ABSTRACT: The rapid development of marine industry requires more and more marine engineering materials, including white copper, which is widely used in ships, some power plants and seawater desalination. Because of the long-term seawater erosion corrosion, if the corrosion resistance does not meet the use requirements, corrosion failure will occur, resulting in huge losses. Therefore, cupronickel has excellent seawater corrosion resistance (especially good erosion corrosion resistance) and corrosion fatigue resistance, excellent cold and hot processing properties, as well as high tensile strength, yield strength and other properties have attracted increasing attention. The composition, application and domestic and foreign manufacturers of different cupronickel alloys are reviewed. The research progress of corrosion resistance of cupronickel alloys is also summarized.

Keywords: Cupronickel; composition; Corrosion Resistance; Author: Lu Jing, classification number: TG146.15, symbol number: A

With the development of marine industry, Seawater cooling system is developed from TUP copper, aluminum brass and stainless steel to cu-ni alloy with good seawater corrosion resistance. Nickel is the main element of Cupronickel, and a small amount of FE, MN and other elements can form a continuous-single-phase solid solution, so that it has good ductility, impact and thermal stability. At the same time, the infinite solution of Cu-ni makes the phase transformation of cu-ni alloy not happen in the later cold and hot working, so the mechanical properties and corrosion resistance of the alloy are little affected. This paper introduces different cupronickel alloys and their applications, and summarizes the research progress of cupronickel alloys. Figure 1 shows the offshore platform system and figure 2 shows the material performance requirements.

Classification of copper-nickel alloys

Table 1 is the grade and element composition of cupronickel alloy. Cupronickel can be divided into manganese cupronickel, iron cupronickel, common cupronickel, aluminum cupronickel and zinc cupronickel. Due to its irreplaceable corrosion resistance and many properties superior to the traditional alloy, so it has a greater application potential.

2. Properties and application of cu-ni alloy

Common Cupronickel is generally a structural copper-nickel alloy, in addition to high corrosion resistance, at high and low temperatures, there are better comprehensive mechanical properties, that is, plasticity and toughness are good, generally as bar or strip. At the same time, the addition of some trace alloying elements such as Fe, MN, Zn and AL on the basis of ordinary cupronickel can meet the special performance requirements of practical application and better meet the industrial requirements.

BFe10-1-1(C70600) and BFe30-1-1(C71500) are the most widely used ferrous cupronickels. When the mass fraction of Ni is 30% and 10% , the passivation interval is wider and the corrosion resistance is the best. The alloy is also known as "Marine Engineering Alloy" for its excellent resistance to seawater erosion and corrosion. The main applications of copper and copper alloys in marine engineering are shown in Table 2. BFe10-1-1 and BFe30-1-1 alloys have the advantages of good resistance to seawater erosion and corrosion, high heat transfer coefficient, excellent mechanical/welding properties, inhibition of adhesion of marine microorganisms, etc. , it is widely used in cooling water pipes of main and auxiliary engines, fire fighting pipes of offshore oil production platforms, heat exchangers of power plants and condensers of Binhai nuclear power stations

Salt water heater for Grade I flash unit [2-4]. BFe30-1-1 alloy has higher strength and is also used in high strength structures such as shafts, fasteners, valve stems and flanges in some offshore installations. The development of BFe30-2-2 alloy with good erosion-corrosion resistance and sand-corrosion resistance is to solve the problem of high sand content in seawater in the east China Sea.

The mechanical properties of BFe10-1-1 and BFe30-1-1 alloy tubes in hard state should be as follows: tensile strength ≥370 MPA, yield strength ≥150 MPA, elongation ≥18% , vickers hardness ≥85; corrosion resistance: Corrosion Rate (50 °C, 3.5% NACL seawater)≤0.025 MM/A; No pitting allowed.

C, 3.5% NACL seawater)≤0.025 mm/a, no pitting is allowed. Manganese white copper (BMN3-12 alloy) has moderate resistance coefficient, small resistance temperature coefficient and relatively stable, because of its good electrical properties, BMn3-12 alloy can be used to make standard resistance and other precision instrument resistance components. With the development of the time, the precision of the instrument is required more and more, so the research on the alloy can not stop at changing the composition and content of the alloy. By annealing, horizontal extrusion and drawing, the BMn3-12 alloy has a special coherent twin boundary, which can improve the strength of the material without affecting its conductivity. BMn40-1.5 alloy is an electrical copper-nickel alloy which was used earlier than BMn3-12 alloy. Because of its low temperature coefficient of resistance, it has good heat resistance and can be used in a wide temperature range. Compared with BMn3-12 alloy, BMn40-1.5 alloy has higher thermoelectric potential to copper, so it is suitable for AC precision resistance, sliding resistance, starting, regulating transformer and resistance strain gauge.Aluminum Cupronickel has both high strength and good plasticity and toughness. Among them, BAl13-3 alloy is often used to make corrosion-resistant parts with high strength, and BAl16-1.5 alloy is used to make flat spring with important use. For a long time, in order to improve the service performance of aluminum white copper, a small amount of micronutrient is often added to strengthen the matrix of aluminum white copper. Because of its high strength, high conductivity and good wear resistance, al-cupronickel can be used as a potential material for lead frame and wear-resistant parts. Zinc cupronickel (BZn18-18BZn15-20 alloy) is also called "German silver" [12] . Due to its excellent tensile strength, fatigue resistance and corrosion resistance, zinc cupronickel is mainly used as the housing of components or crystals, medical appliances, building materials and wind instruments.

