Effects of surface oxide films on hydrogen permeation and susceptibility to embrittlement of X80 steel under hydrogen atmosphere

Hydrogen embrittlement (HE) induced by hydrogen permeation is a serious threat to the hydrogen transmission pipeline. In this study, oxide films were prepared on X80 steel by applying high-temperature oxidation, blackening treatment and passivation in concentrated H₂SO₄, and their effects on hydrogen permeation and HE susceptibility of X80 substrate were studied by conducting hydrogen permeation tests and slow strain rate tension (SSRT) tests. A numerical diffusion model was established to quantitatively determine the resistance of these oxide films to hydrogen permeation. Results showed that the oxide film prepared by high-temperature oxidation presented the highest resistance to hydrogen permeation with the ?_m/?_f value of 3828, and the corresponding HE index decreased from 38.07% for bare X80 steel to only 4.00% for that covered with oxide film. The characteristic of the corresponding fracture surfaces changed from brittle features such as quasi cleavage facets and secondary cracks to typical ductile dimple feature.     

Effects of internal hydrogen and surface-absorbed hydrogen on the hydrogen embrittlement of X80 pipeline steel

Effects of internal hydrogen and surface-absorbed hydrogen on hydrogen embrittlement (HE) of X80 pipeline steel were investigated by using different strain rate tensile test, annealing and hydrogen permeation tests. HE of X80 pipeline steel is affected by internal hydrogen and surface-absorbed hydrogen, and the latter plays a major role due to its higher effective hydrogen concentration. The HE susceptibility decreases with increasing the strain rate because it is more difficult for hydrogen to be captured by dislocations at the high strain rate. Annealing at 200 °C can weakened HE caused by internal hydrogen, while it has little effect on HE caused by surface-absorbed hydrogen. HE of X80 pipeline steel is mainly determined by the behavior of dislocation trapping hydrogen, which can be attributed to the interaction between hydrogen and dislocation.     

Notched-tensile properties under high-pressure gaseous hydrogen: Comparison of pipeline steel X70 and austenitic stainless type 304L, 316L steels

The effect of high-pressure gaseous H₂ on the fracture behavior of pipeline steel X70 and austenitic stainless steel type 304L and 316L was investigated by means of notched-tensile tests at 10 MPa H₂ gas and various test speed. The notch tensile strength of pipeline X70 steel and austenitic stainless steels were degraded by gaseous H₂, and the deterioration was accompanied by noticeable changes in fracture morphology. The loss of notch tensile strength of type 316L and X70 steels was comparable, but type 304L was more susceptible to hydrogen embrittlement than the others. In the X70 steel, hydrogen embrittlement increased as test speed decreased until the test speed reached 1.2 × 10^?3 mm/s, but the effect of test speed was not significant in 304L and 316L steels.     

Damage associated with interactions between microstructural characteristics and hydrogen/methane gas mixtures of pipeline steels

There is no common standard for blended hydrogen use in the natural gas grid; hydrogen content is generally based on delivery systems and end-use applications. The need for a quantitative evaluation of hydrogen-natural gas mixtures related to the mechanical performance of materials is becoming increasingly evident to obtain long lifetime, safe, and reliable pipeline structures. This study attempts to provide experimental data on the effect of H₂ concentration in a methane/hydrogen (CH₄/H₂) gas mixture used in hydrogen transportation. The mechanical performance under various blended hydrogen concentrations was compared for three pipeline steels, API X42, X65, and X70. X65 exhibited the highest risk of hydrogen-assisted crack initiation in the CH₄/H₂ gas mixture in which brittle fractures were observed even at 1% H₂. The X42 and X70 samples exhibited a significant change in their fracture mechanism in a 30% H₂ gas mixture condition; however, their ductility remained unchanged. There was an insignificant difference in the hydrogen embrittlement indices of the three steels under 10 MPa of hydrogen gas. The coexistence of delamination along with the ferrite/pearlite interface, heterogeneous deformation in the radial direction, and abundance of nonmetallic MnS inclusions in the X65 sample may induce a high stress triaxiality at the gauge length at the beginning of the slow strain rate tensile process, thereby facilitating efficient hydrogen diffusion.     

