Rex B. McLellan

The solubility and diffusivity of hydrogen in well-annealed and deformed iron

Abstract It has been shown that a large volume of data for the solubility of hydrogen in iron is affected by spurious surface conditions. Arrhenius plots of solubility data in the temperature range 300–1750 K, which are free of such effects, exhibit a temperature variation which, despite the low H-solubility in the entire temperature range, is not consistent with regular mixing statistics. This departure from regular behavior is consistent with the thermal activation of H atoms into energetically less favorable octahedral sites as the temperature is increased. The enhancement in H-solubility caused by the cold deformation of iron can be understood in terms of a simple Maxwell-Boltzmann distribution of H atoms between “normal” lattice sites and “trapping” sites of depth 34 kJ/mol. The 62 currently existing sets of data for the diffusivity D of hydrogen through b.c.c. iron exhibit a large degree of mutual inconsistency. Exhaustive statistical analysis of this large data mass has shown that only those data obtained by electrochemical methods and H2-gas equilibration methods using UHV techniques and Pd-coated membranes are reliable. In the range −40–80°C the best representation of D is D = 7.23 x 10−8 exp (−Q/RT)m2/s where Q = 5.69 kJ/mol. In the temperature range 50°–550°C, the best D-representation is D = (1−2.52) x 10−7 exp (−|Q/RT)m2/s with Q in the range 6.70–7.12 kJ/moI. These differing diffusivities are consistent with an increase with temperature of the fraction of H-atoms hopping from octahedral rather than tetrahedral sites. The problem of H-diffusion in deformed iron has been analysed using a semi-quantitative model in which the retarding effect of trapping sites on the diffusivity is partially compensated by a “pipe” diffusion contribution along dislocations. It is shown that this model is in accord with the diffusivities measured in deformed iron when data not encumbered by spurious surface effects are considered.

Diffusion of carbon and nitrogen in B.C.C. iron

Abstract It is well known that Arrhenius plots of the diffusivity of carbon in b.c.c. iron exhibit positive departures from linearity. It has been demonstrated that such an effect is shown by nitrogen but the strength of the deviation is only about one half that found for carbon. The theoretical models proposed to account for this effect have been discussed and it is tentatively concluded that there is a change in the intrinsic jump frequency of the diffusing species due to the change in that portion of the motion energy due to the magnetic exchange energy.

The solubility of hydrogen in rhodium, ruthenium, iridium and nickel.

Abstract The temperature variation of the solubility of hydrogen in rhodium, ruthenium, iridium and nickel in equilibrium with H2-gas at 1 atm pressure has been measured by a technique involving saturating the solvent metal with hydrogen, quenching and analyzing in resultant solid solutions. The solubilities determined are small (atom fraction of H is in the range 5 × 10−4−10−5) and the results are consistent with the simple quasi-regular model for dilute interstitial solid solutions. The relative partial enthalpy and excess entropy of the dissolved hydrogen atoms have been calculated from the solubility data and compared with well-known correlations between these quantities.

A quasi-chemical treatment of interstitial solid solutions: It application to carbon austenite

Abstract A first order quasi-chemical treatment of binary interstitial solid solutions has been given. The enumeration of the nearest neighbor pairs is simplified by only considering those pairs whose number depends on the atomic configuration of the solution. Simple expressions have been derived for the partial Gibbs free energy and thermodynamic activity of an interstitial solute atom in terms of the pairwise interaction energy between two nearest neighbor solute atoms. It is shown that the equations yield the appropriate results in the limits when the binding energy between solute pairs becomes zero or infinite. Finally the equations are shown to be compatible with the known activity of carbon in austenite and a computer fitting technique has been used to derive the value of the solute-solute binding energy concomitant with the closest fit of the experimental data fo the theoretical activity equation.

