Lift-Off of Surface Oxides During Galvanizing of a Dual-Phase Steel in a Galvannealing Bath

Focused ion beam (FIB) and transmission electron microscopy (TEM) were used to characterize the interfacial structure of a dual-phase steel after galvanizing in a galvannealing bath with a dissolved aluminum concentration of 0.125 wt pct. Improved specimen preparation overcame preferential milling difficulties so that the interface region of the zinc-coated steel specimen was well preserved. The results show that the surface MnO oxide, a product of heat treating prior to galvanizing, was lifted off the surface of the substrate and subsequently entrapped in the Zn-Fe phase during galvanizing. A discontinuous inhibition layer was formed directly on top of the substrate. In regions where the inhibition layer was absent, Zn-Fe crystals grew directly on top of the substrate. This behavior significantly differs from the parallel study in which a DP steel was galvanized in a Zn bath with a higher Al concentration, where no lift-off of the MnO oxide was observed and the inhibition layer grew directly on top of the MnO oxide film. Possible mechanisms that are responsible for these significant differences were discussed.