Apparent Hydrogen Diffusion Coefficient Research Articles

Fabrication and hydrogen permeation resistance of dense CrN coatings

Hydrogen embrittlement has become a hot research topic due to the rapid development of hydrogen energy and coating technology is regarded as the most efficient method to mitigate hydrogen embrittlement. However, the grain gaps in coatings frequently serve as paths for hydrogen permeation thus decreasing the protecting effects. In this work, ion bombardment is performed during magnetron sputtering deposition of dense CrN coatings to decrease the formation of grain gaps. The compactness of the CrN coatings is improved by disrupting the growth of grains using energetic ion bombardment. Hydrogen permeation tests reveal that the apparent hydrogen diffusion coefficient and hydrogen permeability of the dense CrN coating 52.6 and 24.1 times less than those of the unprotected substrate. The tensile test also reveals excellent hydrogen embrittlement resistance compared to the X70 substrate and conventional coatings.

The Positive Role of Nanometric Molybdenum-Vanadium Carbides in Mitigating Hydrogen Embrittlement in Structural Steels.

The influence of hydrogen on the fracture toughness and fatigue crack propagation rate of two structural steel grades, with and without vanadium, was evaluated by means of tests performed on thermally precharged samples in a hydrogen reactor at 195 bar and 450 °C for 21 h. The degradation of the mechanical properties was directly correlated with the interaction between hydrogen atoms and the steel microstructure. A LECO DH603 hydrogen analyzer was used to study the activation energies of the different microstructural trapping sites, and also to study the hydrogen eggresion kinetics at room temperature. The electrochemical hydrogen permeation technique was employed to estimate the apparent hydrogen diffusion coefficient. Under the mentioned hydrogen precharging conditions, a very high hydrogen concentration was introduced within the V-added steel (4.3 ppm). The V-added grade had stronger trapping sites and much lower apparent diffusion coefficient. Hydrogen embrittlement susceptibility increased significantly due to the presence of internal hydrogen in the V-free steel in comparison with tests carried out in the uncharged condition. However, the V-added steel grade (+0.31%V) was less sensitive to hydrogen embrittlement. This fact was ascribed to the positive effect of the precipitated nanometric (Mo,V)C to alleviate hydrogen embrittlement. Mixed nanometric (Mo,V)C might be considered to be nondiffusible hydrogen-trapping sites, in view of their strong hydrogen-trapping capability (~35 kJ/mol). Hence, mechanical behavior of the V-added grade in the presence of internal hydrogen was notably improved.

Hydrogen Diffusivity in Different Microstructures of 42CrMo4 Steel

It is well known that the presence of hydrogen decreases the mechanical properties of ferritic steels, giving rise to the phenomenon known as hydrogen embrittlement (HE). The sensitivity to HE increases with the strength of the steel due to the increase of its microstructural defects (hydrogen traps), which eventually increase hydrogen solubility and decrease hydrogen diffusivity in the steel. The aim of this work is to study hydrogen diffusivity in a 42CrMo4 steel submitted to different heat treatments—annealing, normalizing and quench and tempering—to obtain different microstructures, with a broad range of hardness levels. Electrochemical hydrogen permeation tests were performed in a modified Devanathan and Stachursky double-cell. The build-up transient methodology allowed the determination of the apparent hydrogen diffusion coefficient, Dapp, and assessment of its evolution during the progressive filling of the microstructural hydrogen traps. Consequently, the lattice hydrogen diffusion coefficient, DL, was determined. Optical and scanning electron microscopy (SEM) were employed to examine the steel microstructures in order to understand their interaction with hydrogen atoms. In general, the results show that the permeation parameters are strongly related to the steel hardness, being less affected by the type of microstructure.

