Environmentally Assisted Cracking Research Articles

Stress corrosion cracking in fuel-grade ethanol: The role of the testing methodology

The increase in fuel-grade ethanol (FGE) consumption demands efficient and safe storage and transportation. However, depending on the raw material used for ethanol production, stress corrosion cracking (SCC) might be a major concern. This work evaluated the API 5L X70 steel susceptibility to SCC in simulated fuel-grade ethanol (SFGE), corn, and sugar cane FGE. The maximum stress intensity factor (Kmax.) was determined from KR curves, and the stress intensity factor threshold for environment-assisted cracking (KEAC) was obtained from the direct current electric potential difference (DCEPD) versus load-line displacement (LLD) curves. The results showed that Kmax. was weakly affected by the environment (different variants of ethanol), although brittle fracture comprising cleavage-like features, intergranular facets, and secondary cracks was observed for SFGE and corn FGE. Under the specific set of testing conditions adopted here, the parameter KEAC was effective in showing that the API 5L X70 steel is quite sensitive to SCC in SFGE and corn FGE, and not susceptible to this progressive cracking process in sugar cane FGE.

Coupling Fracture Mechanics Experiments and Electrochemical Modeling to Mitigate Environment-Assisted Cracking in Engineering Components

Environment-assisted cracking (EAC) is a pertinent failure mode for many applications and industries, but the design of robust EAC mitigation strategies can be challenging due to the number of material and environmental factors that affect EAC behavior. In this study, a coupled experimental-modeling approach for designing EAC mitigation strategies in a standard panel-and-fastener geometry is presented. Fracture mechanics-based testing is executed on a high-performance steel (Pyrowear 675) immersed in 0.6 M NaCl to assess the effect of electrode potential and loading rate on EAC susceptibility. Finite element modeling (FEM) is then used to calculate the electrode potential distribution across the panel for four realistic EAC mitigation strategies (anodized fastener, fully coated panel, selectively coated panel, and bare panel/fastener). The FEM and EAC susceptibility data are synthesized to inform the efficacy of each proposed mitigation strategy. Results demonstrate that the anodized fastener and fully coated panel approaches are likely to promote EAC, while the selectively coated panel and all-bare strategies mitigate EAC. The benefits and limitations of this coupled approach for mitigating EAC are then discussed.

Effect of thermal aging on the environmental assisted cracking behavior of 308L weld metal in high temperature water

The effect of thermal aging on environmental assisted cracking (EAC) of 308 L weld metal was investigated. Experimental results reveal that thermal aging promotes corrosion fatigue (CF) crack growth with extensive inter-dendritic cracking, but has a limited effect on the stress corrosion cracking (SCC) growth under constant loads. The higher crack tip strain rate and the enhanced preferential oxidation of deteriorated δ-ferrite are responsible for the enhanced CF susceptibility after thermal aging. The dispersed δ-ferrites and the decreased crack tip strain rate account for the high SCC resistance under constant load regardless of the thermal aging.

A sampling of environmental data, and its presentation, from a multi-role U.S. coast guard aircraft

Abstract This work presents a summary of environmental data from 3600 h of data collected during operations of a U.S. Coast Guard HC-130H aircraft performing its routine missions over a five-year period. The aircraft operated from multiple U.S. and deployed locations during the data collection period in the tropical and mid-latitudes at altitudes up to 8.5 km (28 kft). The aircraft collected pressure, temperature and humidity data during the period. No aerosol components were sampled (chlorides, sulfur dioxide, etc.). While there was no Time of Wetness sensor, the ISO 9223 definition is used for the present work as an estimator. These data are presented in the context of research into corrosion and environmental severity, or the potential conditions for it. About 40% of the data were collected while the aircraft was parked on the ground at various based and deployed locations. The purpose of this work is to present and summarize the flight data as an aid in providing some context for future research into environmental effects as they relate to both corrosion and environmentally assisted cracking (EAC) of aircraft structure, and perhaps for comparisons with existing or future datasets from aircraft having other missions. Prediction models need realistic, real-world inputs, such as those presented here. An example for the use of these data in corrosion prediction based on the ISO 9223 international standard is presented. Perhaps not surprisingly, these data show that, for this aircraft, conditions most favorable to corrosion occurred while the aircraft was on the ground.

