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

Abstract

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 (ΔG) and the trap densities (N) are determined by fitting the theoretical model to the experimental permeation data. The steel presents a high density of weak traps, |ΔG| < 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 ΔG values ranged from 57 to 72 KJ/mol.

Effect of Nitriding on the Hydrogen Diffusion Coefficient through AISI 4340

“…The hydrogen that might be reversibly trapped at high energy trap sites during the rise transients diffuses slowly along the lattice diffusion path during the decay transient, re-sulting in a lower effective diffusion coefficient. 33 The slow desorption of hydrogen during the decay transient relative to the rise transient has been observed by several authors in Fe alloys. [34][35][36] The slight decrease in effective diffusion coefficient values of subsequent rise transients for the as received specimens might be due to the formation of an oxide layer on the charging surface, which slows the transport of hydrogen.…”

Effects of anisotropy and regime of diffusion on the measurement of lattice diffusion coefficient of hydrogen in metals

“…Trap sites are structural defects such as vacancies, voids, grain boundaries, dislocations or carbide precipitates. McNabb & Foster [24] introduced the kinetics of trapping and detrapping of hydrogen atoms for a single trap type by observing the mass conservation law. According to this, the rate of change of total concentration of atoms is equal to the flux of diffusing atoms through lattice.…”