Role of Hydrogen in the Separation of Interfaces in S13Cr Supermartensitic Stainless Steel

This paper outlines experimental procedures and numerical analyses to investigate hydrogen embrittlement (HE) of S13Cr supermartensitic stainless steel (SMSS) subjected to various cathodic potentials. The hydrogen diffusion behavior was investigated using two electrochemical permeation techniques, namely, the double potentiostatic method (DPM) and the step method (SM), along with thermal desorption spectroscopy (TDS) tests. The apparent hydrogen diffusion coefficients varied from 1.4 × 10−13 to 4.7 × 10−12 m2/s, and TDS revealed the existence of deep traps, such as interfaces around precipitates and between retained austenite and ferritic matrix. The effects of HE in terms of a reduction in ductility were analyzed through tensile tests and fractographic analysis. A maximum reduction in elongation of approximately 14% was measured, and a majority of brittle fracture along the entire net section was observed in test samples pre-charged under -1.5 V/SCE and -1.7 V/SCE. A computational simulation was performed using the finite element method to predict the loss of ductility using the obtained experimental data. The computational model used a fracture-controlled method under static structural conditions, which links the reduction in elongation of the tensile specimens to the decrease in critical fracture energies, which, in turn, were calculated through a new mean field approach that used thermodynamic excess variables. The observations presented here suggest that S13Cr has good resistance to HE and that the computational model is reliable because only slight deviations in the magnitude (5%) were observed between the experimental and numerical results.

Effect of vanadium on hydrogen embrittlement susceptibility of high-strength hot-stamped steel

Based on the chemical composition of traditional hot-stamped steel (e.g., 22MnB5 and 30MnB5), Nb and V microalloying elements are added into 30MnB5 steel to meet the requirements of ultra-high strength, excellent ductility and potent resistance to hydrogen embrittlement (HE) at the same time. The influence of hot-stamped steel on HE was studied by conducting a hydrogen permeation method and pre-charged hydrogen slow strain rate test. Meanwhile, the experimental steel microstructures and corresponding fracture surfaces are observed and analyzed to characterize HE behavior. The results show that a finer microstructure, a lower apparent diffusion coefficient of hydrogen and a smaller percentage of strength and plasticity reduction are obtained due to the addition of the vanadium element into hot-stamped steel. Compared to the V free experimental steel, the steel with 0.14 wt.% V has a large number of dispersive precipitates and more grain boundary areas, which makes hydrogen atoms dispersedly distribute.

Hydrogen diffusion and trapping in 42CrMo4 quenched and tempered steel: Influence of quenching temperature and plastic deformation

Medium- and high-strength steel components working in contact with hydrogen-rich environments must be appropriately designed in order to provide safe and reliable service during their entire lifespan. In this context, the present paper aims to study hydrogen diffusion and trapping in a quenched and tempered 42CrMo4 steel by means of electrochemical permeation methods. The effect of quenching from a high austenitizing temperature and applying a plastic deformation of 10% and 20%, were also evaluated.In general terms, the results showed that with increasing cathodic current (amount of hydrogen introduced in the steel specimen), the apparent hydrogen diffusion coefficient also increases up to a maximum constant value that corresponds to the lattice hydrogen diffusion coefficient, DL. Quenching the steel from a high austenitizing temperature decreases hydrogen diffusivity and increases the density of traps. On the other hand, microstructural trapping increases (increase in the dislocation density) and the diffusion coefficients decrease after applying a cold plastic deformation. All the aforementioned modifications related to the permeation of hydrogen throughout the microstructure of the steel were finally correlated with changes in the steel's hardness.

Experimental study and finite element simulation of hydrogen permeation resistance of Ti-6Al-4V alloy strengthened by laser peening

The hydrogen permeation resistance of Ti-6Al-4V alloy treated by laser peening (LP) was investigated. Electrochemical hydrogen charging, hydrogen permeation experiments were carried out respectively. The residual stress, micro-hardness of hydrogenated and non-hydrogenated specimens were analyzed. The apparent hydrogen diffusion coefficient, lattice hydrogen concentration and hydrogen trap density were compared. LP process and hydrogen diffusion process were numerically simulated by ABAQUS software. The results show that LP can effectively increase the compressive residual stress (CRS) and micro-hardness on the surface of Ti-6Al-4V alloy. LP also promoted the closure of the original micro-cracks by forming a dense strengthening layer on the surface of the material, which reduced the hydrogen concentration and the diffusion coefficient in the crystal lattice. The laser power was inversely related to the hydrogen diffusion coefficient. The numerical simulation results reveal that the hydrogen concentration in the depth direction of LPed specimen decreases significantly, and the maximum drop of 50% for hydrogen concentration was appeared at the depth of 0.6 mm from the top surface. Moreover, the distribution trend of hydrogen concentration in the LP area is consistent with that of CRS, which indicates that CRS plays a positive role in the inhibition of hydrogen diffusion.

