Exploring Hydrogen Embrittlement: Mechanisms, Consequences, and Advances in Metal Science

Abstract

Hydrogen embrittlement (HE) remains a pressing issue in materials science and engineering, given its significant impact on the structural integrity of metals and alloys. This exhaustive review aims to thoroughly examine HE, covering a range of aspects that collectively enhance our understanding of this intricate phenomenon. It proceeds to investigate the varied effects of hydrogen on metals, illustrating its ability to profoundly alter mechanical properties, thereby increasing vulnerability to fractures and failures. A crucial section of the review delves into how different metals and their alloys exhibit unique responses to hydrogen exposure, shedding light on their distinct behaviors. This knowledge is essential for customizing materials to specific applications and ensuring structural dependability. Additionally, the paper explores a diverse array of models and classifications of HE, offering a structured framework for comprehending its complexities. These models play a crucial role in forecasting, preventing, and mitigating HE across various domains, ranging from industrial settings to critical infrastructure.

1. Introduction

Hydrogen embrittlement (HE) is a well-documented phenomenon observed in high-strength materials. It contributes to subcritical crack growth, fracture initiation, and ultimately catastrophic failure, leading to a decrease in mechanical properties such as ductility, toughness, and strength [1]. The concept of HE presumably dates back to Zappfe [2], who proposed it as the planar pressure theory in 1941, likely marking the earliest theory of HE. Over time, the global scientific community has embraced the concept of HE, recognizing it as a multifaceted phenomenon with far-reaching implications for the performance characteristics of metals and alloys. It is noteworthy that the detrimental effects of hydrogen can manifest even at relatively low concentrations, measuring less than 1 ppm [3].

Currently, the primary hurdles in advancing the hydrogen sector pertain to the storage and conveyance of hydrogen. An earlier statistical investigation, which examined 215 hydrogen-related incidents, highlights that over 80% of these occurrences are linked to activities involving hydrogen production or storage [4].

Hydrogen stands as the universe’s most abundant and elemental constituent. Its omnipresence, lightweight nature, and rapid mobility pose a challenge when attempting to gauge individual hydrogen atoms within a material, whether it be a metal or alloy. Hydrogen has a tendency to infiltrate materials, and even very small concentrations of hydrogen (<1 ppm) can progressively establish conditions for embrittlement. Hence, it becomes paramount to detect the presence of hydrogen within materials, quantifying it accurately, even down to the measurement of single atoms. Molecular hydrogen enters into reactions with a few chemical elements but, turning into a radical, interacts with many. Saturation of a metal with hydrogen leads to hydrogenation and destruction [5]. The free movement of hydrogen causes deformation and accelerated destruction of materials. Hydrogen, together with mechanical stresses, causes specific damage inside materials.

Hydrogen tends to be located in hydrogen-rich regions within the structure of materials [6]. At the same time, the accumulation of hydrogen in the structure of materials to the maximum possible concentrations significantly changes the mechanical properties of the materials. For this, it is important to determine not only the average concentration but also the distribution of hydrogen in a metal/alloy, taking into account its interaction with the material. At first glance, the metal does not show signs of corrosion but becomes brittle at impact strength—an indicator of the influence of hydrogen on the property of the metal, characterized by HE.

Under certain conditions, negligible concentrations of hydrogen can lead to the initiation of cracks in metals and premature failure of products. This problem has become unusually acute for high-strength steels and a number of new metals and their alloys, which have found applications in new technology [7]. The influence of hydrogen on the mechanical properties of a metal can be carried out as a result of facilitating ductile fracture, which is usual for a given metal, or as a result of a change in the nature of fracture from ductile intragranular to brittle intergranular [7,8,9]. Under the influence of hydrogen, the sensitivity of metals to the presence of cracks increases significantly. This makes real the danger of catastrophic brittle fracture of structures that have sufficient bearing capacity under normal conditions [10].

1.1. The Purpose and Methodology of the Review

This review paper aims to provide a comprehensive and up-to-date examination of HE in metals, with a focus on understanding the diverse mechanisms of hydrogen accumulation, its profound effects on the mechanical properties of metals, its impact on specific metals and their alloys, and the various models and classifications of HE. By synthesizing existing knowledge and recent research findings, this paper seeks to offer a holistic overview of HE, shedding light on its multifaceted aspects and implications for materials science and engineering. Through an in-depth exploration of these key facets, the objective is to enhance our understanding of HE, its underlying mechanisms, and its implications for real-world applications, ultimately guiding the development of strategies to mitigate the detrimental effects of hydrogen in metallic materials.

This comprehensive review employs a methodology that amalgamates and meticulously analyzes a broad spectrum of data sources, encompassing scientific literature, peer-reviewed research articles, and industry reports. The overarching objective is to deliver an exhaustive portrayal of the prevailing landscape of HE in diverse metals. This multifaceted exploration traverses the intricate facets of HE, spanning its fundamental mechanisms, the multifarious effects it exerts on the mechanical properties of metals, the intricate interplay of hydrogen with specific metals and their alloys, and the comprehensive typologies and models that have evolved to elucidate HE phenomena. By undertaking a rigorous curation and analysis of these diverse data sources, the review offers a comprehensive and contemporary overview of HE that serves as a valuable resource for researchers, engineers, and practitioners across the materials science and engineering spectrum.

1.2. Relevance and Knowledge Gap Statements

Despite significant advancements in understanding HE [1,2,3,4,5], there remains a critical need for a comprehensive review paper that synthesizes the diverse influence of hydrogen on various alloy materials. While some individual studies have explored specific aspects, a cohesive overview encompassing mechanisms, models, industrial impacts, and mitigation strategies is currently lacking. This gap hinders a holistic grasp of the complex phenomenon of HE, impeding progress in fields relying on high-strength materials.

This review paper on HE addresses a crucial knowledge gap by providing a structured and inclusive analysis of its effects on a wide range of materials and alloys. By discussing mechanisms, models, industrial implications, and mitigation strategies, this paper offers valuable insights for researchers, engineers, and professionals in industries where high-strength materials are integral. Its comprehensive approach ensures that readers gain a thorough understanding of HE, paving the way for informed material design and engineering practices. Given the pervasive impact of HE, this review is a timely and essential contribution to the field.

2. Hydrogenation Process and Its Effects

2.1. Hydrogenation Process

The process of hydrogenation of a metal begins with the interaction of its surfaces with conjugated microelements and microparticles found in the environment. As a result of the interaction of the unsaturated force fields of a solid body with the force fields of gas molecules moving towards a solid surface, or the interaction of a liquid in contact with a solid body, the surface of the latter is covered with a film of substances contained in the environment: gases, water vapor, and other liquids. In addition, the film contains microparticles of various substances and microorganisms from the surrounding space in contact with the surface of the solid. The nature of the interactions of the resulting films with a metal is determined both by the properties of the metal itself and the influence of the environment on it at the interfaces, as well as by the intensity of physical and chemical processes occurring on the metal surfaces and the separating layer itself [11,12,13,14].

Hydrogen, as a rule, is introduced into the material during:

• Electrochemical processes (electrochemical corrosion, etching, electroplating), when hydrogen is ionized in the electrolyte and absorbed by the material, passing into the state of a quasi-ion—a proton of a metal shielded by electrons [15]. It is important to note that this process can take place at room temperature. As a result of electrochemical corrosion and cathodic treatment [16,17], internal delamination [18], as well as bubbles and cracks emerging on the surface, can occur in a metal.
• The contact of a metal with a hydrogen-containing gaseous medium at elevated temperatures and pressures as a result of thermal dissociation of hydrogen [19]. In this process, hydrogen can also enter into chemical reactions with the structural components of technical metals and alloys, for example, carbides.
• The interaction with reactive hydrogen-containing substances (for example, hydrogen sulfide). In this case, chemical reactions of metals with hydrogen compounds produce hydrogen, which is absorbed by the material. This phenomenon is widespread in the chemical and oil and gas industries [20].

The introduction of hydrogen into the material manifests itself in three stages [21]:

• Adsorption. It consists of the accumulation of hydrogen atoms on the surface of materials. There is physical (low-temperature) and chemical (high-temperature) adsorption. During physical adsorption, Van der Waals forces act on hydrogen atoms; during chemical adsorption, the forces of chemical interaction act on these atoms.
• Absorption–dissolution of adsorbed hydrogen atoms in materials. Absorption can occur both with the release and absorption of heat—it depends solely on the nature of the material under study.
• Diffusion. Hydrogen, due to its small atomic radius, diffuses much more easily than, for example, carbon and nitrogen. The diffusion coefficient of hydrogen for pure iron is 1.5 × 10⁻⁵ cm²/s, for ferritic steel 10⁻⁶ cm²/s, and for austenitic steel 2.3 × 10⁻⁸ cm²/s.