The experiment study to assess the impact of hydrogen blended natural gas on the tensile properties and damage mechanism of X80 pipeline steel

Environmental hydrogen embrittlement has become a non-negligible problem in the hydrogen blended natural gas transportation. To qualitatively study the degradation mechanism of X80 steel used in the natural gas pipelines, the slow strain tensile experiments are carried out in this work. The nitrogen and hydrogen are adopted to simulate the hydrogen blended natural gas to explore the tensile properties of X80 steel. According to the volume proportion of hydrogen, the test atmospheres are divided into the reference atmosphere and the hydrogen-contained atmospheres of 1%, 2.2% and 5%. The tensile experiments of the smooth and notched specimens are conducted in the above gas atmospheres. Mechanical properties and fracture morphologies after stretching are further analyzed. The results show that the hydrogen blended natural gas has little effect on the tensile and yield strengths. Distinguished from the hydrogen volume proportion of 1% and 2.2%, with the increase of hydrogen proportion, the effect of hydrogen on mechanical properties of specimens increases significantly. Moreover, the deteriorated mechanical properties of notched specimens are more seriously than those of smooth specimens. This work provides the basis for safe hydrogen proportion for X80 pipeline steel when transporting hydrogen blended natural gas.     

Influence of low tensile stress on the kinetics of hydrogen permeation and evolution behavior in alkaline environment

The hydrogen behavior the surface of X70 pipeline steel in alkaline environment after applying low tensile stress was investigated by electrochemical tests. It is found by hydrogen permeation tests that the steady-state hydrogen permeation current density (i_∞) and sub-surface hydrogen concentration (C₀) greatly increased, whereas apparent diffusivity (D) was almost unchanged after applying low tensile stress. LSV and EIS measurements indicated that the activity of hydrogen evolution reaction (HER) was improved by elastic tensile stress. The mechanism of stress enhanced the hydrogen embrittlement sensitivity was conducted by Iyer-Pickering-Zamanzadeh (IPZ) and surface effect model. The results demonstrated that the Volmer reaction was facilitated, and the Tafel reaction was restricted by the application of tensile stress. The activation energy obtained by the Arrhenius equation indicated that when the specimen suffered from tensile stress, the adsorption activation energy decreased, and the desorption activation energy increased, leading to the remarkable increase of C₀.     

Quantifying the hydrogen embrittlement of pipeline steels for safety considerations

In a near future, with an increasing use of hydrogen as an energy vector, gaseous hydrogen transport as well as high capacity storage may imply the use of high strength steel pipelines for economical reasons. However, such materials are well known to be sensitive to hydrogen embrittlement (HE). For safety reasons, it is thus necessary to improve and clarify the means of quantifying embrittlement. The present paper exposes the changes in mechanical properties of a grade API X80 steel through numerous mechanical tests, i.e. tensile tests, disk pressure test, fracture toughness and fatigue crack growth measurements, WOL tests, performed either in neutral atmosphere or in high-pressure of hydrogen gas. The observed results are then discussed in front of safety considerations for the redaction of standards for the qualification of materials dedicating to hydrogen transport.     

Crack propagation analysis of hydrogen embrittlement based on peridynamics

We introduced a coupled peridynamic hydrogen diffusion and fracture model to solve the hydrogen embrittlement fracture of low alloy steel AISI 4340. In this model, the influence of temperature on hydrogen diffusion coefficient is considered, and a new peridynamic constitutive analysis method is used to simulate the crack propagation of hydrogen embrittlement. We verified the model in 3D using the experimental test of the hydrogen embrittlement cracking process of AISI 4340 steel in 0.1 N H₂SO₄ solution from the literature. Considering different ambient temperatures, it is found that the crack propagation is highly similar to the experimental results. Based on the numerical analysis of peridynamics, the model can numerically simulate the hydrogen embrittlement fracture of AISI 4340 steel, and obtain a visual demonstration of the entire process of hydrogen atom diffusion and crack growth.     

