Corrosion and protection in urea production

A company has built a melamine tail gas co-production urea unit (aqueous solution full cycle method) with an annual output of 110kt urea. The main raw material is the tail gas generated during the production of melamine. This is the first case in China. The traditional urea production process raw materials must be added with fresh CO2. The biggest feature of this set of equipment is that there is no addition of fresh CO2. The mass fractions (design values) of each component in the exhaust gas are respectively: CO2 36.35%, NH3 43.65%, H2O 20%. The function of the high-pressure ammonia preheater is to preheat high-pressure ammonia, reduce the load of the methylammonium preheater, so that methylammonium (NH2COONH4) can be fully decomposed before entering the urea synthesis tower, and provide favorable conditions for the synthesis of NH3 and CO2 into urea (CO(NH2)2) in the urea synthesis tower. Two years after the device was put into operation, the high-pressure ammonia preheater leaked frequently, which affected the normal production and required timely resolution. 1 Introduction to high-pressure ammonia preheater The high-pressure ammonia preheater preheats the ammonia at the outlet of the high-pressure ammonia pump from about 40°C to 150°C, mixes it with methylammonium, and heats it again in the high-pressure methylammonium preheater. The methane ammonium decomposes into NH3 and CO2 and enters the urea synthesis tower together with the high-pressure NH3 from the high-pressure ammonia preheater. The urine produced by the co-production urea plant is sent to large urea plants for granulation or used as raw material for melamine plants. The high-pressure ammonia preheater is a horizontal U-shaped tube heat exchanger with a heat exchange area of ​​72 m2. The tube and tube plate connection method is strength welding + expansion. ； The main gasket material is outer L4+inner ring PTFE/0Crl8Ni9+graphite, the tube box material is 16MnR, the tube plate material is 16MnRⅣ, and the heat exchange tube material is 20 (100 pieces). management: Step 4, the process medium is liquid NH3, the design pressure is 21.9 MPa, the design temperature is 200℃, the working temperature is 40℃ (inlet), 150℃ (outlet) ； shell process: In process 1, the process medium is steam, the design pressure is 2.55 MPa, the design temperature is 245°C, and the working temperature is 219°C. 2 Leakage and Cause Analysis 2.1 Leakage Hazards Once the high-pressure ammonia preheater leaks, it will have an impact on the normal operation of production. (1) The leaked NH3 enters the steam condensate pipe network, and the contaminated steam and steam condensate cannot be recycled. At 100% load, 55 t/h of steam condensate must be discharged on site. (2) The high-pressure ammonia preheater leaks, the amount of NH3 added into the urea synthesis tower is insufficient, the ammonia-to-carbon ratio is too low, and the synthesis conversion rate decreases, which increases the circulation volume of the system, leading to a vicious cycle, and the water-to-carbon ratio of the urea synthesis tower will increase. A high water-to-carbon ratio will promote the decomposition of ammonium cyanate and produce more corrosive cyanate radicals, which will reduce the pH value of the medium and increase the corrosiveness of the medium to high-pressure equipment. (3) According to the synthesis reaction of NH3 and CO2, the ratio of the added NH3 and CO2 substances should be 2, but at this time the medium will be very corrosive, and excess NH3 needs to be added during production to inhibit corrosion. Excessive NH3 can partially neutralize the acidity of the urea carbamate solution, increase the pH value, inhibit the production of highly corrosive cyanide radicals ((CNO)-), and reduce corrosion caused by the presence of water. Insufficient NH3 is added to the inlet of the urea synthesis tower, resulting in a low ammonia-to-carbon ratio, which will increase the corrosiveness of the urea synthesis tower. 2.2 Leakage situation: The tubes of the high-pressure ammonia preheater were blocked three times and 5 tubes were blocked (Figure 1). The first leakage site was inside the tube, and the second leakage site was at the mouth of the tube. http://www.nmtech.com.cn/jishuwang/upload1/0709141710208431.jpg The last time you opened it to check the status: (1) The pipes in the 4th pass are severely corroded, while the leakage in the 1st pass is serious. (2) There are methylammonium crystals at the outlet of the 4th pass tube (that is, the outlet of NH3) (as shown in Figure 2), which indicates that the two check valves at the outlet of the high-pressure ammonia preheater are also corroded. When the high-pressure ammonia preheater tube leaks, the pressure suddenly drops, and the outlet check valve does not work. Methyl ammonium pours into the outlet of the high-pressure ammonia preheater. This is also the reason for the serious corrosion of the 4th pass tube. http://www.nmtech.com.cn/jishuwang/upload1/0709141710537714.jpg (3) The partition plate between the 1st and 4th pass tubes is severely corroded (as shown in Figure 3), causing part of the high-pressure NH3 to enter through the inlet and then go through a short circuit without being heated by the preheater. It is directly mixed with the high-pressure ammonium methane liquid from the high-pressure ammonium methane preheater through the 4th pass outlet. http://www.nmtech.com.cn/jishuwang/upload1/0709141711243155.jpg 2.3 Cause analysis: Overtemperature at the top of the medium-pressure absorption tower (operating indicators: ≤44℃), CO2 frequently flows up, polluting the ammonia system, and the high-pressure ammonia preheater enters methylammonium, which is highly corrosive under high temperature and high pressure conditions. At present, there are two main theories about methylammonium corrosion:: (1) Corrosion of carbamate (COONH2)-. Japan's Mitsui Toka Co., Ltd. and the Netherlands' Stamica Company believe that the carbamate radicals dissociated from the ammonium carbamate solution in water are reducing and can prevent the formation of oxide films on the surface of passivating metals, causing metal corrosion. The corrosiveness of the medium increases with the increase of methane ammonium content. This is an obvious fact in production, so the explanation of this corrosion mechanism is relatively easy to be accepted by everyone. (2) Corrosion of cyanoxylate. This view believes that the corrosiveness of urea carbamate solution is due to the fact that urea will produce the isomeric ammonium cyanate (NH4CNO) under high temperature and pressure. In the presence of water, ammonium cyanate can dissociate to produce cyanate radicals. The cyanoxylate radicals have strong reducing properties, making it difficult for passivating metals to form a passivation film in it, resulting in serious activation corrosion. When the load of the melamine plant is reduced, the NH3 content in the exhaust gas is high. To control the urea synthesis tower, it is necessary to adjust the amount of NH3 entering the inlet. When adjusting the load of the high-pressure ammonia pump, it is easy to jump due to high temperature of the electrical heat sink and overload, causing the NH3 in the high-pressure ammonia preheater to come and go, causing sudden cooling and sudden heating. In addition, the high-pressure ammonia preheater is not sufficiently preheated before it is put into operation. After feeding, in order to ensure that the temperature of the urea synthesis tower rises too fast, it is subject to the effect of temperature difference stress, which will inevitably cause leakage over a long period of time. The leakage of the high-pressure ammonia preheater head and first-stage tubes is mainly caused by this reason. 3 Countermeasures (1) Because if NH3 is not added to the inlet of the urea synthesis tower, a low ammonia-to-carbon ratio will aggravate the corrosion of the urea synthesis tower, so the load of the melamine unit is reduced to reduce the impact of the load of the co-production urea unit on corrosion. Usually the methods to adjust the ammonia-to-carbon ratio of the urea synthesis tower are:: Increase the amount of NH3 added to the raw material (melamine tail gas, that is, the amount of NH3 fed into the melamine reactor) ； Increase the load of urea high-pressure ammonia pump ； Increase the amount of reflux NH3 in the urea medium pressure absorption tower. At this time, when the high-pressure ammonia preheater cannot be put into use, the NH3 content in the melamine tail gas can only be adjusted to a slightly higher level (that is, the amount of NH3 added to the melamine reactor is increased), and the reflux NH3 of the medium-pressure absorption tower is increased to reduce the corrosion rate of the urea synthesis tower. However, if the ammonia-to-carbon ratio of the urea synthesis tower is increased in this way, the high-pressure methane-ammonium pump will easily vaporize due to the increased NH3 content in the imported material. At this time, the inlet of the high-pressure methane-ammonium pump must be frequently added with water. In this abnormal situation, the CO2 conversion rate of the urea synthesis tower is already low. Adding water at the inlet of the high-pressure methane-ammonium pump causes the conversion rate to drop even lower. The water circulation volume of the entire system increases, and the system enters a vicious cycle. At the same time, this is detrimental to the corrosion resistance of the urea synthesis tower. The temperature of the medium and low pressure decomposers can be slightly lowered to allow more water to flow toward the urea preconcentration, thereby reducing the amount of water circulation in the urea system. (2) Add water and replace the high-pressure ammonia preheater for pressure test and hand it over to check for leaks. The maintenance personnel will perform maintenance and adjust the system to maintain operation. (3) Considering that the high-pressure ammonia preheater has a high probability of entering methane ammonium during operation, and the existing head gasket material of the preheater has poor resistance to methane ammonium corrosion, it was replaced with a gasket made of 316L material. (4) Considering that the high-pressure ammonia preheater is frequently inspected and put into use, this is very detrimental to the operation of the equipment itself and will increase the risk of leakage. In addition, because methylammonium is highly corrosive only in high-temperature and high-pressure environments, the high-pressure ammonia preheater is only put into use NH3, and no steam is put into the shell side. Cold NH3 is used to preheat through high-pressure methane. device to prevent high-temperature corrosion due to the channeling of