Hydrogen Embrittlement

In high-strength steels it is often difficult to distinguish between hydrogen embrittlement and various other brittle failure mechanisms. The objective of this work was to develop a sensitive analytical procedure based on secondary ion mass spectrometry (SIMS) that would allow in-service identification of local hydrogen accumulation, either during quality control or during failure analysis of electroplated items. Dynamic SIMS was found useful in identifying when baking of Cd-plated AISI 4340 steel was not carried out, thus potentially leading to hydrogen embrittlement. In all non-baked samples, an increase in the hydrogen signal was found at the Cd/steel interface. In baked samples, either a peak was not observed at the interface, or it was insignificant based on determination of the ratios between the hydrogen signals in the coating, interface and substrate. This reproducible effect was monitored even after 16 months storage in a desiccator. These observations make the procedure practical in suggesting more accurate, reliable and cost effective recommendations for prevention of failures. The main effect of baking was found to be effusion of hydrogen from the interface and the substrate steel into the atmosphere. A mechanism for delayed failure is suggested.

Hydrogen embrittlement (HE) is an epidemic problem for high strength medium carbon low alloy steel causing reduction in mechanical strength and useful service lifetime. Hydrogen embrittlement of high strength steel is concern for various industrial components used in critical applications. Unpredictable & sudden failure of mechanical equipment/components made of high strength steel (HSS) below designed allowable stress and without appreciable deformation resulted in many catastrophic accidents. HSS being used in various engineering applications such as Aerospace, Nuclear, Marine and Transportation etc. Components typically affected by hydrogen embrittlement are pressure vessels, boilers, automobile frame, fasteners & hardware, shafts, axles, rotors, pipelines etc. Extensive research has been done by scientists & engineers to explore various ways to prevent hydrogen embrittlement. Some of them are i) addition of alloying elements ii) selection of material with less susceptibility to hydrogen iii) use of barrier layers to prevent hydrogen diffusion iv) change in manufacturing process and application environment v) use of advance Nano coatings to minimize hydrogen penetration etc. Out of all these methods of controlling hydrogen embrittlement, use of advance coatings appears to be more practical, less intricate and economical. In current decade Graphene has achieved a great attention from engineering world because of its excellent chemical, physical, mechanical & thermal properties. Graphene can act as a protective barrier against many environmental induced degradation of alloy steels because of its extraordinary impermeability. This review article summaries the current state of research & available commercial solutions for mitigations of hydrogen embrittlement using graphene based Nano-coatings. Authors have conducted experiments on several available coatings including graphene to investigate the behavior in an environment promoting hydrogen embrittlement and compared the effect on mechanical properties and ductility of high strength steel (EN24/AISI4340/34CrNiMo6)

Many efforts have been made to understand the effects of hydrogen on steels, resulting in an abundance of theoretical models and papers. However, a fully developed and practically applicable predictive physical model still does not exist industrially for predicting and preventing hydrogen damage. In practice, it is observed that different types of damages to industrial boiler components have been associated with the presence and localization of hydrogen in metals. In this paper, a damaged boiler tube made of grade 20-St.20 (or 20G, equivalent to AISI 1020) was investigated. The experimental research was conducted in two distinctive phases: failure analysis of the boiler evaporator tube sample and subsequent postmortem analysis of the viable hydrogen embrittlement mechanisms (HE) in St.20 steel. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack (HTHA) during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, tube wall thickness measurement, tensile testing, hardness measurement, impact strength testing (on instrumented Charpy machine), analysis of the chemical composition of corrosion products-deposit and the microstructural characterization by optical and scanning electron microscopy-SEM/EDX. The HTHA damage mechanism is a primary cause of boiler tube fracture. Based on the multi-scale special model, applied in subsequent postmortem investigations, the results indicate a simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. The model is based on the correlation of mechanical properties to the SEM fractography analysis of fracture surfaces.

The API 5L X70 and X52 pipeline steel weld fracture toughness parameters are measured in a hydrogen environment and compared to the ones in air. The hydrogen environment is created by in situ hydrogen charging, using as an electrolyte a simulated soil solution, with three current densities, namely 1, 5 and 10 mA/cm 2. A specially designed electrolytic cell mounted onto a three-point bending arrangement is used and hydrogen charging is performed during the monotonic loading of the specimens. Ductility is measured in terms of the J 0 integral. In all cases a slight change in toughness was measured in terms of K Q. Reduction of ductility in the base metal is observed, which increases with increasing current density. A more complex phenomenon is observed in the heat affected zone metal, where a small reduction in ductility is observed for the two current densities (1 and 5 mA/ cm 2) and a larger reduction for the third case (10 mA/cm 2). Regarding microstructure of tested X70 and X52 base and HAZ metal, it is observed that the hydrogen degradation effect is enhanced in banded ferriteepearlite formations. The aforementioned procedure is used for calculating the fracture toughness parameters of a through-thickness pipeline crack.

