Hydrogen trapping and repelling in an Al-6 wt % Zn-2 wt % Mg alloy

Hydrogen trapping in an Al-6 wt % Zn-2 wt % Mg alloy aged up to typical stages in the agehardening curve has been studied by measuring the tritium release rate after charging. The distribution of hydrogen in the aged alloy has been studied by tritium electron microautoradiography. It has been found that the Guinier-Preston zones in the alloy do not act as trapping sites but as a repeller for hydrogen, and that precipitate does not trap hydrogen, but the interface between the matrix and precipitate acts as a trapping site for hydrogen. Dislocation has been found to be capable of trapping hydrogen, while trapped hydrogen by the grain boundary has not been observed.     

Hydrogen trapping in an Al-2.1 wt % Li alloy

Hydrogen trapping in an Al-2.1 wt % Li alloy aged up to typical stages in the age-hardening curve, has been studied by measuring the tritium release rate after charging. The distribution of hydrogen in the aged alloy has been studied by tritium electron microautoradiography. It has been found that the coherent precipitate and the incoherent precipitate act as a trapping site for hydrogen, while the semi-coherent precipitate does not trap hydrogen. A dislocation has been found to be capable of trapping hydrogen, while hydrogen trapping by the grain boundary has not been observed.     

Effect of alloying elements on hydrogen diffusivity in α-iron

The effect of Cr, Ni, Mo, Si and Cu on the diffusivity of hydrogen in-iron is studied in the temperature range of 160 to 430° C at a hydrogen pressure of 1 atm. The diffusivity of hydrogen was determined by absorption rate experiments using Sievert's type apparatus. The results show that the diffusivity of hydrogen in iron alloys decreases and the activation energy increases, as the concentration of the alloying elements increases, except Cu and Ni. The trapping parameters of hydrogen in iron alloys on the basis of Oriani's approach are calculated. The results show that Si, Mo and Cr are in order of increasing trap energy and that Cu and Ni have a negligible effect on hydrogen trapping. This can be explained by taking account of both chemical affinity effects and elastic strain effects of alloying elements on hydrogen trapping.     

Anodic Hydrogenation of Iron in a Carbonate–Bicarbonate Solution

For the first time, we established a phenomenon of anodic hydrogenation of 1010 steel in a deaerated 1 N NaHCO₃ + 1 N Na₂CO₃ solution at a temperature of 325 ± 0.1 K. Hydrogen was absorbed by the metal surface at anodic potentials of –430 mV and lower, which coincides with the range of thermodynamic stability of FeCO₃ and H₂O. The upper bound of this range is higher than the line a in the Pourbaix diagrams by 170 mV. For the potentials of formation of a magnetite-maghemite film [–320 to –410 mV], hydrogen was not absorbed. To carry on experiments, we created a system for measurement of hydrogen flows with a sensitivity of the order of 10^–11 mole/(m²·sec) and proposed two methods for detection of weak flows of absorbed hydrogen. Anodic hydrogenation of the steel in a 1 N NaHCO₃ + 1 N Na₂CO₃ solution may testify to the role of hydrogen in carbonate corrosion cracking.     

Hydrogen trapping phenomena in carbon steel

Hydrogen trapping phenomena in carbon steel with different amounts of trapping sites were investigated by thermal analysis and permeation experiments. In thermal analysis, the relative amount of trapped hydrogen and the activation energy for evolution from various lattice defects were calculated by monitoring the pressure change caused by the release of hydrogen from hydrogen-charged specimens heated at a uniform rate. Hydrogen release peaks were observed at 116, 205 and 387° C, respectively, when the hydrogen-charged specimens with various defects were heated at a constant heating rate of 2.6° C min^–1. Analysis suggested that the peak at 116° C corresponded to release from ferrite-cementite interfaces and the peak at 205° C corresponded to release from dislocations. The activation energy for evolution of trapped hydrogen determined experimentally from the measured peak temperature at different heating rates was found to be 18.4 kJ mol^–1 in the ferrite-cementite interface. The hydrogen energy level around the trapping site was suggested from the trap activation energy and expected saddle-point energy. It was observed that most of the hydrogen is trapped in dislocations in spheroidized 0.49 wt% carbon steel.     

