The influence and mechanism of alloying elements on the corrosion resistance of brass
Brass is a copper alloy with zinc as the main alloying element. The zinc content is generally between 10% and 50%. The zinc content of industrial brass is less than 50%. It is a single-phase α brass and a two-phase brass. α+β brass [1]. Compared with pure copper, brass not only has the general characteristics of copper and copper alloys, but also has better mechanical properties than pure copper, as well as the advantages of low price and beautiful color, making it the most widely used and most economical material. Copper alloy.
The corrosion resistance of brass is an extremely important performance. Corrosion-resistant brass is widely used as heat exchange materials such as condenser tubes in power plants and ships due to its excellent thermal conductivity and corrosion resistance. However, brass still has the problems of dezincification corrosion and stress corrosion cracking during use, which brings many hidden dangers to industrial production. Further improving the corrosion resistance of brass and preventing corrosion failure of brass tubes is of great significance to the safe and economical operation of related industrial sectors.
1. The influence of alloying elements on the corrosion resistance of brass
In order to inhibit the dezincification of brass, researchers have taken many measures. The most effective method is to add alloying elements. The alloying elements currently used include tin, aluminum, nickel, manganese, arsenic, boron, antimony, rare earth, etc. . When adding a certain alloying element alone, there will generally be an optimal amount of addition to achieve the best corrosion resistance; when adding multiple alloying elements, there will be an optimal amount of addition among them. and proportion, thereby creating a synergistic effect, which further improves the corrosion resistance of brass relative to brass with the addition of a single element. Selecting a reasonable combination of several alloy elements and determining their optimal addition amount and proportion to improve the corrosion resistance of brass are key issues in alloy composition design.
However, adding alloying elements will inevitably have adverse effects on some other properties of the alloy. Therefore, while using alloying methods to improve corrosion resistance, avoiding or reducing harmful effects on other properties, especially ensuring good comprehensive forming and processing capabilities, is another key issue in alloy composition design. Listed below are the effects of commonly used alloying elements in complex brasses on their properties and the synergies they have with each other.
1.1 Effects of arsenic
In 1928, R. May[2] reported that adding trace amounts of arsenic to brass can inhibit dezincification of brass. Subsequently, domestic and foreign scholars conducted a large number of studies on the mechanism of arsenic inhibiting dezincification of brass. There are two main views. One view is that the addition of arsenic inhibits the cathodic process, that is, the redeposition process of copper, thereby inhibiting dezincification. R. May[2] proposed that when α-brass added with As is exposed to seawater, a layer of As film will be deposited on the surface of the copper alloy. This film acts as an oxygen carrier and can oxidize Cu+ to Cu2+, and then Cu2+ is converted into insoluble alkali. Formula chloride is deposited on the substrate, which reduces the concentration of copper ions near the interface and inhibits the redeposition process of copper. Luo[3] believed that the addition of arsenic reduces the overpotential of hydrogen on α-brass, causing hydrogen to be reduced before copper at the cathode position, thereby inhibiting the redeposition of copper. Lucey[4] believes that only Cu2+ can be reduced to copper by α-brass, and trace amounts of arsenic reduce Cu2+ to Cu+, keeping the concentration of Cu2+ at a very low level and inhibiting the redeposition of copper. Another view is that arsenic inhibits dezincification by inhibiting the anodic process, that is, the preferential dissolution process of zinc. Langenger[4] studied the mechanism of arsenic in CuCl2 or CuCl 5% HCl medium. He believed that arsenic interacts with copper and zinc to form Cu-As-Zn at the grain boundaries of brass. protective layer that blocks the zinc Dissolve preferentially. Yao Lu'an[5] et al. used positron annihilation technology to study α brass and α+β dual-phase brass, and confirmed that arsenic inhibits the diffusion of double vacancies, and believed that arsenic formed a "double vacancy-arsenic pair" in brass. The migration of this complex is more difficult than that of free divacancies, which reduces the transport capacity of zinc, that is, reduces the diffusion capacity of zinc, thereby inhibiting the preferential dissolution of zinc. Although arsenic can effectively inhibit the dezincification of brass and greatly improve the corrosion resistance of brass, because arsenic is a highly toxic element, the toxic gases and dust in the production process will seriously pollute the environment and endanger people's health. Arsenic can also negatively affect other processing properties of the alloy. Therefore, in a world where environmental pollution is becoming increasingly serious, researchers hope to find a replacement element for arsenic to eliminate arsenic pollution in the brass industry.
