Effects of process-generated hydrogen on RPV walls

3. Hydrogen in steel

Figure 3.3: Temperature and pressure dependent solubility of hydrogen in iron. [36]

3.3 Hydrogen trapping In a perfect lattice structure, the dissolved hyrogen will only be found in the interstitial lattice positions as explained above. However, industrial steels are not made of these perfect single grain materials. All types of defects in a microstructure can attract and bond hydrogen atoms or molecular hydrogen. Figure 3.4 gives an overview of some of the different types of hydrogen traps. [37] One can discriminate two major types of hydrogen traps: Reversible hydrogen traps and irreversible hydrogen traps. Whether a hydrogen trap is considered as reversible of irreversible is determined by the ease of hydrogen release from the trap. When the binding energy of the hydrogen atom to the trap is smaller then 50 to 60 kJ/mol, the trap is said to be reversible, while traps with larger binding energies are considered irreversible. [38, 39] Of course this is an arbitrary limit to easily differentiate between both types and does not give any fundamental information on the trapping mechanism. In general, reversible traps are characterized by a reversible reaction for the capture and release of hydrogen atoms. The reversible reaction means that there is no hysteresis involved in the process and capture and release occur at the same rate. [39] Irreversible traps on the other hand are considered as only capturing hydrogen and not releasing it, since their binding energy is very high. Of course this has to be seen in perspective since giving enough energy to the bonded hydrogen atom, i.e. by increasing the temperature, will result 24