Study On The Process And Anticorrosion Mechanism Of Hot Dipping Zinc-Aluminium-Magnesium Alloy Coating

Hot galvanizing process is an economical and effective method to treat surface of steel materials. It was widely used because of its simple technology and good corrosion resistance. But the technology is developing and the society is progressing, more and more attention has been paid to the further improvement of the anticorrosion behaviors, so new types of alloy coating with excellent property are wished to be exploited.The process of adding right quantity of Al and Mg in coating has many distinct advantages. It can improve the surface appearance and corrosion resistance of the coating. The Zn,Zn-55%Al and Zn-5%A1 alloy coatings have been further studied. A new Zn-Al-Mg alloy coating is developed on the basis of the previous studies and the production process,corrosion resistance and anticorrosion mechanism are also studied.Mg has low density and active chemical property, it riches on surface of molten Zn and is oxidized very sharply. So mix proportion of molten alloy is not be sure, and oxides affect greatly on coating. It is suggested to use Zn-Mg alloy when adding Mg, and to select a new type of covering agent by tests. This kind of agent covers the surface of molten alloy to low the quantities of Mg loss and oxides, and to move scum. A new type of hot dipping auxiliary is selected by lots of tests for Zn-Al-Mg alloy coating. The coating surface with this auxiliary is smooth and has few limitations. Besides, the coating bonds fast to base.The weight loss rate by corrosion of Zn-Al-Mg alloy coatings with different components is measured by full-dipping testing and neutral salt spray testing. As a result, with increase of Mg content, corrosion resistance of Zn-Al-Mg alloy coating is improving and its corrosion resistance is equivalent to the one of Zn-5%A1 alloy coating when the content of Mg is 2.5%.The corrosion current density and polarization resistance are measured by AC impedance and polarization curve measurements. It was found that the corrosion resistance of Zn-Al-Mg alloy coating is improving with the increasing of Mg content.Corrosion products of coating in Na₂SO₄ solutions are measured by XRD. Zn₄SO₄(OH)₆·H₂O appeared in coating corrosion products and it can form compact protective film on the surface of the coating. So the coatings can be prevent from corroding sequentially.Surface appearance of coating is observed by SEM. Crystal particles of coating are being diminished and structure tends to uniformity with growing Mg content. The compact coating is formed by Zn/Al/ Zn₂Mg Ternary Eutectic. It could effectively inhibit Intergranular Corrosion.The hot dip time and temperature of the new alloy is equivalent with the Zn alloy for the low Al and Mg content in the coatings. On the basis of the original galvanizing unit it can produce the new alloy coatings, so the coatings have wide application prospect.