Hi, It is found that Al atoms in TWIP steel cause a localized dilation that better accommodates hydrogen, this phenomen expressed via an Al-H binding energy. This binding energy implemented in trapping theory, indicates that the presence of Al allows the TWIP steel to absorb more hydrogen than Al-free steel and the diffusion coefficient of hydrogen is affected by Al element.

Al plays multiple roles in hydrogen embrittlement resistivity. Al increases stacking faults energy leading to the suppression of martensite transformation and deformation twinning, preferential sites for crack initiation. Additionally, an increase in stacking fault energy enhances cross slip resulted in suppression of dislocation pile-ups. And also depends the method of charging, if electrochemically, formation of an Al2O3 on the surface layer prevent hydrogen diffusion.

There are a lot of papers in this regards such as:

https://doi.org/10.1016/j.ijhydene.2013.12.171 Article Hydrogen Embrittlement in Al-Added Twinning-Induced Plastici...

Victor Alcántara The physical mechanism behind increased resistance to hydrogen embrittlement after alloying TWIP steels with Al is the increase in the stacking fault energy (SFE). As a result, the cross-slip of dislocations is easier and the amount of the dislocations piled-up at the grain boundary during plastic deformation decreases, ultimately lowering the transport of H atoms and the associated concentration of H atoms at the grain boundaries.

For example, see the following paper:

Article Role of copper and aluminum additions on the hydrogen embrit...

Aluminium, like Titanium, is a strong carbide builder that strengthens grain boundaries.