Thermodynamics of Gaseous Hydrogen and Hydrogen Transport in Metals

Abstract

The thermodynamics and kinetics of hydrogen dissolved in structural metals is often not addressed when assessing phenomena associated with hydrogen-assisted fracture. Understanding the behavior of hydrogen atoms in a metal lattice, however, is important for interpreting materials properties measured in hydrogen environments, and for designing structurally efficient components with extended lifecycles. The assessment of equilibrium hydrogen contents and hydrogen transport in steels is motivated by questions raised in the safety, codes and standards community about mixtures of gases containing hydrogen as well as the effects of stress and hydrogen trapping on the transport of hydrogen in metals. More broadly, these questions are important for enabling a comprehensive understanding of hydrogen-assisted fracture. We start by providing a framework for understanding the thermodynamics of pure gaseous hydrogen and then we extend this to treat mixtures of gases containing hydrogen. An understanding of the thermodynamics of gas mixtures is necessary for analyzing concepts for transitioning to a hydrogen-based economy that incorporate the addition of gaseous hydrogen to existing energy carrier systems such as natural gas distribution. We show that, at equilibrium, a mixture of gases containing hydrogen will increase the fugacity of the hydrogen gas, but that this increase is small for practical systems and will generally be insufficient to substantially impact hydrogen-assisted fracture. Further, the effects of stress and hydrogen trapping on the transport of atomic hydrogen in metals are considered. Tensile stress increases the amount of hydrogen dissolved in a metal and slightly increases hydrogen diffusivity. In some materials, hydrogen trapping has very little impact on hydrogen content and transport, while other materials show orders of magnitude increases of hydrogen content and reductions of hydrogen diffusivity.

Tritium Barriers and Tritium Diffusion in Fusion Reactors

“…For materials with low solubility and relatively large E t , the effective diffusivity can be substantially reduced compared to the lattice diffusivity ( Figure 2). The wide variation of reported diffusivity of hydrogen in α-iron at low temperature is a classic example of the effect of trapping on hydrogen transport [2,20]: while the diffusivity of hydrogen at high temperature is consistent between studies, the effective diffusivity measured at low temperature is significantly lower (in some cases by orders of magnitude) compared to the Arrhenius relationship established from measurements at elevated temperature. Moreover, the range of reported values of effective diffusivity demonstrate the sensitivity of the measurements to experimental technique and test conditions.…”

“…Although binding energies and densities of hydrogen traps vary substantially, the majority of reported values for RAFM steels are in the range 40 to 60 kJ mol -1 and 10 -3 to 10 -5 traps per metal atom, respectively. The traps are attributed primarily to boundaries [105] and result in a significant reduction in the apparent diffusivity at temperature less than about 573 K. At higher temperature, the traps are essentially unoccupied and do not affect diffusion [20].…”

“…If the amount of trapped tritium (c T ) is large relative to the amount of lattice tritium (c L ), the effective diffusivity can be several orders of magnitude less than the lattice diffusivity [20]. Moreover, the effective diffusivity is a function of composition of the hydrogen isotopes, depending on the conditions of the test as well as sensitive to the geometry and microstructure of the test specimen.…”

“…Therefore, the amount of trapped hydrogen (in the absence of irradiation and implantation damage) is relatively low at elevated temperature. Moreover, due to the high solubility of hydrogen and its isotopes in austenitic stainless steels, the density of trapping sites would need to be impractically high to measurably increase the inventory of hydrogen and its isotopes in the metal [20]. For these reasons, trapping from a microstructural origin is anticipated to have little if any impact on the transport of hydrogen and its isotopes in austenitic stainless steels at temperatures greater than ambient.…”

A quantitative assessment of the hydrogen storage capacity of the UK continental shelf

“…(2) = /( + ) Where ρ is density, P is pressure, R is the gas constant (4160 J/kg K for hydrogen [60]), T is temperature, and b is the co-volume (15.84 cm 3 /mol for hydrogen [61]). Mass was converted to energy using the higher heating value (HHV) for hydrogen (39.41 kWh/kg [62]) to allow a comparison to energy demand in the UK.…”