In order to understand the hydrogen embrittlement of steel, it is necessary to understand the hydrogen solid solution properties in metals. Firstly, the hydrogen concentration in the thermal equilibrium metal is considered. The H2 molecule is first physically adsorbed on the metal surface, dissociated into two hydrogen atoms and stably chemically adsorbed on the metal surface, and then enters the metal lattice through the thermal activation process. This process is reversible, and its reflection can be expressed by Equation ( 17-1 ) :
H2 ( g ) = H2 ( a ) = 2H ( a ) = 2H ( s ) ( 17-1 )
Here, g, a, s denote the gas hydrogen, respectively. Adsorbed hydrogen and solid solution hydrogen.
Pressure has little effect on solid phase and liquid phase equilibrium, and is an important factor for gas-containing reactions. For example, at a certain temperature, the maximum solubility of gas in metal will increase significantly with the increase of gas pressure, so the change of pressure makes the shape of gas-metal two-circle phase diagram change significantly. The relationship between the pressure P of hydrogen in the thermal equilibrium state and the maximum solubility S ( [ H ] / [ M ] ) of hydrogen in the metal obeys Sieverts ' law ( referring to diatomic gas ) :
