Hydrogen interaction with multiple traps: Can it be used to mitigate embrittlement?

The presence of reversible traps in steels with large or small binding energies has been thought to influence the kinetics of hydrogen-induced degradation and possibly the mechanism of embrittlement. In this study, we try to quantitatively describe the interaction of multiple trap states with diffusible hydrogen at a crack tip in a model steel system under conditions simulating hydrogen uptake through the inner-diameter surface of a pipeline. We assume reversible traps whose hydrogen populations are in equilibrium with interstitial lattice hydrogen and binding energies representative of both strong and weak traps. We find that strong traps act as hydrogen sinks that slow down diffusion till they get saturated regardless of their density. Weak reversible traps are being filled as diffusible hydrogen becomes available and their occupancy spatial profiles follow that of the diffusible hydrogen. To explore the hydrogen trap interactions further, we parametrically vary the yield stress of the material, the trap density of the weak tarps, and the hydrogen gas pressure. In general, we cannot infer from the current study that strong trapping states can be used to mitigate hydrogen embrittlement.

► Hydrogen interaction with multiple trap states in a cracked body in hydrogen gas. ► Effect of yield stress, trap density, and gas pressure on hydrogen trapping. ► Hydrogen saturates strong traps before it becomes available to weaker traps. ► Strong or weak traps only modulate rate of hydrogen supply to fracture process zone. ► Embedding irreversible traps in a material does not mitigate hydrogen embrittlement.