Selective adsorption and hydrogen embrittlement

This paper presents a definite correlation between hydrogen embrittlement and adsorption. The effects of the presence of gaseous additives on hydrogen embrittlement and hydrogen adsorption were studied. Those gaseous additives which halt a running crack in 4340 steel loaded in a hydrogen atmosphere also halt hydrogen adsorption. Those gaseous additives which accelerate the crack growth increase the supply of hydrogen atoms at the metal surface. It is concluded that the effect of gaseous additives to inhibit or promote crack growth is a consequence of their ability to increase or decrease the supply of hydrogen atoms at the metal surface by some chemical process.      

Selective adsorption and hydrogen embrittlement

This paper presents a definite correlation between hydrogen embrittlement and adsorption. The effects of the presence of gaseous additives on hydrogen embrittlement and hydrogen adsorption were studied. Those gaseous additives which halt a running crack in 4340 steel loaded in a hydrogen atmosphere also halt hydrogen adsorption. Those gaseous additives which accelerate the crack growth increase the supply of hydrogen atoms at the metal surface. It is concluded that the effect of gaseous additives to inhibit or promote crack growth is a consequence of their ability to increase or decrease the supply of hydrogen atoms at the metal surface by some chemical process.     

Modeling hydrogen entry and exit in metals exposed to multiple charging processes

A model is presented to evaluate hydrogen entry or exit in metals exposed to an aqueous environment. The model may be used to analyze data obtained from hydrogen permeation experiments through metal membranes. The model takes advantage of the ideal behavior predicted from the Nernst Equation and Sievert’s Law. It provides a single parameter,κ, to quantify the deviation from Sievert/Nernstian behavior. It is sufficiently general to allow arbitrary chemical potentials of hydrogen on both sides of a sheet, in addition to an arbitrary initial hydrogen distribution within a metal sheet. Simulated permeation curves are presented to show the influence of the model parameters upon permeation behavior. The model is applied to solution agitation during permeation of a low-carbon steel sheet as an illustration. By curve-fitting the model to experimental data, the diffusivity, surface solubilities, andκ on both sides of a metal sheet may be obtained for a given electrochemical charging condition. When compared to other popular models, the model presented by this article fit the experimental data well. The parameters obtained by the model may be used to characterize a given charging process. As such, the effect of one or more processes may be evaluated by using the model to calculate hydrogen distributions in a metal sheet.     

The rate of absorption of nitrogen into liquid iron containing oxygen and sulfur

The rate of nitrogen absorption into and desorption from liquid iron containing sulfur and/or oxygen was measured by employing a constant-volume technique with a highly sensitive pressure transducer. Critical evaluation of the results demonstrated conclusively that the chemical rate at high oxygen or sulfur contents is second order with respect to nitrogen content in the metal and probably controlled by the dissociation of the nitrogen molecule on the surface. The effect of sulfur on the rate is complex because of the influence of 1) liquid-phase mass transfer at low sulfur contents, 2) the chemical rate on vacant iron sites at intermediate sulfur contents, and 3) the rate on the adsorbed sulfur layer or the limiting rate at high sulfur contents. However, at intermediate concentrations the limiting case for the adsorption isotherm for sulfur is adhered to and the rate is inversely proportional to the sulfur concentration. For Fe-O melts the rate is inversely proportional to the oxygen content except at low oxygen levels where mass transfer affects the rate. The rate for Fe-S-O melts can be calculated reasonably well from the results for the Fe-S and Fe-0 alloys, assuming that oxygen does not effect the adsorption of sulfur andvice versa and that there is nearly complete coverage of the surface with oxygen and sulfur atoms.     

Brittle failure resistance of steels and weld metals for pressure vessels of light-water reactors

The influence of the hydrogen effect during welding of great thicknesses of steels induced into welded joints particularly by moisture contained in the filler material used, is followed up in this paper. The results are compared with the effect of hydrogen induced into the weld metal by controlled cathodic hydrogenization, i. e. by simulation of service conditions. The development of cracks was observed by their propagation in both the undercladding area and surface weld deposit. In the paper the dependence of the crack propagation rate on the stress intensity factor range is expressed. It was found that even if the effect of metallurgically induced hydrogen has no unfavourable consequences on mechanical properties, it is necessary to minimize the hydrogen content in weld metal and avoid subcladding initiations and crack formation and protective cladding overlay reduction. Undercladding initiations and surface defects are the intercepting spots for the hydrogen diffusion, where damaging or substantial reduction of cohesive strength between grains appears.     

