality assurance tool in the petroleum industry, leveraging fracture mechanics principles to assess the environment-assisted cracking (EAC) resistance of metals and alloys. The DCB test evaluates the fracture toughness KISSC of materials in H2S-containing environments via assessment of the crack arrest, which serves as a vital parameter in structural integrity assessments to mitigate the risk of service-related failures from sulfide stress cracking (SSC). However, various studies suggest that different test parameters, such as arm displacement and initial notch, significantly influence KISSC. This work presents a detailed numerical investigation on a comprehensive simulation of the DCB test, examining the effects of different test parameters on KISSC prediction. A coupled deformation-diffusion phase field framework is adopted to simulate SSC in DCB specimens arising from a complex interplay between material deformation, hydrogen diffusion, and fracture. The numerical results show good agreement with experimental results reported in the literature and provide deeper insights into the factors affecting crack growth and arrest in DCB testing.

6th International conference on environmentally-assisted cracking (Reston, Washington, DC, USA, July 16–21, 2023)

6th International conference on environmentally-assisted cracking (Reston, Washington, DC, USA, July 16–21, 2023)

Environmentally Assisted Cracking of Duplex and Lean Duplex Stainless Steel Reinforcements in Alkaline Medium Contaminated with Chlorides

Herein, the corrosion performance of different stainless steel (SS) reinforcing bar grades in alkaline solution is presented, including UNS S32205 duplex stainless steel (DSS), UNS S32304 and UNS S32001 lean DDS (LDSS). The electrochemical dissolution kinetics were studied by potentiodynamic polarization and the Tafel slope method. The environmentally assisted cracking (EAC) mechanisms of the different SS grades in the presence of Cl− were revealed with the slow strain rate test (SSRT). The higher activation of the anodic branch and the loss of toughness were related to the austenite-to-ferrite phase ratio. UNS S32205 DSS presented the slowest anodic dissolution kinetics, mainly due to the higher austenite content compared to the other LDSS; however, it suffered a more severe EAC than the UNS S32304 LDSS. In the case of UNS S32001 LDSS, even while having the lowest Ni content (i.e., large ferrite α-phase ratio), it experienced the least decrease in elongation as well as low anodic dissolution kinetics for Cl− contents up to 8 wt.%, where the Cl− threshold was reached.

On the role of surface stress in environment-assisted fracture

Abstract Environment effects in plasticity and fracture of metals, well studied for several decades, still pose many unanswered questions. A micro-mechanics explanation of how dislocation activity is influenced by the material surface state, that can answer these questions, has been elusive. We build on a recently discovered effect in metal cutting – organic monolayer embrittlement (OME) – wherein metal surfaces are rendered brittle by long-chain organic adsorbates, to explore how material state variables influence surface plasticity and fracture. In particular, cutting experiments with Al containing Self Assembled Monolayers (SAMs), show that the OME is controlled by surface stress (f) induced by the adsorbates. This is contrary to many instances of environment-assisted fracture which are usually attributed to surface energy changes, and wherein f is largely ignored. Other contributions include (a) a cantilever technique to measure surface stress, (b) demonstration of strong effect of SAM molecule chain length on f, (c) characterization of how dislocation activity at crack-tips is affected by adsorbate-induced f, and (d) large improvements in machining processes enabled by controlled environment-assisted fracture. We make the case that surface stress, due to adsorbates, likely influences all environmentally assisted cracking (EAC) phenomena, warranting a revisit of extant models of EAC.

Effects of surface hardening by laser shock peening and shot peening on a nickel-based single-crystal superalloy CMSX-4

Improving the expected life of nickel-based single-crystal superalloy turbine components by surface hardening treatments including laser shock peening (LSP) and mechanical shot peening (MSP) are of particular interest for mitigation of life limiting damage such as environmental assisted cracking in hot section components of gas turbines. In the present study the effects of LSP and MSP on the surface roughness, microhardness and work hardening of a nickel-based single crystal superalloy CMSX-4® have been assessed. Surface roughness was measured using laser profilometry. The degree of work hardening was measured using electron backscattered diffraction with local misorientation analysis. The analysis showed evidence for a work hardening layer in the MSP sample to a depth of approximately 70 μm. Sets of slip bands extending far into the bulk of the sample were observed in the LSP-treated sample, without any evidence of a work hardening layer. Microhardness measurements used to gauge the depth of residual stress showed that LSP produced a much deeper hardness profile than MSP, with compressive residual stress depths of 1000 μm and 200 μm in LSP and MSP respectively. The retention of hardness after a heat treatment of 50 h at 700 °C was more prominent in the LSP sample than in the MSP sample. LSP and MSP have therefore been shown to be at the opposite ends of the spectrum of surface hardening treatments of CMSX-4, with LSP giving milder hardening, but to a greater depth.

