P. J. Ficalora

A study of the reaction kinetics for the formation of rare earth-transition metal laves compounds

The kinetics of the reaction A(s) + 2B(s) =AB2(s) was studied by differential thermal analy sis. TheA component was Pr, Gd, Dy or Er, theB was Fe, Co, or Ni and the resultingAB2 compound was a MgCu2 Laves intermetallic compound. A generalized search method to de termine the reaction mechanism was developed and applied to the above reactions. The re actions were found to be diffusion controlled and the kinetics best described by the Valensi-Carter equation.

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 H2+ 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 H2+ ion is the rate controlling process in gaseous hydrogen embrittlement. At temperatures above 300 K, the decreasing net adsorption rate of H2+ becomes the limiting process.

The dependence of the fracture toughness of 4340 steel on an external chlorine gas environment

The effect of an external chlorine gas environment on the fracture toughness of a HSLA steel was investigated. Cracking occurred via a fast fracture. Premature fracture resulted from an adsorption induced process occurring on the surface. (FS)

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.

Diffusion in transition metals and alloys

The activation energies for diffusion in transition metals is correlated against the localizedd electron population. Thed electron population is determined by the Engel-Brewer theory. A quasichemical model of a binary solid solution is used to determine the distribution of atoms and vacancies. The use of this model enables one to determine the average number of bonds affected by the formation of a vacancy and the jumping of an atom into the vacancy. The activation energy is considered to consist of energies of vacancy formation and atom migration. The correlative curve and an empirical ratio is used to separate the activation energies into the above two parts. Finally the results of the quasichemical model and the correlation are combined. The calculated activation energies of impurity atoms in pure metals and of self diffusion in binary alloys (Co-Ni, Fe-Ti, Fe-Cr and Fe-V) are compared with experimental results.

A magnetization-chemisorption study of hydrogen and sulfur dioxide on iron

Magnetization-pressure isotherms are measured for hydrogen and sulfur dioxide on small iron particles. The hydrogen isotherms are shown to relate directly to hydrogen uptake and obey the Temkin isotherm. The bonding of hydrogen to iron surfaces at different temperatures is determined from the data and related to other work. This conclusion is that the bond type of hydrogen on iron changes as a function of temperature. The formation of two hydrogen species on iron and at defects in the iron lattice is related to anomolous hydrogen diffusion in iron and to hydrogen embrittlement. The reactions of SO2 and hydrogen on an iron surface are shown to explain the inhibiting effect SO2 has an hydrogen embrittlement.

Kinetic aspects of slow crack growth in the gaseous hydrogen embrittlement of steels

An explanation of the slow crack-growth phenomenon in gaseous hydrogen embrittlement is suggested on the basis that chemisorption is the rate-limiting step. The basis of the analysis is the existence of a mobile adsorbed species which is a prerequisite to the occurrence of slow crack growth. The disappearance of the mobile species with increasing temperature results in the observed crack-velocity dependence on temperature. The analysis is able to account qualitatively for the observed dependence of crack velocity with pressure in the different temperature regions of crack growth.

Work function changes during H2 adsorption on polycrystalline Fe and hydrogen embrittlement

It was demonstrated that the concentration of atomic hydrogen (H/sup -/) on an iron surface is consistent with a surface migration rate limited reaction at room temperature. The concentration of atomic hydrogen (H/sup -/) as it varies with time and hydrogen pressure is likely the controlling factor in the gaseous hydrogen embrittlement phenomena.

Equilibrium crack growth measurements and the adsorption model of hydrogen embrittlement

The adsorption model of Oriani and Josephic is consistant with available data. However this paper must not be construed as a proof of the validity of the adsorption model concerning hydrogen embrittlement. (FS)