Hydrogen Concentration Gradient Research Articles

Electromagnetic–thermal–mass transport triple hybrid model for energy-efficient hydrogen release from metal particles by induction heating

Hydrogen storage in metals is anticipated to be a highly promising technical approach for managing hydrogen in diverse energy applications, emphasizing the need to establish efficient operating conditions. In this study, we developed a numerical model that can simultaneously calculate hydrogen and heat transport combined with electromagnetic induction heating of metal particles. Using the developed numerical simulator, we investigated the behavior of the hydrogen release process from the hydrogen-absorbing metal and proposed important indicators for the operation. The fractions of the electromagnetic energy absorbed by the metal particle used for the hydrogen release, sensible heat, interfacial heat conduction, and thermal radiation were systematically evaluated. Considering the trade-off between temperature rise and the hydrogen concentration gradient, an optimal diameter of metal particles existed for the maximum hydrogen release rate. Similarly, the trade-off between temperature rise and energy consumption revealed an optimal electromagnetic power density for the heat utilization fraction for hydrogen release. Our electromagnetic–thermal–mass transport triple hybrid model constitutes a useful numerical simulator for the design of energy-efficient hydrogen storage systems.

Effect of an edge dislocation on hydride growth in zirconium: elastic complex potential study

The formation of hydrides can significantly decrease the mechanical reliability of Zr servicing in the nuclear reactor. This study investigates the effect of the local stress field resulting from the hydride eigenstrain and dislocations on hydrogen transport and explores the hydride growth regulation. The stress functions, which can be extended and further applied to the realistic crystal structure and servicing status, are derived using the dislocation-based strain nucleus model and elastic complex potential method. The nonhomogeneous equation of hydrogen transport governed by diffusion and hydrostatic stress gradient is analytically solved. The results show that: (1) In the absence of dislocations, the hydride grows toward 112¯0 direction under the hydrostatic stress gradient resulting from the hydride eigenstrain. (2) The lower the temperature is, the higher the hydrogen concentration gradient in the adjacent area of the hydride is. The higher hydrogen fraction can also accelerate the growth rate. (3) Dislocation defects are capable of inducing triangle-shaped and asymmetric growth tendencies and providing new nucleation loci for hydrides. Based on the significant hydrostatic stress gradient led by dislocation singularity, this study analytically explains the autocatalysis and new-nucleation-locus effects raised by the in-situ observations.

An Experimental Study on the Large-Volume Liquid Hydrogen Release in an Open Space

Liquid hydrogen is one of the high-quality energy carriers, but a large leak of liquid hydrogen can pose significant safety risks. Understanding its diffusion law after accidental leakage is an important issue for the safe utilization of hydrogen energy. In this paper, a series of open-space large-volume liquid hydrogen release experiments are performed to observe the evolution of visible clouds during the release, and an array of hydrogen concentration sensors is set up to monitor the fluctuation in hydrogen concentration at different locations. Based on the experimental conditions, the diffusion of hydrogen clouds in the atmosphere under different release hole diameters and different ground materials is compared. The results show that with the release of liquid hydrogen, the white visible cloud formed by air condensation or solidification is generated rapidly and spread widely, and the visible cloud is most obvious near the ground. With the termination of liquid hydrogen release, solid air is deposited on the ground, and the visible clouds gradually shrink from the far field to the release source. Hydrogen concentration fluctuations in the far field in the case of the cobblestone ground are more dependent on spontaneous diffusion by the hydrogen concentration gradient. In addition, compared with the concrete ground, the cobblestone ground has greater resistance to liquid hydrogen extension; the diffusion of hydrogen clouds to the far field lags. The rapid increase stage of hydrogen concentration at N8 in Test 7 lags about 3 s behind N12 in Test 6, N3 lags about 7.5 s behind N1, and N16 lags about 8.25 s behind N14. The near-source space is prone to high-concentration hydrogen clouds. The duration of the high-concentration hydrogen cloud at N12 is about 15 s, which is twice as long as the duration at N8, increasing the safety risk of the near-source space.