HDDR Process and Hydrogen-Absorption Properties of Didymium–Aluminum–Iron–Boron (Dd12.3Al1.2Fe79.4B6) Alloy

We investigate the interaction of hydrogen with the alloy of the Dd–Al–Fe–B system containing (in wt. %) 28.6% Dd, up to 0.5% Al, 1.1% B, and the rest Fe. At an initial hydrogen pressure of 1.0 MPa, hydride with a hydrogen content of 0.5 wt. % is formed. Hydrogen adsorption is accompanied by an increase in the lattice constants (a grows by 1.4% and c does by 1.1%) under a total volume increase of 4.1%. According to the kinetics of hydrogen adsorption, the duration of a full saturation by hydrogen is 3–5 h at an initial gas pressure of 0.1–0.2 MPa. Under heating in the hydrogen medium, the alloy disproportionates at a temperature of 1003 K. X-ray diffraction data indicate that the ferromagnetic phase disproportionates into didymium hydride DdH _x [a = 0.5443(2) nm, -Fe (a = 0.2865(1) nm) and a = 0.5112(8) nm, c = 0.423(2) nm] and iron boride Fe₂B. Heating of the products of disproportionation in vacuum to a temperature of 1123 K results in the recombination of the initial ferromagnetic phase.     

Control of hydrogen cracking in the welded steel using microstructural traps

Hydrogen diffusion into steel can embrittle the material in H₂S environments, but this effect can be offset by suitable hydrogen trapping sites in the microstructure. Fine Ti(C,N) inclusions have been studied as the trapping sites in high strength low alloy (API X-70) welds, with Ti additions ranging from 0.004 to 0.16?wt.%. The trapping sites were investigated by electron microscopy and thermal desorption spectroscopy. Manganese sulphide particles were the main initiation sites for hydrogen induced cracking as expected. The optimum Ti addition was around 0.02% with no evidence of cracking in the weld. The estimated values of trapping activation energy for dislocations, microvoids, MnS and Ti(C, N) were approximately 25.9, 34.6, 65.1 and 87.6?kJ?mol^?1, respectively.     

Effect of electrical operation on the defect states in organic semiconductors

We have investigated the role of the trapping process in degradation mechanisms of poly(9,9-dihexylfluorene-co-N,N-di(9,9-dihexyl-2-fluorenyl)-N-phenylamine) (PF) based diodes, after aging (at half lifetime) by electrical stress. By using the Charge based Deep Level Transient Spectroscopy, we have determined the trap parameters in PF light emitting devices. The mean activation energies of the traps are in the range 0.13–0.60 eV from the band edges with capture cross sections of the order of 10^–18 to 10^–20 cm². The trap densities are in the range of 10^–16 to 10^–17 cm^?3. Upon aging, no new trap levels have been found indicating that the electrical stress did not create additional defect level in the polymer in contrast to previous investigations on other organic materials, which reported that the degradation of devices in humid atmosphere lead to the onset of new traps acting as recombination centers. Furthermore, aging would not affect uniformly the defect levels in the polymer. Shallow trap states (below 0.3 eV) remain stable, whereas the enhancement in trap density of deeper trap levels (above 0.3 eV) have been observed, suggesting that degradation by electrical stress leads to an increase in density of deep levels.     

Theoretical evaluation of the role of crystal defects on local equilibrium and effective diffusivity of hydrogen in iron

ABSTRACT

Hydrogen diffusion and trapping in ferrite is evaluated by quantum mechanically informed kinetic Monte Carlo simulations in defective microstructures. We find that the lattice diffusivity is attenuated by two to four orders of magnitude due to the presence of dislocations. We also find that pipe diffusivity is vanishingly small along screw dislocations and demonstrate that dislocations do not provide fast diffusion pathways for hydrogen as is sometimes supposed. We make contact between our simulations and the predictions of Oriani's theory of ‘effective diffusivity’, and find that local equilibrium is maintained between lattice and trap sites. We also find that the predicted effective diffusivity is in agreement with our simulated results in cases where the distribution of traps is spatially homogeneous; in the trapping of hydrogen by dislocations where this condition is not met, the Oriani effective diffusivity is in agreement with the simulations to within a factor of two.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