Hydrogen environment embrittlement Aluminum bronze Copper alloy a b s t r a c t Aluminum bronze CW307G was tensile and fatigue tested in 10 MPa hydrogen, 10 MPa helium and 0.1 MPa air atmosphere. Neither tensile nor SeN fatigue properties were affected when testing in H 2 . Fractography on the fatigue specimens revealed similar striation morphology on the specimens tested in H 2 and He. Dissociative chemisorption as well as hydrogen absorption were identified as potential rate limiting processes being responsible for the impassivity to HEE of this alloy. (T. Michler).

A wide range of TIG welds of austenitic stainless steels and filler materials were investigated with respect to their microstructure and cryogenic toughness. Depending on the combination of base and filler material the resulting structure of the weld seam can show either long cross-sectional δ-ferrite dendrites, isolated δ-ferrite islands or no δ-ferrite at all (fully austenitic). The δ-ferrite content varied

In the current paper, instead of the Oriani formulation used by Sofronis and McMeeking, a more general formulation introduced by McNabb and Foster is adopted and applied successfully to a non-symmetric hydrogen diffusion scheme. The results are compared to Sofronis and McMeeking's and Krom et al.'s work for the hydrogen diffusion near a blunting crack tip. Results represent that the McNabb-Foster formulation produces drastically different hydrogen distribution from Oriani's equilibrium theory in both trap and lattice sites particularly when the loading time is significantly small.

There is a need for numerical models capable of predicting local accumulation of hydrogen near stress con-centrators and crack tips to prevent and mitigate hydrogen assisted fracture in steels. The experimental char-acterisation of trapping parameters in metals, which is required for an accurate simulation of hydrogen transport, is usually performed through the electropermeation test. In order to study grain size influence and grain boundary trapping during permeation, two modelling approaches are explored; a 1D Finite Element model including trap density and binding energy as input parameters and a polycrystalline model based on the assignment of a lower diffusivity and solubility to the grain boundaries. Samples of pure iron after two different heat treatments-950 • C for 40 min and 1100 • C for 5 min-are tested applying three consecutive rising permeation steps and three decaying steps. Experimental results show that the finer grain microstructure promotes a diffusion delay due to grain boundary trapping. The usual methodology for the determination of trap densities and binding energies is revisited in which the limiting diluted and saturated cases are considered. To this purpose, apparent diffusivities are fitted including also the influence of boundary conditions and comparing results provided by the constant concentration with the constant flux assumption. Grain boundaries are char-acterised for pure iron with a binding energy between 37.8 and 39.9 kJ/mol and a low trap density but it is numerically demonstrated that saturated or diluted assumptions are not always verified, and a univocal determination of trapping parameters requires a broader range of charging conditions for permeation. The relationship between surface parameters, i.e. charging current, recombination current and surface concentrations, is also studied showing that trapping phenomena are stronger during the diluted steps and that recombination currents are much higher than the steady state obtained flux.

Characteristics of tempered martensite embrittlement (TME), hydrogen embrittlement (HE), and stress corrosion cracking (SCC) in high-strength steels are reviewed. Often, it is important to determine unambiguously by which of these mechanisms failure occurred, in order to suggest the right actions to prevent failure recurrence. To this aim, samples made of high-strength AISI 4340 alloy steel were embrittled by controlled processes that might take place, for example, during the fabrication and service of aircraft landing gears. The samples were then fractured and characterized using light and scanning electron microscopy, microhardness tests, and X-ray diraction. Fractography was found to be the most useful tool in determining which of these mechanisms is responsible for a failure, under similar conditions, of structures made of AISI 4340 alloy steel. #

Hydrogen embrittlement in 304L austenitic stainless steel fabricated by laser powder-bed-fusion (LPBF) was investigated and compared to conventionally produced 304L samples with two different processing histories; casting plus annealing (CA) and CA plus thermomechanical treatment (CA-TMT). Interestingly, no significant difference in the amount of deformation-induced α ′ martensite between the LPBF and CA-TMT samples was observed, suggesting that the solidification substructure in the LPBF sample enhanced the strength without promoting the harmful hydrogen embrittlement effect. These results are discussed in terms of the chemical in-homogeneity, hydrogen-assisted cracking behavior, and hydrogen diffusion and trapping in the present 304L samples.

