Hydrogen Trapping Sites Research Articles

Measurement of hydrogen trapping in cold-work dislocations using synchrotron X-ray diffraction

In water-cooled nuclear power plants containing zirconium alloys as the clad material, embrittlement of the cladding can occur from hydrides produced by the water-zirconium alloy interaction, thus limiting the life of the fuel assembly (Motta et al., 2019). The hydrogen produced at the water-zirconium interface can also be trapped in neutron irradiation-induced microstructural defects, reducing the amount of hydride present at any given time/temperature. Knowledge of the extent of hydrogen trapping by various microstructural features in zirconium and its alloys is therefore key for accurate predictions of hydride-induced embrittlement. A novel method for quantifying both trap capacity and trap binding energy using synchrotron X-ray diffraction is described and demonstrated for a cold-worked, Zircaloy-4 sample in which line dislocations provide an analogue for irradiation-induced dislocation loops as hydrogen trap sites. This study has experimentally determined the following parameters for cold-work dislocations: Thermal stability; Binding energy; Maximum capacity for hydrogen; Number of equivalent hydrogen trap sites. These parameters may be used in modelling hydrogen embrittlement of zirconium alloys, as experimental estimates for their analogue of irradiation-induced dislocation loops. More importantly, this pilot study has successfully demonstrated the methodology and results of a novel technique that may be applied generally to irradiated material to quantify hydrogen trapping at other irradiation-induced microstructural defects.

The role of delta ferrite in hydrogen embrittlement fracture of 17-4 PH stainless steel

The role of δ-Fe in hydrogen embrittlement (HE) of 17-4 PH steel is studied in this work. Scanning Kelvin probe force microscopy result indicates that δ-Fe is a hydrogen trapping site. Accordingly, δ-Fe can reduce the hydrogen concentration of surrounding martensite and prior austenite grain boundaries (PAGBs) and imped the brittle fracture along lath boundaries and PAGBs, which can be beneficial to the HE resistance improvement. However, a cleavage fracture of δ-Fe can occur under the synergetic action of hydrogen-enhanced localized plasticity (HELP) and hydrogen enhancement of the strain-induced generation of vacancies (HESIV). These findings indicate a new path to improving HE resistance of high strength martensitic steels.

Visualization of Hydrogen-trapping Sites in Steels by Atom Probe Tomography

Visualization of Hydrogen-trapping Sites in Steels by Atom Probe Tomography

Study on Corrosion Resistance and Hydrogen Permeation Behavior in Inter-Critically Reheated Coarse-Grained Heat-Affected Zone of X80 Pipeline Steel

We studied the effects of peak temperature and cooling rate in the secondary welding thermal cycles on the martensite/austenite (M/A) constituents’ characteristics (including fraction, average size and distribution), corrosion resistance and hydrogen permeation behaviors in the inter-critically reheated coarse-grained heat-affected zone (ICCGHAZ) of X80 pipeline steel. We observed that the M/A constituents’ characteristics mainly depend on the secondary peak temperature and cooling rates, while the microstructure style and prior austenite grain size are dependent on the first peak temperature. In addition, the variations in the M/A constituents’ characteristics result in different corrosion resistance and hydrogen permeation behaviors by changing the micro-galvanic effect and the number of hydrogen trapping sites. The high fraction and coarse grain size of M/A constituents are against the corrosion resistance and hydrogen permeation in ICCGHAZs, and their functional relationships are established. Moreover, the effects of the fraction of M/A constituents on the corrosion resistance and hydrogen permeation behaviors are much greater than those of the average size.

Preferential Locations of Hydrogen Accumulation and Damage in 1.2 GPa and 1.8 GPa Grade Hot-Stamped Steels: A Comparative Study

