Numerical Evaluation on Analysis Methods of Trapping Site Density in Steels Based on Hydrogen Permeation Curve

Abstract

Hydrogen permeation techniques have been widely utilized to extract hydrogen effective diffusivity, as well as hydrogen trapping site characteristics in steels. Several methods have been proposed to examine reversible and irreversible trapping site characteristics. However, the inappropriate utilization of these simplified models, as well as incorrect value assignment to the key parameters, can result in several orders of magnitude difference in hydrogen trapping site density. Therefore, in order to evaluate these models and verify their application prerequisites, a serial of hydrogen permeation tests were numerically simulated and examined, separately considering reversible and irreversible hydrogen trapping sites. In the meantime, suggestions were given to conduct hydrogen permeation test more effectively, and evaluate hydrogen trapping site characteristics more precisely.

Influence of traps reversibility on hydrogen permeation and retention in Eurofer97

“…In the second part, the influence of each trapping site on the permeation dynamics is investigated with simulations of permeation experiments performed with mhims. In accordance with previous observations [19][20][21][22][23][24], irreversible trapping sites are shown to have an impact on permeation. A definition of reversibility is given in [25]: 'a reversible trap is one at which hydrogen has a short residence time at the temperature of interest with an equivalent low detrapping energy.…”

“…In order to extend this notion to fusion materials operating at higher temperatures, the present work investigates trap reversibility as a function of temperature. This analysis shows in particular that the high-energy trapping site in Eurofer97 is irreversible in the entire operational temperature range of Eurofer97 [28], which impacts the kinetics of permeation in the case of cyclic loading as already observed in iron and steels [20][21][22]29].…”

“…As observed figures 3 and 4, the first through simulations may be much longer than the next lags when the simulation includes several permeation cycles and at least one irreversible trapping site. This effect has been witnessed experimentally in steels and iron [20][21][22]29]. An example of this effect in Eurofer97 at 523 K is shown in figure 11, where the breakdown of the detailed inventory shows that the extra time corresponds to the filling of the irreversible trap.…”

“…The influence of a given trapping site on permeation is linked to its reversibility, which states whether a trapping site containing hydrogen is likely to release its content [19][20][21][22][23][24]. Reversibility as a notion can be used independently of temperature (e.g.…”

“…Subsections 4.2 and 4.3 have shown that each trapping site has an effect on diffusivity in a limited temperature range. This has been shown to be caused by the reversibility of the trapping sites [19][20][21][22][23][24]. Usually, trapping sites with a low detrapping energy are considered reversible, as they allow hydrogen to easily detrap.…”