| Modern industry needs to improve the properties of galvanized steel sheet. It is necessary to develop new types of alloy coatings with excellent properties. The Zn-Al-Mg and Zn-Al-Mg-Si alloy coatings was studied in this paper. The hot-dip galvanized alloys were made up using the intermediate alloy-melting salt cover method. The Zn-0.5wt%Al-x Mg(x=0,0.5,1.0,1.5wt%),Zn-2wt%Al-x Mg(x=0,0.5,1.0,1.5wt%) and Zn-2wt%Al-1.5wt%Mg-x Si(x=0.15,0.3wt%) alloy coatings were gained by a process developed by ourself on Q235 steel sheet. The microstructure and peroperties of alloy coatings were observed and analyzed. The effect rules of Al, Mg and Si on the microstructure and peroperties of alloy coatings were investigated. The main research results were obtained as follows.The surface of alloy coatings is smooth, and no defects of zinc melanoma and leakage in coatings. The coating brightness get to dim with increase of Al and Mg content. The surface brightness of Zn-Al-Mg alloy coatings are lower than that of Zn-Al alloy coatings. The surface brightness of Zn-0.5wt%Al alloy coating is excellent.The alloy coating microstructure can be refined with increasing Al, Mg and Si contents. The Al, Mg and Si contents are high in the grain boundary. The Al and Si contents are higher in steel substrate surface. The Zn-0.5Al alloy coating is composed with Zn phase, Al phase and Fe2Al5 phase, and the Zn-0.5Al-1.5Mg and Zn-2Al-1.5Mg alloy coatings are composed with Zn phase, Al phase, Fe2Al5 phase and Mg Zn2 phase. The microstructure of Zn-Al-Mg alloy coating are composed with bulk Zn phase and eutectic structure, thereinto, Zn-0.5Al-1.5Mg alloy coating is lamellar binary eutectic Zn+Mg Zn2, and Zn-2Al-1.5Mg alloy coating is granulous ternary eutectic Zn+Al+Mg Zn2. The microstructure of Zn-2Al-1.5Mg-0.3Si alloy coating is composed with Zn phase, eutectic structure and acicular phase with the length of 2 μm, width of 100 nm.The coating hardness increases with increase of Mg and Si contents. With increase of Mg content, the running-in time of Zn-0.5Al alloy coating decreases; the running-in time of Zn-2Al alloy coating increases firstly and then decline, and the running-in time of 1.0wt%Mg alloy coating is the longest. With increasing Si content the running-in time of Zn-2Al-1.5Mg alloy coating increases firstly and then decline when add Si element in the range of 0~0.3wt%.The study on the corrosion resistance of alloy coatings reveals that, The rank of corrosion rate for alloy coatings from low to high is Zn-Al-Mg-Si alloy coatings <Zn-Al-Mg alloy coatings <Zn-2Al alloy coating <Zn-0.5Al alloy coating. With the increase of Mg and Al contents, the corrosion rate of alloy coating declines, and corrosion resistance of alloy coating is enhanced. The corrosion resistance of Zn-2Al-1.5Mg-0.15 Si alloy coating is the best. |
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