Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

3.2. Solubility of hydrogen in steel

only 0.19 Å. Figure 3.2 shows both types of interstitial sites. As the octahedral sites are much smaller, the lattice expansion for a hydrogen atom occupying an octahedral position is much larger compared to the tetrahedral positions, which explains the preferred tetrahedral positions. As a result of the compact microstructure of the Fe lattice, the hydrogen solubility in Fe is very limited, since the lattice is distorted in order to accommodate the hydrogen atoms. However, also the interstitial lattice sites are very close together due to the small lattice parameter of Fe, resulting in a very high mobility of the individual hydrogen atoms. [35]

Figure 3.2: Different types of interstitial lattice sites in a bcc lattice, as for steel in the α structure. a) octahedral positions in a unit cell. b) tetrahedral positions in a unit cell. [35] In contrast to atomic hydrogen, molecular hydrogen cannot migrate in a metal lattice. The molecule is more than twice the size of the individual atom and therefore too big to be accommodated in the interstitial lattice sites. It is however possible for 2 dissolved hydrogen atoms to recombine with each other and form molecular hydrogen in micro-/macrovoids, microcracks and around inclusions or precipitates. When this happens the hydrogen molecule is generally accepted to be trapped at that position and can only migrate by dissociating again in atomic hydrogen and diffuse as individual atoms in the metal lattice. [27] The amount of hydrogen absorbed in the steel is dependent on the temperature as both the microstructure and lattice parameters change with temperature. Also the equivalent hydrogen pressure in contact with the steel is an important parameter for the hydrogen solubility in steel. Figure 3.3 gives the hydrogen solubility in pure iron as a function of the hydrogen pressure and temperature. The solubility is expressed in Normal cm 3 per 100 g of iron, which is the equivalent hydrogen gas volume at 20 ◦ C and a pressure of 1 atm. [36] 23