The incubation period is a very important characteristic that determines the conditions for the occurrence of HE by electrochemical processes. The duration of the incubation period depends primarily on the continuously applied voltages. At a voltage above the short-term strength limit, the sample collapses almost instantly and the duration of the incubation period is zero. As the applied voltage decreases, the incubation period increases. The incubation period decreases sharply with increasing hydrogen content in steel. A crack initiated at the end of the incubation period begins to propagate spontaneously without increasing external stresses [22]. Analysis of the experimental data made it possible to identify the following patterns in the manifestation of HE:

• hydrogen-induced brittleness occurs at low strain rates [23,24,25];
• an increase in the hydrogen content of a material worsens its strength and plastic characteristics [26,27];
• hydrogenated metal is subject to delayed destruction, i.e., destruction under constant or slightly varying loads [28];
• the mechanical characteristics of a hydrogenated metal in a stressed state can be at least partially restored during the process of rest after stress relief [29,30];
• with the tightening of the stress state pattern, the intensity of embrittlement increases noticeably [31].

2.2. Effects of Hydrogenation

Hydrogen penetrating into the metal lattice of an alloy adversely affects the strength of the material, and regardless of its concentration in the crystal lattice of the metal, there is a sharp decrease in its strength and plastic properties, thereby strengthening micro- and macro-discontinuities (the appearance of single sharp cracks, porosity), which is an indicator of HE. The following materials can be endangered by this process: titanium alloys (which are used in the aircraft industry); nickel alloys (used in well drilling); aluminum (at high temperatures); hardened iron; and, in particular, high-strength steels.

As a rule, HE is associated with the complete or almost complete separation of single-crystal metal grains from each other (destruction of grain boundaries), or the growth of hydrides inside metals, which are stress concentrators and also lead to destruction. The interaction of materials with hydrogen in the external environment as well as corrosion processes lead, for all materials, to the accumulation of hydrogen. The accumulation of hydrogen always leads to HE [32]. A peculiarity of the behavior of hydrogen in materials is its tendency to localize, which can lead to serious consequences under operating conditions even at negligible hydrogen concentrations [3].

Furthermore, speaking of ductility, it is worth noting that hydrogen often leads to a decrease in macroscopic ductility without significantly changing the microscopic modes of destruction and without introducing any brittle fracture mechanism. In the limiting case, the microscopic mechanism will remain a highly ductile process as the nucleation, development, and coalescence of micropores occur, and macroscopic ductility becomes almost zero. A radically opposite case is also possible when, with the introduction of hydrogen, the metal on a macroscale remains plastic, and microfracture occurs by chipping, which is associated with the formation of hydrides under the action of tensile stresses [8].

The cycles of the interaction of matter can be conventionally characterized:

• penetration
• diffusion process
• absorption
• destruction of material in a local place
• changing material parameters
• inverse influence on the nature of penetration with corresponding diffusion transfer

For low-strength, highly plastic materials, the influence of hydrogen on the mechanical properties is a result of facilitating the “pitting” ductile fracture that is usual for these materials. For strong materials, whose characteristics are similar to those of the transition from a ductile to a brittle state, the effect of hydrogen consists in changing the nature of fracture under the action of hydrogen from “normal” ductile fracture, including the nucleation and growth of pores, to a low-plastic intra- and intergranular cleavage [8,21].

The presence of hydrogen leads to an increase in the fragility of all metals without exception; in not a single case was an increase in the ductility of the metal observed upon the occlusion of hydrogen. It has been established that the result of HE of steel is a decrease in impact strength, relative elongation, and relative contraction. The harmful effect of hydrogen on plastic properties is more pronounced in chromium–nickel, chromium–molybdenum, and chromium–nickel–molybdenum steels. Significant embrittlement of steel containing hydrogen occurs in the temperature range of −100 to +100 °C; the maximum of HE occurs at temperatures close to room temperature, and at a temperature of −196 °C, HE of steel is practically not observed [33].

2.3. Factors Contributing to Hydrogenation

The occurrence of environmental HE is contingent on the complex interplay of various factors. Among these, the type of hydrogenated environment plays a pivotal role, encompassing parameters such as pressure, temperature, hydrogen purity, form, and source. Another critical determinant is the specific metal under consideration, which includes aspects ranging from its fundamental crystal structure to microstructural features, heterogeneities, substructural conditions, phase stability, strength level, and surface conditions, among others [34]. The third vital factor is the stress field, which takes into account factors such as the type of load (monotonic or cyclic), the state of applied stress, and the presence of residual stress. While the individual effects of these factors have been extensively investigated over time, comprehending their synergistic interplay remains a complex challenge [35]. Figure 1 provides a schematic representation of the interdependence of these influencing factors on the susceptibility of industrial equipment to HE [36].

Generally speaking, the higher the strength of a material, the higher the sensitivity to HE. In addition to hardness, special attention should be paid to the following parts of a material:

• Parts’ safety factor. For safety-relevant parts, hydrogen purification should be increased.
• Parts with small cross-sectional area, such as small springs, thinner springs, etc.
• Toothed parts, which are prone to stress concentration.

The quality of the metal surface has a great influence on the kinetics of hydrogen uptake by the metal. Hydrogen collects in any discontinuities or defects on the metal surface. Therefore, the more the defects and discontinuities on the surface of zircon products (even defects allowed by technical conditions: scratches, prints, scratches, punctures, etc.), the more hydrogen the metal absorbs. Oxide films greatly hinder the penetration of hydrogen into metals [37].

The accumulation of hydrogen in most metals and alloys leads to an increase in the concentration of vacancies by several orders of magnitude [38,39,40,41]. Excessive formation of hydrogen-stabilized vacancies is a prerequisite for the formation of microdamages and is the initial moment of HE [42,43,44,45,46,47].

Despite extensive research on HE in bulk materials, the effects of hydrogen on the deformation and fracture of nanograined materials remain largely unexplored [48]. However, due to the recent emergence of numerous nanoscale materials that could benefit from HE research, such as those detailed in references [49,50,51], further investigation is warranted.

For instance, in the study [52], the authors conducted relaxation experiments and simulations, which indicated that hydrogen adsorbed on the surface can inhibit the nucleation of dislocations on the surface of NWs. However, merely increasing the resistance to surface nucleation uniformly does not fully explain the observed localization of plasticity. There must be non-uniform interactions between surface dislocations and hydrogen. This non-uniformity likely occurs during both the initiation of surface nucleation and the subsequent nucleation events.

Furthermore, the study [53] investigates hydrogen segregation and HE mechanisms in nanograined iron (Fe) with varying grain sizes using classical molecular dynamics simulations. The findings reveal that while the hydrogen segregation ratio increases as grain size decreases, the local hydrogen concentration at grain boundaries (GBs) decreases for a given bulk hydrogen concentration. Additionally, the results indicate that hydrogen atoms increase the yield stress in nanograined models, regardless of grain size.

3. Mechanisms, Types, and Models of HE

3.1. Mechanisms and Models

HELP. The concept of the hydrogen-enhanced localized plasticity (HELP) mechanism, responsible for hydrogen-related fractures, was originally introduced by Beachem [54] and is now widely recognized. According to this model, the accumulation of hydrogen near the crack tip reduces resistance to dislocation movement, thereby increasing the flexibility of the dislocation. This allows the dislocation to serve as a carrier for plastic deformation in the metal lattice. However, it was also observed that there was no direct correlation with the planned or proposed HELP model. Different fracture modes can be observed, such as intergranular, transgranular, and semi-ductile, depending on factors such as hydrogen concentration, microstructure, and stress intensity at the crack tip [55].

Analyzing HE mechanisms poses a significant challenge, primarily due to the increased plasticity of the material and the heightened dislocation activity and mobility resulting from hydrogen-dislocation interactions. Even in cases of macroscopically brittle fractures, localized plasticity effects have been observed at both micro- and nanoscales. This mechanism hinges on the enhanced mobility of dislocations induced by hydrogen, affecting dislocation slip behavior around a crack tip. This, in turn, leads to material softening and a decrease in yield strength [35].

HEDE. Hydrogen-enhanced decohesion (HEDE) was originally introduced by Troiano in 1959. It relies on reducing the strength of the material in the crack tip region through the absorption of hydrogen atoms. The strength of the differential bond between molecules decreases as the 1s electron from the hydrogen enters the 3D orbital of the iron atom. Decohesion occurs when the tip of the crack reaches tip removal [56].

The HEDE theory posits that an abundance of diffusing hydrogen in grain boundaries, interstitials, and precipitates weakens the interatomic bonds in metals under stress, ultimately causing atomic binding to break in order to accommodate slip. Initially introduced to explain phenomena such as load relaxation during tensile tests and intergranular failure, dislocation activity may occur during decohesion, as Lynch [57] suggested. This leads to increased local stresses at specific sites, such as atomically sharp crack tips affected by adsorbed hydrogen, as well as near cracks where dislocation entanglements result in a maximum tensile stress.

