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The phenomenon of hydrogen trapping in precipitates of steels is well-recognized, though a precise atomic level understanding remain elusive, leading to ongoing debates. This study highlights the specific roles of interfaces under various coherent conditions through first-principle calculations, incorporating local stress level, which is a novel aspect seldom explored in previous studies. It was found that coherent interfaces exhibit varying hydrogen-trapping capacities depending on their defect states, with the lowest hydrogen solution energy reaching −0.83 eV, primarily functioning as reversible traps. In contrast, derivative interfaces demonstrate superior hydrogen-trapping capacity, with hydrogen solution energy reaching −1.28 eV, classifying them as irreversible sites. Specific tensile or compressive stresses do not necessarily enhance the entrapment or release of hydrogen at precipitate interfaces; instead this depends on the characteristics of polyhedral interstices within the interfaces, with residual stress from hydrogen segregation also playing a critical role. These findings help address certain apparent controversies and advance understanding of the hydrogen embrittlement mechanism. Comprehensive investigation of H trapping characteristics of precipitates with various coherent conditions under stress. [Display omitted] • Perfect coherent precipitates exhibit very limited H-trapping capacity. • Coherent interfaces with Es reaching −0.83 eV mainly serve as reversible traps. • Derivative interfaces with Es reaching −1.28 eV function as irreversible traps. • Various stress levels can enhance H entrapment or release at precipitate interfaces. [ABSTRACT FROM AUTHOR]

Copper (Cu) in steel production can be a residual element, causing challenges during steel processing, as well as an alloying element, improving corrosion resistance and providing hardenability by nanosized precipitates. For the transition toward a green economy, increased recycling rates in steel production and alternative energy carriers, such as hydrogen, are of vital importance. As hydrogen is known for its embrittling effect on high-strength steels, this work sought to explore possible mitigation strategies for hydrogen embrittlement (HE) with the help of Cu precipitates. Hydrogen trapping at Cu/Fe interfaces following the complex phase transformations in the Cu precipitation sequence from body-centered cubic (bcc) to the so-called 9R structure to face-centered cubic (fcc) was addressed by a series of systematic density functional theory calculations. In combination with thermodynamic calculations, the pressing question regarding which of the precipitate structures was most desirable for the tackling of HE was alluded to. We found that hydrogen trapping at the Cu/Fe interfaces increased from −0.05 to −0.18 eV following the precipitation sequence. Despite this relatively weak hydrogen trapping, which was in the range of dislocations, we showed through thermodynamic calculations that fcc Cu precipitates could still contribute to lowering the risk of triggering the hydrogen-enhanced localized plasticity (HELP) mechanism of HE. [ABSTRACT FROM AUTHOR]

This review examines hydrogen embrittlement (HE) in precipitation-hardenable aluminum alloys, focusing on the role of precipitates as hydrogen traps. It covers hydrogen entry mechanisms, the effects of microstructural features such as dislocations and grain boundaries, and secondary phase evolution during heat treatment. The interaction between hydrogen and precipitates, including the role of coherent and incoherent interfaces, is analyzed in view of the impact on HE susceptibility. Various techniques used to assess the interaction between hydrogen and aluminum alloys are also compared. The goal is to summarize the state-of-the-art understanding of the microstructural factors influencing the resistance of aluminum alloys to HE. [ABSTRACT FROM AUTHOR]

Hydrogen has emerged as a promising alternative to meet the growing demand for sustainable and renewable energy sources. Underground hydrogen storage (UHS) in depleted gas reservoirs holds significant potential for large-scale energy storage and the seamless integration of intermittent renewable energy sources, due to its capacity to address challenges associated with the intermittent nature of renewable energy sources, ensuring a steady and reliable energy supply. Leveraging the existing infrastructure and well-characterized geological formations, depleted gas reservoirs offer an attractive option for large-scale hydrogen storage implementation. However, significant knowledge gaps regarding storage performance hinder the commercialization of UHS operation. Hydrogen deliverability, hydrogen trapping, and the equation of state are key areas with limited understanding. This literature review critically analyzes and synthesizes existing research on hydrogen storage performance during underground storage in depleted gas reservoirs; it then provides a high-level risk assessment and an overview of the techno-economics of UHS. The significance of this review lies in its consolidation of current knowledge, highlighting unresolved issues and proposing areas for future research. Addressing these gaps will advance hydrogen-based energy systems and support the transition to a sustainable energy landscape. Facilitating efficient and safe deployment of UHS in depleted gas reservoirs will assist in unlocking hydrogen’s full potential as a clean and renewable energy carrier. In addition, this review aids policymakers and the scientific community in making informed decisions regarding hydrogen storage technologies.

In this study, single-crystalline and poly-crystalline CrCoNi alloys are utilized as model systems to analyze the distinct roles of each GB and interstitial lattice sites. To effectively reveal hydrogen behavior, both electrochemical and gaseous hydrogen pre-charging methods are applied. Hydrogen content, diffusivity, and trap behaviors are quantified using thermal desorption analysis and hydrogen permeation tests, which determines (1) changes in hydrogen behavior depending on the presence of GB and (2) alterations in hydrogen behavior depending on lattice crystallographic orientation. The results indicate that GB and interstitial lattice sites exhibit comparable binding energies for hydrogen trapping. However, the introduction of GB alters the primary trapping sites from interstitial lattice sites to GB. In this case, the hydrogen content in the poly-crystalline alloy is determined by the trap site density of the primary trapping site. On the other hand, in the single-crystalline alloy, where only interstitial lattice sites exist, the crystallographic orientation of the hydrogen-charged plane is an important variable that determines hydrogen content and hydrogen diffusivity. Such insights contribute to a deeper understanding of hydrogen behavior within a more intricate microstructure, suggesting the alloy design approach to enhance resistance to HE.

