ABSTRACT: corrosion-resistant brass is widely used as heat exchange material for condensing tubes in power plants and marine vessels. The effect of alloying elements on the corrosion resistance of brass and its mechanism are introduced in this paper. Adding a certain amount of rare earth into brass can deoxidize the melt, refine the grain structure, improve the shape and distribution of impurity elements in the alloy, and combine with other elements to form corrosion inhibitor, so as to improve the corrosion resistance of the alloy. Adding rare earth is an important direction of developing new environmental protection brass in the future.

Key words: Brass; rare earth; alloy elements; Corrosion Medium Diagram Classification: T G 174.2; T G 146.11 Literature Symbol: A article number: 1005-748 x (2012)07-0605-05 brass is a copper alloy with zinc as the main alloy element, the zinc content is generally between 10% and 50% , the zinc content of industrial brass is all below 50% , single phase brass and two phase + brass [1]. Compared with pure copper, brass not only has the general characteristics of copper and copper alloys, but also has the advantages of superior mechanical properties, low price and beautiful color, which makes it the most widely used and most economical copper alloys. The corrosion resistance of brass is an extremely important service property. Corrosion-resistant brass is widely used as heat-exchange material in power plants and marine vessels for its excellent thermal conductivity and corrosion resistance. However, the problems of dezincification corrosion and stress corrosion cracking still exist in the use of brass, which bring many hidden troubles to the industrial production. It is of great significance to improve the corrosion resistance of brass and prevent the corrosion failure of brass tubes for the safety and economic operation of relevant industrial departments.

1. The effect of alloy elements on the corrosion resistance of brass many measures have been taken by the researchers to prevent the dezincification of brass, and the most effective way is to add alloy elements, at present, the alloy elements used are tin, aluminum, nickel, manganese, arsenic, Boron, antimony, rare earth and so on. A single addition of a certain alloy element will generally have an optimal amount of addition to achieve the best corrosion resistance, while the addition of a variety of alloy elements will have an optimal amount and proportion among them, resulting in synergy, the corrosion resistance of brass is further improved compared with that of brass added with single element. It is a key problem of alloy composition design to select reasonable combination of several alloy elements and determine the best addition amount and proportion to improve the corrosion resistance of brass. However, the addition of alloying elements will inevitably adversely affect some other properties of the alloy. Therefore, it is another key problem of alloy composition design to improve the corrosion resistance of the alloy while avoiding or reducing the harmful influence on other properties, especially to ensure the good processing ability of the alloy. The following is a list of the effects of alloying elements commonly used in complex brass on their properties and the synergies between them.

1.1 effect of arsenic

 in 1928, R. May [2] reported that the addition of trace arsenic to brass inhibited the dezincification of brass. Subsequently, the mechanism of arsenic inhibiting dezincification of brass has been studied by many scholars at home and abroad. One view is that the addition of arsenic inhibits the CATHODIC process, that is, the copper redeposition process, thereby inhibiting, dezincification. R. M A Y [2] suggests that when brass with as is added is exposed to seawater, a s film is deposited on the surface of the copper alloy as an oxygen carrier, which can oxidize C u + to C u 2 + , and then C u 2 + is deposited on the substrate in the form of insoluble basic chloride, in this way, the concentration of copper ion near the interface is reduced, and the redeposition process of copper is inhibited. It is believed that the addition of arsenic reduces the overpotential of hydrogen on brass, so that hydrogen is reduced before copper at the Cathode, thus inhibiting the re-deposition of copper. L U C E Y [4] argues that only C u 2 + can be reduced to copper by brass, and trace arsenic reduces C U 2 + to C U + , which keeps the concentration of C u 2 + at a very low level and inhibits copper redeposition. Another view is that arsenic inhibits dezincification by inhibiting the anodic process, the preferential dissolution of zinc. The mechanism of action of arsenic has been studied in 5% HCL medium of cu-cl-2 or cu-cl. It is suggested that the interaction of arsenic with cu and zn forms a protective layer of cu-as-z-n at the grain boundary of brass, which prevents the preferential dissolution of Zn. The results show that arsenic can inhibit the diffusion of double-vacancy, and that arsenic forms a double-vacancy-arsenic pair in brass, the migration of the complex is more difficult than that of the free double vacancy, which reduces the transport capacity of zinc, that is, the diffusion capacity of zinc, thus inhibiting the preferential dissolution of zinc. Although arsenic can effectively inhibit the dezincification of brass and greatly improve the corrosion resistance of brass, as arsenic is a highly toxic element, the poisonous gas and dust in the production process will seriously pollute the environment and endanger people's health, the addition of arsenic also has a negative effect on the other technological properties of the alloy. Therefore, in this increasingly polluted world, the researchers hope to find a replacement element of arsenic, so as to eliminate arsenic pollution in the brass industry.

