Hydrogen Permeation In Metals Research Articles

Hydrogen permeation in metals and alloys: A Brief Review

This is an integrative review deals with the hydrogen-metal interaction mechanisms, reversible and irreversible traps and highlights the use of electrochemical permeation as an important tool of evaluation the Hydrogen embrittlement (HE) mechanism. Due to its simplicity, flexibility, low cost and low risk, this technique is widely used in the study of hydrogen transport and diffusion parameters, contributing to the selection of more resistant steels and alloys. On the other hand, it´s important toothose new efforts should be made to develop and implement new techniques that can be applied to different temperature and pressure conditions.

Hydrogen Permeation in Zirconium Using a Molten Salt Devanathan-Stachurski Electrochemical Cell

This paper explores whether the Devanathan-Stachurski method for the measurement of hydrogen permeation in metals can be extended to measure hydrogen diffusion zirconium at temperatures corresponding to the operating conditions of nuclear power plant. The investigation consisted of two steps, the selection of suitable electrodes and (molten salt) electrolytes for high temperature use, and the subsequent investigation of hydrogen charging and hydrogen electrochemical oxidation in the chosen electrolyte. An alternative electrochemical charging and oxidation method was devised using molten hydroxide (NaOH-KOH) as a suitable electrolyte at 250-300 °C.

Modelling the influence of trapping on hydrogen permeation in metals

Abstract Hydrogen permeation (diffusion) is strongly influenced by the local hydrogen concentration and by the density and depth of traps. Consequently hydrogen diffusion cannot be described by a constant effective diffusivity as suggested in standards and several papers. A modelling study for diffusion of hydrogen in metals with traps is presented. Simulations are performed for a charging and discharging process, showing a remarkable asymmetry. The actual chemical diffusivity ranges over several orders of magnitude depending on the hydrogen concentration as well as the density and depth of traps. Consequently, the concept of effective diffusivity outlined in standards and many publications fails.

Hydrogen permeation in metals near room temperature by a tritium tracer technique

In a fusion reactor, tritium retention and permeation in structure materials are very important safety concerns. Most data for diffusion and permeation of hydrogen in metals so far available have been limited for rather higher temperatures and, in particular, no data are available for high-Z metals near room temperature (RT). We have tried to observe gaseous hydrogen permeation through metals near RT applying a tritium tracer technique, which is a very powerful tool to detect quite small amount of hydrogen (tritium) by a liquid scintillation counting (LSC) method. After confirming the reliability of the method for the determination of diffusion and permeation coefficients in pure Ni, it was applied to hydrogen permeation in W near RT, and diffusion and permeation coefficients of hydrogen in W were determined, D=(3.42±0.68)×10-9exp((-37.8±1.2)(kJ/mol)/RT),m2s-1, and Φ=(1.21±0.24)×10-5exp((-57.8±0.9)(kJ/mol)/RT),molm-3s-1Pa-1/2.

Chemistry of excited states: Its impact on plasma wall interactions in fusion devices

Differently from molecular hydrogen, energetic hydrogen escaping from plasma is directly implanted into plasma facing materials depositing its energy to excite electrons, to displace lattice atoms and to excite phonons and finally thermalized in the target. This makes subsequent migration of the thermalized hydrogen in the target very complex. Electrons and photons escaping from plasmas are superposed to influence the hydrogen migration, too. Simultaneously, various particles are emitted from the surface, including reflected, sputtered, and re-emitted particles, secondary electrons and photons. Those emitted particles are not necessarily equilibrated thermodynamically with the temperature of the target surface but are often in higher excited states and some are even ionized. Hence the surface exhibits various chemical reactions with impurities and/or chemical change. Enhanced erosion of carbon materials by implanted hydrogen producing methane, the chemical sputtering of carbon, is one of the most well-known examples. Even atomic hydrogen emission from carbon is observed at such a low temperature as 1000°C. The super permeation of hydrogen in metals is also caused by impinging of energetic hydrogen. Re-emitted hydrogen molecules are not necessarily in thermal equilibrium at the surface and bring hyperthermal energy released upon recombination. Then the recombination model based on the surface equilibrium should be modified. And the hydrogen recombination process may not be the rate limiting process for hydrogen re-emission. Taking into account the present knowledge of surface physics and chemistry, the kinetics and mechanisms of those phenomena given by energetic hydrogen injection which are important in fusion devices are discussed.

