Effects of process-generated hydrogen on RPV walls

3.4. Conclusion

in a partial or complete dissolution of these precipitates or in annihilation of some hydrogen traps, like dislocations and grain boundaries. Furthermore, the history of the material, meaning the mechanical and thermal treatments have a major influence on the total grain boundary area and concentration of dislocations in the grains. Therefore, this again will have a very large influence on the amount of hydrogen that can be trapped in the material and thus on the solubility of hydrogen in steel. The result is a very large variation in the solubility parameters for hydrogen in steel. The hydrogen traps do not only influence the solubility of hydrogen in steel, but can also affect the diffusion coefficient of hydrogen in steel. The hydrogen traps will bind the hydrogen and therefore stop its migration, until the hydrogen has enough energy to break the bond. Therefore, the hydrogen migration will be delayed or slowed down compared to the case where hydrogen would be completely dissolved in a perfect lattice at the interstitial positions. The result will be a decreased effective diffusion coefficient compared to the one expected from pure interstitial hydrogen diffusion. [43] There have been some efforts trying to model the diffusion of hydrogen including these hydrogen traps by Louthan and Caskey [43], Maroef et al. [39] and others, but all of them concluded that more fundamental information is needed on the mechanisms of hydrogen trapping to make models with an acceptable accuracy. The effect of hydrogen trapping on the solubility and the diffusivity can be seen in Figure 3.5. One can see the very large variation in both solubility constant and diffusion coefficient for hydrogen in steel.

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