Determination of hydrogen pęrmeation parameters in alpha titanium using the mass spectrometer

The kinetic parameters of hydrogen permeation through α-Ti were determined from mass spectrographic measurements between 400° and 800°C and 0.004 and 2.0 torr H₂. The specimens were hollow cylinders and the hydrogen source was gaseous hydrogen. Hydrogen entered the specimen wall at the inside surface and emerged from the specimen at the outside surface. The permeated hydrogen flux is given byP = 1.6 × 10⁻⁵ p exp (-14,900 ± 856/RT) mole torr/s cm² where the hydrogen pressure isp and the permeation activation energy is 14,900 cal mole⁻¹ The surface dependent nature of hydrogen permeation in α-Ti is established in these experiments by the observations that 1) permeation is not proportional to the square root of input hydrogen pressure, 2) the concentration gradient calculated from experimental permeation data is lower than it would be if equilibrium solubility had developed, and 3) the diffusional frequency factor as determined under nonsteady-state conditions is low by a factor of three compared with values determined by others.     

Influence of metal-wustite separation on the reduction of wustite with hydrogen

The reduction rate of wustite with hydrogen at 1133 to 1233 K and for hydrogen pressures between 1.6 and 5 mbar has been measured gravimetrically upon varying the separation between tablets made of wustite and iron or nickel. The reduction rate of wustite is increased by a factor 2.4 prior to precipitation of iron on wustite when the wustite-metal distance is changed from 2 mm to 0.35 at H₂ pressures of 1.6 mbar and 2.3 mbar respectively. It has thus been demonstrated for the first time, that the catalytic effect of the metal phase on wustite reduction also exists when metal and wustite are separated. To describe this catalytic effect mathematical relations have been derived on the basis of a reaction model. These relations are in good accord with the results of the measurements. From recently published experimental findings, it may be concluded that, as a result of heterogeneous reactions, translationally ‘hot’ H₂ molecules are desorbed from the metal surface. These induce a rapid reaction at wustite unless they have been deactivated by molecular collisions within the bulk gas between the tablets.     

Investigations of hydrogen storage in palladium decorated graphene nanoplatelets

Carbon based nanomaterials are ideal media for hydrogen storage due to their highly porous structure, low density, and large surface area. The hydrogen uptake capacity of different carbon nanomaterials can be enhanced by spill over mechanism from a supported transition metal catalyst. In the present work graphene nanoplatelets (GNP) were prepared by thermal exfoliation method and decorated with palladium nanoparticles (Pd/GNP) by ethylene glycol reduction method. The high pressure hydrogen adsorption/desorption measurements were carried out for GNP and Pd/GNP using Sieverts apparatus in the temperature and pressure ranges of 25–100°C and 0.1–4 MPa, respectively. The hydrogen storage capacity of GNP and Pd/GNP were found to be 0.28 wt% and 1.21 wt% respectively at 25°C and 3.2 MPa pressure. Uniform dispersion of palladium nanoparticles over the surface of GNP enhances the hydrogen storage capacity of GNP by 70% due to spill over mechanism.     

Measurement and evaluation of hydrogen trapping in thoria dispersed nickel

The trapping of hydrogen in nickel-2 vol pct thoria has been measured by permeation techniques. The theory of McNabb and Foster was used to determine the density of trap sites, the rate constant for trapping, and the rate constant for untrapping. Annealed material has trap sites associated with the thoria-nickel interface with the density of sites equal to about a monolayer of hydrogen at the interface. Cold-rolling up to 59 pct reduction in thickness increases the trap site density, probably by opening more sites at the nickel-thoria interfaces. The trapping energy was determined to be about 7.1 kcal/mol H. The trapping rate constant has an activation energy similar to the activation energy for hydrogen diffusion. It was shown that trapping models assuming equilibrium between lattice hydrogen and trapped hydrogen do not apply to the nickel-thoria system. For this system trapping is relatively rapid and untrapping is slow, particularly at low temperatures.     