methylammonium from the CO2 in the medium-pressure absorption tower into the high-pressure ammonia preheater. However, in this way, we cannot rely on the outlet temperature of the high-pressure ammonia preheater to judge whether the tubes will still leak. We can only rely on whether the explosion-proof plate on the shell side of the high-pressure ammonia preheater explodes (frost will form on the sealed vent line after the explosion-proof plate explodes). ； The shell side of the high-pressure ammonia preheater is normally opened and poured backward. If the tube leaks, NH3 will emerge during the backward pouring. Since the second method is easier to find, the second method is adopted. The impact of cold NH3 entering the high-pressure ammonia preheater on the urea system in a short period of time before the new equipment of the high-pressure ammonia preheater arrives.: Increased steam load of high pressure ammonium methane preheater ； Cold NH3 directly enters the ammonium methane preheater, which increases the temperature difference stress on the methane ammonium preheater and the preceding mixer, increasing the risk of leakage. Other than that, there is not much impact. For details, see Table 1. Process analysis data of the urea synthesis tower before and after the high-pressure ammonia preheater cold NH3 enters. The presence of iron does not have a great impact on the corrosion of the urea synthesis tower. Generally, the mass fraction can be controlled within 10×10-6. ； The increase in the nickel content of the urea synthesis tower is detrimental to its corrosion resistance. This adverse effect is even worse under anoxic conditions, but under normal oxygen conditions, this adverse effect is compensated because it can evenly distribute chromium. The urea synthesis tower cannot cut off the passivation air, so the urea synthesis tower can still withstand the above nickel content. (5) If the high-pressure ammonia preheater leaks again, consider adding a bypass line to temporarily maintain system operation. (6) Considering that during operation, the high-pressure ammonia preheater has a high probability of entering methane and ammonium, and the equipment is severely corroded, and the existing material of the high-pressure ammonia preheater cannot resist methane corrosion, a high-corrosion-resistant equipment (the overall material is 316L) is prepared for replacement. Before the new equipment arrives, pay close attention to the analysis of the iron and nickel content of the urea synthesis tower to prevent the ammonia-to-carbon ratio of the synthesis tower from being too low and aggravating corrosion. (7) Make the main pipe of the tray flushing water of the medium-pressure absorption tower thicker and move all the valves to the bottom of the tower to reduce labor intensity and increase the flushing effect. It takes more than 0.5 hours to flush the tray each time and the temperature through the tower rises to about 100°C to achieve the effect. Strictly control the temperature at the top of the medium-pressure absorption tower to prevent CO2 from passing up and polluting the ammonia system. (8) The liquid level in the medium-pressure absorption tower should not be too high to prevent the gas phase from carrying liquid and causing CO2 to escape into the ammonia system. (9) When adjusting the load of the high-pressure ammonia pump, always contact the electrical personnel to check the pump current to prevent frequent tripping due to overheating of the radiator, and try to stabilize the ammonia-to-carbon ratio and water-to-carbon ratio of the urea synthesis tower. http://www.nmtech.com.cn/jishuwang/upload1/0709141712289966.jpg

There are approximately 182 aqueous solution full cycle urea units in our country. Although these units were built in different years and have different process flows, most of the medium-pressure systems have now been changed to pre-separation-pre-distillation processes. The operation conditions are generally good. However, some plants in Shandong, Shanxi, and Henan have corrosion of pre-distillation towers, and the corrosion is very serious and shocking. Therefore, it cannot help but attract our attention. The author is willing to express his opinion on this issue and discuss it with colleagues in the industry. ?1 Corrosion situation of pre-distillation tower? The original designed production capacity of a certain device was 180t/d, which was later modified to 260~270t/d, and then to 350t/d. The pre-separation-pre-distillation process is adopted. The pre-separator is φ1200mm, V=4m3, and the installation height is 11.7m (the vertical distance from the pre-separator outlet to the urine inlet of the pre-distillation tower). The pre-distillation tower is φ1100mm, H 8500mm, and contains 5 bubble cap trays, with 41 bubble caps on each layer. When the production capacity is 260-270t/d, no corrosion of the pre-distillation tower is found. After the production capacity was increased to 350t/d, it was found that the pre-distillation tower was corroded, and the metal on the upper wall of the corroded tray lost its luster and turned white. ； The tray is corroded, especially the bubble cap. It needs to be replaced every 7 to 10 days. The tray is yellow. The urine pipeline from the outlet of the pre-distillation tower to the first heater needs to be replaced every 7 days. ； The Ni content of finished urea is (0.7~1.2)×10-6. Due to the corrosion of the pre-distillation tray, liquid leakage occurred, which lowered the temperature of the second-stage decomposition to 153°C. It was calculated that the decomposition rate of methylammonium in the medium-pressure decomposition system dropped from the original 86% to 60%, and the total ammonia distillation rate dropped from the original 78% to 54%. This also affected the operation of the second-stage decomposition system. ? 2 Analysis and preventive measures for corrosion ? 2.1 Oxygen deficiency is an important cause of corrosion? The pre-separation-pre-distillation process is a big technological progress compared with the pre-separation process. It is beneficial to the system water balance, increases the heat energy recovery amount of the heat utilization section of the evaporator, and increases the production capacity of the heater. However, when applying the pre-separation-pre-distillation process, while the urine from the urine tower is separated into NH3, CO2, H2O and other gases in the pre-separator, oxygen is also separated from the urine. When developing the pre-separation-pre-distillation process, the corrosion phenomenon of the pre-distillation tower was discovered. Later, a method was adopted to supplement oxygen from the outlet of the pre-distillation tower to the urine pipeline of the first heater to effectively control the corrosion of the pre-distillation tower. The existing production equipment has a large production capacity, and after the pre-separator is increased in height, the separation effect is good. NH3, CO2, and H2O in the urine are desorbed. At the same time, more oxygen in the urine is also desorbed, resulting in the oxygen content in the urine being less than

In the urea production process, the corrosion problem of urea high-pressure vessels has always been the focus of equipment maintenance and management. It is particularly important to use appropriate detection methods to monitor corrosion conditions, and provide a reasonable original basis for formulating maintenance and repair plans through analysis and processing, so as to achieve effective suppression of corrosion. At present, domestic large-scale urea units are mainly of two types: STAC (CO2 stripping method) and SNAM (NH3 stripping method). In addition, there is also an ACES unit (energy-saving type, similar to CO2 stripping method). In recent years, our company has carried out continuous tracking and testing of urea high-pressure vessels in these three types of devices. Now we will make a summary of the corrosion testing situation. 1 Urea synthesis tower Among the three types of devices, regardless of equipment structure or corrosion characteristics, the most common equipment is the urea synthesis tower, see Table 1. Table 1 Equipment overview of urea synthesis tower Device type Corrosion-resistant layer welding material ACES head: Strip welding, type 25-22-2, δ=8mm barrel section: Loose lining, 316L-UG type, δ=8mm 25-22-2 type STAC head: Strip welding, type 25-22-2, δ=8mm barrel section: Loose lining, 316L-UG type, δ=10mm 25-22-2 type STAC integral loose lining, 316L-UG type, δ=10mm 19/15H type SNAM integral loose lining, 316L-UG type, δ=5mm 19/15H type 1.1 Determination of ferrite content of weld seam 1.1.1 The original value of ferrite content is related to the type of welding material used in the manufacturing process. For welds where the welding material is 25-22-2, the maximum original value of ferrite content is 0.28%, the minimum is 0, and the average value does not exceed 0.1%. ； For welds with 19/15 welding materials, the original ferrite content has a maximum original value of 1.1%, a minimum of 0.1%, and an average of no more than 0.6%. 1.1.2 The relationship between the ferrite content of the weld and the corrosion status of the weld. On the premise that no repair is carried out, the development status of the corrosion of the weld has little relationship with the original value of the ferrite content of the weld in the same part. ； For different parts, the original value of ferrite content in the weld is related to the corrosion degree of the weld meat itself. 1.1.3 Analysis of the reasons why the original value of ferrite content exceeds the standard. The original value of ferrite content exceeds the standard. It is not due to the wrong use of welding materials, because the excessive value does not affect the entire welding line, and the ferrite content does not exceed 2%. ； Rather, the manufacturing or repair process is not strictly controlled, and impurities appear in the surface deposited metal due to grinding or welding process problems. Table 2 shows the measurement results of weld ferrite content. Table 2 Measurement statistics of ferrite content of weld seam of urea synthesis tower Welding material detection time Maximum value/% Minimum value/% Average value/% Repair situation description 19/15H2002 1.10.10.35 Partial repair exceeding 0.6% in 2003 0.5 0.1 0.28 Repair of cause-free ferrite exceeding the standard in 2004 0.48 0.1 0.26 Repair of cause-free ferrite exceeding the standard 25-22-2 2002 0.26 0 0.08 Repair of cause-free ferrite exceeding the standard in 2004 0.28 0 0.08 Repair of cause-free ferrite exceeding the standard 1.2 Fixed-point thickness measurement of the corrosion-resistant layer 1.2.1 Longitudinal comparison of the strip cladding layer made of 25-22-2 from a single manufacturer: The local maximum corrosion rate is 0.08mm/a, and the local minimum corrosion rate is 0.02mm/a. ； The average corrosion rate is not greater than 0.05mm/a (instrument measurement accuracy is 0.01mm). Lining material: 316L-UG: The local maximum corrosion rate is 0.30mm/a, the local minimum corrosion rate is 0.06mm/a, and the average does not exceed 0.15mm/a (instrument measurement accuracy is 0.10mm). The measurement results of a certain factory are shown in Table 3. Table 3 Statistics of the corrosion rate of the corrosion-resistant layer of a factory's urea synthesis tower. Time corrosion rate/mm/a. Local maximum value. Local minimum value. Average value. Head end with pole overlay welding straight section lining. Gas-liquid interface. Head end with pole overlay welding. Straight section lining. Gas-liquid interface. Head end section. 