1.2 The influence of boron and the synergistic effect of boron-arsenic
In 1984, Toivanen [6] *** added trace element boron into cast Cu-Zn duplex brass, and confirmed that trace element boron can effectively inhibit dezincification of brass. Moreover, he believes that this is the result of boron occupying the vacancies created after dezincification and preventing the migration of zinc atoms. Wang Jihui et al. [7] conducted a systematic study on the structure, mechanical properties, corrosion resistance, and abrasion resistance of HAl77-2 aluminum brass after boron was added, and found that after boron was added to aluminum brass, the grains were refined. , the hardness is increased, and the corrosion resistance and abrasion resistance are significantly improved. They used positron annihilation experiments to study the mechanism of boron and believed that boron atoms can fill grain boundaries and double vacancies, enhance the bonding force in these places, and hinder the diffusion and migration of zinc atoms through double vacancies and grain boundaries.
Shift; the optimal boron content in HAl77-2 is 0.01%. At the same time, Wang Jihui et al. [8] also used the same method to conduct a systematic study on boron- and arsenic-added HAl77-2 aluminum brass. The research results were compared with HAl77-2 aluminum brass that only added boron and only arsenic. It was found that the combined addition of arsenic and boron can inhibit the dezincification corrosion of brass more effectively than adding boron or arsenic alone, and in ** Under the optimal boron and arsenic content, the dezincification coefficient of brass is almost equal to 1, that is, dezincification is almost completely suppressed. Moreover, they also calculated that the optimal atomic percentage of boron and arsenic added to aluminum brass is approximately 1:1, and the content is approximately 5×10-4. Therefore, they believe that the combination of arsenic and boron works in the form of an As-B pair. Although boron and arsenic are added separately, the "double vacancy-boron atom" complex and the "double vacancy-arsenic atom" complex formed can occupy the double vacancy, reduce the diffusion capacity of the double vacancy and inhibit dezincification, but because they cannot completely Filling double vacancies, but they can only slow down, but not prevent, the migration of double vacancies; the As-B pair formed by the synergistic effect of arsenic and boron can completely fill the double vacancies generated after corrosion, thereby blocking the percolation channel and preventing the migration of double vacancies. Migration, thus making it possible to completely inhibit dezincification of brass.
Zhang Zhiqiang et al. [9] studied the composition, structure and corrosion resistance of HSn70-1 tin brass added with boron and arsenic, and confirmed that the synergistic effect of arsenic and boron improved the corrosion resistance of the alloy; Ling Jinsong [10 ] studied the stain resistance and corrosion resistance of HSn70-1 tin brass added with boron and arsenic, and found that the stain resistance and corrosion resistance of tin brass were improved under the synergistic effect of arsenic and boron, and It is believed that the addition of boron changes the defect structure of the surface cuprous oxide, making the cuprous oxide film more uniform and dense, and less susceptible to erosion.
1.3 The influence of tin
The addition of tin will simultaneously improve the strength, hardness and corrosion resistance of brass. It is generally believed that tin continuously accumulates on the corroded surface of brass during the corrosion process of the anode, forming a dense tetravalent tin compound film. This film has the function of blocking the corrosion of the anode of the substrate, inhibiting the dezincification of brass, and making it corrosion resistant. Sexuality is greatly improved. After studying dual-phase brass, Seungman Sohn [11] also believed that the role of tin is to promote the formation of the surface passivation film, and that the film nucleates in the α phase, and then gradually grows to cover the β phase. However, Liu Zengcai [12] studied that adding Sn to brass strengthens the grain boundaries, thereby greatly improving the corrosion resistance of α brass HSn70-1A. However, for the duplex brass HSn62-1, Sn can be present at the phase boundary. and α phase grain boundary enrichment, which inhibits dezincification, but cannot completely prevent corrosion from connecting along phase boundaries and grain boundaries. Tin brass is widely used in marine environments such as seagoing ships and coastal power plants, so it is also known as "naval brass". However, too much tin will reduce the plasticity of the alloy. Commonly used tin brass contains about 1% tin.
