Site-Specific Studies on the Interfacial Structures of Galvanized Dual Phase Steels

Focused ion beam and transmission electron microscopy were employed as site-specific techniques to study the interfacial structures of galvanized steel. Four samples, processed under varying fuel-to-air ratio in the annealing chamber prior to hot-dipping, were analyzed. The specimen with the highest fuel-to-air ratio had a discontinuous external oxide layer only, which comprised of manganese and silicon oxides. As the fuel-to-air ratio decreased, the amount of internal oxides increased. For the specimen with the lowest fuel-to-air ratio, the internal oxides were observed approximately 500 nm in the steel subsurface. Due to the increasing amount of oxygen potential as the fuel-to-air ratio is decreased, the amount and depth of internal oxides increased since oxygen could diffuse deeper into the steel subsurface. The external oxides in two samples were found entrapped between the inhibition layer and the Zn coating, which indicates the inhibition layer is undercutting the oxides by dissolution of iron.