520000.060.20.30.030.10.10.030.110.1420020.070.270.290.040.1 20.130.050.140.1520040.070.180.250.040.060.090.050.080.121.2.2 The corrosion rate of the strip cladding layer made of 25-22-2 material from relevant manufacturers is relatively consistent, and the average corrosion rate does not exceed 0.05mm/a. The linings made of 316L-UG all have local large thinning areas to varying degrees, and the corrosion rates vary greatly. The average corrosion rate is between 0.07 and 0.15mm/a, see Table 4. Table 4 Comparison of corrosion rate of corrosion-resistant lining of urea synthesis tower Time corrosion rate/mm/a Local maximum value Local minimum value Average value Factory A, Factory B, Factory C, Factory A, Factory B, Factory C, Factory A, Factory B, Factory C 2002 0.290.30.30.130.00.10.140.070.152003/0.20.2/0.00.0/0.080.082004 0.250.30.40 .060.10.00.080.100.091.2.3 Corrosion status of the corrosion-resistant layer (1) The corrosion rate of the corrosion-resistant layer is closely related to the type of corrosion-resistant material and the quality of its surface passivation film formation ； (2) The corrosion thinning status of the corrosion-resistant layer. The average corrosion rate value can only be used as a general reference. The maximum local corrosion rate is the more important data. ； (3) The areas with severe lining thinning are usually near the gas-liquid interface and the weld HAZ zone. ； (4) Due to the existence of severe local thinning, the situation of lining corrosion and thinning is not optimistic. 1.3 Macro-corrosion inspection 1.3.1 Internal parts (1) The welds of the central downcomer all have corrosion and perforation defects of varying degrees. ； It is common for metal particles to fall off the base material of downcomers made of 316L-UG material, and some manufacturers have obvious intergranular corrosion defects, as shown in Figure 1. (2) Compared with other internal parts, the tray lugs are severely corroded. In addition to the obvious local end grain corrosion characteristics, some manufacturers have experienced fillet weld cracks/pinholes and even lugs being corroded and falling off, as shown in Figure 2. (3) Some hook head bolts are severely corroded due to inconsistent material properties, see Figure 3. http://www.nmtech.com.cn/jishuwang/upload/0603031402268849.jpg http://www.nmtech.com.cn/jishuwang/upload/0603031404139001.jpg Figure 1 Corrosion of the central downcomer of a certain plant Figure 2 Corrosion of the lugs of a certain plant http://www.nmtech.com.cn/jishuwang/upload/0603031405556562.jpg Figure 3 Corrosion of hook bolts in a factory 1.3.2 Corrosion-resistant layer (1) The overall passivation of the electrode cladding layer is good, and there are no serious abnormal corrosion defects ； However, sporadic intergranular corrosion cracks have begun to appear in individual factories, as shown in Figure 4. http://www.nmtech.com.cn/jishuwang/upload/0603031406502400.jpg http://www.nmtech.com.cn/jishuwang/upload/0603031407519671.jpg Figure 4 Corrosion cracks on the lower head of a urea synthesis tower in a factory (2) Affected by manufacturing quality, operating stability, process conditions and maintenance quality, the passivation quality of the linings of urea synthesis towers of different manufacturers varies greatly. However, there are phenomena such as surface roughness and metal particles falling off, and some even have serious corrosion defects such as local activation and groove corrosion, as shown in Figures 5 and 6. http://www.nmtech.com.cn/jishuwang/upload/0603031409064817.jpg http://www.nmtech.com.cn/jishuwang/upload/0603031411249857.jpg Figure 5 The surface condition of the lining of a certain plant Figure 6 The circumferential groove of the lining of a certain plant (3) Affected by the solid solution treatment conditions of the plate, operating reaction conditions and other factors, the corrosion environment of each barrel section of the same urea synthesis tower is different, and the passivation conditions are different, especially the gas-liquid interface is the most obvious, see Figure 7. http://www.nmtech.com.cn/jishuwang/upload/0603031416155697.jpg Figure 7 Corrosion morphology of gas-liquid interface of urea synthesis tower 1.3.3 Corrosion-resistant welds 1.3.3.1 Corrosion-resistant welds whose welding material is 25-22-2 are in generally good condition, as follows:: (1) Except for local subcutaneous pores, undercuts and other welding defects exposed during manufacturing, there are no other serious abnormal corrosion defects. ； (2) The thinning trend in the HAZ area is obvious, and the weld fusion line has begun to show signs of knife line corrosion. 1.3.3.2 The overall condition of corrosion-resistant welds with welding materials of 19/15H is worrying, as follows:: (1) The HAZ area is severely thinned, and the weld fusion line is obviously corroded. ； (2) The weld structure is loose and black, and the ferrite structure has obvious selective corrosion characteristics ； (3) Intergranular corrosion of welds caused by improper repairs often occurs ； (4) Leakage accidents of synthesis towers caused by weld corrosion penetration occur frequently, and some manufacturers even have pressure shell corrosion penetration accidents. The corrosion conditions of corrosion-resistant welds of different welding materials are shown in Figures 8 to 12. http://www.nmtech.com.cn/jishuwang/upload/0603031417394633.jpg Figure 8 HAZ zone thinning of 25-22-2 type weld Figure 9 HAZ zone thinning of 19/15H type weld http://www.nmtech.com.cn/jishuwang/upload/0603031418491482.jpg Figure 10 Microstructure-type selective corrosion of 19/15H type weld Figure 11 Knife line corrosion of 19/15H type weld http://www.nmtech.com.cn/jishuwang/upload/0603031420035063.jpg Figure 12 Corrosion caused by improper repair of type 19/15H welds 2 High-pressure scrubber The SNAM device does not have a high-pressure scrubber. The high-pressure scrubber of the ACES device has a stuffing tank structure, and the corrosion-resistant material is 316L-UG plate + 25-22-2 weld. The corrosion characteristics are the same as those of the urea synthesis tower. The high-pressure scrubber of the STAC device is a heat exchanger, which is divided into two types: American-Dutch type and French type. Next, a brief introduction will be given to the detection of high-pressure scrubbers of STAC devices. 2.1 Macro-corrosion inspection 2.1.1 US-Dutch type device (1) The corrosion-resistant materials of pipe boxes, overflow tubes, explosion-proof panels and other components are 316L-UG plates + 25-22-2 welds ； The corrosion characteristics are the same as those of the urea synthesis tower. (2) Before equipment replacement, some manufacturers had tube leakage caused by pinhole defects in the fillet welds of heat exchange tubes. (3) For the original 316L-UG heat exchange tube, the terminal crystal corrosion that appeared at the upper tube mouth (Figure 13) is a serious corrosion defect common to this type of equipment. ； However, after replacing the 25-22-2 type heat exchange tube, the terminal crystal corrosion phenomenon is no longer obvious. http://www.nmtech.com.cn/jishuwang/upload/0603031421069784.jpg Figure 13 Terminal crystal corrosion of the original 316L-UG type tubes 2.1.2 French type device (1) The phenomenon of terminal crystal corrosion of heat exchange tubes is not obvious. (2) Affected by manufacturing quality, there are many corrosion defects in the cladding layer of the spherical explosion-proof space. ； In addition to the loose and black welding strip, there is also a large amount of intergranular corrosion (see Figure 14). http://www.nmtech.com.cn/jishuwang/upload/0603031422224626.jpg Figure 14 Corrosion morphology of the strip cladding layer of a high-pressure scrubber in a certain factory 2.2 Eddy current flaw detection of heat exchange tubes (1) There are no countable outer wall defect signals in the heat exchange tube sections outside the tube plate. (2) For some manufacturers whose Cl- content has always been high, the 316L-UG heat exchange tube once had an outer wall defect signal in the lower tube plate section, and the tube was blocked after being confirmed by ammonia leakage inspection. Therefore, the possibility of SCC occurring within a high-pressure scrubber cannot be ignored. (3) A very small number of 316L-UG heat exchange tubes have inner wall defect signals. After tracking and inspection, it was found that the defect signals not only did not become larger, but were eliminated. Obviously, this is caused by metallurgical defects in the tubes and is not a corrosion defect. 