Strain rate sensitivity of the hydrogen embrittlement of ferritic steels

The susceptibility to hydrogen embrittlement of ferritic steels is known to increase with decreasing strain rates down to strain rates of 10-5 - 10-7 s- 1. The literature attributes this strain rate sensitivity to the diffusion of hydrogen. However, for a specimen pre-charged with hydrogen, lattice diffusion dominates over trap kinetics and is too rapid to have an effect at such low strain rates. Here, we present a model to rationalise the observed strain rate dependence of hydrogen embrittlement in a uniaxial tensile test in the context of the Hydrogen Induced Fast-Fracture (HIFF) mechanism. In this mechanism, egress of hydrogen gas from the matrix fills a cavity around a debonded inclusion. This hydrogen gas initiates a fast-propagating crack from the surface of the cavity that ultimately results in fracture of the specimen. Our calculations show that the hydrogen desorption rates from the cavity surfaces is the dominant kinetics that governs the strain rate sensitivity. Using the measured desorption enthalpy for hydrogen from an Fe surface, our predictions of the strain rate sensitivity of embrittlement are in remarkable agreement with observations. The HIFF model is also known to rationalise the disconnect between the effect of hydrogen on the fracture toughness and tensile strength as well as explain why hydrogen embrittlement depends only on the lattice and not total hydrogen concentration. Combined with our current predictions for the strain rate sensitivity, it seems that the HIFF model provides a relatively complete picture of the mechanisms of hydrogen embrittlement in ferritic steels.