Difference In Hydrogen Concentration Research Articles

Adapted Galvanostatic Charge Method for cell-individual in-situ characterization of PEM fuel cell stacks and systems

Increasingly challenging lifetime requirements for automotive Polymer-Electrolyte-Membrane (PEM) fuel cells call for onboard characterization methods to enable state-of-health (SOH) adaptive operation strategies. In this work, an adapted Galvanostatic Charge Method (GCM) is presented, that allows for cell-individual electrochemical characterization without nitrogen flush, making it system- and potentially onboard-compatible. Key innovation is to utilize the system’s standard shutdown procedure, i.e. oxygen depletion with closed air valves, to create an inert atmosphere in the cathode volume in which the characterization takes place. The electrochemical processes triggered during the characterization with GCM are now embedded in a closed-volume-problem instead of a steady-flow-environment. This causes the hydrogen concentration difference between anode and cathode to be dynamic instead of constant during the characterization. In this work, a 0D-simulation model of the closed-volume-problem and complementary measurements on a system testbench are presented. Additionally, a novel model-based evaluation method is introduced which extracts the SOH-indicators electrochemical active surface area (ECSA), double layer capacity, hydrogen permeability of the membrane and short circuit resistance from the measurements by fitting simulated to measured voltage curves. The simulation model is thoroughly analyzed and a systematic error, evoked by the fluidic coupling of the individual cells, is found to influence the results obtained for membrane permeability. The influence of different initial conditions (among them initial hydrogen concentration) on the evaluation results is experimentally investigated and found to have no significant influence. For further studies, improvement potential was found in modeling, measurement procedure and evaluation method.

(Invited) Regenerative Fuel Cell System Utilizing a High Differential Pressure Water Electrolysis Technology

The expansion of manned activities in space requires energy system technology that utilizes sunlight, and hydrogen fuel cells are one such means. By electrolyzing water with sunlight, energy is stored with oxygen and hydrogen, and electricity is generated by fuel cells. The generated water can be recycled, and energy can be stored and supplied without external replenishment. This is commonly called a regenerative fuel cell. In the space domain, such energy systems must be compact and lightweight, and gas must be stored under high pressure. However, mechanical compressors that are generally used for compressing are heavy, and there are also issues of vibration and noise.Since the 1990s, Honda has worked on the development of high-differential-pressure water electrolysis technology that generate hydrogen from renewable energy and compress without a mechanical compressor, in parallel with the development of fuel cell technology. High-differential-pressure water electrolysis is a technology that generates high pressure hydrogen by moving the protons generated by electrolysis to the cathode through the Proton exchange membrane (PEM) . The energy required is only the potential generated by the difference in hydrogen concentration, which is superior to small mechanical compressors whose efficiency is reduced by friction loss. However, a large differential pressure is generated across the PEM because only one side is at a high pressure. Honda has achieved a maximum pressure increase of 70 MPa by using its own structure. Moreover, this principle can be applied not only to the generation and pressurization of hydrogen, but also to oxygen. Honda is working on the development of RFC technology that integrates these hydrogen technologies with the aim of applying them not only to sustainability on Earth, but also to the space domain. Currently, we are undertaking a JAXA project and are promoting the production of a breadboard model.The feature of Honda's RFC technology is that it applies high-differential-pressure water electrolysis technology, not only to generate oxygen and hydrogen, but also to pressurize each gas. By not using a mechanical compressor, it is possible to downsize the system, and it is also no vibration or noise. Utilizing the characteristics of this technology, we are planning to build an energy system that can store energy at high density and apply it to various manned missions.Honda's RFC consists of the high-differential-pressure water electrolysis system that simultaneously electrolyzes water and pressurizes oxygen, a Electrochemical hydrogen compressor system (EHC system) that electrochemically pressurizes the hydrogen and a fuel cell system. In the electrolysis stack, oxygen is generated and compressed to the target pressure by the differential pressure water electrolysis. The generated hydrogen is sent to the EHC stack that also applies the technology of high-differential pressure water electrolysis, where the hydrogen is pressurized. As a result, both oxygen and hydrogen gases were generated and compressed without a mechanical compressor. For the fuel cell system, the compact and highly efficient fuel cell technology that has been developed for FCVs is applied. When the fuel cell is used on the ground, oxygen is supplied from the atmosphere, but in space RFC it is necessary to generate electricity using pure oxygen and to circulate the system. A dedicated circulating FC system was constructed to achieve stable power generation. The water produced by power generation is used in the next water electrolysis cycle, and the proof of principle as RFC has been completed. In addition to these basic functions, for space application, we must address issues not found on Earth, such as heat management in a vacuum, radiation, and vibrations during launch. Starting with the lunar exploration mission scheduled for 2030, Honda will develop this technology for the spread of manned activities. Figure 1

Modeling the Hydrogen Redistribution at the Grain Boundary of Misoriented Bicrystals in Austenite Stainless Steel.