2.3 Eddy current thickness measurement of heat exchange tubes (1) The tubes are in a uniformly thinned state. Except for the special tube plate section, there are no obvious local corrosion thinning areas in the other tube sections. (2) During the inspection of the original 316L-UG type tubes, there was a phenomenon that the wall thickness of the tube plate section was about 0.3mm smaller than the wall thickness of the entire tube. ； For the currently used 25-22-2 type tubes, this phenomenon is not obvious due to the short use time, but attention should be paid to the wall thickness detection of the tubes in the tube sheet section. (3) Tubes with relatively small wall thickness are mostly distributed at the edge of the tube sheet, see Figure 15. http://www.nmtech.com.cn/jishuwang/upload/0603031423524864.jpg Figure 15 Distribution of wall thickness of high-pressure scrubber tubes in a factory 3. High-pressure methane-ammonium condenser. The equipment materials and structural forms of the high-pressure methane-ammonium condenser are shown in Table 5. Table 5 High-pressure ammonium methane condenser equipment overview Device type Corrosion-resistant material Structure form Tube box Heat exchange tube ACES loose lining DP-12DP-12 straight tube vertical 2 units in series SNAM loose lining 316L-UG2RE69U type horizontal STAC loose lining 316L-UG316L-UG straight tube vertical STAC strip surfacing 25-22-22RE69 straight tube vertical 3.1 Macro-corrosion inspection 3.1.1 SNAM type device (1) The corrosion characteristics of the corrosion-resistant layer of the pipe box are the same as those of the urea synthesis tower ； The main defects are thinning in the HAZ zone of the lining weld and corrosion of the fusion line knife line, see Figure 16. http://www.nmtech.com.cn/jishuwang/upload/0603031426058491.jpg Figure 16 Knife line corrosion of the 25-22-2 type lining weld in the methane condenser (2) Endless corrosion of the heat exchange tube occurs. (3) Pinholes appear in the fillet welds of heat exchange tubes, and some manufacturers even cause leakage due to penetrating defects, see Figure 17. http://www.nmtech.com.cn/jishuwang/upload/0603031427277537.jpg Figure 17 Leakage status of the methylammonium condenser in a factory 3.1.2 ACES type device (1) Corrosion defects are mainly concentrated in the upper pipe box. (2) The passivation condition of the DP-12 duplex steel lining is better than that of the 316L-UG austenitic stainless steel, and the average corrosion rate is around 0.05mm/a. (3) The lining weld is DP-12 type. The main corrosion characteristics are weld fusion line grooves and tissue selective corrosion. The overall corrosion condition is not as good as that of the 25-22-2 type weld, see Figure 18. http://www.nmtech.com.cn/jishuwang/upload/0603031428292818.jpg Figure 18 Corrosion status of the lining welds of the ammonium methane condenser of the ACES unit (4) The heat exchange tubes and their fillet welds are all made of DP-12 material. Selective corrosion is obvious at the arc closing points of all fillet welds and the end faces of individual tubes. Burn-through and weld collapse during manufacturing are relatively common, see Figures 19 and 20. http://www.nmtech.com.cn/jishuwang/upload/0603031429338989.jpg Figure 19 Corrosion morphology of the tube plate on the ammonium methane condenser of the ACES unit Figure 20 Corrosion of the end face and fillet weld of the DP-12 tubes 3.1.3 STAC type device (1) The corrosion characteristics of the corrosion-resistant layer and welds of the pipe box are similar to those of the urea synthesis tower and have common characteristics. (2) For heat exchange tubes made of 2RE69, the terminal crystal corrosion phenomenon is not obvious. (3) The heat exchange tubes are made of 316L-UG type, and the tube ends are severely corroded. In addition to the obvious characteristics of terminal crystal corrosion, some manufacturers even have severe knife-like corrosion of the fusion line of the pipe wall, as shown in Figure 21. http://www.nmtech.com.cn/jishuwang/upload/0603031430325883.jpg Figure 21 Corrosion morphology of the pipe end of the 316L-UG type heat exchange tube of a methylammonium condenser in a certain factory (4) Due to the reason that the sulfur content in CO2 gas has always seriously exceeded the standard, some manufacturers' heat exchange tubes have activation corrosion, see Figure 22. http://www.nmtech.com.cn/jishuwang/upload/0603031431431468.jpg Figure 22 Activated corrosion of 316L-UG type heat exchange tubes in a plant's ammonium methane condenser 3.2 Eddy current flaw detection (1) ACES type device, no measurable inner and outer wall defect signals were found in the heat exchange tubes. (2) In both SNAM type devices and STAC type devices, a small number of heat exchange tubes have inner wall defect signals. ； Except for a few tube tube defects that tend to increase in size, the remaining defects have basically remained unchanged, similar to the situation in high-pressure scrubbers. (3) For some SNAM manufacturers, the heat exchange tubes in the upper half (the shell side is in the gas phase) also have measurable outer wall defect signals. ； The axial position of the defect is between the inside of the tube sheet and the fifth baffle plate ； However, the corrosion cause of the defects remains to be analyzed and verified. (4) STAC type devices all have measurable outer wall defects in the heat exchange tubes. ； The axial position of the defect is mainly about 500mm from the upper/lower nozzle. Some manufacturers even have C-level defects 3.8m away from the lower nozzle. (5) For STAC type devices, SCC induced by high Cl- content in the condensate is still the main phenomenon of tube corrosion. A factory with a high Cl- content had frequent condenser leaks due to SCC, and had to replace the equipment. 3.3 Eddy current thickness measurement (1) SNAM type device, the entire heat exchange tube made of 2RE69 is in a uniform corrosion state, and there is no obvious thinning area. (2) The STAC type device ignores the pipe section factors at the location of outer wall defects, and the thinning of the heat exchange tubes is similar to that of a high-pressure scrubber. (3) ACES type device, the entire heat exchange tube section of the tube section is in a uniform corrosion state, with no obvious thinning area. ； The tube plate section of the 1# condenser (the tube side is methane ammonium medium, and the shell side is steam and condensate) has no obvious thinning, but the wall thickness of the tube plate section of the 2# condenser (the tube side/shell side is both methane ammonium medium) is 0.3 to 0.5mm smaller than the tube section wall thickness, and the minimum wall thickness is 1.40mm. (4) Excluding the influence of the stability of the operating process, in the corrosive environment of methane-ammonium medium, the corrosion resistance of DP-12 material is the best, followed by 2RE69 material and 316L-UG material is the worst. See Table 6 (the comparison time is one detection cycle). Table 6 Comparison of corrosion rates of heat exchange tubes in high-pressure ammonium methane condensers Corrosion rate/mm/a Single tube maximum value Single tube minimum value Average value DP-122RE69316L-UGDP-122RE69316L-UGDP-122RE69316L-UG 0.100.100.200 .00.00.050.040.050.09 4 Stripper The stripper equipment materials are shown in Table 7. Table 7 Stripping tower equipment overview Device type Equipment name Corrosion-resistant material pipe box Corrosion-resistant layer Heat exchange tube ACESCO2 stripping tower pine lining DP-12DP-12STACCO2 stripping tower pine lining 316L-UGX2CrNiMo25-22-2STACCO2 stripping tower strip welding 25-22-22RE69SNAMNH3 stripping tower pine lining B265 Gr.1B338 Gr.3SNAMNH3 stripping tower pine lining 25-22-225-22-2+Zr4.1 Macroscopic inspection 4.1.1 ACES device (1) There are no abnormal corrosion defects in the heat exchange tube end face, fillet welds, inner wall HAZ area and other parts. (2) The passivation quality of the corrosion-resistant lining base material is good, and there is no abnormal thinning in the HAZ zone of the weld. (3) The lining weld is severely corroded. In addition to the obvious selective corrosion characteristics of ferrite, phenomena such as weld fusion line grooves and intergranular corrosion and shedding of the weld structure are also significant, see Figure 23. http://www.nmtech.com.cn/jishuwang/upload/0603031432517042.jpg Figure 23 Corrosion status of welds in the stripper lining of the ACES unit (4) In addition to selective corrosion, there is also obvious intergranular corrosion in the internal welds, as shown in Figure 24. http://www.nmtech.com.cn/jishuwang/upload/0603031433498079.jpg Figure 24 Corrosion status of welds in the internal parts of the stripping tower of the ACES unit 4.1.2 STAC device (1) The corrosion characteristics of the corrosion-resistant lining and welds are similar to those of the urea synthesis tower, and the overall corrosion degree is relatively mild, see Figure 25. (2) The corrosion characteristics of the electrode cladding layer are the same as those of the high-pressure scrubber of the French-type device, and the corrosion degree is relatively serious, as shown in Figure 26. http://www.nmtech.com.cn/jishuwang/upload/0603031435188991.jpg Figure 25 The morphology of the corrosion-resistant surfacing layer at the lower head of a CO2 stripping tower in a certain factory Figure 26 The corrosion morphology of the electrode cladding layer of a CO2 stripping tower in a certain factory (3) Except for the local end crystal corrosion at the upper tube mouth of some manufacturers, in general, the end crystal corrosion phenomenon of the heat exchange tubes is not obvious. (4) Leakage may occur due to penetrating pinholes in the tube fillet welds. 4.1.3 SNAM type device (1) The stripping tower whose tube material is 25-22-2+Zr has no abnormal corrosion defects in the corrosion-resistant lining, lining welds, tube end faces, etc. (2) In a stripping tower whose tube material is Ti (B338 Gr.3), the main corrosion defect is corrosion at the end of the heat exchange tube, see Figures 27 and 28. http://www.nmtech.com.cn/jishuwang/upload/0603031436422226.jpg http://www.nmtech.com.cn/jishuwang/upload/0603031437346901.jpg Figure 27 Gap on the end surface of the heat exchange tube Figure 28 Corrosion holes on the sealing surface of the heat exchange tube This is an inevitable corrosion phenomenon in this type of gas tower, because the Ti material has poor corrosion resistance against crevice corrosion and erosion corrosion. ； Of course, the quality of the sealing ring and the assembly quality of the swirl tube are also closely related. (3) Some manufacturers have dents in the lining cover. (4) Some manufacturers have experienced leakage in the linings of custom pipe boxes. ； The inspection of the fillet welds of the lining cover should be strengthened, and technical means such as ammonia leakage should be used if necessary. (5) Attention should be paid to the alignment quality and inner wall weld quality when replacing the pipe head to avoid uneven liquid film falling, which will aggravate the corrosion and thinning of the tubes. 