3.2. Reversible and Irreversible Embrittlement

Some researchers divide HE into reversible and irreversible [8]. With reversible HE, if all the hydrogen is removed from the material, then the brittleness will disappear. The reversibility of HE is understood as the restoration of metal plasticity as a result of hydrogen desorption from the metal during aging at room temperature or as a result of annealing or tempering in air or in vacuum. If there are phenomena associated with HE that cannot be eliminated, then the HE is called irreversible. Such irreversible phenomena include bubbles, flocks, and cracks in the heat-affected zone during welding [20].

Hydrogen brittleness due to decomposition processes that develop during deformation can be called irreversible brittleness of the second kind in the sense that if, after a long action of stresses, the load is removed, the ductility of the alloys is not restored, and during subsequent tests at high speed, carried out any time after removal of the load, brittle fracture is detected. The embrittlement process is not irreversible. Under certain conditions, it can be reversed. A “damaged” part can be repaired and returned to service [58].

In the crystal lattice, hydrogen causes reversible brittleness, which is accompanied by a decrease in ductility with decreasing test speed or at constant stress. This is due to the fact that under load in areas with increased stress, hydrogen is redistributed, even in the case of testing at room temperature [59]. No other impurity is as mobile at room temperature as hydrogen. Phosphorus, sulfur, antimony, etc., remain in the position they took during high-temperature heat treatment and do not have mobility, unlike hydrogen [55].

The embrittlement effect of hydrogen, due to its high pressure, should be more pronounced if the pores are not spherical, but wedge-shaped. Such cavities can arise during the plastic deformation of metals. Indeed, L. S. Moroz and T. E. Mingin [60] found that steel electrolytically hydrogenated after tensile hardening by 8–10% is significantly more prone to irreversible HE than undeformed steel.

In recent years, reversible HE has been discovered in many transition metals. In all cases, the following patterns are characteristic of reversible HE of the second kind [7]:

• Brittleness of this kind manifests itself in a certain temperature range, which depends on the rate of deformation, the nature of the alloys, and their chemical composition.
• With an increase in the strain rate, the temperature interval for the drop in plasticity decreases, plastic characteristics increase, and if the rate exceeds a certain limit, then brittleness is not detected.
• The transition temperature from ductile to brittle fracture increases with increasing hydrogen content.
• Fracture at a low strain rate occurs along grain boundaries.
• Hydrogen has little effect on yield strength and elongation until necking occurs and greatly reduces lateral contraction.

To date, it has been established that reversible HE is observed in many transition metals, regardless of their crystal structure. It is found in metals with a body-centered lattice, including iron, niobium, vanadium, tantalum [61], and nickel [62,63,64]; in austenitic steels [65,66], having a face-centered cubic structure; and in α titanium alloys, with a hexagonal close-packed structure [67,68,69]. This brittleness is observed in metals that absorb hydrogen endothermally, such as iron and nickel, and also in exothermically absorbing metals, such as titanium, vanadium, niobium, and tantalum. In the former metals, hydrogen can develop huge internal pressures, while in the latter, this pressure is negligible. The basic laws of reversible HE are the same for all metals, regardless of whether hydrides are formed in them or not.

4. Current Knowledge Regarding HE in Specific Metals and Alloys

4.1. Steels

For steels, there are several main channels for hydrogen accumulation [32]:

The first avenue, often referred to as the “metallurgical” route, is linked to chemical processes that involve the thermal dissociation of water during steel production. Additionally, the metal becomes impregnated with hydrogen through the natural dissociation of water vapor present in the ambient air. This mechanism primarily results in the gradual accumulation of minute concentrations of hydrogen within a metal.

The second pathway is associated with various stages of product processing. Actions such as quenching in oil or water, rolling, tempering, and the application of diverse surface coatings can either diminish or amplify the initial hydrogen levels several-fold. Throughout mechanical or thermomechanical loading, such as rolling, a notable redistribution of hydrogen within the metal structure takes place. Even at this production stage, microcracks, pores, or other defects associated with hydrogen’s influence can emerge.

The third channel involves the introduction of hydrogen from the external environment during the operational phase of the finished product. A classic illustration of such accumulation is the elevation of hydrogen concentration in the walls of gas and oil pipelines.

In recent years, various mechanisms of the action of hydrogen on the structure and properties of steels have been proposed: a coupled model of diffusion–elastoplasticity [70], a mechanochemical model [59], models for describing the kinetics of delayed fracture of high-strength steels in inactive and hydrogen-containing media [47], and others [71,72,73,74]. However, the listed models describe irreversible HE, although it is noted that destruction under the influence of diffusion-mobile hydrogen (reversible HE) is poorly predictable and most dangerous [75].

The authors of [76] utilized an electrochemical technique to pre-charge specimens of low-alloy Cr–Mo steel AISI 4130 with hydrogen. They conducted a series of experimental tests to evaluate the steel’s susceptibility to HE. Additionally, they aimed to compare the electrochemical pre-charging method with the more commonly used techniques involving gaseous hydrogen and an H2S environment. The study concluded that low-alloy Cr–Mo AISI 4130 steel is significantly sensitive to HE. In hydrogen-charged specimens, the fatigue crack growth rate increased by two to three orders of magnitude. The relationship between crack growth rate and the stress intensity factor (ΔK) can be divided into three distinct phases: an initial rapid increase in growth rate, a plateau phase with a constant crack growth rate, and a final phase where the growth rate accelerates rapidly. Furthermore, the study highlighted the influence of loading frequency; at lower frequencies (0.1–1 Hz), crack growth rates were higher compared to those at higher frequencies (10 Hz).

In the work [77], a comparison is made between the stress–strain behavior of a microstructure in two states: one without hydrogen and the other with a pre-hydrogen-charged condition held for 20 days (at −38 mA/cm²). The uncharged microstructure exhibits a wide range of ductility (Figure 2), featuring strain hardening and the transition to plastic instability, which are characteristic traits of duplex stainless steel, as previously reported in studies. The pre-hydrogen charging, however, led to HE, evident from the decrease in elongation to failure without the initiation of plastic instability. Additionally, hydrogen exposure tests were conducted for 72 days (at −38 mA/cm²) and similar behavior was observed.

The sensitivity of steel to HE depends on many factors: first of all, on the level of strength, and then on the state, composition, and structure of the steel, as well as the properties of individual heats. Zapffi and Sims [78] were pioneers in proposing the theory of HE attributed to the pressure of molecular hydrogen. According to their theory, the brittleness of steel is linked to the pressure exerted by molecular hydrogen accumulated in specific regions. This pressure can reach extremely high levels, up to several tens of thousands of atmospheres, surpassing steel’s elastic limit. This excessive pressure results in metal deformation and the eventual formation of cracks [7]. Hydrogen is present in steel in the interstices of the crystal lattice in the form of atoms in a solid solution, or in pores (hydrogen collectors) in molecular form. In collectors, hydrogen is able to create high internal pressure and plastically deform the metals. Such hydrogen causes irreversible HE, which increases the defect density. The appearance of hydrogen brittleness is accompanied by a sharp decrease in the ductility of steel both during static and impact tests, but this does not affect the occurrence of delayed brittle fracture [8].

The mechanism of initiation and propagation of cracks depends on the mechanical properties of steel (strength level, for example): in mild steels, hydrogen leads to destruction by the mechanism of transgranular cleavage [79].

Two theories of crack propagation can be distinguished [8]:

• The first is Oriani’s theory [80] about the gradual (“atom by atom”) development of a crack as a result of breaking bonds at its tip [8].
• The second is the Troiano theory [81], which proceeds from the initiation of secondary cracks in front of the top of the main crack and their subsequent merging.

Cracking is accompanied by the formation of areas of HE (hydrogen through cracks); the regularity is that these areas alternate. On the same fracture, traces of brittle fracture and traces of ductile fracture can be seen. The study of methods for preventing HE of structural materials is complicated by the multivariance of its manifestation: cracks and pockmarks at the submicroscopic level, reduction in impact strength, hydrogen corrosion, reduction in resistance to deformation, flocks, and other factors (effect of temperatures, concentrations in the alloy).

The influence of hydrogen on the mechanism of crack initiation in steel can be due to [79]:

• high pressure of molecular hydrogen in microvolumes;
• decrease in surface energy;
• decohesion of the lattice, intergranular, and interphase boundaries.

