Effect of electrodeposited metals on the permeation of hydrogen through iron membranes

The permeability of electrolytically charged hydrogen through annealed Ferrovac E iron membranes was found to decrease significantly upon coating the charging surface of iron with thin layers of either Pt, Cu or Ni (Watts or electroless). The absorption of hydrogen was delayed for a period which depends on the nature and the thicRness of the metallic coating. The results show that such coatings do not have to be thicR or even continuous to be effective, in which case a catalytic mechanism is proposed to explain the marRed reduction in hydrogen permeation through the iron. Experimental confirmation is presented of this catalytic mechanism and of the barrier mechanism which is operative in the presence of dense continuous coatings. It is also shown that a decrease in catalytic activity occurs with time (aging) and is pronounced in the presence of As³⁺ ion. Formerly Research Associate, The Pennsylvania State University     

Effect of trapping on hydrogen permeation

Approximate solutions were obtained to the nonlinear partial differential equations derived by A. McNabb and P. K. Foster for a general model of diffusion-plus-trapping. The finite-difference method was applied to their equations for boundary conditions appropriate to hydrogen permeation and evolution in a plane. Solutions are shown to be stable and to converge to the exact solutions in certain limiting cases. The effect of trapping on hydrogen permeation and evolution was explored for a range of parameter values from computer generated tables of concentrations of diffusing and trapped hydrogen. General characteristics of the transient stages of permeation in the presence of trapping are inferred from the results.     

Effect of trapping on hydrogen permeation

Approximate solutions were obtained to the nonlinear partial differential equations derived by A. McNabb and P. K. Foster for a general model of diffusion-plus-trapping. The finite-difference method was applied to their equations for boundary conditions appropriate to hydrogen permeation and evolution in a plane. Solutions are shown to be stable and to converge to the exact solutions in certain limiting cases. The effect of trapping on hydrogen permeation and evolution was explored for a range of parameter values from computer generated tables of concentrations of diffusing and trapped hydrogen. General characteristics of the transient stages of permeation in the presence of trapping are inferred from the 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,h _m, which is defined as the surface-limited mass-transfer coefficient. Theh _m coefficient can account for all possible surface effects and may be experimentally evaluated.     

Diffusion of hydrogen in iron

A mass spectrometer was used to study hydrogen diffusion and trapping phenomena in fullyannealed and slightly cold-worked pure iron specimens which were in contact with distilled water or dilute acidic Buffer solutions. In the case of fully-annealed iron and slightly coldworked iron, hydrogen can diffuse into iron only when the iron contacts water directly. This diffusion phenomenon of hydrogen increased markedly with temperature and was accelerated by abrasion and hydrogen ion concentration in dilute acid. Abrasion and hydrogen ions in a dilute acidic Buffer solution did not affect the diffusion coefficient of hydrogen,D, but increased the hydrogen concentration at the iron surface contacting water or Buffer solution,C _s . The permeability of hydrogen in fully-annealed iron in contact with distilled water and the diffusion coefficient of hydrogen in fully-annealed and slightly cold-worked iron for the temperature range 10° to 100°C were measured. Trapping parameters in the slightly cold-worked iron were calculated.     

Hydrogen permeation through oxide and passive films on iron

Hydrogen permeation through thin films of Fe_I–Y O on iron and through chemically polished iron were investigated by the sensitive electrochemical technique. The oxide was formed on the exit side of the sample membrane. The hydrogen arriving at the iron/oxide interface is in an atomic or protonic state which renders the hydrogen uptake by the oxide possible. The wustite films were formed by oxidation in a H₂O-H₂-atmosphere. The dependence of the hydrogen permeation current on temperature and film thickness and different degrees of nonstoichiometry in Fe_I–Y O was studied. Hydrogen permeation through these oxides is possible, but very low permeation coefficients have been found, of the magnitude of at 25°C. The diffusion coefficient of diffusible hydrogen was determined to be about 4 · 10^?10 cm²/s. Measurements of the potential dependence of permeation across the film indicate that hydrogen migrates in the oxide as a charged particle (proton). In the case of the passive surface film on iron formed by chemical polishing, the dependence of the permeation current on temperature and anodic potential was measured. The electrochemical behaviour of the film was studied by cyclic voltametry. Electron transfer reactions were investigated by means of the hexacyanoferrate (II/III) redox system. Further information on the film composition were obtained by Auger electron spectroscopy. On the one hand, electron transfer across the film can occur, but on the other hand, the film is nearly impermeable for hydrogen, even if the hydrogen is in the atomic or protonic state. Cyclic voltamograms show the formation of an oxygen adsorption layer on the film in a range of anodic potential.     