Effect of internal hydrogen on the tensile properties of different CrMo(V) steel grades: Influence of vanadium addition on hydrogen trapping and diffusion

The influence of hydrogen on the mechanical behaviour of different quenched and tempered CrMo steels with or without vanadium were investigated by means of tensile tests. Smooth and circunferentially-notched round-bar specimens pre-charged with gaseous hydrogen in a high pressure hydrogen reactor were tested. The degradation of the tensile properties was correlated with the interaction between hydrogen atoms and microstructure, which was analysed by means of thermal desorption analysis (TDA) and permeation tests. A LECO DH603 hydrogen analyzer was used to study the activation energies of the different microstructural traps and also to study the hydrogen eggresion kinetics at room temperature. Moreover, electrochemical hydrogen permeation tests were also used to determine the apparent hydrogen diffusion coefficients and the density of traps present in the different steel grades.Hydrogen embrittlement measured in notched specimens was much greater than that observed in the smooth samples, being this effect more notable in the steel grades with higher yield strengths, tempered at the lowest temperatures, where a change in the fracture micromechanism from ductile in the absence of hydrogen to intermediate and brittle in the presence of internal hydrogen was clearly observed, especially in tests performed at the lowest displacement rates. Results were discussed through FEM simulations of local stresses acting on the process zone. On the other hand, the V-added steel grades were less sensitive to hydrogen embrittlement due to the effect of the submicrometric VC precipitated during the tempering treatment, which might be considered non-diffusible hydrogen-trapping sites, in view of their strong hydrogen-trapping capability (35–41 kJ/mol). In these steels (V-added grades), diffusible hydrogen and hydrogen accumulation in the process zone decrease, improving hydrogen embrittlement resistance.

The Role of Hydrogen on the Separation of Interfaces of  Supermartensitic Stainless Steel S13Cr

This paper outlines experimental procedures and numerical analyses to research Hydrogen embrittlement (HE) of the supermatensitic stainless steel (SMSS) S13Cr submitted to different cathodic potentials. Hydrogen diffusion behavior was investigated by means of two electrochemical permeation techniques, namely the double potentiostatic method (DPM) and the step method (SM), and it was also performed thermal desorption spectroscopy (TDS) tests. Apparent Hydrogen diffusion coefficients vary from 1.4 x 10-13 m²/s to 4.7 x 10-12 m²/s and TDS reveals the existence of deep traps, such as interfaces of precipitates and between retained austenite and ferritic matrix. HE effects in terms of reduction of ductility were analyzed through tensile tests and fractographic analysis, where a maximum reduction of elongation of around 14% was measured and a majority of brittle fracture along entire net section was observed in test samples pre charged under -1.5 V/SCE and -1.7 V/SCE. A calculation procedure based on computational simulation via finite element method has been performed, in order to predict mentioned loss of ductility using the experimental data mentioned before. The computational model used a fracture-controlled method under static structural condition, which links reduction of elongation of tensile specimens to the decreasing of critical fracture energies, which, in turn, were achieved through a new mean field approach regarding thermodynamic excess variables. Herein presented observations make it possible to suggest, that S13Cr has good resistance to HE and that the mentioned calculation procedure is reliable, since slight deviations in the magnitude of 5% were observed.

Effects of hydrogen on the fracture toughness of 42CrMo4 steel quenched and tempered at different temperatures