1.2 the effect of Boron and the synergistic effec

t of Boron and arsenic. In 1984, T O I v A N E N [6] added Micronutrient Boron for the first time to the CAST C U Z N biphase brass. It was proved that Micronutrient Boron could effectively inhibit the dezincification of brass. What's more, he thinks this is the result of Boron occupying the vacancies created by Dezincification, preventing the migration of zinc atoms. [7] The microstructure, mechanical properties, corrosion resistance and wear resistance of Al Brass H al 77-2 containing Boron were systematically studied. It was found that the addition of Boron to Al Brass resulted in finer grains and higher hardness, the corrosion resistance and wear resistance are obviously improved. They studied the mechanism of Boron by using Positron annihilation experiments, and found that Boron atoms can fill into the crystal boundary and double vacancy, strengthen the bonding force in these places, and hinder the diffusion of zinc atoms through the double vacancy and the migration of the crystal boundary The optimum content of Boron in h a l 772 is 0.01% . At the same time, Wang Jihui et Al. [8] also made a systematic study of h Al 77-2 brass with Boron and arsenic. The results were compared with that of Al Brass H AL 772 containing only Boron and only arsenic. It was found that the combined addition of arsenic and Boron could inhibit the dezincification corrosion of brass more effectively than the addition of Boron or arsenic alone, the dezincification coefficient of brass is almost equal to 1 under the optimum content of Boron and arsenic, that is, the dezincification is almost completely inhibited. Moreover, they calculated that the optimum amount of Boron and arsenic in Atomic ratio brass was approximately 1∶1 and about 510-4. So they think the arsenic-boron combination works in the same way as the a s-b pair. Although the "double vacancy-boron atom" complex and the "double vacancy-arsenic atom" complex can occupy the double vacancy and reduce the diffusion ability of the double vacancy and inhibit the dezincification, however, as they can not completely fill the double vacancies, they can only slow down the migration of the double vacancies, while the as-b pairs formed by the synergistic action of arsenic and Boron can completely fill the double vacancies after corrosion, thus blocking the percolation channel, thus, it is possible to completely inhibit the dezincification of brass. Zhang Zhiqiang et al [9] studied the composition, microstructure and corrosion resistance of hsn70-1 tin brass containing Boron and arsenic. It was proved that the synergistic effect of arsenic and Boron in hsn70-1 tin brass improved the corrosion resistance of hsn70-1 alloy Ling Jinsong [10] studied the stain resistance and corrosion resistance of hsn70-1 tin brass with Boron and arsenic. It was found that the stain resistance and corrosion resistance of tin brass with arsenic and Boron were improved, it is believed that the addition of Boron changes the defect structure of the surface Cuprous oxide, which makes the Cuprous oxide film more uniform and compact and not easy to be corroded

1.3 The addition of tin will improve the strength, hardness and corrosion resistance of brass. It is generally believed that tin accumulates on the surface of brass during the anodic corrosion process and forms a compact tetravalent tin compound film, which can retard the anodic corrosion of the substrate and inhibit the dezincification of brass, the corrosion resistance is greatly improved. After the study of biphase brass, it is also considered that tin action is responsible for the formation of the passivation film on the surface, and that the film is formed at the phase nucleus, and then gradually grows up to cover the phase. However, Liu Zengcai [12] considered that the addition of sn to brass strengthened the grain boundary, which greatly improved the corrosion resistance of brass hsn70-1a. However, for biphase brass hsn62-1, sn could enrich in the phase boundary and grain boundary and inhibit the dezincification, however, the corrosion can not be completely prevented along the phase boundary and grain boundary. Tin Brass is widely used in marine vessels, coastal power plants and other marine environment, so it is also known as "Navy brass" . But containing too much tin, will reduce the alloy's plasticity, commonly used tin brass containing about 1% tin.

