Pulverization mechanism of hydrogen storage alloys on microscale packing structure

In-situ and transient visualizations of the packing structure of a hydrogen storage alloy bed are carried out using an X-ray computed tomography (CT) system. The packing structure is clearly observed on the microscale using the CT system. When the alloy bed is subjected to hydrogen absorption–desorption cycles, the pulverization progresses from the lower to the upper regions of the bed. After several hydrogen absorption–desorption cycles, the packing structure in the lower region of the bed changes and the microstructural void decreases slightly. Based on these results, we propose a pulverization mechanism of the packed bed in which the friction between particles affects the pulverization process.     

A study on crystal structure and chemical state of TiCrVMn hydrogen storage alloys during hydrogen absorption-desorption cycling

The evolution of crystal structure and chemical state of the V-based hydrogen storage alloy (Ti_0.32Cr_0.46V_0.22)₉₆Mn₄ during hydrogen absorption/desorption cycling was examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Reasons for the degradation in cycling capacity of the alloy are presented and discussed. One reason is the continuous reduction of the V-based cell volume during cycling, which cannot hold further hydrogen atoms. The decrease in cycling capacity can also be attributed to the oxidation of Ti, V, and Cr elements during cycling.     

Compatibility of materials with hydrogen. Particular case: Hydrogen embrittlement of titanium alloys

A review of the effect of hydrogen on materials is addressed in this paper. General aspects of the interaction of hydrogen and materials, hydrogen embrittlement, low temperature effects, material suitability for hydrogen service and materials testing are the main subjects considered in the first part of the paper. As a particular case of the effect of hydrogen in materials, the hydride formation of titanium alloys is considered. Hydrogen absorption and the possible associated problems must be taken into account when considering titanium as a candidate material for high responsibility applications. The sensitivity of three different titanium alloys to the Hydrogen Assisted Stress Cracking phenomena has been studied by means of the Slow Strain Rate Technique (SSRT). The testing media have been sea water and hydrogen has been produced on the specimen surface during the test by cathodic polarization. Tested specimens have been characterized by metallography and scanning electron microscopy. Results obtained show that the microstructure of the materials, particularly the β-phase content, plays an important role on the sensitivity of the studied alloys to the Hydrogen Assisted Stress Cracking Phenomena.     

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.     

Atomistic simulations of hydrogen embrittlement

It is well known that hydrogen weakens strengths of metals, and this phenomenon is called hydrogen embrittlement. Despite the extensive investigation concerning hydrogen related fractures, the mechanism has not been enough clarified yet. In this study, we applied the molecular dynamics method to the mode I crack growth in α-Fe single crystals with and without hydrogen, and analyzed the hydrogen effects from atomistic viewpoints. We estimated the hydrogen trap energy in the vicinity of an edge dislocation in order to clarify the distribution of hydrogen atoms, using the molecular statics method. We also evaluated the energy barrier for dislocation motion under a low hydrogen concentration. Based on these results, we propose a mechanism for hydrogen embrittlement of α-Fe under monotonic loading.     

Crystal structure and hydrogen storage properties of Ti-V-Mn alloys

The compositions of TiMn _{(100-x, Ti/Mn=5/8)}V_x (x = 25, 30, 35, 40, 45 and 50) alloys have been investigated comprehensively for their microstructure and hydrogen absorption/desorption properties. The proportion of BCC and C14 Laves phases changes with the V content, and BCC phase increases with increasing V content. With increasing BCC phase, more number of cycles are needed to reach to the saturated hydrogen absorption, and the hydrogen storage capacity first increases and then decreases after 40 at.% of the V content. It is indicated that the brittle C14 Laves phase plays as the “path” for hydrogen atom diffusion into the BCC phase. For the samples of V₄₅Ti₂₁Mn₃₄ and V₅₀Ti₁₉Mn₃₁ with less content of C14 Laves phase, it is difficult for hydrogen to diffuse into the BCC phase leading to low absorption capacity. The results of XRD and DSC analyses show that hydrides are less stable in V-poor samples. V₄₀Ti₂₃Mn₃₇ has the best hydrogen storage properties in this study: Its maximum hydrogen absorption capacity is 3.5 wt% at 293 K, dissociation enthalpy is 34.88 kJ/mol H₂, and desorption plateau platform is 0.05 Mpa at 303 K.     