Notch sensitivity and short cracks tolerance in a super 13Cr stainless steel under sulfide stress corrosion cracking conditions

Abstract Notch sensitivity under sulfide stress corrosion cracking (SSC) conditions is quantified in this work, considering its tolerance to short cracks that start at notch tips and become non-propagating after growing for a short while. The considered material is an UNS S41426 super 13Cr5Ni2Mo martensitic stainless steel, frequently used in pipelines to transport aggressive fluids in offshore applications. Tolerance to short SCC cracks depends on the notch size and shape, on the stress gradient ahead of its tip, and on basic material resistances to crack initiation S SSC and growth K ISSC inside the aggressive environment. The proposed notch-sensitivity model can be a powerful and economical alternative design tool to substitute traditional pass/non-pass criteria normally used to choose materials for SSC and similar environmentally assisted cracking (EAC) service conditions. This model is validated by testing the steel into an aqueous solution of 100 g/L of Cl−, pH 4.0, 25 kPa of H2S, and 75 kPa of CO2, at 23 ± 2 °C. After measuring S SSC (461 ± 23 MPa) and K ISSC (36.9 ± 0.6 MPa√m), short crack tolerance predictions are verified on notched specimens loaded under a peak stress at their tips σ max = 0.95⋅S Y >> S SSC, where S Y = 826 MPa is the yield strength of the tested steel.

Environment-Assisted Crack Initiation in Aluminum Alloys Studied by Local Probe Techniques

Environment-assisted crack initiation in aluminum alloys is strongly related to the physical and chemical reactivity of intermetallic particles (IMPs) whatever their location, i.e., at grain boundaries or in the matrix. Therefore, this paper first focuses on the most recent contribution of microscale local probe techniques to the study of processes occurring at the coarse IMPs that need to be taken into account in the initiation stage of environment-assisted cracking (EAC). A critical review of microscale electrochemistry, chemistry, and analysis of the influence of stress on IMPs leading to the microscale analysis of crack incubation is presented. Moreover, the contribution of hydrogen to the initiation of EAC remains a widely debated issue. Therefore, the second part of this paper more specifically reviews and summarizes the contribution of some specific local probe techniques to a better understanding of the contribution of hydrogen enrichment to environment-assisted crack initiation. The remaining challenges in future studies needed to fully elucidate hydrogen-assisted cracking mechanisms at the microscale are discussed.

Susceptibility to Pitting and Environmentally Assisted Cracking of 17-4PH Martensitic Stainless Steel Produced by Laser Beam Melting.

Materials produced by additive manufacturing (AM) often have different microstructures from those obtained using conventional metallurgy (CM), which can have significant impacts on the materials’ durability, and in particular, resistance to corrosion. In this study, we were concerned with the susceptibility to pitting and environmentally assisted cracking (EAC) of 17-4PH martensitic stainless steel (MSS). We focused on the evolution from pitting to EAC, and the behaviour of MSS produced by AM was compared with that of its CM counterpart. Potentiodynamic polarisation tests were combined with chronoamperometry measurements performed without and with mechanical loading to study both stable and metastable pitting and the influence of stress on these processes. EAC tests were carried out and combined with observations of fracture surfaces. MSS produced by AM was more resistant to pit initiation due to fewer and finer NbC particles. However, the propagation kinetics of stable pits were higher for this MSS due to a higher amount of reversed austenite. The stress was found to stabilise the metastable pits and to accelerate the propagation of stable pits, which resulted in an increased susceptibility to EAC of the MSS produced by AM. These results clearly highlighted the fact that the reversed austenite amount has to be perfectly controlled in AM processes.