1.4 Impact of Aluminum
Compared with other alloying elements, aluminum can significantly improve the strength and corrosion resistance of brass. Since the standard potential of aluminum is more negative than that of zinc, it has a greater ionization tendency and takes precedence over oxygen in the environment to form a dense and hard aluminum oxide film, which can prevent further oxidation of the alloy. The formed Al2O3 film has Retards substrate corrosion. Moreover, because the protective film is dense and hard, it can still resist the impact and friction of seawater even in flowing seawater. At the same time, its complete anti-corrosion product film can reduce the porosity to a minimum, which can be achieved to a large extent. Avoid localized corrosion. Adding aluminum to brass will significantly shift the α phase region toward the copper angle. When the aluminum content is high, a hard and brittle γ phase will appear, increasing the strength and hardness of the alloy. At the same time, its plasticity is greatly reduced. Adding Sn, Sb, Bi, Te, Si, Ni and other elements to aluminum brass can further improve its corrosion resistance.
The effect of 1.5 is synergistic with nickel-tin
The addition of nickel expands the α phase area of brass, that is, when the Zn and Al contents are increased, a single α phase structure can still be maintained, improving the strength, toughness and hot and cold pressure processing properties of brass. Seungman-Sohn et al. [11] studied the effects of tin and nickel on the corrosion performance of H60 brass. The results showed that simply adding nickel could not improve the corrosion performance of the alloy. The addition of nickel can only be significant when tin is present in the brass. The corrosion resistance of the brass is improved to a greater extent than that achieved by simply adding tin. This also shows that there is a synergistic effect between nickel and tin. When the content of tin is about 0.7% and the content of nickel is equal to or slightly lower, nickel and tin precipitate in the form of a compound, which affects the yellow color. The corrosion products on the surface of copper have a protective effect and prevent further corrosion, thus improving the corrosion resistance of the alloy.
1.6 Effect of Manganese
The added Mn element dissolves into the copper, causing the copper lattice to be distorted and generating distortion energy, so that the alloy is solid solution strengthened. At the same time, after aging, Mn and Si in the alloy combine to precipitate in the form of Mn5Si3 particles. These dispersed Mn5Si3 compounds can hinder the movement of dislocations, greatly improving the strength of the alloy. It can be seen that adding manganese can improve the strength and hardness of brass. Combined with its excellent corrosion resistance in seawater, chloride and superheated steam, manganese brass is more widely used in shipbuilding and military industries.