Hydrogen transport and embrittlement in structural metals

The close relationship between hydrogen transport and embrittlement is indicated by evidence of hydrogen absorption preceding degradation of mechanical properties. Concentration of hydrogen at a crack or flaw by diffusion or by transport with moving dislocations is probably necessary also. Experimental studies show that hydrogen permeation is significantly influenced by surface conditions, particularly oxide films, and internal defects and impurities that trap diffusing hydrogen. The usual thermodynamic and diffusion relations, therefore, do not predict accurately the final distribution of hydrogen and the kinetics of the processes.Investigation of the effects of hydrogen on the mechanical properties of approximately fifty structural alloys at ambient temperature and pressures up to 69 MPa indicates that all alloys show evidence of susceptibility to hydrogen embrittlement. The degree of hydrogen embrittlement appears to be related to the amount of hydrogen absorbed and its distribution within the metal lattice. Surface condition, defect structure, hydrogen purity, and hydrogen pressure influence embrittlement. Of major importance is the transport of hydrogen with moving dislocations, a mechanism for concentration and redistribution of hydrogen that is operative at temperatures lower than for diffusion.     

Composite structure and hydrogen storage properties in Mg-base alloys

In order to improve the hydrogen absorption/desorption kinetic properties of Mg and Mg–Ni alloys, composite hydrogen storage alloys in the form of powder and film have been synthesized and investigated. For fabricating the composite powder, Mg or Mg–Ni powder was mechanically alloyed with _MmNi3.5(CoAlMn_)1.5${\mathrm{MmNi}}_{3.5}(\mathrm{CoAlMn}{)}_{1.5}$ alloy. For the preparation of the film with a composite structure, evaporation deposition and magnetron sputtering methods have been used to fabricate Mg–Ni film with multi-phase structure and Mg/Mm–Ni and Mg–Ni/Mm–Ni multi-layer film. By controlling the fabrication process, the microstructure feature, such as phase constituent, grain size, interlayer distances, interlayer boundary structure, of the composite can be modified. To reveal the influence of the composite structure on the hydrogen absorption/desorption kinetic properties of Mg and Mg–Ni alloys, hydrogen storage properties of the composite were measured with their microstructure features varied systematically. The present work shows that the hydrogen sorption properties of Mg and Mg–Ni-based alloys can be substantially improved by forming composites having proper microstructure features.     

Origin of hydrogen embrittlement in vanadium-based hydrogen separation membranes

Hydrogen embrittlement in metals is a challenging technical issue in the proper use of hydrogen energy. Despite extensive investigations, the underlying mechanism has not been clearly understood. Using atomistic simulations, we focused on the hydrogen embrittlement in vanadium-based hydrogen separation membrane. We found that, contrary to the conventional reasoning for the embrittlement of vanadium, the hydrogen-enhanced localized plasticity (HELP) mechanism is the most promising mechanism. Hydrogen enhances the nucleation of dislocations near the crack tip, which leads to the localized plasticity, and eventually enhances the void nucleation that leads to the failure. Those results provide an insight into the complex atomic scale process of hydrogen embrittlement in vanadium and also help us design a new alloy for hydrogen separation membranes.     

Role of solute atoms and vacancy in hydrogen embrittlement mechanism of aluminum: A first-principles study

Hydrogen trapping performances of Al with solute atoms X (X = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn) and X-vacancy defects are investigated using First-principles method. For X-doped Al supercells, most structures show strong alloying ability. Among the solute atoms studied, Cr is the most useful element to trap H due to its lowest H trapping energy. For vacancy@X-doped Al supercells, the strong interactions of X-vacancy are explored. All vacancy@X-doped Al supercells are more favorable to capture H than X-doped Al supercells. In addition, both elastic and chemical interactions should comparably contribute to H-X or H-X-vacancy interactions in Al. Solute atoms and vacancy may regulate electron distribution of Al to enhance the ability of capturing H. Overall, our insights present the quantitative role of solute atoms and vacancy in H trapping for Al, and guide the design of new alloys with high resistance to hydrogen embrittlement.     