The aim of this paper is to study the effect of hydrogen on the fracture toughness of 42CrMo4 steel quenched and tempered at 500, 550, 600, 650 and 700 °C. The influence of hydrogen was assessed by means of fracture toughness tests carried out on compact CT specimens, pre-charged with gaseous hydrogen in a pressurized reactor at 19.5 MPa and 450 °C for 21 h. Thermal desorption analysis (TDA) and finite element simulations (FEM) were employed to study the hydrogen diffusivity and solubility of the different steel grades. Additionally, X-ray diffraction (XRD) was used to determine the dislocation densities present in these steel microstructures and scanning electron microscopy (SEM) to analyse them and to identify the fracture micromechanisms that took place during the fracture toughness tests.According to the obtained results, dislocation density, hydrogen solubility and residual hydrogen strongly trapped in the steel microstructure were seen to decrease with increasing tempering temperature, following the apparent hydrogen diffusion coefficient the opposite trend. It was also observed that hydrogen embrittlement was much greater in the steel grades tempered at the lowest temperatures (with higher yield strength), and in tests performed at the lowest displacement rates. Moreover, a change in the fracture micromechanism was detected, from ductile (microvoids coalescence, MVC) in the absence of hydrogen, to intermediate (plasticity-related hydrogen induced cracking, PRHIC) in the case of the hydrogen pre-charged steels with relatively low yield strengths under low displacement rates and, finally, to a fully brittle behaviour (mostly intergranular fracture, IG) in the case of steels with the highest strengths, tested at low displacement rates.It was finally demonstrated that only diffusible hydrogen, hydrogen atoms able to move at room temperature, is responsible for hydrogen embrittlement.

Hydrogen embrittlement of high strength steam turbine last stage blade steels: Comparison between PH17-4 steel and PH13-8Mo steel

Hydrogen embrittlement behaviors of PH17-4 steel and PH13-8Mo steel used as steam turbine last stage blades were investigated by means of slow strain rate tensile tests and hydrogen permeation tests. The results indicate that PH13-8Mo steel exhibits higher strength level mainly due to higher precipitate strengthening and lath strengthening compared with PH17-4 steel in the absence of hydrogen. Moreover, pre-hydrogen charging does not affect tensile strength of PH17-4 steel, but decreases tensile strength of PH13-8Mo steel. The plasticity of PH17-4 steel and PH13-8Mo steel is degraded after hydrogen charging and the deterioration is accompanied with considerable change in fracture mode. The loss of plasticity of PH13-8Mo steel is higher than that of PH17-4 steel, indicating that PH13-8Mo steel exhibits lower resistance to hydrogen embrittlement, which is mainly related to higher hydrogen diffusion behaviors and higher strength level of PH13-8Mo steel. Additionally, PH17-4 steel exhibits lower apparent hydrogen diffusion coefficient and higher apparent hydrogen solubility in comparison with those of PH13-8Mo steel, which implies that incoherent Cu-rich precipitates in PH17-4 steel possess higher capability to trap hydrogen atoms than that of coherent NiAl precipitates in PH13-8Mo steel.

Hydrogen absorption/desorption characteristics of Mg-V-Ni hydrogen storage alloys

In this study, Mg+5at%V+5at%Ni alloys are prepared by wet mechanical alloying (MA) in an organic solvent under a purified argon atmosphere. The hydrogen absorption capacities of the samples are evaluated at 473 K at an initial hydrogen pressure of 0.2 MPa in a Sieverts apparatus. First, to obtain the ideal hydrogen absorption and desorption characteristics, organic solvents for milling are changed. Among the alloy samples milled with different organic solvents, the maximum hydrogen absorption capacity is obtained for the alloy prepared with benzene. After determining the organic solvent, the milling time was varied from 3 to 8 h. The maximum hydrogen absorption capacity of 5.15 mass% can be achieved in 7 h-milling. Furthermore, the apparent hydrogen diffusion coefficients are obtained by fitting the temporal change of the hydrogen absorption capacity with a theoretical curve. The theoretical curves are calculated, assuming that the particle shape is a sphere and the size of the sample is 40 μm.

Effect of plastic deformation at room temperature on hydrogen diffusion of S30408

Understanding the influence of plastic deformation on diffusion is critical for hydrogen embrittlement (HE) study. In this work, thermal desorption spectroscope (TDS), slow strain rate test (SSRT), feritscope, transmission electron microscope (TEM) and TDS model were used to study the relation between plastic deformation and hydrogen diffusion, aiming at unambiguously elucidating the effect of plastic deformation on hydrogen diffusion of austenitic stainless steel, S30408. An effective method was developed to deduce apparent hydrogen diffusion coefficient of austenitic stainless steel in this paper. Results indicate apparent hydrogen diffusion coefficient decreases firstly and then increases with increasing plastic deformation at room temperature. Hydrogen diffusion effected by plastic deformation is a complicated process which is suggested to be divided into two processes controlled by dislocation and strain-induced martensite, respectively, and the transition point is about 20% strain demonstrated by experiments in this case.