Common alloying elements such as Mn and Al can significantly influence the local dynamics of Hydrogen in steel, promoting or attenuating the mechanisms associated with Hydrogen induced Embrittlement (HIE). Here, we propose a first principles-based framework to systematically unlock the physical underpinnings of such influence in Mn/Al-alloyed γ-Fe. Our framework can be readily adapted to analyse H behaviour in the bulk phase of any face-centred cubic (FCC) Fe-X-Y alloy, provided that solutes X and Y substitute the Fe sites. In our scheme, all thermodynamically stable substitutional solute sites were identified (≤5.4 wt% Mn; ≤4 wt% Al) up to the third nearest neighbour (NN) shell of a single H atom. The impact of Mn/Al on H-binding was quantitatively evaluated, indicating a surprisingly strong correlation with the local Al distribution regardless Mn content, and indirect stabilization by Al when present in the 2nd NN shell. Nonetheless, Al strongly repels H bonding. The contradictory role of Al was explained in terms of bonding/anti-bonding orbitals occupancy in H-M interactions (M = Al, Mn, Fe). The barriers to H hopping between adjacent local environments and the corresponding jump frequencies were subsequently calculated, providing insights into the limits imposed by the presence of Al and Mn on H mobility in Mn/Al-alloyed γ-Fe. Most notably, presence of Al in the 2nd NN shell of H severely reduces the H jump frequency, leading to potential irreversible trapping at high Al contents. Such behaviour may critically contribute to mitigate H-induced delayed fracture in Al-rich austenite steel. [Display omitted] • H-binding strength is strongly correlated with local distribution of Al, regardless Mn content. • Al repels H but indirectly strengthen the H-binding when present in the 2nd NN shell. • The unique dual role of Al on H-binding is explained via COHP analysis. • Reversible and irreversible trapping sites are isolated for Fe-Mn-Al alloys. • Irreversible H-trapping may contribute to mitigate H-induced delayed fracture. [ABSTRACT FROM AUTHOR]

The effect of different tempering temperatures on the hydrogen embrittlement (HE) resistance of a V + Nb-microalloyed medium-carbon bolt steel was studied by slow strain rate tensile testing and hydrogen thermal analysis. The HE resistance represented by notch tensile strength σ bN of hydrogen-charged specimens increases with increasing tempering temperature T tem , and the σ bN of the in situ hydrogen-charged sample is considerably lower than that of the pre-hydrogen-charged sample. Both the diffusible hydrogen content C diff and hydrogen-trapping capability of the precipitates show a significant increase-decrease change trend with T temp. The ratio of C diff primarily trapped by nanoscale V-rich MC precipitates increases linearly with increasing T temp. The control of V-rich MC through suitable increasing T temp is vital for obtaining the best HE resistance although at the cost of decreased strength. It is thus suggested that a compromise between strength and HE resistance should be considered for the practical application of this type of steel. [Display omitted] • Effect of tempering temperature T tem on HE of V + Nb-added bolt steel was studied. • T tem has significant effect on both HE resistance and diffusible hydrogen content. • Control of V-rich MC carbide by T temp is vital for obtaining the best HE resistance. • Mechanisms for the variation of HE resistance with T tem were explored. • A compromise between strength and HE resistance should be considered. [ABSTRACT FROM AUTHOR]

As hydrogen reduces the fatigue life of 100Cr6 bearing steel significantly, extensive research on the interaction of hydrogen with 100Cr6 is necessary. This study investigated the influence of rolling/sliding tribotesting performed on a micro-pitting-rig on the hydrogen absorption and trapping behaviour of 100Cr6 bearing steel. Thermal desorption mass spectrometry was used to compare the hydrogen desorption spectra of 100Cr6 samples after tribological tests and static heated oil-immersion tests to untested reference samples. The approach was chosen to further understand the influence of both microstructural deformation as well as steel-oil contact on the hydrogen absorption and trapping behaviour of 100Cr6. The tribological test showed a stable friction behaviour and mild wear which was dominated by local plastic deformation of surface asperities. Despite the mild wear, a change in de-trapping temperatures was found for tribotested samples compared to oil-immersed and untested reference samples. This finding indicates that even mild tribotesting conditions alter the hydrogen trapping behaviour of 100Cr6 bearing steel. [ABSTRACT FROM AUTHOR]

Controversy surrounds the potential of grain refinement in suppressing metal hydrogen embrittlement (HE). The literature presents two methods for introducing hydrogen into metals: hydrogen pre-charging (HPC) and hydrogen in-situ charging (HISC). This study compares the effect of grain refinement on HE under electrochemical HPC and HISC conditions, focusing on type 304 austenitic stainless steel with a grain size range of 0.5–35 μm. HE behaviors were evaluated through tensile testing and fracture morphology analyses, while hydrogen diffusion and concentration were examined using hydrogen uptake amount measurements and hydrogen permeation tests, complemented by finite element (FE) simulations. Results indicate that coupling with plastic deformation significantly accelerated hydrogen transport in the HISC condition, leading to higher apparent hydrogen diffusivity compared to the HPC condition, with grain boundaries (GBs) act as hydrogen traps. Under the HISC, grain refinement reduced HE susceptibility by decreasing the apparent diffusivity due to an increased trapping effect of GBs and suppressing intergranular fracture by reducing the hydrogen amount distributed to each GB (hydrogen occupancy of the GBs). However, grain refinement to below 1 μm increased HE under the HPC condition. Hydrogen permeation tests showed that under the HPC, grain refinement slightly decreased hydrogen diffusivity. In the ultrafine-grained specimen, as GBs trapped more hydrogen atoms, they not only provided hydrogen to stress-concentrated lattices and newly-formed dislocations by detrapping but also maintained a higher hydrogen occupancy of the GBs, thus increasing HE. It is concluded that whether grain refinement suppresses HE depends primarily on the resulting hydrogen occupancy of the GBs, which is influenced by the charging conditions rather than the quantity of traps. • A comparison of effect of grain size on HE in 304 steel is conducted under in-situ H charging and pre-charging. • Coupling with plastic deformation accelerates H transport in in-situ charging condition. • Grain refinement reduces HE in in-situ charging, but it slightly decreases and then increases HE in pre-charging. • The former is attributed to decreased H diffusivity and reduced H occupancy of grain boundaries (GBs). • The latter is due to that H-filled GBs act as H reservoirs, providing H to stress concentrations and dislocations. [ABSTRACT FROM AUTHOR]

