Characterization of the Fe-Al interfacial layer in a commercial hot-dip galvanized coating

Recently there has been substantial interest in the formation of transient Fe-Al interfacial layers during hot-dip galvanizing and galvannealing. These layers delay the formation of Fe-Zn intermetallic compounds by preventing the interdiffusion of Zn and Fe. Despite the industrial importance of this inhibition phenomenon and the resulting research on the subject, there remains considerable uncertainty about the nature of the Fe-Al inhibition layer. In this study, the inhibition layer on commercially produced hot-dip galvanized steel is characterized using a combination of conventional and analytical SEM and TEM and X-ray diffraction, A reproducible technique for extracting the interfacial layer for the purpose of making plane-view TEM specimens is presented. Using this combined TEM, SEM, X-ray diffraction method, it is shown that the inhibition layer is composed of a layered structure of Fe2Al5 and FeAl3. It is also demonstrated that X-ray diffraction alone may fail to detect the FeAl3 phase and that a combined method using both TEM and X-ray diffraction is best suited for characterizing these interfacial layers. Additionally, it is shown that grain boundary diffusion of Zn within the Fe-Al interfacial layer may play an important role in the eventual transformation of the inhibition layer.