Hydrogen embrittlement, as one of the major concerns for austenitic stainless steel, is closely linked to the diffusion of hydrogen through the grain boundary of materials. The phenomenon is still not well understood yet, especially the full interaction between hydrogen diffusion and the misorientation of the grains. This work aimed at the development of a robust numerical strategy to model the full coupling of the hydrogen diffusion and the anisotropic behavior of crystals in 316 stainless steel. A constitutive model, which allows easy incorporation of crystal orientation, various loading conditions, and arbitrary model geometries, was established by using the finite element package ABAQUS. The study focuses on three different bicrystal models composed of misoriented crystals, and the results indicate that the redistribution of hydrogen is significant closely to the grain boundary, and the redistribution is driven by the hydrostatic pressure caused by the misorientation of two neighboring grains. A higher elastic modulus ratio along the tensile direction will lead to a higher hydrogen concentration difference in the two grains equidistant from the grain boundary. The hydrogen concentration shows a high value in the crystal along the direction with stiff elastic modulus. Moreover, there exists a large hydrogen concentration gradient in a narrow region very close to the grain boundary to balance the concentration difference of the neighboring grains.

Effect of phosphoric acid-doped polybenzimidazole membranes on the performance of H+-ion concentration cell

H+-ion concentration cell, which can harvest thermal energy to generate electricity by hydrogen concentration difference principle with a fuel cell structure, is an innovative thermoelectric conversion device. In this system, phosphoric acid-doped polybenzimidazole (PA-doped PBI) membrane is a key component influencing the power generation performance of the cell. Herein, 30, 45, 60, 75, and 90 μm thick PBI membranes are successfully synthesized and doped with phosphoric acid. To achieve a good compromise between the proton conductivity and durability, the properties of PA-doped PBI membranes are experimentally evaluated to clarify the effect of the acid doping time and membrane thickness on cell performance. The results indicate that the higher the acid doping level, the worse the dimensional stability of the membrane. Also the thinner the PBI membrane, the smaller the membrane resistance to ions motion, while the poorer the stability. Upon reaction at 170 °C, this cell can boast a power density from 3.0 to 8.0 W m−2, which results in a thermoelectric conversion efficiency of 5.97–14.32%. This study potentially boosts the practical application of thermal-to-electrical conversion technology.

In-situ hydrogen production and storage in (0 0 2) oriented TiO2 thin films

Hydrogen production and in-situ storage from water splitting is an attractive technology for clean energy. Here (0 0 2) oriented rutile TiO2 thin films are prepared by the hydrothermal method for photocatalytic water splitting. The H2O molecule efficiently dissociates on the high energy rutile (0 0 2) surface and the highest hydrogen production rate of 28.4 mmol h−1 g−1 is achieved in the film with the lowest density, at a rate comparable to powder TiO2 photocatalysts. The produced hydrogen is stored in-situ in the films with the help of the atomic hydrogen concentration difference between the surface and in the crystal lattice. Theoretical calculations show that water is dissociated on the rutile (0 0 2) surface with a negligible energy barrier. Hydrogen atoms diffused into the sublayers are confined in the TiO2 lattice owing to a high energy barrier for them to overcome to escape from the rutile (1 1 0) surface. The theoretical storage capacity is up to 1.2%, at room temperature and atmospheric pressure, demonstrating the potential for practical room-temperature hydrogen production and storage.