The present study aims at investigating the interactions between hydrogen and deformation-induced defects in a 316L austenitic stainless steel (SS). The studied material was pre-strained up to different levels. The resulting microstructures were characterized by optical and electron microscopy and by X-ray diffraction. Dislocation densities were systematically quantified showing a more than 50-fold increase between the unstrained and the most pre-strained sample. Cathodic charging was employed to introduce deuterium into the material. By means of Thermal Desorption Spectrometry (TDS) and Secondary Ions Mass Spectrometry (SIMS) analyses, deuterium energetic and spatial distributions were evaluated both just after charging and after an additional isothermal aging treatment. Thanks to aging treatments, three different energetic states of deuterium in the 316L SS were evidenced: interstitial 2H, low-energy trapped 2H and high-energy trapped 2H. Probing the low-energy trapping was challenging – probably due to the proximity between the migration enthalpy and the associated detrapping energy – but aging highlighted it. This low-energy contribution, displayed in TDS results, increased with the pre-strain level and the dislocation density, and was associated with dislocations elastic fields. Contrary to our expectations, the high-energy trapping could not be correlated directly neither to the pre-strain level nor to the dislocation density. Trapping at vacancies was suggested. • Aging after hydrogen charging evidenced two types of strain-induced trap sites. • The low-energy trapping was attributed to the elastic fields of the dislocations. • The amount of hydrogen in this energy state increased with the pre-strain level. • The high-energy trapping was assigned to the dislocations cores or vacancies. [ABSTRACT FROM AUTHOR]

Hydrogen diffusivity can play a crucial role in hydrogen-assisted degradation of steels. Herein, we investigate the hydrogen diffusion behavior in two X65 pipeline steels using desorption and permeation techniques. Gas charged samples were used for desorption experiments, while permeation tests were conducted using electrochemical charging. The Vintage steel, having a banded ferrite-pearlite microstructure, exhibits a significant influence of strong traps. A modified solution of Fick's 2nd law is proposed to account for the strong traps. At room temperature, the effective diffusivity in the Modern steel, which has homogenous ferrite-bainite microstructure, has a scatter factor of three, whereas for the Vintage steel, the factor is 17. The difference is explained by considering a concentration-dependent effective diffusivity and a tortuous diffusion path. Comparable values of the highest measured effective diffusivity at room temperature were obtained for the Modern and Vintage steels: 2.70 × 10−6 and 3.34 × 10−6 cm2/s, respectively. • Effect of microstructure, technique, and charging condition on measured diffusivity evaluated. • A modified solution to Fick's 2nd law was proposed to account for strong traps. • Significance of method used to determine effective diffusivity varied with steel. • Multiple major trap sites are present in the banded ferrite-pearlite microstructure. • Pearlite bands with low diffusivity can cause a tortuous diffusion path. [ABSTRACT FROM AUTHOR]

This work aims to demonstrate that the resistance to hydrogen embrittlement of conventional advanced high-strength steel (AHSS) with a bainitic ferrite structure can be significantly enhanced through two metallurgical strategies. Firstly, a smaller fraction of cementite in a mixture of bainitic ferrite and lath-martensite structures is realized by an intercritical annealing process with rapid cooling. Secondly, a higher fraction of alloy carbides (MC) with a mean size of less than 30 nm is formed through the combined addition of Ti, Mo, and V as micro-alloying elements. The reduction of cementite precipitation in the matrix leads to an increased resistance to hydrogen evolution/adsorption on the surface under cathodic polarization. Moreover, a large number of V-bearing nanoprecipitates ((Ti,Mo,V)-carbides and V-carbides) provides intermediate trapping for hydrogen atoms, effectively delaying the diffusion kinetics of hydrogen towards potential crack-forming areas in the matrix. Consequently, the lower strain reduction during slow strain rate test and the higher resistance to cracking during the four-point bending test in acidic aqueous solutions are manifested in the newly designed AHSS.

This review examines hydrogen embrittlement (HE) in precipitation-hardenable aluminum alloys, focusing on the role of precipitates as hydrogen traps. It covers hydrogen entry mechanisms, the effects of microstructural features such as dislocations and grain boundaries, and secondary phase evolution during heat treatment. The interaction between hydrogen and precipitates, including the role of coherent and incoherent interfaces, is analyzed in view of the impact on HE susceptibility. Various techniques used to assess the interaction between hydrogen and aluminum alloys are also compared. The goal is to summarize the state-of-the-art understanding of the microstructural factors influencing the resistance of aluminum alloys to HE.

The 3D printing of Ti-Al6-V4 alloy is subject to much current investigation, with Laser Beam Powder Bed Fusion (PBF-LB/M) being one of the most applied technologies. Ti-Al6-V4 alloy, despite its great material properties, is susceptible to hydrogen penetration and consequent embrittlement. The level of susceptibility to hydrogen penetration depends on the microstructural state of the alloy. In this work, we compare the effect of electrochemical charging by hydrogen on Ti-Al6-V4 alloy prepared by PBF-LB/M, either in the as-built state or annealed, and conventionally prepared alloy. At the same charging conditions, considerably different hydrogen concentrations were achieved, with the as-built 3D-printed material being the most susceptible. The changes in mechanical properties are discussed in relation to changes in microstructure, studied using microscopy, X-ray, and electron diffraction techniques. [ABSTRACT FROM AUTHOR]