Hydrogen attack behavior of the heat affected zone of a 2.25Cr-1Mo steel weldment

The kinetics of hydrogen attack (HA) has been studied in the heat affected zone (HAZ) in a 2.25Cr-1Mo steel weld to determine the relative rates of attack and bubble nucleation in the HAZ, base metal, and weld metal. The HAZ was found to suffer hydrogen attack at nearly twice the rate of the base metal, but not as rapidly as the weld metal. Nucleation of bubbles does not occur during HA of the HAZ of a 2.25Cr-1Mo steel, on exposure to hydrogen pressure of 20.5 MPa or less, but does occur at higher pressures up to 31.5 MPa (4500 psi) at 550 °C, or up to 27.5 MPa (4000 psi) at 580 °C. Such nucleation results in enhancement of the HA rate by a factor of six. The weak dependence of nucleation effects on hydrogen pressure and the saturation of the nucleation effects in a short time suggest some thermally activated nucleation of fresh bubbles. Formerly with The Ohio State University.     

Crack propagation of high strength steels in a gaseous hydrogen atmosphere

The crack propagation behavior in delayed fracture was investigated at room temperature under pressure up to 784 kPa of hydrogen gas for high strength steels with the tensile strength of 1500 or 2000 MPa. For specimens with the tensile strength of 1500 MPa, the crack propagation rateda/dt increased with C content from 0.21 to 0.42 wt pct, and then it decreased with increasing C content up to 0.53 wt pct.da/dt increased rapidly with Mn content from 0.009 to 0.84 wt pct, and it increased gradually with Mn content from 0.84 to 2.13 wt pct. The permeation flow of hydrogen from the crack tip surface could be qualitatively estimated from the dependence ofda /dt upon the hydrogen pressure based on the simple assumption. In addition to the grain boundary embrittlement caused by tempering and/or hydrogen, the permeation flow of hydrogen was used to explain qualitatively the dependence ofda/dt upon C or Mn content.     

The rate of absorption of hydrogen into iron and of nitrogen into Fe-Cr and Fe-Ni-Cr alloys containing sulfur

The rate of absorption of hydrogen into liquid iron and of nitrogen into liquid Fe-Cr alloys containing various levels of sulfur was measured by using a constant-volume Sieverts apparatus employing a sensitive pressure transducer. The rate for the absorption of hydrogen was measured by using H2 containing a small amount of H₂S(<0.2 pct) such that the activity of sulfur on the surface of the melt was the same as in the bulk metal. The hydrogen-absorption rate for Fe-S melts containing up to 0.72 pet sulfur was virtually independent of sulfur content and controlled by liquid-phase mass transfer. The liquidphase mass-transfer coefficient for hydrogen in liquid iron, calculated from the results, was comparable to that for nitrogen transfer in liquid iron. The rate of absorption of nitrogen into Fe-Cr melts with low-sulfur contents was controlled by liquid-phase mass transfer. For melts containing significant amounts of sulfur it was controlled by both mass transfer and the chemical rate of the dissociation of nitrogen on the surface in series. Equations were developed to calculate the chemical rate from the measured rate, correcting for mass transfer. The chemical rate decreased with increasing sulfur content as expected, because sulfur is strongly adsorbed on the surface and increased with chromium content at constant sulfur activity, possibly because available Cr sites promote nitrogen dissociation. Formerly with United States Steel Corporation, Monroeville, PA     

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.     

Effects of O,Se, and Te on the rate of nitrogen dissolution in molten iron

The effects of oxygen, selenium, and tellurium on the rate of nitrogen dissolution into molten iron have been investigated at 1973 K using an isotope-exchange reaction and the results are summarized as follows. The rate constant of nitrogen dissolution measured at lower oxygen concentration ([mass pct O] < 0.015) is larger than previously reported ones under an atmospheric pressure and agrees well with the value from desorption rate under reduced pressures. Selenium and tellurium retard the nitrogen dissolution into liquid iron more significantly than oxygen, and the degree of the retarding effect is in the order of tellurium, selenium, and oxygen. The adsorption coefficients are calculated to be K_O = 144, K_Se = 1120, and K_Te = 1640 with respect to [1 mass pct solute] from present results. A model that surface active elements and nitrogen are adsorbed on the same site at the interface and the dissociation reaction of nitrogen molecule on the site represented by the equation is the rate-determining step reasonably expresses the retarding effect of the surface active elements on the reaction rate on the assumption that all sites at the metal surface have a uniform adsorption energy for each solute.     

