Analysis and Improvement of Corrosion of Valve Bolts in Nuclear Power Plants
2020-02-06 15:06:04
Abstract: Nuclear power plants convert nuclear energy into heat energy, which produces steam for steam turbines. It drives the generator through steam and finally realizes nuclear power generation. In the nuclear power plant, valve bolts are an important part of nuclear power equipment. Valve bolts seem to be small, but they have a direct impact on the final power generation results. From the past literature, it can be found that the valve bolts of nuclear power plants are prone to rust.
Key words: nuclear power plant; valve bolt; cause of corrosion; analysis improvement
Valve bolts are important components of nuclear power plant machinery and equipment. From the analysis of valve bolt structure in nuclear power plants, there are many connection parts of valve bolts, such as valve covers, nuts, bolts, gaskets, etc., so there is a certain gap between them, if it is in machinery During the operation, acidic liquid or passivation paste enters into it, which will easily cause corrosion of valve bolts in nuclear power plants, which will have certain impact on the safety of nuclear power production. Therefore, it is necessary to further improve the corrosion measures of valve bolts in nuclear power plants. analysis.
1 Analysis of the cause of valve bolt corrosion in nuclear power plant The valve bolt of nuclear power plant is composed of different metal materials. Take 3RIS058VP valve bolt as an example, which involves more than five kinds of metal materials, including CR, Mn, Ni, Cu, V, etc. The material table is as follows:
The corrosion products produced by different materials have different acidity and alkalinity. Under normal circumstances, through the PH test, if the higher the measured pH, the higher the OH- in the etching solution, the more stable the protective film will be formed, and the charge transfer resistance will be increased, thereby improving the resistance. Corrosive. On the contrary, if the pH value in the etching solution is smaller, the H+ in the solution will be more. Since the cathode of H+ has depolarization, it is difficult to form a protective oxide film, which leads to some charge direction in the process. The small resistance is mainly transferred, which reduces the corrosion resistance of the material [1]. In addition, the standard potential of the oxygen reduction reaction in the acidic solution is 1.28v higher than the H+ reduction standard, so the oxygen molecule is also easier to reduce than H+, but since the solubility of oxygen in water is relatively small, The ultimate diffusion current density is also relatively small, about a few hundred microamperes per square meter. Specifically, the main reactions of the corrosion cell include an anodic reaction and a cathodic reaction, wherein the anode reaction is Fe→Fe2++2e, and The cathode reaction is 2H++2e→H2. In this reaction change, the anode product is hydrated to obtain Fe2+, so the surface of the valve bolt of the nuclear power plant is active, indicating that no passivation film will be produced, and Rapid corrosion, but different at the cathode, it can be seen that the acidic environment is an important cause of corrosion of valve bolts in nuclear power plants. In addition, due to the presence of certain gaps in the bolted joints, these gap locations are more susceptible to corrosion by acidic environments.
Secondly, through the author's investigation and research, it is found that many nuclear power plants often use passivation paste for pickling and passivation treatment when repairing valve bolts. The main components of the passivation paste are nitric acid and hydrofluoric acid. After cleaning, although the surface of the valve bolt of the nuclear power plant will become silvery white, and the overall corrosion resistance will be improved, in the pickling passivation process, the passivation paste enters the gap of the valve bolt of the nuclear power plant. In the middle, the corrosion will occur in the acidic environment, which is also an important reason for the corrosion of valve bolts in nuclear power plants. Therefore, in order to better ensure the safe production of nuclear power, it is necessary to prevent corrosion of valve bolts in nuclear power plants. Measures to make further analytical studies.
2 Improved method for valve bolt corrosion in nuclear power plants 2.1 Optimized valve bolts are installed in the nuclear power production process, and the valve bolts are easily corroded by acidic substances. For this reason, the author believes that the valve can be further installed to control the valve corrosion. Taking the domestic seawater butterfly valve as an example, the domestic seawater butterfly valve is often made of CF8M material in the process of designing the valve body and the butterfly plate, and the bolt is made of 0Cr18NiMo2Ti material. The overall valve structure design is more complicated, although there is a sealing seat and a sealing pressure. Circles, etc., but there will be some gaps in the sea water, and the areas of these gaps are most prone to corrosion, so it is necessary to optimize the bolt installation in the future, so that the valve bolts are reduced [2]. Specifically, we can change the bolt shape to the countersunk head, and at the same time, the threaded part is applied with the thread locking glue during the installation of the valve bolt, or the oxygen can be filled in the position of the gap and the bolt hole. Resin, at the same time, can also apply high-solid epoxy anti-corrosion materials on the surface of the valve bolts of nuclear power plants. Through such optimized installation, the exposed valve bolt gaps can be prevented from being acidified and corroded, thus ensuring the steady development of China's nuclear power industry.
