Study On The Interfacial Reaction Of Hot-dip Galvanizing Of High-strength Steels

Due to the excellent corrosion resistance, galvannealed coating of high-strength steels is the preferred material choice for exposed Autobody panels. There is only a couple of continuous galvanizers in China can produce galvannealed coatings; and the quality of the coatings is frequently problematic. To improve the surface quality, aluminum is frequently added to zinc bath. The alloying elements, such as Cr and Mn etc., coming from the steel will interact with the molten Zn-Al alloy, modifying the Zn-Fe reaction and deteriorating the quality of the coating. Focusing on the interaction between Zn, Fe, Al and Cr, the related phase diagrams were determined and the effect of alloying elements on coatings and simulation on the interfacial reaction were carried out, which is important to control the microstructure of coating, and improve the surface quality of Autobody panels.As a series of investigations, the Zn-Cr system and the isothermal section of the Zn-Fe-Cr system was determined using the equilibrated alloys with the aid of diffusion couple approach in the present work. The specimens were investigated by means of scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). These experimental data can be used in the thermodynamic modeling of the Zn-Fe-Cr system and the determination of phase diagrams with multiple element alloys.Based on the interatomic potentials obtained using the lattice inversion method, an atomistic study on the impact of alloy elements on the microstructure of coating was carried out. By X-ray diffraction combined with Rietveld structure refinement, the crystal structure of FeZn₁₃ was determined experimentally in this study. The structure of FeZn₁₃ and the site preferences and solubility of the alloying element T in FeZn₁₃ structure was investigated in atomistic-scale, which accumulate fundamental datum for investigation on the alloying elements' impact. A good agreement between the experimental results and the theoretical calculations was achieved.The microstructure of hot dip galvanized coatings of the pure iron by dipping in several zinc baths containing Al and Cr was studied by means of SEM and EDS. The results show that: a quaternary phase,Ï„₁, was found in the coatings developed from Cr-added zinc bath. Adding 0.1 wt% Al to zinc bath, coatings are composed ofδ,ζ,ηandÏ„₁. Adding 0.2 wt% Al to zinc bath, the thickness ofδlayer decreases,ζlayer changes from banding to impact blocks, and diffuse-Δemergent between theζblocks . At 480℃,ζis absent from coating. Based on the diffusion path theory, the reason that the coating microstructure is changed after adding Al and Cr into the zinc bath is also discussed and a model describing the coating formation has been modeled.Considering the diffusion of aluminum in zinc bath is the rate-limiting process, a reasonable model was developed so as to simulate the kinetics of interface reaction. There are three stages included: the quick dissolution of iron, the nucleation and growth of intermetallics compound and the solid-state diffusion growth of inhibition layer. From this model, the following parameters: the time for nucleation, the time forming complete inhibition layer, total consumption of iron and aluminum and the thickness of inhibition layer, can be calculated. This study is beneficial to establishing the optimal Al level, controlling the structure of coating and depressing the amount of zinc-dross, and it accumulates fundamental theories for developing zinc-bath management tools.