The detrimental effect of hydrogen at dislocations on the hydrogen embrittlement susceptibility of Fe-C-X alloys: An experimental proof of the HELP mechanism

The hydrogen trapping ability of 15 Fe-C-X alloys is compared in this work. Five types of carbides, i.e. Ti, Cr, Mo, W and V based carbides, and their effect on the hydrogen embrittlement susceptibility is considered while three carbon contents are prepared for each carbide former. Two conditions are compared for each alloy to evaluate the hydrogen/material interaction: an as quenched and quenched and tempered condition in which carbides are introduced. Next to the material characterization, also the interaction of hydrogen with the materials is completely elaborated. At first, in-situ tensile tests are done to determine the hydrogen induced ductility loss. To interpret the obtained degrees of hydrogen embrittlement, hot/melt extraction is done to determine the hydrogen content, whereas thermal desorption spectroscopy is performed to assess the hydrogen trapping capacity of the tempered induced precipitates and the different other potentially hydrogen trapping microstructural features. These measurements are done after hydrogen pre-charging till saturation. The tempered induced TiC and V₄C₃ are capable of trapping a significant amount of hydrogen, while the Mo₂C and Cr₂₃C₆ particles only trap a limited amount of hydrogen. The W₂C precipitates, however, are not able to trap hydrogen. The size and coherency of the carbides are considered to be the main factor determining their trapping ability. The degree of hydrogen embrittlement is correlated with the hydrogen present in the alloys. Three amounts of hydrogen were determined by the strength by which they were trapped by combining the different hydrogen characterization techniques, i.e. total, diffusible and mobile hydrogen. It was confirmed that hydrogen trapped by dislocations plays a determinant role. This further confirms the importance of an enhanced dislocation mobility in the presence of hydrogen, as described in the HELP mechanism.     

Hydrogen effects on X80 pipeline steel in high-pressure natural gas/hydrogen mixtures

Blending hydrogen into existing natural gas pipelines has been proposed as a means of increasing the output of renewable energy systems such as large wind farms. X80 pipeline steel is commonly used for transporting natural gas and such steel is subjected to concurrent hydrogen invasion with mechanical loading while being exposed to hydrogen containing environments directly, resulting in hydrogen embrittlement (HE). In accordance with American Society for Testing and Materials (ASTM) standards, the mechanical properties of X80 pipeline steel have been tested in natural gas/hydrogen mixtures with 0, 5.0, 10.0, 20.0 and 50.0vol% hydrogen at the pressure of 12 MPa. Results indicate that X80 pipeline steel is susceptible to hydrogen-induced embrittlement in natural gas/hydrogen mixtures and the HE susceptibility increases with the hydrogen partial pressure. Additionally, the HE susceptibility depends on the textured microstructure caused by hot rolling, especially for the notch specimen. The design calculation by the measured fatigue data reveals that the fatigue life of the X80 steel pipeline is dramatically degraded by the added hydrogen.     

Hydrogen diffusion in Fe4N: Implication for an effective hydrogen diffusion barrier

The diffusion behavior of hydrogen in steel plays a significant role in understanding the mechanism of hydrogen embrittlement which may lead to the failure of steel. In the present work, density functional theory based first-principles calculations were performed to investigate the hydrogen diffusion in Fe₄N at atomic level. The results showed that the hydrogen atom in the Fe₄N structure didn't diffuse until the temperature increased up to 1350K, indicating a layer of Fe₄N covering on the iron could be a good candidate to trap the highly diffusive hydrogen atoms and prevent the aggregation of hydrogen which is the precursor of hydrogen embrittlement.     

Hydrogen environment embrittlement of an ODS RAF steel – Role of irreversible hydrogen trap sites

The microstructure and the effects of 10 MPa hydrogen atmosphere on the tensile properties of a oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel were investigated. The microstructure consists of a fine grained ferritic matrix with Me₃O₄ (Me = Cr, Fe or Mn), VN and Cr₂₃C₆ grain boundary precipitates as well as dispersed yttrium oxide nano precipitates in the ferritic matrix. The yield and ultimate tensile strength were unaffected by the H₂ atmosphere whereas elongation at fracture and reduction in area were markedly reduced. In H₂ atmosphere, the fracture morphology was found to be a mixture of intergranular H-assisted fracture and a smaller amount of transgranular hydrogen enhanced localized plasticity (HELP) fracture. The sensitivity of the ODS RAF steel to hydrogen embrittlement is attributed to the large number grain boundary precipitates which enhance the tendency for intergranular fracture.     