Electroplated Zn, Ni, Cu, Al, PVD-Ti-DLC and electroless NiP coatings as well as carbon, nitrogen and oxygen diffusion layers were investigated for their suitability to reduce hydrogen environment embrittlement (HEE) of 304 austenitic stainless steel. The mechanical stability of the coatings was evaluated by interrupted slow strain rate tensile testing. The Zn, Ni, Ti-DLC and NiP coatings as well as

Hydrogen environment embrittlement (HEE) is a well-known phenomenon in materials science. For the automotive industry, this topic is of special importance due to the upcoming hydrogen technology. Results on a wide variety of commercially austenitic stainless steels are presented which show that the Nickel content is one of the main parameters to meet HEE resistance. In slow strain rate tensile testing no loss in reduction of area at À50 C was found for Ni contents above 12.5 wt%. It was also found that the influence of minor alloying elements and impurities like Mo, Mn, Ti and S is negligible within the range investigated here. Another important parameter is local metallurgy. It is shown that the HEE resistance strongly depends on the type of the semi-finished product especially between a Nickel range of 10 and 12.5 wt% Drawn bar materials with a very homogeneous structure show a better resistance compared to rolled plate materials with segregations despite identical nominal chemical compositions.

Room temperature cathodic hydrogen embrittlement in alloy 718 was investigated by means of slow strain rate tensile tests conducted on specimens charged either prior to or during deformation. Tensile tests performed on precharged specimens at strain rates of 5 ×10 − 7 , 5×10 − 5 and 5×10 − 3 s − 1 suggest that hydrogen embrittlement is correlated with hydrogen segregation to moving dislocation and transport by these dislocations. Observations of 1 mm planar cleavage microfacets on fracture surfaces of specimens charged either prior to or during deformation support the idea that embrittlement occurs by strong hydrogen-deformation interactions. In light of these results, the role of the cathodic hydrogen produced by the corrosion process inside a crack during stress corrosion cracking is discussed in conjunction with the corrosion enhanced plasticity model, proposed some years ago by one of the authors. It is suggested that hydrogen transport by dislocations and localisation along active slip planes may be the controlling stage of the cracking process.

This paper presents an updated review of the external corrosion and failure mechanisms of buried natural gas and oil pipelines. Various forms of external corrosion and failure mechanisms such as hydrogen-induced cracking (HIC), hydrogen embrittlement (HE), corrosion fatigue (CF), stress corrosion cracking (SCC) and microbiologically influenced corrosion (MIC) for oil and gas pipelines are thoroughly reviewed. The factors influencing external corrosion and possible forms of environment-assisted cracking (EAC) of pipeline steels in the soil are also reviewed and analyzed in depth. In addition, the existing monitoring tools for the external corrosion assessment and the models for corrosion prevention and prediction, failure occurrence, and remaining life of oil and gas pipelines, are analyzed. Moreover, the articles on external corrosion management, reliability-based models, risk-based models, and integrity assessment including machine learning and fuzzy logic approaches, are also reviewed. The conclusions and recommendations for future research in the prevention and prediction of external corrosion are presented at the end.

Stress corrosion cracking and hydrogen cracking are two important forms of environmental cracking which can cause serious failures in the oil and gas, pipeline, process and many other industries. Both forms of cracking result in normally ductile metals failing in a brittle manner. In many if not most cases, analysis clearly shows which of these failure modes has occurred. However, there can be situations where the two can be confused. This paper reviews the differences and similarities between the two modes, concentrating on the metals and environments, especially steels in sulphide environments, where confusion can arise. It looks at the various forms of cracking, the nature of cracking, its causes and how it is prevented. Sulphide stress cracking can be especially troublesome as, unlike other forms of hydrogen damage, the cracking initiates at the surface. A review of the mechanism of sulphide stress cracking is given, along with its relationship to stepwise cracking. There can also be confusion between these two forms of damage, as a case study from the literature shows.

Hydrogen evolution and permeation occur during electroplating, corrosion, and cathodic protection. Hydrogen accumulates in areas of high stress and may reach a critical concentration, potentially causing fractures and catastrophic damage. This chapter describes hydrogen permeation and hydrogen-induced damage in metals and alloys. It also discusses hydrogen evolution kinetics, theoretical diffusion solutions, and basic hydrogen permeation models. Models are used as a diagnostic tool to determine the effectiveness of various metals and alloys as hydrogen permeation inhibitors. It then explains experimental atomic hydrogen permeation transient determination and hydrogen absorption rate constants and diffusivity evaluation into metals using case studies. Hydrogen embrittlement, hydrogen-induced cracking, hydrogen blistering, and hydrogen stress cracking are also discussed to show the relationship to hydrogen permeation and hydrogen-induced cracking mechanisms. Finally, various techniques used to prevent and control hydrogen damage of metals and alloys are described.

