1. Different states of existence of hydrogen in steels

As mentioned above, hydrogen exists in many forms. We can distinguish :

• hydrogen in proton form $\left(\underset{\_}{\text{H}}\right)$

  According to Friedel [1] , monoatomic hydrogen donates its electron to the metal's 3d conducting layer; this is known as a screened proton in an interstitial position in the lattice, and this form is mobile (interstitial diffusion);

• hydrogen in molecular form (H ₂ )

  When a defect (crack, cavity, pore...) has sufficient volume,...