1.4 the effect of aluminum on the strength and corrosion resistance of brass is most significant compared with other alloying elements. Because the standard charge of aluminum is more negative than that of zinc, there is a greater tendency of ionization, which takes precedence over oxygen binding in the environment, and gives priority to the formation of a micro-dense and hard alumina film, which can prevent further oxidation of the alloy, the AL 2O 3 film formed has the effect of blocking the corrosion of Matrix. Moreover, due to its compactness and hardness, the protective film can resist the impact and friction of seawater even in flowing seawater, and its complete corrosion-resistant product film can reduce the porosity to a minimum and avoid local corrosion to a great extent. When aluminum is added to brass, the phase zone will move to the copper angle obviously. When the aluminum content is high, a hard and brittle phase appears, which improves the strength and hardness of the alloy. At the same time, its plasticity is greatly reduced. Adding SN, SB, Bi, Te, SI and NI TO AL brass can further improve its corrosion resistance.

1.5 and the synergistic effect of nickel and tin, the addition of nickel enlarges the phase zone of brass, that is, when the content of Zn and Al is increased, the single phase structure can still be maintained, and the strength, toughness and cold-hot-press machinability of brass can be improved. The effect of Tin and nickel on the corrosion resistance of H60 brass was studied. It was found that the corrosion resistance of H60 brass could not be improved by nickel addition alone. Only when tin was present in the brass, the corrosion resistance of H60 brass could be improved by nickel addition, that is, the increase is larger than that of adding tin alone. When the content of tin is about 0.7% and the content of nickel is equal to or slightly lower than that of tin, nickel and tin precipitate out in the form of a compound, which can protect the corrosion products on the surface of brass, the corrosion resistance of the alloy is improved by preventing the corrosion from going further.

1.6 Mn has an effect on the solid solution strengthening of the alloy by the addition of mn into copper, which leads to the distortion of the Copper Crystal lattice and the distortion energy. At the same time, after aging, the M N and SI in the alloy are combined and precipitated as m n 5 S I 3 particles. These dispersed M N 5 s I 3 compounds can hinder the dislocation movement and greatly enhance the strength of the alloy. It can be seen that the strength and hardness of brass can be improved by adding manganese. Combined with its excellent corrosion resistance in seawater, chloride and superheated steam, manganese brass is more widely used in shipbuilding and military industry.

1.7 influence of rare earths Xie Bing et Al. [14] it is considered that the addition of rare earths to copper and copper alloys can play the role of degassing and impurity removal, and can improve the microstructure, strength, hardness and thermal stability of copper and copper alloys, it can also enhance the corrosion resistance and wear resistance of copper alloy. Tan Rongsheng et Al [15-16] studied the effect of rare earth addition on the corrosion resistance and corrosion mechanism of hs n 70-1 tin brass. It is considered that the addition of rare earth into tin brass has the following effects on improving the corrosion resistance: 1 degassing, removing impurity, purifying metal and refining grain, it can make the alloy structure compact and increase the diffusion resistance of zinc atom, 2 it is easy to form oxide film on the interface to prevent the diffusion of zinc atom, 3 it can inhibit the decomposition of C u 2cl 2, and prevent the transformation from C u + to C u 2 + , and reduce the redeposition of C U 2 + . At the same time, the high temperature properties of hsn70-1 tin brass with mixed rare earth addition and arsenic addition were compared. The results are as follows: (1) adding proper amount of mixed rare earth can refine the microstructure of the alloy and inhibit the dendrite growth in the microstructure, the results show that the microstructure tends to be equiaxed, while the dendrite crystals in the as-added hsn 70-1 alloy are well developed. 2 The elongation at high temperature and hot workability of tin brass can be improved by adding proper amount of mixed rare earth, while the elongation at high temperature can be reduced by adding arsenic, the high temperature strength of tin brass is improved slightly by adding mixed rare earth, but the effect of adding arsenic is not obvious. Zhang Zhiqiang [17] found that the corrosion resistance of hsn 70-1 condenser tube added with rare earth cerium was further improved, but he did not report the mechanism of action of Cerium, only observed the change of structure caused by the addition of cerium, that is, a large number of black dot-shaped second phase. When SB, AL and re were added to hsn 70-1 at the same time, the corrosion resistance of hsn 70-1 was improved greatly. The function of antimony is to form a SB 2O 3 oxide film, which prevents the new diffusion and the new preferential dissolution. But the effect of antimony is not as strong as that of arsenic, and the corrosion depth is larger. When antimony, aluminum and rare earth are added at the same time, the three elements will not only have a comprehensive effect, but also inevitably produce a synergistic effect, which not only reduces the sloughing layer, but also eliminates the penetrating layer, thus obtaining a good effect with the lowest corrosion depth, its corrosion resistance is comparable to that of hsn 70-1 containing arsenic.

