Application of rare earth in hot dip galvanizing

It is well known that rare earths have many excellent properties in various materials, such as: in metal materials, it can refine matrix grains, strengthen matrix structure, improve matrix mechanical and mechanical properties, and improve corrosion resistance of metal substrates. Wait. In the hot-dip galvanizing process, adding appropriate rare earth metal can not only lower the melting point of the zinc liquid, increase the fluidity of the zinc liquid, make the hot-dip galvanizing layer thin and uniform, increase the surface finish of the coating, and greatly enhance the coating. Corrosion resistance and mechanical properties, prolong the protection time of the coating [2~3]. China is one of the countries with the richest reserves of rare earths in the world. It has carried out intensive research and development of rare earth hot-dip galvanizing technology, which has very important economic and social benefits.

I. Application and development of rare earth in hot dip galvanizing

1. Origin and development of rare earth application in hot dip galvanizing

The application of rare earth in the hot-dip galvanizing industry was first studied by the International Lead and Zinc Research Organization (ILZRO) and the Leige Metallurgical Research Center (CRM). In 1980, Zn - 5 % Al- 0.05% RE heat was successfully studied. Galvanized aluminum rare earth coating - Galfan coating [4]. In 1982, Fical of France realized the hot-plated Galfan coating on steel wire.

Industrial production. The coating formability, corrosion resistance, adhesion and the like are better than the traditional hot-dip galvanized layer, the coating is dense and uniform, and the salt spray test and the SO2 atmosphere test result show that the corrosion resistance is much higher than that of the conventional coating [5]. In 1984, the Inland Steel Corporation of the United States and the Leige Metallurgical Research Center of Belgium jointly researched and produced the Galfan alloy coated steel wire with an average coating thickness of 12 μm. The atmospheric corrosion resistance is equivalent to a conventional coating of 20 μm. The salt spray test results show that it is resistant to corrosion. The sex is 2 to 3 times higher than that of galvanized wire [6]. In the 1990s, as the demand for galvanized materials continued to expand, the Galfan alloy plating process achieved rapid development and achieved great success.

2. Development and research status of rare earth hot dip galvanizing in China

In the 1980s, China began to study the hot zinc rare earth process technology on a large scale and achieved a series of important research results. In the early 1980s, Hebei Metallurgical Research Institute cooperated with Beijing University of Science and Technology and Hebei Gucheng Wire Rope Factory for the first time in China for Zn-(0.02-0.3)% RE-(0.005-0.3)% Al-(0.05-0.50% Mg alloy). , influencing the performance of hot-dip galvanized coatings, and successfully developed the Zn-0.2Al-RE coated steel wire and steel strand process in 1986 [7]. In 1986, the former Tianjin Fourth Metal Products Factory and Baotou Rare Earth Research Institute, based on the introduction of foreign patents on Galfan, has been the first in China to apply this alloy hot-dip galvanizing technology to the production of machine-made mesh fence steel wire in 1989, and passed the appraisal of the Ministry of Metallurgy in 1989. In the same period, many domestic enterprises and researchers have also been involved in research and development in this field, and have achieved good results [3]. Since the 1990s, the research work of heat Zn- 5% Al-RE as anti-corrosion coating has been achieved. In 1994, Tianjin Steel Wire Factory introduced the Galfan alloy hot-dip galvanizing process for the first time, and introduced the US steel wire hot-dip galvanizing alloy production line, which achieved good economic benefits. Since then, many domestic enterprises and research institutes have been waiting.

Chen Jinhong from the Materials Research Institute of South China University of Technology studied the properties of Zn-5%Al-RE and Zn-Ni alloy coatings [8]. The results show that Zn-5%Al-RE and Zn-Ni alloy coatings and pure Compared with the Zn coating, it has excellent corrosion resistance and processing properties. The extremely thin alloy phase layer is the main reason for the good adhesion of the alloy coating. The addition of alloying elements greatly enhances the corrosion resistance of the coating.

Gong Mingming used orthogonal experiments to investigate the effects of Al, Mg and rare earth on the Zn content on the coating and the corrosion resistance of the coating [9]. Gong Mingming believes that in the low Al-Zn alloy, the corrosion resistance of the alloy containing 0.2% aluminum is the best. The proper amount of rare earth and magnesium can improve the corrosion resistance of the coating. The corrosion resistance of the Zn-Al-RE-Mg alloy coating is 2 to 3 times that of pure Zn.

Du Pengxiang et al studied the effect of rare earth elements on hot dip 55% Al-Zn alloy [10]. Studies have shown that in hot dip 55 % Al-Zn alloy, 0.1% rare earth elements are added, and rare earth elements are concentrated in the plating. The liquid surface layer has a good protection effect on the plating solution, prevents and reduces the oxidation of the plating solution, and improves the surface performance of the plating layer; meanwhile, the concentration of the rare earth element in the plating layer may be lower than the concentration in the plating solution. Hu Wenjie et al. used hot-dip galvanizing of steel tubes with Zn-Al alloys added with rare earths [11]. Compared with ordinary hot-dip Zn layers, the coatings were thinner by 18%-20% on average, and Zn consumption was reduced. 10%; at the same time, compared with the ordinary hot-dip Zn layer, the corrosion resistance of the rare earth Zn-Al alloy hot-dip Zn layer is improved by 1 to 2 times.

