Abstract: After steel sheets (0.37wt％Si) pre-electroplated with a thin layer of pure Fe were immersed in molten zinc for various time, the change in the microstructure of the galvanized coating on the steel and the change of α-Fe/ Γ interface were studied. The EDS (energy dispersive sepectroscopy) results show that excessive silicon accumulates on the surface of the steel substrate due to the low solubility of silicon in the Γ layer after the Fe layer is depleted by the increasing growth of the compound layers. With the movement of α-Fe/ Γ interface towards the substrate by the Fe/ Zn reaction, silicon-rich α-Fe peels off from the substrate and breaks into particles. The particles, much like an inert marker in a Kirkendall effect experiment, move towards the δ layer through the Γ layer because silicon-rich α-Fe can not be adsorbed in the Γ layer. On reaching the δ/ Γ interface, the particles quickly dissolve in the δ layer, and accelerate its growth, resulting in the gradual disappearance of the Γ layer. At the same time, the normal coating is quickly changed into coatings typical of reactive steels as silicon dissolved in the δ layer soon diffuses toward the ζ layer. A similar process may happen in the initial stage of galvanizing reactive steels on a small scale, although it is hard to be observed.

Key words: Zn coating