1.7 Impact of rare earths
Xie Bing et al.[14] studied that after rare earths are added to copper and copper alloys, they can degas and remove impurities, improve the microstructure of copper and copper alloys, increase their strength and hardness, and enhance thermal stability. Can enhance the corrosion resistance and wear resistance of copper alloys. Tan Rongsheng et al. [15-16] studied the effect of adding rare earths on the corrosion resistance and corrosion mechanism of HSn70-1 tin brass. They believed that adding rare earths to tin brass has the following effects in improving the corrosion resistance: ① Except Gas, remove impurities, purify metal, refine grains, make the alloy structure dense, and increase the diffusion resistance of zinc atoms; ② easily form an oxide film on the interface to prevent the diffusion of zinc atoms; ③ inhibit the decomposition of Cu2Cl2 and hinder the transformation of Cu+ to Cu2+ , reduce Cu2+ redeposition. At the same time, they will also conduct a comparative study on the high-temperature properties of HSn70-1 tin brass with added mixed rare earths and arsenic added. The results are as follows: ① Adding an appropriate amount of mixed rare earths can refine the alloy structure, inhibit the growth of dendrites in the microstructure, and make The crystal structure tends to be equiaxed, while dendrites are developed in the HSn70-1 alloy with arsenic added; ② Adding an appropriate amount of mixed rare earths can significantly increase the high-temperature elongation of tin brass and improve the hot workability, while adding arsenic reduces its temperature Elongation, deteriorating hot workability; ③ Adding mixed rare earths slightly improves the high-temperature strength of tin brass, while adding arsenic has little effect. Zhang Zhiqiang [17] found that the corrosion resistance of HSn70-1 condenser tubes added with rare earth cerium was further improved, but he did not report the mechanism of action of cerium, but only observed the structural changes caused by the addition of cerium, that is, there was a problem A larger number of black dot-like second phases. Sun Lianchao et al. [16] added antimony, aluminum and rare earth to HSn70-1 at the same time, which had a good effect on improving the corrosion resistance of the alloy. The role of antimony is to form an Sb2O3 oxide film to prevent new diffusion and inhibit new preferential dissolution. However, the effect of antimony is not as strong as that of arsenic, and the corrosion depth is greater. After adding antimony, aluminum and rare earth at the same time, in addition to the comprehensive effect, the three elements will inevitably produce a synergistic effect, which not only reduces the shedding layer, but also eliminates the penetration layer, and obtains the good effect of the shallowest corrosion depth. Its corrosion resistance is equivalent to that of HSn70-1 with arsenic added.
2. Mechanism of action of rare earths
2.1 Physical and chemical effects of rare earths
Industrial copper and copper alloys generally contain a variety of impurities, and the total amount of impurities can even reach 0.05% to 0.8%. Some of these impurities, although not large, often seriously affect the excellent properties of pure copper or copper alloy materials. . For example, brittle compounds (Cu2O and Cu2S) formed by oxygen, sulfur and copper reduce the conductivity, corrosion resistance and welding performance of copper. Since rare earth metals have high chemical activity and large atomic radius, adding rare earth additives to copper or copper alloys can effectively degas and
Remove impurities, improve and enhance various properties.
2.2 Purification effect of rare earths
(1) Deoxidizing rare earth is a strong deoxidizer. After the rare earth completes the deoxidation reaction, the generated oxide will float on the surface of the copper liquid in a solid phase and enter the slag phase to be removed, thereby achieving the purpose of purifying copper and removing oxygen. If we explain it from a thermodynamic point of view, taking rare earth yttrium as an example, its general deoxygenation reaction formula is: x[RE]+y[O]→ RExOy(S)
(2) Desulfurization The principle of desulfurization 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: Cu2S + Ce→ 2Cu+CeS·According to thermodynamic data, it can be calculated that this desulfurization reaction is above the melting point temperature of copper alloy, and the relationship between the standard free energy of formation and temperature T is: ΔG0T =-192360+9.2TlogT-11.8T at 1400K, ΔG0T=-707103J/mol. At this time, the equilibrium constant of the desulfurization reaction is Kp=4.461×1026. It can be seen that in molten copper, the thermodynamic trend of the rare earth desulfurization reaction is very large, and it can remove a small amount of sulfur impurities in copper.
(3) The dehydrogenation process of dehydrogenated rare earths in copper liquid can be approximately described as: H2→ 2[H]CuRE+[H]→Cu[REH] solid solution[REH] solid solution+ (x-1)[H] ] →CuREH The reaction between rare earth metals and hydrogen to form REH type stable hydride is a strong exothermic reaction. During the copper processing process, adding rare earths to the copper melt with dissolved hydrogen can quickly absorb and dissolve the atomic hydrogen from the copper, and react with it to generate hydride under certain conditions. Hydride easily floats to the surface of the copper liquid and is thermally decomposed again at high temperatures, releasing hydrogen gas or being oxidized.
