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The phenomenon of hydrogen trapping may be considered in this context. Diffusion studies of iron and steel have shown lag time for hydrogen diffusion through these materials before a steady-state diffusivity compatible with that expected theoretically is achieved. The lag time is attributed to the interaction of hydrogen with impurities, structural defects, or microstructural constituents in the metal, which is referred to as "trapping." Hydrogen accumulates at these internal interfaces, called "traps." These hydrogen traps may be mobile (such as dislocation and stacking faults) or stationary (such as solute atoms, particle interfaces, grain boundaries, cracks, and voids). The traps have been classified as reversible or irreversible [8]. Short-duration trapping of hydrogen is referred to as reversible, whereas a long residency time for hydrogen characterized by a high binding energy is termed irreversible trapping. Deep or reversible traps act as sinks for hydrogen and reduce the population of hydrogen at the crack tip, thus increasing the resistance to HIC.