Electrochemical/Microstructural Studies on the Corrosion of Prestressed Steel Strand in Concrete with Naturally Ingressing Chlorides

The occurrence of environmentally assisted cracking (EAC) is as dependent on the aggressiveness of the environment as the susceptibility of the material and the presence of static tensile stresses. However, the influence of the environment has not been adequately considered in past investigations on EAC mechanisms of prestressing strands. This study utilizes various characterization techniques to evaluate the surface/bulk deterioration of corroded pretensioned concrete (PTC) specimens after natural chloride exposure (by diffusion through cover concrete). Corroded strands in two PTC prism specimens (3,000 mm × 150 mm × 200 mm) were characterized using electrochemical impedance spectroscopy (EIS) and other microanalytical techniques. The EIS and scanning electron microscope (SEM) images obtained after 1 y and 2 y of exposure revealed a negligible residual protectiveness of the passivated surface (although the concretes possessed high resistivity). Raman spectra, SEM, and x-ray computed tomography images of extracted corroded strands also provided unique insights into the pattern of corrosion propagation in PTC systems subjected to realistic chloride exposure. Microcracks in the bulk metal beneath flat-bottomed corrosion pits revealed the possible EAC at low chloride levels expected in service (<0.6% by weight of binder). The findings serve as a basis to define chloride-induced passive-to-active transition as the end of risk-free service life of PTC structures, and for considering it as the limit state for both service life design and corrosion assessment to avoid the onset of EAC.

Dealloying of Ni- and Fe-Based Alloys in Boiling Caustic Environments and the Parameters Influencing the Geometry of a Dealloyed Layer

Nickel alloys and stainless have been the material of choice for a wide range of industrial applications like power and petrochemical plants that involve exposure to corrosive environments. For instance, Alloy 800 (Fe-30Ni-20Cr) has been used as the tubing material in CANDU and Siemens steam generators (SGs) for decades with excellent in-service performance. However, despite their good resistance to general to localize corrosion, Ni- and Fe-based alloys are susceptible to environmentally assisted cracking (EAC) in certain environments, such as hot caustic aqueous solutions. Laboratory experiments have shown that these materials are susceptible to dealloying in hot caustic environments due to the selective removal of Fe, Cr and Mo (less noble elements). As a result of dealloying, a brittle nanoporous film forms on the surface that is enriched in Ni (the more noble element). This film has been proposed to act as a precursor to stress corrosion cracking (SCC) by a film-induced cleavage (FIC) mechanism, where the brittle film injects a high speed/energy crack to the substrate material.The fast kinetics of dealloying requires a connected percolating pathway of less noble element atoms to exist within the alloy that extends from the surface into the material. The presence of this pathway depends on the composition of the alloy. The parting limit defines the minimum amount of the more reactive element(s) in a particular alloy system that makes it susceptible to dealloying. Below the parting limit composition, dealloying will be suppressed by surface diffusion of the more noble element, even at very oxidizing potentials, and the surface of the material will show a passive-like behavior (general dissolution would eventually occur at high anodic overpotentials). Besides the alloy composition, dealloying highly depends on the electrode potential. Below a threshold potential (dealloying critical potential), surface diffusion of the more noble element will overcome the dissolution of the less noble element(s) and suppress dealloying.In this study, we examine different factors that affect dealloying and the geometry of a dealloyed layer through electrochemical measurements and nano-scale characterization techniques. Several engineering Ni- and Fe-based alloys were tested in deaerated boiling caustic solutions to understand the effect of alloy composition on the susceptibility to dealloying. Results indicate that an increase in Ni content will shift the dealloying critical potential to more positive potentials and subsequently the resistance to dealloying. Although Ni is the alloying element that provides nobility and resistance in hot caustic environments, our results show that the thermodynamic and kinetic behaviour, and the content of the less noble elements play an important role in these environments. A comparison between different alloys with similar Ni-content but different Cr and Mo contents shows the beneficial and detrimental role of Cr and Mo in dealloying, respectively. The air-formed chromia-based oxide on the surface of alloys with high Cr-content slowly dissolves in the caustic environment, delays the dissolution of less noble elements, and increases the resistance to dealloying.Changes in electrode potential and time of exposure to the caustic environments, alter the geometry of the dealloyed layer. Results show an increase in the diameter and coarsening of ligaments with increasing time of exposure to the caustic environment. Coarsening of ligaments is believed to impair the ability of a dealloyed layer to inject a micro-crack to the underlying material and initiate SCC. Also, results show that an increase in anodic overpotential of only a few millivolts in the vicinity of dealloying critical potential would change the geometry of the dealloyed layer significantly. The surface diffusion of the more noble element (i.e. Ni) cannot keep up with the fast removal of atoms of the less noble elements (i.e. Cr, Mo, and Fe) at more positive electrode potentials compared to the dealloying critical potential which results in a weak bonding at the film/metal interface. The weak bonding may not be able to support the transmission of a high-speed crack to the substrate material, reducing the likelihood of SCC initiation. Figure 1