It is known that the basis of all carbon alloys is iron. It is characteristic that iron–carbon alloys contain useful and harmful impurities that determine their physical and chemical properties, one of which is hydrogen. The harmful effects of hydrogen on iron alloys (in particular steel) were first noted in the early 1870s. Since then, much effort has been made to characterize and understand the phenomenon of HE [56]. The critical values of hydrogen with a lower diffuse mobility are tens of times higher; this hydrogen is released from steels during production only during severe deformation—rolling and hot stamping. In this case, the formation of hydrogen-containing metal defects—flocks—is possible [32].

In the study [82], the authors investigated the impact of hydrogen on various steel types. The first type is S355, which is a structural carbon steel that is cost-effective when external loads are relatively low. The second type, H8, is a quenched and tempered alloyed steel used in naval settings, where hydrogen can infiltrate from the saline aqueous environment. Figure 3 visually depicts HE through SEM images of the fracture areas of two steel variants, showing the effects of pre-hydrogenation on the steel’s microstructure. The surfaces of H-free S355 (Figure 3a) and H8 (Figure 3c) display a typical ductile morphology, characterized by the nucleation, growth, and coalescence of microvoids (MVC). In S355 steel, the fracture surface remains ductile, with hydrogen’s effect limited to larger and flatter microvoids, indicative of HELP. Conversely, the presence of hydrogen in H8 steel significantly transforms its fracture behavior from ductile (Figure 3c) to brittle, with evident cleavage features (Figure 3d). H8 steel demonstrates the highest HE index (HEI) and the most pronounced alteration in fracture micromechanism.

4.2. Aluminum and Its Alloys

Aluminum is capable of accumulating relatively small concentrations of hydrogen compared to other metals. The accumulation and redistribution of hydrogen occurs from the external environment and internal sources due to diffusion [32].

Aluminum is not prone to HE. The only defect that occurs in aluminum and its alloys under the action of hydrogen is gas porosity, which affects the mechanical properties of the alloys. The decrease in tensile strength appears to be a consequence of embrittlement due to porosity [83]. When the critical value is exceeded, the material matrix is destroyed. For most aluminum alloys, HE manifests at a hydrogen concentration value of 0.5 ppm. In an aluminum matrix, hydrogen fills mainly lattice defects and grain boundaries [84]. The mechanisms of hydrogen accumulation in aluminum are similar to those in steels, but its influence is much stronger [32].

The widespread use of aluminum alloys implies the possibility of exposure to various environmental factors that initiate thinning of the upper layer of the protective oxide film, which leads to the development of corrosion processes and reduces the overall safety of structures. Aluminum alloys are generally inert to corrosion processes; however, this is true only for alloys with a low degree of supersaturation in a solid solution. Alloys with a high concentration of alloying elements, which include alloys of the duralumin system, if they exist in aggressive environments, are, on the contrary, prone to stress corrosion cracking. The cause of such cracking, among other things, can be hydrogen [85,86,87,88,89].

In [90], the authors investigated the effect of HE on Al-Mg weld. Figure 4 shows the microstructure of both the base metal AA5083-H112, which is around 95% Al, and Al-Mg weld bead at different concentrations of hydrogen. The findings indicate that as the hydrogen content increases, the impact toughness of the weld diminishes. Moreover, the weld shows signs of impurity decohesion from the matrix and the development of porosity (Figure 4).

4.3. Titanium and Its Alloys

Another structural material, titanium, has a fundamentally different type of interaction with hydrogen than aluminum [8]. The main differences are the ability to form chemical compounds with hydrogen–hydrides, as well as the presence of phase transitions of titanium, which are influenced by hydrogen and a fundamentally different level of critical hydrogen concentrations. For aluminum and titanium alloys, the HE threshold differs by more than 100 times. Hydrogen has a dual effect on the mechanical properties of titanium and its alloys. At room temperature, it leads to their embrittlement [8], and at high temperatures, it plasticizes [91].

The presence of hydrogen in titanium causes a sharp deterioration in the plastic properties of a metal during tension and other types of deformation, reduces the resistance to impact fracture, and negatively affects the long-term strength characteristics and other service properties of the metal. Technical titanium is more sensitive to HE than high-purity titanium [83].

Williams [92] suggested that HE, which develops in titanium alloys at low strain rates, differs from the brittleness that manifests itself at high strain rates only in the different nature of the kinetics of hydride release.

The impact of the strain rate on the susceptibility of titanium alloys to HE is attributed to two factors: the diffusion-controlled growth rate of hydrides and the speed at which the sample reaches failure. At high strain rates, as well as at low temperatures, the hydrides in the sample either do not have time to nucleate at all during the deformation or do not reach sizes sufficient to participate in destruction. Brittle fracture occurs only at stresses that are sufficient to initiate nucleation, but not so high that ductile fracture occurs faster than the hydrides reach the size required for fracture [7].

Titanium alloys absorb hydrogen less intensively than commercially pure titanium [93]. The HE of titanium is heavily influenced by the presence of impurities within the metal. At high strain rates, hydrogen brittleness in titanium is linked to the formation of lamellar hydrides [94]. Research on steel, titanium, and their alloys indicates that hydrogen promotes the initiation and propagation of cracks in these metals [7]. Aluminum reduces titanium’s tendency toward HE [95]. In one of his first works on the HE of titanium, Burte [96] found that hydrogen causes embrittlement of alloys tested at high tensile rates at room temperature after holding under load at elevated temperatures.

To illustrate the formation of hydrides followed by secondary cracking in titanium, we refer to the study in [97] conducted on Ti Gr-12 alloy specimens. Figure 5 shows the hydride channels extending from the surface of a Ti Gr-12 alloy specimen, creating pathways for brittle crack development during straining. The tests were performed at a strain rate of 2 × 10⁻⁷ s⁻¹ in seawater. The authors also suggest that, in the case of the Ti Gr-5 alloy, the β-phase content is sufficient to dissolve the hydrogen generated on the specimen surface during testing, preventing both hydride precipitation and secondary cracking.

4.4. Zirconium and Its Alloys

In addition, it has long been known that hydrogen can accumulate efficiently in the crystal lattice of some metals and alloys. Alloys based on titanium and zirconium are possible storage materials for hydrogen [98,99,100,101].

The hydrogenation of zirconium alloys is affected by the alloying elements that are included in their composition. They can affect both individually and in combination with other elements. A small amount of nickel significantly increases its corrosion resistance, but at the same time, the absorption of hydrogen is enhanced. The addition of tin does not significantly affect either the corrosion resistance or the absorption of hydrogen by the metal. Iron, chromium, and antimony in small amounts (up to 0.2%) significantly reduce the proportion of absorbed hydrogen (in relation to its entire amount released during corrosion), and chromium and iron, in addition to the above, increase corrosion resistance. When a small amount of lead, bismuth, arsenic, and tellurium is introduced into zircaloy, the corrosion resistance and hydrogenation ability of the alloy remain almost unchanged. Niobium helps to reduce the absorption of hydrogen by the alloy, while the higher the content of niobium, the greater the resistance of the alloy to hydrogenation [101,102,103].

Since hydrogen, even at very low concentrations, sharply embrittles zirconium, there was a concern that zirconium and its alloys could not be used in water-cooled reactors [104]. Indeed, the first studies [105,106] have already shown that hydrogen does not significantly affect the strength characteristics of zirconium, but significantly reduces its impact strength at low temperatures. HE manifests itself when the content is 0.001% H₂.

Studies have established the dependence of hydrogen absorption by zirconium on the nature of surface treatment, preliminary heat treatment, work hardening of the metal, the presence and nature of impurities contained in it, and especially on the nature of surface films on the metal [107,108,109].

The absorption of hydrogen by zirconium alloys can cause their embrittlement and subsequent failure. The degree of embrittlement of zirconium alloys as a result of hydrogenation depends on the amount of absorbed hydrogen and the form of its presence in the alloy structure: in solid solution or in the form of a hydride phase, which is determined by the limiting solubility of hydrogen in this alloy [110].

Additionally, the authors of [111] assert that at relatively low temperatures, a significant amount of hydrogen exists as precipitates, which increases the likelihood of HE failure. Figure 6 illustrates the ratio of solid solution hydrogen to precipitated hydrogen at various temperatures in Zr-4 cladding. Generally, as the temperature increases, the solid solubility of hydrogen also increases. For cladding containing 200 ppm hydrogen, nearly all of the hydrogen is in the form of precipitated hydrides at room temperature. However, at 350 °C, the fraction of precipitated hydrogen drops to less than 40%. At 550 °C, hydrogen only begins to precipitate in the cladding when its content exceeds 600 ppm. The impact of these two forms of hydrogen on the mechanical properties of Zr-4 cladding will be discussed in detail in the subsequent subsections.

