Hydrogen isotopic effect during the graphite high-temperature corrosion in water vapours

This paper presents the results on a study the processes of physicochemical interactions of water with graphite. The main regularities of the formation of H₂, HD and D₂ molecules on the graphite surface were determined. It was shown that the fraction of D₂ and HD in the gaseous outcome increases in the process of heating, and the quasi-equilibrium state of the graphite's absorption of hydrogen isotopes at the initial stages of interaction is significant: the flow of dissolved atoms into the sample volume is higher than the desorption flow. We suppose that this is due to the higher rate of dissolution of hydrogen atoms in the volume of graphite. We also estimated also the separation factor for the graphite surface-volume system for hydrogen atoms, which was 1.53 for the selected experimental conditions. The temperature dependence of the effective rate constant K_s for the formation of hydrogen isotope molecules in the interaction of graphite with water vapour in the range of 1100 °C–1200 °C was determined. It turned out that K_S(D₂) > K_S(HD) > K_S(H₂).     

Effects of hydrogen on fatigue crack growth behavior of austenitic stainless steels

The effect of hydrogen on fatigue crack growth behavior of three stainless steels has been investigated from the viewpoint of microscopic fatigue mechanisms, martensitic transformation and hydrogen content. Fatigue crack growth rates in the hydrogen-charged SUS304 and SUS316 were accelerated with respect to crack growth rates in uncharged specimens. The crack growth rate in the hydrogen-charged SUS316L was only slightly higher than that in the uncharged SUS316L. Martensitic transformation on the fatigue fracture surfaces was detected using X-ray diffraction both in the hydrogen-charged and uncharged specimens of SUS304, SUS316 and SUS316L. Materials with increased tendency for martensitic transformation also showed increased acceleration in fatigue crack growth rate due to hydrogen. It was concluded that martensitic transformation in the vicinity of the fatigue crack tip increased the local diffusion of hydrogen thus increasing crack growth rate.     

Evaluation of polymer electrolyte membrane fuel cells by electrochemical impedance spectroscopy under different operation conditions and corrosion

Electrochemical impedance spectroscopy (EIS) was employed for in situ diagnosis for polymer electrolyte membrane fuel cells during operation. First, EIS was measured as a function of operation parameters such as applied current density, gas flow rates and gas humidification temperature. The resistance that correlated with conductivity of the membrane and the contact resistance between bipolar plate and gas diffusion layer (GDL) was set as R_m in the assumed equivalent circuit. The charge transfer resistances were considered for cathode (R_ct(C)). The value of R_ct(C) was sensitive to the parameters that affected cell voltage. Additionally, the diffusion resistance (R_d) was ascribed to the effect of oxygen supply and drainage of generated water. Second, the influence of corrosion of type 430 stainless steel bipolar plates was evaluated by EIS method during operation. Corrosion of the stainless steel bipolar plates resulted in an increase in the value of R_d.     

Creep fatigue crack development in dissimilar metal welded joints between steels and nickel based alloy

Abstract

Creep and strain controlled cyclic/hold creep fatigue tests have been performed at temperatures in the range of 550–575°C on specimens extracted from dissimilar metal welded (DMW) joints between two classes of steel and a nickel based alloy. The details and results of the tests are described. While crack development in the cyclic/hold creep fatigue test specimens tends to be creep dominated, the microstructural paths followed in the steels in the vicinity of their heat affected zones are not identical to those observed in creep rupture testpieces taken from the same DMW joint. In pure creep tests, cracking may occur adjacent to the fusion line and/or in the fine grain heat affected zone (FGHAZ), with rupture location being dependent on temperature stress and microstructural condition. In contrast, creep dominated creep fatigue cracking typically occurs in the intercritical heat affected zone/FGHAZ or the overtempered parent material on the steel side of such weldments, depending on the composition of the joint.     

