Hydrogen compatibility of structural materials in natural gas networks.

Abstract

There is growing interest in utilizing existing infrastructure for storage and distribution of hydrogen. Gaseous hydrogen, for example, could be added to natural gas in the short-term, whereas entire systems can be converted to transmission and distribution networks for hydrogen. Many active programs around the world are exploring the safety and feasibility of adding hydrogen to these networks. Concerns have been raised about the structural integrity of materials in these systems when exposed to hydrogen. In general, the effects of hydrogen on these materials are grossly misunderstood. Hydrogen unequivocally degrades fatigue and fracture resistance of structural steels in these systems, even for low hydrogen partial pressure (~1 bar). In most systems, however, hydrogen effects will not be apparent because the stresses in these systems remain very low. Another misunderstanding results from the kinetics of the hydrogen effects: hydrogen degrades fatigue and fracture properties immediately upon exposure to gaseous hydrogen, and those effects disappear when the hydrogen environment is removed, even after prolonged exposure. There is also a misperception that materials selection can mitigate hydrogen effects. While some classes of materials perform better in hydrogen environments than other classes, for most practical circumstances, the range of response for a given class of material in gaseous hydrogen environments is rather narrow. These observations can be systematically characterized by considering the intersection of materials, environmental, and mechanical variables associated with the service application. Indeed, any safety assessment of a hydrogen pressure system must quantitatively consider these aspects. In this report, we quantitatively evaluate the importance of the materials, environmental, and mechanical variables in the context of hydrogen additions to natural gas piping and pipeline systems with the aim of providing an informed perspective on parameters relevant for assessing structural integrity of natural gas systems in the presence of gaseous hydrogen.