Xiang Changxiang et al studied the application and development of Zn-Al-Ni-RE quaternary alloy rare earth in the hot dip galvanizing industry under laboratory conditions. The effect of Zn-Al-Ni-RE ternary alloy rare earth on the hot dip galvanizing of Si-containing steel (16Mn) [12], research shows Adding alloying elements such as Al, Ni and Ce to the Zn solution has a synergistic effect on the anomalous growth of the ζ phase in the inhibition of zinc-iron reaction, in which the effect of Al is most obvious, followed by Ce and Ni, and in the coating, Al It is evenly distributed in each phase of the coating. Ni is distributed at the iron-based interface, and Ce is distributed on the grain boundary of the ζ phase. This not only inhibits the growth of the ζ phase, but also makes the grain of the coating fine and dense, and it is improved to some extent. The coating is resistant to intergranular corrosion and enhances the coating's protective properties. Xu Xiuqing et al studied the effect of adding trace rare earth Ce on the corrosion resistance of hot-dip galvanizing layer in the plating solution [13]. The results show that adding about 0.05% rare earth Ce in the plating solution can significantly improve the coating in acidic medium. Corrosion resistance; if the addition amount of rare earth Ce is increased to 0.1%, the corrosion resistance of the coating to the neutral corrosive medium is significantly improved.

Tan Juan et al. systematically studied the fluidity of the plating solution, the thickness of the plating layer, the microstructure of the coating and its corrosion resistance by GI galvanizing solution (Al content of 0.15 wt% to 0.22 wt% in zinc ingot) [14]. influences. The study believes that the addition of rare earth has the effect of refining the surface dendrites of the coating, improving the fluidity of the plating solution, reducing the thickness of the coating and improving the corrosion resistance of the coating. Among them, there is an optimum range of the effect of rare earth on the corrosion resistance of the coating. Under the experimental conditions, when the rare earth content is between 0.045% and 0.069%, the salt spray corrosion resistance of the coating is about 2.8 times that of the pure zinc coating. In addition, Yin Jingqun et al. studied the effects of adding trace amounts of La, Ce rare earth and Al on the properties of hot-dip galvanized layer [15];

Yu Guofeng et al [16] conducted in-depth research and discussion on the performance of hot dip Zn-5% Al rare earth alloy coating on the sling of the bridge and the mechanical properties and corrosion resistance of the steel wire.

Zhongsheng et al. have done a lot of research on the microstructure and properties of hot-dip Zn Al rare earth alloy layer [17].

The above research results show that the addition of trace amounts of rare earth elements can not only improve the mechanical properties and forming properties of the hot dip galvanized layer, but also control the thickness of the coating and reduce the Zn consumption. At the same time, adding an appropriate amount of rare earth elements to the plating solution can improve The surface properties of the coating enhance the corrosion resistance of the coating.

Second, the role and influence of rare earth elements on hot-dip galvanizing layer

Du Pengxiang et al [10] studied the influence of adding rare earth elements on the alloy coating in 55% Al-Zn alloy plating solution, and concluded that the distribution of rare earth elements in 55% Al-Zn alloy plating solution is not uniform, it is easy to be rich. It accumulates on the surface of the plating solution, thus providing a good protection against the surface of the plating solution.

Literature [14] believes that the surface tension of hot-dip galvanizing bath is reduced after adding a trace amount of rare earth in the zinc bath [18]. According to the kinetic conditions of non-uniform nucleation [19], the surface tension of the plating solution is reduced, resulting in a decrease in the specific surface energy of the solid-liquid interface, a decrease in the critical nucleation radius of the nucleation, an increase in the nucleation rate, and a refinement of the surface grains. size. The grain boundary density of the coating surface increases, the impurity distribution is uniform, and the subgrain boundary gradually becomes shallower, which makes the corrosion resistance of the subgrain boundary be improved. However, the addition of rare earth has an optimum range. Above this range, the corrosion resistance of the coating will decrease with the addition of rare earth. Experiments show that the optimum range of rare earth addition is not more than 0.069%.

Zhang Jianqiang et al. [20], the addition of trace rare earth to Zn- 5% Al alloy can improve the wettability of the interface between steel wire and alloy coating, refine the grain of alloy coating and reduce intergranular corrosion. Experiments show that the corrosion resistance of the coating is related to the added rare earth content. When the rare earth addition amount is 0.06%, the salt spray corrosion resistance of the coating is optimal.

In the literature [21], Yang Yun et al. studied the effect of adding rare earth on hot dip galvanizing of Zn- 5% Al alloy. The results show that adding 0.1% to 0.2% rare earth in Zn-5% Al alloy can reduce The surface tension of the alloy plating solution reduces the wetting angle between the solution and the steel plate, increases the rising height of the solution on the steel plate and the adsorption work, thereby improving the wettability between the alloy plating solution and the steel plate, and improving the protective performance of the alloy plating layer.