4.5. Nickel and Its Alloys

It has also been established that the presence of hydrogen leads to a sharp decrease in the plastic properties of nickel, and the nature of this process has much in common with the process of embrittlement of steel and a number of other metals with a cubic lattice. However, unlike steel, where the pressure leads to irreversible changes due to plastic deformation, nickel deforms only elastically: after the hydrogenation stops, the deformation gradually disappears.

Hydrogen forms hydrides and solid solutions with nickel. Therefore, it can be expected that hydride brittleness and reversible brittleness, which manifests at low strain rates, will develop in nickel. The authors of [112] found that during electrolytic hydrogenation on the surface of the samples, hydrides appear, and the samples are brittle. Nickel’s propensity for HE decreases with increasing chromium and iron content. In the work [113], it was shown that the HE of nickel can be due to molecular hydrogen. The most convincing data on the tendency of nickel to acquire HE at low strain rates were obtained by Troyano et al. [81,114,115].

In [114], nickel and Monel metal were shown to be prone to static HE. Static HE was found at temperatures of 450–533 K in a rather wide range of stresses. This embrittlement is quite similar to that observed in steel. However, due to the lower mobility of hydrogen in face-centered metals, delayed fracture is observed at temperatures above room temperature.

In [116], the hydrogen solution energy in Ni and its alloys is shown. Figure 7a,b illustrate the distributions of hydrogen solution energy in pure Ni and high-entropy alloy (HEA) structures, respectively. It is evident that HEAs exhibit a broader range of hydrogen solution energies compared to Ni, indicating significant inhomogeneity. In HEAs, the diffusion barriers for hydrogen vary greatly across different regions, suggesting a more rugged diffusion pathway and greater hindrance to hydrogen atom movement. However, the specific impact of the HEA’s chemical composition and local chemical environment on hydrogen diffusion behavior and the extent to which the hydrogen diffusion coefficient is reduced due to the inhomogeneous hydrogen solution energy distribution in HEAs remain unresolved.

4.6. Tantalum

The fact that hydrogen leads to the brittleness of tantalum has been known for a long time [117,118]. It was found that tantalum, despite its high corrosion resistance, can be hydrogenated in aggressive environments to such concentrations that it becomes brittle [119]. Doping with O₂, C, Fe, Si, Ni, Nb, Ti, and W does not protect tantalum from hydrogenation when operating in corrosive environments and does not prevent the development of HE. The most effective protection for tantalum is contact with platinum. A 10,000:1 ratio of the contact surfaces of tantalum to platinum held in concentrated hydrochloric acid at 463° K does not lead to HE even after 10,000 h. Tantalum is least prone to HE. The durability of hydrogenated tantalum is also significantly lower than that of non-hydrogenated tantalum.

In the work [120], the authors discuss the development of a protective layer for tantalum (Ta). Figure 8 illustrates HE as a function of corrosion time for both pristine Ta and samples subjected to the highest implantation fluence. An untreated Ta sample becomes completely embrittled in less than 10 days, breaking immediately when bent to 90°. In contrast, the Pt-surface-alloyed sample demonstrates remarkable resistance, showing minimal embrittlement in the first week. Even after six months, the embrittlement remains below 50%. No significant difference was observed between the metallic and oxidized samples, indicating that the oxide film does not significantly improve resistance under severe corrosion conditions.

4.7. Vanadium and Its Alloys

In [121], Brown’s data are given, according to which plastic vanadium becomes brittle after cathode saturation. HE appears at more than 0.01% (by mass) of H₂, while the solubility of hydrogen in vanadium is 0.039% (by mass). At low hydrogen concentrations and sufficiently low temperatures, the brittleness of vanadium belongs to the reversible hydrogen brittleness of the second kind.

Figure 9 illustrates the mechanism of void formation in pure vanadium single crystals, as simulated under molecular dynamics (MD) tensile loading. The samples were subjected to continuous tensile stress at 650 K, the typical operational temperature for metallic hydrogen separation membranes. The hydrostatic stress, von Mises stress, and stress triaxiality calculations are also shown. At first, the specimen deforms elastically. With increased loading, the deformation becomes plastic, reducing the von Mises stress and creating a peak on the stress curve, referred to as the “yield point”. After the yield point, hydrostatic stress rises until void nucleation starts. When void nucleation occurs, there is a sharp drop in hydrostatic stress, resulting in another peak on the stress curve [122].

In the design of metallic membrane materials, it is highly advisable to steer clear of the formation of second-phase particles to enhance resistance against embrittlement. During the void nucleation process mentioned earlier, the characteristics of these second-phase particles hold less significance because void nucleation does not occur due to the rupture of particles, but rather due to the triaxial stress state of the matrix near these particles. This means that the presence of second-phase particles leads to embrittlement, regardless of whether they are hydrides or not. This particular trait provides a sound explanation for an intriguing aspect of the experimental study conducted in [123]. The authors observed that membranes made of quenched V–10 at%Ni and V–15 at%Ni alloys lacking V–Ni intermetallic compounds exhibit relatively robust resistance against embrittlement. Conversely, the same alloys with V–Ni intermetallic compounds display severe embrittlement.

4.8. Niobium and Its Alloys

In a number of metals, for example, niobium, hydrogen dissolves in large quantities before the formation of hydrides or any other embrittling phases. The pressure of hydrogen in discontinuities in niobium is negligible; nevertheless, hydrogen transforms niobium into a brittle state. This brittleness can be explained by the fact that hydrogen, distorting the niobium lattice, transfers it from a plastic state to a brittle one. It is similar to cold brittleness, to which the oxygen, nitrogen, and phosphorus dissolved in metals lead [7].

The niobium–hydrogen system was studied in detail in [124]. At sufficiently high hydrogen contents (>120 ppm), hydride embrittlement should be expected [125]. Since hydrogen strongly distorts the niobium lattice, large internal stresses must arise, leading to brittleness at high strain rates. The solubility of hydrogen in niobium is very high; therefore, it can be expected that reversible HE will develop in it, which manifests itself at low strain rates.

The hydrogen brittleness of niobium alloys also manifests in tensile tests at a low strain rate. With this type of test, HE also appears in a certain temperature range, which depends on the composition of the alloys. An increase in the content of niobium in alloys leads to an increase in the plasticity of hydrogenated samples.

Furthermore, in [126], the authors studied the influence of Ru on the HE of Nb. Three samples were prepared: Nb–5Ru, Nb–10Ru, and Nb–15Ru with Ru content equal to 5%, 10%, and 15%, respectively. Figure 10 presents the pressure–composition–isotherm (PCT) curves for all three samples measured at 673 and 773 K. For comparison, the graph also includes data for pure niobium, as reported by Veleckis et al. [127]. It can be observed that as the ruthenium content in the alloy increases, the PCT curve shifts to the left and upward, indicating a decrease in the amount of hydrogen dissolved in niobium at a given pressure with more ruthenium. Additionally, the equilibrium hydrogen concentration decreases further with increasing temperature.

4.9. Copper and Its Alloys

Hydrogen’s most notable impact on copper includes gas porosity in castings and hydrogen-induced damage. Exposure to hydrogen leads to a significant reduction in the ductility of copper, necessitating caution regarding embrittlement during technological processes such as the widely practiced bright annealing of copper products [33]. In contrast, gold does not display these effects due to hydrogen’s minimal solubility [7].

In copper, hydrogen is responsible for ingot porosity [128]. Research indicates that HE in aluminum-copper alloys stems from the high pressure of molecular hydrogen within pores [129]. This embrittlement occurs as voids form and expand under stress, eventually merging into a crack. This crack propagates in a zigzag pattern, weakening the specimen until it fractures along a conical surface.

In the study [130], researchers introduced a high-strength metal alloy with exceptional resistance to HE. The alloy, precipitation-hardened copper–beryllium, has a tensile strength (TS) of 1400 MPa. Slow strain rate tensile (SSRT) tests were performed on both smooth and notched specimens in 115 MPa hydrogen gas at room temperature (RT). The results showed that the alloy maintained a relative reduction in area (RRA) of approximately 1 and a relative notch tensile strength (RNTS) of about 1, with no observed degradation in these properties.

Figure 11 illustrates the slow strain rate tensile (SSRT) properties of smooth specimens for CuBe-H and CuBe-HT, both in air and in 115 MPa hydrogen gas at room temperature (RT). In RT air, CuBe-HT exhibits TS and RA values of 1402 MPa and 20.5%, respectively, while CuBe-H displays values of 746 MPa and 70.6%. In 115 MPa hydrogen gas at RT, CuBe-HT demonstrates TS and RA values of 1364 MPa and 23.0%, while CuBe-H shows values of 710 MPa and 71.2%. The designations CuBe-HT and CuBe-H refer to precipitation-hardened and non-hardened alloys, respectively. For CuBe-HT, the Vickers hardness HV of the matrix is 406, whereas for CuBe-H it is 242.

