Study On Microstructure And Corrosion Resistance Of Coating On Hot Galvanized Plates

In order to reveal the microstructure of coating on hot galvanized plates,middle annealing was chosen in this thesis.To research the effect of annealing time on the surface and cross-section microstructure of coating,scanning electron microscope(SEM),energy spectrum analyzer(EDS),X-ray diffractometer(XRD)and other testing equipments and analysis methods were used.Neutral salt spray experiment was used to study the relationship between coating thickness and corrosion resistance and the corrosion of hot galvanized plates during practical use.At the same time,the causes and mechanism of white and red rust formation were thoroughly discussed.The results of SEM,EDS and XRD of coating after annealing show that the order of the iron and zinc compounds on surface of the coating is η→ζ→δp→δk.The Г1 phase rarely appeared on the coating surface,it will appear only when the annealing time is long enough.While the order of dominant phase on coating cross-section is: ζ→δp→δk→Г+Г1.In addition,the original plating surface is relatively flat.After annealing,a large number of small particles form on the coating surface.As the annealing time increases,small particles aggregate into large particles,and the number of large particles continuously increases.The results of the neutral salt spray experiment show that the corrosion extends from surrounding to the internal,the matrix to the surface and zinc layer to the internal.When the coating thickness is thin,increasing the coating thickness can effectively promote the resistance to white rust.However,when the coating thickness is thick,the efficiency of resisting white rust is reduced by increasing the coating thickness.And in actual production,the relationship should be comprehensively considered between coating cost and the resistance of white rust.Based on the experimental data,corrosion kinetics model was established based on the Johnson-Mehl-Avrami(JMA)equation,which has good fitting degree and provides a basis for the quantitative analysis of corrosion on hot galvanized plates.The results of this study clearly define the effects of annealing time on the surface morphology and microstructure of the coating,clarify the relationship between the coating thickness and corrosion resistance on hot-dip galvanized sheet,which provide theoretical basis in the actual production and prolong service life of hot galvanized plates.