Electrochemical Methods for Measuring Diffusivities of Hydrogen in Palladium and Palladium Alloys

A palladium bielectrode has been used in a viscous electrolyte consisting of glycerin and phosphoric acid. The transport of molecular hydrogen from and to the electrode is remarkably diminished in this electrolyte in comparison with aqueous electrolytes. Therefore, changes of the electrode potential are unambiguously related to concentration changes caused by diffusion within the electrode. A single current pulse and a potentiostatic technique were applied to determine the diffusion coefficients of H in Pd over a wide range of concentration and at different temperatures. New evaluation methods for current‐time and potential‐time curves also yield values of the initial concentration, which are in excellent agreement with the values calculated from previous coulometric titrations. A special arrangement of two bielectrodes also allows the measurement of small differences in the diffusivities of hydrogen in the two electrodes. Results for hydrogen diffusion in pure palladium are in excellent agreement with Gorsky effect measurements.

Thermodynamics and diffusion behavior of interstitial solute atoms in non-perfect solvent crystals

Abstract The effect upon the thermodynamic functions and diffusion behavior of the interaction of interstitial solute atoms with lattice discontinuities has been calculated. The calculations are based on first-neighbor first-order statistics and the absolute rate reaction theory. It is shown that the calculations are consistent with the observed behavior of hydrogen in b.c.c. iron if it is assumed that the iron contains a density of ‘trapping’ sites in the range 10 −6 –10 −3 such sites per Fe atom and the depth of the sites is ~ −27 kJ/mole. The form of the distribution function relating the ratio of the number of solute atoms in ‘normal’ lattice sites to these located in ‘trapping’ sites to temperature is a modified Fermi-Dirac function.

The diffusivity of hydrogen in aluminum

Abstract Using an electrolytic method employing a viscous electrolyte, the diffusivity of hydrogen in aluminum has been measured in the temperature range 285–328 K. The results show that H diffuses by a single-stage process from 285 K up to the melting temperature and no departures from Arrhenius behavior due to trapping effects involving lattice vacancies are observed.

Temperature dependence of the Young’s modulus and shear modulus of pure nickel, platinum, and molybdenum

A pulse-echo technique utilizing a wave guide operating at high temperatures has been used to measure the variation in the Young’s modulus and shear modulus of pure platinum, nickel, and molybdenum in the temperature range from 25 to 1000°. With the exception of Ni in the temperature range below the Curie point, linear behavior was found for all three metals. The significance of the linear relation between temperature and the elastic constants for Mo to the observed nonlinearity of the Arrhenius plot of the diffusivity of carbon in Mo is discussed.

The solubility of hydrogen in nickel and cobalt

Abstract The solubility of hydrogen in nickel and cobalt at atmospheric pressure has been measured over large temperature ranges. Large departures from Arrhenius behavior have been found for polycrystalline nickel and cobalt, but equilibrations using a nickel single crystal did not show curvature in the Arrhenius plot. The measured solubility data have been interpreted using a solid solution model involving sites of differing energy at which solute atoms can be located.

Hydrogen interactions with metals

Review of the literature on the nature and extent of hydrogen interactions with metals and the role of hydrogen in metal failure. The classification of hydrogen-containing systems is discussed, including such categories as covalent hydrides, volatile hydrides, polymeric hydrides, and transition metal hydride complexes. The use of electronegativity as a correlating parameter in determining hydride type is evaluated. A detailed study is made of the thermodynamics of metal-hydrogen systems, touching upon such aspects as hydrogen solubility, the positions occupied by hydrogen atoms within the solvent metal lattice, the derivation of thermodynamic functions of solid solutions from solubility data, and the construction of statistical models for hydrogen-metal solutions. A number of theories of hydrogen-metal bonding are reviewed, including the rigid-band model, the screened-proton model, and an approach employing the augmented plane wave method to solve the one-electron energy band problem. Finally, the mechanism of hydrogen embrittlement is investigated on the basis of literature data concerning stress effects and the kinetics of hydrogen transport to critical sites.