The
hot dip galvanized coating on Si containing steel is usually gloom and over thick and has poor adhesion to the substrate.
Thus tin was introduced into the zinc bath for the hot dip galvanization of 0.37%Si steel so as to modify the growth and microstructures of the galvanized coating. The thickness and microstructures of the resulting galvanized coatings prepared by immersing in the zinc bath containing various contents of Sn (with the mass fraction of Sn to be 0, 3%, 5%, 7%, and 12%, respectively) for different durations were determined and studied by means of electron probe microanalysis and scanning electron microscopy. The results showed that it was feasible to get rid of the over thickness of the coating by introduction of a proper amount of Sn into the zinc bath. As the Sn content in the zinc bath was less than or equaled to 5%, the coating thickness greatly decreased with increasing Sn content. When the Sn content was above 5%, the coating thickness assumed little change with further increase of the Sn content. Moreover, the addition of Sn to the zinc bath led to changes in the
microstructure of the coating. Namely, at a Sn content over 5%, the δ layer became thick and compact, the ζ layer was much thin, and the microstructure of the ζ layer changed from loose massive crystal to regular columnar one. It was supposed that the coating thickness decrease in the presence of Sn was due to the formation of a Sn rich zone, which dominated over the formation of the ζ layer and acted to delay the Fe/Zn diffusion.