Influence of the crystalline microstructure on the diffusivity of hydrogen in palladium galvanostatic permeation and X-ray diffraction measurements

The influence of crystalline microstructure upon the apparent diffusion coefficient of hydrogen in Pd samples are reported. The apparent diffusion coefficient was measured by the galvanostatic permeation method (at 20°C), while the structure was characterized by X-ray diffraction measurements. Pd membrane electrodes of different thicknesses, l = 5 × 10⁻³cm and l = 10⁻²cm, were used. The structure of Pd membrane electrodes was changed by successive annealing and sequences of absorption and desorption of hydrogen accompanied by the α⇌β phase transition. Both the irreversible and reversible traps affect the mobility of hydrogen in Pd. The irreversible traps manifest in the variation of the apparent diffusion coefficient as a function of the average stationary bulk concentration of hydrogen, while the diffusion coefficient of the free hydrogen does not depend on the actual concentration of hydrogen in Pd. This is true provided that the average stationary bulk concentration of hydrogen is higher than the irreversible trap concentration. The diffusion coefficient of free hydrogen depends on the specific internal surface area according to McNabb-Foster-like equation. The value of the diffusion coefficient of hydrogen in single crystal of palladium was estimated, D₀ = (4.2 ± 0.3) × 10⁻⁷ cm²s⁻¹ (20°C).     

Hydrogen diffusion coefficients in the titanium alloys IMI 834, Ti 10-2-3, Ti 21 S, and alloy C

The diffusion coefficients of hydrogen were determined for three beta titanium alloys, which differ in the stability of the beta phase, and, for comparison, for one near-alpha titanium alloy in the temperature range from 40 °C to 500 °C. For this purpose, a step hydrogen concentration profile was produced by charging one side of rods of these materials with hydrogen by means of an electrochemical technique. After subsequent diffusion annealing, the hydrogen concentration profiles were determined analyzing small discs cut from these rods. The corresponding diffusion coefficient was calculated by adapting its value in such a way that a numerical diffusion simulation yields the experimentally determined profile. In all cases studied, the hydrogen diffusion coefficient was found to be independent of the hydrogen concentration and showed an Arrhenius-type temperature dependence. Hydrogen diffusion in alpha titanium was slower as compared to the beta titanium alloys under investigation. Furthermore, the diffusion coefficient of the metastable beta titianium alloys is only slightly affected by the prior heat treatment that determines the morphology and volume fraction of the precipitated alpha phase.     

Hydrogen transport in stagnant molten iron

A new experimental method is described for the study of gas absorption in liquid metals. The technique involves the sudden admission of the gas into a previously evacuated tube containing a column of stagnant liquid metal. Following a short initial period of pressure instability (< 10 sec), absorption rates can be accurately followed by monitoring changes in gas pressure (or volume) using a differential pressure transducer. In the present work, the solution of hydrogen in pure liquid iron was studied and confirmed to be a diffusion controlled process. The mean diffusion coefficient measured for hydrogen absorption in pure iron at 1 atm pressure over the temperature range 1550° to 1720°C was 8.76 x 10⁻⁴ sq cm sec⁻¹. This figure is about 35 pct lower than previously reported values by Parlee and coworkers.⁹⁻¹⁰     

Measurement of hydrogen solubility in liquid iron alloys employing a constant volume technique

The solubility of hydrogen in liquid pure iron and in several liquid binary iron alloys at steelmaking temperatures has been determined by measuring changes in hydrogen pressure in a constant volume system. The solubility of hydrogen corrected to one atmosphere pressure was found to be 27.70 ± 1.28 cc (STP)/100 grams in liquid pure iron at 1600°C with a temperature coefficient of solubility of 2.9 x 10^-2 cc (STP)/°C. This solubility decreases with increasing concentrations of aluminum, boron, or silicon; slightly increases with increasing concentrations of chromium, nickel, and niobium; and is almost independent of the concentrations of copper or sulfur. The data are compared with those of previous investigators who employed the more conventional Sieverts’ or sampling techniques.     