3. copper-nickel alloy production plants and performance requirements

3.1 copper nickel alloy production plant

Cu-ni alloy is often used as alloy pipe in the ocean. It is required to have high performance whether it is seawater desalination or ship drainage pipe. Fig. 3 is a schematic diagram of the production process of cu-ni alloy pipe. FAMOUS OVERSEAS COPPER CONDENSER TUBE PRODUCTION PLANTS ARE: American Brass Anaconda company, Duisburg, Japan Kobe, Sumitomo Group, South Korea Toyoyama and the British Institute of non-ferrous Metals, etc. . At present, the BFe10-1-1 alloy tubes are produced in China with a diameter of 324mm produced by CHALCO and a diameter of 324mm 4mm tested by Jiangsu Gaoxin Zhangtong. German production of diameter 324 mm 16 mm large-diameter white copper pipe, due to technical maturity for export. BFe10-1-1 alloy tubes manufactured in France, with a maximum diameter of 400mm, for military export [15]. The output capacity of some production enterprises is shown in table 3.

3.2 performance requirements for cu-ni alloys

With the rapid development of China's marine shipbuilding industry, Marine Oil and gas industry, marine mining industry, marine electric power industry and seawater desalination industry, the demand for materials is getting higher and higher. The copper-nickel Alloy Tube is widely used, and the copper alloy condenser tube used in ships has been working in seawater environment with high temperature, high pressure and high corrosion. Therefore, it is not enough only to meet the requirements of chemical composition, mechanical properties and seawater corrosion resistance of the products, but also to strictly require such indexes as geometric size accuracy, technological properties and internal organization, at the same time, the copper-nickel alloy pipe is required to have good corrosion resistance, high heat transfer coefficient, large diameter, high precision, excellent mechanical welding technology, and better inhibition of the adhesion of marine microorganisms.

At present, domestic processing equipment problems, leading to large-caliber copper tube alloy can not mass production, mainly rely on foreign imports. Therefore, the production difficulties of large-caliber copper tubes still need to be overcome.

4. Research progress of cu-ni alloy

4.1 corrosion mechanism of cu-ni alloy

Fig. 4 shows the processes of corrosion reaction of Cupronickel in oxygen-containing seawater, and the AB line shows the CATHODIC reaction of cupronickel in seawater, it can be expressed as: 1/2[ o 2] + [ h 2O ] + 2e something = 2[ Oh something ](1) the CD line in Fig. 4 is an anodizing process in which cupronickel does not passivate in seawater. The curves t 1, t 2 and t 3 of Cu = cu + e something (2) are the anodic reactions of Cupronickel passivated in seawater. The EF wire is a hydrogen evolution reaction of Cupronickel due to oxygen deficiency, so there is neither passivation nor corrosion product in this process. Cu (orCu2O) = Cu2 + + 2e something 2[ CU2 + ] + 3[ Oh something ] + [ Cl something ] = Cu2(OH)3Cl

Study on corrosion resistance of 4.2 cu-ni alloy

In order to meet the requirements of material performance in marine industry, a lot of research has been done to improve the corrosion resistance of cu-ni alloy. Deng chuping et Al. [19] found that the tensile strength and elongation of Cupronickel with CE increased, the grain structure became denser and the addition of CE improved the typical ni-removal corrosion tendency of the alloy in the medium containing sulfur. The effect of different Fe content on the microstructure and properties of B10 alloy was studied by Jiang et Al. . It was found that the corrosion resistance of B10 alloy increased at first and then decreased with the increase of Fe content. By controlling the ratio of Fe/mn to find the best alloy ratio to improve the corrosion resistance of B10, it is found that when Fe/Mn = 3:2, the corrosion resistance of the alloy is the best. It is found that the number of grain boundaries and twin boundaries can be increased by solution + cold rolling deformation + recrystallization annealing, thus the alloy with better corrosion resistance can be obtained.

5. Conclusions

In this paper, the chemical composition and application of different copper-nickel alloys are described, and the famous copper condenser tube production plants at home and abroad are introduced, the performance demand of copper-nickel alloy pipe for offshore pipeline in China and the future trend of development of copper-nickel alloy are summarized, at present, the corrosion resistance of cu-ni alloy is improved mainly by adding trace rare earth elements, adding different content of Fe, changing the ratio of matrix alloy elements and grain boundary engineering. 

Source: Chinanews.com, by Lu Jing