Susceptibility of a Fe-Mn-Al-Ni-Cr shape memory alloy to environment-assisted corrosion cracking

In recent years, iron-based shape memory alloys (SMAs) have become increasingly interesting for application in civil engineering structures. Promising candidates in the field are iron-based Fe-Mn-Al-Ni-X (X = Ti, Cr) SMAs. These alloys have demonstrated the potential to be used in various types of applications. While the fully recoverable martensitic phase transformation can be used for structures where high damping capacities are required, i.e. for high rise buildings in earthquake prone areas, it has been successfully demonstrated that the material can also be used as prestressing element in concrete structures. However, for the application in civil engineering structures the problem of stress corrosion cracking and environment-assisted cracking has to be considered. The simultaneous occurrence of tensile stresses and unfavorable environmental conditions, such as carbonation of concrete or the penetration of salt, can lead to spontaneous failure of the building structure. In the present study, the susceptibility of Fe-Mn-Al-Ni-Cr to stress corrosion cracking was studied in neutral chloride containing solutions. For that purpose, tensile tests under constant load control at different stress levels have been conducted. The pitting corrosion that occurred during immersion in the electrolytic solution has a decisive influence on the martensitic phase transformation and the crack evolution. The high density of dislocations at the martensite-to-austenite interfaces promotes the formation of transgranular cracks as well as stress-induced corrosion cracks originating from corrosion pits. Assessment of the fracture surfaces revealed dominating intergranular crack growth, most likely induced by hydrogen embrittlement. The detrimental mechanisms lead to failure of the specimens well before the targeted test time of 1000 h.

Characterization of Mechanical Heterogeneity and Study of the Mechanical Field at the Tip of the Stationary-Growing Crack in Dissimilar Metal Welded Joints

Abstract The mechanical heterogeneity in local areas of dissimilar metal welded joints and the micro-area mechanical state at the crack tip are key factors in determining Environment-Assisted Cracking (EAC). Traditional methods for acquiring material mechanical properties often result in destructive damage to specimens, while conventional “sandwich” models exhibit abrupt changes in interfacial mechanical properties and a lack of research into the mechanical field at the tip of the stationary or growing crack. In light of these challenges, this study, based on the analysis of microstructures in localized regions of the welded joint and the acquisition of material mechanical properties through indentation tests, developed a user-defined material subroutine (UMAT) to characterize the mechanical properties of non-uniform local areas within the welded joint. Additionally, it investigated the mechanical field at the tip of the stationary—growing crack using an integral method and a de-bond technique. The results indicate that non-destructive indentation tests can accurately acquire the material mechanical properties of local areas in the welded joint. Notably, significant changes in mechanical properties typically occur in the material interface regions, making them vulnerable points for potential failure. Furthermore, under the same load, mechanical heterogeneity significantly influences the distribution of the mechanical field at the crack tip. Crack propagation induces alterations in crack tip stresses, resulting in noticeable residual stresses and strains along the propagation path.

Environmentally-assisted cracking of electropolished 316L stainless steel in molten FLiNaK salt

Environmentally-assisted cracking is a well-known and extensively studied phenomenon in light water reactor environments but remains relatively unexplored in proposed molten salt reactor systems where the mechanisms at play are expected to be quite different. Here, slow strain rate testing has been performed on 316L stainless steel tensile specimens during simultaneous exposure to a molten LiF-KF-NaF eutectic salt environment at 600°C with a strain rate between 1E-5 to 1E-7 s-1. Sample ductility and ultimate tensile strength were both observed to decrease for slower strain rates/longer salt exposures. Post-mortem characterization of fractured specimens revealed Fe deposits and Cr- and O-rich precipitates decorating surface grain boundaries and crack surfaces, suggesting a cracking mechanism driven by embrittlement of the crack tip due to oxide formation. Unlike previous studies, intergranular Cr corrosion into the bulk was not observed, and distinct differences in surface morphology between electropolished and non-electropolished sample surfaces are reported. Trace metal salt chemistry measurements showed accumulation of 316L alloying elements over the course of the tests. It appears likely that the surface oxide layer induced by electropolishing, in addition to oxygen and moisture impurities in the salt, led to the formation of LiCrO2 which had a net passivating effect that prevented significant bulk corrosion, though some interplay between mechanical and environmental stressors is still observed.