4.10. Uranium

In [131], the authors indicate that the harmful effect of hydrogen on the properties of uranium is not limited to HE. In addition, it causes the porosity of castings. Hydrogen in uranium tends to diffuse from the bulk of the metal to the surface and destroys the protective coatings.

The study [132] encompasses both metallographic and mechanical assessments of uranium samples with hydrogen introduced thermally at concentrations ranging from 1.25 to 15 ppm. The crucial finding indicates that the formation of uranium hydride precipitates is the primary factor contributing to embrittlement in uranium–hydrogen alloys. The highest degree of embrittlement is observed in alloys with approximately 2.5 ppm of hydrogen. The research also delves into the impact of strain rate (ranging from 2.6 × 10⁻²/s to 1.3 × 10⁻⁵/s) and grain size (ranging from 0.0075 mm to 0.180 mm) on the fracture stress of thermally loaded hydrogen uranium alloys. Additionally, it was demonstrated that in uranium subjected to cathodic loading with hydrogen, embrittlement is contingent on the orientation of the uranhydride precipitates in relation to the stress axis. Figure 12 represents the electron micrograph precipitation, primarily at the grain boundaries on the sample surface.

4.11. Other Metals

Under real production conditions, the formation of hydrides in beryllium is excluded, and therefore hydride embrittlement is not observed in it. Therefore, the development of hydrogen disease in beryllium is also excluded. Thus, hydrogen is not a hazard to beryllium in a number of applications. Purging liquid beryllium with hydrogen does not change its properties and does not affect the transition temperature from ductile to brittle [133]. After exposure of beryllium samples at 1173° K for several hours in a hydrogen atmosphere, there was also no decrease in plasticity [134].

The interaction of magnesium with hydrogen shows that under real conditions of production and use of magnesium, the formation of hydrides in it is excluded, and therefore, hydride brittleness is not observed in it (with the exception of magnesium alloys with zirconium). The hydrogen issue is not observed in magnesium and its alloys, since magnesium oxides are not reduced by hydrogen even at their melting temperatures. Due to the high solubility of hydrogen in magnesium and its alloys, reversible HE of the second kind can develop in them [7].

Pure magnesium is highly prone to stress corrosion cracking (SCC), and even its alloys can undergo SCC in environments typically considered safe for most other engineering metals, such as distilled water. To mitigate SCC, some researchers propose that the applied stress should be maintained below 50% of the yield strength (YS), which diminishes the appeal of magnesium alloys for structural purposes [135]. The susceptibility of magnesium alloys to SCC rises with higher aluminum content, which is contrary to the beneficial effect of Al in stress-free corrosion rates [136].

Determining the solubility of hydrogen in magnesium is fraught with several challenges. Firstly, the formation of MgH₂ as a surface layer can impede the absorption kinetics of hydrogen primarily due to the extremely low diffusivity of hydrogen within MgH₂ (on the order of 10⁻¹⁶ m²/s at 25 °C). Secondly, magnesium exhibits high vapor pressure and volatility [137].

Studies have also revealed that silver can suffer from hydrogen disease. Annealing silver in a hydrogen atmosphere at temperatures above 773 K results in brittleness and, in some cases, surface bubbles [138,139]. Additionally, brittleness is observed when hydrogenated silver samples are heated in an air atmosphere.

5. Hydrogen Trap and Crack Formation

The effect of low, natural concentrations of hydrogen has been little studied. At the same time, knowing the mechanisms of accumulation and redistribution of hydrogen through traps of various natures, it is possible to predict or even control the processes of material destruction [140]. A trap for hydrogen localizes in a specific place, preventing it from moving and causing destruction.

Hydrogen contained in traps of different natures affects the mechanical properties of materials in different ways. In steels during production, the critical concentrations of diffusely mobile hydrogen in microdefects can be tenths of a millionth. At the same time, they determine both the ductility and corrosion resistance of the metal [141]. One of the most dangerous types of destruction is delayed destruction, since it is sudden and cannot be diagnosed in advance without special instruments. Delayed brittle fracture is the destruction of a solid body that occurs over time, in which there is practically no plastic deformation accompanied by the development of a crack with less energy than with ductile fracture [142].

A number of experimental and theoretical studies have shown that a local increase in the hydrogen concentration in the zone of intense deformation of the material, when the density of defects in the crystalline structure increases sharply, is provided by its dislocation transfer, and hydrogen accumulates in trap defects in the metal [143,144], which reduces its resistance to HE. In the case of fixing an external load, the process of accumulation of traps stops, and at a certain distance from the crack tip, an equilibrium between lattice and trapped hydrogen is established, controlled by the field of hydrostatic stresses [144]. However, it is known that the formation of new defects in the structure of the metal and the development and transformation of existing traps are also possible in the process of saturation with hydrogen [145].

The role of structural metal in the processes of hydrogen transfer increases, particularly at low climatic temperatures, when lattice diffusion is difficult [146]. Thus, at low temperatures, hydrogen is in significant amounts in reversible and irreversible traps, the number of which is related to the imperfection of the material, which strongly depends on the degree of plastic deformation. Lattice hydrogen in this case plays a secondary role. Inclusions and impurities in the material and the structure of its deformation at the micro- and mesolevels greatly increase the number of hydrogen traps. At the same time, hydrogen itself has a significant effect on the properties of the material and its ability to deform and generate defects. In this case, it is necessary to take into account the interdependence of hydrogen transfer and changes in the defectiveness of the material by solving a connected (“multiphysics”) problem [147].

In [148], the Birch–Murnaghan equation was employed to fit the equation of state for calculating the lattice parameter and bulk modulus of body-centered cubic (bcc) iron. According to the study, within the bcc Fe lattice, hydrogen can occupy three potential sites: (i) an octahedral interstitial, (ii) a tetrahedral interstitial, and (iii) a substitutional site (Figure 13).

At normal temperatures, hydrogen has a selective effect on the mechanical properties of a stressed metal in a structure. Mechanical properties vary greatly in the tension zones and practically remain stable in the compression zones of a structure; moreover, in stretched zones, the stronger the change in mechanical properties, the greater the amount of hydrogen penetrated into the corresponding volume of the structure. In addition, the more rigid the stress state scheme (the more complex the stress state at this point), the stronger its effect will be at the same hydrogen concentration. Hydrogen exposure leads to embrittlement of the material, which is not taken into account by the existing calculation standards. This can lead and, in some cases, has already led to accidents [10].

For a long time, it was believed that hydrogen was not involved in the formation of cracks, and brittleness was due to the precipitation of the martensite phase. Subsequent studies [149] using an electron microscope showed that the martensite phase precipitates due to increased hydrogen content. Thus, modern science believes that the strength of a weld depends entirely on the hydrogen content in the diffuse mobile phase. These cracks were the first example of the proven implicit influence of hydrogen when it plays the role of an initiator for other factors of destruction [32]. Hydrogen can penetrate into metals during the casting of ingots, metal forming, heat treatment, welding, pickling, and electrolytic coatings, as well as during the operation of finished products [7].

Crack growth occurs according to the following mechanism: first, hydrogen accumulates in the region of the crack tip; then, under the action of a static load at a critical hydrogen concentration, the crack grows to a distance equal to the region enriched in hydrogen. Then, the same process is repeated again, up to the moment when the dimensions of the crack reach the critical ones [21,81]. Hydrogen moves to the crack tip at low and high stresses by various mechanisms: in the first case, by ascending diffusion, and in the second, by transport by mobile dislocations [21,55]. There are many opinions about the method of transporting hydrogen to the area of volumetric tension: by means of molecular hydrogen pressure [150], lattice decohesion [81,151,152], and adsorption factors [153]. There are also many opinions about the mechanism of crack propagation, but all these opinions boil down to two general methods: the initiation of several cracks and their merging, and the breaking of interatomic bonds at the crack tip when the critical local stress is reached [71].

6. Impact of HE in Different Areas and Industries

6.1. HE in Construction

Many structures are subject to HE during operation. Most commonly, HE is observed in high-strength fasteners in construction, the automotive industry, and nuclear and thermonuclear energy materials. Hydrogen initiates various types of defects in building structures and elements of various engines, including the destruction of rolling bearings and the surface of friction pairs.

Under the influence of various physical fields (temperature, force, radiation, etc.), the effect of the embrittlement of structural materials is observed. In this case, dangerous brittle fractures with catastrophic consequences may occur, particularly in the structures of critical energy facilities [154]. In connection with these circumstances, the problem of embrittlement occupies a special position in the mechanics of materials; therefore, numerous works have been devoted to its study [155].

