Carbon steel anti-corrosion method for typical water quality of thermal power nuclear power plant

Abstract

According to the carbon steel anti-corrosion method for the typical water quality of the thermal power nuclear power plant, the water quality characteristics of demineralized water, tap water and seawater and the corrosion condition of the carbon steel in the demineralized water, the tap water and the seawater are analyzed, and an appropriate anti-corrosion method for the carbon steel in the demineralized water, the tap water and the seawater at the temperature of 50 DEG C is constructed; through a static coupon weightlessness method and an electrochemical test, a rule of inhibiting corrosion of carbon steel in demineralized water, tap water and seawater by a pH value, corrosion inhibition performance of imidazoline to carbon steel in demineralized water, application conditions, applicability of imidazoline in tap water and seawater, and applicability of cathode protection to corrosion prevention of carbon steel in typical water quality of a power plant are obtained. A novel green composite corrosion inhibitor with a corrosion inhibition effect on carbon steel corrosion in tap water and seawater is developed and obtained; the effective corrosion-resistant method for preventing the carbon steel from being corroded in demineralized water, tap water and seawater is developed, the carbon steel is effectively prevented from being corroded in typical water quality of a power plant, and the method has important significance and effects on safe and economical operation of thermal power plants, nuclear power plants and the like.

Description

Technical Field

[0001] The present application relates to a method for resisting corrosion of carbon steel with typical water quality in power plants, and in particular to a method for resisting corrosion of carbon steel with typical water quality in thermal power and nuclear power plants, belonging to the technical field of carbon steel anti-corrosion methods. Background Art

[0002] Carbon steel is commonly used in various water distribution systems and heat exchange systems, such as condensate water and water supply systems in the secondary circuits of thermal power plants and nuclear power plants, even steam drums, water-cooled wall tubes, closed cooling water systems, industrial cooling water systems, raw water transportation and purification systems, and indirect cooling water systems of Harmon air-cooled units. Carbon steel corrosion is very common. When the corrosion is severe, it not only deteriorates the water quality, but also causes thinning and perforation of the pipe wall. Desalted water is the supplementary water for the secondary circuit water, steam circulation system and some closed cooling water systems of thermal power plants and nuclear power plants. Some thermal power plants and nuclear power plants use tap water as the supplementary water for industrial cooling water. Some thermal power plants and nuclear power plants use seawater for cooling the condensers. Therefore, preventing the corrosion of carbon steel in desalted water, tap water and seawater is of great significance to the safe and economical operation of thermal power plants and nuclear power plants.

[0003] Thermodynamic calculations show that carbon steel will spontaneously undergo oxygen corrosion in demineralized water, tap water, and seawater. On the one hand, the corrosion of carbon steel in these three types of water is relatively serious, and on the other hand, the corrosion conditions of carbon steel in these three types of water are different. For example, desalted water is very clean and has very low buffering capacity. Its water quality is easily polluted, especially its pH value and conductivity are particularly susceptible to the influence of the air it contacts. In addition to oxygen corrosion, the corrosion of carbon steel in it may also be caused by carbon dioxide dissolving into the desalted water, which reduces the pH value and leads to carbon dioxide corrosion or weak acid corrosion. The corrosion problem of carbon steel water pipes is relatively common. The corrosion of tap water pipes often leads to a rapid decrease in the residual chlorine content in the water, and a significant increase in the color, turbidity, iron and manganese bacteriological indicators, which has a serious impact on the water quality of the pipe network. The safety of water use cannot be guaranteed. When the water flow speed and water pressure in the pipeline suddenly increase or the direction changes suddenly, the water quality will deteriorate in a short time, and even water quality accidents such as red water and black water will occur. The concentration of corrosive ions in seawater is much higher than that in fresh water. In addition to containing a large amount of salts, it also contains marine organisms and organic matter, which are highly corrosive. Carbon steel will suffer from severe corrosion in seawater.

[0004] For the corrosion protection of carbon steel in desalted water, it is required to resist corrosion while not polluting the desalted water quality. The surface treated covering layer will be released in the water. Due to the low conductivity of desalted water, it is not suitable for cathodic protection with sacrificial anodes. The current required for cathodic protection with impressed current will be very large, which is not economical and not suitable. General surface treatment and electrochemical protection are not suitable for the corrosion protection of carbon steel in desalted water. The general carbon steel-desalted water system is difficult to seal well. For open systems, deoxidation is not realistic, and the anti-corrosion effect of adjusting the pH value is not good. For the protection of equipment and pipelines in the closed-loop desalted water cooling system of power plants, some Na3PO4 is added to increase and maintain the pH of the desalted water at 9.5-11.0, so that the steel is naturally passivated to form a complete protective film for corrosion protection, but it is still severely corroded.

[0005] Among the methods to prevent carbon steel from corrosion in tap water and seawater, surface treatment is not applicable to some equipment and pipelines, and the protection time is limited; among the environmental medium treatment methods, adjusting the pH value and adding corrosion inhibitors are more commonly used; among the electrochemical protection methods, sacrificial anode and impressed current cathodic protection methods can be used. In principle, the pH value can be adjusted to prevent the corrosion of carbon steel in demineralized water, tap water and seawater, but whether it is applicable and to what pH value should be adjusted, the existing technology lacks research.

[0006] Since the salt content of seawater is much higher than that of fresh water, the protection effect of cathodic protection on carbon steel in seawater is better than that in fresh water. The existing technology is not clear about the difference between the application of sacrificial anode method and impressed current method in tap water and seawater, as well as the requirements for water conductivity for the implementation of cathodic protection.

[0007] The problems that need to be solved in the corrosion resistance of typical water-quality carbon steel in power plants in the prior art and the key technical difficulties of this application include:

[0008] (1) Carbon steel is used in water distribution systems and heat exchange systems in thermal power plants and nuclear power plants. Corrosion is very common in these systems. Severe corrosion not only deteriorates the water quality, but also causes thinning of the pipe wall and perforation. Carbon steel will spontaneously undergo oxygen corrosion in desalted water, tap water, and seawater that are exposed to the atmosphere, and the corrosion is relatively serious. On the other hand, the corrosion conditions of carbon steel in these three types of water are different. For the corrosion protection of carbon steel in desalted water, it is required to resist corrosion while not polluting the water quality of the desalted water, and the surface treated covering layer will be released in the water; due to the low conductivity of desalted water, the cathodic protection using sacrificial anodes in the existing technology is not suitable, and the current required for the implementation of cathodic protection using impressed current will be very large, which is not economical and not suitable for use. General surface treatment and electrochemical protection are not suitable for the corrosion protection of carbon steel in desalted water. General carbon steel-desalted water systems are difficult to seal well. For open systems, deoxygenation is not realistic, and the anti-corrosion effect of adjusting the pH value is not good. For the protection of equipment and pipelines in the closed-loop desalted water cooling system of power plants, some Na3PO4 is added to increase and maintain the pH of the desalted water at 9.5-11.0, so that the steel is naturally passivated to form a complete protective film for corrosion protection, but the corrosion is still serious. Among the existing methods for preventing carbon steel from corroding in tap water and seawater, surface treatment is not applicable to some equipment and pipelines, and the protection time is limited. Among the environmental medium treatment methods and electrochemical protection methods, sacrificial anode and impressed current cathodic protection methods can be used. In principle, the pH value can be adjusted to prevent the corrosion of carbon steel in demineralized water, tap water and seawater. However, whether they are all applicable and to what pH value should be adjusted, the existing technology lacks research and corresponding methods.

[0009] (2) The differences in the application of the sacrificial anode method and the impressed current method in tap water and seawater, as well as the requirements for water conductivity for the implementation of cathodic protection, are not clear in the existing technology. At present, there are almost no corrosion inhibitors used for the corrosion protection of carbon steel in desalted water. Therefore, it is urgent to develop corrosion inhibitors that can prevent the corrosion of carbon steel in desalted water, so as to be applied to the shutdown protection of the low-temperature operation section of power generation thermal equipment or the long-term shutdown protection of the whole thermal equipment, the operation protection of the closed cooling water system and the indirect cooling water system of the Harmon air-cooled unit, etc. The corrosion inhibition efficiency of existing carbon steel corrosion inhibitors in seawater is generally low, and the dosage required for most corrosion inhibitors is relatively large. The development and application of corrosion inhibitors cannot ignore environmental protection requirements. Most traditional corrosion inhibitors have certain harm to the environment. The research and development of environmentally friendly green corrosion inhibitors is the current focus. Among them, natural corrosion inhibitors with strong affinity for metal surfaces are the focus of the development of green and environmentally friendly corrosion inhibitors. Although natural corrosion inhibitors have the advantages of less environmental pollution and good biodegradability, they also have the disadvantages of low corrosion inhibition efficiency and large usage. In addition, the composition is relatively complex and the corrosion inhibition mechanism is still unclear. Therefore, we should strengthen the research and development of green and environmentally friendly corrosion inhibitors to prevent carbon steel corrosion in tap water and seawater.