2. How rare earths work

2.1 there are many kinds of impurities in copper and copper alloys for industrial use, and the total amount of impurities can reach 0.05% ~ 0.8% . Some of them are not very high, but they often affect the excellent properties of pure copper or copper alloys. Brittle compounds such as oxygen, sulfur, and copper (Cu 2o and cu 2s) reduce the conductivity, corrosion resistance, and weldability of copper. Due to the high chemical activity and large atomic radius of rare earth metals, the addition of rare earth additives into copper or copper alloys can effectively degassing and removing impurities, and improve and enhance various properties.

2.2 decontamination of rare earth (1) deoxidation. Rare earth is a strong deoxidizer. After deoxidation of rare earth is completed, the oxide formed will float on the surface of copper liquid in solid phase and be removed by entering the slag phase, in order to achieve the goal of purifying copper and removing oxygen. The general formula of deoxidation reaction of rare Earth Yttrium is x [ r e ] + y [ o ]→ r e x o y (s)(2) the principle of deoxidation of rare earth in copper alloy is similar to that of deoxidation. Taking Rare Earth Ce as an example, the reaction formula is as follows: C U 2s 10 c e →2c U + C E s according to thermodynamic data, this desulfurization reaction can be calculated above the melting point temperature of copper alloy. The relationship between the standard formation free energy and the temperature T is g 0 T =-192360 + 9.2 t L O G t-11.8 T at 1400 K, G 0 T =-707103 j/m O L. The equilibrium constant K p = 4.4611026. Thus, the thermodynamic trend of rare earth desulfurization reaction in copper solution is very large, which can remove a small amount of sulfur impurities in copper. (3) the dehydrogenation process of dehydrore in copper solution can be approximately described as: h 2→2[ H ] C U R E + [ h ]→ C U [ R E H ] solid solution [ r e h ] + (x-1)[ H ]→ C U R E H metal reacts with hydrogen to form stable hydride which is a strong exothermic reaction. In the process of copper processing, adding rare earth into the molten copper containing hydrogen can rapidly absorb and dissolve atomic hydrogen from the copper, and react with it to form hydride under certain conditions. The hydride can easily rise to the surface of the copper liquid and decompose again at high temperature to remove the hydrogen or be oxidized into the slag phase.

2.3 The addition of rare earth to the refined microstructure of copper and copper alloy can refine the grain size, reduce or eliminate the columnar grain size and enlarge the equiaxed grain size. The mechanism of grain refinement mainly includes the following views: (1) formation of new nucleation. Rare earths can form high melting point compounds in copper and its alloys, which are often suspended in the melt by very fine particles and become dispersed crystalline cores, thus refining the grains. (2) microcrystallization: Because the atomic radius of rare earth elements (0.174 ~ 0.204 NM) is 3.6% ~ 60% larger than that of copper (0.127 nm) , rare earth atoms can easily fill the surface defects of the new crystalline phase of the growing copper alloy and form a film which can prevent the continuous growth of the grains, so that it can be refined into crystallites. From the point of view of solidification principle and thermodynamics, rare earth accumulates in the liquid phase at the front of the solid-liquid interface. At the same time, a thin neck is produced at the branch node, and a fuse is broken, which increases the crystallization, the core, and thus the fineness and the crystallization of the grains. The addition of rare earth elements into conductive copper, lead brass, aluminum brass, manganese brass and copper-based memory alloys can result in significant grain refinement. As the radius of rare earth atoms is larger than that of copper, the crystal lattice distortion in the copper phase lattice results in the increase of the system energy. In order to maintain the lowest free energy of the system, the rare earth atoms can only be enriched to the grain boundary where the atoms are not closely arranged, most of the rare earths distribute in grain boundaries and retard grain growth. The higher the content of rare earth, the greater the tendency of undercooling, the smaller the dendrite spacing and the finer the microstructure. Such as adding excessive rare earth, the formation of a large number of rare earth compounds, reducing the undercooling effect of rare earth elements, alloy coarsening instead.