Changes in hydrogen states after interactions between hydrogen and stress-induced martensite transformation for Ni–Ti superelastic alloy

ABSTRACTChanges in hydrogen states after dynamic interactions between hydrogen and a stress-induced martensite transformation have been investigated for Ni–Ti superelastic alloy. After homogenised hydrogen concentration by aging, the interactions change the hydrogen thermal desorption behaviour. Hydrogen in a solid solution probably changes to hydrogen trapped in damage regions induced by the interactions, as reported previously. With time after the interactions, the amount of hydrogen desorbed at low temperatures (200–400°C) markedly increases, clearly indicating that hydrogen states further change from the states immediately after the interactions. In cyclic tensile deformation in the stress plateau region caused by stress-induced martensite and reverse transformations, the number of cycles to fracture substantially increases with time after the interactions. The present results indicate that hydrogen states further change to a nominally interacting state after the interactions despite the absence of dynamic structural changes and a hydrogen concentration gradient, thereby suppressing hydrogen embrittlement related to the martensite transformation of the alloy.

Numerical Study of the Hydride Embrittlement in Zirconium Alloy using XFEM

A numerical model for hydride embrittlement in Zirconium alloy (Zr–2.5Nb) is developed utilizing the extended finite element method (XFEM). Hydride embrittlement reduces the ductility and failure time of a metal/alloy. During hydride embrittlement, stress-directed hydrogen diffusion, metal-hydride phase transformation, mechanical deformation, and hydride precipitation occur simultaneously. The present model incorporates all these processes and is able to predict the hydrogen concentration and the hydride fraction distribution under any externally applied stress field. In this work, both the steady and transient hydrogen diffusion cases are evaluated. Further, the XFEM is utilized to develop a model of hydride embrittlement in the presence of a crack. The first step of the hydride embrittlement process is the diffusion of hydrogen. According to Fick’s law of diffusion, hydrogen diffusion is directly dependent on hydrostatic stresses and hydrogen concentration gradient under external stresses. The next step is the hydride precipitation in hydride embrittlement, where the expansion of material takes place that changes the hydrostatic stress field. Thus, studying the effect of precipitation of hydride on hydrostatic stresses is essential. Moreover, the process of hydride embrittlement is highly influenced by residual stresses in the structure. Hence, the effect of residual stress present in the zirconium alloy pressure tube (PT) is also evaluated. The results indicate that the residual tensile stresses contribute to the growth of hydride, which will reduce the material failure time.

Development of in-situ Delayed Hydride Cracking tests using neutron imaging to study the H redistribution in Zr-2.5%Nb

Delayed Hydride Cracking (DHC) is a failure mechanism that occurs in Zr alloys under certain conditions of hydrogen concentration, temperature and stress gradient. In service, hydrogen produced by corrosion reaction can be incorporated in Zr alloys and if the solid solubility is exceeded, hydrogen precipitates as zirconium hydride. The presence of a stress concentrator, such as a crack, generates the hydrogen diffusion and precipitation to the high stress zone beginning the DHC process. In this work, in-situ DHC tests in air at 250°C were performed at ANTARES, the neutron imaging facility of the FRM-II reactor. Samples of Zr2.5%Nb produced from extruded tubes and pressure tubes were studied using a stress rig specially modified to perform DHC tests in the neutron beam. H redistribution during mechanical testing was followed in-situ by registering the changes in neutron transmission. The results were compared with the images obtained by light optical microscopy after the tests. The results highlight the capabilities of neutron imaging to analyze time-dependent H distribution during DHC crack growth.

Evaluation of hydrogen recombination characteristics of a PAR using SPARC PAR experimental results

Passive auto-catalytic recombiners (PARs) are widely used to mitigate a hydrogen hazard. The first step to evaluate the hydrogen safety by PARs is to obtain qualified test data of the PARs for validation of their analytical model. SPARC PAR tests SP8 and SP9 were conducted to evaluate the hydrogen recombination characteristics of a honeycomb-shaped catalyst PAR. To obtain the hydrogen recombination rate from the PAR test data, two methods, Method-1 and Method-2, introduced by the THAI project, were applied. Since a large gradient of hydrogen concentration developed during hydrogen injection can cause a large error in the hydrogen mass obtained by integrating the measured hydrogen concentrations, a gate was installed at the PAR inlet to homogenize hydrogen in the test vessel before the PAR operation in the tests. A computational fluid dynamics (CFD) code with a PAR model was also applied to evaluate the characteristics of the PAR recombination according to the PAR inlet conditions, and the results were compared with those from Method-1 and Method-2. It was confirmed that the recombination rates from Method-1 require a correction factor to be compatible with results from Method-2 and the CFD simulation in the case of the SPARC-PAR tests.