[0010] (3) The corrosion of carbon steel in desalted water, tap water and seawater is mainly oxygen corrosion, but the influencing factors are different, the same influencing factors have different action rules, the pH value has different action rules, the amount of alkali required to adjust the pH value to the same value, the change rule of pH over time, etc. The existing technology lacks the exploration of the law of pH value inhibiting the corrosion of carbon steel in desalted water, tap water and seawater through hanging plate test and electrochemical test, the corrosion inhibition performance of a certain imidazoline on carbon steel in desalted water, the application conditions and whether it has the same corrosion inhibition effect on carbon steel in tap water and seawater, whether cathodic protection (including sacrificial anode method and impressed current method) is suitable for preventing the corrosion of carbon steel in desalted water, tap water and seawater, and finding a suitable method to prevent the corrosion of carbon steel in desalted water, tap water and seawater. There is a lack of research and development on effective anti-corrosion methods for carbon steel in desalted water, tap water and seawater based on the water quality characteristics of desalted water, tap water and seawater and the corrosion conditions of carbon steel therein. The corrosion cannot be reduced or prevented, and the corrosion resistance of carbon steel in desalted water, tap water and seawater cannot be solved in a targeted manner. This not only brings many problems in technology and economy, but also causes problems in energy, resources and environmental protection. Summary of the invention

[0011] The present application analyzes the water quality characteristics of desalted water, tap water and seawater and the corrosion of carbon steel therein, and constructs a suitable anti-corrosion method for carbon steel in 50°C desalted water, tap water and seawater; through the static hanging piece weight loss method, supplemented by electrochemical testing, the law of pH value on the inhibition of carbon steel corrosion in desalted water, tap water and seawater, the corrosion inhibition performance of a certain imidazoline on carbon steel in desalted water, the application conditions and its applicability in tap water and seawater, the applicability of cathodic protection to the corrosion prevention of carbon steel in desalted water, tap water and seawater, and the development of a new type of green composite corrosion inhibitor with a certain corrosion inhibition effect on carbon steel corrosion in tap water and seawater; the development of an effective anti-corrosion method suitable for preventing carbon steel from being in desalted water, tap water and seawater, reducing the harm caused by corrosion, and effectively preventing the corrosion of carbon steel in desalted water, tap water and seawater, which is of great significance and role to the safe and economical operation of thermal power plants, nuclear power plants, etc.

[0012] In order to achieve the above technical effects, the technical solutions adopted in this application are as follows:

[0013] The method for anti-corrosion of carbon steel in typical water quality of thermal power and nuclear power plants is analyzed. The water quality characteristics of desalted water, tap water and seawater and the corrosion of carbon steel in them are analyzed, and a suitable anti-corrosion method for carbon steel in 50℃ desalted water, tap water and seawater is constructed; through the static hanging piece weight loss method, supplemented by electrochemical testing, the law of pH value on the inhibition of carbon steel corrosion in desalted water, tap water and seawater is obtained, the corrosion inhibition performance of a certain imidazoline on carbon steel in desalted water, the application conditions and its applicability in tap water and seawater, the applicability of cathodic protection to the corrosion of carbon steel in desalted water, tap water and seawater, and the development of a new green composite corrosion inhibitor with a certain corrosion inhibition effect on carbon steel corrosion in tap water and seawater;

[0014] A: Corrosion of carbon steel in deionized water: Increasing the pH value to above 11 can inhibit corrosion in a short time; a certain imidazoline corrosion inhibitor with a concentration of 60 mg / L can inhibit corrosion for a long time; cathodic protection method can prevent corrosion of carbon steel in water with a conductivity of more than 200 us / cm;

[0015] B: Corrosion of carbon steel in tap water: Increasing the pH value to above 12 can inhibit the corrosion in a short time. The combination of imidazoline and sodium molybdate has a good corrosion inhibition effect on the corrosion of carbon steel in tap water. Cathodic protection can prevent the corrosion of carbon steel in tap water.

[0016] C: Corrosion of carbon steel in seawater: simply increasing the pH value cannot prevent it. When the pH value is increased, the corrosion inhibition efficiency of the green composite corrosion inhibitor is better. The cathodic protection method can effectively prevent the corrosion of carbon steel in seawater.

[0017] D: Increasing the pH value is only suitable for short-term corrosion protection of carbon steel in clean water with low hardness, otherwise it will not work or will be uneconomical; the imidazoline developed in this application has a good corrosion inhibition effect on carbon steel in desalted water, but has no obvious corrosion inhibition effect on carbon steel in tap water and seawater; the cathodic protection method is suitable for preventing corrosion of carbon steel in water with higher conductivity, and the anti-corrosion effect of cathodic protection on carbon steel in seawater is more obvious.

[0018] Preferably, the corrosion resistance model of carbon steel in deionized water with different pH values: In deionized water with different pH values, if the pH value is not adjusted high enough, the corrosion of carbon steel cannot be well suppressed even in a short time, while the corrosion can be suppressed in a short time when the pH value is adjusted to 11 or above. Under higher pH conditions, OH - As the concentration increases, OH - The adsorption opportunities and coverage on the carbon steel surface increase, and the adsorption of oxygen and other corrosive ions on the carbon steel surface decreases or is inhibited. When the pH value is not high enough, OH - When the carbon steel surface is not completely covered, the carbon steel will still corrode. When the pH value is high enough so that the carbon steel surface is completely covered by OH, its corrosion can be suppressed in a short time.

[0019] Preferably, to prevent corrosion of carbon steel in demineralized water:

[0020] A1: Adjust the pH value to prevent corrosion of carbon steel in deionized water. If the pH value is not adjusted high enough, corrosion cannot be effectively inhibited even in a short time. If the pH value is adjusted to 11 or above, corrosion can be inhibited in a short time.

[0021] A2: The corrosion inhibitor imidazoline has a good corrosion inhibition effect on carbon steel in desalted water, and as the concentration increases, its corrosion inhibition effect on carbon steel becomes better and better. When the concentration of imidazoline reaches 60 mg / L, the corrosion of carbon steel in desalted water can be suppressed for a long time. Imidazoline is an anodic corrosion inhibitor. Adding an appropriate concentration of imidazoline is better than adjusting the pH value.

[0022] A3: The ion content in desalted water is very low and its conductivity is poor. It is difficult for the cathodic protection method to polarize the potential of the carbon steel electrode in the desalted water to the protection potential to prevent the corrosion of the carbon steel material in the desalted water. It is not suitable to use the cathodic protection method. The cathodic protection method is suitable for preventing the corrosion of carbon steel in water with a conductivity of more than 200us / cm.

[0023] Preferably, the corrosion and corrosion resistance model of carbon steel in tap water with different pH values: when the pH value of tap water is adjusted to about 12, the corrosion of carbon steel can be suppressed in a short time, and increasing the pH of tap water can prevent the corrosion of carbon steel, and increasing the OH - Concentration, increase OH - The adsorption opportunities and coverage on the carbon steel surface can be increased, and the adsorption of oxygen and other corrosive ions on the carbon steel surface can be reduced or inhibited to prevent corrosion. That is, the carbon steel surface will be passivated in tap water with a higher pH value, and the passivation trend will increase with the increase of pH value. In desalted water, corrosion can be prevented by simply increasing the pH value to 11.

[0024] Preferably, to prevent carbon steel from corroding in tap water:

[0025] B1: Increasing the pH value of tap water can prevent corrosion of carbon steel in a short period of time, but the pH value needs to be adjusted to above 12, which is not economical;

[0026] B2: No green corrosion inhibitor monomer with good anti-corrosion effect on carbon steel in tap water was found; the combination of imidazoline and sodium molybdate has a good corrosion inhibition effect on carbon steel in tap water, and the corrosion inhibition efficiency reaches 90% when the total concentration reaches 200 mg / L;

[0027] B3: Cathodic protection can prevent corrosion of carbon steel in tap water. When impressed current protection is used, carbon steel will not be adequately protected when the potential is low, and over-protection will occur when the potential is too high. The optimal protection potential range for carbon steel is about -0.9V to -1.4V.

[0028] Preferably, the corrosion inhibition of carbon steel in 50°C seawater by the optimized composite formula is as follows: composite formula 1 is optimized, and formula 1 is prepared according to PESA 30 mg / L + HPMA 6 mg / L + PASP 50 mg / L + sodium citrate 50 mg / L + sodium molybdate 10 mg / L + sodium silicate nonahydrate 100 mg / L.

