Experts share: research Progress of Zn-Al-Mg Hot-dip Alloy [Hot-dip galvanizing Development Forum]

SMM Sept. 14: at the fourth China International Hot Dip galvanizing Technology and Market Application Development Forum 2021 held by (SMM) of Shanghai Nonferrous Network Information Technology Co., Ltd., Gao Bo, professor and doctoral director of the School of Metallurgy of Northeastern University and foreign academician of the Russian Academy of Natural Sciences, introduced in detail the research progress of Zn-Al-Mg hot-dip alloy.

At present, several commercial multi-element hot-dip alloy coatings include Zn-55Al-1.6Si coating (Galvalume), Zn-5Al-0.1RE coating (Galfan), Zn-23Al-0.3Si coating, Zn-4.5Al-0.1Mg coating (Superzinc), Zn-11Al-3Mg-0.2Si coating (Super Dyma), Zn-0.5Mg coating (DYMAZINC), and Zn-6Al-3Mg coating (ZAM), etc. These seven coatings have been produced on a large scale and have been developed by foreign iron and steel companies, but are protected by foreign patents.

Among them, Zn-4.5Al-0.1Mg coated (Superzinc) is a 4.5%Al-Zn-0.1%Mg coated steel plate developed by Japan Iron and Steel Research Center in 1985. the results of exposure experiments in the atmosphere for ten months show that the corrosion performance of hot-dip galvanized magnesium alloy steel sheet is more than twice that of ordinary hot-dip galvanized steel plate. Zn-11%Al-3%Mg-0.2Si coating (Super Dyma) was developed by Nippon Steel Company for the first time in 2000, and the alloy coating with Zn-11%Al-3%Mg-0.2Si composition was first developed by Nippon Steel Co., Ltd. This kind of alloy coating is thin, mainly aluminum-iron, containing a small amount of zinc and magnesium. Its corrosion resistance is 15 times that of hot-dip galvanized steel and 6-8 times that of hot-dip Galfan steel. Super Dyma alloy has good scratch resistance, and its surface hardness is close to 140HV, which is about 3 times that of hot-dip galvanized coating, and the notch corrosion resistance of Galvalume coating is better than that of hot-dip galvanized coating.

In addition, the Zn-6%Al-3%Mg alloy coating (ZAM) is in the 1990s. On the basis of previous studies, Nippon Steel Company of Japan developed a coating alloy material with a higher corrosion resistance composition of Zn-6Al-3Mg, which is known as the fourth generation high corrosion resistance coating material. The corrosion resistance of the Zn-Al eutectic coating with 3% magnesium content is 10-20 times higher than that of galvanized (GI) steel. Galfan coating is 5-8 times higher than that of steel plate, which is the best hot-dip galvanized alloy coating at present.

Taking Zn-6Al-3Mg (ZAM), Zn-55Al-1.6Si (Galvalume) and Zn-23Al-0.3Si galvanized sheet as the research object, our group studied the effects of Si and RE on the corrosion resistance and formability of ZAM coating, and the effects of Mg and RE on the corrosion resistance and formability of Galvalume and Zn-23Al-0.3Si coating.

Study on Modification of ZAM coating

The thickness of Zn-6Al-3Mg-0.1Si coating is about 23 μ m, which is about 10 μ m thinner than that of ZAM coating, and there is no defect in the coating. There is only one alloy layer with a thickness of about 1 μ m at the interface of Zn-6Al-3Mg-0.1Si coating / steel plate. Zn-6Al-3Mg-0.1Si coating is composed of Zn/Al/ MgZn2 ternary eutectic structure and bulk phase, in which the bulk phase is aluminum dendrite.

Effect of Si and RE on thickness and quality of ZAM coating

From the results of neutral salt spray test and electrochemical detection, it can be found that the addition of 0.1% Si and 0.1% RE, can significantly improve the corrosion resistance of ZAM coating.

The effect of Si and RE on the corrosion resistance of ZAM coating was tested by full immersion test in 5% NaCl solution at room temperature. it was concluded that the thickness of Zn-6Al-3Mg-0.1Si-0.1RE coating was only 49.7% of that of ZAM, but its service life in NaCl solution was 1.61 times that of ZAM, indicating that Si and RE significantly improved the corrosion resistance of ZAM coating.