DIFFUSION OF HYDROGEN IN WELDED JOINTS OF STRUCTURAL STEELS

High-strength low-alloy steels are widely used in the construction of welded metal structures. The main advantage of these steels is good combination of strength and toughness, and weldability. However, when welding high strength low alloy steels during cooling of the weld to a temperature below 150-100 °C there may be a risk of formation of bulk crystal structures defects in the weld zone - cold cracks. It was experimentally established that one of the factors contributing to the formation of cold cracks may be the occlusion of hydrogen in the atmosphere of arc plasma in the solidifying weld metal, from which diffusion hydrogen may diffuse to different areas of the weld after cooling. Hydrogen cracking typically has a tendency to slow down i.e. cracks can occur several days after the completion of welding process. As a rule, hydrogen induced cracking occurs either in the original steel in the heat-affected zone or in the weld metal, which is important, topical and long been researched by various scientific schools. Modern technologies of high strength low alloy steels processing have significantly improved the quality of the base material by reducing the amount of carbon and impurities, which has increased the stability of weld in the heat affected zone (HAZ) to hydrogen induced cold cracking. The paper presents modern approaches to the definition of diffusion coefficient of hydrogen in welded joints of high-strength low-alloy steels. Taking into account the temperature, the gradient of chemical potential and continuity conditions there has been considered the process of mass transfer of hydrogen under the influence of diffuse inhomogeneous mediums. It has been shown that the local effects of changing pressure and chemical potential are described using the equation of generalized potential of the diffusing substance. Our paper presents analytical expressions to determine the apparent diffusion coefficient of hydrogen in different local areas of a welded joint depending on temperature.

Hydrogen diffusion and trapping in a low copper 7xxx aluminium alloy investigated by Scanning Kelvin Probe Force Microscopy

The susceptibility to hydrogen embrittlement (HE) of the 7046 aluminium alloy (AA 7046) was investigated. Samples of AA 7046 corresponding to different ageing temperature/time couples were hydrogenated by cathodic charging in a H2SO4 solution. Scanning Kelvin Probe Force Microscopy (SKPFM) combined with global hydrogen amount measurements allowed apparent hydrogen diffusion coefficients (Dapp) to be measured: the decrease of the Dapp values with the increase of the ageing duration was attributed to hydrogen trapping by hardening η’ and η precipitates for the aged alloy. Additional SKPFM measurements were carried out on hydrogen charged samples after desorption at 25°C and combined with SEM observations of the fracture surfaces after tensile tests. Results showed that hydrogen could be trapped at the grain boundaries leading to brittle intergranular fracture. However, hardening precipitates could act as efficient trapping sites and reduce hydrogen trapping at the grain boundaries. Conclusion is that the most critical microstructural parameters for HE of AA 7046 correspond to the grain boundaries while ageing could contribute to improve the resistance to HE of the alloy by a well-controlled precipitation.

Effect of Degree of Order on Hydrogen Diffusion in (Fe,Ni)<sub>3</sub>V Alloys

The effect of hydrogen on the mechanical properties of disordered and ordered (Fe,Ni)3V alloys has been investigated. The diffusion behaviors of hydrogen in the disordered and ordered (Fe,Ni)3V alloys were reported. The results showed that the depth of intergranular (IG) fracture on a surface of tensile fracture of (Fe,Ni)3V alloy increased linearly with increasing pre-charging temperature at the same pre-charging time. An apparent hydrogen diffusion coefficient was calculated by the time lag method. The apparent hydrogen diffusion coefficient in the disordered (Fe,Ni)3V alloy was greater than that in the ordered (Fe,Ni)3V alloy. The relationship between the apparent hydrogen diffusion coefficient and pre-charging temperature in (Fe,Ni)3V alloy agreed with Arrhenius equation. The activation energies of apparent hydrogen diffusion in the disordered and ordered (Fe,Ni)3V alloys were 34.6 kJ/mol and 42.2 kJ/mo1, respectively.

