Effect of Hydrogen Concentration on Hydrogen Diffusion Coefficient in Tempered Martensitic Steel

The effect of hydrogen concentration on the hydrogen diffusion coefficient was investigated by analyzing the relationship between hydrogen permeation current density and hydrogen diffusion coefficient. The hydrogen diffusion coefficients were obtained using the half-rise time method, which analyzes the build-up behavior of the hydrogen permeation current densities, and were correlated with the steady-state values of the hydrogen permeation coefficients. The hydrogen diffusion coefficient decreased as the hydrogen permeation coefficient decreased, indicating that the diffusion coefficient depends on the hydrogen concentration. Subsequently, a hydrogen diffusion coefficient model was developed based on the Fermi–Dirac distribution to express this concentration dependence. The experimental relationship between the hydrogen permeation and hydrogen diffusion coefficients was fitted using this model. Assuming the presence of more than two hydrogen trap sites was necessary to achieve an accurate fit. Therefore, two types of trap sites were assumed in this study. The hydrogen trap energies at these trap sites were determined using the Choo–Lee method under the same hydrogen charging conditions as those used in the hydrogen permeation tests. The hydrogen occupancy at each trap site was treated as a fitting parameter in this model. The estimated values from the model agreed well with the experimental data. Furthermore, validation under varying temperature conditions exhibited a similar good agreement between the experimental data and the model predictions.