4.2 Eddy current flaw detection (1) Under normal detection conditions, no countable outer wall defect signals were found in the heat exchange tubes of the stripping tower of various devices. (2) In the stripping tower of the STAC device, the 25-22-2 type heat exchange tube occasionally has inner wall defect signals, similar to the situation in other heat exchangers. (3) However, in the stripping tower of the STAC unit, there have been special cases of Class D defects in the heat exchange tubes in the upper tube plate section. ； Analysis suggests that it was caused by leakage from the plug. 4.3 Eddy current thickness measurement 4.3.1 ACES type device (1) There is a certain corrosion and thinning area in the heat exchange tubes, which is generally located within the range of 1m to 2m from the upper tube sheet. (2) The thinning rate of tube wall thickness is relatively constant, with a maximum value of 0.10mm/a for a single tube, a minimum value of 0.05mm/a for a single tube, and an average of 0.06mm/a, see Table 8. Table 8 ACES device stripping tower eddy current thickness measurement statistics in 2002 and 2004 wall thickness/mm number of root/root wall thickness/mm number of root/root 2.6532.6042.7032.65122.7514 2.70592.80922.75762.851642.801692.901722.851712.951892.902433.001892.95 2133.052373.002293.102723.051603.152673.101423.201433.15663.25533.20323 .30363.25253.35163.307 The average wall thickness of the tubes is 3.03. The average wall thickness of the tubes is 2.94 (3) The relatively thin-walled tubes are distributed more concentrated on the tube plate, see Figure 29. http://www.nmtech.com.cn/jishuwang/upload/0603031439227291.jpg http://www.nmtech.com.cn/jishuwang/upload/0603031440262397.jpg Figure 29 Distribution of wall thickness of stripper tubes in ACES unit 4.3.2 STAC type device (1) There is an obvious corrosion and thinning area in the heat exchange tubes, which is generally located in the range of 3m to 3.8m from the upper tube sheet, but there are also a few individual manufacturers with thinning sections at 1.5m or 4.5m from the upper tube sheet. (2) Since 2002, a considerable number of manufacturers have replaced new strippers, so the corrosion rates of various manufacturers are incomparable. (3) For the "old" stripper that has been used for a long time, a few tubes have abnormal corrosion and thinning, and the overall corrosion rate value is too large, see Tables 9 and 10. Table 9 Comparison of eddy current thickness measurement data of a stripping tower of a certain STAC device Detection time Wall thickness value/mm Corrosion rate/mm/a Maximum value Minimum value Average value of single root Maximum value of single root Minimum value Total average value 2002 2.451.501.94 2003 2.401.501.840.300.050.10 2004 2.301.501.730.300.050.11 Table 10 The statistics of the thin-walled tube array inspection of a STAC device stripping tower are based on detection time: 1.50mm1.55mm1.60mm1.65mm1.70mm. Cumulative tube plugging in 2002: 27101463142003: 42053182323332004: 8671552776345 (3) The distribution position of relatively thin-walled tubes on the tube plate is relatively concentrated, see Figure 30. http://www.nmtech.com.cn/jishuwang/upload/0603031441306269.jpg Figure 30 The wall thickness of the stripping tower of a certain STAC unit is ≤1.70mm and the tube distribution status 4.3.3 SNAM type unit 4.3.3.1 The entire tube (1) made of 25-22-2+Zr is uniformly corroded, and there is no obvious corrosion thinning area. (2) The thinning rate of tube wall thickness is relatively constant, with a maximum value of 0.01mm/a for a single tube, a minimum value of 0 for a single tube, and an average of 0.01mm/a, see Table 11. Table 11 Statistics of eddy current thickness measurement of stripping tower of SNMA device (25-22-2+Zr) in 2002 and 2004. Wall thickness/mm number of roots/root Wall thickness/mm number of roots/root 2.65182.654 72.708752.709892.758472.759622.803952.802242.851102.85352.90122.90 The average wall thickness of the tubes is 2.75. The average wall thickness of the tubes is 2.734.3.3.2. Tubes made of Ti (1) There are obvious corrosion and thinning areas in the heat exchange tubes, which are generally located in the range of 1m to 1.5m from the upper tube plate. However, there are also a few individual manufacturers with thinned tube sections 0.5m away from the upper tube plate. (2) There are a few cases of abnormal corrosion and thinning of tubes, mainly concentrated in the middle of the tube sheet. (3) When replacing tubes with poor tube head quality, the corrosion thinning area will move upward, and at the same time, the thinning rate of a single tube will be too high. (4) The corrosion rates of tubes from different manufacturers are different. Table 12 Comparison of eddy current thickness measurement data of stripper of SNAM device (Ti) Detection time wall thickness value/mm Corrosion rate/mm/a Maximum value Minimum value Average single root maximum value Single root minimum value Total average value Factory A, Factory B, Factory A, Factory B, Factory A, Factory B, Factory B, Factory A, Factory B, Factory A, Factory B, Factory B, Factory A, Factory B, Factory B, 2002 3.553.802.001.452.802.86 2003 3.55 2.00 2.70 0.25 0.00 0.10 2004 3.503.751.901.102.612.770.600.300.100.050.090.06 (5) Different manufacturers have different distribution conditions of relatively thin and thick wall tubes on the tube plate, see Figures 31 and 32. http://www.nmtech.com.cn/jishuwang/upload/0603031442464811.jpg Figure 31 Distribution of tubes with wall thickness value ≤ 2.40mm in Plant A Figure 32 Distribution of tubes with wall thickness ≤ 1.90mm in Plant B 5 Conclusion The corrosion of urea high-pressure vessels is not only restricted by the use of corrosion-resistant materials of the equipment itself, but is also closely related to equipment manufacturing quality, process operation, maintenance quality and other factors. To achieve a complete and accurate analysis of the corrosion status of urea high-pressure vessels, various corrosion detection methods need to be applied, and the detection items should be as complete and the detection range as wide as possible to achieve effective and comprehensive monitoring of the equipment. How to use simple and effective means to detect leaks in the pipe boxes on the Ti NH3 stripping tower is a subject that needs further discussion. During the operation of the equipment, process procedures and process disciplines must be strictly operated to avoid the phenomenon of competing for equipment in pursuit of output. Ensure long-term safe operation of equipment. It should be further emphasized to comply with the basic technical requirements for the maintenance of urea high-pressure equipment, strictly abide by the precautions for maintenance work, comply with the repair and welding operating procedures and precautions, strengthen the technical level training of welding personnel, improve the technical level of welding personnel, and ensure the repair quality of urea equipment.

1 Overview Jintianhua’s 520,000-ton-annual urea plant adopts the ammonia stripping process introduced in my country in the late 1980s. It was put into operation in August 1993 and has undergone a total of 7 overhauls. Judging from the results of several overhauls, the corrosion of the equipment is relatively serious. The synthesis tower trays were all replaced during the overhaul in October 1999.； In 1997, 8 pipe heads on EO1 were overhauled due to corrosion and perforation. From 1998 to now, more than 90 pipe heads have been replaced with new ones. In May 2000, it was detected that there was a corrosion perforation in the weld of the E01 upper pipe box cylinder, and repair welding was performed on it. Corrosion of high-pressure equipment has become the biggest hidden danger for stable urea production. How to reduce corrosion as much as possible, extend the service life of equipment, and eliminate hidden dangers has become a major issue after the stable operation of urea. This article analyzes and studies corrosion control in order to find out economically feasible anti-corrosion measures. 2 Corrosion of high-pressure equipment 2.1 Synthesis tower R01 The synthesis tower is one of the most critical equipment in the urea production line (see Figure 1). During the first overhaul in 1994, it was discovered that the synthesis tower was severely locally corroded, with various types of corrosion pits existing on the lining of each tube section and the melon lining of the head. During the overhaul in 1997, the uniform corrosion of the synthesis tower developed seriously, the color of the gas phase lining turned black, and the surface was extremely rough. ； The liquid lining is silvery white, with grains falling off, and feels rough, like fine sandpaper. ； The surface of the downcomer and the tray are uniformly corroded and thinned, and the surface of the tray is extremely rough, with grains falling off. During the overhaul in 1999, it was discovered that there was a large amount of arc corrosion in the circumferential and longitudinal welds of the lining, with the deepest depth reaching 3 mm and more than 200 places. There was slight knife edge corrosion in all the welds of the lining, and the thinnest part of the lining measured 4.3 mm. The tray welds were corroded and perforated in many places, and the thickness was very serious. All trays have been replaced with domestically produced parts. http://www.nmtech.com.cn/jishuwang/upload/0605151015378215.jpg 2.2 During the inspection of the gas stripping tower in 1995, except for slight scale corrosion at the lower part of the gas stripping tube, no corrosion was found in the rest. During maintenance in 1997, it was discovered that eight heat exchange tubes had corrosion and perforations at their nozzles, and dozens of other heat exchange tubes had their nozzles severely thinned, with the thinnest part less than 1 mm and abnormally sharp (see Figure 2), so the tube heads were replaced. In October 1999, several lugs (titanium material) of the riser tube fixed flower plate were eroded and corroded, and there were grooves 4 mm deep, 4 mm wide, and 7 mm long on the surface. More than 80 nozzle perforations on the air lift tube were thinned. The titanium lining thickness measurement found no obvious thinning. http://www.nmtech.com.cn/jishuwang/upload/0605151016237000.jpg 2.3 During the maintenance of the high-pressure ammonium methane condenser in 1994, it was found that a circle 20 mm away from the sealing surface felt rough. In 1997, it became more serious, the grains were coarse, and the longitudinal weld seam of the split-range partition plate cracked 197 mm, and the crack was on the weld bead. 3 Corrosion mechanism 3.1 Chemical corrosion (1) Carbamate corrosion 2NH3+CO2+H2O→NH4COONH2+H2O NH4COONH2+H2O→NH4+ +COONH2—+H2O COONH2— is reducing and can prevent the formation of oxide films on the surface of passivated metals, causing active corrosion of metals. The higher the concentration and temperature of methylammonium, the stronger the corrosion. (2) Urea isomerization under cyanate corrosion at high temperature: NH2CONH2=NH4CNO=NH4+ +CNO— CNO— has strong reducing properties and destroys the passivation film. (3) Formation of ammonia complex Under high temperature and high pressure, the ammonia in the urea carbamate solution forms a complex with the metal oxide, thereby destroying the oxide film on the surface of the stainless steel and showing activated corrosion. (4) Formation of carbonyl compound Nickel is easier to form nickel carbonyl. 3.2 Electrochemical corrosion of cathode: The metal becomes ions and enters the solution to be oxidized, and the electrons are transferred to the anode Me—e→Me+ anode: H ions or oxygen molecules in the solution take electrons from the anode and are reduced 2H+ +2e→H2↑ O2+2H2O+4e→4OH. Stainless steel and titanium may both produce reactive corrosion and maintain electrochemical corrosion. 