Influence of Aluminum Concentration in Zinc Bath on Galvanizing Behavior of a Dual Phase High Strength Steel

The increased Mn and Si concentration in advanced high strength steels (AHSSs) poses as a challenge to galvanizing processes. In this work, we investigated the interfacial structure of a dual phase (DP) steel that was hot-dipped into a galvanizing (0.2wt%Al-Zn) bath (GI bath) and a galvannealing (0.125wt%Al-Zn) bath (GA bath). The resultant interface between the steel substrate and the Zn coating was examined with transmission electron microscopy (TEM). It was found that the Al% in the zinc bath has an important impact on the interfacial structure. For the GI bath, a continuous inhibition layer was observed on top of the continuous external MnO oxides with a thickness of ~20 nm, followed by the Zn-rich phases. For the GA bath, the external MnO oxide was lifted-off from the substrate surface and was entrapped in the Zn-rich phase and a discontinuous inhibition layer was observed. Possible mechanisms will be discussed.