Characterization of hydration in the mantle lithosphere: Peridotite xenoliths from the Ontong Java Plateau as an example

We report concentrations of hydrogen (H) in upper mantle minerals of peridotites (olivine and pyroxenes) transported by alnöitic lavas, which erupted on the southwestern border of the Ontong Java Plateau (Malaita, Solomon Islands, West Pacific). Unpolarized FTIR analyses show that olivine, orthopyroxene, and diopside contain 2–32 ppm, 162–362 ppm and 159–459 ppm wt H2O, respectively. In the studied lherzolites, garnets are anhydrous. The concentration of hydrogen within individual olivine and pyroxene grains is almost homogeneous, indicating no evidence of dehydration or hydration by ionic diffusion. In the lherzolite, the concentration of hydrogen in olivine tends to increase weakly with depth (based on geothermobarometry), consistent with the increase of water solubility with increasing water fugacity as a function of pressure, but concentrations remain well below water-saturation values determined experimentally. The highest concentration of H in olivine (32 ppm wt H2O) is, however, found in refractory spinel harzburgites, which equilibrated at depths of 85 km., while deeper specimens as the high-temperature spinel harzburgites, and some of the garnet lherzolites, contain less hydrogen in olivine. Olivines from pyroxene- or pargasite-rich peridotites have also lower hydrogen concentrations. We interpret the high hydrogen concentrations in olivine from the refractory spinel harzburgites as due to (1) simultaneous hydration and metasomatism of the lithospheric mantle by a water-rich silicate melt/fluid, during which hydrogen follows MREE and where spinel harzburgite have experienced ‘stealth’ metasomatism, and/or (2) to a late ‘fleeting’ hydrogen metasomatism, which would hydrate the rock after this first ‘stealth’ metasomatism event. In the second case, the composition of the ‘fleeting’ percolating fluid (small volume fraction of very evolved fluids, with high volatiles concentration and transient properties) is likely to be linked to the decrease of the plume activity and resulting in the downwelling of the lithosphere asthenosphere boundary. The difference in hydrogen concentration between harzburgites and lherzolites could be linked to variation in trace elements in olivine induced by the first ‘stealth’ metasomatism alone or associated to the late ‘fleeting’ hydrogen metasomatism, triggered by the lack of secondary crystallization of clinopyroxenes in the harzburgite. To conclude, H concentrations in upper mantle minerals may thus be correlated to metasomatic events, with the later yielding variation of the mineral assemblage and trace element composition that constitute the peridotites.

Study of hydride blisters in Zr-alloy using neutron tomography

Formation of hydride blisters in Zircaloy pressure tubes of pressurized heavy water reactor (PHWR) is a major life limiting factor which hinders the safe and uninterrupted operation of the reactor. Nondestructive detection and evaluation of location and size of these blisters as well as hydride distribution in the matrix surrounding them may help in damage quantification and residual life extension. In this article we present the neutron tomography studies carried out on simulated hydride blister samples grown on Zircaloy tubes. Characterization on samples having various levels of hydrogen concentrations were also carried out for quantification of the detectability of our neutron tomography system. We could identify the spatial in-homogeneity of hydride concentration present in the samples. Quantitatively hydrogen concentration difference up to 25 wppm has been observed experimentally and calibrated against image intensity in the reconstructed image. This study establishes neutron tomography as a potential non-destructive evaluation tool for the estimation of the severity of damage in the integrity of the pressure tubes and provides valuable information about kinetics of blister formation.

Cyclic deformation behavior and low cycle fatigue life of normalized medium carbon steel with hydrogen charging

The effects of hydrogen on low cycle fatigue properties have been fundamentally examined using normalized JIS S45C steel. With the aid of hydrogen thermal desorption analysis, the influence of the increase in carbon content has been discussed. The S45C steel easily pick-up hydrogen. But it is relatively insensitive to hydrogen absorption. This increased amount of hydrogen pick-up is firstly caused by large amount of pearlite structure. Further sulfur takes an important role for hydrogen pick up during cathodic charging. Hydrogen plays two kinds of roles during fatigue tests. One is hydrogen in dislocation core and makes plastic deformation easier in the first monotonic tension stage. The other is hydrogen/vacancy pair, and suppresses dislocation motion and increases stress amplitudes. The latter should be the fundamental issue for fatigue life. The amount of absorbed hydrogen itself is not only important, but the difference of hydrogen concentration between surface and inside is more important. When the fatigue crack initiates inside, the fatigue life is severely damaged to less than 10% of non-charged one. Taking the lower limit value of hydrogen to form internal fish-eye type cracking, S45C is less sensitive to hydrogen in cyclic straining. This comes from the pearlite structure that resists crack propagation.