Hydrogen gas is a renewable energy source for electrical and transportation fuel for vehicular applications. However, the storage and transportation of hydrogen gas are challenging because of its very nature and impact on pipelines and storage tank/facility materials. This paper investigates the influence of hydrogen on the candidate fracture toughness (K Q) of low carbon steel immersed in acidic hydrogen environments for one year which has limited previous research. Steel specimens were coated from all sides except one surface to accurately quantify the influence of hydrogen diffusing from the environments into the specimens. Specimens were tested for crack tip opening displacement (CTOD) fracture toughness at six-and twelve-month intervals of immersion in acidic environments. Before K Q testing at various intervals, the hydrogen contents of the specimens were determined by an electrochemical approach. Based on test results, models for the degradation of K Q of steel were developed in accordance with the proposed hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) model (HELP + HEDE model) of hydrogen embrittlement. Furthermore, fractography of the specimens was performed to observe the synergistic action of HELP and HEDE mechanisms (HE), and their subsequent effects on the microstructure and fracture resistance of steel. The significance of the research is highlighted by its practical application for assessing the durability of steel structures and infrastructure against hydrogen environmental assisted cracking (HEAC). Furthermore, this paper highlights the synergistic activity of HELP and HEDE mechanisms of HE in steel and the importance of developing structures for storing hydrogen on a large scale.

Component failures due to the hydrogen embrittlement (HE) were observed in different industrial systems, including high-pressure hydrogen storage tanks, aircraft components, high-strength alloy components, and high-strength steel fasteners. The contemporary approach in studying the effects of hydrogen on the mechanical properties of steels and iron at different scales is based on the implementation of various multiscale (macro, micro-meso, and nano-atomic) modeling approaches and the applications of advanced experimental methods. A large number of contemporary studies confirmed the multiple effects and activity of different HE mechanisms in steels and iron. The coexistence and synergistic activity-concurrent action and effects in a cooperative manner of different HE mechanisms, including the hydrogen-enhanced localized plasticity (HELP) and the hydrogen-enhanced decohesion (HEDE), were recently detected and confirmed through computations-simulations, as well as experimentally in different grades of steel. However, the critical evaluation and quantification of synergy between the HELP and HEDE mechanisms, enhanced plasticity and decohesion, hydrogen-deformation/dislocation interactions and their simultaneous effect on the mechanical properties (hardening and softening), still do not exist. In this review paper, the multifaceted nature of the synergistic interplay of HE mechanisms is covered through extensive literature overview regarding the chronological development of ideas related to the HELP + HEDE concept and HELP mediated HEDE model. The particular emphasis is given to the proposal of the novel and unified HELP + HEDE model based on the specific microstructural mapping of the dominant HE mechanisms with implications on the fracture process and resulting hydrogen-assisted fracture modes. Most of up-to-date experimental and modeling approaches, current trends and future challenges in the investigation of the synergistic interplay of HE mechanisms in different grades of steel, including the most advanced, and iron, are also included and critically discussed.

Many efforts have been made to understand the effects of hydrogen on steels, resulting in an abundance of theoretical models and papers. However, a fully developed and practically applicable predictive physical model still does not exist industrially for predicting and preventing hydrogen damage. In practice, it is observed that different types of damages to industrial boiler components have been associated with the presence and localization of hydrogen in metals. In this paper, a damaged boiler tube made of grade 20 – St.20 (or 20G, equivalent to AISI 1020) was investigated. The experimental research was conducted in two distinctive phases: failure analysis of the boiler evaporator tube sample and subsequent postmortem analysis of the viable hydrogen embrittlement mechanisms (HE) in St.20 steel. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack (HTHA) during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, tube wall thickness measurement, tensile testing, hardness measurement, impact strength testing (on in-strumented Charpy machine), analysis of the chemical composition of corrosion products – deposit and the microstructural characterization by optical and scanning electron microscopy – SEM/EDX. The HTHA damage mechanism is a primary cause of boiler tube fracture. Based on the multi-scale special model, applied in subsequent postmortem investigations, the results indicate a simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. The model is based on the correlation of mechanical properties to the SEM fractography analysis of fracture surfaces.

Ever more stringent regulations on greenhouse gas emissions from transportation
motivate efforts to revisit materials used for vehicles1. High-strength aluminium alloys
often used in aircrafts could help reduce the weight of automobiles, but are
susceptible to environmental degradation2,3. Hydrogen ‘embrittlement’ is often
indicated as the main culprit4; however, the exact mechanisms underpinning failure
are not precisely known: atomic-scale analysis of H inside an alloy remains a challenge,
and this prevents deploying alloy design strategies to enhance the durability of the
materials. Here we performed near-atomic-scale analysis of H trapped in
second-phase particles and at grain boundaries in a high-strength 7xxx Al alloy. We
used these observations to guide atomistic ab initio calculations, which show that the
co-segregation of alloying elements and H favours grain boundary decohesion, and
the strong partitioning of H into the second-phase particles removes solute H from
the matrix, hence preventing H embrittlement. Our insights further advance the
mechanistic understanding of H-assisted embrittlement in Al alloys, emphasizing the
role of H traps in minimizing cracking and guiding new alloy design.