The diffusion of antimony in alpha iron

Diffusion coefficients of antimony in α-iron were determined in the temperature range 700 to 900°C using the residual activity method. Specimens were large-grained polycrystals for the higher temperature measurements and single crystals for the low temperature measurements. Above 800°C the data may be represented by the equationD _sb(cm²/s) = (440 ± 200) exp [- (270,000 ± 7000)/RT]. The activation energy (reported in J/mole) is approximately equal to that measured for iron self-diffusion in this same temperature range, although the antimony diffusion coefficients are a factor of ten larger than the iron self diffusion coefficients. The potential for strongly coupled vacancy-antimony motions is demonstrated, based on the observed enhancement of iron self diffusion in dilute iron-antimony alloys. Finally molybdenum is shown to have a negligible effect on the diffusion of antimony in α-iron. These results are discussed in relation to the phenomenon of temper brittleness in steels. Embrittlement kinetics in iron-antimony alloys are shown to be consistent with an antimony diffusion controlled segregation mechanism.     

Ferric nitrate leaching of uranium and radium from uranium ores

A dilute aqueous acidic ferric nitrate solution of a relatively low concentration of 0.02 M is found to remove up to 97% of the uranium and 93% radium from ores occurring in the Elliot Lake area of Canada, after an initial flotation-separation of the sulfide minerals from the ore. A temperature of 75°C, a contact time of 24 h and a liquid/solid ratio of 1 ml/g are found optimum for leaching giving tailings which are effectively sulfide-free and with radium levels approaching a low value of 25 pCi/g. Radium may be removed from the leachate by adsorption and uranium by solvent extraction. An amount of 80–87% of the ferric nitrate may be recirculated for further leaching. Because of the low reagent concentrations and the recycle, it is possible to keep nitrate ion levels in the effluent below the prescribed level of 10 mg/l.     

Extraction of nickel,cobalt and other metals [Cu,Zn, Fe(III)] with a commercial β-diketone extractant

The extraction of nickel, cobalt, copper and zinc from ammoniacal solutions of ammonium carbonate or ammonium sulphate by solutions of Hostarex DK-16 in kerosene has been investigated as a function of phase contact time, aqueous-phase pH and organicphase reagent concentration. Besides copper, Hostarex DK-16 also partially extracts iron (III) from moderately acidic solutions whereas nickel, cobalt(II), copper and zinc are extracted from neutral or ammoniacal ammonium sulphate and ammonium carbonate solutions. Extraction decreases in the following order of metals: Cu > Co > Ni > Zn. Cobalt(III) is not extracted, but the complex of cobalt(II) with Hostarex DK-16 is slowly oxidized to a cobalt(III) complex which cannot be stripped even when 10 N sulphuric acid is used. Absorption spectra for cobalt complexes with Hostarex DK-16 (purified by preparative thin-layer chromatography) in benzene also suggest oxidation of cobalt(II) to cobalt(III) in the organic phase. Nickel, cobalt(II), zinc and copper can be stripped easily from organic solution with dilute solutions of sulphuric acid. Hostarex DK-16 extracts iron very slowly, nickel moderately rapidly and copper, cobalt(II) and zinc rapidly. Slope analysis and extraction isotherms suggest that the complexes CuR₂, NiR₂ ·HR and CoR₂·HR are present in the organic phase. Nickel can easily be separated from cobalt by extraction with Hosterex DK-16 after oxidation of cobalt in aqueous ammoniacal solution by hydrogen peroxide; however, LIX 64N seems to be a more promising extractant owing to the higher extraction of nickel under analogous conditions and the poorer extraction of zinc in comparison with Hostarex DK-16.     

Solubility of iron in liquid selenium below 585°C

Abstract

With the use of a JXA-3A microprobe the diffusion characteristics of indium in lead single crystals were determined in the temperature range from 160 to 200°C. For the temperature dependence of the heterodiffusion coefficient of indium in lead in dilute solid solution the relation

D = 33.exp {?(26.79 ± 0.76)/RT} cm²/sec

was found. A special technique for measuring the concentration gradient was developed. This enables one to eliminate the influence of surface relief, formed during the microtome preparation of the sample, on the microprobe measurements.

Résumé

Au moyen d'une micro-sonde JXA-3A les auteurs ont détermine les caractéristiques de diffusion de l'indium dans des cristaux uniques de plomb, entre 160°C et 200°C. En solution étendue, le coefficient de diffusion hétérdgène de l'indium dans le plomb est relié à la température par la relation

D = 33.exp {?(26.79 ± 0.76)/RT}cm²/sec

Une technique nouvelle mise au point par les auteurs élimite l'influence de la rugosité de surface produite au cours de la préparation d'éprouvettes par microtome.     

