[ materials ]
Compatibility of metals with hydrogen gas
As hydrogen becomes more prominent in the portfolio of technologies for reducing carbon emissions , more attention has been directed towards its safe transport , storage , and end use . Most hazard scenarios involve the unintended release of hydrogen , which can result from improper design , assembly , or operation of hydrogen containment systems . One important aspect of design is the selection of materials , particularly those comprising the pressure boundaries of hydrogen containment components . From the perspective of unintended hydrogen release , materials selection is focused on two particular concerns related to hydrogen ingress : 1 ) hydrogen permeation rates in materials , which dictate the leak rates through pressure boundaries , and 2 ) the potential for hydrogen to degrade the mechanical properties of materials , which can lead to failure of pressure boundaries and sudden release of hydrogen . This article focuses on the second concern , and more specifically on the hydrogeninduced degradation of metals . In gaseous hydrogen environments at near-ambient temperatures , failure resulting from hydrogen-induced degradation of metals is typically referred to as ‘ hydrogen embrittlement ’.
By Dr . Brian Somerday , Materials Engineering Consultant , Somerday Consulting , LLC
The potential for hydrogen embrittlement starts with the ingress of hydrogen into the metal . When hydrogen is in molecular form ( H 2
), it cannot enter directly into the metal since the hydrogen molecule size is greater than the ‘ free volume ’ between metal atoms . Consequently , hydrogen can only enter into metals that can catalyze the molecular hydrogen to dissociate into individual hydrogen atoms ( H ) on the metal surface . These hydrogen atoms can then enter and penetrate ( diffuse ) further into the metal by occupying the free volume between metal atoms . During this diffusion process hydrogen atoms can encounter metallurgical defects , and it is these hydrogen-defect interactions that lead to material degradation . These degradation events at the micro-scale are recognized as hydrogen embrittlement at the macro-scale by the formation of cracks . Such cracks can then propagate through the pressure boundaries of hydrogen containment components , leading to unintended hydrogen release . Since the dissociation of molecular hydrogen and diffusion of atomic hydrogen are time-dependent in nature , the formation and propagation of cracks are similarly time-dependent .
While the terms ‘ hydrogen embrittlement ’ and ‘ material compatibility ’ are often invoked interchangeably , the phenomenon of hydrogen embrittlement is not solely a material compatibility issue . This perspective is illustrated from the diagram in Figure 1 , in which the circles represent three types of variables : material properties , environmental conditions , and mechanical stress characteristics . Hydrogen embrittlement is activated at the intersection of these three variable types , indicating that the phenomenon depends not only on material properties but also on environmental conditions and mechanical stress characteristics . In practical terms , a metal comprising the pressure boundary in a hydrogen containment component may function safely under one combination of environmental conditions and mechanical stress characteristics , but this same metal may suffer hydrogen embrittlement under a different combination of these variable-types .
22 Hydrogen Tech World | Issue 9 | April 2023