2.2 Improvement of valve bolt materials Nuclear power plants provide important power energy for the city. Although the valve bolts of nuclear power plants have certain corrosion resistance with the continuous improvement of the process, the valve bolts of nuclear power plants are in a special working environment. For this reason, corrosion can occur inevitably. In view of this, the author believes that evolution should further optimize the valve bolt material of nuclear power plants. First of all, high-nickel ductile iron is a kind of corrosion-resistant material. The uniform corrosion rate of this material is about 0.1mm/a, and the resistance to seam corrosion and pitting corrosion is much better than that of general 316 stainless steel. It is recommended that nuclear power plants choose high-nickel ductile iron materials when selecting valve bolt materials in the future. Secondly, ASTM A439 material has good machinability and casting performance, and has good impact resistance and wear resistance. Therefore, it is also an ideal material for valve bolts in nuclear power plants [3]. For example, when making bolts for water pump valves in nuclear power plants, ASTM A439 D2 materials can be selected, and the corrosion resistance has been professionally verified. According to data survey, the material will not be in the epoxy coating below 300μm. Significant corrosion. Therefore, in the process of casting the valve bolts of nuclear power plants, the conventional CF8M materials or 0Cr18NiMo2Ti materials can be changed to ASTM A439 D2 materials while improving the valve structure foundation.
2.3 Strengthening Valve Bolt Corrosion Maintenance Nuclear power production is a complex fission process. Therefore, it is inevitable that there will be corrosion problems in the valve bolts of nuclear power plants. When the party corrosion problem occurs, it is very important to do the corrosion repair of the valve bolts in nuclear power plants. Two aspects of the proposal. The first is a comprehensive inspection to see if the crystal is deformed. If the surface grain of the valve bolt of the nuclear power plant is separated by grain boundary corrosion, this means that the valve bolt of the nuclear power plant has serious intergranular corrosion and must be replaced. In addition, the observation of the human eye is bound to be very limited. Therefore, if necessary, the scanning electron microscope can be used for observation. The full view of the valve bolt of the power plant can be observed by scanning electron microscopy. If the corrosion region is found to be corrosive and porous, the valve bolt of the nuclear power plant is also present. Severe intergranular corrosion must be replaced [3]. For the second maintenance and cleaning, the nuclear power plant needs to clean and repair the valve bolts from time to time. The valve bolts can be maintained by passivation treatment, but it is worth noting that the pickling passivation process needs to be further developed in the future. Perfection, such as some of the relatively small gaps, is not recommended for acidification passivation, and if it is necessary to deal with, it is recommended to dispose of the parts after disassembly or before loading. In addition, in the process of cleaning and curing, the final cleaning should use demineralized water, and use PH test paper to check to ensure that the PH value is controlled between 6 and 8 during the whole process, so that the valve bolts of the nuclear power plant can be better. Raise care while avoiding corrosion problems.
to sum up:
In the context of industrialization, nuclear power production provides power energy for cities, and nuclear power production is a complex fission process. Valve bolts seem to be small, but they have a direct impact on nuclear power safety production. In nuclear power production, valves Bolts are easily corroded by acidic substances. For this reason, nuclear power plants must pay attention to the management and maintenance of mechanical valve bolts in the future, and make further analysis and research on the prevention and control measures of valve bolt corrosion in nuclear power plants, so as to truly promote the steady development of China's nuclear power industry.
references:
[1] Sun Guocai, Zhang Congran, Liao Xuebo, Li Weiguang. Analysis and improvement of valve bolt corrosion in a nuclear power plant [J]. General Corrosion Control, 2017, 31(11): 40-42.
[2]LIN Wenliang, YAN Xiaolai, RU Hongyu, CHEN Junbo, YAN Baokai.Finite Element Simulation of Assembly Process of Valve Ring Bolt Assembly[J].Mechanical Engineer,2015(03):166-167.
