Analysis of the Fe-Zn interface of galvanized high Al-low Si TRIP steels

TRIP-assisted steels are ideal for lightweight automotive applications due to their high strength and ductility. The selective oxidation of the alloying elements Mn, Al and Si during annealing prior to galvanizing can cause poor reactive wetting during galvanizing, resulting in bare spot defects. Despite the selective oxidation of alloying elements two high Al-low Si TRIP steels were successfully galvanized. It was determined that good reactive wetting occurred by the presence of an Al-rich interfacial layer composed of Fe2Al5 and FeAl3. Two mechanisms by which reactive wetting occurred were identified; (1) aluminothermic reduction of MnO by the Al in the Zn bath and (2) Zn bridging of oxide particles. When aluminothermic reduction occurred there was a localized depletion of Al in the Zn bath resulting in inhibition breakdown and the formation of Fe-Zn intermetallics at the Fe-Zn interface.