Hydrogen embrittlement is a common, dangerous, and poorly understood cause of failure in many metal alloys. In practice, it
is observed that different types of damage to industrial components have been tied to the presence and localization of hydrogen in metals. Many efforts have been made at understanding the effects of hydrogen on materials, resulting in an abundance of theoretical models and papers. However, a fully developed and practically-applicable predictive physical model still does not exist industrially for predicting and preventing hydrogen embrittlement. The connection of microstructure-based behaviors of materials and effects on the macroscopic measurable characteristics (stress levels, hardness, strength, and impact toughness) is of the utmost importance to achieve a unified model for hydrogen embrittlement. This paper gives an overview of the application of a model for structural integrity analysis of boiler tubes made of
plain carbon steel exposed during operation to a local corrosion process and multiple hydrogen assisted degradation processes: hydrogen embrittlement and high-temperature hydrogen attack. The model is based on the correlation of mechanical properties to scanning electron microscopy fractography analysis of fracture surfaces in the presence of simultaneously active hydrogen embrittlement micromechanisms. The proposed model is practical for use as a predictive maintenance in power plants, as it is based on the use of standard macro-mechanical tests.

The hydrogen embrittlement (HE) behavior of low carbon steel was investigated. Hydrogen-rich acidic corrosive environments were used, and their effects analyzed. Hydrogen assisted cracking, blister, and grain boundary deterioration were detected. Nanoelastic and nanomechanical properties changes due to HE were investigated. Nanoelastic modulus reduced to 45% in some of the grains due to HE. Low carbon steel Hydrogen-assisted cracking Hydrogen embrittlement Corrosion Elastic modulus and Nanomechanical properties a b s t r a c t This paper aims to investigate the effect of hydrogen-induced mechanical degradation of low carbon steel at macro-, micro-and nano-levels in the hydrogen-rich acidic environments. From the test results of specimens, a relationship in hydrogen concentration and corrosion propagation was observed that led to the significant reductions of bulk elastic modulus after 28 days of exposure to the hydrogen-rich acidic environments. Through microstructural analysis, the deformation of larger grains, cracks, and blisters caused by hydrogen penetration was found as the possible cause for this reduction. Moreover, by performing nanoindentation on the areas of interest of various specimens at planned time periods, the influence of hydrogen on the nano-elastic and nano-hardness properties of grains was determined. The 3D surface profiles of the nano-elastic modulus and nano-hardness of various specimens are presented in this paper.

Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.

Efforts are being made to produce highly pressurised electrolysers to increase the overall energy efficiency by eliminating mechanical compression. However, in-depth modelling of electrolysers suggests that electrolysis at atmospheric pressure is electrically more energy efficient if parasitic energy consumption and gas losses are incorporated in both cases. The reversible cell voltage increases with increasing pressures. The electrode activation and Ohmic losses, leakage current and inevitable heat losses increase the electrolysis voltage beyond the thermoneutral voltage and consequently heat removal from the stack becomes essential. The expected gas loss at various operating pressures is incorporated to reveal the energy consumption that would occur in practice.

Component failures due to the hydrogen embrittlement (HE) were observed in different industrial systems, including high-pressure hydrogen storage tanks, aircraft components, high-strength alloy components, and high-strength steel fasteners [1,2]. The contemporary approach in studying the effects of hydrogen on the mechanical properties of steels at different scales is based on the implementation of various multiscale (macro, micro-meso, and nano-atomic) modeling approaches and the applications of advanced multiscale experimental methods, Fig. 1 [3]. Simultaneous action in a cooperative manner of the hydrogen-enhanced localized plasticity (HELP) and the hydrogen-enhanced decohesion (HEDE) mechanism of HE, according to the HELP + HEDE model [4] of HE, were confirmed to be active, depending on the local concentration of hydrogen in steel [2-5], Table 1 [2]. Further investigations of the mechanical properties of steels at different length scales exposed to hydrogen damage should open a new window in the research related to the simultaneous action of HE mechanisms. It will also provide improved maintenance and the accurate service life prediction of various industrial components exposed to multiple active HE mechanisms [5,6].

This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement.

This work aims to evaluate the effects of hydrogen in three high- strength steel grades. The phenomena of hydrogen (H) entry, transport and trapping inside the metals, together with the different types of damages due to the presence of hydrogen are presented. The study materials are a range of AHSS steel grades: Dual Phase Steel (DP 1000 and DP 1200) and Martensitic Steel (M 190). The hydrogen entry was performed by cathodic charging, which is suitable for industrial applications. In order to evaluate the influence of H on the steel mechanical properties, the following tests were done: H charging, to measure total H content (saturation point) and diffusible H content (embrittlement susceptibility); uniaxial tensile test of uncharged samples to determine notched tensile strength values and the strength levels at the end of elastic region and constant load tensile testing carried out in hydrogen environment, to determine the threshold values where hydrogen has an effect on the material. DP 1200 and M 190 were strongly affected by H pre-charging, as shown by the significant drop in stress required to break them. On the other hand, DP 1000 showed a lower embrittlement susceptibility, which is attributed to its lower mechanical strength. The current densities effects (0.2 up to 1.0 mA/cm²) were evaluated during H charging to measure diffusible H content. All steels showed a drop in the tensile strength i.e. experienced hydrogen embrittlement. Steels with higher tensile strength, as DP 1200 and M 190, showed a much bigger drop that is related to the favorable characteristics of martensitic microstructure regarding to the hydrogen permeability and diffusivity.