[0029] Preferably, to prevent corrosion of carbon steel in seawater:

[0030] C1: Adding alkali to increase the pH value of seawater will cause precipitation to continuously form in the seawater. It is not feasible to adjust the pH value of seawater to prevent the corrosion of carbon steel in it.

[0031] C2: Without adjusting the pH value, 200 mg / L sodium silicate has a good inhibitory effect on the corrosion of carbon steel in seawater in a short period of time. The corrosion inhibition effect deteriorates significantly with time. In a shorter and longer period of time, the optimized composite formula 1 has a good corrosion inhibition effect on carbon steel in 50°C seawater. After adding the optimized composite formula 1 and increasing the pH value of seawater, the corrosion inhibition effect is better. The optimized composite formula 1 is a mixed corrosion inhibitor composed of polymers PESA, PASP, HPMA, molybdate, citrate and silicate. Among them, PESA, PASP and HPMA are all non-toxic polymers and have good scale inhibition performance. Molybdate, citrate and silicate are all environmentally friendly corrosion inhibitors. The optimized composite formula 1 is a green composite corrosion inhibitor;

[0032] C3: Cathodic protection is used to prevent corrosion of carbon steel in seawater, and the protection effect is good. The optimal protection potential range of carbon steel in seawater is about -0.9V to -1.1V (vsSCE). Compared with tap water, the external voltage required for complete protection of carbon steel electrodes in seawater is smaller;

[0033] For cathodic protection of carbon steel in seawater, impressed current is more appropriate than sacrificial anodes, which have a short service life and must be replaced after consumption.

[0034] Preferably, the applicable water quality of the three anti-corrosion methods of improving pH value, imidazoline and cathodic protection is improved: according to the water quality characteristics of desalted water, tap water and seawater and the corrosion of carbon steel therein, the suitable anti-corrosion method of carbon steel in 50°C desalted water, tap water and seawater is obtained by static hanging weight loss method and electrochemical test method, as well as the applicable water quality of the three anti-corrosion methods of improving pH value, corrosion inhibitor and cathodic protection;

[0035] D1: Raising the pH value of desalted water to above 11 can prevent the corrosion of carbon steel in a short time. The pH value of tap water needs to be raised to 12 to prevent the corrosion of carbon steel in a short time. Adding alkali to seawater will continuously produce precipitation. Raising the pH value is only suitable for the corrosion prevention of carbon steel in clean and low hardness water. Otherwise, it will not work and is not economical. In addition, raising the pH value can only inhibit corrosion in a short time. Over time, corrosion will occur again.

[0036] D2: The imidazoline developed in this application has a good corrosion inhibition effect in desalted water. In desalted water with different concentrations of imidazoline, its corrosion inhibition effect on carbon steel becomes better and better with the increase of imidazoline concentration in a short time. When the imidazoline concentration reaches 60 mg / L, the corrosion of carbon steel in desalted water can be suppressed even for a long time. However, imidazoline has almost no corrosion inhibition effect on carbon steel in tap water and seawater.

[0037] D3: The protective effect of cathodic protection is closely related to the conductivity of the medium. The ion content in desalted water is very low, and the conductivity is very poor. The external driving force required to polarize the carbon steel electrode to the protective potential is very large. Therefore, cathodic protection is not suitable for desalted water.

[0038] Cathodic protection is suitable for preventing corrosion of carbon steel in water with a conductivity of more than 200us / cm. Cathodic protection can be used to protect carbon steel in both tap water and seawater. The protection in seawater is better. When the impressed current method is used, there is an optimal protection potential range. When the potential is low, it will not provide adequate protection. Too high a potential will cause over-protection. The conductivity of seawater is very high and the polarization resistance is very small. The impressed voltage required to protect carbon steel in seawater is lower than that in tap water. It is more economical to use impressed current cathodic protection in seawater. Cathodic protection can also be used when there is no more effective and economical anti-corrosion method in tap water.

[0039] Compared with the prior art, the innovations and advantages of this application are:

[0040] (1) This application analyzes the water quality characteristics of desalted water, tap water, and seawater and the corrosion of carbon steel therein, and constructs a suitable anti-corrosion method for carbon steel in 50°C desalted water, tap water, and seawater; through the static hanging piece weight loss method, supplemented by electrochemical testing, the law of pH value on the inhibition of carbon steel corrosion in desalted water, tap water, and seawater, the corrosion inhibition performance of a certain imidazoline on carbon steel in desalted water, the application conditions and its applicability in tap water and seawater, the applicability of cathodic protection to the corrosion prevention of carbon steel in desalted water, tap water, and seawater, and the development of a new type of green composite corrosion inhibitor with a certain corrosion inhibition effect on carbon steel corrosion in tap water and seawater; the development of an effective anti-corrosion method suitable for preventing carbon steel from being in desalted water, tap water, and seawater, reducing the harm caused by corrosion, and effectively preventing the corrosion of carbon steel in desalted water, tap water, and seawater, which is of great significance and role to the safe and economical operation of thermal power plants, nuclear power plants, etc.

[0041] (2) The present application has reached important conclusions on preventing the corrosion of carbon steel in tap water: Increasing the pH value of tap water can prevent the corrosion of carbon steel in it in a short period of time, but the pH value needs to be adjusted to above 12, which is not economical; no green corrosion inhibitor monomer with good anti-corrosion effect on carbon steel in tap water has been found; the combination of imidazoline and sodium molybdate has a good corrosion inhibition effect on carbon steel in tap water, and the corrosion inhibition efficiency reaches 90% when the total concentration reaches 200 mg / L; cathodic protection can prevent the corrosion of carbon steel in tap water; when external current protection is used, carbon steel will not be adequately protected when the potential is low, and over-protection will occur if the potential is too high. The optimal protection potential range for carbon steel is about -0.9V to -1.4V, which is of great value in preventing carbon steel from corroding in tap water and ensuring the safety and stability of the power generation system.

[0042] (3) The present application has obtained important conclusions on preventing the corrosion of carbon steel in desalted water: adjusting the pH value to prevent the corrosion of carbon steel in desalted water. If the pH value is not adjusted high enough, the corrosion cannot be well inhibited even in a short time; if the pH value is adjusted to 11 or above, its corrosion can be inhibited in a short time; the corrosion inhibitor imidazoline has a good corrosion inhibition effect on carbon steel in desalted water, and as the concentration increases, its corrosion inhibition effect on carbon steel becomes better and better. When the imidazoline concentration reaches 60 mg / L, the corrosion of carbon steel in desalted water can be inhibited for a long time. Imidazoline is an anodic corrosion inhibitor. Adding an appropriate concentration of imidazoline is better than adjusting the pH value; the ion content in desalted water is very small and the conductivity is poor. It is difficult for the cathodic protection method to polarize the potential of the carbon steel electrode in the desalted water to the protection potential to prevent the corrosion of carbon steel materials in the desalted water. It is not suitable to use the cathodic protection method. The cathodic protection method is suitable for preventing the corrosion of carbon steel in water with a conductivity of more than 200 us / cm. A method for preventing the corrosion of carbon steel in desalted water is obtained.

[0043] (4) The present application has reached an important conclusion on preventing the corrosion of carbon steel in seawater: adding alkali to increase the pH value of seawater will cause precipitation to continuously form in the seawater, and it is not feasible to adjust the pH value of seawater to prevent the corrosion of carbon steel therein; without adjusting the pH value, 200 mg / L sodium silicate has a good inhibitory effect on the corrosion of carbon steel in seawater in a short period of time, and the corrosion inhibition effect obviously deteriorates with time. In both short and long periods of time, the optimized composite formula 1 has a good corrosion inhibition effect on carbon steel in 50°C seawater. The corrosion inhibition effect is even better after adding the optimized composite formula 1 and increasing the pH value of seawater. The optimized composite formula 1 is composed of polymer PESA, PASP, HPMA, molybdate, citrate and silicate. A mixed corrosion inhibitor is prepared, among which PESA, PASP and HPMA are all non-toxic high molecular polymers with good scale inhibition performance. Molybdate, citrate and silicate are all environmentally friendly corrosion inhibitors. The optimized composite formula 1 is a green composite corrosion inhibitor. Cathodic protection is used to prevent the corrosion of carbon steel in seawater, and the protective effect is good. The optimal protection potential range of carbon steel in seawater is about -0.9V to -1.1V (vsSCE). Compared with tap water, the external voltage required for complete protection of carbon steel electrodes in seawater is smaller. For cathodic protection of carbon steel in seawater, it is more appropriate to use an impressed current than a sacrificial anode. The service life of the sacrificial anode is short and it must be replaced after consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a graph showing the corrosion and corrosion resistance of carbon steel in desalted water with different pH values ​​at 50°C.