A feature of the operation of loaded structures subjected to low-temperature hydrogenation is that the change in the mechanical properties of the material in tensioned zones occurs more intensively than in compressed zones. An uneven change in properties causes a redistribution of the stress field, which in turn affects the distribution of the hydrogen field. This process of redistribution of stresses and the hydrogen field over the volume of the structure will be unsteady until the state of the structure either stabilizes or collapses. Under low-temperature hydrogenation, the kinetics of HE are controlled by the kinetics of hydrogen transport. If the hydrogen concentration exceeds the maximum allowable value, then HE develops [33].

Non-hardened fasteners of low-strength classes made of low- and medium-carbon steels are not affected by HE (screws, bolts, cotter pins, screws, dowels, washers, self-tapping screws, etc.). In the works [156,157,158,159], the effect of hydrogen on the defect structure of steels and zirconium was studied by the thermopower method. Hydrogen can exert its negative effect on the strength of the product at all stages of destruction, starting with the appearance of a crack, and its slow growth, and ending with an accelerated unstable growth of its size. Moreover, HE occurs in any hydrogen-containing medium [160]. As a result of the action of metallurgical and technological processes in the weld metal, the amount of diffusion-mobile hydrogen may be sufficient for the occurrence of a pre-fracture site. In this case, the gradient of hydrostatic stresses near stress concentrators and cracks causes the inhomogeneity of the chemical potential of hydrogen and causes its transfer to the zone of high-tensile hydrostatic stresses, which is controlled by the mechanism of the so-called “up-hill” diffusion. Thus, it is assumed that as a result of the diffusion process, the combined action of hydrogen, which embrittles the metal, and the stress field in the pre-fracture zone leads to the formation, growth, and abrupt propagation of a cold crack [146]. This or that nature of HE is associated with the nature of the phases formed during the interaction of metals with hydrogen, and with a change in the structure of these phases during subsequent heat treatment. For this reason, it is possible in a number of cases to predict in advance how HE will manifest itself in a particular metal and to outline ways to eliminate it.

In some industries, the use of standard alloys is being abandoned and the use of single-crystal materials is being adopted, which makes it possible to increase their structural strength. The absence of intergranular boundaries—favorite places of accumulation of hydrogen—excludes the possibility of intergranular destruction. However, other dangerous defects appear in single crystals—micropores and microcracks—that occur in the processes of crystallization, homogenization, etc. They have a strong effect on the mechanical properties of materials. In many cases, as for interstitial boundaries, pores and cracks are sites of hydrogen accumulation. Therefore, the transition to the use of new materials did not solve the problems associated with the destructive effect of low hydrogen concentrations [161].

The quality of the surface has a great influence on the kinetics of hydrogen uptake by a metal. For example, spacer grids are thermally oxidized, fuel element claddings are anodized, and guide channels and central tubes are not subjected to additional surface treatment. Hydrogen collects in any discontinuities or defects on the surface. The more the defects (including defects allowed by technical specifications: risks, prints, scratches, punctures, etc.), the more the hydrogen penetrates into the metal [162].

Togliatti scientists proved that destruction under the action of hydrogen and as a result of classical low-temperature embrittlement are fundamentally different from each other. In the future, experimental data can be used in the development of physical and mathematical models for calculating the durability of steel products operating under HE risk conditions, as well as in the development of steels resistant to HE [163].

6.2. HE in Plumbing

Main pipelines often operate in conditions associated with the risk of hydrogenation. As a rule, such objects are made of low-carbon and low-alloy steels, which, at low hydrogen concentrations, are practically not subject to HE [164]. Structures are made of metals that are embrittled by hydrogen from the very beginning as a result of technological production: casting; welding (arc); chemical, electrochemical, and thermal treatments; hot forming, etc.

In [165], the authors investigated the impact of hydrogenation on certain pipeline steels. It was observed that when specimens were exposed to hydrogen, there was a significant increase in the crack growth rate, reaching up to two orders of magnitude within the range of the explored physical parameters. The influence of hydrogen on the crack rate is clearly depicted in Figure 14, which illustrates the relationship between crack length and the number of loading cycles in F22 low alloy steel. The effect of hydrogen on the number of loading cycles to failure is striking. For instance, when a hydrogen-charged specimen was tested at room temperature and a frequency of 1 Hz, it endured only 5500 cycles before failure. In contrast, an uncharged specimen withstood an impressive 340,000 cycles before reaching failure. This indicates that the presence of hydrogen reduces the number of loading cycles to failure by a factor of approximately 60.

6.3. HE in the Industries of Oil and Gas

For the oil and gas industry, the influence of hydrogen is especially important. An analysis of statistical data over the past 30 years has shown that the most common accidents in oil tanks are brittle fractures (63.1%), followed by explosions and fires (12.4%) [166]. Brittle fracture is a characteristic feature of the hydrogen degradation of mechanical properties. The use of new high-strength steels capable of withstanding high specific loads can aggravate the situation in the industry [32].

One of the main reasons for the leakage of oil products into the environment is corrosion damage to the walls of pipelines. It is important to distinguish between general corrosion, leading to wall thinning; local corrosion, which forms ulcers and fistulas; and corrosion cracking (both hydrogen-induced and sulfide). One of the most dangerous types of corrosion damage is considered to be the hydrogenation of metals, leading to embrittlement and corrosion cracking of equipment [167].

In design, on a completely smooth surface, various kinds of defects (from one to several cracks) arise due to the hydrogen content and the imperfection of the crystal lattice of an alloy. The study of the effect of various types of surface treatment on the diffusion and absorption characteristics of hydrogen in metals and alloys is of paramount importance for many industries, such as hydrogen energy, the oil and gas industry, and other areas that deal with structures operating in the environment of hydrogen isotopes and any hydrogen-containing environment [168,169,170,171].

7. HE Test Methods and Mitigation

7.1. Testing Methods

Key testing techniques include the constant extension rate test (CERT), linearly increasing stress testing (LIST), temperature desorption spectroscopy (TDS), and the hydrogen microprint technique (HMT). CERT, also known as slow strain rate testing (SSRT) [172], involves applying a constantly increasing elongation over time until the specimen fails or fractures. A drawback of this method is that after reaching the threshold stress, the sample extends further, making the failure process time-consuming and cumbersome.

To address this issue, the LIST technique was introduced. Similar to SSRT, LIST gradually increases the applied stress until failure occurs. The load is applied via the movement of a weight, with its rate of motion controlled by a motor, offering advantages over SSRT.

TDS, also known as temperature-programmed desorption (TPD), is a widely accepted method for studying HE and hydrogen-induced failure [173,174]. The literature indicates that various techniques exist to measure diffusible hydrogen in steel and understand its role in failure. Hydrogen mobility is crucial in HE, and TDS effectively qualifies and quantifies diffusible hydrogen.

HMT is used to trace the paths of diffusible hydrogen in metals. By identifying these paths, the specific microstructure can be recognized, and the impact of hydrogen can be understood. HMT is simple and unique and provides high accuracy and resolution [175]. This technique is applied to study hydrogen distribution in stress fields, such as notched and deformed steel. HMT can be used on various materials, including low-carbon steel, high-strength steel, and austenitic stainless steel [176,177].

In a conventional strain rate test (CSRT), the hydrogen content in a sample reflects the cumulative hydrogen concentration from the combined loading test (CLT) and slow strain rate test (SSRT). This test is performed at a high strain rate, which results in minimal hydrogen diffusion. Thus, the hydrogen content at the crack site represents the average hydrogen content in a specimen. The main benefit of this method is its significantly short testing duration. However, it requires a highly uniform hydrogen charging process before testing. Chida et al. (2016) investigated susceptibility to HE in low-alloy steels using SSRT, CLT, and CSRT. Both SSRT and CLT showed a similar relationship between diffusible hydrogen content and nominal fracture stress, as they allow hydrogen diffusion during testing. CSRT showed a lower susceptibility to HE, even with the same local hydrogen concentration, likely due to limited interactions between hydrogen and dislocations [178].

For early detection and control of HE in metals and alloys, it is necessary to study the interaction of dislocations and hydrogen–vacancy complexes. Among the known control methods, the most sensitive and suitable for solving the problem are acoustic and electrophysical methods: thermopower and electrical resistance measurements [156].

The method of acoustic emission (AE) has found wide application in industry and scientific research due to significant progress in the field of electronic and computer technology, as well as fundamental research in the field of physics of materials [156,157,179]. Registration of signals and determination of the parameters and coordinates of AE signal sources make it possible at the early stages of structural changes to control the rate of accumulation of defects and to assess the degree of the danger of embrittlement and destruction of materials caused by the presence of hydrogen [158,159,180]. A promising method is to find a significant correlation between the speed of sound waves and the lifetime of positrons in metals depending on the hydrogen content [158,180,181,182,183].