Literature [22] pointed out that the addition of trace rare earth elements reduces the melting point of the plating solution, improves the fluidity of the alloy, improves the formability of the coating, and reduces the zinc consumption and energy consumption to some extent. At the same time, trace elements make the alloy coating resistant to electrochemical corrosion and chemical corrosion resistance 2 to 4 times higher than ordinary coatings.

In the literature [23], Wu Junlin et al. found that the effect of adding trace rare earth elements on the properties of hot-dip galvanizing layer in zinc-based alloys shows that when rare earth elements are added to pure zinc, the rare earth can purify impurities and reduce The active cathode phase in the alloy prevents the cathode process from proceeding, thereby improving the corrosion resistance of the alloy coating. At the same time, the rare earth element in the alloy is a surface active material, which has a tendency to enrich the surface, and can form a dense and uniform oxide layer on the surface of the plating layer, hindering the diffusion of external impurity atoms into the interior of the coating layer, and slowing down the oxidation and corrosion process of the coating layer. In addition, the addition of trace rare earth elements can reduce the surface tension of the plating solution, reduce the critical size of the nucleation, increase the core, and provide a heterogeneous crystal nucleus for the crystallization of the plating solution, while the rare earth element that is not a heterogeneous nucleus is rich. It is concentrated on the front of the alloy crystal, hinders grain growth, promotes grain refinement, makes the matrix structure finer, and thus makes the coating more protective. It also acts to hinder the diffusion of external atoms into the interior of the coating, delaying the corrosion process. This conclusion is also consistent with the relevant research conclusions in the literature [24~28].

In the literature [2], the researcher Song Renying et al. conducted in-depth research on the action and influence of trace rare earth elements in Zn-based hot-dip alloys. The results show that the rare earth can reduce the surface tension of the plating solution, reduce the wetting angle, improve the fluidity of the plating solution, and improve the formability of the coating. At the same time, the rare earth makes the coating alloy structure more uniform, fine, grain refinement, and rare earth pair The alloy also has the effect of purifying and refining the metamorphism, which can delay the oxidation of the surface of the alloy. In addition, after the addition of trace rare earth, the loose layer of ZnO as the main component of the alloy coating will be converted to ZnCl2·4Zn(OH)2. The dense film layer, and the film layer is poor in conductivity, can effectively inhibit the further progress of the corrosion reaction, and improve the corrosion resistance of the plating layer. Although the current mechanism of action on rare earths is not clear, there is no uniform standard for the amount of rare earth added in actual operation, but the consensus is that the rare earth content should not be too high, generally not more than 0.2%. Since the radius of the rare earth atom is much larger than the radius of the zinc atom according to the solid solution theory in which the atomic radius is similarly soluble, its solid solubility in the zinc solution is small. If the amount of rare earth added is too much, the excess rare earth may react with Zn to form a Zn-RE intermetallic compound, which affects the quality of the coating, which unnecessarily increases the zinc consumption and wastes the rare earth material, and also causes the corrosion of the zinc pot to be accelerated [ 30~31].

Third, the current problems in the research of rare earth hot-dip galvanizing

It can be seen from the above discussion that the addition of trace rare earth elements to the hot-dip galvanizing alloy can improve the properties of the hot-dip galvanized layer, especially improving its corrosion resistance, while also reducing zinc consumption and energy conservation to a certain extent. Production costs and increase production efficiency. However, if rare earths are to be used in large-scale industrial applications in the hot-dip galvanizing industry, there are still many difficult problems to be solved, including:

(1) The mechanism of action of rare earths in alloy coatings is still uncertain and needs to be further studied and discussed;

(2) At present, most of the hot-dip galvanized rare earth alloys remain in the laboratory stage. It is still a long way to go to research and develop rare-earth hot-dip galvanized alloys that can be used for industrial production;

(3) Applying the latest technology and scientific research results, research and development of new functional hot-dip alloy materials and process methods will be an important direction for the future development of the hot-dip galvanizing industry;

(4) At present, many domestic galvanizing plant equipment are old and aging, not only have low automation, low production efficiency, high production cost, but also serious environmental pollution. Therefore, actively researching and developing new energy-saving and environment-friendly galvanizing equipment and supporting equipment has become an urgent solution. Major issues.

Conclusion

With the improvement of various performance requirements of hot-dip galvanizing materials, traditional hot-dip galvanizing technology has been difficult to meet people's requirements. The rare earth hot-dip galvanizing process, as an improved production process based on the traditional hot-dip galvanizing, not only greatly improves the corrosion resistance and surface properties of the hot-dip galvanized layer, but also does not change the original production process, It will not increase production costs, so it is increasingly valued by major manufacturers. At the same time, the theoretical research on rare earth hot-dip galvanizing will also usher in a new development climax.

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Label: hot-dip galvanized material

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