A study of the Pd/highly oriented pyrolytic graphite electrodeposition system by in situ electrochemical scanning tunnelling microscopy

Abstract Palladium electrodeposition has significant industrial application in the plating of electrical contacts and academic importance in studies of hydrogen permeation in metals. We used a simple palladous chloride electrolyte and a highly ordered pyrolytic graphite substrate to study the early stages of Pd electrodeposition in situ by means of potentiodynamics and scanning tunnelling microscopy, also by conventional electrochemical measurements and scanning electron microscopy. The initial nucleation and growth of Pd crystallites occurred predominantly at surface defects; alterations in the electrodeposition conditions were found to affect the size, distribution and morphology of the Pd deposit. For the in situ scanning tunnelling microscopy experiments, a tip field-induced modification of the local electrochemical behaviour at the surface was observed that restricted Pd nucleation and growth within a region of substrate underneath the tip.

Effects of visible light on hydrogen permeation in metals

The influence of light on hydrogen electropermeation through a thin membrane of iron, stainless steel and palladium, respectively, has been investigated. Hydrogen is cathodically evolved at one side of the membrane and anodically monitored at the other side. Illumination of the inlet side produces a decrease of the permeation current due to combined heating and “photo” effects which seem to reduce the degree of surface coverage with hydrogen. On the other hand, three different processes are observed when illuminating the anodically polarized outlet side: hydrogen electro-oxidation, hydrogen photo-oxidation (both to produce protons) and water photo-oxidation. As the photocurrent is proportional to the hydrogen flux at the metal surface, it is possible to monitor with an optical probe the local distribution of the diffusing hydrogen atoms at the metal surface. This technique allows in situ and continuous non-destructive control of all those metal which are susceptible to hydrogen damage.

Models for hydrogen permeation in metals

Hydrogen transport in metals is affected by processes occurring in the bulk, the surface/bulk, and gas/surface interfaces. Several models have been proposed to explain the permeation process. Here the development of the various models is discussed and the similarities and differences among them are highlighted. By considering the limiting case of high pressure, it is shown that the dissociative chemisorption model of Wang [Proc. Cambridge Philos. Soc. 32, 657 (1936)] is sufficient to explain the observed experimental results.

Modeling surface effects on hydrogen permeation in metals

A new mathematical model for analyzing hydrogen permeation in solids, in which surface effects and traps influence hydrogen transport, is presented and solved. The new model combines the McNabb and Foster equations for diffusion with concomitant trapping[1] and a surface-limited mass-transfer boundary condition. An important result of the new model is the introduction of a new variable,hm, which is defined as the surface-limited mass-transfer coefficient. Thehm coefficient can account for all possible surface effects and may be experimentally evaluated.

Contribution of cathodic charging to hydrogen storage in metal hydrides

A review of recent permeation studies shows the strong detrimental influence of superficial layers (such as oxides) on the absorption kinetics of gaseous hydrogen in metals. This effect was shown to be primarily the consequence of a decrease in the dissociation rate of H 2 molecules on the surface. Cathodic charging short circuits this H 2 dissociation rate-controlling step and enhances hydrogen permeation in metals. The role of different parameters in the enhanced permeation of cathodic hydrogen is discussed and the possible applications of this technique to hydrogen storage in metal hydrides are illustrated.

Hydrogen Permeation in Metals During Exposure to a Process Gas Environment

Hydrogen permeation of nickel-base heat-resistant alloys in a process gas environment is investigated in a high-temperature range up to 1273 K. Time-dependent permeation behavior of candidate alloys (R, NSC-1, SZ, KSN, 113M, and Hastelloy XR-51) for intermediate heat exchangers of a high-temperature gas-cooled reactor is examined in a reducing gas of 80% H2 + 15% CO + 5% CO2. The result in the reducing gas is compared to that of the permeation in pure hydrogen. For both measurements, a helium carrier gas method is used, simulating the practical configuration of the heat exchangers. The permeation rate decreased proportionally to the inverse of the square root of time in the reducing gas and had a square root dependence on hydrogen pressure at a constant thickness of the oxide layer. These results are discussed on the basis of a two-layer diffusion model.

