INFLUENCE OF SILICON IN STEELS ON GALVANIZED COATINGS

摘要

After steel sheets (0.37wt percent Si) pre-electroplated with a thin layer of pure Fe were immersed in molten zinc for various time, the change in the micro structure of the galvanized coating on the steel and the change of alpha-Fe/GAMMA 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 GAMMA layer after the Fe layer is depleted by the increasing growth of the compound layers. With the movement of a-Fe/GAMMA interface towards the substrate by the Fe/Zn reaction, silicon-rich alpha-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 delta layer through the GAMMA layer because silicon-rich alpha-Fe can not be adsorbed in the GAMMA layer. On reaching the delta/GAMMA interface, the particles quickly dissolve in the delta layer, and accelerate its growth, resulting in the gradual disappearance of the GAMMA layer. At the same time, the normal coating is quickly changed into coatings typical of reactive steels as silicon dissolved in the delta layer soon diffuses toward the zeta 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.