In order to meet the growing demand for electrical energy in the 21st century, China's thermal power generation is moving toward high-parameter, high-capacity units. With the improvement of the unit parameters and the enhancement of heat transfer, the corrosion inside the boiler is obviously intensified. Corrosion of the water wall is the main cause of water wall leakage and bursting. Furnace water and its steam are the medium in which the water wall tubes are in direct contact, and their properties are directly related to the corrosion of the water wall. There are many reports on the causes of water wall bursting accidents at home and abroad, but there are few reports on the nature of boiler water and the influence of trace components in furnace water on corrosion. In order to clarify the influence of trace component dissolution and temperature on the corrosion of water wall, this study simulates the phosphate water treatment condition. The water wall material is made of carbon steel No. 20, and the simulated coupon corrosion test is carried out in the autoclave. The corrosion rate is weight. Method; surface analysis using electron probe (EPMA), X-ray diffraction (XRD); find the corrosion law, provide the basis for the optimal conditions of on-site furnace water control.
The theoretical value in Table 1 is the data obtained under the equilibrium of two phases of pure water and water vapor. The experimental values ​​are simulated vapor pressure data obtained during the very slow heating of the autoclave. As the temperature increases, the difference between the theoretical value and the experimental value increases exponentially. As the temperature approaches the critical point, the vapor pressure rises linearly with temperature. The deviation between the real side value and the theoretical value is increasing. The main reason is that the measured value is obtained under the unidirectional heating process, that is, under unbalanced conditions, and a trace amount of salt is dissolved in the water. Due to the variation of the vapor pressure of water with temperature, other properties of water have similar changes. Such as the density of water, the surface tension of water, the dielectric constant of water and so on. As the temperature increases, the density of water decreases (equal to water vapor at the critical point); the surface tension of water decreases (zero at the critical point of water); the dielectric constant of water also decreases (at the critical point with water) The steam is equal). These changes in properties cause changes in the properties of the minor components. For example, the vapor pressure is increased from 14 MPa to 22 MPa, and the partition coefficient of trisodium phosphate in the gas-liquid two phases is increased by 4 times, and the salt carrying amount is also increased by 4 times. Another example is the dielectric constant of water from 81 at 25e to 514 at the critical point, that is, some organic solvents when water is similar to 25e near the critical point. The dielectric constant of a medium indicates its ability to store or hold a charge, and the dielectric constant of water indicates their ability to hold ions. For furnace water, when it is close to the critical point, the solubility of salt in the furnace water is greatly reduced, and the ion content is lowered; this indicates why the solubility of some substances (such as phosphate) in the high temperature furnace water decreases with increasing temperature. Therefore, these salts are either transferred to the gas phase (such as chloride) or deposited on the surface of the furnace tube (such as phosphate, carbonate, sulfate, etc.) as they approach the critical point. Therefore, the water quality of high-parameter boiler water must be strictly monitored. Otherwise, it will deposit on the water wall tube and cause scale corrosion. If the salt is transferred to the gas phase, it is condensed and concentrated on the turbine blades, causing corrosion to the turbine blades. Therefore, the salt content in the boiler water must be strictly controlled. This is also the main reason for the use of low-phosphate or ultra-low-phosphate water treatment conditions for subcritical steam drum furnaces at home and abroad. Then, under this water condition, the temperature of the furnace water and the effect of dissolved O on the corrosion of the water wall are discussed below.
2 Corrosion test of dissolved oxygen on water wall 211 Test method and test piece treatment Carbon steel corrosion test in high temperature furnace water is usually carried out by static simulation using an autoclave. The simulated furnace water is poured into a clean kettle body, and the pre-treated carbon steel test pieces are suspended in water with nickel-chromium wire. After nitrogen and oxygen removal, the temperature is slowly increased, and the temperature is set to a constant temperature for several hours (t). After the test was completed, the mixture was cooled to room temperature, and the test piece was taken out, immediately rinsed with demineralized water, and then washed with alcohol and acetone in a desiccator. Weigh (W) after the same time interval as the pretreatment. Simultaneous determination of total iron concentration in water (C in order to ensure good reproducibility of the test results, custom carbon steel test strips need to be treated on the surface. Sanding on the flat plate, followed by 200 mesh, 400 mesh, 600 From coarse to fine, the direction of sanding changes 90b when changing sandpaper, and each time it is polished until the previous grinding marks are not visible. Then use absorbent cotton, rubbing alcohol and acetone to remove oil, and wrap it in brightener with bright paper. After a certain period of time, the surface area was weighed and measured. (Measurement of dissolved oxygen in water in W212 autoclave and measurement of dissolved oxygen were carried out using a SJG-203A dissolved oxygen meter.