4 Analysis of factors affecting corrosion 4.1 The impact of machinery manufacturing on the corrosion of the synthesis tower. During the first overhaul in 1994, it was found that the lining of the R01 synthesis tower was severely corroded locally, with dozens of corrosion pits. The reason was due to improper welding on the lining during manufacturing or arc ignition on the lining and welding spatter. It showed obvious sensitization corrosion grains falling off in pieces, and some corrosion pits showed the silvery white color of the base material at the bottom. For such manufacturing defects, we perform grinding and repair welding, and the grinding depth sometimes exceeds 3mm. Use 25-22-2 welding wire and TIG repair welding. During grinding, it is found that there are delamination defects in the base metal, and repair welding is performed after polishing the delamination area. 4.2 The impact of materials on equipment corrosion. The E01 lining and heat exchange tubes are made of titanium. Titanium is resistant to high temperature corrosion but not erosion. Judging from the overhaul situation over the past few years, E01 erosion corrosion is relatively serious. The nozzle erosion corrosion and thinning of the gas lift tube are serious. More than 100 pipe heads have been replaced. ； The operating temperature of E01 is 207°C, which is the highest operating temperature part of the urea unit, but the average corrosion rate is significantly lower than 316L. The lining thickness was tested in October 1999 and no thinning was found. The material of R01 is 316L urea grade stainless steel. Its anti-corrosion performance under oxygen-deficient and aerobic conditions is far inferior to that of dual-phase steel (25Cr-7Ni-3Mo) and 2RE69 (25Cr-22Ni-2Mo). http://www.nmtech.com.cn/jishuwang/upload/0605151018492390.jpg 4.3 In terms of installation and maintenance, the upper nozzle of the heat exchange tube of the gas stripping tower is seriously eroded and corroded. The main reason is that the processed surface of the pipe end is deformed after welding. When the distribution pipe and the heat exchange tube are installed, the inner PTFE sealing gasket is extruded and deformed, which makes the gap between the upper nozzle and the distribution pipe locally enlarged. Urine washes the gap and causes corrosion. The corrosion locations are all on the outer wall of the pipe end (see Figure 1). 4.4 Process factors 4.4.1 Operating temperature The increase in temperature will aggravate the corrosion of ureamethane solution. This is because the increase in temperature accelerates the oxidation-reduction process of the cathode and anode, that is, it increases the electrochemical corrosion rate. For stainless steel below 165°C, temperature has little effect on the corrosion rate. However, when the temperature increases from 165°C to 200°C, the corrosion rate increases by approximately 3 to 4 times. There is some experience in the upper temperature limit of the main materials used in urea engineering. For example, titanium is 205~210℃, zirconium is 230℃, and 316L mod stainless steel is 190℃. According to the actual situation of our factory, the liquid temperature of the synthesis tower generally does not exceed 190°C, and is best controlled below 189°C. The operating temperatures of R01 in recent years are shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload/0605151019353068.jpg It can be seen from Table 2 that the reason why the average corrosion rate of R01 develops rapidly is inseparable from the over-temperature range of R01. If the operating temperature is too high, the corrosion of R01 will inevitably be aggravated. Therefore, on the premise of ensuring the CO2 conversion rate, appropriately lowering the operating temperature of the synthesis tower is beneficial to reducing the corrosion rate and extending the service life of R01. 4.4.2 Effect of NH3/CO2 Increasing the NH3/CO2 of the solution can reduce the corrosion rate of stainless steel caused by the methylammonium solution. This is because NH3 can partially neutralize the acidity of the solution and increase the pH value of the solution. ； NH3 can inhibit the formation of cyanic acid or ammonium cyanate which is corrosive to stainless steel. ； NH3/CO2 has a very obvious effect on corrosion within the range of 2 to 2.8. When NH3/CO2>2.8, it has little effect on corrosion. Our factory's NH3/CO2 design is 3.3~3.6, but the actual control is 3.0~3.2, so the influence of NH3/CO2 is almost non-existent. 4.4.3 H2O/CO2 Increasing H2O/CO2 will promote the dissociation of ammonium cyanate and produce more corrosive cyanooxycyanates.: NH4CNO→NH4+ +CNO—. Therefore, it is not appropriate to control an excessively high water-to-carbon ratio in the urea production process. According to the actual operation conditions of our plant, H2O/CO2 is much higher than the design value of 0.67. In recent years, H2O/CO2 is shown in Table 3. http://www.nmtech.com.cn/jishuwang/upload/0605151020239822.jpg The above data shows that the water-to-carbon ratio control is relatively high, which will aggravate corrosion to a certain extent. 4.4.4 Amount of Oxygen Added The oxygen in the solution acts as a corrosion inhibitor for metals such as stainless steel and titanium. It can form a dense passivation film on the surface of stainless steel and titanium to separate the solution from the metal. Since ureamethane solution is a reducing medium, in order to maintain the stainless steel in a passive state, oxygen must be continuously added. Our factory's design value is (0.25~0.3)% (v), and the actual operating conditions average around 0.28%, with a minimum of 0.22%. Especially during an emergency stop or a carbon dioxide compressor interlock trip, if there is no time to add more O2 to the system, the tower will be shut down. This will inevitably cause less and less dissolved oxygen in the system, and a large amount of oxygen will overflow from the liquid phase. The formation of passivation film is the reaction between the metal elements in the stainless steel and the oxygen in the liquid phase. The lack of oxygen in the solution will make the nickel element easily form nickel ammonia complex or nickel carbonyl in the urea carbamate solution, so that the nickel in the surface layer can be selectively dissolved. Under the condition that there is no lack of oxygen in the solution, oxide films such as Cr2O3 and Mo2O3 can be formed immediately to prevent further penetration of the solution. When the dissolved oxygen in the liquid phase overflows, chromium and molybdenum lack enough dissolved oxygen to react with them, and cannot form passivation films such as chromium oxide or molybdenum oxide for protection, thus aggravating the corrosion rate. In this case, it is very easy for the iron and nickel in urea to exceed the standard, especially the nickel content will exceed the normal value by dozens of times, and the color of the urea will turn red. 4.4.5 Tower sealing time The tower sealing time should not be too long, otherwise the oxygen dissolved in the methylammonium solution will easily escape. Because in a static state, the saturated solubility of oxygen in ureamethane solution is only 0.1×10-6, while under normal oxygenation conditions, the oxygen content is 80~200×10-6 * * If it is supersaturated, the shutdown holding time allowed for sealing the tower should be the time it takes for the oxygen content in the liquid phase to drop to the critical oxygen content required to maintain passivation. The parking and tower closing time must be strictly controlled. 5 Preventive measures In order to reduce the corrosion of equipment caused by urea solution, manufacturers in operation can only strictly control the process operation and inspection and maintenance. (1) The operating temperature is the main process parameter for controlling corrosion in urea production. Obvious corrosion aggravation will occur if the temperature exceeds 1 to 2°C. This can be seen from the Ni content analysis results. Therefore, the liquid temperature TI203 of the synthesis tower should be controlled below 189°C, and the maximum cannot exceed 190°C. (2) The actual value of H2O/CO2 in our factory has always exceeded the design value, and should be controlled as close to the design value as possible. This is not only beneficial to reducing the corrosion rate, but also increases the carbon dioxide conversion rate, which is another important control indicator for optimizing operations. At present, our factory has three water adding points into the system, namely P02 seal flushing water, T03 water adding, and E07 inlet adding process condensate and tail gas ammonia water. The inlet water volume of E07 is relatively high, and the water volume of P02 is also higher than the design value. Therefore, to reduce the water-to-carbon ratio of the system, the water volumes of these two streams must be reduced. The amount of P02 seal flushing water is limited by the design conditions of the mechanical seal and is difficult to adjust. The water at the E07 inlet is entirely for condensation to absorb the low-pressure decomposer and hydrolyzed gas phase. It is recommended to add a water cooler to cool down the decomposed water gas (see Figure 3) and directly return it to V06. This can not only reduce the amount of water added to the low-pressure E07 inlet, but also alleviate the problem of low pressure and overpressure caused by overload condensation of E08. http://www.nmtech.com.cn/jishuwang/upload/0605151021224476.jpg (3) The O2 content should be increased to (0.4~0.45)% before shutting down the tower. If there is no time to add more during emergency shutdown, the tower can be drained or more ammonia added to the system to alleviate corrosion, and the tower sealing time should not exceed 24 hours. (4) When reinstalling the E01 distribution pipe, special attention should be paid to ensure that the PTFE gasket is not extruded and deformed, causing gaps at the connections. The pipe head should be replaced for the existing pipe mouth that has been severely corroded and thinned. (5) Material selection for urea high-pressure equipment In the urea-methylammonium liquid medium, nickel will reduce the corrosion resistance of stainless steel. Especially under anoxic conditions, the preferential and selective dissolution of nickel is more obvious. Although the corrosion resistance of titanium is better than that of 316L, it is not resistant to erosion, and scaling is difficult to deal with, so better materials are chosen instead. For example, 25-22-2 is more corrosion-resistant than 316L, and zirconium is more resistant to high-temperature corrosion than titanium.