The concept of hydrogen-enhanced entropy (HEENT) for hydrogen embrittlement prediction was firstly introduced and discussed through experimental validation methods, multiscale characterization, and trapping studies. The accumulated generated entropy reaches a constant value (fatigue fracture entropy (FFE)) regardless of hydrogen content. Hydrogen-enhanced localized plasticity (HELP) is the dominant mechanism for hydrogen-assisted cracking in the studied material, due to quasi-cleavage fracture pattern with serrated marking and H-enhanced planar slip. Fatigue life is reduced due to increasing hydrogen uptake with increasing current density of H charge, and the accumulated generated entropy reaches a constant FFE value. As H content increases, a fraction of weakly trapped hydrogen at interstitials and dislocations in the ferrite increases, and the possibility of reaching hydrogen to crack tip and slip planes increases. Direct evidence of HELP mechanisms with a well-developed dislocation substructure is reported for the first time in pearlitic steels with nanoscale observations near the H-assisted crack path. The contribution of microplasticity due to the HELP mechanism to the total entropy compensates for the total entropy reduction generated due to reduced fatigue life. This introduces the hydrogen-enhanced entropy (HEENT) concept for hydrogen embrittlement prediction. • Hydrogen-enhanced entropy (HEENT) concept introduced for H embrittlement prediction. • H-enhanced localized plasticity (HELP) connected to fracture fatigue entropy (FFE). • Accumulated generated entropy reaches a constant FFE value regardless of H content. • The contribution of microplasticity due to HELP mechanism to the total entropy. • Direct evidence of HELP mechanisms with a well-developed dislocation substructure. [ABSTRACT FROM AUTHOR]

Rapid developments in the field of hydrogen energy have prompted the need for safe and efficient hydrogen transportation and storage. Steels form the backbone of the current energy infrastructure and thus offer a fast and cost-effective solution. Their excellent mechanical properties are attributed to the underlying microstructure which comprises of finely dispersed nano-precipitates. However, one major factor restricting their application is their susceptibility to Hydrogen Embrittlement (HE). In the past decade, experimental and theoretical works have been carried out to understand if the nano-sized carbides can aid in reducing the susceptibility to HE along with providing strengthening. Within this ab-inito study, we investigated the effectiveness of fully coherent nano-carbides (i.e. TiC, VC and NbC) to limit the diffusible hydrogen content in bcc Fe. Our study revealed that the interplay between hydrogen and carbon vacancies, local atomic environment at interface as well as elastic strain fields at the interface can lead to significantly increased hydrogen solubilities. While in TiC, the deepest traps were found to be in the bulk of carbides, in VC and NbC, the elastic strain fields around the interface led to the strongest trapping. Further, the formation of a two-hydrogen-vacancy complex was found to be favourable in VC. Finally, the migration barriers for hydrogen trapping in bulk TiC as well as across the Fe/TiC coherent interface indicate that these deep traps in the form of carbon vacancies are fairly accessible. • DFT investigation of effectiveness of TiC, VC and NbC as hydrogen traps in bcc Fe. • Preferential segregation of hydrogen in the bulk or at the interface. • Hydrogen migration barrier in bulk carbides and at the coherent interface. • Novel insights into vacancy and hydrogen-vacancy complex formation. [ABSTRACT FROM AUTHOR]

The Fe3C/α-Fe interface in pearlite is the main hydrogen trapping site. The first-principles calculation method based on density functional theory was employed to investigate the bonding and hydrogen trapping mechanism of the Fe3C/α-Fe interface with three different orientation relationships, specifically the Bagaryatsky orientation, Isaichev orientation, and Pitsch-Petch orientation. The results showed that by calculating the interface energy and separation work, the theoretical bonding strength and stability of the three interfaces presented the following order: Isaichev orientation > Pitsch-Petch orientation > Bagaryatsky orientation. Hydrogen segregation energy revealed that H atoms were more likely to be trapped at the Fe3C/α-Fe interface region and the hydrogen trap ability of three interfaces was in the following order: Isaichev orientation > Pitsch-Petch orientation > Bagaryatsky orientation. The maximum absolute value of segregation energy was found at the interface with Isaichev orientation, which was 0.38 eV. The differential charge density, density of states, and Mulliken bond population calculations revealed that the formation of the interface was related to the covalent bond between C atoms in cementite and Fe atoms in ferrite, and affected by the spatial distribution of C atoms. The C atoms also repelled the H atoms, so the strength of H–Fe bond was identified as the dominant factor of hydrogen trapping behavior. Overall, these insights can potentially aid in improving the design and performance of materials with pearlite structures.

To improve the performance of hydrogen-permeable alloys, quantitatively elucidating the metal–hydrogen interactions and identifying the controlling factors that govern the bottleneck step of the hydrogen permeation process are essential. In this study, based on the density functional theory, we characterized the interactions of hydrogen with point defects (such as antisite atoms and vacancies) in B2-type non-stoichiometric PdCu alloys, which are promising candidate materials for low-temperature working membranes for hydrogen purification. We proposed a "double defect" mechanism that produces a new type of defect complex with an atomic configuration similar to the Cu vacancy in B2-type PdCu alloys. We found that both Cu vacancies and double defects can act as strong hydrogen-trapping sites and are thus responsible for the marked decrease in hydrogen diffusivity. In contrast, antisite Cu atoms only repel H atoms and do not significantly hinder hydrogen diffusion. Whereas the binding energy of hydrogen to Cu vacancies and double defects exceeded 0.3 eV when relatively few (one or two) H atoms were trapped, the binding energy of hydrogen to Pd vacancies was substantially smaller (0.02 eV at most). Therefore, in Cu-rich non-stoichiometric PdCu alloys, the double defects may be the major hydrogen trapping sites. Furthermore, the effects of hydrogen on the formation of Pd vacancies, Cu vacancies, and double defects in B2-type PdCu alloys were clarified. Notably, double-defect formation from Pd vacancies in B2-type PdCu was significantly enhanced by the presence of nearby H atoms. This phenomenon, in which vacancies in B2-type intermetallic compounds transform into defect complexes in the presence of hydrogen, is postulated for the first time in this study. The findings of this study contribute to further understanding the driving forces for the formation of vacancy-hydrogen complexes in PdCu alloys at the atomic level and provide theoretical support for identifying the lattice defects that inhibit hydrogen permeability in Pd-based membranes. [Display omitted] • Point defects responsible for reduced H diffusivity in B2-type PdCu are identified. • A "double defect" mechanism for forming a new type of defect complex is proposed. • Cu vacancies and double defects trap H atoms and significantly reduce H diffusivity. • Antisite Cu atoms repel H atoms, but their effects on H diffusion are minor. • Formation of double defects from Pd vacancies is enhanced in an H environment. [ABSTRACT FROM AUTHOR]

