Hydrogen accumulates at microstructural defects in steel

For nearly a century, scientists have known that hydrogen gas can cause steel to become brittle, sometimes leading to equipment failure. Embrittlement limits industrial applications of hydrogen—for example, in the transportation and energy sectors—because inexpensive steel pipes and tanks can’t be used to deliver and store the gas. Researchers have suspected that this degradation is caused by the way hydrogen interacts with defects in steel’s internal microstructure. But because hydrogen atoms are difficult to image, researchers have been unable to prove directly that hydrogen actually resides at defect sites—information that’s key to confirming the mechanism and overcoming the problem. A team led by Julie M. Cairney of the University of Sydney and Hongzhou Lu of Citic Metal in Beijing heated niobium-carbon steel samples in deuterium gas because hydrogen’s heavy isotope diffuses through steel more slowly than ordinary hydrogen does. Then, while maintaining the samples at cryogenic temperatures to lock the atoms in place, the team used atom-probe tomography to determine the samples’ 3-D crystal structures. They found that hydrogen atoms—or in this case deuterium atoms—do indeed accumulate at defects such as dislocations and grain boundaries and can be trapped at interfaces between precipitates and the surrounding steel matrix (Science 2020, DOI: 10.1126/science.aaz0122).