Influence of copper as an alloying element on hydrogen environment embrittlement of austenitic stainless steel

A Cu alloyed (18Cr–10Ni–3Cu) and a Cu free (18Cr–12.7Ni) austenitic stainless steel were tensile tested in gaseous hydrogen atmosphere at 20 °C and −50 °C. Depending on the test temperature, the Cu alloyed steel was extremely embrittled whereas the Cu free steel was only slightly embrittled. Austenite stability and inherent deformation mode are two main criteria for the resistance of austenitic stainless steels against hydrogen environment embrittlement. Based on the well known austenite stability criteria, the austenite stability of both steels should be very similar. Interrupted tensile tests show that martensite formation upon plastic deformation was much more severe in the Cu alloyed steel proving that the influence of Cu on austenite stability is overestimated in the empirical stability equations. When tested in high pressure H₂, replacing Ni by Cu resulted in a fundamental change in fracture mode atmosphere, i.e. Ni cannot be replaced by Cu to reduce the costs of SS without compromising the resistance to hydrogen environment embrittlement.     

Flameless combustion for hydrogen containing fuels

In this paper, flameless combustion was promoted to suppress thermal-NO_x formation in the hydrogen-high-containing fuel combustion. The PSRN model was used to model the flameless combustion in the air for four fuels: H₂/CH₄ 60/40% (by volume), H₂/CH₄ 40/60%, H₂/CH₄ 20/80% and pure hydrogen. The results show that the NO_x emissions below 30 ppmv while CO emissions are under 50 ppmv, which are coincident with the experimental data in the “clean flameless combustion” regime for all the four fuels. The simulation also reveals that CO decreases from 48 ppmv to nearly zero when the hydrogen composition varies from 40% to 100%, but the NO_x emission is not sensitive to the hydrogen composition. In the highly diluted case, the NO_x and CO emissions do not depend on the entrainment ratio.     

The role of hydrogen in microwave plasma valorization of producer gas

Plasma methods are given significant attention in the context of conditioning the producer gas derived from biomass gasification. The goal of this work is to present the impact of hydrogen on the other producer gas compounds during microwave plasma valorization. These compounds include main producer gas components (CO, CO₂, CH₄, N₂) and minor impurities (tar compounds, H₂S and NH₃). The results prove a beneficial impact of hydrogen addition on the conversion of CH₄ and toluene, increasing it from ca. 68%–95% and ca. 97%–100%, respectively. Additionally, the presence of hydrogen changes the distribution of the products, inhibiting soot and aromatics production and promoting C₂ compounds. In the case of CO₂, the conversion increases from ca. 18%–63% when compared to nitrogen plasma, with CO being the resulting product. The presence of hydrogen inhibits H₂S conversion and does not affect CO and NH₃     

Numerical study of hydrogen addition to DME/CH4 dual fuel RCCI engine

In this study, the effect of hydrogen addition to DME/CH₄ dual-fuel RCCI (Reactivity Controlled Compression Ignition) engine is investigated using three dimensional calculations coupled with chemical kinetics. A new reduced DME (Dimethyl Ether) oxidation mechanism is proposed in this study. With the addition of H₂, the ignition time is advanced and the peak cylinder pressure is increased. The addition of hydrogen has a greater effect on the beginning stage of combustion than the later stages of combustion. The CH₄ emission is reduced with the addition of H₂. However, as the flame does not propagate throughout the charge, the CH₄ emission is still high. The CO emission is reduced and most of the remaining CO is produced by the combustion of the premixed CH₄. With the addition of hydrogen, NO emission is increased. The simulation shows that the final NOx emissions are significantly determined by the injection strategy and quantity of the pilot fuel during dual fuel operation conditions.     

