Microstructure and Corrosion Resistance of Two-step Hot-dipped ZAM Alloy Coating on Structural Steel
WU Changjun1,2, YAO Lili1,3, ZHOU Zhisong1,3, XUE Feng4, ZHU Chenlu1,3
1 Jiangsu Xingda Steel Tyre Cord Co., Ltd., Taizhou 225721, China 2 School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China 3 Jiangsu Key Laboratory for Structural and Functional Metal Composite Materials, Taizhou 225721, China 4 School of Materials Science and Engineering, Southeast University, Nanjing 211189, China
Abstract: To obtain Zn-6Al-3Mg (ZAM) alloy coating, which has high corrosion resistance, the Zn-6Al-3Mg alloy coatings were prepared on Q235 steel by two-step hot-dip method. The Q235 steel, which was pretreated by conventional galvanizing method, was pre-dipped in pure zinc bath at 450 ℃ for 3—30 s and then immersed in ZAM alloy bath for 5—120 s. The influences of pre-dipping time and immersion time on the microstructure and corrosion resistance of the coating were investigated. Experimental results showed that the ZAM alloy coating with excellent surface quality can be obtained by two-step hot-dip method. The coating was composed of Fe2Al5-Znx, FeAl3-Znx+Z (solidification structure of the liquid phase) layer and the solidified ZAM alloy layer, while no Fe-Zn compound existed. After immersing in the ZAM bath for 10 s, the Fe-Zn compounds formed during pre-dipping had been totally corroded and turned to FeAl3-Znx+Z two phases. This layer could stably exist in the coating and its thickness increased with the pre-dipping time. At the same time, very thin Fe2Al5-Znx layer existed at the interface between steel substrate and coating. It was found that the thickness of both Fe2Al5-Znx and FeAl3-Znx+Z two phase layer changed a little with the immersing time in ZAM bath. In addition, it was confirmed that the corrosion resistance of the two-step hot-dipped ZAM coating was better than that of pure zinc coating. This kind of Zn-6Al-3Mg (ZAM) alloy coating with FeAl3-Znx+Z two-phase layer and high corrosion resistant will help to develop new kind of coa-ting on steel parts.