Effects of crack position on fatigue life of large seamless storage vessels made of 4130X for hydrogen refueling station

Fatigue property of CrMo steel is significantly degraded by hydrogen embrittlement (HE), which probably influences the fatigue life of hydrogen storage vessels made of CrMo steel. In this study, tests of fatigue crack growth rate (FCGR) for specimens extracted from the cylinder, juncture and shoulder of a seamless storage vessel made of 4130X for hydrogen refueling station were performed in 45 MPa hydrogen gas. Based on the test results, a finite element method (FEM) based on adaptive grid technique was applied to calculate fatigue life of the vessel with an initial crack at different positions. 3D laser scanning technology was used to build the model which can well conform to the actual vessel. Results indicate that the FCGR of 4130X in 45 MPa hydrogen is approximately 10–15 times of the FCGR in air. The FCGR of specimens from the shoulder is approximately 1.2–1.5 times of that from the cylinder and juncture. However, as the influence of stress distribution, the fatigue life of a vessel with a crack in the middle of the cylinder is lower than that of the vessel which exists crack with the same size at the juncture and shoulder.     

Hydrogen isotope effects on the structural phase transition of NH3BH3

A systematic study of the structural phase transition of NH₃BH₃ and of its fully deuterated analogue was performed combining DSC and anelastic spectroscopy measurements. The transition is accompanied by a latent heat, and therefore is of the 1st order. On the deuterated sample the enthalpy variation is reduced of more than 20%, from 1.29 to 1.01 kJ/mol and the transition is shifted by ∼1.5 K toward higher temperatures. Both NH₃BH₃ and ND₃BD₃ display a temperature hysteresis between cooling and heating, thus denoting that the phase transition is of first-order. In addition, this hysteresis is extremely small (∼0.5 K) indicating that the coexistence region between the two phases is very narrow. During isothermal ageing, the transformation of the low-temperature orthorhombic phase into the high-temperature tetragonal one occurs with a time constant of ∼16 min in NH₃BH₃ and ∼64 min in ND₃BD₃, evidencing a drastic slowing down of kinetics in the deuterated compound.     

Hydrogen production by aluminum corrosion in aqueous hydrochloric acid solution promoted by sodium molybdate dihydrate

Recycled aluminum alloys corrosion system was studied for hydrogen production in 1.4 M HCl solution at low temperature, 333 K. Molybdate ions were used as corrosion promoter. Commercial Na₂MoO₄·2H₂O was tested, varying its concentration (0–0.036 M) in order to analyze the promoter effect in H₂ production. Moreover, Na₂MoO₄·2H₂O particles were synthesized via sonochemical method and by ultrasonic method. Later, synthesized promoters were tested in order to analyze the effect of their physicochemical properties in H₂ production. Gas composition produced during the reaction was analyzed by gas chromatography. Results showed that reaction rate was notorious affected by molybdate ions concentration. The highest hydrogen volume and the greatest reaction rate obtained was with the use of 1050 aluminum foil alloy with very low promotor concentration (0.004 M), becoming, therefore, a low-cost process for high purity hydrogen production.     

Fatigue crack growth behavior of JIS SCM440 steel near fatigue threshold in 9-MPa hydrogen gas environment

In this study, stress intensity factor range (ΔK) decreasing tests were conducted and the in-situ observations were used to investigate the fatigue crack growth behavior of JIS SCM440 steel near the fatigue threshold in a 9-MPa hydrogen gas environment. The fatigue crack growth rate reflected the threshold behavior of the material, although the crack propagation knee point immediately before the threshold stress intensity factor range (ΔK_th) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before ΔK_th. In contrast, the knee points in a helium gas environment and air were very distinct. Fractographic analysis further revealed the existence of intergranular facets, which were observed immediately before ΔK_th in the hydrogen gas environment. Conversely, no facet was observed immediately before ΔK_th in the helium gas environment and air. The formation of the facets was considered to be one of the causes of the uneven crack propagation immediately before ΔK_th in the hydrogen gas environment.     

Hydrogen transport in solution-treated and pre-strained austenitic stainless steels and its role in hydrogen-enhanced fatigue crack growth

Hydrogen solubility and diffusion in Type 304, 316L and 310S austenitic stainless steels exposed to high-pressure hydrogen gas has been investigated. The effects of absorbed hydrogen and strain-induced martensite on fatigue crack growth behaviour of the former two steels have also been measured. In the pressure range 10–84 MPa, the hydrogen permeation of the stainless steels could be successfully quantified using Sieverts' law modified by using hydrogen fugacity and Fick's law. For the austenitic stainless steels, hydrogen diffusivity was enhanced with an increase in strain-induced martensite. The introduction of dislocation and other lattice defects by pre-straining increased the hydrogen concentration of the austenite, without affecting diffusivity. It has been shown that the coupled effect of strain-induced martensite and exposure to hydrogen increased the growth rate of fatigue cracks.     