Investigation of surface treatment effects on the environmentally-assisted cracking behaviour of Alloy 182 in boiling water reactor environment

ABSTRACT Surface treatments of primary circuit components in light water reactors are regarded as possible ways to mitigate environmentally-assisted cracking (EAC). To date, it is not fully conclusive which surface condition is suitable to reduce the EAC initiation susceptibility. Constant extension rate tensile (CERT) tests were performed by several labs using flat tapered tensile specimens with different surface conditions (ground, industrial face milled, advanced face milled and shot peened), exposed to a boiling water reactor normal water chemistry environment at 288°C. Despite some scatter in the results, the CERT tests revealed that the EAC initiation susceptibility seems lowest for the advanced face milled surface and highest for the shot peened surface. However, it must be emphasised that the differences were moderate and that the surprising behaviour of the shot peened surface can be explained. The mechanical grinding of the surface did not significantly retard EAC initiation compared to industrial face milling.

Large-scale serial sectioning of environmentally assisted cracks in 7xxx Al alloys using femtosecond laser-PFIB

In order to better understand the mechanisms of environmentally-assisted cracking (EAC) of 7xxx aluminium aerospace alloys, large scale serial sectioning using the newly developed femtosecond laser plasma focused ion-beam (laser PFIB) has been performed on both AA7050 and AA7085 alloys containing environmentally-assisted cracks introduced during accelerated laboratory testing. The samples were firstly scanned using X-ray computed tomography in order to reveal the long-range crack morphology and allow for targeted lift out of the crack tip regions for serial sectioning. This paper describes the optimisation of the methodology for both the liftout procedure and subsequent serial sectioning using laser-PFIB. Electron backscatter diffraction (EBSD) mapping at each slice allowed for 3D reconstruction of statistically valid volumes of material, up to 0.5 mm3 with a fine voxel size (1 μm). High resolution secondary electron imaging at each slice allowed for 3D reconstruction of the crack volume, and the datasets were combined in order to investigate the interaction of the crack with the microstructure. Optimisation of both the liftout geometry and the slicing procedure resulted in significant reduction in laser-induced curtaining artefacts and higher quality data acquisition. For these alloys, laser slicing alone was sufficient to provide a deformation free surface allowing for EBSD indexing rates up to ~95% acquired at high speeds of ~2000 Hz leading to reasonable acquisition times of ~4–5 days for each dataset. The datasets have shown that the grains in these alloys are much larger and more complex than previously realised, and the interaction of these cracks with the microstructure have shown that complex-shaped recrystallised grains can lead to significant crack deflection in these alloys and so may help to explain the observed differences in EAC behaviour.

Environmental assisted cracking and strength attenuation effect computing on the mechanical properties of casing steel P110 for industrial revolution 5.0 applications in sour well environments

Environmental assisted cracking and strength attenuation effect computing on the mechanical properties of casing steel P110 for industrial revolution 5.0 applications in sour well environments

On the loading rate dependence of environment-assisted cracking in sensitized AA5456-H116 exposed to marine environments

The influence of the applied loading rate (dK/dt) on the environment-assisted cracking (EAC) behavior of sensitized AA5456-H116 in 0.6 M NaCl at applied potentials ranging from −800 to −900 mVSCE is assessed via a rising-K testing framework. The applied potential strongly affects the dK/dt-dependence of EAC with results suggesting a minimal influence for potentials more positive than −830 mVSCE and a stronger dK/dt-dependence for potentials more negative than −830 mVSCE. Crack growth rates measured using rising versus static K testing are compared, which demonstrates that rising K methods consistently yield conservative EAC metrics with increased efficiency.