Numerical approach to theoretical aspects of wedge-induced oblique detonation wave in a hydrogen concentration gradient

The present study conducts numerical simulations of oblique detonation wave (ODW) induced on a wedge in the concentration gradient of a hydrogen–air mixture. As a continuation of the author's previous work on the morphology of the initiating flame in a non-uniform mixture, the concentration gradient is provided only to the ODW front to address its theoretical characteristics: the propagation velocity and structures of post-shock reactive flow associated with the Chapman–Jouguet and Zeldovich–von-Neumann–Doering theories, respectively. Applying a Gaussian distribution of the hydrogen mole fraction to the ODW front induces a curved shape that is concave or convex in fuel-rich or fuel-lean compositions, respectively. The local wave angle on a curved ODW matches the one-dimensional theory in a uniform mixture, which proves its robustness in predicting the detonation velocity in a non-uniform mixture. Furthermore, tracing streamlines with different compositions reveals that the flow path and variations in temperature and pressure are almost coincident with those predicted by one-dimensional and uniform assumptions. The slight variation among the different conditions is attributed to the effects of two-dimensional convergence/divergence that are intensified at stronger gradients. The understanding achieved in the present study will also benefit the evaluation of propagating detonation in a non-uniform mixture layer formed in propulsion devices.

Critical verification of the effective diffusion concept

Knowing the hydrogen distribution c(x,t) and local hydrogen concentration gradients grad(c) in ferritic steel components is crucial with respect to hydrogen embrittlement. Basically, hydrogen is absorbed from corrosive or gaseous environments via the surface and diffuses through interstitial lattice sites into bulk. Although, the lattice diffusion coefficient DL∼0.01mm2/s is in the order of magnitude of those for well-annealed pure iron, trapping sites in the microstructure retard the long-range chemical diffusion jL=−Dchem(c)grad(c), causing local hydrogen accumulation in near surface regions in limited time. Considering pure ferritic crystals without trapping sites in the microstructure, the limited characteristic diffusion depth xc∼Defft is proportional to the square root of the effective diffusion coefficient Deff and of time t. Effective diffusion coefficients are measured independently for hydrogen using the electrochemical permeation technique. For pure crystals, the effective diffusion coefficient is constant at given temperature and allows accurate calculations of the diffusion depths. However, with trapping sites in the microstructure the effective diffusion coefficient is not a material property anymore and becomes dependent on the hydrogen charging conditions. In the present work, the theory of hydrogen bulk diffusion is used to verify the concept of effective diffusion. For that purpose, the generalized bulk diffusion equation was solved numerically by using the finite difference method (FDM). The implementation was checked using analytical solutions and a comprehensive convergence study was done to avoid mesh and time dependency of the results. It is shown that effective diffusion coefficients can vary by magnitudes depending on the sub-surface lattice concentration. This limits the application of the effective diffusion concept and also the calculation of the characteristic diffusion depth.

Investigation of a Method of Evaluating Proton Transport Resistance in PEFC Catalyst Layers