In this work, hydrogen segregation and damage sites in 1.2 GPa and 1.8 GPa grade hot-stamped steels were comparatively investigated by hydrogen permeation experiments and the hydrogen microprint technique (HMT). Compared with 1.2 GPa steel, 1.8 GPa steel exhibited a lower hydrogen diffusion coefficient (Deff) and a higher number of hydrogen trapping sites (Nt) due to its finer microstructure and richer nano-sized precipitates. The results of HMT showed that the grain boundaries in both steels played a role in initial hydrogen segregation, and then the martensitic laths became the locations of hydrogen accumulation. For 1.2 GPa and 1.8 GPa steels, however, hydrogen accumulation appeared preferentially on martensitic laths and grain boundaries, respectively, resulting in various damage behaviors. The introduced nano-sized carbides as “good hydrogen traps” played an important role in hydrogen diffusion, accumulation, and damage, which greatly alleviated hydrogen-induced cracking for the 1.8 GPa steel. Moreover, electron backscatter diffraction (EBSD) analysis further revealed that the damage behavior was also controlled by the low-angle grain boundary, stress distribution, and recrystallization fraction of the samples.

Hydrogen trapping and desorption affected by ferrite grain boundary types in shielded metal and flux-cored arc weldments with Ni addition

Hydrogen trapping behavior and diffusion induced by the microstructure of shielded metal and flux-cored arc weldments (SMAW and FCAW) were characterized using a combination of high-resolution microstructural characterization methods, hydrogen trap site studies, and a modeling technique. H trapping by HAGBs that was found by TDS was confirmed by NanoSIMS with a cryogenic stage. Cellular automaton modeling results showed that in grain sizes smaller than a critical grain size, the hydrogen diffusion coefficient decreases with decreasing grain size, indicating that H trapping dominates short-circuit diffusion mechanism along high-angle grain boundaries (HAGBs). These results firstly show that smaller grain size and high HAGB density in the FCAW specimen results in a lower H diffusion coefficient and higher density of relatively strong HAGB traps, and a lower total desorbed hydrogen content in the FCAW specimen. Also, it was suggested that the fraction of acicular ferrite grains can define the HAGB content in the alloy, and can be a determinant factor in the behavior of weldments in H-containing media.

Comparison of Hydrogen Embrittlement Susceptibility of Different Types of Advanced High-Strength Steels.

This study investigated the hydrogen embrittlement (HE) characteristics of advanced high-strength steels (AHSSs). Two different types of AHSSs with a tensile strength of 1.2 GPa were investigated. Slow strain rate tests (SSRTs) were performed under various applied potentials (Eapp) to identify the mechanism with the greatest effect on the embrittlement of the specimens. The SSRT results revealed that, as the Eapp increased, the elongation tended to increase, even when a potential exceeding the corrosion potential was applied. Both types of AHSSs exhibited embrittled fracture behavior that was dominated by HE. The fractured SSRT specimens were subjected to a thermal desorption spectroscopy analysis, revealing that diffusible hydrogen was trapped mainly at the grain boundaries and dislocations (i.e., reversible hydrogen-trapping sites). The micro-analysis results revealed that the poor HE resistance of the specimens was attributed to the more reversible hydrogen-trapping sites.

Hydrogen diffusion and trapping in nickel-based alloy 625: An electrochemical permeation study

This work studied the effect of cathodic charging conditions on hydrogen diffusion and trapping behavior in a nickel-based Alloy 625 using the electrochemical permeation approach. Results showed that the effective diffusion coefficient of hydrogen can be expressed as Deff=1.82×10−7exp(−44.46kJmolRT)m2/s. A sinusoidal trend in Deff with charging current density was observed showing a deflection point at -40 mA/cm2, which contributes to the most efficient hydrogen permeation. In addition, the thermodynamic relationship between hydrogen activity and electrochemical potential was investigated. Furthermore, the energy associated with the solubility energy of interstitial hydrogen and reversible trapping site was determined as 9.90 kJ/mol and 26.04 kJ/mol, respectively.

Hydrogen trapping by irradiation-induced defects in 316 Lstainless steel: A combined experimental and modeling study

