Effect of Ni addition in Al-Si hot-dip bath on microstructure, corrosion resistance, and hydrogen embrittlement of hot-press-formed steels

Hot-press forming (HPF) excels at manufacturing ultra-high-strength steel through a combination of elevated-temperature forming and in-die quenching. A critical component of this process is the application of Al-Si alloy coatings, which are essential for preserving the mechanical properties of steel by preventing decarburization during high-temperature processing. Adding Ni to these coatings shows strong promise for better properties, driven by the development of Ni-based intermetallics like Al₃Ni and Al₉FeNi. Ni additions at 0, 1, 3, and 5 wt% were introduced to the molten Al-Si bath to investigate changes in coating microstructure, hydrogen embrittlement (HE) susceptibility, and corrosion behavior post-HPF. With Ni present, intermetallics like Al₉FeNi and Al₃Ni developed on the coating surface as well as within the layer structure. During the HPF process, these intermetallics transformed into Al₅FeNi and localized on the surface of the coating. Ni incorporation substantially limited crack development through the coating thickness while blocking substrate contact with the electrolyte, thus boosting coated steel corrosion resistance. Moreover, Ni addition was found to decrease the formation of surface voids caused by hydrogen atoms; these voids typically occur because of the high reactivity of Al with moisture. Notably, the diffusible hydrogen content in samples containing 5 wt% Ni was reduced from 1.74 to 0.76 wppm immediately after the HPF process, indicating a substantial decrease in HE.