Introduction In polymer electrolyte fuel cells (PEFCs), the catalyst layers, consisting of electrocatalysts and ionomers, greatly affect current-voltage (IV) performance. In other words, not only catalytic activity and electronic conductivity, but also proton conductivity become important to achieve higher performance in PEFCs. Even though the proton conductivity within the catalyst layer is one of the most important factors, there is no method to directly measure the proton conductivity in catalyst layers. Therefore, the objective of this study is to establish a method to directly measure the proton resistance and to quantitatively evaluate the proton conductivity within the catalyst layer. Experimental A schematic diagram for the proton conduction measurement used in this study is shown in Fig. 1. A commercial cell holder commonly used for PEFCs was utilized in our measurement system. As show in Fig. 1, Pt/C gas diffusion electrodes and water-repellent GDLs were used for both the anode and cathode sides. The sample to be analyzed was sandwiched between two Nafion® membranes in order to block electrons. In detail, the 1 cm2 gas diffusion electrode was prepared by spray printing 46.5% Pt/KB (TEC10E50E) on each Nafion®212 film. Then, for one of the two Nafion® membranes, the sample to be analyzed was also spray-printed on the reverse side. Finally, two Nafion® membranes were hot pressed first and assembled with two GDLs as described in Fig. 1. With this setup, ohmic resistance was measured using a current interrupt method, in which direct current (DC) was applied to the sample but electrons were blocked. Since it is important to select a reaction with a small overvoltage for the DC reaction, we have used hydrogen oxidation/reduction reaction in this study and flowed hydrogen on the anode side and nitrogen on the cathode side. Results and Discussion Before the actual sample measurement, a Nafion®212 was first sandwiched instead of the sample to be analyzed and tested. When the H2 generated at the cathode side was quickly removed along with the N2 flow, a gradient of hydrogen concentration was produced. The resulting concentration cell generated the additional voltage. Also, unlike a fuel cell reaction, since water was not produced during the reaction, the Nafion® membrane dried up as the reaction proceeded, leading to increase in resistance. Therefore, in order to avoid formation of a concentration cell, the flow rate of gases was minimized and also N2 gas was stopped at the cathode right before the measurement. Additionally, the water-repellent GDLs normally used in PEFCs were replaced with GDLs without a water-repellent treatment. Using the optimized measurement system described in the above, two samples, a catalyst layer containing Pt/Vulcan, and a Vulcan carbon support layer without Pt, were tested. As a result, ohmic resistance of the Pt/Vulcan layer was very small like 0.011 Ω. On the other hand, the Vulcan catalyst layer showed 0.095 Ω. The reason for this is most likely because the presence of Pt in the layer brings up some other chemical reactions, causing the electron transfer within the sample layers, and the electron movement cannot be completely blocked.Consequently, by directly evaluating the Pt-free carbon support layer, deriving the proton conductivity in the catalyst layer becomes the best way. Figure 1

Thermodynamic analysis and optimization of solar methane dry reforming enhanced by chemical hydrogen separation

Thermodynamically limited reactions in membrane reactors for hydrogen generation require a high hydrogen concentration gradient for separation, which imposes an energy requirement that reduces the process energy efficiency. To decrease the required separation energy, chemical hydrogen separation by CO2 is proposed by combining solar-driven dry reforming of methane (DRM) in a hydrogen permeable membrane (HPM) reactor with either reverse water gas shift (RWGS) or methanation. This system has the benefit of using solar renewable energy as well as reducing CO2 emissions. The performances of the proposed membrane reactor configurations for in situ hydrogen consumption are compared to DRM and super-dry reforming of methane (SDRM) in a fixed-bed reactor. The thermodynamic, kinetic, and environmental performances are analyzed and compared from 300 °C to 1000 °C. 2.01 mol CO2 can be reduced to CO per mole CH4 consumed at 1000 °C by coupling DRM with RWGS in a membrane reactor, resulting in a CO2 reduction rate of 1064 kg m−2 yr−1. Alternatively, the minimum energy consumption per mole CO2 converted is 0.79 MJ at 860 °C. This study demonstrates the feasibility of DRM enhanced by chemical hydrogen separation in HPM reactors to convert CO2 into fuels and store solar thermal energy as chemical energy.

Bulk Thermodynamics Determines Surface Hydrogen Concentration in Membranes

AbstractThe functioning of hydrogen selective metal membranes relies on the fine‐tuned interaction of their surfaces with the inner bulk. The link between hydrogen permeation and surface concentration is unveiled using a novel combination of thin film preparation and surface science analysis to probe and intentionally modify the surface properties of palladium vanadium composite membranes. The in situ preparation allows for observation of ultra‐clean hydrogenation and the quantification of the permeation as a function of temperature and pressure. The sub‐surface hydrogen concentration is determined by operando reflecting electron energy loss spectroscopy (REELS) and surface elemental characterization by X‐ray photoelectron spectroscopy and Auger electron spectroscopy under oxygen‐free conditions as a function of pressure and temperature, from which the surface hydrogen pressure‐composition isotherms are derived. The energy dependence of REELS reveals that the VHx layer exhibits a hydrogen concentration gradient. Modeling of the permeation behavior and surface hydrogen concentration yields activation energies in good agreement with similar hydrogen absorbing thin film systems for the overall process, the dissociation at the Pd surface, the diffusion in the membrane, and the dissociation at the V surface. The bulk thermodynamic properties of the metal hydride govern the desorption process.