[0045] Figure 2 This is a graph showing the corrosion and corrosion resistance of carbon steel in demineralized water with different concentrations of imidazoline added.

[0046] Figure 3 It is the implementation flow chart of PSO-connected network.

[0047] Figure 4 This is a diagram showing the effect of sacrificial anode protection after adjusting the conductivity of desalted water with different salts.

[0048] Figure 5 This is a graph showing the corrosion and corrosion resistance of carbon steel in tap water with different pH values ​​at 50°C.

[0049] Figure 6 This is a diagram of the corrosion inhibition effect of the combination of imidazoline and sodium molybdate.

[0050] Figure 7 This is a diagram showing the protection effect of applying different impressed voltages to tap water using the impressed current cathodic protection method.

[0051] Figure 8 This is a graph showing the corrosion inhibition of carbon steel by green corrosion inhibitor monomer in 50°C seawater.

[0052] Fig. 9 This is a graph showing the corrosion inhibition of carbon steel in 50°C seawater by the composite formula and 100 mg / L sodium silicate.

[0053] Fig.10 This is a graph showing the corrosion inhibition of carbon steel in seawater at 50°C and adjusted pH value by the optimized composite formula.

[0054] Fig.11 This is a diagram showing the anti-corrosion effects of different impressed voltages in the seawater impressed current cathodic protection method. DETAILED DESCRIPTION

[0055] Below, in conjunction with the accompanying drawings, the technical solution of the carbon steel anti-corrosion method for typical water quality in thermal power and nuclear power plants provided by the present application is further described so that technicians in this field can better understand the present application and implement it.

[0056] This application analyzes the water quality characteristics of desalted water, tap water, and seawater and the corrosion of carbon steel therein, and constructs a suitable anti-corrosion method for carbon steel in 50°C desalted water, tap water, and seawater; through the static hanging piece weight loss method, supplemented by electrochemical testing, obtains the law of pH value on the inhibition of carbon steel corrosion in desalted water, tap water, and seawater, the corrosion inhibition performance of a certain imidazoline on carbon steel in desalted water, the application conditions and its applicability in tap water and seawater, the applicability of cathodic protection (including sacrificial anode method and impressed current method) to the corrosion protection of carbon steel in desalted water, tap water, and seawater, and develops a new type of green composite corrosion inhibitor that has a certain corrosion inhibition effect on carbon steel corrosion in tap water and seawater;

[0057] A: Corrosion of carbon steel in deionized water: Increasing the pH value to above 11 can inhibit corrosion in a short time; a certain imidazoline corrosion inhibitor with a concentration of 60 mg / L can inhibit corrosion for a long time; cathodic protection method can prevent corrosion of carbon steel in water with a conductivity of more than 200 us / cm;

[0058] B: Corrosion of carbon steel in tap water: Increasing the pH value to above 12 can inhibit the corrosion in a short time. The combination of imidazoline and sodium molybdate has a good corrosion inhibition effect on the corrosion of carbon steel in tap water (the corrosion inhibition efficiency can reach 90% when the total concentration is 200 mg / L). Cathodic protection can prevent the corrosion of carbon steel in tap water.

[0059] C: Corrosion of carbon steel in seawater: Simply increasing the pH value cannot prevent corrosion. When the pH value is increased, the corrosion inhibition efficiency of the green composite corrosion inhibitor can reach 76%. The cathodic protection method can effectively prevent the corrosion of carbon steel in seawater.

[0060] D: Increasing the pH value is only suitable for short-term corrosion protection of carbon steel in clean water with low hardness (such as desalted water), otherwise it will not work or will be uneconomical; the imidazoline developed in this application has a good corrosion inhibition effect on carbon steel in desalted water, but has no obvious corrosion inhibition effect on carbon steel in tap water and seawater; the cathodic protection method is suitable for preventing the corrosion of carbon steel in water with higher electrical conductivity (such as tap water and seawater), and the cathodic protection has a more obvious anti-corrosion effect on carbon steel in seawater.

[0061] 1. Methods for preventing corrosion of carbon steel in desalted water

[0062] Compared with general industrial circulating water, desalted water is weakly acidic and more corrosive. Thermodynamic calculations show that carbon steel will spontaneously undergo oxygen corrosion in the nearly neutral desalted water that is connected to the atmosphere, and the corrosion is actually quite severe.

[0063] (I) Corrosion resistance model of carbon steel in desalted water with different pH values

[0064] Figure 1 This is the short-time (12h) corrosion and corrosion resistance of carbon steel in deionized water with different pH values ​​at 50℃.

[0065] In deionized water with different pH values, if the pH value is not adjusted high enough, the corrosion of carbon steel cannot be well suppressed even in a short time. However, if the pH value is adjusted high enough (11 and above), the corrosion can be suppressed in a short time. Under higher pH conditions, OH - As the concentration increases, OH - The adsorption opportunities and coverage on the carbon steel surface increase, and the adsorption of oxygen and other corrosive ions on the carbon steel surface decreases or is inhibited. However, increasing the pH value not only reduces or inhibits corrosion, but also increases the conductivity of the desalted water and accelerates electrochemical oxygen corrosion. This is a contradiction. Therefore, when the pH value is not high enough, OH - When the carbon steel surface is not completely covered, the carbon steel will still corrode. When the pH value is high enough so that the carbon steel surface is completely covered by OH, its corrosion can be suppressed in a short time.

[0066] (II) Corrosion and corrosion resistance of carbon steel in demineralized water with different concentrations of imidazoline corrosion inhibitor

[0067] Figure 2It is the corrosion and corrosion resistance of carbon steel in desalted water at 50℃ with different concentrations of imidazoline corrosion inhibitor. In desalted water with different concentrations of imidazoline, the corrosion inhibition effect on carbon steel is better as the concentration of imidazoline increases in a short time. When the imidazoline concentration reaches 60mg / L, the corrosion of carbon steel in desalted water is suppressed even for a long time. Imidazoline corrosion inhibitor inhibits corrosion by adsorption, forming a firm film on the metal surface, preventing the corrosive medium from contacting the metal, blocking the corrosion process of carbon steel, and playing a role in slowing down or preventing corrosion. The higher the concentration of the corrosion inhibitor, the better the covering layer and the better the corrosion inhibition effect.

[0068] In order to further study the corrosion inhibition mechanism of imidazoline, electrochemical tests were carried out. After adding imidazoline, the open circuit potential of the carbon steel electrode shifted significantly positively, and the higher the imidazoline concentration, the more positive the open circuit potential of the carbon steel electrode.

[0069] After adding the corrosion inhibitor imidazoline, the Tafel slope of the anode increased significantly, while the Tafel slope of the cathode did not change significantly, indicating that imidazoline is an anodic corrosion inhibitor.

[0070] Compared with blank deionized water, after adding imidazoline, the corrosion current density obtained from the strong polarization zone is greatly reduced, and the polarization resistance obtained from the linear polarization is significantly increased. The larger the polarization resistance, the greater the adsorption of imidazoline on the metal surface. Through adsorption, a film is formed on the surface of the carbon steel electrode, which effectively blocks the direct contact between the medium and the steel surface, increases the difficulty of corrosion, and thus reduces the corrosion rate.

[0071] (III) Cathodic protection of carbon steel in desalted water

[0072] When the external current makes E corr When it drops to the same potential as the starting potential of the microbattery anode, the anodic corrosion current is 0. In order to fully protect carbon steel, it must be cathodically polarized to the equilibrium potential of Fe.

[0073] In cathodic protection, if the self-corrosion potential of steel materials can be shifted negatively by 200 to 300 mV, it is believed that the corrosion behavior of steel will be inhibited.

[0074] (1) Impressed current cathodic protection to prevent corrosion of carbon steel in desalted water

[0075] The protective potential of carbon steel in deionized water is unknown in advance and difficult to calculate accurately. It is determined by the potential shift method. The DC power supply is adjusted to gradually increase the applied voltage. The potential difference between the two electrodes is monitored regularly to obtain the relationship between the applied voltage, carbon steel electrode potential and corrosion conditions. Figure 3 .

[0076] The measured open circuit potential of carbon steel electrode in deionized water is about -0.8V (relative to platinum electrode). In order to make the potential of carbon steel shift 200-300mV negative from the self-corrosion potential, the potential difference between the carbon steel electrode and the reference electrode (platinum electrode) measured by a multimeter should be around -1V. Due to the extremely low ion content in deionized water and the extremely poor conductivity, it is difficult to polarize the carbon steel electrode to the protection potential by the impressed current method, and the impressed voltage required to polarize to below -1V is very large, which is very uneconomical. The surface of the carbon steel electrode still corrodes immediately when the impressed voltage is as high as 1.5V. Therefore, the corrosion of carbon steel materials in deionized water is not suitable for protection by the impressed current method.

