Studies of the morphology of the Al-Rich interfacial layer formed during the hot dip galvanizing of steel sheet

A small addition of Al in the liquid Zn bath inhibits the Fe/Zn reactions during hot dip galvanizing of steel sheets. Although it is well known that Al and Fe are tied up in an layer located at the interface between Fe and Zn, the effect of the galvanizing parameters on the formation of this Al-rich interfacial layer is not. This study has been carried out to determine the effect of the galvanizing parameters on the formation of this Al-rich layer. Five zinc baths containing 0.10, 0.12, 0.13, 0.16, and 0.18 wt pct Al were used to produce galvanized coatings of commercial ultra-low-carbon (ULC) steel substrate. Full inhibition of the Fe/Zn reactions was achieved in baths with Al content above 0.15 wt pct. The Al-rich layer is isomorphous to Fe₂Al₅ crystals and the Fe/Al ratio is close to Fe₂Al₅ with large amounts of Zn present (22 to 28 wt pct). The morphology of the Al-rich layer is strongly related to the Al content of the bath. Indeed, bath contents above 0.15 wt pct are associated with two sublayers of Fe₂Al₅ crystals, making up colonies of grains of similar crystallographic orientation and directly associated to the crystallographic orientation of the steel grains. In baths with contents below 0.15 wt pct, the Al-rich layer has only one sublayer of crystals and shows colonies of grains with similar orientation. Finally, the Fe₂Al₅ crystals have a strong crystallographic texture. X-ray diffraction (XRD) reveals that the (200) planes are parallel to the surface of the substrate.