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.

Hydrogen diffusion under the effect of stress and temperature gradients

In this paper, a finite element (FE) model is developed to investigate lattice hydrogen diffusion in a solid metal under the influence of stress and temperature gradients. This model is applied to a plate with a circular hole which is subjected to temperature and hydrogen concentration gradients. It is demonstrated that temperature gradients significantly influence hydrogen diffusion and hence susceptibility to hydrogen embrittlement when utilizing hydrogen for gas turbines.

An Enhanced Sherwood Number to Model the Hydrogen Transport in Membrane Steam Reformers.

It is well known that membrane reactors are inherently two-dimensional systems in which species concentrations vary as a consequence of both the reaction and permeation across the membrane, which occurs in the direction perpendicular to that of the main gas flow. Recently, an expression for an enhanced Sherwood number was developed to describe the hydrogen concentration gradients arising in methane steam-reforming membrane reactors as a consequence of the combined effect of hydrogen production, dispersion, and permeation. Here, the analysis is developed in further detail with the aim of (i) assessing the validity of the simplifying assumptions made when developing the 1D model and (ii) identifying the operating conditions under which it is possible to employ the 1D model with the enhanced Sherwood number.

The effects of hydrogen-induced lattice distortion and Nb-D formation on its permeation through niobium

Abnormal permeation behavior of hydrogen through niobium has been investigated in this paper, i.e. the permeation flux saturated with long-term decrease after reaching a maximum. The diffusivity and permeability have been deduced from the decay edge of permeation transient. Three kinds of polycrystalline niobium foils with different annealing temperature have been compared, to verify the effect of defects and grain properties on the permeability and diffusivity. In the temperature range of (773–1023) K, the heat treatment along with the permeation cycles could either reduce or increase the permeability and diffusivity depending sensitively on temperature and showing a temperature threshold around 950 K. The permeation flux is proportional to square root of pressure, revealing that the abnormal permeation was still bulk diffusion-limited. The diffusivity gradually decreased with permeation cycles, and became more and more sensitive to pressure. The niobium foil expanded macroscopically along the gradient of hydrogen concentration, which reveals the strong and unrecoverable lattice distortion in this temperature and pressure range. The X-ray diffraction studies showed that splitting of all the Nb peaks and shifting of Nb-D peaks along with hydrogen loadings. The phase transition was expected to eliminate the lattice strain during hydrogen loading and which in turn acted as a diffusion barrier.

Влияние электролитического насыщения водородом на механизмы деформации аустенитной нержавеющей стали 01Х17Н13М3 при холодной прокатке

In this work, we investigated the effect of the current density during electrolytic hydrogen-charging before deformation on the mechanisms of plastic deformation of stable austenitic stainless steel Fe-17Cr-13Ni-3Mo-0.01C during rolling. Using transmission electron microscopy and backscattered electron diffraction, it has been shown that plastic deformation by rolling at room temperature with 25 % reduction causes the formation of a high density of crystal lattice defects in the initially coarse-grained steel samples. The main deformation mechanism during rolling is dislocation slip, which is accompanied by mechanical twinning as an additional mechanism contributing the structure fragmentation. Regardless of the mode of preliminary hydrogen-charging, it promotes more active development of twinning. The higher the current density during hydrogen pre-charging, the higher the density of twin boundaries and dislocation density in the steel structure during rolling. After hydrogen-charging at a current density of 200 mA / cm2, the γ → ε transformation is also realized upon deformation. At the same time, an increase in the current density upon hydrogen pre-charging from 10 to 200 mA / cm2 does not contribute to a significant increase in the concentration of hydrogen adsorbed by the material (0.0017 – 0.0018 mass.%), which, together with microstructural studies, indicates a different distribution of hydrogen in the samples immediately before deformation and the important role of the hydrogen concentration gradient on the deformation mechanisms of the steel during subsequent rolling. With a decrease in the deformation temperature (due to the cooling of the samples before rolling to the temperature of liquid nitrogen), hydrogen-charging also causes an increase in the linear density of twin boundaries and the formation of ε-martensite, but these effects are more significant compared to room-temperature deformation. The data obtained indicate that at similar values of the concentration of adsorbed hydrogen in the structure of steels, the saturation mode affects the deformation mechanism and phase transformations in austenitic steel.