The irradiation-induced defects in stainless steel internal components of pressurized water reactors combined with hydrogen uptake during the oxidation process could be a key parameter in the mechanism for Irradiation-Assisted Stress Corrosion Cracking (IASCC). A heat-treated 316 L SS containing a low amount of defects was Fe3+ ions-implanted; irradiation-induced defects types and depth distributions were characterized by Transmission electron microscopy (TEM). Deuterium was then inserted in the specimens either by cathodic charging or by sample exposure to deuterated primary water. Secondary Ions Mass Spectrometry – SIMS – permitted to access the deuterium distribution at the implanted stainless steel surface. A finite difference numerical solver accounting for hydrogen diffusion/trapping coupling was used to simulate the hydrogen transport in the implanted material, taken into consideration the specific heterogeneous character of the irradiation-induced defects distribution in matter. Taking as input data the experimental defects distribution associated with Frank loops or voids, the main trapping sites for hydrogen were assigned to voids, not Frank loops. Such numerical approach was able to deal accurately with the problem of hydrogen transport in a heterogeneous material as well as to differentiate two potential trap sites contributions to the experimental deuterium distribution. In addition, according to results obtained after primary water exposure, trapping at voids was still effective at 320 °C, signature of a high binding energy of hydrogen in voids.

Investigation of the effect of carbon on the reversible hydrogen trapping behavior in lab-cast martensitic Fe C steels

The present study evaluates the active hydrogen trapping sites of three martensitic Fe C alloys with a carbon content of 0.2 wt%, 0.4 wt% and 1.1 wt% by thermal desorption spectroscopy (TDS). The absence of additional alloying elements reduces the microstructural complexity and allows focussing on the carbon effects only. The TDS spectra are extrapolated towards cryogenic temperatures, enabling to deconvolute the desorption spectrum in Gaussian curves corresponding with H detrapping from lattice positions, dislocations, high angle grain boundaries and cementite. The activation energy for hydrogen desorption and the amount of H trapped at each site is further profoundly evaluated. It is found that the carbon content controls the amount of hydrogen trapped at dislocations and its activation energy for detrapping decreases with increasing carbon content. The trap density of the high angle grain boundaries is controlled only by the prior austenitic grain size and the corresponding activation energy for H desorption is independent of the carbon content. Hydrogen trapping at cementite was only detected in the samples with the highest carbon content (Fe-1.1C). • Interaction between hydrogen and martensitic Fe C steels is investigated by TDS. • The main hydrogen trapping sites are dislocations and high angle grain boundaries. • C content strongly influences the H trapping characteristics at dislocations. • Amount of H trapped at HAGBs merely depends on the prior austenitic grain size. • H trapping at cementite is only relevant for hypereutectoid compositions.

高強度鋼板中の水素拡散挙動に及ぼす張出し加工の影響

The hydrogen diffusion behavior in a tempered martensitic steel sheet with 1-GPa grade tensile strength was investigated utilizing a newly developed hydrogen visualization technique using an Ir complex, whose color changes from yellow to orange due to its reaction with hydrogen. The hydrogen permeation through the steel sheet could be monitored from the color change of the Ir complex. Furthermore, the breakthrough time of hydrogen through the specimen could be qualitatively evaluated from the changes in the brightness of the Ir complex. This hydrogen visualization technique was also applied to a steel sheet stretch-formed using a hemisphere punch to simulate press-forming of automotive structural parts. The hydrogen breakthrough time around the top of the specimen was long and decreased with increasing the distance from the top. According to the plastic strain distribution of the specimen calculated by the finite element method, the hydrogen breakthrough time increased with increasing plastic strain. It was considered that the introduction of plastic strain decreased the hydrogen diffusion coefficient due to the introduction of dislocations acting as hydrogen trap sites, resulting in the increase of hydrogen breakthrough time.

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.

Effect of grain orientation and interface coherency on the hydrogen trapping ability of TiC precipitates in a ferritic steel

The way of capturing hydrogen has always been the focus of research in steels. Through the in-situ SKPFM study, a great Volta potential difference has been found between the TiC precipitates (∼0.82 μm) and the matrix after hydrogen charging, especially at the phase interface, which is demonstrated as an effective hydrogen trapping site. Moreover, the TiC carbides precipitated in the ferrite (111) plane show a higher average potential increase than that in the (110) plane. The difference of hydrogen trapping ability can be attributed to the deviation of Baker-Nutting orientation relationship of the precipitate/matrix interface, since the interface coherency on the (111) plane is destroyed more severely due to a lower Schmid factor of (111) orientation during the coiling process.

