Research On The Influence Of Zinc Bath Characteristics On The Crystallization And Diffusion Of Hot-dip Coatings

Hot dipping is a traditional protective technology with a long history.Hot diping is still the best protection methods from its development for steel materials.Hot dipping has been widely used because of its high bond strength,good corrosion resistance and process flexibility.Previously,the research has focused on the microstructure,corrosion resistance and surface aesthetics of the coating on hot dip galvanizing,poor studies have been done on the relationship between the characteristics of dipping bath and the effect of zinc bath on the quality of the coating,and there is no complete system in the hot dip galvanizing industry,therefore,following the study of zinc bath characteristics,the relationship between zinc bath characteristics and coating quality was studied based on the National Natural Science Foundation project"Pre-control of zinc bath characteristics and coating performance"to promote the further development of hot dip galvanizing.Based on the thermal analysis technology,the cooling curves of the workpiece cooling curve in hot dipping were determined in this paper as follows:temperature:430⁴80°C,time:5³00s,components:Zn,Zn-0.05%Al,Zn-0.1%Al,Zn-0.2%Al,Zn-0.3%Al,Zn-0.5%Al,Zn-0.2%Al-0.2%RE,Zn-0.2%Al-0.3%RE.The effects of different temperatures,times and compositions on the crystallization of the coating were analyzed with the crystallization parameters that obtained by means of the differential curve;The series of hot dipping parameters as temperature:450°C and480°C,time:180³00s,components:Zn,Zn-0.05%Al,Zn-0.1%Al,Zn-0.2%Al,Zn-0.3%Al,Zn-0.5%Al,Zn-0.2%Al-0.2%RE,Zn-0.2%Al-0.3%RE on Kirkendall effect in the microstructure of the coating was analyzed and the coating were observed by means of metallographic microscope.Through these studies,The mian conclusions were as follows:With the increase of the immersion temperature and the decrease of immersion time,the degree of super-cooling of the coating decreases.When immersed in a pure zinc bath at 450°C for more than 60s,the degree of super-cooling in coating is in the range of 14.1°C to 15.0°C;at 480°C for more than 60s,the degree of super-cooling in the coating is in the range of 32.6°C to 38.8°C.Under the condition of 5s,aluminum content of 0.05%to 0.2%and the same temperature,the degree of super-cooling increased.When the aluminum content was0.3%,the degree of super-cooling decreased,and the degree of super-cooling increased when the aluminum content was 0.5%.The degree of super-cooling of crystallization in the coating was 7.2°C measured at 450°C in zinc bath.The addition of rare-earth reduced the characteristic value of crystalline in the coating.The degree of super-cooling of crystallization in the Zn-0.2%Al-0.2%RE bath is in the range of 10.7°C¹3.4°C,and in the Zn-0.2%Al-0.3%RE bath is in the range of 9.4°C¹1.1°C.In the pure zinc bath,cracks appeared at 480°C for 240s and 300s.In the Zn-Al alloy bath,the alloy layer at 480°C for the same time was decreased or even disappeared,rare-earth elements promoting the diffusion between Fe and Zn by inhibiting the layer of Fe₂Al₅.The growth of alloy coating was controlled by interfacial diffusion at 450°C,and the growth kinetics index increased with the increase of aluminum content in zinc-aluminum alloy bath.