The diffusion of antimony in alpha iron

Diffusion coefficients of antimony in α-iron were determined in the temperature range 700 to 900°C using the residual activity method. Specimens were large-grained polycrystals for the higher temperature measurements and single crystals for the low temperature measurements. Above 800°C the data may be represented by the equationD _sb(cm²/s) = (440 ± 200) exp [- (270,000 ± 7000)/RT]. The activation energy (reported in J/mole) is approximately equal to that measured for iron self-diffusion in this same temperature range, although the antimony diffusion coefficients are a factor of ten larger than the iron self diffusion coefficients. The potential for strongly coupled vacancy-antimony motions is demonstrated, based on the observed enhancement of iron self diffusion in dilute iron-antimony alloys. Finally molybdenum is shown to have a negligible effect on the diffusion of antimony in α-iron. These results are discussed in relation to the phenomenon of temper brittleness in steels. Embrittlement kinetics in iron-antimony alloys are shown to be consistent with an antimony diffusion controlled segregation mechanism.     

Diffusivity and solubility of hydrogen as a function of composition in Fe-Ni alloys

An electrochemical method has been used to determine the permeability,P, diffusion coefficient,D, and solubility,c, of hydrogen in alloys of the Fe-Ni system. The heats of activation for diffusion and the heats of solution have been derived.D falls from ≃10⁻⁴ sq cm per sec for pure iron to ≃10⁻¹⁰ sq cm per sec for 40 wt pct of Ni in the alloy. Thereafter it rises slightly to that for pure nickel,c rises by about 10³ between pure iron and 40 wt pct Ni, then remains constant up to pure nickel. The resultantP doubles at 5 wt pct Ni and then falls by 10³ times up to 40 wt pct Ni, afterwards rising slightly to that for pure nickel. Between 0 and 40 wt pct Ni the dominant factor in controlling the value ofP is the fall of the mole fraction of the α phase in the alloy. This hypothesis gives a reasonable quantitative calculation of theP-composition relation. Between 40 and 100 wt pct, the crystallographic phase is allγ and the major effect is the bonding of hydrogen in the alloy, the small changes noted being reasonably calculable. The negligible change of solubility in this region reflects the negligible change ind character of the alloy from 40 to 100 wt pct Ni. The hydrogen permeability of Fe-Ni (5 wt pct) is greater than that of palladium atT > 200°C. The corrosion rate and hydrogen permeability (hence, susceptibility to hydrogen embrittlement) pass through a minimum at about 50 wt pct Ni. A remarkable parallelism exists between corrosion rate and hydrogen permeation in Fe-Ni alloys. An interpretation is suggested. Formerly with the University of Pennsylvania Formerly with the University of Pennsylvania Work carried out by P. K. SUBRAMANYAN in partial fulfillment of the requirements for the degree of Doctor of Philosophy, University of Pennsylvania, 1970.     

Electromigration of hydrogen and deuterium in tantalum: Isotope effect

Electromigration experiments at steady-state were conducted to determine the effective valence of hydrogen isotopes in tantalum specimens at room temperature (25 °C). Varying solute concentrations were obtained by gas-phase and electrolytic charging of the specimens. The transient concentration profiles of several specimens were monitored by a resistance technique allowing a controlled current through the samples. Steady-state concentration profiles were obtained by direct hot-vacuum extraction measurements. The results indicate a strong concentration dependence for the effective valence and evidence of a possible reversal in the direction of solute migration. While the reversal in the case of deuterium migration is not certain, an isotope effect on the effective valence as a whole is substantial. Deuterium has a 30 pct higher effective valence. The hypothesis that solute segregation at lattice defects and differing electronic structure of the defects and the perfect lattice are responsible for the concentration dependence of effective valence is found invalid as both cold-worked and annealed specimens did not indicate any microstructure effect on Z*. The concentration dependence of diffusion coefficient does not justify the strong dependence of Z* on concentration observed experimentally.     