[3]Zhang Xiaoqing. Optimization of assembly process of valve ring bolt group and its experimental research [D]. Harbin Institute of Technology, 2016.
Key words: nuclear power plant; valve bolt; cause of corrosion; analysis improvement
Valve bolts are important components of nuclear power plant machinery and equipment. From the analysis of valve bolt structure in nuclear power plants, there are many connection parts of valve bolts, such as valve covers, nuts, bolts, gaskets, etc., so there is a certain gap between them, if it is in machinery During the operation, acidic liquid or passivation paste enters into it, which will easily cause corrosion of valve bolts in nuclear power plants, which will have certain impact on the safety of nuclear power production. Therefore, it is necessary to further improve the corrosion measures of valve bolts in nuclear power plants. analysis.
1 Analysis of the cause of valve bolt corrosion in nuclear power plant The valve bolt of nuclear power plant is composed of different metal materials. Take 3RIS058VP valve bolt as an example, which involves more than five kinds of metal materials, including CR, Mn, Ni, Cu, V, etc. The material table is as follows:
The corrosion products produced by different materials have different acidity and alkalinity. Under normal circumstances, through the PH test, if the higher the measured pH, the higher the OH- in the etching solution, the more stable the protective film will be formed, and the charge transfer resistance will be increased, thereby improving the resistance. Corrosive. On the contrary, if the pH value in the etching solution is smaller, the H+ in the solution will be more. Since the cathode of H+ has depolarization, it is difficult to form a protective oxide film, which leads to some charge direction in the process. The small resistance is mainly transferred, which reduces the corrosion resistance of the material [1]. In addition, the standard potential of the oxygen reduction reaction in the acidic solution is 1.28v higher than the H+ reduction standard, so the oxygen molecule is also easier to reduce than H+, but since the solubility of oxygen in water is relatively small, The ultimate diffusion current density is also relatively small, about a few hundred microamperes per square meter. Specifically, the main reactions of the corrosion cell include an anodic reaction and a cathodic reaction, wherein the anode reaction is Fe→Fe2++2e, and The cathode reaction is 2H++2e→H2. In this reaction change, the anode product is hydrated to obtain Fe2+, so the surface of the valve bolt of the nuclear power plant is active, indicating that no passivation film will be produced, and Rapid corrosion, but different at the cathode, it can be seen that the acidic environment is an important cause of corrosion of valve bolts in nuclear power plants. In addition, due to the presence of certain gaps in the bolted joints, these gap locations are more susceptible to corrosion by acidic environments.
Secondly, through the author's investigation and research, it is found that many nuclear power plants often use passivation paste for pickling and passivation treatment when repairing valve bolts. The main components of the passivation paste are nitric acid and hydrofluoric acid. After cleaning, although the surface of the valve bolt of the nuclear power plant will become silvery white, and the overall corrosion resistance will be improved, in the pickling passivation process, the passivation paste enters the gap of the valve bolt of the nuclear power plant. In the middle, the corrosion will occur in the acidic environment, which is also an important reason for the corrosion of valve bolts in nuclear power plants. Therefore, in order to better ensure the safe production of nuclear power, it is necessary to prevent corrosion of valve bolts in nuclear power plants. Measures to make further analytical studies.
2 Improved method for valve bolt corrosion in nuclear power plants 2.1 Optimized valve bolts are installed in the nuclear power production process, and the valve bolts are easily corroded by acidic substances. For this reason, the author believes that the valve can be further installed to control the valve corrosion. Taking the domestic seawater butterfly valve as an example, the domestic seawater butterfly valve is often made of CF8M material in the process of designing the valve body and the butterfly plate, and the bolt is made of 0Cr18NiMo2Ti material. The overall valve structure design is more complicated, although there is a sealing seat and a sealing pressure. Circles, etc., but there will be some gaps in the sea water, and the areas of these gaps are most prone to corrosion, so it is necessary to optimize the bolt installation in the future, so that the valve bolts are reduced [2]. Specifically, we can change the bolt shape to the countersunk head, and at the same time, the threaded part is applied with the thread locking glue during the installation of the valve bolt, or the oxygen can be filled in the position of the gap and the bolt hole. Resin, at the same time, can also apply high-solid epoxy anti-corrosion materials on the surface of the valve bolts of nuclear power plants. Through such optimized installation, the exposed valve bolt gaps can be prevented from being acidified and corroded, thus ensuring the steady development of China's nuclear power industry.