1 Overview Since the introduction of the first urea plant in my country in the 1960s, the problem of equipment corrosion in the urea production process has not been well solved, and scientific research institutions and manufacturers have not found ideal anti-corrosion means. Taking the aqueous solution full cycle urea unit as an example, the most serious corrosion cases are the synthesis tower and the sub-tower in the pre-distillation process. Many urea plants use the anti-corrosion method of adding 0.5% to 0.8% O2 to the raw CO2 gas and adding air to the heater inlet. Although using oxygen as a preservative has many disadvantages, such as occupying the effective volume of the synthesis tower, reducing production capacity, and increasing ammonia emissions, it is still used as an effective anti-corrosion method at this stage. Now, based on the situation of our company, we will focus on some superficial understanding of the anti-corrosion of synthetic towers and one-step towers. 2 Causes of corrosion Generally speaking, the corrosion of steel by CO2, NH3 and H2O, the raw materials for synthesizing urea, is relatively slight. It is generally believed that the causes of corrosion are as follows. (1) Corrosion of ammonium carbamate Methyl ammonium can dissociate into carbamate radicals in water, and this substance is highly corrosive. http://www.nmtech.com.cn/jishuwang/upload/0605291548535321.jpg (2) Corrosion of urea isomer ammonium cyanate Ammonium cyanate can dissociate into extremely corrosive cyanate radicals in water. http://www.nmtech.com.cn/jishuwang/upload/0605291549353509.jpg (3) Metals can form polycarbonyl compounds with the medium, and can also form easily soluble complexes with ammonia and suffer corrosion. 3 Factors affecting corrosion Urea synthesis towers and one-stage towers generally use Crl8Ni2Mo2Ti (or 316L). During normal production, a certain amount of air or oxygen is introduced to form a dense oxide protective film on the stainless steel surface of the inner wall of the tower, which puts the metal surface in a passivated state and prevents oxidative corrosion of the inner metal to achieve the purpose of anti-corrosion. In actual production, there are many factors that affect corrosion. Internal factors include material, and external factors include oxygen content, hydrogen sulfide, chloride ion content, medium temperature, flow rate, etc. 3.1 The content and composition of each element in stainless steel with different materials and the stress existing in the metal are different. Increasing the chromium and molybdenum content of stainless steel used in urea is beneficial to its corrosion resistance, while increasing the nickel content is unfavorable. This adverse effect is more obvious in the anoxic urea reaction solution, but under normal oxygenation, the adverse effects are compensated because nickel makes the chromium content evenly distributed. 3.2 Oxygen content It is generally believed that the oxygen content in the melt can form a passivation film on the metal surface to prevent metal corrosion. Even if the passivation film is damaged, a new passivation film can be formed immediately. Stainless steel is an easily passivated metal, and its surface can be naturally passivated in the air. In a solution with oxygen, the surface of stainless steel is in a passivated state, the corrosion potential increases, and the corrosion rate decreases. But for easily passivated metals, if there is excess oxygen in the solution or at high temperatures, the passivation area may also corrode. 3.3 The hydrogen sulfide ions in the hydrogen sulfide solution can be adsorbed on the surface of easily passivated metals such as stainless steel, taking away the oxygen in the passivation film and destroying the passivation film, thereby aggravating the corrosion of stainless steel. Therefore, the corrosion rate of metals such as stainless steel increases as the H2S content in the solution increases. Within a certain range, increasing the oxygen content can offset part of the impact of H2S. It is generally believed that when the H2S content in carbon dioxide gas is 5 to 10 mg/m3, it requires 0.5% to 1% oxygen. If the H2S content is above 15 mg/m3, passivation cannot be maintained no matter how much oxygen is added. 3.4 Chloride ions (brought in by soft water or condensate) Chloride ions are strongly adsorbed ions and are easily adsorbed on the metal surface, weakening the atomic affinity of the strained metal, that is, weakening the surface properties of the metal, which is conducive to the occurrence and development of cracks. Therefore, chloride ions are the main factor causing stress corrosion and pitting corrosion of stainless steel in the urea reaction solution. Generally, high-temperature water containing a few parts per million of chloride ions can cause stress corrosion cracking in commonly used stainless steel. When oxygen is present, stress corrosion will intensify. 3.5 Composition of the molten liquid: The high methylammonium content (or CO2 content) in the molten liquid makes it highly corrosive. The high NH3/CO2 (molecular ratio) or low H2O/CO2 (molecular ratio) weakens the corrosiveness. This is because ammonia can partially neutralize the acidity of the solution, increase the pH value of the solution, inhibit the formation of ammonium cyanate that is corrosive to stainless steel, and reduce corrosion caused by the presence of large amounts of water. After the increase of H2O/CO2, the concentration of ammonia in the solution is reduced, which promotes the formation of ammonium cyanate and increases the corrosion of stainless steel. 3.6 Temperature As the temperature rises, corrosion intensifies. Generally, the operating temperature of stainless steel for urea should not exceed 190°C. When it exceeds 195°C, the corrosion rate accelerates, that is, it exceeds the allowable corrosion rate several times or even more than ten times. 3.7 Flow rate of the molten liquid If the molten liquid flows through the metal surface too fast, it is easy to destroy the passivation film and aggravate corrosion. The high flow rate erosion destroys the passivation film on the metal surface and easily causes the metal surface to crack, thereby accelerating the erosion effect. The amount of erosion and corrosion is greater than the amount of wear and corrosion combined. However, if the flow rate of the solution is too low, a stagnation zone will be formed. In the stagnation zone, the oxygen in the solution cannot be replenished after consumption, which will also cause corrosion. 4 Control means and measures For the anti-corrosion of the synthetic tower and one-step tower, the author believes that the following work should be done. (1) In normal production, the oxygen content in the raw gas should be controlled at about 0.5% ； The amount of air added to the heater inlet should not be less than 1 m3 per ton of urea. One hour before planned shutdown, the oxygen content of the raw gas should be controlled at about 0.8% to avoid a large amount of O2 in the liquid phase escaping after the tower is sealed and aggravating corrosion. (2) The tower should be sealed under high pressure (16~18MPa) as much as possible. The shorter the tower sealing time, the better, and it should not exceed 24 hours at most. (3) During winter production, the air volume and operation conditions of the first tower should be carefully inspected to prevent pipeline freezing and blocking. If freezing blocking is found, it should be dealt with promptly. (4) NH3/CO2 should be controlled at 3.8~4.2 as much as possible, because excess ammonia can reduce the acidity of the system. (5) The water-to-carbon ratio of the feed to the synthesis tower should be less than 1.0. When the water-to-carbon ratio increases, the pH value of the solution decreases, which will promote the formation of reducing acid radicals, methylammonium radicals and cyanate radicals, and intensify corrosion. (6) During equipment maintenance, procedures should be strictly followed, such as replacing internal parts of the equipment, repair welding, etc. Special materials should be used for materials and welding rods. (7) Dead corners and stagnant areas should be reduced, and the liquid level gauge and temperature and pressure pipes of the one-part tower should be shortened as much as possible. (8) During startup, shutdown or production, it is forbidden to open and close P4, H3 and other cut-off valves or regulating valves violently, and the material flow must be smooth. (9) During hot washing, the chloride ion content of the incoming water should be strictly controlled. It is forbidden to use water and condensate containing chloride ions as flushing water, and it is strictly forbidden to enter the system at a time. (10) Monitor the Ni content in the finished urea regularly. The Ni content should be less than 0.2×10-6. When the Ni content exceeds 0.2×10-6, find the cause and take measures. (11) Stabilize the decarburization system and stabilize the CO2 flow and pressure, which is beneficial to the stability of the amount of O2 added to the raw gas. Strictly control wet and dry desulfurization indicators, and prohibit excessive H2S in CO2. (12) The synthesis tower cannot recover after O2 is cut off for more than 10 minutes, and the system should be shut down for a short period of time. (13) Strictly control the temperature of the synthesis tower not to exceed 190°C. (14) Monitor the leak detection pipe of the synthesis tower and deal with problems in a timely manner. 5 Conclusion The anti-corrosion of urea system is a relatively complex issue involving many factors, which requires us to strictly control various process indicators in normal production, pay attention to anti-corrosion work ideologically, and conduct systematic research and analysis. At the same time, attention should be paid to the fact that after adding O2 for anti-corrosion, the system will form an explosive hazard. Corresponding measures should be taken to prevent explosion and ensure the safe and stable operation of the device. With the advancement of science and technology, there will be more scientific, economical and safe methods for anti-corrosion of urea system. We must keep learning * , master more knowledge and do this work well.