Rate of the reduction of the iron oxides in red mud by hydrogen and converted gas

The drying and gas reduction of the iron oxides in the red mud of bauxite processing are studied. It is shown that at most 25% of aluminum oxide are fixed by iron oxides in this red mud, and the other 75% are fixed by sodium aluminosilicates. A software package is developed to calculate the gas reduction of iron oxides, including those in mud. Small hematite samples fully transform into magnetite in hydrogen at a temperature below 300°C and a heating rate of 500 K/h, and complete reduction of magnetite to metallic iron takes place below 420°C. The densification of a thin red mud layer weakly affects the character and temperature range of magnetizing calcination, and the rate of reduction to iron decreases approximately twofold and reduction covers a high-temperature range (above 900°C). The substitution of a converted natural gas for hydrogen results in a certain delay in magnetite formation and an increase in the temperature of the end of reaction to 375°C. In the temperature range 450–550°C, the transformation of hematite into magnetite in red mud pellets 1 cm in diameter in a converted natural gas is 30–90 faster than the reduction of hematite to iron in hydrogen. The hematite-magnetite transformation rate in pellets is almost constant in the temperature range under study, and reduction occurs in a diffusion mode. At a temperature of ～500°C, the reaction layer thickness of pellets in a shaft process is calculated to be ～1 m at a converted-gas flow rate of 0.1 m³/(m² s) and ～2.5 m at a flow rate of 0.25 m³/(m² s). The specific capacity of 1 m² of the shaft cross section under these conditions is 240 and 600 t/day, respectively. The use of low-temperature gas reduction processes is promising for the development of an in situ optimum red mud utilization technology.     

Interaction of the oxyfluoride melts of Ca,Al, Mg,Si, Ba,and Cr with refractories based on alumina and magnesia

The interaction of slags consisting of the oxides and fluorides of Ca, Al, Mg, Si, Ba, and Cr with refractories based on Al₂O₃, MgO, and MgO · Al₂O₃ is experimentally studied at 1600–1700°C and p _Ar = 0.3 MPa. Laser emission microprobe analysis is used to study the fracture surfaces in the slag-crucible contact zone, to analyze the depth profile of the chromium concentration in the bulk of the refractory material, and to determine the diffusion coefficient of chromium. The values of the diffusion coefficient are related to the slag melt composition. These relations are established using the optical basicity of the slag.     

Leaching of Chalcopyrite Concentrate with Hydrogen Peroxide and Sulfuric Acid in an Autoclave System

In this study, leaching of chalcopyrite concentrate was investigated in an autoclave system using hydrogen peroxide and sulfuric acid. By decomposition of hydrogen peroxide, the active oxygen formed can provide both high oxidation potential and high pressure in a closed vessel for leaching. Preliminary studies showed that hydrogen peroxide can be used as an oxidant instead of oxygen gas in the autoclave. Central composite design (CCD) was used to examine the effects of the experimental parameters on the copper and iron extraction as a response. The proposed model equation using CCD showed good agreement with experimental data, the correlation coefficients R ² for copper and iron being 0.84 and 0.86, respectively. The optimum conditions to obtain the main goal of maximum copper and minimum iron extraction from chalcopyrite were determined as to be sulfuric acid concentration of 2.5 M, hydrogen peroxide concentration of 2.3 M, leaching time of 24 minutes, chalcopyrite amount of 3.17 g (in 50-mL solution), stirring speed of 630 rpm, and leaching temperature of 351 K (78 °C). Under the optimum condition, 76 pct of copper and 9 pct of iron were extracted from chalcopyrite concentrate. Extraction yield results of metals indicate that selective leaching of chalcopyrite can be achieved using hydrogen peroxide and sulfuric acid in an autoclave system.     

Thermodynamics of the Fcc Fe−Ni−C and Ni−C alloys

The activity of carbon in the fcc solid solution of the Fe?Ni?C system has been measured at 800°, 1000°, and 1200°C by comparison with observed values in the Fe?C binary by equilibration with methane-hydrogen mixtures. Defining the lattice ratioz _C≡n _C/(n _Fe+n _Ni?n _C), the activity coefficient Ψ_C≡a _C/z _C has been determined as a function of temperature and composition. At infinite dilution log Ψ_C goes through a maximum at about 70 pct Ni in agreement with Smith. The partial molar free energy of carbon in the dilute solution referred to graphite is not a linear function of the base alloy composition, but has a large deviation with maximum at about 60 pct Ni. Similar maxima occur in both ΔH _C ^° and ΔS _C ^° . Linear equations are derived for the activity coefficient of carbon in three composition ranges of Fe?Ni?C alloys; a simplified equation applicable to nickel steels is included. The solubility of graphite in nickel has been determined. The marked deviation from linearity is ascribed to the existence of iron atoms in two electronic states, γ₁ and γ₂ which differ in energy and are antiferromagnetic and ferromagnetic, respectively.     