The hydrogen embrittlement (HE) susceptibility of X70 grade, Acid-resistant, submarine pipeline steel with Mg treatment was studied via constant loading and slow strain rate tension (SSRT). The hydrogen diffusion and trapping behaviour in the tested steels treated with different Mg content was analysed by electrochemical hydrogen permeation testing and thermal desorption spectroscopy (TDS). The results suggested that micro/nanocomposite inclusions with “core-shell” structures obtained via Mg treatment have a greater activation energy of hydrogen desorption. On the one hand, this kind of inclusions can trap more hydrogen and reduce the diffusion coefficient of hydrogen in steel; on the other hand, the internal rigid core of Al–Mg–Ti–O and external soft shell of MnS not only play a role in stress shielding but also hinder the initiation and expansion of stress cracks. For the X70 grade high-strength, acid-resistant submarine pipeline steel, an optimal Mg addition amount is 0.003% (mass fraction).

This study evaluates the hydrogen absorption and trapping in carbide containing steels from a very low hydrogen partial pressure atmosphere at elevated temperatures. Two generic titanium-containing Fe–C steels, differing in carbon and titanium content, are studied. The steels are subjected to a quench and temper treatment, where tempering is performed in a dilute hydrogen gas atmosphere. Detailed microstructural analysis via SEM and TEM together with characterization via thermal desorption spectroscopy and hot extraction are performed to analyze the hydrogen trapping ability of the different types of carbides. A significant amount of hydrogen is introduced in the Fe–C–Ti alloys during tempering. Moreover, the hydrogen appears to be strongly trapped. This could be related to trapping at the carbon-vacancies inside the TiC, and more specifically the large undissolved carbides. The binding energy is estimated to range between 80 kJ/mol and 90 kJ/mol and appears to be independent of the undissolved carbide size. [Display omitted] • TiC containing steels are annealed in dilute H 2. • High temperature TDS peak identified as hydrogen trapped in bulk carbon-vacancies. • Undissolved carbides are major traps due to high carbon-vacancy concentration. • Theoretically determined binding energy ranges between 80 kJ/mol and 90 kJ/mol. • No influence of carbide size on trap binding energy is observed. [ABSTRACT FROM AUTHOR]

To avoid hydrogen flaking in rail production, it is of crucial importance to understand the differences in hydrogen diffusion and trapping between different production steps. Therefore, as-cast unfinished material was compared with two finished rails, hot-rolled and head-hardened, using electron backscattered diffraction (EBSD), electrochemical permeation, and thermal desorption spectroscopy (TDS). A significant increase in dislocation density was in the head-hardened rail compared with the other material states. This leads to an effective hydrogen diffusion coefficient of 5.8 × 10−7 cm2/s which is lower by a factor of four than the diffusion coefficients examined in the other states. Thermal desorption spectroscopy analyses show a clear difference between unfinished and finished rail materials. While a peak in activation energy between 32 and 38 kJ/mol is present at all states, only as-cast unfinished material shows a second peak with an activation energy of 47 kJ/mol, which is related to microvoids. The results show that in the investigated material, the effect of increasing dislocation density has a stronger influence on the effective diffusion coefficient than the presence of a second active trapping site. [ABSTRACT FROM AUTHOR]

The initial growth of a porous alumina layer and the hydrogen absorption during galvanostatic anodization were studied using high-resolution electron microscopy, thermal desorption spectroscopy, and hydrogen microprint technique. The nanostructure of the alumina layer depends strongly on the anodization time. The embryo of pores grows as the thickness of the oxide layer increases, and a porous alumina layer is formed until the voltage reached its maximum value. Eventually, the connected pores to the substrate appear in a steady-state voltage region that acted as hydrogen pathways. The substrate does not show delayed embrittlement after the early and late stages of anodization, which is attributed to the low amount of absorbed hydrogen during the anodization. In the middle stage of the anodization, a higher amount of hydrogen is trapped in the substrate/layer interface and then migrates inward into the alloy when the specimen is subjected to stress resulting in delayed hydrogen embrittlement.

Solid-state hydrogen storage is the best choice for balancing economy and safety among various hydrogen storage technologies, and hydrogen storage in the secondary phase might be a promising solid-state hydrogen storage scheme. In the current study, to unmask its physical mechanisms and details, a thermodynamically consistent phase-field framework is built for the first time to model hydrogen trapping, enrichment, and storage in the secondary phases of alloys. The hydrogen trapping processes, together with hydrogen charging, are numerically simulated using the implicit iterative algorithm of the self-defined finite elements. Some important results are attained: 1. Hydrogen can overcome the energy barrier under the assistance of the local elastic driving force and then spontaneously enter the trap site from the lattice site. The high binding energy makes it difficult for the trapped hydrogens to escape. 2. The secondary phase geometry stress concentration significantly induces the hydrogen to overcome the energy barrier. 3. The manipulation of the geometry, volume fraction, dimension, and type of the secondary phases is capable of dictating the tradeoff between the hydrogen storage capacity and the hydrogen charging rate. The new hydrogen storage scheme, together with the material design ideology, promises a viable path toward the optimization of critical hydrogen storage and transport for the hydrogen economy. [ABSTRACT FROM AUTHOR]

