Hydrogen Compatability of Structural Materials in Natural Gas Networks

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 (-l 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.