The effect of transition metals on hydrogen migration and catalysis in cast Mg-Ni alloys

The inexpensive fabrication technique of casting is applied to develop new Mg-Ni based hydrogen storage alloys with improved hydrogen sorption properties. A nanostructured eutectic Mg-Mg₂Ni is formed upon solidification which introduces a large area of interfaces along which hydrogen diffusion can occur with high diffusivity. After a few cycles of hydrogenation and dehydrogenation, an ultrafine porous structure formed in the eutectic Mg-Mg₂Ni and some cracks developed along the interface between the eutectic and the α-Mg matrix. This indicates that hydrogen atoms introduced into the alloys preferentially migrate along the interfaces in the nanostructured eutectic which enables effective short-range diffusion of hydrogen. Furthermore, transition metals (TMs) such as Nb, Ti and V in the range 240-560 ppm are added directly to molten Mg-10 wt% Ni alloys and are found to form intermetallic compounds with Ni during solidification. The alloys can store 5.6-6.3 wt% hydrogen at 350 °C and 2 MPa. TM-rich intermetallics distributed homogeneously in the cast alloys appear to play a key role in accelerating the nucleation of Mg from MgH₂ upon dehydrogenation. This leads to a significant improvement in the hydrogen desorption kinetics.     

Overview of hydrogen embrittlement in high-Mn steels

Hydrogen and fuels derived from it will serve as the energy carriers of the future. The associated rapidly growing demand for hydrogen energy-related infrastructure materials has stimulated multiple engineering and scientific studies on the hydrogen embrittlement resistance of various groups of high performance alloys. Among these, high-Mn steels have received special attention owing to their excellent strength – ductility – cost relationship. However, hydrogen-induced delayed fracture has been reported to occur in deep-drawn cup specimens of some of these alloys. Driven by this challenge we present here an overview of the hydrogen embrittlement research carried out on high-Mn steels. The hydrogen embrittlement susceptibility of high-Mn steels is particularly sensitive to their chemical composition since the various alloying elements simultaneously affect the material's stacking fault energy, phase stability, hydrogen uptake behavior, surface oxide scales and interstitial diffusivity, all of which affect the hydrogen embrittlement susceptibility. Here, we discuss the contribution of each of these factors to the hydrogen embrittlement susceptibility of these steels and discuss pathways how certain embrittlement mechanisms can be hampered or even inhibited. Examples of positive effects of hydrogen on the tensile ductility are also introduced.     

Sensitivity of pipelines with steel API X52 to hydrogen embrittlement

The following cases of hydrogen influence on pipeline metal were considered: gaseous hydrogen under internal pressure in notched pipes and electrochemically generated hydrogen on external pipe surface from soil aqueous environment. The burst tests of externally notched pipes under pressure of hydrogen and natural gas (methane) were carried out after the pipe has been exposed to a constant “holding” pressure. It has been shown that even for relatively “soft” test conditions (holding pressure p = 20 bar and ambient temperature) the gaseous hydrogen is able to penetrate into near surface layers of metal and to change the mechanism of local fracture at notch. The sensitivity to hydrogenating of given steel in deoxygenated, near-neutral pH NS4 solution under soft cathodic polarisation was studied and the assessment local strength at notches in pipeline has been made for this conditions. Here, the relationship between hydrogen concentration and failure loading has been found. The existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was also shown.     