Concerning electrochemical measurements of hydrogen permeation in metals

It is clearly true that the rate of development of heat in a conductor, whether an electrolyte or electrode, is a function of the (square of the) current that is passed. As mentioned earlier, however, the rate of development of heat — i.e., Joule heating — is not the same as the rate of increase of temperature in a conductor. Joule heating may, therefore, certainly occur in a permeation cell, or any electrochemical cell for that matter, but the temperature rise depends on the heat capacity of the material as well as on the rate of heat loss. We believe that in our studies the rate of temperature change and the corresponding increase in anodic current density that may be expected are not compatible with the observed transients. It is our view at this stage that the observed transients in nickel reflect transport of hydrogen predominantly by lattice diffusion in unstrained membranes and by dislocations in membranes deformed at a constant strain rate during cathodic charging. Of course, the point raised by Otsuka and Isaji cannot be dismissed in general without further study. What might perhaps prove convincing would be to examine a series of alloys, in some of which dislocation transport is considered to occur — for example, in nickel serrated yielding in hydrogen-charged single crystals indicates mobile dislocation-hydrogen interactions (26) — and others in which dislocation transport is considered less likely. It is interesting in the latter sense to cite the recent work of Berkowitz (27) in which electrolytic hydrogen permeation on straining electrodes of 4130 steel is found to decrease at the yield point in contrast to the work on nickel (1 and 3). This has been interpreted to reflect dislocation trapping of hydrogen rather than transport. Similar observations have been recently made in our laboratory on a 2 1 4 Cr-1Mo steel (28). In any case, it seems unlikely that Joule heating of the magnitude suggested (1) could account for the latter behavior. In short, we do acknowledge having certain reservations about the details of the analysis of permeation transients on straining electrodes. These have been under study in our laboratory. Joule heating was not one of these details. Our view is that while Joule heating may certainly occur when a current passes through a conductor, much of the phenomenology that we have observed on straining membranes (a grain size dependence, etc.) is best understood in terms of a hydrogen permeation process.

Thermodynamics of the Solubility and Permeation of Hydrogen in Metals at High Temperature and Low Pressure

The solubility and permeation of hydrogen in metals is analyzed on the basis of classical thermodynamics plus a minimum of kinetic theory. Expressions are derived for x, the number of H atoms absorbed per solid atom, and J, the permeation rate, in the limiting cases of continuum and free-molecule flow. For most conditions of temperature (T) and total pressure (pt = pH + pH2), the expressions reduce to the familiar equations for solubility and permeation, i.e., a curve for constant pt appears as a straight line on Arrhenius plots of logx vs 1 / T and of logJ vs 1 / T. However, for extreme conditions of high temperature and low pressure, the expressions predict that a curve of constant pt will become increasingly nonlinear as T increases, and it may even pass through a maximum when T becomes sufficiently high to produce almost complete dissociation of the gaseous H2. Also, under certain conditions neither x nor J is proportional to the square root of the total pressure. The analysis provides a simple explanation of existing experimental data on the solubility and permeation of hydrogen in tungsten and in molybdenum at high temperature and low pressure.

Hydrogen permeation in metals as a function of stress, temperature and dissolved hydrogen concentration

An investigation of the diffusion of electrolytic hydrogen through membranes of: (1) polycrystalline Armco iron; (2) single crystal Armcoiron; (3) zone refined iron; and (4) A.I.S.I. 4340 has been made. The temperature and stress dependence of the permeation rate through (1) and (4) was investigated, while (1) to (3) have been investigated under varying concentrations of hydrogen in the metal. From the results concerning Armco iron polycrystals and single crystals, and zone-refined iron it has been concluded that trace impurities and grain boundaries have negligible effects on the hydrogen permeation rate. Stress has been shown not to affect D , but it increases the solubility of hydrogen in the lattice. The D 0 and Δ H p value for α-iron and the stress dependence of the hydrogen solubility indicate that the rate-determining step in diffusion is the formation of a cavity (a dilated octahedral hole) to accommodate the diffusion hydrogen. Maxima observed in the relation of the rate of permeation to time were explained in terms of ‘blister´ formation; the temperature dependence of the critical hydrogen concentration necessary to form these blisters is in concordance with this hypothesis. The nucleation sites for such blisters are aggregates of dislocations. The mechanism proposed to explain the maxima in the permeation transients was used to rationalize the existing discrepancies in diffusion data found in the literature. It is suggested that the crack initiation site for stress corrosion cracking may be a highly localized density of dislocations, on a metal surface, generated by blister formation due to hydrogen.