2.4 there are four main types of impurity forms and distribution: (1) reducing or eliminating dendrite and columnar crystals in the alloy structure; This is related to the formation of refractory compounds and dispersion of rare earth with some impurities. (2) The mechanical and workability of metals and alloys are improved or improved by changing some bar, sheet or block impurities (some of which can form eutectic at low melting point) into point or sphere. (3) some harmful impurities (such as s, P, PB, BI, etc.) in the alloy are changed from concentrated distribution in Dendrite or grain boundary to more uniform distribution in the whole crystal, so that the impurities can be redistributed in the microscopic volume of the metal or affect the macrosegregation of the impurities, resulting in a variety of performance improvements. (4) reducing the amount of harmful impurities at the grain boundary with low melting point, thus weakening the temper Brittleness of the alloy at high temperature.

2.5 The solubility of rare earths in copper is very small, usually only a few thousandths to a few thousandths of a  a thousandth. Their toughness and strength at room temperature are one to several times higher than that of common pure copper. Some rare Earth Metals (such as Y, CE, etc.) may have high-temperature oxidation resistance to copper, it can improve the mechanical properties, heat resistance and high temperature oxidation resistance of copper and copper alloys. [20] a safe, effective and environmental-friendly passivating solution for Copper Alloy was prepared by mixing rare earth with BTA (Benzotriazole) . This is because the addition of appropriate rare earth salt can improve the corrosion resistance of passivation film, so that it can maintain its original luster in the atmosphere, weak acid environment for a long time. The synergistic effects of rare earth elements La, C E, Y and B t a were studied. The results show that the corrosion resistance of red copper can be improved by the combination of trace rare earth elements La, CE, Y and BTA, and the effect of Rare Earth Element La is the most significant. The reason is that rare earth elements make the passivation film formed on the surface of red copper more compact and complete, and improve the adhesion between the film and the Base metal. Rare Earth elements such as LA 3 + , CE 3 + , y 3 + can form a series of complexes with inorganic and organic ligands in aqueous solution. Moreover, due to the large volume of these rare earth ions, the coordination number of the complexes will be higher according to the space requirement of ligand arrangement. The major difference between rare earth complexes and d-zone transition-group elements, such as Z-n and C-r, is that rare earth ions can form complex with high coordination number. Coordination Numbers 4 and 6 are the characteristic coordination numbers of transition elements in d region, but the coordination numbers of rare earth elements are often larger than 6, with 7,8,9,10, even up to 12. Since water is a strong ligand for rare earth ions, the most common rare earth complexes in aqueous solutions are hydrated ions, such as [ La (H 2O) N ]3 + . However, in the presence of other ligands in aqueous solution, the LIGAND and water will compete with the rare earth ions for coordination, only oxygen-containing ligands or chelates, which have a stronger ability to bind to rare earth ions than water, can penetrate the hydrate layer and replace the water with rare earth ions. Rare Earth elements such as LA 3 + , CE 3 + , Y 3 + can form oxygen containing ligand complexes and nitrogen containing ligand complexes with oxygen and nitrogen atoms. There are a lot of oxygen-containing ligands (such as organic carboxylic acids) and nitrogen-containing ligands (Bta) in the solution. These ligands have strong coordination ability with rare Earth Ions, so it is speculated that organic carboxylic acids, Bta and rare earth ions may be formed in the solution, b T A can also be complexed with copper to form a polymerized C U (I) b t a complex on the surface. After adding rare earth ions, the complexes of organic carboxylic acid, BTA and rare earth ions adsorbed on the surface of copper and complexed with copper, thus changing the structure of the film and improving the corrosion resistance of the passivation film.

3. Conclusion because arsenic can effectively inhibit the dezincification of brass, at present, the method of adding alloy element arsenic is mainly used to improve the corrosion resistance of brass. However, arsenic is a highly toxic element, which will cause serious pollution to the environment and endanger human health during the production and use of arsenic-containing corrosion-resistant brass. Therefore, it is an important goal to seek the substitute element of arsenic and eliminate the arsenic pollution in brass production and use. Adding a certain amount of rare earth into brass can deoxidize the melt, refine the grain structure, improve the shape and distribution of impurity elements in the alloy, and combine with other elements to form corrosion inhibitor, so as to improve the corrosion resistance of the alloy. Adding a certain amount of rare earth into brass is not only necessary for the sustainable development of copper processing industry in China, but also for the high-speed development of electric power industry in China. 

Source: Chinanews.com, by Zhang Juan

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