Corrosion resistance analysis of nuclear power Inconel 690 evaporator heat transfer tube
People use nuclear energy to generate electricity for the benefit of mankind and meet human needs for energy; however, nuclear is a double-edged sword, and the safe and stable operation of nuclear power plants is the premise to ensure that it can achieve its beneficial effects on mankind. Once a nuclear power plant leaks, it will be a disaster for mankind. The evaporator heat transfer tube of a nuclear power plant plays a very important role in the safe operation of the nuclear power plant. In order to continuously improve the safety and economy of nuclear power plants, the optimization and corrosion resistance of evaporator heat transfer tube materials are extremely important.
After more than 50 years of development, the materials for nuclear power evaporator heat transfer tubes have undergone the evolution process of Inconel 600, Incoloy 800 and Inconel 690. Inconel 690 material is considered to be the most promising third-generation nuclear power plant steam generator heat transfer tube material with its significant oxidation resistance and excellent stress corrosion resistance. Due to the high technical content and difficulty in production of nuclear power plant evaporator heat transfer tubes, Inconel 690 heat transfer tubes for nuclear power plant evaporators in my country are still all imported, and are key materials that urgently need to be localized. The supply of Inconel 690 alloy tubes is mainly monopolized by three major companies in the world: Valinox of France, Sumitomo of Japan, and Sandvik of Sweden, and their production technology is strictly confidential. At present, a few steel companies in my country have begun to study Inconel 690 alloy tubes. With the development of the nuclear power industry, if the localization of nuclear power evaporator heat transfer tubes can be achieved, it will play an important role in promoting the development of nuclear power and the adjustment of steel industry product structure.
1. Selection of materials for nuclear power plant evaporator heat transfer tubes
Nuclear power plants are usually composed of two major systems: the primary circuit and the secondary circuit. Among them, the evaporator is the key equipment of the nuclear power plant and the hub connecting the primary and secondary circuits. The flowing medium in the evaporator heat transfer tube is high-purity water. Through the circulation of high-purity water, the heat generated by nuclear fission in the reactor is carried to the evaporator, and then the heat is transferred to the secondary circuit medium water through thousands of heat transfer tubes, so that it boils to produce high-pressure steam, and then the high-pressure steam drives the steam turbine generator set to generate electricity. Since the inner and outer surfaces of the evaporator U-shaped heat transfer tube work under two different media respectively, their use conditions are also different, so the requirements for them are very strict. A 1 million kilowatt ordinary nuclear power plant requires 1255 tons of various steel pipes, of which 200 tons are U-shaped heat transfer tubes for reactor evaporators. The reliability of heat transfer tubes directly affects the technical performance and safety of nuclear power plants.
The materials of evaporator heat transfer tubes have basically gone through the following three stages:
The first stage was in the 1950s and 1960s, the early stage of nuclear power development, mainly using 18-8 stainless steel tubes. The evaporator heat transfer tubes used in early pressurized water reactor nuclear power plants all used 18-8 chromium-nickel stainless steel. However, this steel has poor resistance to chloride ion corrosion, and a large number of steel pipe rupture accidents occurred.
The second stage was in the 1970s. Based on the experience of petrochemical engineering, stress corrosion-resistant Inconel 600 nickel-based alloy pipes were selected; Inconel 600 nickel-based alloy suffered caustic stress corrosion damage from 1970 to 1972. Some Western European countries began to use the third-generation stress corrosion-resistant material Incoloy 800 alloy to replace Inconel 600 nickel-based alloy in 1972. The Ni content of Incoloy 800 alloy is much lower than that of Inconel 600 nickel-based alloy, so a large amount of nickel is saved. In high-temperature and high-pressure water, since the nickel content of this alloy is in a range that neither transgranular nor intergranular stress corrosion cracking occurs, and the price of Incoloy 800 alloy is much lower than that of Inconel 600 nickel-based alloy, Incoloy 800 alloy has become the third-generation alloy that can be used for pressurized water reactor evaporators after 18-8 stainless steel and Inconel 600 nickel-based alloy.
The third stage is after the 1980s. In order to solve the stress corrosion problem of Inconel 600 alloy tubes in evaporator media, France, the United States, Japan and other countries jointly developed the fourth-generation Inconel 690 with excellent stress corrosion resistance. Inconel 690 is a nickel-based alloy with excellent resistance to corrosion by various aqueous media and high-temperature atmospheres. The alloy has better stress corrosion cracking resistance than Inconel 600, Incoloy 800 and 18-8 stainless steel in pure water and low-concentration caustic soda solutions. It also has high strength, good metallurgical stability and excellent processing characteristics, making it an important material for nuclear evaporator pressurized water reactors. Since the early 1990s, highly corrosion-resistant Inconel 690 pipes have been officially used in engineering.
Since the development of nuclear power in my country, nuclear evaporator heat transfer pipes have basically been imported from abroad. The types of pipes introduced vary with the partners and the technical status at that time. Initially, 18-8 stainless steel pipes were used. Later, the Qinshan Phase I Nuclear Power Plant, which was designed by itself, had more contact with Europe and used the Incoloy 800 alloy pipes favored by the Germans at that time. The Daya Bay Nuclear Power Plant designed by France used the Inconel 690 pipes that were recently developed. The Qinshan Phase II and Ling'ao Nuclear Power Plants both used the most popular Inconel 690 pipes.
Currently, almost all evaporators of newly designed and under-construction nuclear power plants use Inconel 690 pipes as heat transfer pipes, and many evaporator units that originally used 18-8 stainless steel pipes or Inconel 600 alloy pipes have also been replaced with Inconel 690 heat transfer pipes.
2. Chemical composition of Inonel 690 material
Inonel 690 alloy (nominal composition Ni-30Cr-9Fe) is an austenitic nickel-based corrosion-resistant alloy with a C content of no more than 0.03% and a Cr content of about 30%. The main difference between the chemical composition of Inconel 690 and Inconel 600 alloys is that Inconel 690 increases the Cr content (28%-30%) and correspondingly reduces the Ni content (>58%). Inconel 600 alloy contains 15.5% Cr. Compared with Inconel 600 alloy, Inconel 690 has the characteristics of simple composition and excellent performance.