Hydrogen permeation tests and tensile mechanical tests were performed at room temperature on API 5CT P110 steel to characterize hydrogen trapping and to evaluate their hydrogen embrittlement susceptibility. The hydrogen trap density was calculated from two consecutive hydrogen permeation transients plotted using an electrochemical cell. Slow strain rate tensile tests on hydrogen-charged samples through cathodic polarization at different potentials were performed to evaluate the hydrogen embrittlement susceptibility. Thereby it was established the lowest potential that characterizes the onset of the cathodic overprotection for studied steel. After mechanical tests was observed a decrease in ductility as the protection potential became more negative and that the fracture mode was changed from ductile microvoid coalescence on the as-received steel to extended quasi-cleavage on the hydrogen-charged steel. The results showed that API 5CT P110 steel has high susceptibility to hydrogen embrittlement conditioned by a predominance of reversible traps in microstructure and by high hydrogen solubility.

The 3D printing of Ti-Al6-V4 alloy is subject to much current investigation, with Laser Beam Powder Bed Fusion (PBF-LB/M) being one of the most applied technologies. Ti-Al6-V4 alloy, despite its great material properties, is susceptible to hydrogen penetration and consequent embrittlement. The level of susceptibility to hydrogen penetration depends on the microstructural state of the alloy. In this work, we compare the effect of electrochemical charging by hydrogen on Ti-Al6-V4 alloy prepared by PBF-LB/M, either in the as-built state or annealed, and conventionally prepared alloy. At the same charging conditions, considerably different hydrogen concentrations were achieved, with the as-built 3D-printed material being the most susceptible. The changes in mechanical properties are discussed in relation to changes in microstructure, studied using microscopy, X-ray, and electron diffraction techniques.

The hydrogen embrittlement of 15Cr martensitic stainless steel, for steam turbine last stage blades, was systematically studied by using slow strain rate tensile (SSRT) test and constant loading tensile (CLT) test at room temperature and 80 °C to simulate the service conditions. It was shown that, despite the lower hydrogen concentration absorbed during SSRT, the hydrogen-induced fracture strength of 15Cr steel for SSRT was lower than the threshold fracture strength for CLT. This was due to the remarkable enhancement in local hydrogen concentration due to the transportation of hydrogen by mobile dislocation during SSRT. In addition, although the higher hydrogen concentration was absorbed during SSRT at 80 °C, the hydrogen embrittlement susceptibility of 15Cr steel for SSRT at 80 °C was lower than that at room temperature, because the degree of local hydrogen accumulation decreased at a higher temperature. • Hydrogen embrittlement behavior of 15Cr is studied at simulated service condition. • The risk of fracture is higher for slow strain rate test than for constant load test. • The accumulation degree of hydrogen at room temperature is greater than that at 80 °C. • Hydrogen embrittlement behavior is related to hydrogen accumulation. [ABSTRACT FROM AUTHOR]

Austenitic steels are known to exhibit a low hydrogen diffusion coefficient and hence a good resistance to hydrogen embrittlement. Therefore, it is an experimental challenge to investigate their hydrogen diffusion properties. In this study, the electrochemical permeation technique is used to determine the hydrogen diffusion coefficients in different pre-deformed states (φ = 0, 0.32, 0.39, 0.49) of the high-alloy austenitic TRIP steel X3CrMnNiMoN17-8-4 in a temperature range of 323 K–353 K. In combination with microstructural analysis, a correlation between phase transformation from γ-austenite to α′-martensite and dislocation density is shown. As a result of the lattice transformation from fcc to bcc, the diffusion rate of hydrogen is significantly increased (D app, φ = 0 = 3.6 × 10−12 cm2 s−1, D app, φ = 0.32 = 1.6 × 10−11 cm2 s−1at 323 K). With higher degrees of deformation, the dislocation density also increased in the martensite islands, resulting in a degressive growth of the diffusion coefficient (D app, φ = 0.39 = 5.3 × 10−11 cm2 s−1, D app, φ = 0.49 = 1.1 × 10−10 cm2 s−1at 323 K). Moreover, detailed calculations are performed to describe the way of hydrogen trapping and to give a possible mechanism of diffusion. [Display omitted] • Diffusion coefficients of H in austenitic steel (e.g. D 323 K = 3.6 × 10−12 cm2 s−1). • Accelerated hydrogen diffusion due to strain-induced α′-martensite formation. • Amount of trapped hydrogen strongly decreases with increasing temperature. • Solubility of hydrogen significantly decreases with increasing α′-martensite. [ABSTRACT FROM AUTHOR]

This study explores the hydrogen trapping capability and hydrogen embrittlement (HE) of a second-generation single crystal (SX) superalloy. Embrittlement susceptibility is greater as the duration of hydrogen charging is extended. The presence of hydrogen induces cleavage, micro-crack formation and denser slip traces. Moreover, hydrogen facilitates the formation of nano-voids and the activation of high-density dislocations, leading to denser slip bands which serve as initiation sites for micro-cracks. In addition, the desorption activation energy of hydrogen trapped at dislocations and soluble in the γ matrix is 33.7 kJ/mol, and hydrogen trapped at the γ/γ′ interface and vacancies is 42.4 kJ/mol. First-principles calculations have indicated that hydrogen reduces the binding strength at the γ/γ′ interface, which promotes the propagation of micro-cracks. • Hydrogen facilitates the initiation of more micro-cracks along slip bands in Ni-based SX superalloy. • The desorption activation energy of hydrogen trapped at the γ/γ' interface and vacancies is 42.4 kJ/mol. • Hydrogen reduces the bonding strength of Ni-Ni bond and particularly Ni-Al bond at the γ/γ' interface. • HE failure in SX superalloy is due to nano-voids, γ/γ' interface decohesion, and stress concentration around cracks. [ABSTRACT FROM AUTHOR]

