Process And Mechanism Of Hot Dipping Zn-Al-Mg Alloy By Reduction Method

Zn-Al-Mg alloy coating has become a research hotspot for its excellent corrosion resistance and trimming protection.However,the traditional solvent method for hot dipping Zn-Al-Mg alloy has some problems,such as difficult plating,poor coating quality and environmental pollution.Therefore,it is of great theoretical significance and practical value to study the process and mechanism of hot dipping Zn-Al-Mg alloy by reduction method.In this paper,the process of hot dipping Zn-6Al-3Mg alloy by reduction method was optimized by orthogonal test.The effects of the surface condition of hot rolled steel plate,H₂content in reduction atmosphere,reduction time,steel entry temperature,temperature of zinc solution and dipping time on the structure and properties of the coating were studied.In addition,microstructure defects and formation mechanism of the coating were analyzed.The results showed that the removal of oxide scale from hot rolled steel plate by mechanical grinding could improve the surface quality of the coating and the interface bonding between the coating and the steel matrix.Under the conditions of 30%H₂in reduction atmosphere,reduction time for 30 s,steel entry temperature at 670?,temperature of zinc solution at 460? and dipping time for 20 s,the surface quality,interface bonding and corrosion resistance of the coatings obtained by dipping of polished steel plate were better.At this time,the Zn-6Al-3Mg alloy coating was composed of surface solidification layer and alloy layer near the matrix.The surface solidification layer consisted of Zn-rich phase,MgZn₂,Zn/MgZn₂binary eutectic structure,Zn/Al/MgZn₂ternary eutectic structure,and the alloy layer consisted of Fe₄Al₁₃.The thickness of Fe-Al alloy layer increased with the increase of H₂content in reduction atmosphere when the reduction time was 30 s.The thickness of the whole coating and Fe-Al alloy layer increased with the reduction time when the H₂content in reduction atmosphere was 30%.Leakage on the surface of the coating occurred when the steel entry temperature was lower than 500?.With the increase of steel entry temperature,the interface bonding between coating and matrix became worse and the corrosion resistance decreased.When the steel entry temperature was 900?,the phenomenon of separation of Fe₄Al₁₃alloy layer into solidification layer from the matrix occurred,and severe stripe protrusion defects were formed on the surface of the coating.It was found that the oxide scale on the surface of the unpolished steel plate could effectively inhibit the formation and separation of Fe₄Al₁₃alloy layer.Due to the reaction of water vapor produced during reduction of steel plate with Mg atom in zinc solution,petal-like hollow holes consisting of zinc-rich massive structure appeared in the coating.