12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Two medium-Mn advanced high strength steels, a reference 0.1C-6Mn-2Si alloy and a 0.05 wt pct Sn added grade, were hot-dip galvanized in a conventional Zn-0.2Al and a Zn-2Al-2Mg bath in order to compare the reactive wetting behavior of these model third generation alloys. Despite being known for providing superior corrosion protection and higher performance, the steels immersed in the Zn-Al-Mg bath exhibited inferior coating appearance and adherence compared to those that were galvanized in the Zn-0.2Al bath, where a variety of coating defects such as bare spots and micro-cracks in the MgZn2 phase were detected. TEM+EELS analysis of the Fe-Zn interfaces determined that partial formation of an inhibition layer had occurred during immersion in all cases; however, it was also found that the higher Al content of the ternary bath did not necessarily translate into a higher overall reactivity vs the conventional galvanizing bath. Aside from the macro-scale defects and bare areas in the Zn-2Al-2Mg coatings, which are primarily rooted in bath management and dross formation problems, the presence of sub-micron dross particles along with brittle MgZn2 intermetallics at the Fe-Zn interface had also contributed to the poor coating adhesion and flaking of these ternary coatings upon bending.