Reduction of perovskite oxide LnCoO3(Ln = La-Eu) in a hydrogen atmosphere

The reduction of LnCoO₃(Ln = La-Eu) in a hydrogen atmosphere of 2×10⁶ Pa at 25 to 600° C was investigated by X-ray diffraction and thermogravimetric analysis, which were carried out to pursuein situ the reaction of reduction of LnCoO₃. The amount of lattice oxygen consumed at 600° C increases on going from LaCoO₃ to EuCoO₃. It was shown that the reduction process proceeded through the formation of a series of oxygen-deficient structures in keeping the perovskite structures; for example, the final X-ray powder diffraction pattern of NdCoO_3–x (x=1.1) could be indexed on a cubic cell (a = 0.39 nm) and an orthorhombic cell (a₀=0.522 nm,b ₀ = 0.559nm, c₀ = 0.795nm), the relationship with the cubic cell beinga ₀ 2^1/2 a _c,b ₀ 2^1/2 a _c,c 2a_c.     

Behaviour of hydrogen in pure aluminium,Al-4 mass% Cu and Al-1 mass% Mg2Si alloys studied by tritium electron microautoradiography

The effect of lattice defects and microstructures on hydrogen distribution in pure aluminium, Al-4 mass% Cu and Al-1 mass% Mg₂Si alloys has been studied by tritium electron microautoradiography. It was found that in pure aluminium and both the alloys, dislocations and grain boundaries act as short-circuiting diffusion paths and also as trapping sites for hydrogen. It was found that the Guinier-Preston (GP) zones in the Al-1 mass% Mg₂Si alloy do not act as trapping sites but as repellants for hydrogen, in contrast to the GP zones in the Al-4 mass% Cu alloy in which they act as trapping sites for hydrogen. In the Al-1 mass% Mg₂Si alloy, the interfaces between the matrix lattice and the metastable precipitate and between the matrix lattice and the equilibrium precipitate, were proved to be strong trapping sites for hydrogen. In the Al-4 mass% Cu alloy, the equilibrium precipitate itself has been found to be able to trap hydrogen.     

Hydrogen permeation through mild steel in temperature range 20–500°C measured by hydrogen collection method

Abstract

Hydrogen permeation through steel plate maintained at various constant temperatures between 20 and 500°C was investigated using the hydrogen collection method. Fused salts were employed at elevated temperatures. Hydrogen entry was induced by step changes in cathodic charging current density. Emanating flux transients obtained at the hydrogen exit face corresponded closely to a model for permeation determined exclusively by bulk hydrogen diffusion through steel, induced by stepped changes in hydrogen concentration at the steel entry face subsurface, and zero hydogen at the exit face subsurface. An Arrhenius plot of log (D) v. 1/T, using diffusion coefficients D derived from permeation transients was approximately linear in the range 20–310°C. The values derived for activation energy E _a of 17 kJ mol ^-1 and for the pre-exponential factor D ₀ of 2·6 × 10^-3 cm2 s^-1, according to D = D ₀ exp (-E _a/RT) were similar to literature values. Between 310 and 500°C, stable permeation conditions were difficult to obtain, but flux measurements were repeatable and unaffected by moisture in air.     

Equivalent Hydrogen Fugacity during Electrochemical Charging of 980DP Steel Determined by Thermal Desorption Spectroscopy

Thermal desorption spectroscopy (TDS) is used to analyze hydrogen in 980DP after (i) electrochemical charging, and (ii) gaseous charging. The hydrogen concentration increases with (i) a more negative charging potential and (ii) an increasing hydrogen gas pressure. For charging in 0.1 M NaOH, the hydrogen fugacity for 980DP is similar to that for (i) low interstitial steel, and (ii) MS1500, and is greater than that for the 3.5NiCrMoV steel. This indicates an influence of steel chemistry on the hydrogen evolution reaction. The de‐trapping activation energies are 40.5 and 50.2 kJ mol^?1, indicating hydrogen traps at boundary defects.