Effect of Mg on Microstructure of Galvalume coating

The surface of Zn-23Al-0.3Si coating is mainly dendritic structure, and the coating surface is mainly composed of aluminum-rich phase and pure zinc phase, as well as zinc-aluminum binary eutectic structure around and inside the pure zinc phase. When a small amount of Mg is added to the alloy, the aluminum-rich phase on the surface of the coating still maintains its dendritic structure, but the difference is that the striped zinc-aluminum-magnesium eutectic structure appears in the white interdendritic zone of pure zinc. With the further increase of Mg content, defects such as pores and cracks begin to appear in the coating structure.

Effect of Mg on corrosion Resistance of Galvalume coating

It is known from the experimental data that the corrosion rate of Zn-23Al-0.3Si-2.0Mg coating reaches the minimum value (0.0245 g ·m-2 ·h-1), and the corrosion resistance of the coating is considered to be the strongest. Therefore, in the process of hot-dip plating, adding appropriate amount of alloy element Mg, to the coating is helpful to improve the corrosion resistance of the coating.

Effect of Mg on cross-sectional structure and thickness of Galvalume coating

With the addition of Mg, the intermetallic layer of the coating becomes more uniform and its thickness becomes thinner.

Effect of Mg on Section formability of Zn-23Al-0.3Si coating

Cracks occurred in the Zn-23Al-0.3Si coating during the deep drawing test. Because of the cracks, when the coating is in a corrosion environment after machining, the corrosion medium can reach the steel plate substrate directly through the cracks and corrode the steel plate. A few cracks still exist in the Zn-23Al-0.3Si-2.0Mg coating, but the crack width becomes narrower obviously and the formability of the coating is greatly improved.

Effect of Mg on corrosion types of Zn-23Al-0.3Si coating

After the corrosion products are removed on the surface of Zn-23Al-0.3Si coating, it can be seen that the corrosion layer of the coating is thicker. Due to the existence of cracks, the corrosion of the coating is more serious, and the corrosion extends to the interior of the coating, which is very disadvantageous to the coating.

From the low-power and high-power topography of the Zn-23Al-0.3Si-2.0Mg coating after removing the corrosion products, it can be seen that the overall surface corrosion of the coating is not serious, and the surface is basically flat, but part of the interdendritic zone is shallowly corroded, indicating that the zinc-rich interdendritic zone of the coating has been strengthened with the addition of 2.0wt.% Mg, thus the corrosion resistance of the coating has been improved to a certain extent.

Study on the effect of waste Aluminum on Zn-5Al-0.1RE coating

New aluminum scrap refers to the process waste and unqualified waste products produced in the process of aluminum processing. Old aluminum scrap refers to scrapped aluminum after social consumption, such as aluminum doors and windows, automobile aluminum castings, cans, etc., as well as new aluminum waste flowing into the society for recycling.

Effect of waste Aluminum on corrosion performance of Zn-5Al-0.1RE coating

Through the comparative test of the surface morphology of the coating before and after the salt spray test, it is found that the Zn-5Al-0.1RE coating with different content of waste aluminum has different degrees of corrosion, and the larger the black area of the surface after corrosion is, the more serious the corrosion is. By comparison, it can be found that the Zn-5Al-0.1RE coating is more corrosion-resistant than other coatings.

Effect of waste Aluminum on formability of Zn-5Al-0.1RE coating

The Zn-5Al-0.1RE coating has a two-layer structure, and the inner layer is Zn-Fe-Al layer, and there are cracks in the inner layer after deep drawing. when 90% waste aluminum is added, there are only slight cracks in the Zn-5Al-0.1RE coating, which is almost invisible, indicating that the addition of waste aluminum enhances the formability of the coating, does not produce cracks when the coating is bent, and improves the service life of the coating.

Effect of waste Aluminum on corrosion products of Zn-5Al-0.1RE coating

After adding 90% waste aluminum, the corrosion products of the coating have the passivation effect of Al2O3,Al2O3, which can play a protective role, which is also the reason for the improvement of corrosion resistance after adding waste aluminum. Through the experimental comparison, it is found that the intergranular corrosion occurs in the coatings of Zn-5Al-0.1RE, Zn-5Al-0.1RE, Zn-5Al-0.1RE and Zn-5Al-0.1RE. Compared with the Zn-5Al-0.1RE coating with 90% waste aluminum, the corrosion degree of the Zn-5Al-0.1RE coating is much lighter and more uniform, and the surface of the corroded coating is still very flat.

Generally speaking, after adding waste aluminum, Zn-5Al-0.1RE coating is more resistant to corrosion than other coatings, and enhances the formability of the coating, so that the coating does not produce cracks when bending, and improves the service life of the coating.