Hydrogen embrittlement is a complex phenomenon, involving several length- and timescales, that affects a large class of metals. It can significantly reduce the ductility and load-bearing capacity and cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Despite a large research effort in attempting to understand the mechanisms of failure and in developing potential mitigating solutions, hydrogen embrittlement mechanisms are still not completely understood. There are controversial opinions in the literature regarding the underlying mechanisms and related experimental evidence supporting each of these theories. The aim of this paper is to provide a detailed review up to the current state of the art on the effect of hydrogen on the degradation of metals, with a particular focus on steels. Here, we describe the effect of hydrogen in steels from the atomistic to the continuum scale by reporting theoretical evidence supported by quantum calculation and modern experimental characterisation methods, macroscopic effects that influence the mechanical properties of steels and established damaging mechanisms for the embrittlement of steels. Furthermore, we give an insight into current approaches and new mitigation strategies used to design new steels resistant to hydrogen embrittlement.

We calculate the rate coefficient as a function of temperature for lattice diffusion of hydrogen and its isotopes in α-iron; and also for trapping and escape from a vacancy. We employ Monte-Carlo and molecular dynamics methods based around the Feynman path integral formulation of the quantum partition function. We find large quantum effects including tunnelling at low temperature and recrossing at high temperature due to the finite extent of the particle probability density. In particular these serve to increase the rate of trapping and to decrease the rate of escape at low temperature. Our results also show very clear non classical isotope effects

The effect of hydrogen content on the microstructure, mechanical properties, and fracture mechanisms of low-carbon lath martensitic steel was investigated using both experimental methods and atomistic modeling. Tensile testing revealed a transition in the fracture behavior with increases in hydrogen concentration. Specifically, at a hydrogen content of 0.44 wppm, a shift from transgranular to intergranular fractures was observed. The most probable cause of hydrogen embrittlement was identified to be HELP-mediated HEDE. As the hydrogen concentration increased, the dislocation density in close-packed planes, such as (111) and (100), was found to rise. The key differences between the hydrogen-free and hydrogen-charged specimens were the localization and density of dislocations, as well as the change in the distribution of slip bands. Atomistic modeling supported these experimental findings, showing that “quasi-cleavage” cracks predominantly initiate at block boundaries with higher local hydrogen accumulation. These results underscore the significant role of hydrogen in modifying both the microstructural characteristics and fracture behavior of low-carbon martensitic steel, with important implications for its performance in hydrogen-rich environments.

We have developed an atomistic kinetic Monte Carlo (kMC) model describing carbon diffusion, trapping and detrapping in the stress field generated by a 1/2[111] screw dislocation in bcc iron. By performing long time scale carbon diffusion simulations we study the formation of carbon Cottrell atmospheres around screw dislocations at different temperatures and background carbon concentrations. The kMC simulations allow us to predict the rate of formation and strength of carbon atmospheres which control the dislocation's behavior resulting in dynamic strain aging in steel. From the kMC simulations of carbon diffusivity in a block of iron crystal containing a screw dislocation we extract the effective diffusion constants. Our results show that except at low carbon concentrations the diffusivity resulting from carbon transport inside the dislocation core dominates the effective diffusivity, which indicates that pipe diffusion in a screw dislocation is likely. The kMC approach, which explicitly accounts for the behavior of individual carbon atoms, offers an atomistic view of the carbon drag mechanism by which mobile dislocations can collect and transport carbon within their cores. We simulate the carbon drag mechanism at different temperatures, background carbon concentrations, and dislocation velocities and estimate the maximal dislocation velocity at which the atmosphere of carbon atoms can follow a moving screw dislocation.