Effects of process-generated hydrogen on RPV walls

3. Hydrogen in steel

metal surface. In the final stage, the different hydrogen atoms are absorbed by the metal and occupy subsurface interstitial lattice sites. Once the hydrogen atoms are present in these subsurface positions, their motion in the metal lattice will be defined by diffusion. [33]

Figure 3.1: Reaction mechanism for the absorption of molecular hydrogen in a metallic material. a) Diffusion of the H 2 molecule towards the metallic surface. b) Physical adsorption of H 2 on the metallic surface. c) Dissociation of the H 2 molecule into atomic H. The different hydrogen atoms are chemically adsorbed on the metallic surface. d) The hydrogen atoms move towards subsurface interstitial lattice sites. Further movement of the hydrogen atoms is defined by diffusion in the metal lattice. [33] Similarly, for hydrogen originating from water molecules, hydrogen atoms have to be separated from the molecule in order to be able to enter the steel. This process happens by corrosion reactions of the steel in the aqueous environment. Depending on the pH of the environment, this cathodic reaction will be different. In acidic solutions the following reaction will take place: H 3 O + + M + e − −−→ M · H ads + H 2 O (3.1) while for alkaline environments the main corrosion reaction will be: H 2 O + M + e − −−→ M · H ads + OH − (3.2) One can see that one of the resulting products remains the same for both reactions, being a chemically adsorbed hydrogen atom on the metal surface. In a sequential stage this hydrogen atom can again move towards a subsurface interstitial position, like the final stage for the molecular hydrogen reactions. [34] Once the atomic hydrogen is present in the bulk of the steel, it can very easily diffuse in the crystal lattice. Since the covalent radius of the H atom is only 0.37 Å, the atom is small enough to easily hop between interstitial lattice sites. It was found that the hydrogen atoms prefer the tetrahedral interstitial positions in the very compact pure iron bcc lattice, α structure, over the octahedral sites. The tetrahedral sites have a “radius” of 0.36 Å, while for the octahedral interstitial lattice sites this is 22