Thermodynamics of Gaseous Hydrogen and Hydrogen Transport in Metals
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1098-HH08-01
Thermodynamics of Gaseous Hydrogen and Hydrogen Transport in Metals Chris San Marchi, and Brian P Somerday Hydrogen and Metallurgical Sciences, Sandia National Laboratories, 7011 East Ave, Livermore, CA, 94550 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. INTRODUCTION Molecular hydrogen adsorbs on metal surfaces and dissociates producing atomic hydrogen, which dissolves into the metal where it interacts with the microstructure and stress fields. If the atomic hydrogen in the metal reaches sufficient levels, it can enhance fracture processes by a number of mechanisms, collectively called hydrogen-assisted fracture or hydrogen embrittlement. In order to understand hydrogen-assisted fracture, it is necessary to understand the concentration and distribution of hydrogen in the metal (thermodynamics) and hydrogen transport in the metal (kinetics). The thermodynamics and kinetics are influenced by a number of factors including the fugacity of hydrogen, surface phenomena, stress fields, and trapping of hydrogen by microstructural features.
We focus on three physical aspects of hydrogen in this brief communication: (1) the real gas behavio
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