The Study On The Effect Of Si Addition On The Structure And Property Of Hot-dip Zn-Al Coatings

Hot-dip Zn-Al has the advantages of hot dip galvanized and aluminum.Compared with hot-dip galvanized,it has better corrosion resistance and high temperature oxidation resistance.Compared with hot-dip aluminum plating,it has the advantages of low cost,good abrasion resistance and strong adhesion.Adding trace elements in the zinc-aluminum bath not only improves the structure of the coating,but also improves the performance of the coating.In order to understand the effect of trace element silicon in the bath on the microstructure and properties of the coating,different amounts of silicon were added to the Zn-20%A1 bath and the Zn-50%A1 bath.Plating samples were obtained at different immersion times.The high-temperature oxidation resistance test was performed on the Zn-50%A1 coating with high Al content,while the neutral salt spray corrosion test was performed on the Zn-20%A1 coating sample with low A1 content.Scanning electron microscope,energy spectrum analyzer and X-ray diffractometer were used to observe and analyze the microstructure,corrosion and oxidation products of the coating.The result of Si element on the Zn-20%Al coating shows that,the coating is divided into two layers,the inner layer is an intermediate alloy layer composed of an Fe-A1 phase adjacent to the substrate,and the outer layer is a free layer composed of an Al-rich dendritic phase and an intercrystalline zinc-rich phase.Adding 0.1%of silicon to the bath results in a sharp reduction in the thickness of the intermediate alloy layer.When the content of Si in the bath is less than 0.1%,periodic lamellar structure is formed in the coating.When the content of Si in the bath increases to 0.8%,the intermediate alloy layer changes from FeAl3 phase to T4 phase.The salt spray corrosion test results show that a dense compound Al3.21Si0.47 and the corrosion product Zn0.63Al0.37(OH)2(Cd3)0.185·H2O are formed on the surface of the plating under the corrosive environment,which play a role in insulating the corrosive environment and improving the corrosion resistance of the coating.When the content of Si is less than 1.5%,the corrosion resistance of the coating gradually increases as the content of Si increases,and when the content of Si exceeds 1.5%,the surface quality of the coating deteriorates,resulting in a gradual deterioration of the corrosion resistance of the coating.The result of Si element on the Zn-50%Al coating shows that,when no or less than 0.2%of silicon is added in the bath,periodic lamellar structure is produced in the coating.The lamellar structure consists of a gray black FeA13 phase and a dark black Zn-A1 phase alternately arranged parallel to the contact surface between the base and the bath.The formation of periodic layer structure requires a suitable immersion time,in a certain time range,FeA13 flaked off from the front of the reaction and diffused into the free layer,gradually forming a lamellar shape.When the immersion time is too long,the FeA13 phase in the lamellar layer gradually broken up and distributed in a free layer.Periodic layer structure disappears.In the case where the Si content in the bath is higher than 0.4%,the inhibition of the Fe-Al reaction by the Si element restricts the growth of the alloy layer,and the thickness of the plating layer is reduced from several hundred micrometers to less than twenty micrometers.The results of high temperature anti-oxidation experiments show that The FeAl3 phase in the intermediate alloy layer transforms into FeAl and Fe3Al phases under high temperature oxidation conditions.The cross-section energy spectrum of the coating indicates that there are two thick oxide layers mainly composed of A12O3 on the surface of the coating to protect the plating.The high-temperature oxidation resistance of silicon-containing Zn-50%Al coating is related to the silicon content.With the increase of Si content,the high-temperature oxidation resistance of the coating gradually increases.