Critical verification of the Kissinger theory to evaluate thermal desorption spectra

Multiple types of hydrogen trapping sites in advanced high-strength steels (AHSS) are often experimentally characterized by means of thermal desorption spectroscopy (TDS). The evaluation is regularly based on the peak deconvolution procedure combined with Kissinger's theory, which provides distinctive desorption energies of hydrogen trapping sites at microstructural defects. However, the desorption energies published in literature are often non-conclusive and from time to time contradictive in nature. Therefore, it is of utmost importance to verify the evaluation procedures according to Kissinger's theory for multiple types of hydrogen trapping sites. For that purpose, theoretical TDS spectra were simulated using a bulk diffusion model according to Oriani's theory. Binding energies and trap densities were chosen for providing TDS spectra with clearly separated as well as overlapping TDS peaks. Finally, the desorption energies according to Kissinger's theory were compared with the theoretical trapping energies used in the models. Based on this theoretical work, it is strongly recommended to apply the Kissinger theory only for the evaluation of single or well separated TDS peaks. If peaks overlap, complementary microstructural variation and characterization are a perquisite to correctly evaluate the TDS spectra.

Hydrogen-assisted failure in Inconel 718 fabricated by laser powder bed fusion: The role of solidification substructure in the embrittlement

The influence of hydrogen on the mechanical behavior of Inconel 718 fabricated by laser powder-bed-fusion was investigated through a series of tensile experiments. Samples subjected to two different post-fabrication heat treatments, viz. direct aging (DA) and homogenization plus aging (HA), were tested. Detailed microstructural characterization showed that a solidification substructure including a high density of dislocations and precipitates prevails in the DA sample while the HA sample is free from such a substructure. The DA sample exhibited a comparatively higher strength, but a lower resistance to hydrogen embrittlement. By recourse to a statistical analysis of the hydrogen-assisted cracks, the severe hydrogen embrittlement of the DA sample was proven to be due to the significant portion of hydrogen-assisted intergranular cracks that occurred without the aid of slip localization. These results are discussed in terms of the changes in microstructure upon heat treatments, and their influences on the hydrogen trapping sites.

Influence of microstructure-driven hydrogen distribution on environmental hydrogen embrittlement of an Al–Cu–Mg alloy

The hydrogen trap sites and corresponding hydrogen binding energies in an Al–Cu–Mg alloy with the different microstructures were investigated to unravel the environmental hydrogen embrittlement (HE) behavior of the alloy. The results showed that hydrogen can reside at interstitial lattices, dislocations, S′-phase, and vacancies. In the aged specimen with the highest hydrogen content, it was firstly reported that hydrogen resided at S′-phase particles with relatively high binding energy, which is a determinant factor on HE resistance of the alloy. In the cold-rolled specimen, high content of hydrogen trapped at dislocations with a reversible nature leads to intergranular hydrogen-assisted cracking. In the solution-treated specimen, hydrogen migration to the surface due to low trap density results in low hydrogen content and prevents the GBs from reaching critical hydrogen concentration. The obtained results clearly reveal that trap site density, and the nature of trap sites can determine environmental HE susceptibility of the alloy.

Effect of microstructure on hydrogen embrittlement susceptibility in quenching-partitioning-tempering steel

Hydrogen embrittlement (HE) restricts the application of high strength steel in sustainable energy productions. As one type of efficient hydrogen trap sites, the NbC carbide precipitation is a superior approach to mitigate the HE susceptibility. The microstructure, mechanical properties and HE susceptibility were investigated in different high strength steels treated by quenching and partitioning (Q&P), quenching-partitioning-tempering (Q-P-T) and intercritical annealing quenching and partitioning (IAQP) processes in this study. The results show that the NbC particles can significantly improve the HE resistance of high strength steel treated by Q-P-T process. The NbC carbide precipitation has four different morphologies with varied sizes, which can effectively trap a large amount of diffusible hydrogen atoms. The retained austenite phases with different morphologies and locations have different hydrogen trap ability, but ferrite phase does not show strong hydrogen trap ability. Meanwhile, the NbC carbide precipitation can enhance the yield strength of steel effectively through precipitation strengthening, but has little effect on its ductility.