Phase-structural transformations in a metal hydride battery anode La1.5Nd0.5MgNi9 alloy and its electrochemical performance

This work was aimed at in situ studies of the phase-structural transformations in an Mg- and Nd-modified LaNi3 intermetallic having a bulk composition La1.5Nd0.5MgNi9, as related to its performance as battery anode material in the Ni-Metal Hydride batteries. The interactions in the La1.5Nd0.5MgNi9 system were studied by in situ neutron diffraction in a temperature range 300–1248 K. Two alternative processes of preparation of the alloy were studied: (a) Synthesis from a mixture of the powders La0.75Nd0.25Ni5 + La0.75Nd0.25MgNi4; and (b) annealing of the alloy prepared by induction melting. Various transformations take place in the samples on heating, leading to the formation of four new phases, all with the stacking structures of the intermetallic alloys having three variable ratios (RE + Mg)/Ni, 1:3, 2:7 and 5:19. Further to (La0.75Nd0.25)2MgNi9 with a rhombohedral PuNi3 type, there were formed (La0.75Nd0.25)3MgNi14 with a rhombohedral Gd2Co7 structure and two polymorphic modifications of (La0.75Nd0.25)4MgNi19 compound, a high temperature hexagonal Pr5Co19 type and a low temperature rhombohedral Ce5Co19 type. In every case the unit cell parameters showed shrinking as compared to the La-based analogues. Nd substitution for La in La2MgNi9 promotes the formation of a more homogeneous alloy, with a predominant formation of the target AB3 intermetallic.Properly homogenized La1.5Nd0.5MgNi9 alloy shows a more flat and longer plateau, both in the course of hydrogen absorption–desorption and under electrochemical charge–discharge. An apparent hydrogen diffusion coefficient in alloy bulk appears to be higher as compared to Nd-free alloy, contributing to a better reversibility under a high-rate operation at high discharge current densities, while maintaining a good cycle stability.

The Effect of Stoichiometry on Hydrogen Embrittlement of Ordered Ni3Fe Intermetallics

The effects of Fe stoichiometry on hydrogen embrittlement and hydrogen diffusion in ordered Ni3Fe intermetallics were investigated. The experimental results show that the ordered Ni3Fe alloy with the normal stoichiometry has the lowest mechanical property, the highest susceptibility to hydrogen, and the highest ability of catalytic reaction. The mechanical properties, the susceptibility to hydrogen embrittlement, and the amount of adsorbed hydrogen of the ordered Ni3Fe alloy are dependent of degree of order of the alloy. The apparent hydrogen diffusion coefficient of the ordered Ni3Fe alloy is independent on degree of order of the alloy but depends on Fe stoichiometry. The activation energy of hydrogen diffusion decreased linearly with Fe stoichiometry for the ordered Ni3Fe alloy.

Hydrogen permeation in iron and nickel alloys around room temperature

Hydrogen permeation and diffusion coefficients in alloys of iron (Fe) and nickel (Ni) with the Ni content of 5, 9, and 20at.% and a crystal structure of α/α′ phase have been examined around room temperature (RT) using a tritium-tracer hydrogen-permeation experiment. Hydrogen permeation coefficients around RT agree well with values extrapolated from literature data obtained at higher temperatures for the respective alloys. On the other hand, apparent hydrogen diffusion coefficients determined using the time-lag method are several orders of magnitude smaller than extrapolated from the literature data. This could be caused by surface blocking and/or barrier effects due to surface oxide and/or other impurities. Initially, hydrogen permeation is suppressed by the existence of the surface oxide. It appears that hydrogen, mostly at the upstream side or even at the downstream side, can reduce and remove the surface oxides so that normal hydrogen steady-state permeation can occur without surface blocking or barrier effects. Thus, true hydrogen diffusion coefficients for respective Fe–Ni alloys during steady-state permeation must be much larger than those estimated from the time-lag method.

Measurement of lattice and apparent diffusion coefficient of hydrogen in X65 and F22 pipeline steels

The permeation through and desorption of hydrogen from cathodically charged micro-alloyed API 5L X65 and low alloy ASTM A182 F22 steels were investigated with the aim to analyse the trapping parameters by means of the electrochemical method developed by Devanathan and Stachurski. The lattice diffusivity of hydrogen as well as the amounts and distribution of the diffusible and trapped hydrogen were found by means of partial permeation transient.