Effects of Ni-precoating on hydrogen intrusion/emission behavior in Al-Si-coated hot-press-forming steel sheets

In Al-Si-coated hot-press-forming (HPF) steels, high reactivity between Al and moisture often leads to the generation of hydrogen (H) atoms at elevated temperatures, resulting in hydrogen embrittlement. In this study, a thin Ni layer of approximately 0.5 mu m in thickness was electroplated on the steel substrate surface before the conventional Al-Si coating process. The formation of a Ni-enriched zone was found to affect hydrogen intrusion and emission behavior positively. Prior to the HPF simulation, the Ni-enriched zone was formed inside Fe2SiAl7 intermetallic compounds, and migrated toward the outer FeAl(Si) layer during the subsequent HPF simulation. This migration served as an impediment to elemental diffusion, thereby resulting in a prolonged simulation time for the complete formation of coating microstructures. Consequently, the presence of the Ni-enriched zone functioned as a high-energy barrier, impeding the diffusion of H and rendering H intrusion more challenging, while simultaneously enhancing corrosion resistance. Complete H emission occurred within 3 days in the Niprecoated sheet, whereas a significant amount of H content remained even after 17 days in the non-Niprecoated sheet. This difference could be attributed to the reduced amount of H intrusion in the Ni-precoated sheet, despite similar H emission rates observed in both sheets.