Optimization of exposure time for hot dip galvanizing and study the effects of preflux bath additives on the microstructures of galvanized steel

The objective of this research work is to optimize the time of exposure for hot dip galvanizing of mild steel, so as to achieve such a microstructure of zinc coating that protects mild steel for long against atmospheric corrosion. In order to decide the above mentioned parameter the polished samples, one at a time, were dipped in to 500 g molten zinc at 460 to 470 oC in the crucible for a different period of time for example 2, 5, 10, 15 minutes. The samples of mild steel (25 mm x 40 mm x 5 mm) were abraded on series of emery papers (1/0, 2/0, 3/0 and 4/0 grades) before galvanizing. After abrading, the samples were washed in tap water and air dried before hot-dip galvanizing. The microstructures of the coatings were observed under optical microscope at 200X magnification. It is observed that defect-free uniform layers of different phases were produced on 5 and 10 minutes of exposure to molten zinc in hot-dip galvanizing process. Hot dip galvanizing normally produces four phases in the form of successive layers in the coating; starting with substrate, referred as gamma, delta, zeta and eta phases. These phases are found in Fe-Zn phase diagram on varying Fe content. These observations show that with 5 and 10 minutes time of exposure to molten zinc, good adhesion and uniform layers of different layers of Zn-Fe phases are formed. Hot Dip Galvanizing process includes various pre treatment steps prior to immersion of the ferrous substrate in the molten zinc. The final step of these processes is fluxing which takes place right before immersion in the molten zinc. In the present investigation we have selected various preflux baths such as ZnCl2 .NH4 Cl, CdCl2, Nicl2 and SnCl2 with the identical salt concentration (50%wt) so that, their effects on the microstructures of galvanized steel can be compared. As Cd and Zn belong to the same group of periodic table their properties are expected to be similar, hence their behavior concerning galvanizing have to be alike. But the anomaly is observed in the microstructure of specimen dipped in to the preflux bath containing CdCl2 is showing wavy layer of Zeta phase. When the concentration of NiCl2 reaches 50% in preflux bath the needle like crystals are not observed in zeta phase which is composed only by small grains thus resulted into compact coating layer. Sn addition seems to be inactive with regard to coating structure. Keywords— Hot-dip Galvanizing, mild steel, coating and atmospheric corrosion, prefluxing baths