Correlation study between hydrogen absorption property and lattice structure of Mg-based BCC alloys

Nanostructured Mg₆₀Ni₅Co_mX_35 − m (X = Co, B, Al, Cr, V, Pd and Cu) body centered cubic (BCC) alloys were synthesized by mechanical alloying method. These Mg-based alloys with different lattice parameters can show significantly different hydrogen absorption properties. The BCC alloys with lattice parameter in the range of 0.300∼0.308 nm absorb large amount of hydrogen at 373 K and the BCC alloys with the parameter larger than 0.313 nm have difficulty to absorb hydrogen at this temperature. Geometric effect is thought to be one of the dominant factors to affect the hydrogen absorption property of interstitial alloys. Nanostructure, fresh surface area and defects produced during mechanical alloying process are also important facts that make Mg-based alloys absorb hydrogen at 373 K.     

Recovery of hydrogen from hydrogen sulfide with metals or metal sulfides

The following two types of reactions were investigated for the recovery of hydrogen from hydrogen sulfide: Type 1 H₂S → H₂ + S⁰, Type 2 H₂S + O₂ → H₂ + SO₂ Each type of reaction is constructed by a two-step cycle, in which H₂S is reacted with metal or metal sulfide and then the resulting sulfide undergoes thermal decomposition or oxidation. Ag₂S, FeS, Co₉S₈, Ni₃S₂, and the double sulfide CuFeS₂ were examined in the former type of reaction, while Ag, Cu, Ni, liquid Pb, and liquid BiAg alloy were used as an intermediate in the latter.     

Investigation on hydrogen production using multicomponent aluminum alloys at mild conditions and its mechanism

A series of Al alloys with low melting point metals Ga, In, Sn as alloy elements were fabricated using mechanical alloying method. The phase compositions and morphologies of different Al alloys were characterized by XRD and SEM techniques. The reaction of the Al alloys with water for hydrogen evolving at mild conditions (at room temperature in neutral water) was studied. The results showed that there were no hydrogen yields for binary Al–Ga, Al–In, Al–Sn and the ternary Al–Ga–Sn alloys. The hydrogen yields were observed for Al–Ga–In and Al–In–Sn ternary alloys. The Al–In–Sn alloys showed an even faster hydrogen generation rate and higher yields than Al–Ga–In alloys. Based on the ternary Al–Ga–In and Al–In–Sn system, the hydrogen production property of quaternary Al–Ga–In–Sn was greatly improved. The hydrogen conversion efficiency of the optimized Al–3%Ga–3%In–5%Sn alloy was nearly 100% in tap water. The highest hydrogen generation rate reached 1560 mL/g min in distilled water or deionized water. It was suggested that both the embrittlement of Al by liquid Ga–In–Sn eutectic and the active points formed by intermetallic compounds In₃Sn and InSn₄ may be attributed to the high activity of Al–Ga–In–Sn alloys at room temperature.     

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.     

Microstructural aspects upon hydrogen environment embrittlement of various bcc steels

Several commercial bcc steels with various combinations of ferritic, pearlitic, bainitic and martensitic microstructures were tensile tested in gaseous hydrogen (10 MPa) at room temperature.     

Dependence of hydrogen embrittlement on hydrogen in the surface layer in type 304 stainless steel

Hydrogen embrittlement (HE) together with the hydrogen transport behavior in hydrogen-charged type 304 stainless steel was investigated by combined tension and outgassing experiments. The hydrogen release rate and HE of hydrogen-charged 304 specimens increase with the hydrogen pressure for hydrogen-charging (or hydrogen content) and almost no HE is observed below the hydrogen content of 8.5 mass ppm. Baking at 433 K for 48 h can eliminate HE of the hydrogen-charged 304 specimen, while removing the surface layer will restore HE, which indicates that hydrogen in the surface layer plays the primary role in HE. Scanning electron microscopy (SEM) and scanning tunnel microscopy (STM) observations show that particles attributed to the strain-induced α′ martensite formation break away from the matrix and the small holes form during deformation on the specimen surface. With increasing strain, the connection among small holes along {111} slip planes of austenite will cause crack initiation on the surface, and then the hydrogen induced crack propagates from the surface to interior.