Hydrogen embrittlement and associated surface crack growth in fine-grained equiatomic CoCrFeMnNi high-entropy alloys with different annealing temperatures evaluated by tensile testing under in situ hydrogen charging

The effect of the annealing temperature after cold rolling on hydrogen embrittlement resistance was investigated with a face-centered cubic (FCC) equiatomic CoCrFeMnNi high-entropy alloy using tensile testing under electrochemical hydrogen charging. Decreasing annealing temperature from 800 °C to 750 °C decreased grain sizes from 3.2 to 2.1 μm, and resulted in the σ phase formation. Interestingly, the specimen annealed at 800 °C, which had coarser grains, showed a lower hydrogen embrittlement susceptibility than the specimen annealed at 750 °C, although hydrogen-assisted intergranular fracture was observed in both annealing conditions. Because the interface between the FCC matrix and σ was more susceptible to hydrogen than the grain boundary, the presence of the matrix/σ interface significantly assisted hydrogen-induced mechanical degradation. In terms of intergranular cracking, crack growth occurred via small crack initiation near a larger crack tip and subsequent crack coalescence, which has been observed in various steels and FCC alloys that contained hydrogen.     

The effect of nanosized NbC precipitates on electrochemical corrosion behavior of high-strength low-alloy steel in 3.5%NaCl solution

The effect of nanosized NbC precipitates on electrochemical corrosion behavior of high-strength low-alloy (HSLA) steels in 3.5%NaCl solution has been investigated by the means of precipitate modulation, microstructure observation, electrochemical and immersion tests. The results showed that NbC precipitates markedly enhance the corrosion resistance of the H-contained steel, and the mechanism is that the plentiful and highly dispersed nanosized NbC particles acting as massive and effective hydrogen traps play a decisive role in the resistance to hydrogen activated corrosion. Moreover, it is evident that the inhibiting effect is related with the amount, size and distribution of the precipitates, and the optimized microstructures and precipitated phases improve the mechanical properties and resistance to hydrogen activated corrosion of HSLA steel.     

Hydrogen addition effect on NO formation in methane/air lean-premixed flames at elevated pressure

The present study investigates freely propagating methane/hydrogen lean-premixed laminar flames at elevated pressures to understand the hydrogen addition effect of natural gas on the NO formation under the conditions of industrial gas turbine combustors. The detailed chemical kinetic model which was used in the previous study on the NO formation in high pressure methane/air premixed flames was adopted for the present study to analyze NO formation of methane/hydrogen premixed flames. The present mechanism shows good agreement with experimental data for methane/hydrogen mixtures, including ignition delay times, laminar burning velocities, and NO concentration in premixed flames. Hydrogen addition to methane/air mixtures with maintaining methane content leads to the increase of NO concentration in laminar premixed flames due to the higher flame temperature. Methane/hydrogen/argon/air premixed flames are simulated to avoid the flame temperature effect on NO formation over a pressure range of 1–20atm and equivalence ratio of 0.55. Kinetic analyses shows that the N₂O mechanism is important on NO formation for lean flames between the reaction zone and postflame region, and thermal NO is dominant in the postflame zone. The hydrogen addition leads to the increase of NO formation from prompt NO and NNH mechanisms, while NO formation from thermal and N₂O mechanisms are decreased. Additionally, the NO formation in the postflame zone has positive pressure dependencies for thermal NO with an exponent of 0.5. Sensitivity analysis results identify that the initiation reaction step for the thermal NO and the N₂O mechanism related reactions are sensitive to NO formation near the reaction zone.     