Permeation of hydrogen through alpha iron

The permeation rate of hydrogen through two alpha iron specimens was measured by means of a low pressure steady-state permeation technique under conditions such that diffusion was the rate controlling process for permeation. The observed permeation rate in these samples can be described by the equation: J=5.55 ±.52 × 10^-9 p ^1/2 exp - 8095 ± 88/RT mole s^-1 cm^-1 torr^-1/2 over a range of hydrogen pressures from 21 torr to 766 torr and over a temperature range of 342 to 619 K. Variation of permeation rate with the square root of hydrogen pressure was used as evidence that the permeation process was diffusion rather than surface reaction controlled. The permeation data were used in conjunction with the solubility expression of Gonzalez¹ to determine the diffusivity of hydrogen through alpha iron as:D = 1.01 × 10^-3 exp - 1595/RT cm²/s     

Thermal analysis of trapped hydrogen in pure iron

The relative amount of trapped hydrogen and the activation energy for its evolution from various lattice defects in iron were calculated by monitoring the pressure change caused by release of hydrogen from charged specimens heated at uniform heating rates. Hydrogen release peaks were observed at 385 K, 488 K, and 578 K, respectively, when the hydrogen charged specimen were heated at 2.6 K per minute. Analysis suggests that the peak at 385 K corresponds to hydrogen release from grain boundaries, and the peak at 488 K corresponds to release from dislocations, while the peak at 578 K results from release from micro voids. The activation energies for evolution of trapped hydrogen were determined experimentally from measured peak temperatures at different heating rates and were found to be 17.2 KJ/mol, 26.8 KJ/mol, and 35.2 KJ/mol, respectively, in grain boundaries, dislocations, and microvoids. It was also observed that most of hydrogen is trapped on dislocations if the density of specimen is greater than 98.95 pct, and in microvoids if less than 98.95 pct.     

The diffusion of hydrogen in liquid iron alloys

Rates of absorption of hydrogen in stagnant liquid iron and ten (Fe-X) binary iron alloy systems were studied by an unsteady-state gas-liquid metal diffusion cell technique. These rates were found to be controlled by diffusion of hydrogen in the liquid phase. Chemical diffusion coefficients (D _h) were measured in pure iron and Fe-X alloys in the following (at. pct) composition ranges: Mn (0 to 5), Cr (0 to 25), V (0 to 25), Nb (0 to 10), Mo (0 to 25), W (0 to 5), Ni (0 to 75), Co (0 to 75), Sn (0 to 10), and Cu (0 to 25). All measuredD _H values at 1600°C lie between 7 × 10^-4 and 16 × 10^-4 sq cm per sec. The diffusion coefficients found for pure iron can be represented by D_H ^Fe = 4.37 × 10⁻³ exp (−4134 ± 1012)/RT cm²/sec where the uncertainty in the activation energy, Q, in cal per mole, corresponds to the 90 pct confidence level. A linear relationship was found between the logarithm of the hydrogen diffusion coefficient D_H ^Fe-X and the interaction parameter ε_H ^X for low and medium concentrations of alloying elementX, when applied to a fixed concentration ofX(5 or 25 at. pct) and to individual periods in the periodic table. A useful linear correlation also appears to exist between logD_H ^Fe-X and hydrogen solubility for fixed concentration ofX and with respect to the period in whichX is found. Formerly Research Assistant, Department of Mineral Engineering, Stanford University, Stanford, Calif. This paper is based upon a thesis submitted by P. J. DEPUYDT in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Stanford University and part of a presentation made at the 1970 Annual AIME Meeting.     