[0077] (2) Sacrificial anode protection to prevent carbon steel corrosion in desalted water

[0078] Since the conductivity of desalted water is very small, a large external driving force is required to polarize carbon steel to the protection potential or below. It is speculated that sacrificial anode protection is not effective for carbon steel in desalted water. Experiments using zinc as a sacrificial anode show that the carbon steel specimen connected to the zinc anode exhibits visible corrosion soon after being placed in desalted water, which is not significantly different from the corrosion of carbon steel in blank desalted water, that is, sacrificial anode protection has basically no protective effect on carbon steel in desalted water.

[0079] In order to explore the applicable water quality conditions of the sacrificial anode protection method, different salts were added to the desalted water to adjust its conductivity. The relationship between the protective effect of the sacrificial anode protection on carbon steel in the desalted water and the conductivity of the water and the salt substances in the water was tested. The test results are as follows: Figure 4 .

[0080] Depend on Figure 4 It can be seen that when the conductivity of desalted water is adjusted with the same salt, the higher the conductivity, the better the protective effect of the sacrificial anode. When the conductivity is increased to a certain value, carbon steel is almost not corroded. The rules obtained by adjusting the conductivity with different salts are basically the same. This shows that the protective effect of sacrificial anode protection on carbon steel materials in water is closely related to conductivity. The higher the conductivity, the better the protective effect. It is also related to the type of substance that adjusts the conductivity. In order for sacrificial anode protection to have a good protective effect on carbon steel materials in water, the conductivity of water should be above 200us / cm.

[0081] Since the open circuit potential of zinc is relatively positive, and the effective potential difference with carbon steel is only about 0.2V, the test results obtained are more practical. It is not feasible to prevent the corrosion of carbon steel in desalted water by increasing the conductivity, because no matter how the desalted water is defined or prepared, its conductivity cannot meet the conductivity requirements of sacrificial anode protection for the corrosion of carbon steel materials in water.

[0082] (IV) Preventing carbon steel from corroding in desalted water

[0083] A1: Adjust the pH value to prevent corrosion of carbon steel in desalted water. If the pH value is not adjusted high enough, corrosion cannot be effectively inhibited even in a short period of time. If the pH value is adjusted to 11 or above, corrosion can be inhibited in a short period of time. The reason is that desalted water is passed through the atmosphere, and its pH value will continue to decrease due to the dissolution of carbon dioxide in the atmosphere.

[0084] A2: The corrosion inhibitor imidazoline has a good corrosion inhibition effect on carbon steel in desalted water, and as the concentration increases, its corrosion inhibition effect on carbon steel becomes better and better. When the concentration of imidazoline reaches 60 mg / L, the corrosion of carbon steel in desalted water can be suppressed for a long time. Imidazoline is an anodic corrosion inhibitor. Adding an appropriate concentration of imidazoline is better than adjusting the pH value.

[0085] A3: The ion content in desalted water is very low, and the conductivity is poor. It is difficult for the cathodic protection method to polarize the potential of the carbon steel electrode in the desalted water to the protection potential. Therefore, it is not suitable to use the cathodic protection method to prevent the corrosion of carbon steel materials in desalted water. The cathodic protection method is suitable for preventing the corrosion of carbon steel in water with a conductivity of more than 200us / cm.

[0086] 2. Methods to prevent corrosion of carbon steel in tap water

[0087] (I) Corrosion and corrosion resistance model of carbon steel in tap water with different pH values

[0088] Figure 5 It is the corrosion and corrosion resistance of carbon steel in tap water with different pH values ​​at 50℃. When the pH value of tap water is adjusted to a high enough level (about 12), the corrosion of carbon steel can be suppressed in a short time. Increasing the pH value of tap water can prevent the corrosion of carbon steel and increase the OH - Concentration, increase OH - The adsorption opportunities and coverage on the carbon steel surface can be increased, and the adsorption of oxygen and other corrosive ions on the carbon steel surface can be reduced or inhibited to prevent corrosion. That is, the carbon steel surface will be passivated in tap water with a higher pH value, and the passivation trend will increase with the increase of pH value. In desalted water, it is only necessary to increase the pH value to 11 to prevent corrosion. The reason is that the conductivity of tap water is relatively high. The higher the conductivity, the more serious the electrochemical oxygen corrosion. Therefore, a higher pH value and a larger OH concentration are required to inhibit corrosion.

[0089] (II) Corrosion inhibitors for preventing carbon steel corrosion in tap water

[0090] (1) Corrosion inhibition effect of some green corrosion inhibitor monomers

[0091] Firstly, some green corrosion inhibitor monomers are selected for hanging plate test, and the corrosion inhibition effect of some green corrosion inhibitor monomers on carbon steel in tap water is obtained through hanging plate test.

[0092] The corrosion inhibitors in the prior art have unsatisfactory corrosion inhibition effects on carbon steel in tap water, and some even have negative effects. We selected the corrosion inhibitor monomers with higher corrosion inhibition efficiency and compounded them in pairs. We found that imidazoline and sodium molybdate have good synergistic effects. Therefore, we compounded them in different ratios and conducted coupon tests. The test results are as follows: Figure 6 .

[0093] When sodium molybdate and imidazoline are compounded, the corrosion inhibition efficiency increases with the increase of total concentration. When the total concentration reaches 200 mg / L, the test piece is basically not corroded and the corrosion inhibition efficiency is about 90%. When the total concentration remains unchanged and the ratio of sodium molybdate and imidazoline is changed, the difference in corrosion inhibition efficiency caused by different ratios is not obvious.

[0094] The total concentration of sodium molybdate and imidazoline was 200 mg / L. They were added to carbon steel specimens in tap water according to 5 different ratios. The surface of the specimens basically remained as bright as when they were just polished, with a few places experiencing minor corrosion. The well-polished parts were not prone to corrosion, while the small holes and other parts that were difficult to polish were prone to pitting corrosion, resulting in deeper pits and greater weight loss.

[0095] After adding the corrosion inhibitor composed of sodium molybdate and imidazoline, the open circuit potential of the carbon steel electrode shifted significantly to the positive side, indicating that the corrosion inhibitor composed of sodium molybdate and imidazoline is an anodic corrosion inhibitor; the open circuit potential of each polarization curve was reset to zero, and the corrosion rate of carbon steel was significantly reduced after adding the corrosion inhibitor.

[0096] Compared with blank tap water, after adding sodium molybdate and imidazoline compound corrosion inhibitor, the corrosion current density obtained from the strong polarization zone is greatly reduced, and the polarization resistance obtained from linear polarization is significantly increased. The larger the polarization resistance, the greater the polarization resistance, indicating that the sodium molybdate and imidazoline compound corrosion inhibitor is adsorbed on the metal surface, and a film is formed on the surface of the carbon steel electrode through adsorption, which effectively blocks the direct contact between the medium and the steel surface, increases the difficulty of corrosion, and thus reduces the corrosion rate.

[0097] (III) Cathodic protection of carbon steel in tap water

[0098] (1) Impressed current protection against corrosion of carbon steel in tap water

[0099] The protective potential of carbon steel in tap water is also unknown in advance and difficult to calculate accurately. It is determined by the potential shift method based on empirical data and rough calculations. The DC power supply is adjusted to gradually increase the applied voltage, and the potential difference between the two electrodes is monitored regularly to obtain the applied voltage, carbon steel electrode potential and corrosion conditions.

[0100] The measured open circuit potential of the carbon steel electrode in tap water is about -0.65V (vsSCE). In order to shift the potential of the carbon steel 200mV to 300mV negative from the self-corrosion potential, the potential difference between the carbon steel electrode and the reference electrode (platinum electrode) measured by a multimeter is -0.9V.

[0101] Depend on Figure 7 It can be seen that when the applied voltage rises to about 1.2 V and the electrode potential of carbon steel decreases to -0.95 V (vs SCE), corrosion can be suppressed. However, when the applied voltage rises to 1.7 V, small bubbles appear on the electrode surface.