Thermodynamic assessment of effect of ammonia,hydrazine and urea on water gas shift reaction

In the present study, thermodynamic calculations have been carried out on water gas shift reaction in presence of some selected chemical additives such as ammonia, hydrazine, and urea to understand their impact on the hydrogen generation and carbon suppression. The calculations were performed in a temperature range of 300–1300 K at constant pressure (1 bar) while varying the amount of additives from 0.5 to 2 mol. The results suggest that urea has the highest potential for hydrogen enrichment; however, it also increases carbon formation within the investigated conditions, as compared to other additives, ammonia and hydrazine, which suppress carbon formation along with assisting in hydrogen production. Hydrazine was found to be the most effective in reducing carbon and a molar ratio of N₂H₄:CH₄:CO of 1.5:1:1 was sufficient for completely removing carbon throughout the temperature range of 300–1300 K, as compared to 2:1:1 M ratio for NH₃:CH₄:CO. Both, ammonia and hydrazine, being hydrogen storage materials release hydrogen along with suppressing carbon, which results in suitable conditions for sustained long term catalytic experiments where catalyst poisoning by coking can be eliminated.     

Effects of temperature on corrosion performances of TiO2/SS316L in supercritical water for hydrogen production

Temperature is the most important factor for hydrogen generation during supercritical water gasification process. However, the increasing temperature could accelerate the corrosion of the reactor material, at the presence of oxygen, as less amount of oxygen can promote the hydrogen production. In this study, we prepared a 0.1 mm thick of TiO₂ coating on the surface of 316L stainless steel (SS316L) to enhance the corrosion resistance of SS316L during hydrogen production process in supercritical water. The influences of temperature (400–500 °C) on surface morphologies and corrosion depth and rate of TiO₂/SS316L were evaluated at 25 MPa with 1000 mg/L oxygen for 80h. Results showed that cracks and pores were present on the surface of TiO₂/SS316L after corroded in SCW for 80h. The crack width and corrosion rate was aggravated at higher temperature. The remained thickness of the coating at 400 °C, 450 °C, 500 °C were 0.08 mm, 0.05 mm and 0.03 mm, respectively. NiO and NiFe₂O₄ were generated around the crack on the surface of TiO₂/316L at 500 °C, the coating had a tendency to peel off the substrate.     

The effect of hydrogen on phase transformation and mechanical properties of a β containing γ–TiAl based alloy

The effect of hydrogen on phase transformation and mechanical properties of the unhydrogenated Ti–45Al–5Nb–0.8Mo–0.3Y (in at.%) alloy and Ti–45Al–5Nb–0.8Mo–0.3Y alloy with 1.5 at.% hydrogen addition is investigated in the temperature range 1323–1473 K with a strain rate of 0.01 s⁻¹. The results show that the flow stresses of the hydrogenated alloy are lower than those of the unhydrogenated alloy. H as an interstitial atom occupies B2 phase tetrahedral interstice. The clearance radius of tetrahedron of B2 phase is decreased by approximately 4.6% after B2→ω_o transformation. So, H can impede such phase transformation by means of impeding the decrease of clearance radius of tetrahedron. The volume fractions of B2 phase of the hydrogenated alloy are more than those of the unhydrogenated alloy under all deformation conditions due to hydrogen-stabilized β phase. The lower flow stress of the hydrogenated alloy compared with the unhydrogenated alloy is mainly attributed to hydrogen-increased B2 phase, hydrogen-induced twinning and stacking faults of γ phase, and hydrogen-impeded formation of ω_o and α₂ phases.     

Hydrogen evolution and corrosion products on iron cathodes in hot alkaline solution

To better understand the reactivation of nickel cathodes by iron, the study was performed on iron membranes in 25% KOH at 80 °C. Membranes were cathodically treated and anodically polarised, and simultaneously hydrogen permeation was measured with the electrochemical technique. During cathodic potential sweeps, three peaks of current and of hydrogen permeation occurred. The peaks at about −0.80 V and −1.30 V vs. SHE were ascribed to reduction of Fe₃O₄ to HFeO₂⁻ and H⁺, and of HFeO₂⁻ to Fe, respectively. Electric charge of anodic oxidation of cathodically pretreated membranes was by over two orders of magnitude higher than the charge of desorbing hydrogen, and it increased with the pretreatment time. This was ascribed mainly to the oxidation of iron and its corrosion products. It was proposed that the reactivating effect of iron on Ni cathodes can be associated with the enhanced reactivity of iron freshly deposited by reduction of HFeO₂⁻.     