Surface stress can initiate environment-assisted fracture in metals.

Controlling environmental effects in surface plasticity/fracture of metals is of interest for areas as diverse as manufacturing processes, product performance, and structural safety. The key to controlling these effects is understanding the effect of adsorbates on surface energy (γ) and surface stress (f). While γ has been well studied, the role of surface stress has received much less attention. We characterize surface stress induced in metals by adsorption of organic monolayers. Linear alkanoic acids of varying chain length (3-18) are deposited by molecular self-assembly onto one side of an aluminum cantilever, several centimeters in length. The surface stress is estimated from in situ measurement of the cantilever deflection. We find that the organic adsorbates induce large surface stress of -4 to +30N/m. Furthermore, we show that f may be tuned by varying adsorbate-molecule chain length. The stress data explain beneficial embrittlement of metal surfaces by organic adsorbates in cutting and comminution processes, and point to a critical role, hitherto ignored, for f in environment assisted cracking (EAC) phenomena. Our results suggest opportunities for utilizing controlled environment-assisted fracture as an aid-fracture as a friend-to enhance material removal processes, apart from using surface stress itself as an experimental probe to explore various manifestations of EAC.

A materials selection framework for fastener-in-panel geometries using FEM and LEFM, to mitigate coating degradation and environmentally assisted cracking

A finite element method (FEM) model was used to determine the potential, current, and current density distributions for a multi-material fastener/hole-in-plate configuration in an atmospheric environment. The plate consisted of bare Pyrowear 675 with or without a ZnNi coating, and the fastener consisted of Ti-6Al-4V with or without an anodization surface treatment. The potential distribution within the fastener hole was the focus of this work, as this region would represent the location of highest stress within the fastener/panel geometry. Separately, existing literature data were augmented by fracture mechanics-based evaluations of the environment-assisted cracking behavior of Pyrowear 675 as a function of electrode potential Pyrowear 675 in 3.0 M NaCl. Combining the fracture mechanics and FEM results allowed for a unified framework which was able to narrow a material selection matrix of 13 total scenarios to three scenarios, in an atmospheric environment of 0.6 M NaCl, specifically. The delayed drying out of occluded regions after the surface electrolyte has been evaporated was accounted for, with only one of the studied material selection combinations able to reduce the crack susceptibility under those conditions. A single pit within the fastener hole was also considered, and it was determined that the size of the pit had a larger impact on the potential than the location of the pit, which will aid in future computational assumptions.

The influence of heat treatment on microstructure and mechanical response of a newly developed non-equimolar AlCrCuFeNi high-entropy alloy: Experiments and numerical modeling

High Entropy Alloys (HEA), or more generally, Complex Concentrated Alloys (CCA) have recently shifted the manufacturing and design paradigms of metallic alloys which are more resistant and strong to mechanical loadings as well as environmental-assisted cracking. Although an extensive body of results on these special alloying systems has accrued over recent years, there are still many unknowns related to composition and microstructure and their influences on plastic deformation and failure. In this exploratory study, a new non-equimolar Multi-Principal Element Alloy along with a few annealed variant configurations is investigated by means of microstructure characterization techniques along with computational modeling. The latter is implemented to unveil the interaction of distinct mechanisms controlling the deformation process and failure in this system. For macroscopic behavior, a phenomenological approach is used to understand the plasticity and fracture under different stress states, while a mesoscale-level crystal plasticity model is carried out to determine slip system activity and its influence on plastic deformation. Overall, the new alloy exhibits rising strain hardening curves regardless of the annealing time period, but the onset of fracture is highly sensitive to heat treatment time.

Effect of Chloride Concentration on the Environment-Assisted Cracking Behavior of an Al-Mg Alloy in Atmospheric Environments