Influence of hydrogen and frequency on fatigue crack growth behavior of Cr-Mo steel

Fatigue tests on Cr-Mo steel quenched at 860°C and tempered at 580°C were carried out under the frequencies of 0.2, 2, and 20 Hz with specimens containing a small artificial hole. An additional test in which the test frequency was alternately switched between 0.02 and 2 Hz was carried out. Hydrogen charging to the specimens was carried out by an immersion method. The fatigue life of the hydrogen-charged specimens remarkably decreased in comparison with that of the uncharged specimens. The fatigue crack growth rate da/dN increased with decreasing the test frequency f. The acceleration of da/dN saturated at \({\Delta K\,<\,17\,{\rm MPa}\sqrt{m}}\) for f ≤ 2 Hz. The presence of the upper bound for the fatigue crack growth acceleration was found with respect to the effects of hydrogen and test frequency in a hydrogen environment. The test switching the frequency between 0.02 and 2 Hz resulted the difference in fatigue crack growth morphology which is presumed to be caused by the difference in hydrogen concentration in the vicinity of crack tip. The particular crack morphology under the low test frequency with hydrogen was the localization of the slip around the crack tip and the linearization of the crack growth path. The hydrogen-enhanced striation formation model which was proposed to explain the effect of hydrogen on the fatigue crack growth for an austenitic stainless steel and low carbon steel can be applied also to the quenched and tempered Cr-Mo steel in this study.

Modelling and numerical simulation of permeated hydrogen dispersion in a garage with adiabatic walls and still air

The dispersion of permeated hydrogen from a storage tank in a typical garage with adiabatic walls and still air is studied analytically and numerically. Numerical simulations are performed based on an original approach of a hydrogen mass source term introduction in the hydrogen conservation equation in control volumes around the tank surface. The maximum hydrogen concentration in an enclosure is always on the top surface of the tank and never reaches 100% by volume. Both the analytical analysis and numerical simulations have demonstrated that diffusion and buoyancy contributions to the hydrogen transport from the tank surface are balanced within 1 min from the start of the process. The quasi-steady state conditions within the enclosure with approximately linear distribution of hydrogen from the top to the bottom are established in about 1 h for both considered permeation rates: 1 NmL/hr/L of tank capacity and 45 NmL/hr/L the last being an equivalent to the SAE J2578 requirements. Finally, the numerical simulations demonstrated that the difference in hydrogen concentration between the garage ceiling and floor is negligible compared to the lower flammability limit of 4% by volume of hydrogen.

Alloying Effects on the Hydrogen Diffusivity during Hydrogen Permeation through Nb-Based Hydrogen Permeable Membranes

The hydrogen solubility and the hydrogen permeability have been measured for Nb-based alloys in order to investigate the alloying effects on the hydrogen diffusivity during hydrogen permeation. The hydrogen diffusion coefficient during hydrogen permeation is estimated from a linear relationship between the normalized hydrogen flux, , and the difference of hydrogen concentration, C, between the inlet and the outlet sides of the membrane. It is found that the hydrogen diffusion coefficient during the hydrogen permeation is increased by alloying ruthenium or tungsten into niobium. On the other hand, the activation energy for hydrogen diffusion in pure niobium under the practical permeation condition is much higher than the reported values measured for dilute hydrogen solid solutions. It is interesting that the activation energy for hydrogen diffusion decreases by the addition of ruthenium or tungsten into niobium.

Alloying effects of Ru and W on hydrogen diffusivity during hydrogen permeation through Nb-based hydrogen permeable membranes

The hydrogen permeability have been measured for pure niobium and Nb-5 mol%X ( X = Ru and W) alloys in order to investigate the alloying effects of ruthenium and tungsten on the hydrogen diffusivity during hydrogen permeation. The hydrogen diffusion coefficient during hydrogen permeation is estimated from a linear relationship between the normalized hydrogen flux, J· d, and the difference of hydrogen concentration, Δ C, between the inlet and the outlet sides of the membrane. It is found that the addition of ruthenium or tungsten into niobium increases the hydrogen diffusion coefficient during the hydrogen permeation. On the other hand, the activation energy for hydrogen diffusion in pure niobium under the practical permeation condition is much higher than the reported values measured for dilute hydrogen solid solutions. It is interesting that the activation energy for hydrogen diffusion is decreased by alloying of ruthenium or tungsten into niobium.