Effect of tensile stress on the hydrogen adsorption of X70 pipeline steel

The effect of stress on the cathodic hydrogen evolution behavior of X70 pipeline steel was investigated by electrochemical tests, tensile tests, and microstructural characterization. The results indicated that the tensile stress enhanced the activity of hydrogen adsorption sites on the metal surface, which was considered as the dominating factor a?ecting generation, adsorption, and permeation of hydrogen atoms. The subsurface hydrogen atom concentrations quantified by Cyclic voltammetry (CV) tests and the data calculated by hydrogen permeation experiments showed a good correspondence. The results indicated that the tensile stress enhanced the adsorption of hydrogen atoms on the surface and an inhibitory effect on the Tafel and Heyrovsky reaction, thereby leading to the increase of the subsurface hydrogen atom concentration, enhance the hydrogen embrittlement susceptibility of the X70 steel material as demonstrated by plasticity loss in the tensile tests.     

Effect of the loading mode and temperature on hydrogen embrittlement behavior of 15Cr for steam turbine last stage blade steel

The hydrogen embrittlement of 15Cr martensitic stainless steel, for steam turbine last stage blades, was systematically studied by using slow strain rate tensile (SSRT) test and constant loading tensile (CLT) test at room temperature and 80 °C to simulate the service conditions. It was shown that, despite the lower hydrogen concentration absorbed during SSRT, the hydrogen-induced fracture strength of 15Cr steel for SSRT was lower than the threshold fracture strength for CLT. This was due to the remarkable enhancement in local hydrogen concentration due to the transportation of hydrogen by mobile dislocation during SSRT. In addition, although the higher hydrogen concentration was absorbed during SSRT at 80 °C, the hydrogen embrittlement susceptibility of 15Cr steel for SSRT at 80 °C was lower than that at room temperature, because the degree of local hydrogen accumulation decreased at a higher temperature.     

Effect of low partial hydrogen in a mixture with methane on the mechanical properties of X70 pipeline steel

In this study, the effect of a low partial hydrogen in a mixture with natural gas on the tensile, notched tensile properties, and fracture toughness of pipeline steel X70 is investigated. An artificial HE aging is simulated by exposing the tested sample to the mixture gas condition for 720 h. In addition, a series of tests is conducted in ambient air and 10 MPa of 100% He and H₂. Overall, 10 MPa of 100% H₂ significantly degrades the mechanical properties of an X70 pipeline steel. However, it is observed that the 10 MPa gas mixture with 1% H₂ does not affect the mechanical properties when tested with a smooth tensile specimen. In the notched tensile test, a significant reduction in loss in the area is observed when tested with a notched specimen with a notch radius of 0.083 mm. It is also confirmed that a 10-MPa gas mixture with 1% H₂ causes a remarkable reduction in the toughness. The influence of the exposure time to 1% hydrogen in a mixture with natural gas was found to be minor.     

Effects of stress concentration on the mechanical properties of X70 in high-pressure hydrogen-containing gas mixtures

This study aims to investigate the mechanical properties of X70 pipeline steel under the synergistic influence of hydrogen and stress concentration. Slow strain rate tensile tests and low-cycle fatigue tests were performed on the specimens with different stress concentration factors (Kt) in 10 MPa nitrogen/hydrogen mixtures. Results show that the degradation degree of the ductility and fatigue life of X70 steel induced by hydrogen increases with the increase of Kt, and as the hydrogen partial pressure in mixtures increases, the influence of Kt on hydrogen-induced degradation increases as well. In addition, finite element analysis was performed via a modified hydrogen diffusion/plasticity coupled model to study the effect of Kt on hydrogen distribution in the specimens, which can influence the mechanical properties of X70. The maximum hydrogen concentration consistently appears at the notch tip of the specimen and increases with the increase of Kt, which is proposed to be one of the reasons for the severe hydrogen embrittlement of the specimens with large Kt. As the axial tensile force on the specimen increases, the maximum hydrogen concentration at the notch tip begins to be dominated by hydrogen in the normal interstitial lattice sites and, subsequently, in the trapping sites.