Analysis of Characteristics of Spent Fuels on Long-Term Dry Storage Condition

Currently, the interim storage pools of spent fuels in South Korea are expected to become saturated from 2024. It is required to prepare an operation plan of a domestic dry storage facility during a long-term period, with the researches on safety evaluation methods. This study modified the FRAPCON code to predict the spent fuel integrity evaluation such as the axial cladding temperature, the hoop stress and hydrogen distribution in dry storage. The cladding temperature in dry storage was calculated using the COBRA-SFS code with the burnup information which was calculated using the FRAPCON code. The hoop stress was calculated using the ideal gas equation with spent fuel information such as rod internal pressure. Numerical analysis method was used to calculate the degree of hydrogen diffusion according to the hydrogen concentration and temperature distribution during a dry storage period. Before 50 years of dry storage, the cladding temperature and hoop stress decreased rapidly. However, after 50 years, they decreased gradually and the cladding temperature was below 400 K. The initial temperature distribution and hydrogen concentration showed a parabolic line, but hydrogen was transferred by the hydrogen concentration and temperature gradient over time.

Hydrogen-air explosion with concentration gradients in a cubic enclosure

An inhomogeneous hydrogen-air mixture is induced once hydrogen leakage occurs, and an explosion may be triggered due to an accidental ignition with sufficient strength. The effects of different hydrogen concentration gradients on flame behavior and overpressure were experimentally investigated in a closed cubic vessel with a volume of 0.125 m3. The transverse concentration gradients of hydrogen injected from the top face of the vessel were recorded using five oxygen sensors. A high-speed photography was employed to capture the flame shape evolution and the flame tip velocity was derived. Three piezoelectric pressure transducers were mounted on the top and side walls to measure the pressure-time profiles. The results show that the maximum overpressure in homogeneous mixtures is larger than that in inhomogeneous mixtures for fuel-lean hydrogen-air mixtures. However, the maximum overpressure in inhomogeneous mixtures is equal to or larger than that in homogeneous mixtures for fuel-rich hydrogen-air mixtures. Due to a larger difference between the upward and downward flame speeds, a mushroom-like shape flame was recorded in inhomogeneous hydrogen-air mixtures in fuel-lean to slightly fuel-rich hydrogen-air mixtures. The flame speed in two directions has the almost same value in inhomogeneous fuel-richer mixtures. The average flame speed in inhomogeneous mixtures is significantly larger than that in homogeneous mixtures for fuel-rich hydrogen air mixtures.

A Computational Study of the Mass Transport Behavior of Hydrogen in In-Service Carbon Steel Pipeline Fillet Welds During Deposition and Desorption

Numerical calculations were used to study the diffusion and thermotransport of hydrogen during deposition and diffusion of hydrogen during desorption of in-service carbon steel pipeline fillet welds prone to hydrogen cracking. During deposition, only a small amount of hydrogen migrates into the HAZ and little migrates into the pipe. Thermotransport decreases the hydrogen concentration in the HAZ and regions of the weld closer to the HAZ, and increases the concentration toward the outer edge of the weld. This effect increases the rate of desorption of hydrogen from the weld, which increases with weld size/pipe wall thickness. The activation energy for desorption varies with the fraction of hydrogen removed and appears to be a function of the activation energy for diffusion and mass fraction of hydrogen in the weld, HAZ, and pipe. The differences in the desorption rate due to thermotransport appear to be due to differences in the activation energy for desorption and hydrogen concentration gradients. Maintaining a 121 °C preheat/interpass temperature during welding significantly reduces post-weld heating times necessary to reach hydrogen levels considered to be safe from hydrogen cracking. Calculated results and experimental results from another study show good agreement for desorption at 132 °C for up to 30 minutes.