HYDROGEN DIFFUSION IN PALLADIUM BY GALVANOSTATIC CHARGING (original) (raw)
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A mechanistic analysis of hydrogen entry into metals during cathodic hydrogen charging
Scripta Metallurgica, 1988
Entry of hydrogen into metals is of serious concern to metallurgists and engineers, since it severely degrades the mechanical properties of metals (1). These problems arise in the cathodic protection of metals, power plants, and in environments where H2S is present as in petroleum refining (2) where the hydrogen evolution reaction (h.e.r.) and hydrogen permeation reaction take place in the corroding or cathodically polarized metal. By performing hydrogen charging experiments of thin samples using the Devanathan-Stachursld cell, the permeation characteristics have been extensively studied (3,4). The present paper seeks to analyze the h.e.r, mechanism and to predict the relationship between the permeation flux and the charging and evolution (recombination) fluxes. A thorough development of the model and actual computations of rate constants and hydrogen coverages will appear elsewhere (5).
Study of hydrogen transport in metal hydride electrodes using a novel electrochemical method
Journal of Electroanalytical Chemistry, 2000
A novel and relatively simple electrochemical method is described to determine the hydrogen diffusion coefficient (D) and the surface area (A) of hydrogen-absorbing alloy particles. In this method, which is called 'potential step chronoamperometry (PSCA)', hydrogen is dissolved into the alloy until the hydrogen concentration is uniform, and then the dissolved hydrogen is extracted electrochemically at a sufficiently high potential step (larger than +0.2 V vs. Hg HgO) with an anodic hydrogen ionization current that changes with time. From an electrochemical kinetic analysis and Fick's law for diffusion to a spherical particle, the variation of j (current density, A g − 1) as a function of t − 1/2 (t is time, s) is found to be linear over a small time range, i.e. tB a 2 /D (a is the particle radius) (less than about 7 500 s) and the diffusivity and surface area can be determined from the intercept and slope of this line. The value of the diffusion coefficient of hydrogen in a LaNi 4.7 Al 0.3 alloy is found to be in the range 3.1 ×10 − 14-8.6×10 − 13 m 2 s − 1. In the hydrogen concentration region that was examined, i.e. H/M\ 0.06, the value of the diffusion coefficient declines sharply with increasing hydrogen concentration in the alloy particles. This dependence of the diffusion coefficient of hydrogen on its concentration is discussed in terms of the nature of both the diffusing species and the diffusion medium. It is also found that the hydrogen transport is governed solely by solid-state diffusion in the alloy particles only after a certain period of time (approximately 40-70 s).
Effects of cathodic charging on hydrogen permeation in a Pd 80Rh 20 alloy
Journal of Alloys and Compounds, 2004
Samples of Pd 80 Rh 20 alloy were submitted to electrochemical hydrogen permeation tests at 40 • C, using a 0.1N NaOH electrolyte, to study hydrogen permeation in a Pd 80 Rh 20 alloy for different cathodic hydrogen generating currents varying from 0.1 to 20 mA. It was found that the apparent hydrogen diffusivity and the hydrogen flux increased with increasing cathodic charging. For high levels of applied current, equal to or above 5 mA the hydrogen permeation curves present a double sigmoidal shape which corresponds to hydride formation during the test. The hydrides were characterized by X-ray diffraction after exposure of the sample to hydrogen. Phase separation was also observed in the alloy, having been provoked, for high applied currents, by the formation of a PdRhH hydride and a Rh-rich phase.
Journal of Electroanalytical Chemistry, 1999
Effects of diffusion induced stress on the hydrogen absorption into plate form electrodes of b-phase PdH x are discussed numerically based on the Volmer-Tafel route of the hydrogen evolution reaction, and thermodynamic considerations involving stress fields and non-ideal interactions of hydrogen in the electrode. It is found that the self-induced stresses enhance the absorption rate and may exceed the yield stress, especially when the thickness of the plate and/or charging current (or negative potential) increase. On the other hand, a plate with both sides exposed to electrolyte absorbs hydrogen more rapidly than that with only one side exposed to electrolyte under the same equivalent thickness and other conditions. Of course, the stresses developed in the former plate are always greater than those of the latter.
Some metals and alloys solubilize hydrogen during service, and this absorption can reduce their mechanical properties. To evaluate the susceptibility of metals to hydrogen degradation the procedures prescribed in ASTM G148-97 standard are used. However, some interfacial and volumetric phenomena occurred in metal and the conditions considered in analytical solutions of this standard are no longer valid. In this sense, new boundary conditions are proposed to fit experimental results with the simulated. The solution of the Fick equation with these new conditions needs a numerical solution, and a computational modeling used finite elements to solve the equation with this new situation. The simulated results are compared with experimental one and they agree very well.
Study of the Hydrogen-Metal Systems
Acta Physica Polonica A
Hydrogen accumulation in samples of a palladium and 12Kh18N10T steel at the hydrogen charging by the electrolytic method and hydrogen release from these samples at its electron and X-ray irradiation are studied. Palladium was used as a comparison material (as most efficiently solvent hydrogen known among the simple materials). It is established that a capture effectiveness of hydrogen from an electrolyte (1 M H 2 SO 4 at current density is 0.5 A cm −2) for palladium is 3-4 orders more than for steel. The hydrogen yield nonlinearly increases with growing of electron current density and electron energy is more than 40 keV under electron irradiation of saturated palladium and 12Kh18N10T steel samples. About 90% of the hydrogen had removed from hydrogen saturated palladium samples and only 60% from steel under electron beam with energy 40 keV and current density ≈ 20 µA cm −2 for 1 h of irradiation. It is necessary to increase the energy of electrons from 40 to 100 keV for the more effective removal of hydrogen.
Journal of Surface Investigation. X-ray, Synchrotron and Neutron Techniques, 2010
Hydrogen accumulation during electrolytic saturation of 12Kh18N10T and 12Kh12M1BFR steels, as well as during thermally and radiation stimulated hydrogen release from the same materials, was studied. It was shown that there is a critical hydrogen concentration in the sample, which is reached in 50 h for this saturation method (1 M H 2 SO 4 electrolyte, current density is 0.5 A/cm 2). Initially, hydrogen is trapped at low temperature (400-500°C) traps of several types in surface layers. At saturation times of 50 h and longer, hydrogen penetrates to high temperature (800-900°C) traps in the sample bulk. Under electron irradiation of saturated samples, the hydrogen yield nonlinearly increases with electron current density and energy above 40 keV. It was concluded that electronic processes (Auger process and plasmon excitation) play a dominant role in hydrogen diffusion and desorption activation.