Neutron imaging of hydrogen in iron and steel

Abstract

Neutron radiography and tomography have been used for a time resolved in situ analysis and a 3D mapping of hydrogen diffusion in iron and steel. Samples were electrochemically charged with hydrogen and afterwards neutron transmission images were taken. Hydrogen diffusion coefficients in duplex stainless steel were determined at 623 K by measuring and comparing the sample’s mean intensity with a hydrogen-free reference sample and subsequent normalisation to standards with known hydrogen content. In technical iron and in supermartensitic stainless steel the hydrogen distributions have been investigated. The radiographic images in iron show blisters, cracks and the distribution of molecular hydrogen inside cracks. The analysis of the diffusion behaviour of hydrogen out of a blister illustrates the capabilities of the method with respect to time and spatial resolution. The neutron tomography of supermartensitic tensile stressed samples illustrates the capability to visualise hydrogen distributions three-dimensionally.     

The role of dislocations during transport of hydrogen in hydrogen embrittlement of iron

In order to assess the role of hydrogen transport in hydrogen embrittlement, one of the kinetic aspects of hydrogen embrittlement, the strain rate dependence, was analyzed in terms of hydrogen transport. The results have been analyzed in terms of a new model which takes into account atomic and macroscopic diffusion in describing dislocation-hydrogen interactions. It was found that the strong strain rate dependence of hydrogen embrittlement can be explained by a dynamic trapping effect, which is associated with an increase in the population of dislocations during deformation.     

Direct reduction of iron in low temperature hydrogen plasma

Abstract

This paper describes the effects of various parameters on the reduction of hematite in the presence of microwave assisted non-thermal hydrogen plasma. The parameters include microwave power, hydrogen flowrate, pressure, microwave power density and temperature. It has been shown that hydrogen flowrate, pressure and microwave power are interrelated to effect the microwave power density that controls the plasma temperature. The experimental conditions encounter three temperatures: surrounding the sample, associated with the plasma and at the plasma/substrate interface. It has been deduced that the third one is the most effective in determining the rate of the reaction, and in the present case, activation energy of 20 kJ mol^?1 is reported.     

Alteration in hydrogen absorption by and hydrogen permeation through a high-strength low-alloy steel due to plasma source ion implantation of nitrogen

Plasma source ion implantation of nitrogen on a high-strength steel substrate carried out at relatively low temperatures produced no discernible nitrides. The hardness increase caused by these treatments was discernible. There was indirect evidence of nitrogen in solid solution. The plasma treatments produced changes in the hydrogen permeation through the substrate as a result of alteration in the subsurface hydrogen solubility as well as the hydrogen diffusivity. The alteration in hydrogen permeation behavior depended on the time and temperature of plasma treatment.     

An adsorption study of hydrogen on iron and its relation to hydrogen embrittlement

The chemical reactions of hydrogen gas on iron surfaces have been determined by simultaneously measuring the volume of gas adsorbed and the corresponding magnetization change. The combination of these experimental results as well as the kinetics of the reactions is used to explain the temperature dependence observed in crack growth studies performed in gaseous hydrogen. The reaction is shown to be a two step process involving the formation of an adsorbed molecular precursor prior to the formation of the embrittling hydrogen ion. The adsorption isotherm and magnetizationadsorption isotherm for H₂ on Fe at 77 K were determined to be Langmuirian. This, plus a first order adsorption rate are given as evidence for the existence of a chemisorbed molecular H ₂ ⁺ precursor at this temperature. The mechanical test data of other investigators for slow crack growth in gaseous H₂, which show a nonmonotonic change of crack growth rate with temperature, become explainable based on a measured decrease in the adsorption of H₂ at temperature above 300 K and the two step adsorption process. At temperatures below 300 K the formation of an H^- ion from the adsorbed precursor H ₂ ⁺ ion is the rate controlling process in gaseous hydrogen embrittlement. At temperatures above 300 K, the decreasing net adsorption rate of H ₂ ⁺ becomes the limiting process. Formerly Graduate Student, Syracuse University.     

Experimental study of hydrogen transport during plastic deformation in iron

A series of experiments has been performed in order to analyze hydrogen transport during plastic deformation. The results have been analyzed in terms of a new model which takes into account macroscopic diffusion in describing dislocation-hydrogen interactions. The dynamic trapping effect and hydrogen transport by mobile dislocations, which are associated with an increase in the population and the motion of dislocations, respectively, have been experimentally investigated. These results have been applied to reveal the relationship between hydrogen transport and local hydrogen distribution.