The effect of water content on the electrochemical impedance response and microstructure of Ni-CGO anodes for solid oxide fuel cells

Using electrochemical impedance spectroscopy, the high frequency (HF) and low frequency (LF) impedance of nickel–gadolinium-doped ceria (Ni-CGO) symmetrical cells (Ni-CGO/YSZ/Ni-CGO) in dry and moist atmospheres were studied. The HF component of the impedance response in moist H₂ varied with operating temperature, while the LF component varied with the H₂ content in the fuel. The EIS response in dry fuel behaved differently. The LF impedance in dry fuel (97% H₂, 3% N₂) showed a large increase, and varied significantly with both temperature and H₂ content in the fuel, while the HF component varied with temperature in a similar manner to that observed in moist fuel. This suggests that the HF impedance in both moist and dry fuel is associated with the charge transfer resistance. The LF impedance in the case of moist fuel can be reasonably attributed to mass transport effects, while that in the case of dry fuel cannot be attributed to the same mass transport process. The estimated time constant of the LF component was considerably larger in dry atmospheres, compared to that in moist conditions. Scanning electron microscopy (SEM) images showed crack formation in the anode cermets exposed to dry atmospheres, which were not evident in cermets exposed to moist conditions.     

Hydrogen effect on Ti-6.5Al-3.5Mo-1.5Zr-0.3Si parts produced by electron beam melting

In this work, the hydrogen sorption kinetics as well as the hydrogen effect on phase transformations, structure and properties of additively manufactured Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy using electron beam melting (EBM) were studied. In situ X-ray diffraction complex was used to analyze phase transitions in the EBM Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy under hydrogenation in gas atmosphere. The EBM mode is found to affect significantly on the microstructure and the rate of hydrogen sorption by Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy during hydrogenation at a temperature of 650 °C. The measurements have shown that the highest rate of hydrogen absorption is observed in samples manufactured at the beam current of 3 mA and the scanning speed of 150 mm/s. Hydrogenation of the samples leads to redistribution of alloying elements in the titanium alloy resulted in the formation of aluminum-rich α₂-Ti₃Al intermetallic phase and hydrides precipitation.     

Hydrogen generation during the corrosion of carbon steel in crotonic acid and using some organic surfactants to control hydrogen evolution

The purpose of this paper is to describe and evaluate the corrosion of carbon steel in crotonic acid for hydrogen production and using polysorbate 20 (NS), dioctyl sodium sulfosuccinate (AS) and benzalkonium chloride (CS) to control hydrogen evolution. Measurements were conducted in tested solutions using hydrogen evolution and electrochemical impedance spectroscopy (EIS) measurements and complemented by scan electron microscope (SEM) and energy dispersive X-ray (EDX) investigations. It is shown that the hydrogen generation rate obtained during the corrosion of carbon steel in crotonic acid increased with increase in acid concentration, temperature and immersion time. The addition of organic surfactants inhibits the hydrogen generation rate. The inhibition occurs through adsorption of organic surfactants on the metal surface. Adsorption processes followed the Langmuir isotherm. The order of effectiveness of the surfactants was AS > NS > CS. The values of activation energy (E_a) and heat of adsorption (Q_ads) were calculated and discussed.     

The inhibitory effect of carbon monoxide contained in hydrogen gas environment on hydrogen-accelerated fatigue crack growth and its loading frequency dependency

The effect of carbon monoxide (CO) contained in H₂ gas as an impurity on the hydrogen-accelerated fatigue crack growth of A333 pipe steel was studied in association with loading frequency dependency. The addition of CO to H₂ gas inhibited the accelerated fatigue crack growth due to the hydrogen. The inhibitory effect was affected by the CO content in the H₂ gas, loading frequency, and crack growth rate. Based on these results, it was revealed that the inhibitory effect of CO was governed by both competition between the rate of fresh surface creation by the crack growth and the rate of coverage of the surface by CO and time for hydrogen diffusion in the material to the crack tip with reduced hydrogen entry by CO.