Solid Solution and Properties of Hydrogen in Steel_FAQs

To understand hydrogen embrittlement in steel, it is necessary to understand the nature of hydrogen solid solution in metals. First, consider the hydrogen concentration in a metal under thermal equilibrium. As mentioned earlier, H₂ molecules are first physisorbed on the metal surface, dissociate into two hydrogen atoms and become stably chemisorbed on the metal surface, and then enter the metal lattice via a thermally activated process. This process is reversible, and its reaction can be expressed by Equation (17‑1):
H₂（g）= H₂(a)=2H(a)=2H(s) （17-1）
Here, g, a, and s denote gaseous hydrogen, adsorbed hydrogen, and dissolved hydrogen, respectively.
Pressure has a negligible effect on the equilibrium between solid and liquid phases, but is an important factor for reactions involving a gas phase. For instance, at a given temperature, the maximum solubility of a gas in a metal increases significantly with rising gas pressure. Thus, changes in pressure substantially alter the shape of gas–metal binary phase diagrams.
Under thermal equilibrium, the relationship between the hydrogen pressure P and the maximum solubility S of hydrogen in the metal ([H]/[M]) follows Sieverts' Law (for diatomic gases).

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Solid Solution and Properties of Hydrogen in Steel

2019.12.21