The effect of vacancies on hydrogen diffusivity and solubility in pure iron at room temperature

Determination of the apparent diffusivity and effective solubility of hydrogen in pure iron at room temperature by an electrochemical hydrogen permeation technique. Evidence that the effect of vacancies as hydrogen trap sites on hydrogen diffusivity and solubility in pure iron at room temperature is stronger than the influence of the dislocations.     

Hydrogen Diffusion and Trapping Effects in Low and Medium Carbon Steels for Subsurface Reinforcement in the Proposed Yucca Mountain Repository

The electrochemical hydrogen permeation method was used to investigate hydrogen transport, trapping characteristics of low (0.08 pct C) and medium carbon (0.44 pct C) steels proposed for the Yucca Mountain (YM) repository environment. The presence of relatively high amounts of C, Mn, and S increased the density of trapping sites in medium carbon steel. The measured diffusivity of medium carbon steel was lower than that of the low carbon steel due to increased trapping of hydrogen at irreversible sites in the medium carbon steel. Hydrogen concentration values obtained for low carbon steels in YM ground water electrolytes indicate that increased ionic concentration decreases the uptake of hydrogen. The decrease in hydrogen permeation were due the formation of CaCO₃ corrosion products on the surface of steels.     

Effects of NO2 ? and NO3 ? Ions on Corrosion of AISI 4130 Steel in Ethylene Glycol?+?Water Electrolyte

The electrochemical behavior of steel alloy in ethylene glycol?Cwater mixture was investigated by electrochemical methods. The results obtained showed that corrosion rate was decreased with increasing ethylene glycol concentration. The effect of nitrite and nitrate as inhibitor was studied and higher inhibition efficiency was obtained for nitrite. It was found that surface passivation occurred in presence of inhibitor. The inhibiting effect of the nitrite was explained on the basis of the competitive adsorption between the inorganic anions and the aggressive Cl^? ions and the adsorption isotherm basically obeys the Flory?CHuggins adsorption isotherm. Thermodynamic parameters for inhibitor adsorption were determined and reveal that the adsorption process is spontaneous.     

Corrosion of Alloy Steel in 30% Ethylene Glycol Solution and CrO_4~（2-） Under Hydrodynamic Condition

 The electrochemical behavior of steel alloy in 30% ethylene glycol-water solution at different solution rotating speeds was investigated by polarization curves and AC impedance measurements. The results obtained showed that corrosion rate did not change significantly at different rotating speeds. The effect of chromate as the inhibitor was studied and high inhibition efficiency was obtained. It was found that surface passivation occurred in the presence of the inhibitor. The inhibiting effect of the chromate was explained on the basis of the competitive adsorption between the inorganic anions and the aggressive Cl- and the adsorption isotherm basically obeys the Langmuir adsorption isotherm. Thermodynamic parameters for inhibitor adsorption were determined and results reveal that the adsorption process is spontaneous.     

The hydrogen embrittlement of Fe-Ni martensites

Iron alloys containing 20 and 30 pct Ni and 3 to 4 cu cm H per 100 g metal have been subjected to slow strain-rate tensile tests in a study of hydrogen embrittlement. In the lower nickel massive martensite alloy, embrittlement is manifest as the cracking of prior austenite grain boundaries and is severe at room temperature but less marked at -196°C; while in the higher nickel acicular martensite alloy, the embrittlement observed at 20°C does not occur at —196°C. Hydrogen embrittlement in these materials is believed to be the result of high hydrogen contents in the vicinity of the prior austenite grain boundaries combined with stress concentrations caused by boundary perturbations which result from the impact of the martensite shears. During deformation, microcracks form and propagate in the prior austenite grain boundaries, probably assisted by internal hydrogen pressure and the lowering of crack surface energy by hydrogen adsorption. The temperature dependence and the effect of the type of martensite on the embrittlement can be explained by their effects on the hydrogen content and stress concentrations at prior austenite grain boundaries during deformation.     

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