The point at which hydrogen stress cracking initiates is determined by achieving a critical combination of applied and/or residual tensile stress and hydrogen concentration. When the stress level remains below this threshold for a fixed amount of hydrogen, no time-dependent cracking occurs. However, once this threshold is exceeded, subcritical cracking may arise, ultimately leading to delayed failure. Augmenting the quantity of hydrogen lowers the necessary critical stress, and vice versa [154].

Chemical testing primarily involves the quantification of hydrogen content in steel, a well-established technique. However, establishing a strong correlation between hydrogen content and susceptibility to HE poses challenges. One complication arises from the presence of hydrogen in two distinct states within steel: diffusible and trapped. Of these, diffusible hydrogen is considered the most relevant to HE [155].

Hydrogen Extraction: Hydrogen is extracted from steel through a heat treatment process conducted in an inert gas or vacuum environment. The diffusible hydrogen is released when the steel is heated to approximately 400 °C. If the steel is melted at a temperature of 2000 °C, trapped hydrogen is also liberated. The assessment of HE is performed by quantifying the amount of hydrogen in the material. This measurement can be conducted using various methods [156]:

• Isolating hydrogen from the inert gas.
• Employing mass spectrometry.
• Analyzing the conductivity of the emitted gas.
• Measuring the volume of hydrogen gas.
• Conducting gas chromatography.
• Utilizing a heated Palladium filter.

Electrochemical Method: To address the monitoring of hydrogen content in specific environments, researchers have increasingly focused on electrochemical hydrogen sensors. The underlying principle of an electrochemical hydrogen sensor involves the reaction of hydrogen with the sensing electrode material, leading to electron transfer [158]. Hydrogen undergoes oxidation at the anode, while oxygen is reduced at the cathode. The concentration of hydrogen is determined by detecting the resulting change in an electrical signal [158].

Zakroczymski [159] proposed an alternative method for evaluating hydrogen trap density using the electrochemical permeation technique (EPT). This method involves comparing the area under the experimental desorption permeation curve, which includes both diffusible and reversibly trapped hydrogen, with the area under the theoretical permeation curve calculated using the lattice diffusion coefficient (DL) representing only diffusible hydrogen. The difference between these areas allows for the determination of the density of reversible traps. In a subsequent study [160], the authors compared assessments of hydrogen trap density in DP AHSS using both the Dong method and the permeation curve method. The total trap density obtained via the permeation curve model was slightly lower than that obtained from the Dong method. This discrepancy is because the Dong method accounted for only one type of reversible trap when calculating hydrogen trap binding energy and used values for α-Fe to estimate interstitial site density. These approximations could lead to inaccuracies in trap density values. The reversible trap density derived from complete decays was about 2 × 10¹⁸ sites cm⁻³.

In a study [161], hydrogen trapping properties in CP steel were evaluated by fitting permeation curves to theoretical curves calculated using equations based on the McNabb and Foster diffusion-with-trapping model. The average trap density for CP1200 was found to be 4.36 × 10¹⁸ cm⁻³.

7.2. Mitigation of HE

To minimize HE, a multiple analysis of the hydrogen content is carried out: the content and concentration of hydrogen are recorded for the molten and the finished product. The maximum allowable values for each alloy are determined by the technical conditions with a description of the technology and the conditions for conducting the analysis (material selection, welding technique). There are models that provide practical guidance for developing materials that are resistant to HE. According to [162], alloying of steels with elements located to the left of iron in the periodic table (Cr, Mo, Mn, etc.) reduces the likelihood of HE in steel, while those on the right (Ni, Cu, Al, etc.) contribute to hydrogen degradation.

To enhance resistance against HE, researchers have explored the use of microstructural hydrogen traps [163]. These traps immobilize hydrogen in non-detrimental positions, preventing its involvement in crack propagation. For instance, vanadium carbide has demonstrated effectiveness in reducing HE susceptibility in ferritic steels. This is achieved by augmenting the density of hydrogen traps and decreasing hydrogen diffusivity. Similarly, the introduction of copper precipitates has shown the ability to trap hydrogen, consequently diminishing local hydrogen concentrations that could lead to HE [164]. Additionally, the incorporation of calcium into pipeline steels has proven effective in increasing HE resistance. This is attributed to calcium’s capacity to react with sulfur impurities, forming CaS and thereby reducing the presence of MnS inclusions, which are susceptible to HEDE [165].

In many ways, the strength depends on the composition, structure (metal grain size), size of the alloy and its susceptibility to HE. HE is a serious problem associated with the choice of metals and alloys operating under stress and environmental influences. Of fundamental and practical interest is the determination of the relationship between hydrogen and alloy components, distributed impurities, and the boundaries of the inner surfaces of grains of alloys [167]

Special hydrogenation greatly distorts the initial distribution of hydrogen in materials and makes it impossible to model the natural hydrogen saturation that occurs during the deformation and heat treatment of materials. Consequently, this approach does not provide information about the initial stages of hydrogen accumulation and material destruction, which is necessary for the development of methods to combat the phenomenon of HE [21].

8. Key Findings and Future Implications

8.1. Key Findings

• Hydrogen Accumulation Mechanisms: This review paper has presented multiple mechanisms through which hydrogen accumulates in metals. These mechanisms encompass metallurgical processes, product manufacturing, and external environmental factors. Understanding these mechanisms is crucial for effective mitigation strategies. Discussions on mechanisms such as hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE), along with the exploration of various models, contribute to a nuanced understanding of HE.
• Hydrogenation Process: By delving into the hydrogenation process and elucidating different types of HE, the paper provides a valuable context for researchers, engineers, and professionals dealing with high-strength materials in diverse applications. This comprehensive discussion aids in identifying specific challenges and tailoring approaches to address them effectively.
• Hydrogen Effects on Metals: The paper has comprehensively outlined the diverse effects of hydrogen on metals. It established that HE can significantly reduce the mechanical properties of metals, leading to an increased susceptibility to fractures and failures.
• Metal-Specific Responses: The review paper systematically examines the influence of hydrogen on a diverse range of materials and alloys, providing a comprehensive overview of its effects in a structured manner. The paper has explored how different metals and their alloys exhibit distinct responses to hydrogen exposure. This knowledge is invaluable for tailoring materials for specific applications and ensuring structural integrity.
• Modeling and Classification: The paper has presented various models and classifications of HE, providing a framework for understanding the complexities of this phenomenon. These models aid in predicting, preventing, and mitigating HE in different contexts.

8.2. Future Implications

• Advanced Detection Methods: Future research should focus on developing more sensitive and accurate methods for detecting and quantifying hydrogen within metals. This will enable early identification of embrittlement risks.
• Mitigation Strategies: Research should continue to explore innovative mitigation strategies to counteract HE. This includes the development of coatings, materials, and manufacturing processes that are less susceptible to HE.
• Metal-Specific Studies: Investigating the HE behavior of specific metals and alloys remains crucial. Future studies can delve deeper into the underlying mechanisms, enabling the development of tailored solutions.
• Hydrogen Storage and Transportation: Given the importance of hydrogen in emerging energy technologies, research should focus on materials and methods for safe hydrogen storage and transportation, minimizing the risk of embrittlement in critical infrastructure.
• Environmental Factors: As environmental factors can contribute to HE, research should address the impact of environmental conditions on HE, allowing for better risk assessment and management.
• Multiscale Modeling: Advancements in multiscale modeling techniques can provide a more accurate understanding of HE. Future research can employ these models to predict and prevent embrittlement in complex systems.
• Standardization: Establishing standardized testing protocols and guidelines for evaluating HE susceptibility can aid industries in ensuring the safety and reliability of metal components.
• Interdisciplinary Collaboration: Promoting cooperation among engineers, materials scientists, and environmental specialists is crucial for tackling the complex issues presented by (HE).

Incorporating these future implications into research and industry practices will contribute to the development of materials and technologies that are more resilient to HE, ultimately enhancing the safety and performance of critical infrastructure and applications.

9. Conclusions

A qualitative decline in material mechanical properties directly impacts the performance of structural components, with significant implications for critical industries, including oil and gas, chemical processing, nuclear and hydrogen fuel cell technologies, as well as infrastructure for hydrogen storage and transportation, and even architectural structures. Enhancing material resilience to HE is imperative, especially considering the cost constraints associated with introducing entirely new alloys into industrial processes.

Traditional industry materials, while cost-effective, are vulnerable to hydrogen-induced degradation. Consequently, the development of hydrogen-resistant materials represents a complex and time-intensive endeavor. However, the pursuit of such materials holds the promise of averting catastrophic accidents and industrial disasters in the future.

The significance of addressing this challenge is underscored by the keen interest of vital sectors, including chemical, metallurgical, oil and gas, aviation, machinery, shipbuilding, and many others. Invariably, resolving this issue will not only ensure operational safety but also fortify the foundations of these pivotal industries, safeguarding their longevity and reliability.