Many efforts have been made to understand the effects of hydrogen on steels, resulting in an abundance of theoretical models and papers. However, a fully developed and practically applicable predictive physical model still does not exist industrially for predicting and preventing hydrogen damage. In practice, it is observed that different types of damages to industrial boiler components have been associated with the presence and localization of hydrogen in metals. In this paper, a damaged boiler tube made of grade 20 – St.20 (or 20G, equivalent to AISI 1020) was investigated. The experimental research was conducted in two distinctive phases: failure analysis of the boiler evaporator tube sample and subsequent postmortem analysis of the viable hydrogen embrittlement mechanisms (HE) in St.20 steel. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack (HTHA) during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, tube wall thickness measurement, tensile testing, hardness measurement, impact strength testing (on in-strumented Charpy machine), analysis of the chemical composition of corrosion products – deposit and the microstructural characterization by optical and scanning electron microscopy – SEM/EDX. The HTHA damage mechanism is a primary cause of boiler tube fracture. Based on the multi-scale special model, applied in subsequent postmortem investigations, the results indicate a simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. The model is based on the correlation of mechanical properties to the SEM fractography analysis of fracture surfaces.

The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase characteristics. The first one has a ferritic matrix (~74 vol.% ferrite) with embedded austenite and a high dislo-cation density (~10 14 m −2) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (~59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE mi-cromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between fer-rite and the adjacent phase mixture of austenite and strain-induced α'-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms between these two microstructures is related to the spatial distribution of H within them. This observation provides significant insights for future microstructural design towards higher HE resistance of high-strength steels.

The onset of sub-critical crack growth during slow strain rate tensile testing (SSRT) is assessed through a combined experimental and modeling approach. A systematic comparison of the extent of intergranular fracture and expected hydrogen ingress suggests that hydrogen diffusion alone is insufficient to explain the intergranular fracture depths observed after SSRT experiments in a Ni-Cu superalloy. Simulations of these experiments using a new phase field formulation indicate that crack initiation occurs as low as 40% of the time to failure. The implications of such sub-critical crack growth on the validity and interpretation of SSRT metrics are then explored .

The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase characteristics. The first one has a ferritic matrix (~74 vol.% ferrite) with embedded austenite and a high dislo-cation density (~10 14 m −2) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (~59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE mi-cromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between fer-rite and the adjacent phase mixture of austenite and strain-induced α'-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms between these two microstructures is related to the spatial distribution of H within them. This observation provides significant insights for future microstructural design towards higher HE resistance of high-strength steels.

Hydrogen embrittlement affects high-strength ferrite/martensite dual-phase (DP) steels. The associated micromechanisms which lead to failure have not been fully clarified yet. Here we present a quantitative micromechanical analysis of the microstructural damage phenomena in a model DP steel in the presence of hydrogen. A high-resolution scanning electron microscopy-based damage quantification technique has been employed to identify strain regimes where damage nucleation and damage growth take place, both with and
without hydrogen precharging. The mechanisms corresponding to these regimes have been investigated by employing post-mortem
electron channeling contrast imaging and electron backscatter diffraction analyses, as well as additional in situ deformation experiments. The results reveal that damage nucleation mechanism (i.e. martensite decohesion) and the damage growth mechanisms (e.g. interface decohesion) are both promoted by hydrogen, while the crack-arresting capability of the ferrite is significantly reduced. The observations are discussed on the basis of the hydrogen-enhanced decohesion and hydrogen-enhanced localized plasticity mechanisms. We discuss corresponding microstructure design strategies for better hydrogen-related damage tolerance of DP steels.

Numerical investigation of transient hydrogen-natural gas flow in a steel pipeline. Influence of gaseous hydrogen flow on the structural integrity of precracked pipes. Interaction between transient hydrogen-natural gas flow and hydrogen embrittlement. Failure Assessment Diagram (FAD) of a precracked API 5L X52 steel pipeline. The stress intensity and safety factors analysis during transient hydrogen pressure. Rapid closure valve Failure assessment diagram Semi-elliptical crack Finite element analysis a b s t r a c t We are reporting in this study the hydrogen permeation in the lattice structure of a steel pipeline designed for natural gas transportation by investigating the influence of blending gaseous hydrogen into natural gas flow and resulted internal pressure values on the structural integrity of cracked pipes. The presence of cracks may provoke pipeline failure and hydrogen leakage. The auto-ignition of hydrogen leaks, although been small, leads to a flame difficult to be seen. The latter makes such a phenomenon extremely dangerous as explosions became very likely to happen. In this paper, a reliable method is presented that can be used to predict the acceptable defect in order to reduce risks caused by pipe failure due to hydrogen embrittlement. The presented model takes into account the synergistic effects of transient gas flow conditions in pipelines and hydrogen embrittlement of steel material due to pressurized hydrogen gas permeation. It is found that blending hydrogen gas into natural gas pipelines increases the internal load on the pipeline walls due to overpressure values that may be reached in a transient gas flow regime. Also, the interaction between transient hydrogen gas flow and embrittlement of API 5L X52 steel pipeline was investigated using Failure Assessment Diagram (FAD) and the results have shown that transient flow enhances pipeline failure due to hydrogen permeation. It was shown that hydrogen embrittlement of steel pipelines in contact with the hydrogen environment, together with the transient gas flow and significantly increased transient pressure values, also increases the probability of failure of a cracked pipeline. Such a situation threatens the integrity of high stress pipelines, especially under the real working conditions of hydrogen
gas transportation.