The effect of chloride concentration on the environment-assisted cracking (EAC) behavior of AA5083-H131 in atmospheric environments was investigated using high-fidelity fracture mechanics-based testing and concurrent electrochemical potential measurements. EAC susceptibility was found to increase across all environments as chloride concentration increased, denoted by a decrease in the threshold stress intensity and faster stage II crack growth rates. However, EAC susceptibility for a given chloride concentration decreased across all chloride concentrations as cathodic limitation due to solution geometry effects increased. These results are analyzed in the context of the proposed anodic dissolution-enabled hydrogen embrittlement mechanism for EAC in Al-Mg alloys. Specifically, the increase in EAC susceptibility noted at higher chloride concentrations is postulated to arise from an increased overpotential for hydrogen production at the crack tip. Conversely, the decrease in EAC susceptibility observed as the solution geometry becomes more restrictive is attributed to cathodic limitation at the bulk surface decreasing dissolution at the crack tip, resulting in a concomitant less aggressive crack chemistry, and thus lower levels of hydrogen production and uptake at the crack tip. A close correlation between the open-circuit potential on the bulk specimen surface and the crack growth kinetics was also observed across all environments and chloride concentrations, with higher chloride concentrations and cathodic limitations resulting in larger changes in electrochemical potential. This correlates well with known electrochemical potential-dependent EAC observations for these alloys.

Experimental Evaluation of KIEAC of a Carbon Steel Using the Pin-Loaded SENT Geometry

Abstract The experimental determination of the threshold stress intensity factor for environment-assisted cracking (KIEAC) is described by several standards, which allow the use of different specimen geometries and methodologies for crack length estimation. In some cases, the combination of structure, specimen size, or both, main loading direction, and crack orientations of the component that need to be characterized limit the use of standardized geometries. Consequently, alternative geometries must be used. In this study, fatigue pre-cracked pin-loaded single edge notched tension specimens as defined by BS 8571:2018, Method of Test for Determination of Fracture Toughness in Metallic Materials Using Single Edge Notched Tension (SENT) Specimens, were applied for the experimental evaluation of the KIEAC of a carbon steel. The specimens were tested in deaerated substitute ocean water solution saturated with carbon dioxide at 40°C and at 1 and 10 bar (100 and 1,000 kPa) under constant load conditions with incremental (step) loading. The crack length during the tests was monitored by direct current potential drop, which was helpful for defining the applied K corresponding to the onset of subcritical crack growth (KIEAC). Additionally, fractographic analysis of KEAC specimens and results from the fracture toughness evaluation of this material in air at room temperature have also been reported.

The Effect of Crack-Initiating Feature on the Environment-Assisted Cracking Behavior of Sensitized AA5456-H116 in Marine Environments

The influence of crack-initiating feature on the environment-assisted cracking (EAC) behavior of sensitized AA5456-H116 exposed to marine environments is assessed via fracture mechanics-based testing. Specimens that contained either a traditional fatigue precrack or purposefully introduced intergranular corrosion fissures were immersed in 0.6 M NaCl and polarized to select electrochemical potentials while held at a constant force. The measured crack length vs. time relationships from these experiments reveal that the two specimen geometries yield similar crack growth rates at −900 mVSCE and after the onset of accelerated crack propagation at −800 mVSCE. However, precorroded specimens exhibit significantly shorter times to failure than the precracked specimens at −800 mVSCE due to increased crack growth rates at the start of the experiment. The mechanical, environmental, and material factors that could contribute to the initially increased EAC susceptibility of the precorroded specimens are identified using a generalized model for EAC. Analysis of these possible causal factors suggests that the increased susceptibility is due to a residual, initially more deleterious crack chemistry at the occluded corrosion fissure tip from the aggressive galvanostatic polarizations used to accelerate fissure growth. The implications of these results on the efficacy of traditional fracture mechanics-based methods for quantifying EAC susceptibility are discussed.

Effects of sensitization on environmentally assisted cracking of AA5083-H128 Al–Mg alloy

The influence of sensitization on the precipitation behaviour and environmentally assisted cracking (EAC) of AA5083 alloy with the –H128 temper have been investigated. 80 °C/500 h and 80 °C/1000 h sensitized specimens exhibit discontinuous Mg-rich precipitates along the grain boundaries, whereas almost continuous β′ phase is present along the grain boundaries in the 175 °C/100 h sensitized material. EAC susceptibility increases with increasing exposure time and elevated temperature. The relationship between nitric acid mass loss test (NAMLT) values and mechanical test results indicates that specimens with a NAMLT value greater than 25 mg/cm2 exhibit severe EAC. The test environment affects the degree of EAC due to the combined effects of anodic dissolution at the crack tip, hydrogen evolution, and hydrogen embrittlement. AA5083-H128 exhibits lower EAC susceptibility than AA5083-H131, likely due to its lower yield strength. High yield strength can produce more Mg-rich precipitates after sensitization, increase the constraint of the crack tip material and accelerate the diffusion rate of H atoms in the process zone of the crack tip.