• The reaction between moisture and aluminum generates H in the spray forming process. • SF-2195 aluminum alloys contain high content of hydrogen before solution-quenched. • The presence of flattened pores contribute to unusually reduced along-thickness plasticity. • Particles of T 1 phase were found to trap large quantity of hydrogen. • Dislocations were found promote the hydrogen' migration towards dispersoids and T 1. The current research has focused on the hydrogen capturing and its impact on the mechanical properties of spray-formed 2195 (SF2195) aluminum alloy. Microstructure characterization, thermal desorption spectra (TDS) and hydrogen trapping of characteristic microstructure, are used to investigate the generation, migration and trapping of hydrogen. The retention of moisture in the carrier gas is believed to induce high concentration of hydrogen and the amorphous Al 2 O 3 layer within the SF2195 billet, and the local retention of carrier gas brings about pores with the Al 2 O 3 layer on the inner wall. High concentration of hydrogen is captured in the hot-rolled alloy while around 60 % of the hydrogen can be eliminated during the solution-quenching. Particles of T 1 phase trap large quantity of hydrogen and the high-density dislocations developed from the pre-stretch promote the migration of hydrogen towards dispersoids and precipitates. The presence of hydrogen and Al 2 O 3 layer contributes to the reduced plasticity of SF2195 alloy. The spreading of flattened pores can induce intensified hydrogen damage effect in local regions and thus contribute to the unusually decreased ductility along the thickness direction in the T8 aged alloy. [ABSTRACT FROM AUTHOR]

Metals that are exposed to high pressure hydrogen gas may undergo detrimental failure by embrittlement. Understanding the mechanisms and driving forces of hydrogen absorption on the surface of metals is crucial for avoiding hydrogen embrittlement. In this study, the effect of stress-enhanced gaseous hydrogen uptake in bulk metals is investigated in detail. For that purpose, a generalized form of Sievert's law is derived from thermodynamic potentials considering the effect of microstructural trapping sites and multiaxial stresses. This new equation is parametrized and verified using experimental data for carbon steels, which were charged under gaseous hydrogen atmosphere at pressures up to 1000 bar. The role of microstructural trapping sites on the parameter identification is critically discussed. Finally, the parametrized equation is applied to calculate the stress-enhanced hydrogen solubility of thin-walled pipelines and thick-walled pressure vessels during service. [Display omitted] • Improvement of Sievert's law to consider trapping sites and stress states. • Calculation and validation of gaseous hydrogen solubility under extreme conditions. • Hydrogen uptake measured for L450 steel at 200 and 1000 bar. • Calculation of the hydrogen solubility in pipes and pressure vessels. • Comprehensive literature survey of gaseous hydrogen solubility. [ABSTRACT FROM AUTHOR]

In this study, the hydrogen embrittlement (HE) of 316L stainless steel produced by laser powder bed fusion (L-PBF) was investigated by means of hydrogen trapping. The susceptibility of the material to HE is strongly connected to the interaction of hydrogen atoms with volumetric defects in the material. Trapping hydrogen in those defects affects its availability to critical locations where a hydrogen-induced crack can nucleate. Therefore, it is important to study the characteristics of hydrogen traps to better understand the behavior of the material in the hydrogen environment. The hydrogen was introduced into the material via electrochemical charging, and its interactions with various trapping sites were studied through thermal desorption spectroscopy (TDS). The obtained results were compared to conventionally produced 316L stainless steel, and the correlation between microstructure, characteristics of hydrogen traps, and susceptibility to HE is discussed. [ABSTRACT FROM AUTHOR]

Solution and aging twinning-induced plasticity (TWIP) steel were obtained by the solution-treated at 1150°C for 2 h and aging-treated at 700°C for 6 and 24 h. Carbide precipitation is generated in the grain boundary and formed inside grains for aging time prolonging. Scanning Kelvin probe force microscopy was used to investigate the local distribution of hydrogen after hydrogen charging in TWIP steel with carbides in grain boundaries and grains. Results showed that carbides at the grain boundary adsorbed more hydrogen than those in the grain. Carbide precipitated phase as a hydrogen trap can enhance the resistance to hydrogen embrittlement and generated more secondary cracks at the grain boundary. [ABSTRACT FROM AUTHOR]

This study investigated the hydrogen embrittlement (HE) characteristics of advanced high-strength steels (AHSSs). Two different types of AHSSs with a tensile strength of 1.2 GPa were investigated. Slow strain rate tests (SSRTs) were performed under various applied potentials (Eapp) to identify the mechanism with the greatest effect on the embrittlement of the specimens. The SSRT results revealed that, as the Eapp increased, the elongation tended to increase, even when a potential exceeding the corrosion potential was applied. Both types of AHSSs exhibited embrittled fracture behavior that was dominated by HE. The fractured SSRT specimens were subjected to a thermal desorption spectroscopy analysis, revealing that diffusible hydrogen was trapped mainly at the grain boundaries and dislocations (i.e., reversible hydrogen-trapping sites). The micro-analysis results revealed that the poor HE resistance of the specimens was attributed to the more reversible hydrogen-trapping sites. [ABSTRACT FROM AUTHOR]

Hydrogen permeation experiments were performed in quenched and tempered (Q&T) low alloy steels with varying Ni contents exposed to 1 bar H 2 at 30, 50 and 70 °C. From the analysis of build-up and decay transients, it is concluded that the permeation coefficient, the apparent diffusion coefficient (D a p p ) and the concentration of hydrogen in interstitial sites on the charging surface (C 0) decreases as the Ni concentration increases from 0 to 5 wt.%. The total concentration of hydrogen on the charging surface (C 0 , r ), which includes interstitial and reversibly trapped hydrogen, is about an order of magnitude larger than C 0. The hydrogen binding energy (E b) and trap density (N r) were calculated from the D a p p vs. temperature dependence, under the hypothesis of low trap occupancy. C 0 , r , E b and N r do not show a clear correlation with Ni content, indicating that trapping is controlled by a microstructural feature other than Ni atoms in solid solution. • Ni additions refined grain size of a quenched and tempered steel microstructure. • The apparent hydrogen diffusion coefficient decreased with Ni content. • The lattice hydrogen concentration decreased with Ni content. • Total concentration of hydrogen had a weak correlation with Ni content. [ABSTRACT FROM AUTHOR]

