2021 Joint Nanoscience and Neutron Scattering Virtual User Meeting

For decades, the effects of hydrogen embrittlement in metals, particularly ferritic steels, has posed a serious obstacle in designing and manufacturing strong and reliable structural materials for use in energy storage and transportation applications. Pipeline steels, such as those used to transport natural gas, are particularly susceptible to embrittlement and subsequent structural failure due to fatigue, reduction in ductility, and fracture when exposed to hydrogen. Therefore, understanding the fundamental mechanisms governing the embrittlement process, including how hydrogen diffuses within steel microstructures, particularly dislocations, is of great importance. To investigate hydrogen diffusion in ferritic steel, we performed quasielastic neutron scattering (QENS) measurements using the backscattering spectrometer (BASIS) at Oak Ridge National Laboratory. Dislocations were created by subjecting the specimen to multiple strain cycles prior to its exposure to hydrogen. Preliminary QENS spectra (Fig. 1) were collected from the strained steel specimen at hydrogen pressures up to 2.5 kbar in the temperature range 50 K to 300 K. Diffusion coefficients obtained by analyzing the spectra using a jump diffusion model were found to be greater than those reported from bulk bcc Fe, suggesting that hydrogen diffusion through the steel framework is enhanced by the presence of dislocations. Dislocation densities as a function of percent strain applied were quantified using a neutron powder diffractometer at the University of Missouri Research Reactor (MURR). A Williamson-Hall analysis of Bragg peak broadening revealed a factor of 2.3 increase in dislocation density when the specimens were strained to 9%. Fig. 2 shows a neutron diffraction pattern of a strained specimen. Understanding how the presence of dislocations affects hydrogen diffusion within the steel framework is important for assessing models that have been proposed for hydrogen embrittlement. Further QENS measurements on BASIS are in progress to establish whether a high dislocation density in ferritic steel causes a reduction in the activation barrier for hydrogen diffusion. Fig 2: Neutron diffraction pattern of strained ferritic steel. Inset shows a Williamson-Hall plot for an as-received (black) and strained specimen (red). Fig 1: QENS spectra of a strained steel specimen exposed to 2.5 kbar hydrogen. Effect of sidechains on chain conformation of Donor-Accepter conjugated