Numerical Study of Hydrogen Trapping: Application to an API 5L X60 Steel

A numerical finite difference method is developed here to solve the diffusion equation for hydrogen in presence of trapping sites. A feature of our software is that an optimization of diffusion and trapping parameters is achieved via a non linear least squares fit. On the other hand, we have demonstrated that usual electrochemical hydrogen permeation tests are enough to assess hydrogen free energies of trapping in the range of −35 kJ/mol to −70 kJ/mol. These conclusions are obtained by assuming the presence of saturable traps in local equilibrium with hydrogen and are validated by means of simulated permeation and degassing transients. In addition, we check our model performing electrochemical hydrogen permeation tests at 30°C, 50°C, and 70°C, on an API 5L X60 as received steel state to study its trapping and diffusion properties considering only one type of trapping site. The binding energies ( Δ G ) and the trap densities ( N ) are determined by fitting the theoretical model to the experimental permeation data. The steel presents a high densityof weak traps,| Δ G | < 35  KJ/mol, namely, N = 1.4 × 10 − 5 mol cm −3 . Strong trapping sites which alter the shape of the permeation transient are also detected; their Δ G values ranged from 57 to 72 KJ/mol.