This paper considers our recent research on rock bolt stress corrosion cracking (SCC) which studied the influence of metallurgy using a range of (1) existing rock bolt steels and (2) commercial steels. The chemical composition, mechanical properties and microstructures of these steels varied considerably. The aim is to understand this failure mechanism and in particular to apply the knowledge gained from the study of SCC and hydrogen embrittlement (HE) of steels. The resistance of 1008, X65, X70 and 4145H to SCC/HE was similar, an effect which could be hardly expected as these steels have different strengths, microstructures and H-trap distributions. Different steel microstructures (pearlitic-ferritic and tempered martensite) had similar resistance to SCC and HE in the most aggressive conditions (pH 2.1 and À1500 mV SCE ) and had the same dimple rupture fracture surface.

Pressure cycle tests were performed on two types of Cr–Mo steel pressure vessels with notches machined on their inside under hydrogen-gas pressures, between 0.6 and 45 MPa at room temperature. Fatigue crack growth (FCG) and fracture toughness tests of the Cr–Mo steels samples from the vessels were also carried out in gaseous hydrogen. The Cr–Mo steels showed accelerated FCG rates in gaseous hydrogen compared to ambient air. The fracture toughness of the Cr–Mo steels in gaseous hydrogen was significantly smaller than that in ambient air. Four pressure vessels were tested with gaseous hydrogen. All pressure vessels failed by leak-before-break (LBB). The LBB failure of one pressure vessel could not be estimated by using the fracture toughness in gaseous hydrogen K IC,H ; accordingly, the LBB assessment based on K IC,H is conservative and there is a possibility that K IC,H does not provide a reasonable assessment of LBB. In contrast, the fatigue lives of all pressure vessels could be estimated by using the accelerated FCG rates in gaseous hydrogen. Downloaded From: http://manufacturingscience.asmedigitalcollection.asme.org/ on 09/03/2015 Terms of Use: http://www.asme.org/about-asme/terms-of-use

In this work, we study the influence of hydrogen on the deformation behavior and microstructure evolution in an
equiatomic CoCrNi medium entropy alloy (MEA) with an ultimate tensile strength of ∼1 GPa. Upon deforma-
tion, hydrogen-charged samples exhibit enhanced dislocation activity and nanotwinning. Hydrogen shows both
positive and negative effects on the deformation behavior of the CoCrNi MEA. More specifically, it weakens grain
boundaries during loading, leading to intergranular cracking. Also, it promotes the formation of twins which
enhance the material’s resistance to crack propagation. The underlying mechanisms responsible for the hydrogen
resistance of the CoCrNi MEA are discussed in detail.
High entropy alloys (HEAs) containing multi-principal elements
have opened a new field in alloy design [1–4]. The versatility of these
alloys in adjusting compositions over wide ranges and thereby tuning
intrinsic features such as solid solution hardening and the stacking fault
energy for achieving enhanced mechanical properties are attractive
features when designing improved materials [5–7].
Hydrogen in metals is a long-standing research topic owing to its
embrittling effects which can initiate catastrophic failure, a phenom-
enon known as hydrogen-embrittlement [8–11]. Most engineering
metallic alloys, particularly those with strength levels beyond 700 MPa,
are at least to some degree susceptible to this detrimental phenomenon,
yet, possibly due to quite different underlying mechanisms in the in-
dividual materials. Recently, several studies have dealt with hydrogen
embrittlement effects in HEAs [12–16]. The equiatomic CoCrFeMnNi
HEA shows relatively high resistance to hydrogen embrittlement when
exposed to hydrogen concentrations of approximately 80 wt. ppm
[12,17], but it still undergoes embrittlement at higher concentrations
[13]. Although addition of interstitial carbon to this material leads to an
increase in strength [18,19], deformation in the presence of hydrogen
results in the initiation of micro-cracks around carbides and hence C-
doped material shows higher susceptibility to hydrogen embrittlement
compared to the C-free reference alloy [20]. To date, several mechan-
isms have been proposed to explain such embrittlement phenomena in
metals, namely, hydrogen-enhanced localized plasticity (HELP)