[0102] During the cathodic protection process, scale will be generated on the surface of the carbon steel electrode; as the applied voltage increases, the electrode surface gradually turns white, the scale generated becomes more obvious, until the scale can be seen with the naked eye and gradually becomes thicker, and the electrode surface changes from partially corroded to non-corroded and completely covered by scale. Analysis of the ion composition of tap water and research literature shows that the scale layer is mainly a calcium deposit layer, because the reduction reaction of oxygen atoms or hydrogen ions occurs at the carbon steel / tap water interface, and the dissolved oxygen obtains electrons to generate OH, and an oxygen absorption reaction occurs, as shown in Formula 1; but when the cathode potential is too negative, a hydrogen evolution reaction occurs, as shown in Formula 2:

[0103] O2+2H2O+4e→4OH Formula 1

[0104] 2H2O+2e2OH+H2 Formula 2

[0105] The generated OH- will increase the pH value on the surface of the carbon steel electrode. As the reaction proceeds, the pH value at the carbon steel / tap water interface can rise to above 10. OH- reacts with HCOs:

[0106] Under the action of external direct current, the higher the applied voltage of the electric drive, the greater the OH- and CO2 are in the Ca 2+ Mg 2+ The faster the reaction is, the faster the formation of CaCOs and Mg(OH)2, CaCO3 and Mg(OH)2 are the main components of the calcareous deposit;

[0107] When the applied voltage is greater than 1.7V, small bubbles appear on the electrode surface, indicating that hydrogen begins to evolve on the electrode surface, destroying the compactness of the scale layer and making the texture loose. When the applied voltage is around 1.2V, the protection effect is best, which can generate a denser scale layer that appears to the naked eye without causing over-protection.

[0108] After the scale layer is formed, the exposed area of ​​the carbon steel surface in the medium is reduced. The carbon steel electrode that has been protected by an external voltage of 1.2V for 8 hours and has formed a dense deposition layer is powered off. After 10 minutes, corrosion products are clearly shown on the scale surface, indicating that the scale layer has no protective effect.

[0109] In tap water, when the applied voltage is less than 1.2V, it is under-protected, and when it is greater than 1.7V, it is over-protected. The optimal protection potential range for carbon steel is -0.9V to -1.4V (vsSCE), which is consistent with the estimate before the test.

[0110] (2) Sacrificial anode protection against carbon steel corrosion in tap water

[0111] A thin layer of scale will form on the surface of the carbon steel test piece protected by the zinc anode, which increases the quality of the test piece; after washing with pickling solution, the surface of the test piece is as smooth as freshly polished, and no traces of corrosion can be seen. From the sacrificial anode protection test of adding salt to increase the conductivity of desalted water, it can be seen that the sacrificial anode protection can play a good protective role when the conductivity of water is increased to more than 200us / cm; the conductivity of tap water is about 320us / cm, so the sacrificial anode can protect carbon steel in tap water.

[0112] 3.2.4 Preventing Carbon Steel from Corroding in Tap Water

[0113] B1: Increasing the pH value of tap water can prevent corrosion of carbon steel in a short period of time, but the pH value needs to be adjusted to above 12, which is not economical;

[0114] B2: No green corrosion inhibitor monomer with good anti-corrosion effect on carbon steel in tap water was found; the combination of imidazoline and sodium molybdate has a good corrosion inhibition effect on carbon steel in tap water, and the corrosion inhibition efficiency reaches 90% when the total concentration reaches 200 mg / L;

[0115] B3: Cathodic protection can prevent corrosion of carbon steel in tap water. When impressed current protection is used, carbon steel will not be adequately protected when the potential is low, and over-protection will occur when the potential is too high. The optimal protection potential range for carbon steel is about -0.9V to -1.4V.

[0116] 3. Methods for preventing corrosion of carbon steel in seawater

[0117] (I) Corrosion and corrosion resistance model of carbon steel in seawater with different pH values

[0118] When NaOH solution is added to seawater, precipitation will continue to occur in the seawater. Even if the pH value is stabilized at around 10.2, the anti-corrosion effect on carbon steel in seawater is not good. It is not suitable to use the method of increasing the pH value to prevent corrosion of carbon steel in seawater.

[0119] (II) Corrosion inhibitors for preventing carbon steel corrosion in seawater

[0120] (1) Corrosion inhibition of some green corrosion inhibitor monomers on carbon steel in 50°C seawater

[0121] Depend on Figure 8 It can be seen that when the specimens were exposed to 50℃ seawater for 2h, the anti-corrosion effect of 200mg / L green corrosion inhibitor monomers on carbon steel was generally poor, with only HPMA and 2# imidazoline being relatively better. However, there was an obvious light yellow film on the surface of the specimen after HPMA inhibition.

[0122] After the hanging time was changed from 2h to 8h, the corrosion rate of the carbon steel specimen in the seawater with green corrosion inhibitor monomer was reduced, and of course the corrosion rate of the blank sample was also reduced; there was an obvious yellow film on the surface of the specimen with 200mgLHPMA added. In general, 200mg / L sodium silicate has a good anti-corrosion effect, and the corrosion inhibition efficiency is 59.9%. Therefore, these green corrosion inhibitor monomers were compounded (the total concentration of the corrosion inhibitor was 200mg / L), and three green corrosion inhibitor composite formulas were obtained through orthogonal experiments.

[0123] (2) The corrosion inhibition of carbon steel coupons in 50℃ seawater for 8 hours by the green corrosion inhibitor composite formula

[0124] Fig. 9 It is the corrosion inhibition of carbon steel in 50℃ seawater by the green corrosion inhibitor composite formula obtained through orthogonal test (hanging plate for 8h), and there are test results of adjusting the pH value of seawater while adding the composite formula, and there are also test results of adding 100mg / L sodium silicate and adjusting the pH value of seawater at the same time.

[0125] Depend on Fig. 9 It can be seen that after the pH value of seawater with composite corrosion inhibitor is adjusted to around 9 or 10, the corrosion rate of carbon steel is significantly reduced, and the higher the pH value, the lower the corrosion rate, which shows that increasing the pH value within a certain range of corrosion inhibitor concentration can further reduce the corrosion rate of carbon steel; but it is becoming increasingly difficult to continue to increase the pH value, and some flocculent precipitation appears in the seawater; after adding 100 mg / L sodium silicate, increasing the pH value does not help much in reducing the corrosion rate of carbon steel.

[0126] Since all three compound formulas contain EDTA (C 10 H 14 N 28 Na2), and too much N element will cause eutrophication of seawater, excessive growth of algae, and cause environmental problems such as red tides. Therefore, in the following experiments, EDTA was removed and the concentration of polymer was supplemented accordingly to further find a corrosion inhibitor formula that has a better inhibitory effect on carbon steel corrosion in seawater.

[0127] (3) Optimized composite formula for corrosion inhibition of carbon steel in 50°C seawater

[0128] Without adjusting the pH value, the optimized composite formula has a certain corrosion inhibition effect on the corrosion of carbon steel in seawater. After the pH value increases, the corrosion inhibition effect is greatly improved. After the addition of some optimized composite formulas, the corrosion rate of carbon steel has dropped to 0.0893g / (m2h). Several composite formulas with better corrosion inhibition effects were selected, the hanging time was increased, and the corrosion inhibition of carbon steel by the optimized composite formula was continued to be tested.

[0129] When the hanging time is 72h, the optimized composite formula 9 (PESA 31.25mg / L + HPMA 6.25mg / L + PASP 50mg / L + sodium citrate 50mg / L + sodium molybdate 12.5mg / L + sodium silicate nonahydrate 100mg / L) reduces the corrosion rate of carbon steel in seawater at 50℃ and increased pH to about 0.0435g / (m 2 h), which is approximately 0.0485 mm / a, meeting the requirement of GB / T23248-2009 “Specifications for Treatment of Circulating Seawater Cooling Water” that the corrosion rate of carbon steel should be less than 0.075 mm / a.

[0130] Since the dosage of the optimized compound formula 9 is still too large, the formula is further optimized. Fig.10 These are the test results, including the test results of adding 200 mg / L sodium silicate.

[0131] After changing some ingredients in the formula and reducing the dosage, the corrosion inhibition effect was reduced to a certain extent. As time went on, the corrosion rate of carbon steel also decreased to a certain extent. The corrosion inhibition effect of 200 mg / L sodium silicate coupons was not good for 72 hours, indicating that the corrosion inhibition effect of sodium silicate was more obvious in a short time, but poorer for a long time. The optimized composite formula 1 (PESA 30 mg / L + HPMA 6 mg / L + PASP 50 mg / L + sodium citrate 50 mg / L + sodium molybdate 10 mg / L + sodium silicate nonahydrate 100 mg / L) had a good corrosion inhibition effect when the pH was adjusted to 9.6. The corrosion rate of the carbon steel coupon coupons for 72 hours was 0.0422 g / (m 2 h), which is approximately 0.047 mm / a, meeting the requirement of GB / T23248-2009 “Specifications for Treatment of Circulating Seawater Cooling Water” that the corrosion rate of carbon steel should be less than 0.075 mm / a.

