Study On Preparation,Microstructure And Properties Of Hot-dip Zn-Al-Mg-RE Alloy Coatings

Hot dip plating technology is most widely used,cost-effective method and easy-to-operate corrosion protection for steel surface in the world.With the rapid development of hot dip galvanizing,the zinc coating can not meet the needs of the society.It is necessary to develop new hot dip galvanizing products to replace it.In this paper,Zn-Al-Mg and Zn-Al-Mg-RE alloy coatings with different compositions were prepared on Q235 cold-rolled steel plate by changing Mg content and RE content continuously and assisted by ammonium-free plating agent which is the best match for hot-dip alloy.The effect of Mg and Re on the microstructure and properties of the alloy coating was studied by analyzing the microstructure and properties of the alloy coating.The surface of all the alloy coatings was observed.It was found that all the coatings were smooth and bright,and there were no zinc nodules,leakage plating and other defects.There are obvious dendrite structures on the surface of Zn-5Al-1Mg alloy coating.XRD analysis of all alloy coatings showed that the surface of all alloy coatings was mainly composed of Zn and MgZn₂ phases.The results show that the surface and cross section of Zn-5Al-3Mg-0.1RE coating are the most compact.The surface structure of Zn-5Al-2.5Mg-0.05RE coating is flower-like and dendrite is obvious.The thickness of all alloy coatings was measured by magnetic thickness gauge.It was found that the thickness of alloy coatings decreased with the addition of 0.05wt%-0.1wt%RE.The thickness of Zn-5Al-2.5Mg-0.05RE alloy coating is the smallest.The microhardness of all alloy coatings was measured by microhardness tester.It was found that the hardness of several alloy coatings was about 95HV-175HV in the experiment,and there was little difference.After adding Mg and RE elements,the hardness of some alloy coating will increase.The results of Na₂SO₄ immersion test show that with the increase of Mg content,the corrosion resistance of the alloy coating is Zn-5Al-1Mg alloy coating>Zn-5Al-3Mg alloy coating>Zn-5Al-2.5Mg alloy coating.The corrosion rate of the alloy coating decreases with the increase of RE content.The corrosion resistance of the alloy coating with RE addition is Zn-5Al-3Mg-0.1RE alloy coating>Zn-5Al-2.5Mg-0.05RE alloy coating>Zn-5Al-1Mg-0.05RE alloy coating.The electrochemical polarization curves show that the corrosion resistance of the alloy coating can be enhanced by increasing Mg content and RE content.The corrosion resistance is similar to that of Na₂SO₄ immersion test.