A two-fold approach is considered to study hydrogen (H) diffusion characteristics in martensitic steels. Initially, a multi-trap stress coupled H diffusion finite element model was developed to investigate the role of various trap states on effective H trapping during a four point bend test. The calculations show that high angle boundaries are more influential in controlling H diffusion in presence of low initial (bulk) H concentrations, while dislocations can have more pronounced impact, when the bulk H concentration is higher. A microstructural model comprising of prior austenite grains and packets was further developed. The study highlights the importance of packet boundaries (PBs) moderating H diffusion in martensite microstructure. The presence of retained austenite content affecting H diffusion paths was also studied. Overall, this parametric study presents complementary techniques in numerical modeling, as well as implications on the role of various microstructural entities affecting H diffusion. • A multitrap stress-coupled H diffusion FEA model is developed. • Role of various trap states affecting H distribution is presented. • Microstructural model of martensite with two fold hierarchy is developed. • Influence of packet boundaries and retained austenite on H transport is demonstrated. [ABSTRACT FROM AUTHOR]

Environmental-assisted degradation in hydrogen environment, also known as hydrogen embrittlement (HE), has a very important role on physical and mechanical properties of structural materials in the presence of hydrogen. In this research the interactions of high and low fugacity hydrogen with electron beam melted (EBM) Ti–6Al–4V alloy were studied and compared to conventionally manufactured alloy. Parts produced by additive manufacturing (AM) experience relatively high cooling rates and possess unique microstructure features, which might alter the intrinsic properties of the material. Our research describes in details the impact of hydrogen on EBM Ti–6Al–4V alloy by means of hydrogen trapping. The interactions of hydrogen with the material were studied by thermal desorption spectroscopy (TDS), X-ray diffraction, and microstructural observations. Our results suggest that EBM Ti–6Al–4V alloy with relatively fine microstructure has a better resistance for hydrogen induced damage in comparison to a wrought alloy.

The transition metal carbides have been known to act as traps for hydrogen in steels as well as potential materials for hydrogen storage. Despite numerous experimental and a few theoretical studies, what impacts hydrogen trapping and storage in the transition metal carbides is not well understood. In this work, we use density functional theory to systematically investigate the bulk trapping and storage capabilities of the transition metal carbides. We specifically examine how trapping and storage changes with the transition metal, from the group IVB to group VIB, carbon concentration, and structure. Our results demonstrate a strong correlation between the trap energy and the number of valence electrons in the transition metal, suggesting that the group IVB transition metal carbides are the best carbides for trapping and storage. Hydrogen preferentially sits at octahedral interstices, e.g., the carbon vacancies, in the structure except in certain cases near the Me2C concentration when tetrahedral interstices, devoid of nearest neighbor carbon, can become more favorable. Our results further demonstrate that the lower the carbon concentration, the more hydrogen the transition metal carbide can store. These results demonstrate which carbides will act as the best traps for hydrogen.

The effect of the Ni/Cu ratio on hydrogen embrittlement (HE) behavior is studied in the context of the Cu–Ni binary alloy system. When classifying HE sensitivity as a function of the Ni fraction, two regimes are obtained: Regime 1 (wherein Ni fraction is less than 80 wt%) with moderate HE and Regime 2 (where Ni fraction exceeds 80 wt%) with more severe HE, although hydrogen- (H-)induced intergranular (IG) cracking is noted to be the common primary cause of H-induced degradation. Necessities of H-transportation towards grain boundaries (GBs) via H–dislocation interaction and/or dynamic H-diffusion are discussed via the slow strain rate tensile tests at −196 °C. Specifically, in Regime 1, H-transportation is necessary for the triggering of IG cracking. Conversely, in Regime 2, such H-transportation plays a minor role and the initially concentrated hydrogen along GBs can sorely cause IG cracking. • Hydrogen embrittlement (HE) behavior of seven Cu–Ni binary alloys were investigated. • Intergranular cracking was the common, key process for HE in all embrittled alloys. • HE tendency can be categorized into two regimes depending on Ni fraction. • Regime 1: HE was relatively mild and H-supply via dislocation activity was mandatory. • Regime 2: HE became more severe even without H-dislocation interactions. [ABSTRACT FROM AUTHOR]

The hydrogen trap sites and corresponding hydrogen binding energies in an Al–Cu–Mg alloy with the different microstructures were investigated to unravel the environmental hydrogen embrittlement (HE) behavior of the alloy. The results showed that hydrogen can reside at interstitial lattices, dislocations, S′-phase, and vacancies. In the aged specimen with the highest hydrogen content, it was firstly reported that hydrogen resided at S′-phase particles with relatively high binding energy, which is a determinant factor on HE resistance of the alloy. In the cold-rolled specimen, high content of hydrogen trapped at dislocations with a reversible nature leads to intergranular hydrogen-assisted cracking. In the solution-treated specimen, hydrogen migration to the surface due to low trap density results in low hydrogen content and prevents the GBs from reaching critical hydrogen concentration. The obtained results clearly reveal that trap site density, and the nature of trap sites can determine environmental HE susceptibility of the alloy. • H trapping in the cold-rolled sample was dominated by weak traps, i.e. dislocations. • H trapping at dislocation led to H-assisted crack nucleation along grain boundaries. • T6 sample was highly resistant to H due to trapping at strong traps, i.e. S′-phase. • Trap density is effective in the retention and mobility of H in aluminum alloys. • It was firstly reported that S′-phase has critical role on H sensitivity of 2024. [ABSTRACT FROM AUTHOR]