     

Enthalpy relaxation and free volume changes in aged styrene copolymers containing a hydrogen bonding co-monomer

Enthalpy relaxation of polystyrene (PS) and four modified polystyrene copolymers containing co-monomers capable of forming hydrogen bonds of different strengths is described. Values of enthalpy lost ( H(T_a, t_a)) were calculated from experimental data plotted against log (t_a) and modelled using the Cowie-Ferguson (CF) semi-empirical model. This gives a set of values for three adjustable parameters, H(T_a), log (t_c) and . Each of the parameters defines the relaxation process, which was found to be sensitive to changes in hydrogen bond strength. The introduction of hydrogen bonding causes a slower relaxation compared with PS, with a greater overall enthalpy lost measured for the all the copolymers except the styrene-co–4-hexafluoro-2-hydrox isopropyl styrene (SHFHS). Interestingly, the free volume of this copolymer measured using Positron Annihilation Lifetime Spectroscopy (PALS) was greater than that of PS. Furthermore, the SHFHS copolymer had the lowest change in heat capacity ( C_p) of any of the systems on passing through the glass transition, T_g. All experiments indicate that the enthalpy lost by the fully relaxed glass ( H(T_a)) is less than the theoretical amount possible on reaching the state defined by the equilibrium liquid enthalpy line ( H_max(T_a)). The results are discussed with reference to the strength of interaction and free volume.     

Current–Voltage Characteristics of EuGa2S4<Co> Single Crystals

The current–voltage characteristics of EuGa₂S₄ single crystals were measured from 80 to 300 K with the aim of elucidating the mechanism of electrical transport. The results, interpreted as evidence of space-charge-limited currents with an exponential distribution of trap energies, were used to determine the trap depth (E _t = 0.30 eV), trap concentration (N _t = 4 × 10¹³ cm^–3), Fermi energy (E _F = 0.51 eV), equilibrium carrier concentration (p ₀ = 9 × 10⁹ cm^–3), contact potential difference ( = 0.43 eV), and shift of the Fermi level (E _F = 0.09 eV).     

Passivation treatment for inhibition of hydrogen absorption in chromium-plated steel

Hydrogen permeation transients for passivated chromium-plated steels were measured at room temperature as a function of cathodic charging current density. The passivated film on chromium plating is shown to be an effective barrier to hydrogen at low cathodic charging current density (40 and 100 A cm^–2). The passivated film can be reduced to chromium at higher cathodic charging current density (1 and 10 mA cm^–2), promoting hydrogen absorption significantly.     

The Second Order Recombination and Spin-Exchange Relaxation of Atomic Deuterium at Low Temperatures in a Low Magnetic Field

We measured the 2nd order recombination rates and spin-exchange relaxation of atomic deuterium (D) in a ⁴ He coated sample cell, using the hyperfine resonance of (F = 1/2, m_F = –1/2) – (F = 3/2, m_F = –1/2) transition in a low magnetic field (3.9 mT) at temperatures between 0.6 K and 1.2 K. At lower temperatures below 0.9 K, the density decay of D atoms was dominated by D-D recombination on the liquid He surface. We found that the surface recombination cross length was 1_DD = (5.5 ± 1.3) × 10 ^–9 cm and the adsorption energy of D on ⁴ He surface was _a = 3.97 ± 0.07 K. Compared with prior measurements at high magnetic fields by other groups, 1_DD at low field was orders of magnitude smaller than what was expected when the scaling of 1/B² dependence of the direct recombination mechanism was used, and in addition, _a was significantly larger. This was attributed to the onset of the resonant recombination mechanism for the D-D surface recombination at high fields. Above 0.9 K, D-D volume recombination and recombination of D with hydrogen impurity became dominant processes of the density decay of D. The transverse relaxation times were measured and we determined the D-D spin-exchange relaxation rates, G_DD = (1.4 ± 0.6) × 10 ^–10 cm ³ sec ^–1 . It was smaller than theoretical calculations.     

