Effects of process-generated hydrogen on RPV walls

Chapter 3 Hydrogen in steel

3.1 Introduction The introduction of hydrogen in steel can occur via multiple paths. First of all there is the steel-making and final product fabrication process, where hydrogen can enter the material, e.g. dissolved in the scrap for steel-making or by the dissociation of water molecules from the atmosphere or fluxes like CaO added to the steel. [23, 27] The presence of hydrogen in steel can result in serious degradation of the material like hydrogen embrittlement, corrosion fatigue, hydrogen assisted cracking, methane blistering by the attack of carbide phases and many more. [27, 31] All of these damaging effects of hydrogen on materials and steel in particular have a significant influence on all types of industry. Therefore a lot of research has already been done on the properties of hydrogen in steel and its resulting, possibly detrimental, effects. [27, 31, 32] 3.2 Solubility of hydrogen in steel For hydrogen to have an effect on the mechanical properties of a material, it must be absorbed inside the material. The hydrogen can result from hydrogen gas or water in contact with the metallic surface or dissolved in scrap or fluxes. The absorption of hydrogen in steel does not happen directly as molecular hydrogen or complete water molecules, but as atomic hydrogen. When the metal is in contact with hydrogen gas, several reaction stages have to be completed for the hydrogen to enter the metal. Figure 3.1 shows the different subsequent stages. The first step is the diffusion of hydrogen molecules from the outside world towards the metal surface. When there is a collision between the hydrogen molecule and the metal surface, the H 2 -molecule can be physically adsorbed on the surface. Here Van der Waals forces act on the hydrogen molecule and result in a so called “physisorption”. Afterwards, the hydrogen molecule dissociates to form two single hydrogen atoms adsorbed on the metal surface. The interacting forces between the metal and the hydrogen atoms become more chemical than physical in nature and therefore this is called the “chemisorption” of hydrogen atoms on the 21