Impact of Chloride on the Environmentally-Assisted Crack Initiation Behaviour of Low-Alloy Steel under Boiling Water Reactor Conditions

Low-alloy reactor pressure vessel steels have a rather low susceptibility to stress corrosion cracking (SCC) in a boiling water reactor (BWR) environment if the high-temperature water contains no anionic impurities. Recent investigations revealed that under oxidizing BWR normal water chemistry (NWC) conditions extremely small amounts of chloride, can cause very high SCC growth rates in these materials. Therefore, the effect of continuous and temporary chloride additions on the crack initiation behaviour was explored by a series of constant extension rate tensile (CERT) and constant load tests in high-temperature water. In an NWC environment, containing ≥2 ppb of chloride, strain-induced corrosion cracking (SICC) initiation occurred briefly after the onset of plastic yielding and at much smaller strains than in high-purity water. On the other hand, under reducing hydrogen water chemistry conditions with up to 700 ppb chloride, no SICC was detected up to very high strains. CERT experiments, with moderate short-term chloride transients before and during the loading, showed that even serious mechanical loading transients, one day after returning to high-purity water, did not result in early SICC initiation.

Understanding the environmentally assisted cracking (EAC) initiation and propagation of new generation 7xxx alloys using slow strain rate testing

New generation (new-gen) 7xxx series alloys have recently been shown to have higher susceptibility to environmental assisted cracking (EAC) in humid air, compared to AA7050, in T7651 tempers. Hydrogen (H)-EAC fracture features of brittle intergranular (IG) and transgranular (TG) fracture modes were observed with humidity levels as low as 20%. Initiation was favoured at sites of intermetallic particle clusters with gas pores. The higher performance of AA7050 was attributed to the difficulty of initiation in this alloy, where the grain structure, combined with an intrinsic resistance of the grain boundaries to IG-EAC, made the attainment of sustained cracking difficult.

Multiphysics modeling of environment assisted cracking of buried pipelines in contact with solutions of near-neutral pH

This paper presents a multiphysics model to predict environment assisted cracking in buried pipelines exposed to near-neutral pH solutions. In the calculation of crack growth rates, we involve two main cracking modes: hydrogen-facilitated fatigue and hydrogen-facilitated anodic dissolution. The former is fully coupled under the hydrogen-enhanced localized plasticity theory with a focus on hydrogen embrittlement produced by hydrogen diffusion and trapping. This provides formalism to account for contributions of electrochemical corrosion and variation of mechanically-diffusive hydrogen on the crack growth rates. The latter is explicitly quantified using the high-resolution crack tip fields. A competitive-based model is employed to evaluate crack growth rates. The model is verified by comparing experimentally measured and numerically simulated crack growth rates of a full-scale test reported in the literature. The results of this investigation show that the effect of hydrogen on the anodic-dissolution results in conservative predictions of the crack growth rates.

Challenges of Materials and Corrosion Management: Stress Corrosion Cracking (SCC) and Gaping in our Understanding of the Subject

An essential step in materials degradation is the determination of the failure mechanism. For example, stress, metallurgical (microstructure), or environment-assisted cracking (EAC), helps the determination of the fundamental cause of failure and recommendation on proper measures to prevent failure recurrence. In this context, environment-assisted cracking, or environmentally induced failure, is a form of corrosion that produces a failure/fracture in alloys with minimal corrosion. As for example, environmentally assisted cracks in linepipe steels are initiated either because of stresses in combination with environmental effects, as in SCC, or because of trapped hydrogen in the steel or HIC. To understand better the mechanism of the crack initiation process, key metallurgical and environmental elements that can affect the cracking phenomena were investigated and reviewed. The complexity of both cracking phenomena results from the dependence of SCC and HIC on multiple metallurgical, mechanical, and environmental parameters that may all influence both crack initiation and propagation. Cracking of linepipe steels is analyzed critically, with particular attention to the crack initiation process. The paper is divided into two parts: (i) stress corrosion cracking (SCC); and (ii) hydrogen-induced cracking (HIC) also referred to as hydrogen embrittlement (HE). Many non-metallic particles and stringers, and the anomalous microstructures are excellent trapping sites for diffusing H atoms and the initiation and growth of cracking.

