Preparation For Ceramic Coating Used In Inwall Of Hot-Galvanizing Tank And Research On The Corrosion Mechanism Of The Coating

At the operating temperatures for hot-galvanization (460℃-480℃), the dissolving corrosion between melting zinc and steel often happens, which causes quick leakage of the hot-galvanizing tank and becomes one of the most troublesome problems in the field of hot-galvanization.In this paper, the corrosion course of the steels with different C and Si contents has been studied. It has been shown that the Si in the steel will lead to the decomposition ofζphase into a mixed phase which containsζ, FeSi andη. The existence of these phases makes the reaction between Fe and Zn become faster. The C in the Fe₃C increases the proportion of the grain-boundaries, which can promote the intergranular diffusion of Zn, favor to the formation ofζ, phase, and decrease the amount ofδandΓphases.According to the corrosion mechanics, the Al₂O₃ ceramic coating is selected to protect the galvanizing tank. The plasma spraying is adopted to prepare the coating with the effective protecting action for the hot-galvanizing tank.The interface reaction and wetting status between the ceramic coating and molting zinc is investigated. The XRD analysis shows that the elements will migrate along the grain boundaries, and thus the chemical reaction will occur. The phases with the spinel structure, such as the 3ZnO·47Al₂O₃, 4ZnO·11Al₂O₃ and ZnO·Al₂O₃,will slowly form at the surface of the coating. This is caused by the wetting reaction between the ceramic coating and molting zinc. By the thermodynamics and dynamics calculation, it has been proved that the wetting reaction can only occur in a finitely scale, the reaction is slow and incomplete, and is not the main reason for the coating failure.At the temperature for hot-galvanization, the thermal shock resistance of the ceramic coating is investigated. The thermal shock tests have been conducted under the severe conditions. It is proved that the coating has the excellent thermal shock resistance. However, as long as the macroscopic crack, interface crack and mechanical impact damage appear after the thermal shock, the melting zinc will penetrate through these defects. The bond coating and steel substrate are corroded. Subsequently, the desquamation and failure of the coating will occur. This is the main failure reason for the coating. In order to examine the lifetime of the coating, the 08Cu steel is chosen as the body material of the hot-galvanizing tank, and a new welding material is developed for preparing a tiny hot-galvanizing tank. After a long term of corrosion test for three months, it has been shown that the tank wall without the coating has been corroded for nearly 1.5mm while the tank wall with the coating prepared by the plasma-spraying keeps perfect, and the desquamation and cracking of the coating do not occur. It means that the ceramic coating can offer a satisfactory corrosion resistance for the hot-galvanizing tank.