2.2 Improvement of valve bolt materials Nuclear power plants provide important power energy for the city. Although the valve bolts of nuclear power plants have certain corrosion resistance with the continuous improvement of the process, the valve bolts of nuclear power plants are in a special working environment. For this reason, corrosion can occur inevitably. In view of this, the author believes that evolution should further optimize the valve bolt material of nuclear power plants. First of all, high-nickel ductile iron is a kind of corrosion-resistant material. The uniform corrosion rate of this material is about 0.1mm/a, and the resistance to seam corrosion and pitting corrosion is much better than that of general 316 stainless steel. It is recommended that nuclear power plants choose high-nickel ductile iron materials when selecting valve bolt materials in the future. Secondly, ASTM A439 material has good machinability and casting performance, and has good impact resistance and wear resistance. Therefore, it is also an ideal material for valve bolts in nuclear power plants [3]. For example, when making bolts for water pump valves in nuclear power plants, ASTM A439 D2 materials can be selected, and the corrosion resistance has been professionally verified. According to data survey, the material will not be in the epoxy coating below 300μm. Significant corrosion. Therefore, in the process of casting the valve bolts of nuclear power plants, the conventional CF8M materials or 0Cr18NiMo2Ti materials can be changed to ASTM A439 D2 materials while improving the valve structure foundation.
2.3 Strengthening Valve Bolt Corrosion Maintenance Nuclear power production is a complex fission process. Therefore, it is inevitable that there will be corrosion problems in the valve bolts of nuclear power plants. When the party corrosion problem occurs, it is very important to do the corrosion repair of the valve bolts in nuclear power plants. Two aspects of the proposal. The first is a comprehensive inspection to see if the crystal is deformed. If the surface grain of the valve bolt of the nuclear power plant is separated by grain boundary corrosion, this means that the valve bolt of the nuclear power plant has serious intergranular corrosion and must be replaced. In addition, the observation of the human eye is bound to be very limited. Therefore, if necessary, the scanning electron microscope can be used for observation. The full view of the valve bolt of the power plant can be observed by scanning electron microscopy. If the corrosion region is found to be corrosive and porous, the valve bolt of the nuclear power plant is also present. Severe intergranular corrosion must be replaced [3]. For the second maintenance and cleaning, the nuclear power plant needs to clean and repair the valve bolts from time to time. The valve bolts can be maintained by passivation treatment, but it is worth noting that the pickling passivation process needs to be further developed in the future. Perfection, such as some of the relatively small gaps, is not recommended for acidification passivation, and if it is necessary to deal with, it is recommended to dispose of the parts after disassembly or before loading. In addition, in the process of cleaning and curing, the final cleaning should use demineralized water, and use PH test paper to check to ensure that the PH value is controlled between 6 and 8 during the whole process, so that the valve bolts of the nuclear power plant can be better. Raise care while avoiding corrosion problems.
to sum up:
In the context of industrialization, nuclear power production provides power energy for cities, and nuclear power production is a complex fission process. Valve bolts seem to be small, but they have a direct impact on nuclear power safety production. In nuclear power production, valves Bolts are easily corroded by acidic substances. For this reason, nuclear power plants must pay attention to the management and maintenance of mechanical valve bolts in the future, and make further analysis and research on the prevention and control measures of valve bolt corrosion in nuclear power plants, so as to truly promote the steady development of China's nuclear power industry.
references:
[1] Sun Guocai, Zhang Congran, Liao Xuebo, Li Weiguang. Analysis and improvement of valve bolt corrosion in a nuclear power plant [J]. General Corrosion Control, 2017, 31(11): 40-42.
[2]LIN Wenliang, YAN Xiaolai, RU Hongyu, CHEN Junbo, YAN Baokai.Finite Element Simulation of Assembly Process of Valve Ring Bolt Assembly[J].Mechanical Engineer,2015(03):166-167.
[3]Zhang Xiaoqing. Optimization of assembly process of valve ring bolt group and its experimental research [D]. Harbin Institute of Technology, 2016.
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