Non-combustion Related Impact of Hydrogen Admixture - Material Compatibility

The present document is part of a larger literature survey of this WP, aiming to establish the current status of gas utilisation technologies in order to determine the impact of hydrogen (H₂) admixture on natural gas (NG) appliances. This part focuses on the non-combustion related aspects of injecting hydrogen in the gas distribution networks within buildings, including hydrogen embrittlement of metallic materials, chemical compatibility and leakage issues. In the particular conditions of adding natural gas and hydrogen (NG / H2) mixture into a gas distribution network, hydrogen is likely to reduce the mechanical properties of metallic components. This is known as hydrogen embrittlement (HE) (Birnbaum, 1979). This type of damage takes place once a critical level of stress / strain and hydrogen content coexist in a susceptible microstructure. Currently, four mechanisms were identified and will be discussed in detail. The way those mechanisms act, independently or together, is strongly dependent on the material, the hydrogen charging procedure and the mechanical loading type. The main metallic materials used in gas appliances and gas distribution networks are: carbon steels, stainless steels, copper, brass and aluminium alloys (Thibaut, 2020). The presented results showed that low alloy steels are the most susceptible materials to hydrogen embrittlement followed by stainless steels, aluminium, copper and brass alloys. However, the relative pressures of the operating conditions of gas distribution network in buildings, are low i.e. between 30 to 50 mbar. At those low hydrogen partial pressures, it is assumed that a gas mixture composed of NG and up to 50% H2 should not be problematic in terms of HE for any of the metallic materials used in gas distribution network, unless high mechanical stress / strain and high stress concentrations are applied. The chemical compatibility of hydrogen with other materials, and specifically polyethylene (PE) which is a reference material for the gas industry, is also discussed. PE was found to have no corrosion issues and no deterioration or ageing was observed after long term testing in hydrogen gas. The last non-combustion concern related to the introduction of hydrogen in natural gas distribution network is the propensity of hydrogen toward leakage. Indeed, the physical properties of hydrogen are different from other gases such as methane or propane, and it was observed that hydrogen leaks 2.5 times quicker than methane. This bibliographical report on material deterioration, chemical compatibility and leakage concerns coming with the introduction of NG / H₂ mixture in the gas distribution network sets the basis for the upcoming experimental work where the tightness of gas distribution network components will be investigated (Task 3.2.3 WP3). In addition, tightness of typical components that connect end-user appliances to the local distribution line shall be evaluated as well.