[0132] According to the corrosion rate of carbon steel in the coupon test, four test solutions were selected for electrochemical polarization curve test. These four test solutions are: ① blank seawater, ② seawater with 2200mg / L sodium silicate, ③ seawater with optimized composite formula 1 (PESA 30mg / L + HPMA 6mgL + PASP 50mg / L + sodium citrate 50mg / L + sodium molybdate 10mg / L + sodium silicate nonahydrate 100mg / L) and no pH adjustment, and ④ seawater with optimized composite formula 1 and pH adjustment.

[0133] Self-corrosion potential E of carbon steel in four test solutions corrThe order of size is: 1<③≈④<②, the more positive the self-corrosion potential, the smaller the corrosion tendency. After adding the corrosion inhibitor, the difficulty of corrosion is increased, among which 200 mg / L sodium silicate is the most obvious. However, with the progress of polarization, the corrosion current of carbon steel in 200 mg / L sodium silicate test solution slowly approaches that in blank seawater. Under the same polarization potential, the current density of carbon steel in test solutions ③ and ④ is smaller, that is, the corrosion rate is reduced, and the self-corrosion potential is increased. Further optimization of composite formula 1 is a mixed corrosion inhibitor. Increasing pH is more conducive to the formation of film and has better corrosion inhibition effect.

[0134] (III) Cathodic protection of carbon steel in seawater

[0135] (1) Impressed current cathodic protection of carbon steel in seawater

[0136] The protective potential of carbon steel in seawater is determined by the potential shift method based on empirical data and rough calculations. The measured open circuit potential of carbon steel electrode in seawater is about -0.65V (vsSCE). In order to shift the potential of carbon steel 200mV to 300mV negative from the self-corrosion potential, the potential difference between the carbon steel electrode and the reference electrode measured by a multimeter is about -0.9V.

[0137] Adjust the DC power supply to gradually increase the applied voltage, monitor the potential difference between the two electrodes regularly, and obtain the relationship between the applied voltage, carbon steel electrode potential and corrosion conditions. Fig.11 .

[0138] When the applied voltage rises to about 0.8V, the potential of the carbon steel electrode decreases to -0.88V (vsSCE), and the corrosion of the carbon steel is suppressed; when the applied voltage rises to 1.2V, bubbles appear on the electrode surface; a layer of calcium deposits is generated on the surface of the protected carbon steel electrode, and as the applied voltage increases, the deposits generated on the electrode surface gradually become more obvious, thicker, and whiter.

[0139] Therefore, the applied voltage protection range of carbon steel in seawater is 0.8V to 1.2V. It is under-protected when it is less than 0.8V and over-protected when it is greater than 1.2V. The corresponding optimal protection potential range of carbon steel is about -0.9V to -1.1V (vsSCE). Compared with tap water, the applied voltage required for complete protection of carbon steel electrodes in seawater is smaller, but the protection range is also narrower, which is prone to hydrogen evolution.

[0140] (2) Sacrificial anode protection of carbon steel in seawater

[0141] After carbon steel was protected by sacrificial anode method in seawater, a layer of obvious white scale formed on the surface of the test piece, and the quality of the test piece increased. After the white scale on the surface of the test piece was washed away with pickling liquid, the surface of the test piece was smooth as if it had just been polished, with no traces of corrosion, indicating that the sacrificial anode method can well protect carbon steel from corrosion in seawater.

[0142] 3.3.4 Preventing Carbon Steel Corrosion in Seawater

[0143] C1: Adding alkali to increase the pH value of seawater will cause precipitation to continuously form in the seawater. It is not feasible to adjust the pH value of seawater to prevent the corrosion of carbon steel in it.

[0144] C2: Without adjusting the pH value, 200 mg / L sodium silicate has a good inhibitory effect on the corrosion of carbon steel in seawater in a short period of time. The corrosion inhibition effect deteriorates significantly with time. In a shorter and longer period of time, the optimized composite formula 1 has a good corrosion inhibition effect on carbon steel in 50°C seawater. After adding the optimized composite formula 1 and increasing the pH value of seawater, the corrosion inhibition effect is better. The optimized composite formula 1 is a mixed corrosion inhibitor composed of polymers PESA, PASP, HPMA, molybdate, citrate and silicate. Among them, PESA, PASP and HPMA are all non-toxic polymers and have good scale inhibition performance. Molybdate, citrate and silicate are all environmentally friendly corrosion inhibitors. The optimized composite formula 1 is a green composite corrosion inhibitor;

[0145] C3: Cathodic protection is used to prevent corrosion of carbon steel in seawater, and the protection effect is good. The optimal protection potential range of carbon steel in seawater is about -0.9V to -1.1V (vsSCE). Compared with tap water, the external voltage required for complete protection of carbon steel electrodes in seawater is smaller;

[0146] For cathodic protection of carbon steel in seawater, impressed current is more appropriate than sacrificial anodes, which have a short service life and must be replaced after consumption.

[0147] IV. Applicable water quality for three anti-corrosion methods: improving pH value, imidazoline and cathodic protection

[0148] According to the water quality characteristics of desalted water, tap water and seawater and the corrosion conditions of carbon steel in them, the static hanging piece weight loss method and electrochemical test method were used to obtain the appropriate anti-corrosion method for carbon steel in 50℃ desalted water, tap water and seawater, as well as the applicable water quality for the three anti-corrosion methods of increasing pH value, corrosion inhibitor and cathodic protection.

[0149] D1: Raising the pH value of desalted water to above 11 can prevent the corrosion of carbon steel in a short time. The pH value of tap water needs to be raised to 12 to prevent the corrosion of carbon steel in a short time. Adding alkali to seawater will continuously produce precipitation. Raising the pH value is only suitable for the corrosion prevention of carbon steel in clean and low hardness water (such as desalted water). Otherwise, it will not work and is not economical. In addition, raising the pH value can only inhibit corrosion in a short time. Over time, corrosion will occur again.

[0150] D2: The imidazoline developed in this application has a good corrosion inhibition effect in desalted water. In desalted water with different concentrations of imidazoline, its corrosion inhibition effect on carbon steel becomes better and better with the increase of imidazoline concentration in a short time. When the imidazoline concentration reaches 60 mg / L, the corrosion of carbon steel in desalted water can be suppressed even for a long time. However, imidazoline has almost no corrosion inhibition effect on carbon steel in tap water and seawater.

[0151] D3: The protective effect of cathodic protection is closely related to the conductivity of the medium. The ion content in desalted water is very low, and the conductivity is very poor. The external driving force required to polarize the carbon steel electrode to the protective potential is very large, which is very uneconomical. Cathodic protection is not suitable for desalted water.

[0152] Cathodic protection is suitable for preventing corrosion of carbon steel in water with a conductivity of more than 200us / cm. Cathodic protection can be used to protect carbon steel in both tap water and seawater. The protection in seawater is better. When the impressed current method is used, there is an optimal protection potential range. When the potential is low, it will not provide adequate protection. Too high a potential will cause over-protection. The conductivity of seawater is very high and the polarization resistance is very small. The impressed voltage required to protect carbon steel in seawater is lower than that in tap water. It is more economical to use impressed current cathodic protection in seawater. Cathodic protection can also be used when there is no more effective and economical anti-corrosion method in tap water.

Claims

1. A method for anti-corrosion of carbon steel for typical water quality in thermal power and nuclear power plants, characterized in that: Analyze the water quality characteristics of desalted water, tap water, and seawater and the corrosion of carbon steel in them, and construct a suitable anti-corrosion method for carbon steel in 50°C desalted water, tap water, and seawater; obtain the law of pH value on the inhibition of carbon steel corrosion in desalted water, tap water, and seawater through the static hanging piece weight loss method, supplemented by electrochemical testing, the corrosion inhibition performance of a certain imidazoline on carbon steel in desalted water, the application conditions and its applicability in tap water and seawater, the applicability of cathodic protection to the corrosion of carbon steel in desalted water, tap water, and seawater, and develop a new green composite corrosion inhibitor that has a certain corrosion inhibition effect on carbon steel corrosion in tap water and seawater; A: Corrosion of carbon steel in deionized water: Increasing the pH value to above 11 can inhibit corrosion in a short time; a certain imidazoline corrosion inhibitor with a concentration of 60 mg / L can inhibit corrosion for a long time; cathodic protection method can prevent corrosion of carbon steel in water with a conductivity of more than 200 us / cm; B: Corrosion of carbon steel in tap water: Increasing the pH value to above 12 can inhibit the corrosion in a short time. The combination of imidazoline and sodium molybdate has a good corrosion inhibition effect on the corrosion of carbon steel in tap water. Cathodic protection can prevent the corrosion of carbon steel in tap water. C: Corrosion of carbon steel in seawater: simply increasing the pH value cannot prevent it. When the pH value is increased, the corrosion inhibition efficiency of the green composite corrosion inhibitor is better. The cathodic protection method can effectively prevent the corrosion of carbon steel in seawater. D: Increasing the pH value is only suitable for short-term corrosion protection of carbon steel in clean water with low hardness, otherwise it will not work or will be uneconomical; the imidazoline developed in this application has a good corrosion inhibition effect on carbon steel in desalted water, but has no obvious corrosion inhibition effect on carbon steel in tap water and seawater; the cathodic protection method is suitable for preventing corrosion of carbon steel in water with higher conductivity, and the anti-corrosion effect of cathodic protection on carbon steel in seawater is more obvious.