[21–24], hydrogen-enhanced decohesion (HEDE) [25–27], as well as
hydrogen-stabilized superabundant vacancy enrichment [28,29], hy-
dride formation [30,31] and microvoid coalescence [22,32].
In the CoCrFeMnNi alloy family and its lower entropy variants, the
so called medium entropy alloys (MEA), specifically the equiatomic
ternary CoCrNi alloy shows a higher strength-ductility combination
than many other related binary, ternary, and quaternary variants
[33,34]. Also, its fracture toughness is superior to that of the widely
studied equiatomic CoCrFeMnNi HEA [35]. Though the mechanical
properties and deformation mechanisms of the equiatomic CoCrNi MEA
were recently revealed in more detail, nothing is known about its de-
formation behavior in presence of hydrogen. Yet, hydrogen effects in
high and medium entropy alloys are of high interest in this alloy class as
both, detrimental and strain hardening effects were observed and im-
proved hydrogen-resistant alloys are urgently needed for future hy-
drogen-exposed transportation and industry applications. Here we thus
study the influence of hydrogen in the equiatomic CoCrNi MEA by using
tensile testing with hydrogen pre-charging. The mechanisms of hy-
drogen-induced cracking and the evolution of the deformation micro-
structure are probed via the correlative use of electron backscatter
diffraction (EBSD) and electron channeling contrast imaging (ECCI).

The apparent hydrogen diffusivity and the saturated hydrogen content of CreMo and Ni eCreMo steels were determined with high-pressure hydrogen gas. Surface effects on hydrogen entry and exit were also investigated by using palladium-coated samples and by diffusion analysis using the finite-element method. Hydrogen contents of hydrogen-exposed cylindrical specimens of various sizes were measured by means of gas chroma-tographyemass spectrometry to obtain the saturated hydrogen content. The diffusivity was determined by fitting the solution of a diffusion equation to the experimental hydrogen contents determined by desorption at various constant temperatures. In the specimens examined, surface effects were significant at room temperature. The temperature dependences of the diffusivity were reasonably consistent with reference data mainly measured with electrochemical charging. These results were interpreted in terms of hydrogen trapping. Ordinary electrochemical charging represents a more severe condition than exposure to high-pressure hydrogen, for example, at 100 MPa.

such degradation in mechanical behavior have remained an issue of contention for many years, but can be broadly classified into three primary mechanisms , namely (i) decohesion mechanisms, where hydrogen at internal interfaces lowers the cohesive strength there (''hydrogen embrittlement''), (ii) hydrogen-enhanced localized plasticity (HELP), where hydrogen affects the local instabilities associated with plastic flow, and in certain material systems, (iii) hydride formation, where the presence of highly brittle hydride precipitates results in a ''low energy'' fracture path. In addition, there are other hydrogen-related degradation mechanisms involving internal gaseous species; these include blistering, where high hydrogen concentrations, e.g., associated with electrochemical hydrogen charging, result in the reformation of internal gaseous hydrogen at internal interfaces, leading to high internal pressures and the formation of blistering, and hydrogen attack, where at high temperatures and pressures, such internal hydrogen can react with the carbides in steel to form internal methane gas with an associated loss in strength due to decarburization. Although the dominant view is that several of these mechanisms, specifically the decohesion and HELP mechanisms, have been considered to be mutually exclusive, it has been recognized that this may not be the case . Indeed, the present work presents a compelling argument that the two mechanisms may in fact be acting in concert.

This paper addresses the issue of environmentally-assisted fatigue crack propagation in metallic and intermetallic alloys, which is essential to predict the durability of aerospace components. A review of the current understanding of the mechanisms accounting for the detrimental effect of air on fatigue crack growth resistance of conventional alloys is first presented. The adequacy of the two sequential processes identified, namely water vapour adsorption on crack tip surfaces and hydrogen embrittlement of cyclically deformed material within the plastic zone, to account for environmentally-assisted fatigue crack propagation in alloys based on intermetallic compounds is then examined. The last section is devoted to the analysis of environmental effects during creep-fatigue crack growth in an age-hardened aluminium alloy for supersonic aircraft. In particular the influence of environment on the intergranular cavitation damage process is analysed.

The objective of this work is to identify microstructural variables that lead to the large scatter of the relative resistance of 316 grade stainless steels to hydrogen environment embrittlement. In slow displacement rate tensile testing, two almost identical (by nominal chemical composition) heats of SUS 316L austenitic stainless steel showed significantly different susceptibilities to HEE cracking. Upon straining, drawn bar showed a string-like duplex microstructure consisting of a 0 -martensite and g-austenite, whereas rolled plate exhibited a highly regular layered a 0 -g structure caused by measured gradients in local Ni content (9.5-13 wt%). Both martensite and austenite are intrinsically susceptible to HEE.