Because the oxygen concentration after passing the N in the autoclave test is not good, the relationship between the N-pass time and the residual oxygen concentration is measured before a batch of corrosion test, so that the nitrogen-passing time and flow rate can be controlled in the test. Determine the residual oxygen concentration. The dissolved oxygen measurement is carried out under dynamic conditions, and the dissolved oxygen is directly related to the temperature, the flow rate of N, the shape of the container, and the like.
213 The surface of the test piece is made to increase the weight of the test piece due to oxygen participating in the reaction, and the iron is dissolved in water to lose weight of the test piece. Therefore, W represents the weight of oxygen entering the surface, and W represents the weight of iron dissolved in water. The iron concentration C in the simulated furnace water was measured as the iron dissolution rate. Using an electron probe for surface analysis, the relative mass percentage (W/mg/L) of oxygen in the membrane was determined to approximate the oxidation rate.
The corrosion rate of carbon steel in water keeps rising with the increase of dissolved oxygen; that is, the oxidation rate increases; the iron ion concentration changes little with the increase of dissolved oxygen when the dissolved oxygen is low (within 01040mg/L) (at 35mg/L) The oxide film formed is dense and protective, that is, the dissolution rate is low. Then, the increase of dissolved oxygen increases rapidly, that is, the iron dissolution rate is faster, because the oxidation rate of iron is faster when the dissolved oxygen concentration is higher, and the formed oxide film has large pores and is not dense. Carbon steel is not completely isolated from the media. Therefore, the N time should be controlled at 40 min during other corrosion tests. It is not necessary to pass too long, and all dissolved oxygen is removed.
Corrosion test results of 3pH to 20A carbon steel From the thermodynamic data, the quantitative relationship between pH and Fe ion concentration can be derived from LgFe(300e). It is clear from this formula that the pH rises and the Fe concentration decreases. However, since iron is over-plated, its oxide (or hydroxide) is amphoteric, the pH is too high, iron oxide cannot be formed, and even the formed oxide is dissolved, so that the corrosion accelerates, iron The concentration increases. The concentration of Fe directly indicates the degree of corrosion of iron, so it is important to control the corrosion rate of iron by controlling the pH. The optimum pH of the high temperature furnace water should be controlled in what range. For this purpose, a set of corrosion tests at different pH values ​​is designed. The test conditions are in addition to pH.
The N-time dissolved oxygen concentration corrosion rate surface state film is darker and has a dot. After 40 minutes of nitrogen passing, the dots become less and less, but become larger, and the dots brighten under illumination. The oxygen content in the dots is slightly lower than the oxygen content in the outer membrane. No. No. 14 gas liquid surface condition 14 liquid film is more uniform, dense, blue. The gas phase film is also relatively uniform but the film is dark gray. The film of the No. 5 test piece is thin and some areas are bright, as if there is no film.
1 pH is in the range of 912,919, V>0, indicating that the oxidation rate is greater than the dissolution rate of iron, and the iron concentration is small. When the pH was 1,310, the dissolution rate of iron suddenly increased to 327 Lg/L, at which time the test piece lost weight. The opH is in the range of 9121310, and the weight gain of the test piece in the gas phase is larger than that of the liquid phase test piece. It indicates that the oxidation rate in the gas phase is larger than that in the liquid phase, and the effect of pH on the gas phase weight gain rate is smaller than that in the liquid phase. This is because the pH directly affects the liquid phase test piece, and the gas phase test piece is evaporated into water vapor by water. The alkaline substance brought out works. Or the distribution of NaOH in the liquid and gas phases is different. That is, the distribution in the gas phase is less. The pH is between 10109120 and the dissolution rate is small. When pH=13, elemental Fe is transferred to water and corroded due to the formation of solubility. Therefore, pH control is more suitable at 91010.