Effect of surface machining on the environmentally-assisted cracking of Alloy 182 and 316L stainless steel in light water reactor environments: results of the collaborative project MEACTOS

Abstract The main objective of the EU-funded project mitigating environmentally-assisted cracking through optimisation of surface condition (MEACTOS) was to gain knowledge on the ability of different surface machining procedures to mitigate environmentally-assisted cracking (EAC) in typical light water reactor structural materials and environments. Surfaces of cold-worked (CW) type 316L austenitic stainless steel and nickel-based weld metal Alloy 182 flat tapered tensile specimens were machined using different processes. EAC initiation susceptibility of these specimens was evaluated using constant extension rate tensile (CERT) tests under simulated boiling water reactor (BWR) and pressurized water reactor (PWR) conditions and assessed using constant load experiments. More than a hundred tests were performed covering about 10 years of autoclave testing time. Only minor or no measurable improvements in EAC initiation susceptibility as a function of surface treatments (grinding or advanced machining) compared to the standard industrial face milling were demonstrated. In most cases, the stress thresholds for EAC initiation determined in constant load tests confirmed the trend obtained from CERT tests. This paper summarises the most important results and conclusions concerning the EAC initiation behaviour for the CW 316L and Alloy 182 under reducing PWR and oxidizing BWR conditions.

Computational study of the geometrical influence of grain topography on short crack propagation in AA7XXX series alloys

Intergranular Environmentally-Assisted Cracking (EAC) has recently been reported to be an issue of concern in new-generation 7000 series aluminium alloys, such as AA7085, when exposed to humid air. The cracking process occurs in a highly brittle manner almost exclusively along grain boundaries (GB's) and has been attributed to hydrogen embrittlement, probably by GB decohesion within the stress field at the crack tip. Currently, how the highly heterogeneous grain structures found in these partially recrystallized materials impact the growth behaviour of microstructurally short cracks is poorly understood. In particular, there is expected to be a high sensitivity to the grain structure in the transition from initiation to sustained propagation, where the local mechanical driving force is very sensitive to the crack path. Volume Elements, VE's, with synthetic grain structures have been generated from real microstructure and texture data, so that the effects of important grain structure variables can be explored in crystal-plasticity simulations, to understand the extent to which typical grain-structural features affect the driving force for short-crack growth. Specifically, by considering the effect of different uncrystallised grain aspect ratios and embedding recrystallised grains in the model, the strain energy release rate has been calculated as a function of crack path. This has revealed large reductions and fluctuations in the driving force for short cracks in relation to the local grain structure encountered by the crack tip, which have been estimated by the model.

Kinetics of corrosion reactions on press hardened steel in atmospheric conditions under thin electrolyte films

Steels with high mechanical performance are prone to hydrogen embrittlement and environmental assisted cracking. Under atmospheric corrosion conditions, the source of hydrogen can be the steel corrosion process itself or galvanic coupling with a metallic coating. Electrochemical behaviour of Press Hardened Steel (PHS) under electrolyte films of different thicknesses using local electrochemical techniques was studied on a fundamental level. Scanning Vibrated Electrode Technique (SVET) was applied to study the evolution and localization of the corrosion process during PHS immersion in NaCl electrolyte. Kelvin Probe (KP) was used as a reference electrode to obtain cathodic and anodic polarization curves on PHS surfaces which were covered by thin electrolyte films (60 to 500 µm) of 0.1 M NaOH and 0.6 M NaCl. For both electrolytes, a strong increase in the oxygen reduction rate due to the decreasing of electrolyte thickness has been clearly demonstrated. Data are correlated well with a theoretical plot determined by Nernst-Fick equation. The influence of the rust layers on the kinetics of corrosion reactions under thin electrolyte films was investigated using KP.

Environment-assisted cracking of AZ31 magnesium alloy in a borate buffer solution containing ammonium thiocyanate under various potentials

The environment-assisted cracking behavior of AZ31 magnesium alloy was investigated using tensile tests at cathodic and corrosion potentials in a Na2B4O7∙10H2O solution containing NH4SCN and in air. Although the mechanical properties of the AZ31 with an initial strain rate of 10−4 s−1 were independent of the environments, those degraded in the solution with an initial strain rate of 10−6 s−1. The total elongation became smaller at higher potentials. The average hydrogen absorption rate increased with a positive potential shift. These facts imply that environment-assisted cracking became more severe under relatively high potential owing to corrosion accompanied by enhanced hydrogen absorption.