Atomic-scale observation of hydrogen trap sites in bainite–austenite dual-phase steel by APT

It was reported by the thermal desorption analysis (TDA) of hydrogen that bainite–austenite dual-phase steels showed two activation energies of ~30 kJ/mol and ~ 47 kJ/mol under low- and high-fugacity conditions, respectively, and the latter energy corresponded to hydrogen diffusion in austenite; however, the hydrogen trap sites of the former energy were unclear (Sekine et al., 2019, 2018 [ 1 , 2 ]). The hydrogen trap sites under the low-fugacity condition in the steel were investigated by directly observing deuterium-charged specimens using atom probe tomography (APT). The charged deuterium atoms were distributed in carbon-rich regions of the fine irregular structure in the bainite region of the steel. The carbon concentrations in these region were in the range of 12–20 at.% without enrichment of other alloying elements. The diffraction analysis with transmission electron microscopy (TEM) indicated the presence of a substantial amount of fine precipitates of epsilon carbide in the bainite region of the steel. Hence, the hydrogen trap sites were considered to be associated with fine precipitates of epsilon carbide (ε-carbide) with carbon compositions marginally lower than the stoichiometric composition. First principles calculations suggested that the hydrogen trap sites in the steel originated from the carbon vacancies in the surface region of ε-carbide. • Hydrogen trap sites in bainite–austenite dual-phase steel were investigated by APT. • Charged deuterium atoms were observed in carbon-enriched regions in the bainite. • TEM analysis indicated the presence of a large amount of fine ε-carbide precipitates. • The trap energies in ε-carbide were investigated by first principles calculation. • Carbon vacancies in the surface region of ε-carbide are probably the trap sites.

Ultrasonic nanocrystal surface modification for strength improvement and suppression of hydrogen permeation in multi-layered steel

Hydrogen embrittlement of multi-layered steel (MLS) is significantly affected by a degree of hydrogen permeation from surface layers to interfaces. Here, the surface microstructure of multi-layered steel was modified for the first time using ultrasonic nanocrystal surface modification (UNSM), and the roles of UNSM-treated surface layers in strength and hydrogen permeation behavior were investigated by examining microstructural evolutions of the surface layer. Since the UNSM induced the compressive residual stress, grain refinement, and deformation twin formation at the specimen surface, the yield strength greatly improved via synergetic contributions of the grain refinement effect and dislocation strengthening. In addition, the UNSM-affected zone of 150–210 µm along depth direction effectively suppressed the hydrogen permeation by supplying compressive residual stresses and hydrogen trapping sites including grain boundaries, dislocations, and twin boundaries.

Vanadium or copper alloyed duplex lightweight steelwith enhanced hydrogen embrittlement resistance at room temperature

Hydrogen embrittlement is a phenomenon that causes the deterioration of mechanical properties of steel, such as ductility, owing to hydrogen present inside the material and is a crucial factor that determines the reliability of structural materials. Hydrogen embrittlement is controlled by two kinetics—trapping and diffusion of hydrogen present in the material—and these are affected by microstructural factors such as defects and phases. In this study, microstructures of medium-Mn duplex lightweight steel (Fe-0.5C–12Mn–7Al) and lightweight steel containing 0.1 wt% V and 1 wt% Cu were analyzed, and slow strain rate tensile tests were performed after electrochemical hydrogen charging. The correlation between microstructural evolution and hydrogen embrittlement was analyzed by measuring the amount of hydrogen inside the steel through thermal desorption analysis. Resistance to hydrogen embrittlement was improved in the decreasing order of Cu addition, V addition, and no addition of steel. When V is added, the V-rich carbide acts as an effective hydrogen trapping site, improving the resistance to hydrogen embrittlement despite the majority of hydrogen being inside the specimen. When Cu is added, the B2 particles trap hydrogen in a similar way as done by V-rich carbide, and the solute segregation along the boundaries interferes with hydrogen diffusion. Because Cu is an austenite stabilizer, the fraction of FCC, which is a close packed structure, is high. Thus, the amount of hydrogen entering the specimen is the least under Cu addition, and the corresponding steel shows the best resistance to hydrogen embrittlement. Microstructural factors have a significant effect on hydrogen embrittlement, and by adding V and Cu, it is possible to develop a medium-Mn duplex lightweight steel that can accommodate sufficient deformation even when exposed to a hydrogen atmosphere.