2. The carbon steel anti-corrosion method for typical water quality in thermal power and nuclear power plants according to claim 1 is characterized in that: Corrosion resistance model of carbon steel in deionized water with different pH values: In deionized water with different pH values, if the pH value is not adjusted high enough, the corrosion of carbon steel cannot be well suppressed even in a short time. However, if the pH value is adjusted to 11 or above, the corrosion can be suppressed in a short time. Under higher pH conditions, OH - As the concentration increases, OH - The adsorption opportunities and coverage on the carbon steel surface increase, and the adsorption of oxygen and other corrosive ions on the carbon steel surface decreases or is inhibited. When the pH value is not high enough, OH - When the carbon steel surface is not completely covered, the carbon steel will still corrode. When the pH value is high enough so that the carbon steel surface is completely covered by OH, its corrosion can be suppressed in a short time.

3. The carbon steel anti-corrosion method for typical water quality in thermal power and nuclear power plants according to claim 1 is characterized in that: Preventing Carbon Steel Corrosion in Desalted Water: A1: Adjust the pH value to prevent corrosion of carbon steel in deionized water. If the pH value is not adjusted high enough, corrosion cannot be effectively inhibited even in a short time. If the pH value is adjusted to 11 or above, corrosion can be inhibited in a short time. A2: The corrosion inhibitor imidazoline has a good corrosion inhibition effect on carbon steel in desalted water, and as the concentration increases, its corrosion inhibition effect on carbon steel becomes better and better. When the concentration of imidazoline reaches 60 mg / L, the corrosion of carbon steel in desalted water can be suppressed for a long time. Imidazoline is an anodic corrosion inhibitor. Adding an appropriate concentration of imidazoline is better than adjusting the pH value. A3: The ion content in desalted water is very low and its conductivity is poor. It is difficult for the cathodic protection method to polarize the potential of the carbon steel electrode in the desalted water to the protection potential to prevent the corrosion of the carbon steel material in the desalted water. It is not suitable to use the cathodic protection method. The cathodic protection method is suitable for preventing the corrosion of carbon steel in water with a conductivity of more than 200us / cm.

4. The carbon steel anti-corrosion method for typical water quality in thermal power and nuclear power plants according to claim 1 is characterized in that: Corrosion and corrosion resistance model of carbon steel in tap water with different pH values: When the pH value of tap water is adjusted to about 12, the corrosion of carbon steel can be suppressed in a short time. Increasing the pH of tap water can prevent the corrosion of carbon steel and increase the OH - Concentration, increase OH - The adsorption opportunities and coverage on the carbon steel surface can be increased, and the adsorption of oxygen and other corrosive ions on the carbon steel surface can be reduced or inhibited to prevent corrosion. That is, the carbon steel surface will be passivated in tap water with a higher pH value, and the passivation trend will increase with the increase of pH value. In desalted water, corrosion can be prevented by simply increasing the pH value to 11.

5. The carbon steel anti-corrosion method for typical water quality in thermal power and nuclear power plants according to claim 1 is characterized in that: Preventing Carbon Steel from Corroding in Tap Water: B1: Increasing the pH value of tap water can prevent corrosion of carbon steel in a short period of time, but the pH value needs to be adjusted to above 12, which is not economical; B2: No green corrosion inhibitor monomer with good anti-corrosion effect on carbon steel in tap water was found; the combination of imidazoline and sodium molybdate has a good corrosion inhibition effect on carbon steel in tap water, and the corrosion inhibition efficiency reaches 90% when the total concentration reaches 200 mg / L; B3: Cathodic protection can prevent corrosion of carbon steel in tap water. When impressed current protection is used, carbon steel will not be adequately protected when the potential is low, and over-protection will occur when the potential is too high. The optimal protection potential range for carbon steel is about -0.9V to -1.4V.

6. The carbon steel anti-corrosion method for typical water quality in thermal power and nuclear power plants according to claim 1 is characterized in that: Corrosion inhibition of carbon steel in 50°C seawater by optimized composite formula: Optimized composite formula 1, formula 1 is prepared according to PESA 30mg / L + HPMA 6mg / L + PASP 50mg / L + sodium citrate 50mg / L + sodium molybdate 10mg / L + sodium silicate nonahydrate 100mg / L.

7. The carbon steel anti-corrosion method for typical water quality in thermal power and nuclear power plants according to claim 1 is characterized in that: Preventing Carbon Steel Corrosion in Seawater: C1: Adding alkali to increase the pH value of seawater will cause precipitation to continuously form in the seawater. It is not feasible to adjust the pH value of seawater to prevent the corrosion of carbon steel in it. C2: Without adjusting the pH value, 200 mg / L sodium silicate has a good inhibitory effect on the corrosion of carbon steel in seawater in a short period of time. The corrosion inhibition effect deteriorates significantly with time. In a shorter and longer period of time, the optimized composite formula 1 has a good corrosion inhibition effect on carbon steel in 50°C seawater. After adding the optimized composite formula 1 and increasing the pH value of seawater, the corrosion inhibition effect is better. The optimized composite formula 1 is a mixed corrosion inhibitor composed of polymers PESA, PASP, HPMA, molybdate, citrate and silicate. Among them, PESA, PASP and HPMA are all non-toxic polymers and have good scale inhibition performance. Molybdate, citrate and silicate are all environmentally friendly corrosion inhibitors. The optimized composite formula 1 is a green composite corrosion inhibitor; C3: Cathodic protection is used to prevent corrosion of carbon steel in seawater, and the protection effect is good. The optimal protection potential range of carbon steel in seawater is about -0.9V to -1.1V (vsSCE). Compared with tap water, the external voltage required for complete protection of carbon steel electrodes in seawater is smaller; For cathodic protection of carbon steel in seawater, impressed current is more appropriate than sacrificial anodes, which have a short service life and must be replaced after consumption.

8. The carbon steel anti-corrosion method for typical water quality in thermal power and nuclear power plants according to claim 1 is characterized in that: Improve the water quality applicable to the three anti-corrosion methods of pH value, imidazoline and cathodic protection: According to the water quality characteristics of desalted water, tap water and seawater and the corrosion of carbon steel in them, the static hanging piece weight loss method and electrochemical test method are used to obtain the appropriate anti-corrosion method of carbon steel in 50℃ desalted water, tap water and seawater, as well as the applicable water quality of the three anti-corrosion methods of pH value, corrosion inhibitor and cathodic protection; D1: Raising the pH value of desalted water to above 11 can prevent the corrosion of carbon steel in a short time. The pH value of tap water needs to be raised to 12 to prevent the corrosion of carbon steel in a short time. Adding alkali to seawater will continuously produce precipitation. Raising the pH value is only suitable for the corrosion prevention of carbon steel in clean and low hardness water. Otherwise, it will not work and is not economical. In addition, raising the pH value can only inhibit corrosion in a short time. Over time, corrosion will occur again. D2: The imidazoline developed in this application has a good corrosion inhibition effect in desalted water. In desalted water with different concentrations of imidazoline, its corrosion inhibition effect on carbon steel becomes better and better with the increase of imidazoline concentration in a short time. When the imidazoline concentration reaches 60 mg / L, the corrosion of carbon steel in desalted water can be suppressed even for a long time. However, imidazoline has almost no corrosion inhibition effect on carbon steel in tap water and seawater. D3: The protective effect of cathodic protection is closely related to the conductivity of the medium. The ion content in desalted water is very low, and the conductivity is very poor. The external driving force required to polarize the carbon steel electrode to the protective potential is very large. Therefore, cathodic protection is not suitable for desalted water. Cathodic protection is suitable for preventing corrosion of carbon steel in water with a conductivity of more than 200us / cm. Cathodic protection can be used to protect carbon steel in both tap water and seawater. The protection in seawater is better. When the impressed current method is used, there is an optimal protection potential range. When the potential is low, it will not provide adequate protection. Too high a potential will cause over-protection. The conductivity of seawater is very high and the polarization resistance is very small. The impressed voltage required to protect carbon steel in seawater is lower than that in tap water. It is more economical to use impressed current cathodic protection in seawater. Cathodic protection can also be used when there is no more effective and economical anti-corrosion method in tap water.

Images