Corrosion test of 4Cl ion on carbon steel The result of high temperature furnace water Cl promotes corrosion of carbon steel. Therefore, countries have strict regulations on Cl in water under different water treatment conditions. US EPRI balanced phosphate control Cl-99 stipulates the subcritical steam drum furnace phosphate working condition Cl subcritical steam drum furnace water under phosphate working conditions, the amount of aggressive Cl concentration should be controlled, which is the purpose of this test. Corrosion tests were carried out on 20A carbon steel under different chloride ion concentrations, respectively indicating that the corrosion rate of the test piece in the gas phase and the liquid phase indicates the relative oxygen content in the surface film of the liquid test piece.
From the test results, it can be obtained that: 1 When the temperature is constant, the corrosion rate of the carbon steel test piece in the furnace water increases with the increase of the C concentration. When C>014mg/L, the upward trend is very obvious. o As C increases, the surface oxygen content decreases and the oxidation rate decreases. The number of pitting on the surface of the test piece increased significantly. The EPMA secondary electron image of the surface film of the liquid phase test piece shows that when C=012mg/L, the surface state is similar to that of C; the C etch point is large and small; C-1, the etch point is small and dense, and the film is uneven. . Therefore should be controlled below 012mg / L.
1SO on the corrosion of 20A carbon steel in water vapor, when the increase of C increases the oxidation rate; when C decreases. The corrosion of 20A carbon steel in the liquid phase increases with oC, the weight gain rate decreases significantly, and the dissolution rate of iron increases. From the surface state analysis, C has no obvious effect on the gas phase film formation, but has a great influence on the liquid phase film formation. As the concentration increases, the number of etch points increases, and the etch point becomes deeper and denser. The effect of SO on the corrosion rate of carbon steel is similar to that of Cl. The control range can be relaxed below C as appropriate. The analysis of the No. 1 corrosive test solution by ion chromatography revealed that the analysis was mainly caused by the Cl solution originally adsorbed on the wall of the autoclave.
6 Temperature vs. carbon steel corrosion test results According to the general rule of chemical kinetics, the temperature is increased and the corrosion reaction rate is increased. In the operation of the subcritical unit, the water in the water wall tube works near its critical state, and the temperature of the water wall in the high heat load area to the fire side wall is likely to reach the critical point temperature. At present, the existing autoclaves in China are difficult to reach such high temperature and high pressure. For this purpose, a set of corrosion tests under different temperatures (or pressures) are designed to obtain corrosion information, find out the law, and then estimate the water wall (carbon steel) near the water critical point. The surface of the corrosive liquid has no corrosion point on the 13th gas phase film. The liquid film has few bright (etching) points at the lower end of the test piece. From the 4th, there are black spots in the middle of the bright spots in the liquid film, and white spots in the black spots. These eclipses increase with the increase of Cl concentration, the membrane is removed, pinholes appear in the original black spots, the Cl concentration is high, and the needles are empty.
Corrosion test results for 20A carbon steel No. Surface state The gas phase film is uniform, dark gray, and the liquid film black has an etch point. Just like the film detachment, the density of the etch point increases with the increase of the sulfate concentration, and the black spots in the etch point increase. The film color becomes lighter and thinner. When the number of temperature / e surface state is 150200e, the surface of the test piece is yellow, red, brownish yellow, and some areas have blue black and have eclipses. When the temperature is higher than 250e, the surface film is black, and the etching point is small with the increase of temperature, but the liquid phase test spot becomes round and large. It can be seen from the data that the dissolution rate of 1 carbon steel decreases with increasing temperature. The dissolution rate of carbon steel near the critical point of the furnace water is the smallest.
