Process Optimization and Defect of Hot-dip Double-coating

The optimal double-plating process of hot-dip aluminum and Zn-55% Al-1.6% Si on Q235 steel was obtained.Metallurgic microscopy and scanning electron microscopy (SEM) were used to characterize the microstructure of coating defects,and energy dispersive X-ray spectroscopy (EDS) was employed to analyze the composition and forming reason of defects.The optimal process of hot-dip double-plating was determined as follow:carring out secondary dip after complete cooling of pre-plated Zn,dip time and temperature of pure Al and Zn-55% Al-1.6% Si were 60 s,710 ℃,and 30 s,640 ℃ respectively.Nubble was the main defect during double-plating.The restrain method was to control dip time.