Alloying effects of Ru and W on the resistance to hydrogen embrittlement and hydrogen permeability of niobium

The alloying effects of ruthenium and tungsten on the hydrogen solubility, the resistance to hydrogen embrittlement and the hydrogen permeability are investigated for Nb–M (M = Ru and W) alloys. The hydrogen solubility decreases by alloying and also by increasing the temperature of the alloy. As a result, the resistance to hydrogen embrittlement is improved owing to the low hydrogen content in the alloy. On the other hand, the hydrogen flux increases with increasing hydrogen concentration difference, Δ C, between the inlet and outlet sides of the alloy membrane. It is found that Nb–5 mol%X (X = Ru and W) alloys possess excellent hydrogen permeability without showing any hydrogen embrittlement when used under the appropriate permeation conditions of the temperatures and hydrogen pressures.

Initiation and Arrest of Delayed Hydride Cracking in Zr–2.5Nb Tubes

Using Kim’s delayed hydride cracking (DHC) model, this study reanalyzes the critical temperatures for DHC initiation and arrest in Zr–2.5Nb tubes that had previously been investigated with the previous DHC models. At the test temperatures above 180°C, DHC initiation temperatures fell near the terminal solid solubility for precipitation temperatures, requiring some undercooling or ΔT from the terminal solid solubility for dissolution (TSSD) temperatures, and increased toward TSSD with the number of thermal cycles. At the test temperatures below 180°C, DHC initiation occurred at temperatures near TSSD with little ΔT. DHC arrest occurred on heating toward TSSD where the hydrogen concentration difference between the bulk region and a crack tip ΔC decreased to a minimum ΔCmin, under which nucleation of the hydrides was restrained. ΔCmin after the first thermal cycle increased with increasing temperature, demonstrating that nucleation of the hydrides becomes more difficult with increasing temperatures. Different DHC initiation and arrest temperatures with the test temperatures or hydrogen concentrations are discussed in view of a supersaturation of hydrogen (ΔC) for nucleation of hydrides in the zirconium matrix.

Analysis of hydrogen diffusion coefficient during hydrogen permeation through niobium and its alloys

The hydrogen solubility and the hydrogen permeability of Nb-based hydrogen permeable membranes are investigated in order to examine the hydrogen diffusion coefficient during the hydrogen permeation at high temperatures. It is shown that the hydrogen dissolution reaction into niobium and its alloys does not follow the Sieverts’ law at the pressure conditions of the practical hydrogen permeation, in contrast to the case of Pd-based alloys. The hydrogen diffusion coefficients during the hydrogen permeation are evaluated from a linear relationship between the normalized hydrogen flux, J· d, and the hydrogen concentration difference, Δ C between the inlet and outlet sides of the membrane with the thickness of d. It is found that the hydrogen diffusion coefficient for pure niobium is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd–Ag alloy with face-centered cubic (fcc) crystal structure than in pure niobium with body-centered cubic (bcc) crystal structure at 773 K during the hydrogen permeation. It is also found that the addition of Ru or W into niobium increases the hydrogen diffusion coefficient under the practical conditions.

Analysis of hydrogen diffusion coefficient during hydrogen permeation through pure niobium

The hydrogen solubility and the hydrogen permeability of pure niobium at high temperature are investigated in order to analyze the hydrogen diffusion coefficient during the hydrogen permeation. It is shown that the hydrogen dissolution reaction into niobium metal does not follow the Sieverts' law at the practical hydrogen permeation pressures. The hydrogen diffusion coefficient during the hydrogen permeation through pure niobium at high temperature is evaluated from the linear relationship between the normalized hydrogen flux, J· d, and the hydrogen concentration difference, Δ C. It is found that the hydrogen diffusion coefficient under the practical condition is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd–Ag alloy with fcc crystal structure than in pure niobium with bcc crystal structure at 773 K during the hydrogen permeation.