This is because the temperature rises, the dielectric constant of water decreases, the solubility of Fe(OH) decreases, and Fe(OH) is continuously deposited on the surface of the test piece, that is, the temperature is high, which is favorable for film formation, and the film has certain protection. o The increase rate of the rate of increase in the low temperature zone and the high temperature zone is different. There is a transition zone around 200e. The surface state of the test piece is obviously yellow, red and brownish yellow in the low temperature zone, and the blue area of ​​the test piece is blue. The film is thin and uneven, and has an etch point. At high temperature, the film becomes dark, black, uniform, and has few eclipses. The X-ray diffraction analysis of the black film of 360e was carried out. It can be seen that the black film is mainly Fe7. Based on the analysis of the properties of the high-temperature furnace water, the effect of the trace component O in the furnace water and the temperature on the corrosion of the water wall is tested. The results show that: oxygen is below 01040mg / L, iron ion concentration is below 35mg / L, trace dissolved oxygen in furnace water is beneficial to the formation of protective film; Cl is the main cause of pitting corrosion of 20A steel, C corrosion of carbon steel Cl is similar. The control range can be appropriately relaxed than Cl. The C drum pressure is controlled at 310 MPa. When the drug is added, the drum pressure must be lower than 10 MPa, which is higher than 3 MPa. During the dosing process, the protection drum pressure is between 310 MPa.
2.3.3 Keeping time Dosing time is about 2h, in order to facilitate the uniform distribution of BW in the furnace water and promote the formation of protective film. After the end of the dosing, the control steam drum pressure is between 310 MPa, and the main steam temperature is cycled below 480 e for 2 h to further ensure the uniform distribution of the chemical liquid in the furnace water and improve the protection effect. Maintained between 9.610.2, because the pH of the boiler water drops slightly after dosing, so the pH is slightly higher than the running time. The pH of the boiler water must be adjusted to 9.610.2 before dosing.
2.3.5 Operation control When the unit adopts the slide stop protection, before the dosing, the continuous sewage should be turned off and the pH adjusted. Then, when the steam drum pressure and the main steam temperature meet the requirements, the dosing is started, and the water supply is analyzed as required. Furnace water, condensed water, saturated steam, pH of superheated steam, copper, iron, phosphate, etc. After the drug is added, the cycle is 2h. Then, according to the operation method of the sliding stop parameter protection, the furnace is stopped. After the unit is out of service, the protection is over. When the unit is protected by a furnace, the dosing is started and the water quality analysis is carried out. After the medicine is added, it is circulated for 2 hours. Then, according to the hot water discharge and the residual heat drying step, the furnace is shut down. After the unit is out of service, the protection will end.
3 Features of the new method of BW deactivation protection 3.1 Good protection effect The film-forming corrosion inhibitor can not only form a uniform, dense and corrosion-resistant protective film on the surface of boilers, steam turbines, etc., but also shorten the unit when it starts again. Net time, generating significant economic benefits.
3.2 Wide protection range not only protects the boiler body (including steam drums, water wall, economizer, superheater, reheater, etc.), but also protects the turbine.
3.3 Protection time After the surface of the boiler, steam turbine and other equipment is formed into a film, the corrosion time in the atmosphere can reach more than 6 months. The film-forming corrosion inhibitor BW is non-toxic and easily decomposed, and can be dissolved in the demineralized water at room temperature (without any heating).
3.4 The protection process is simple and convenient. The BW film-forming corrosion inhibitor can be added to the furnace by using the on-site feed water plus oxygen scavenger system, without adding a new dosing system. The addition and film formation of the BW film-forming corrosion inhibitor into the furnace can be added and maintained at a certain temperature and pressure for a period of time at a certain temperature and pressure during the sliding process of the unit.
4 Problems to be paid attention to during maintenance 1) Since the BW liquid is in contact with ammonia or hydrazine, it will become viscous, block the pipe, and the liquid cannot be discharged, resulting in maintenance failure. Therefore, before dispensing, the dosing pump and pump buffer tank, temporary medicine box and dosing pipe of the dosing system are thoroughly rinsed with demineralized water.
2) Not only can the chemical vapor removal system be filled in the water vapor system during the maintenance process, but also the water vapor system in the unit should not be flushed after the unit is shut down. So as not to dissolve the already formed film.
3) The concentration of the drug solution is appropriate. The temperature rises and decreases. The high temperature is favorable for uniform film formation, and the main component of the surface film of the 360e test piece is Fe4. The dissolution rate of the carbon steel near the critical point is the smallest.
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