Environment-assisted crack nucleation in La(Fe,Mn,Si)13-based magnetocaloric materials

Cracking in La(Fe,Si)13-based magnetocaloric materials has been observed to predominantly form around La-rich (La2O3) particles and pose a threat to their long-term structural integrity. To understand the formation of these cracks, FIB cross-sectional polishing followed by SEM characterisation was employed to study local microstructural evolution after air exposure. Results suggest that volume expansion and internal degradation associated with a chemical reaction between La2O3 particles and water/moisture can lead to crack nucleation in the 1:13 phase adjacent to La-rich particles. This observation indicates that the formation of La-rich particles should be suppressed, or their size minimised during material processing to ensure the long-term structural integrity of La(Fe,Mn,Si)13 magnetocaloric materials.

Mechanical Stress Effects on 550\xa0\xb0C Hot Corrosion Propagation Rates in Precipitation Hardened Ni-Base Superalloys: CMSX-4, CM247LC DS and IN6203DS

Combinations of temperature, stress and hot corrosion may cause environmentally-assisted cracking in precipitation-hardened Ni-base superalloys, which is little understood. This research aims to increase current understanding by investigating the effects of mechanical stress on the hot corrosion propagation rate during corrosion-fatigue testing of CMSX-4, CM247LC DS and IN6203DS. The parameters used during the tests included a high R-ratio, high frequency, and a temperature of 550 °C. The results showed CMSX-4 experienced a predictable increase in the hot corrosion rate, CM247LC DS also experienced increased rates, but no obvious trend was apparent; whilst IN6203DS showed no evidence of an increased rate. These different behaviours appear to be a result of an interaction between the mechanical stress and microstructural features, which include gamma-prime volume fractions in both the matrix and eutectic regions, along with the distribution of the eutectic structure. The different behaviours in the hot corrosion propagation rate subsequently affected the respective corrosion fatigue results, with both CMSX-4 and CM247LC DS experiencing fracture but with significantly more scatter involved in the CM247LC DS results. All IN6203DS corrosion-fatigue specimens completed the respective tests without fracture and showed no evidence of cracking. It, therefore, appears that precipitation hardened Ni-base superalloys, which are susceptible to environmentally-assisted cracking, also experience increased hot corrosion propagation rates.

Environment assisted cracking of 308L weld metal in high temperature water

• The environmental enhancement factor of 308 L weld metal reached a value of 50 when the frequency was decreased to 0.00001 Hz. • The 308 L weld metal showed excellent SCC resistance regardless of the thermal aging or water chemistry. • No preference of the γ/δ crack path and no elevated EAC either with thermal aging or in oxidizing water was attributed to the no sensitization of the γ/δ boundaries before and after thermal aging. The environment assisted cracking (EAC) behavior, including stress corrosion cracking (SCC) and corrosion fatigue (CF), of a low carbon 308 L stainless steel weld metal was evaluated in high temperature water. The effects of thermal aging (400 °C for 10,000 h) on the crack growth rate (CGR) are discussed. Experimental results reveal an environmental acceleration effect on CGRs with decreasing loading frequency, and the environmental enhancement factor reached a value of 50 when the frequency was decreased to 0.00001 Hz. The 308 L weld metal showed excellent SCC resistance regardless of the thermal aging or water chemistry, which is related to the low carbon content and well-controlled duplex microstructure. After thermal aging, the ferrite/austenite (γ/δ) phase boundaries show negligible sensitization, consistent with the low carbon concentration. No preference of the γ/δ crack path and no elevated EAC either with thermal aging or in oxidizing water was attributed to the no sensitization of the γ/δ boundaries before and after thermal aging.