Hydrogen-inclusion-induced variation of critical current in Nb–AlOx–Nb Josephson junctions

The critical current density (Jc) of Nb–AlOx–Nb Josephson-junction (JJ) arrays was found to depend on their wiring structure. The Jc values of all JJs wired with a niobium electrode covered with a palladium layer increased by about 20%, while the Jc values of those with electrodes without palladium coverage stayed unchanged (except for that of the two junctions directly connected to the pads of an electrical probe covered with palladium.) To explain this Jc increase, we propose a “hydrogen mechanism,” that is, the hydrogen inclusion into niobium electrodes occurs during fabrication, and its desorption occurs after fabrication. Hydrogen atoms incorporated in the electrodes are thought to influence the mechanical and the electronical properties of niobium, resulting in the deviation of critical current density. Hydrogen desorption analysis and measurements on niobium-film properties verified the occurrence of hydrogen incorporation into the niobium films during the fabrication process for superconducting JJ circuits. The incorporation and desorption processes were confirmed to proceed, even in air, if the niobium film is covered with palladium. As hydrogen diffuses quickly in niobium but stops in aluminum or aluminum oxide, differences in hydrogen concentration can happen within a circuit consisting of electrically connected multiple junctions. This hydrogen concentration difference can explain the observation that two junctions with increased Jc exist in the serial junction array without palladium coverage.

Delayed hydride cracking of spent fuel rods in dry storage

Failures of zirconium alloy cladding tubes during a long-term storage at room temperature were first reported by Simpson and Ells in 1974, which remains unresolved by the old delayed hydride cracking (DHC) models. Using our new DHC model, we examined failures of cladding tubes after their storage at room temperature. Stress-induced hydride phase transformation from γ to δ at a crack tip creates a difference in hydrogen concentration between the bulk region and the crack tip due to a higher hydrogen solubility of the γ-hydride, which is a driving force for DHC at low temperatures. Accounting for our new DHC model and the failures of zirconium alloy cladding tubes during long-term storage at room temperature, we suggest that the spent fuel rods to be stored either in an isothermal condition or in a slow cooling condition would fail by DHC during their dry storage upon cooling to below 180 °C. Further works are recommended to establish DHC failure criterion for the spent fuel rods that are being stored in dry storage.

Alloy Design of Nb-Based Hydrogen Permeable Membrane with Strong Resistance to Hydrogen Embrittlement

The alloying effects of Ru and W on the hydrogen solubility, the resistance to hydrogen embrittlement and hydrogen permeability are investigated quantitatively for Nb-based hydrogen permeable alloys. It is found that the hydrogen solubility decreases by the addition of alloying element into niobium or by increasing the temperature. As a result, the resistance to hydrogen embrittlement is improved by reducing the hydrogen concentration. On the other hand, the hydrogen flux, J, through the alloy membrane increases linearly with increasing difference of hydrogen concentration, ΔC, between both sides of the membrane. It is shown that the Nb-5 mol%X (X = Ru and W) alloys possess excellent hydrogen permeability without showing any hydrogen embrittlement when used under appropriate permeation conditions, i.e. temperature and hydrogen pressures. Also, the hydrogen diffusion coefficients during the practical hydrogen permeation at high temperature are evaluated from the linear relationship between the hydrogen flux and the hydrogen concentration difference. It is found that the hydrogen diffusion coefficient of pure Nb is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd-26 mol%Ag alloy with fcc crystal structure than in pure niobium with bcc structure at 773 K during the hydrogen permeation. It is also interesting that the addition of Ru or W into niobium enhances the hydrogen diffusion of the practical hydrogen permeation at high temperature.

Precipitation of crack tip hydrides in zirconium alloys

Zr–2.5Nb cantilever beam (CB) specimens with 60 ppm H were subjected to a thermal cycle where the test temperature of 250 °C was approached by a cooling from a peak temperature of 380 °C. A constant stress intensity factor of 18.4 MPa √m was applied to the CB with a notch of 0.5 mm in depth at three different times during the thermal cycle. It is found that large plastic deformation given during a holding at 380 °C for 50 h promotes a homogeneous precipitation of reoriented hydrides, leading to no delayed hydride cracking (DHC) while little plastic deformation prior to 250 °C causes their inhomogeneous precipitation ahead of the notch tip, thus initiating DHC. It is concluded that the dislocations in the plastic zone enhance precipitation of the hydrides and their inhomogeneous precipitation causes a hydrogen concentration difference or Δ C between the bulk region and the crack tip, and hence DHC. Theoretical rationale of this conclusion is provided by the thermodynamic analyses where the driving force for a preferential precipitation of the hydrides at the crack tip is a decrease of the chemical potential or free energy of hydrogen by an applied stress.