Diffusion of H2-H2O through porous iron formed by the reduction of iron oxides

Measurements were made of the rate of equimolar counterdiffusion of hydrogen and water vapor through porous iron formed by the reduction of dense hematite, magnetite, and commercial iron ore pellets with hydrogen. The experiments were conducted at temperatures between 400° and 1000°C and at pressures between 0.1 and 40 atm. It is demonstrated that the structure of the porous iron is primarily a function of reduction temperature and that the diffusion process at the higher reduction temperatures is normal. The effect of gaseous diffusion on the rate of reduction of dense hematite with hydrogen is discussed. It is shown that gaseous diffusion limits the rate at the higher temperatures and pressures.     

Copper diffusion in iron during high-temperature tensile creep

The diffusion of liquid copper in iron from a notched surface has been studied by metallographic, microanalysis, and sessile drop techniques. The diffusivity of copper was found to be 0.59×10^?6 sq cm per sec at 1100°C and 0.97×10^?6 sq cm per sec at 1130°C. The diffusion factor,D ₀ was 0.78×10^?3 sq cm per sec and the activation energy 19.0 kcal per mole. The predominant mode of copper penetration was along grain boundaries, but when larger volumes of copper at the iron surface were used, surface diffusion increased and grain boundary penetration remained constant. The most frequently occurring dihedral angle for liquid copper was 34 deg at 1100° and 1130°C. The liquid copper/austenite interfacial energy was found to be 444 ergs per sq cm between 1100 and 1130°C. From sessile drop measurements, the contact angle was determined as 35 deg at 1100°C and 28 deg at 1130°C, from which values the respective interfacial energies were calculated to be 387 ergs per sq cm and 301 ergs per sq cm.     

Magnetic properties and microstructures of terbium coated and grain boundary diffusion treated sintered Nd-Fe-B magnets by magnetron sputtering

Tb coating on the surface of commercial sintered Nd-Fe-B magnet was prepared by DC magnetron sputtering.The secondary heat treatment was used to regulate the microstructure for the enhancement of coercivity,namely diffusion treatment and annealing treatment.The coercivity increases significantly from 18.3 to 28.0 kOe,the remanence decreases slightly from 14.1 to 14.0 kGs,and the comprehensive magnetic properties are higher than 75(Hcj+(BH)_max=76.7).SEM results indicate that,on the one hand,950℃is the optimal diffusion temperature.Lower diffusion temperature results in insufficient diffusion of Tb element.Higher diffusion temperature can lead to the main phase grain growth,the decrease of Nd-rich phase,and forming holes in the magnet.On the other hand,500℃is the optimal annealing temperature.Lower annealing temperature can result in the reduction of Nd-rich phase.Higher annealing temperature can generate the non-defined Nd-rich thin layer between grains.     

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.     

Preparation of titania from llmenite by selective H2S sulfidization

Experiments on preferential sulfidization of ilmenite in a static bed by hydrogen sulfide gas with a flow rate of about 20 cu cm per min were conducted at 800°, 900°, 1000°, and 1100°C for different periods. Sulfidization (conversion of about 95 pct of the iron to its sulfide) at 1000°C for 5 hr or at 1100°C for 4 hr was considered quite suitable. The sulfidized samples were leached with boiling dilute hydrochloric acid and the influence of the concentration, amount of the acid, and the duration of leaching was studied. Leaching was most effective by employing 20 pct excess 2 N boiling hydrochloric acid for 1/2 hr. By this treatment, about 95 pct of the iron in ilmenite could be removed and a residue containing about 91 pct TiO₂ has been obtained. X-ray analysis revealed that TiO₂ in the residue has rutile structure. The merits of the process for commercial utilization have been mentioned briefly.