Thermal annealing-induced ageing and structural effects in YBa2Cu3O7−δ superconductors

The YBa₂Cu₃O_7– superconductor loses part of its oxygen by thermal annealing, effectively at annealing temperaturesT _a 400 °C, under reduced air pressure. The release of the oxygen decreases monotonically as a function of time, at givenT _a, and presents two different slopes; fast at the beginning and slow later. The initial slopes (dM/dt) of the isothermal mass loss curves, measured at differentT _a, follow a straight line, as In (dM/dt) is plotted against theT _a ^–1 (K^–1), suggesting the release of the oxygen to be a kinetic process with an activation energyQ ₁=0.45 eV. The oxygen vacancies probably reside along the central-cage Cu-O chains, replacing O(4) ions, which break up spinal Cu-O linkages, and consequently the highT _c superconductivity properties suffer. A YBa₂Cu₃O₇ sample annealed for 30 min at 600 °C (at 10 mbar air pressure) thus gives a considerably reducedT _c at 50 K, compared to 91 K without annealing. However, annealing, peculiarly at a higherT _a 700 °C, for a short period of 10 min, has little effect on the highT _c appearing at 70 K. We believe that the oxygen vacancies possibly populate on O(1) sites along the side Cu-O chains, at this particular temperature, and become positively charged on trapping the hole, thus helping the superconductivity.     

Nuclear magnetic relaxation of3He gas. I. Pure3He

Longitudinal relaxation timesT ₁ have been measured in³He gas, using pulsed NMR, for number densities between 3 × 10²³ and 6 × 10²⁵ spins m^–3 and temperatures between 0.6 and 15 K. Relaxation takes place on or near the walls of the Pyrex sample cells and measurements ofT ₁ give information about the surface phases. A cryogenic wall coating of solid molecular hydrogen was found to delay the formation of a³He monolayer on cooling, andT ₁ measurements were consistent with a binding energy of 13 K for a³He atom to a hydrogen surface. At temperatures below 2 K a completed³He monolayer forms on the H₂ coating. No variation of the areal density of monolayer completion with bulk number density at fixed temperature could be observed and the completed³He monolayer is thought to be a dense fluid. Baking the Pyrex sample cells under vacuum and using an rf discharge in³He gas to clean the walls before sealing in the sample gas were found to increase the observed T₁'s by up to three orders of magnitude. Once a³He monolayer has formed on the H₂ surface in these cleaned, sealed cells, the dipolar interaction between adsorbed spins is thought to be the dominant source of longitudinal relaxation. The data are consistent with a dipolar relaxation model with a correlation time of 2 × 10^–9 sec. This time is long compared to the value of 10^–11 or 10^–12 sec in the 3D fluid. This suggests that if the surface phase is a 2D fluid and the dipolar mechanism is indeed the dominant one, then the atoms in the 2D fluid are less mobile than in three dimensions. This is consistent with recent susceptibility measurements.     

Model of hydrogen behaviour in enamelling grade steels

A model has been developed of the behaviour of hydrogen in enamelling-grade steels in relation to the delayed defect of blow-off of enamelled surface (fishscaling). The model is based on current theories concerning reversible and irreversible trapping of hydrogen in metallic materials. It leads to the establishment of a free hydrogen parameterC _L which can be used to assess the susceptibility of a steel to fishscaling following the usual enamelling processess. The model can also be used to study the effect of both thermomechanical steelmaking cycles and enamelling processes on resistance to the defect.Nomenclature E _aD Activation energy of hydrogen diffusion through normal lattice - E _s Saddle-point energy - E _B Trap binding energy - E _aT Trap activation energy=E_s+E_B - A Trapping site - B Normal lattice site - v ₀ Vibration frequency of hydrogen at a normal lattice site - v ₁ Vibration frequency of hydrogen at a trapping site - N _L Density of normal lattice sites for hydrogen - N _T Density of trapping sites for hydrogen - C _L Concentration of hydrogen on lattice sites - C _T Concentration of hydrogen captured on traps - k Probability of trapping=v₀ exp(–E _s/R T) - p Probability of detrapping =v ₁ N _L exp (–E _aT/R T) - n Fraction of trapping sites occupied with hydrogen atoms among the total trapping sites=C _T/N _T - t Time - T Temperature - R Gas constant