Computational Analysis of Hydrogen Contribution to the Near-Neutral pH Stress Corrosion Cracking

Hydrogen plays a critical role in near-neutral pH SCC in pipelines, but the precise mechanism of its effect on crack initiation and propagation is still not well understood. Fundamentally, the process starts on the atomic level and at the root is dislocation formation and propagation due to various factors. In the present study a molecular statics simulation has been applied for the analysis of the contribution of hydrogen to the near-neutral pH stress corrosion cracking. A 3D crystal structure in which the interatomic forces between the hydrogen-iron and iron-iron atoms were defined, respectively, by the Morse and modified Morse potential functions was tested numerically. The model and the code developed were applied to both a hydrogen-free bcc iron crystal with the premade edge slit and to a bcc iron crystal with the hydrogen atoms aggregated near the crack tip. The width of the reference structure was chosen to be large enough to avoid any significant effects of free boundaries while preserving the basic properties of the structure. The edge slit was obtained by removing of a monolayer of iron; it was assumed that this slit was formed previously as a result of dissolution and the hydrogen-assisted cracking. Simulation results demonstrated that the presence of dissolved hydrogen causes severe distortion of the lattice and results in a weakened zone of interatomic bonds in the vicinity of the hydrogen atom even before the external load is applied to the structure. This phenomenon leads to the nucleation of nano-voids and later to the formation of edge dislocations array, and to the newly nucleated voids coalescing. Consequently the sliding processes start earlier (under the smaller load) leading to a 15–20% loss of residual strength in comparison with the hydrogen free sample.