Snam's ammonia stripping urea unit consists of three high-temperature and high-pressure equipment including a synthesis tower, a stripping tower, and a methane-ammonium separator. The design gauge pressure of the synthesis tower is 16.7MPa and the temperature is 188℃ ； The design gauge pressure of the stripping tower is 16.2MPa and the temperature is 207℃ ； The design gauge pressure of the methylammonium separator is 16.2MPa and the temperature is 155℃. With the extension of operation time, some manufacturers have experienced equipment leakage accidents one after another, and the inner lining and lining welds of the equipment have experienced varying degrees of corrosion. ； Corrosion and perforation of the pipe heads of the stripping column tubes have seriously affected the stability of the distribution pipes and the stripping efficiency. Since the fertilizer plant of Jianfeng Chemical General Plant started operation in 1992, the synthesis tower has leaked five times and the gas tower has leaked once. ； In particular, two leaks occurred in the synthesis tower in 2002 and 2003, indicating that as the operation cycle of the device prolongs, the equipment corrosion trend becomes more and more serious. This article will start from the structural characteristics of the equipment, analyze the causes of equipment corrosion, and propose treatment countermeasures. 1 Equipment Structural Characteristics The design volumes, internal structures and operating parameters of the three high-temperature and high-pressure equipment of the urea plant are different. The synthesis tower is a plate tower with an internal design of 10 trays (some factories change it to 15 trays). CO2 and NH3 enter from the bottom head of the tower. From bottom to top, there are tube sections 1-15 in order. ； The stripping tower is essentially a heat exchanger. The upper tube box is designed with distribution tubes. Its function is to distribute materials into the tubes and strip CO2 and NH3 out from the exhaust holes at the upper end of the distribution tubes. ； The methylammonium separator, like other separators, has a simple structure. Since they are all affected by high temperature, high pressure and strong corrosive media such as urea, methylammonium, and biuret, they have some common characteristics. 1.1 Both the simplified and lined welded cylinders are composite layers. The inner layer is a thinner 316L urea grade corrosion-resistant lining, and the outer layer is a thicker carbon steel strength layer. The lining welds are monitored by leak detection pipes. Due to the irregular structure of the equipment and the limitation of plate size, the inner lining is made of butt-welded layers, with a 90mm wide lining strip along the weld for secondary protection. Lining belt has two structures: One is welded on the surface of the lining weld (see Figure 1), which is a covered structure. The stripping tower adopts this structure. ； One pad is on the back of the weld (see Figure 2), which is a pad structure. The synthesis tower and the ammonium methane separator adopt this structure. http://www.nmtech.com.cn/jishuwang/upload/060208815296482.jpg http://www.nmtech.com.cn/jishuwang/upload/060208816028589.jpg1.2 There is a certain gap between the single lining and the strength layer. Although the inner lining and the strength layer are closely attached, the lining and the strength layer form their own surfaces, and surface unevenness is inevitable during the processing and molding process of the sheet. Therefore, after the inner lining is blasted or otherwise mechanically bonded to the strength layer, it does not form a dense tissue and a certain gap is formed between them. The external leakage of the synthesis tower of the fertilizer plant of Jianfeng Chemical General Plant in April 2003 illustrates this situation. At that time, there was a problem with the 6th lining cylinder section of the synthesis tower, and material crystallization appeared in the 49# and 29# leak detection pipes. After adding medium-pressure flushing water from the 50# and 30# leak detection pipes on the same closed trench, Flush water leaked out through the 93# weld leak detection pipe in the middle discharge pipe of the lining in Section 6, and the 93# leak detection pipe was not in a closed trench with the 49#, 50#, 30#, and 29# leak detection pipes, indicating that the flushing water passed through the gap between the lining and the strength layer and reached the 93# leak detection pipe. However, this gap is generally very small. Under normal pressure, the gap is 0~0.5mm. Under operating conditions, the lining is affected by high internal pressure and is close to the strength layer. The gap appears as random unevenness on the surface of the laminate. When the weld is corroded somewhere, the material first enters the groove on the lining. According to the principle that fluids in multi-channels preferentially choose channels with low fluid resistance, when the groove is unobstructed, the logistics will first choose the groove on the lining to flow to the leak detection pipe, so specific analysis should be carried out based on on-site iron and carbon ion data. 1.3 A closed groove line is formed along the periphery of the lining weld and the lining belt. Whether it is a circular lining at the head, a fan-shaped lining (see Figure 3), or a rectangular lining at the barrel section (see Figure 4), etc., there is a closed groove on the lining belt at each edge. As mentioned above, this groove is connected to the leak detection pipe to protect the material from entering the gap between the lining and the strength layer when the weld leaks, and corroding the strength layer. http://www.nmtech.com.cn/jishuwang/upload/060208816381184.jpg http://www.nmtech.com.cn/jishuwang/upload/060208817122720.jpg2 Common corrosion and faults 2.1 Uniform corrosion of lining Uniform corrosion of lining is the overall corrosion of the inner cylinder wall of the container by the material. With three high-temperature and high-pressure equipment in the urea system, under the action of urea, biuret, methylammonium and other media, the wall thickness of the corrosion-resistant layer lining has changed to a certain extent. Taking the thickness measurement of the three major overhauls of the synthesis tower as an example (see Table 1), the minimum lining wall thickness decreased from 4.8mm in 1998 to 4.2mm in 2001, and the average annual corrosion rate was around 0.15mm. It can be seen that the uniform corrosion of high-temperature and high-pressure equipment by corrosion-resistant layers and highly corrosive media is quite obvious. http://www.nmtech.com.cn/jishuwang/upload/060208817525507.jpg 2.2 Local corrosion of welds Under the action of strong corrosive media, the welds first form intergranular corrosion, and then create crevice corrosion. From the inspection of opening the cover of high-temperature and high-pressure equipment, it can be seen that the corrosion of lining welds generally includes weld knife line corrosion (see Figure 5). The corrosion area is like a mark cut by a knife, and the knife seam corrosion gap penetrates into the inside of the weld. ； The welding meat is corroded (Figure 6) and turns black, with local corrosion pits and the flesh is loose and feathery. ； Fusion line corrosion (Figure 7) forms pits and the like on the separation line between the weld and the base metal. http://www.nmtech.com.cn/jishuwang/upload/060208818461224.jpg http://www.nmtech.com.cn/jishuwang/upload/060208819398229.jpg http://www.nmtech.com.cn/jishuwang/upload/060208820298861.jpg2.3 Pipe head corrosion The pipe head of the stripping tower is subject to medium erosion, gap corrosion between the sealing PTFE ring and the outer wall of the pipe, and uniform corrosion of the inner pipe wall by the medium. Corrosion at the pipe head generally shows signs of carbonization, pipe wall thinning, perforation, and gnawing (see Figure 8). http://www.nmtech.com.cn/jishuwang/upload/060208821065756.jpg 2.4 Leakage from the leak detection pipe As the operation time increases, the hidden dangers of high-temperature and high-pressure equipment are gradually exposed. From the start of the operation in 1992 to the first leakage in the synthesis tower in 1995, there have been 6 leakage accidents (Table 2), 5 times in the synthesis tower and 1 time in the stripping tower. http://www.nmtech.com.cn/jishuwang/upload/060208821544819.jpg 3 Analysis of corrosion causes 3.1 The residual thermal stress of the weld seam and the influence of pores and impurities on the weld seam are the internal causes of cracks in the weld seam. Although the post-weld thermal stress of yustenitic stainless steel is very small, local thermal stress must exist due to reasons such as the level of welding technology. ； At the same time, the weld ferrite may exceed the standard due to the random sampling nature of the test. During welding, defects such as bubbles, inclusions, and cracks may also form. If the defects are in the middle of the weld, it will be difficult to detect them with color flaw detection. Once the weld is uniformly corroded to a certain extent, these defects will be quickly exposed. 3.2 Impact of fluctuations in operating parameters Temperature and pressure are the most important parameters for controlling the operation of equipment. Fluctuations in temperature and pressure have a direct impact on the cracks, inclusions, and pores existing in the weld. The stress generated by materials under conditions of temperature difference and pressure fluctuation can accelerate the expansion of existing weld cracks and generate new cracks. System startup and shutdown are important factors causing temperature and pressure fluctuations. Every time the machine is started, the temperature and pressure will undergo a change from normal temperature and pressure to the equipment's designed operating temperature and pressure (the opposite is true when the machine is stopped), causing fatigue damage to the weld. The more times it is started and stopped, the greater the impact on the service life of the equipment. From the time the fertilizer plant of Jianfeng Chemical General Plant was put into operation until 1996, Class A long-term cycles rarely exceeded 100 days, and there was no record of Class B long-term cycles exceeding 200 days. The system was unstable and caused potential risks for leakage of high-voltage equipment later. 3.3 Effect of Fluid Erosion: The erosion of the lining or internal parts by the medium at high temperatures is prone to high-temperature corrosion, hydrogen corrosion, chlorination, etc. Judging from the inspection of the synthesis tower, CO2 entered the bottom of the synthesis tower at a higher flow rate, causing corrosion in the lower half to be significantly more serious than in the upper half. The weld cracked twice at the butt weld between the lower head and the first cylinder section, once at the butt weld between the first cylinder section and the second cylinder section, once at the weld of the third cylinder section, and once at the weld of the sixth cylinder section. These leaks were all in the lower half of the synthesis tower, indicating that the process fluid had obvious erosion of the welds. Although the stripping tower is a falling film process and the speed is slow, the impact of erosion is also considerable from the corrosion phenomenon at the pipe mouth. In 2000, the stripping tower leaked, and the location was on the pipe box at the liquid inlet end. This accidental leakage to some extent indicates the inevitable existence of impact corrosion. 3.4 Other factors affecting the corrosion of high-temperature and high-pressure equipment In addition to the objective factors mentioned above, human-induced corrosion factors also exist during installation, maintenance, and operation. If the equipment lining is scratched during installation and maintenance, it will cause intergranular corrosion of the equipment. ； When the system is shut down and the tower is closed, the equipment is prone to local high temperatures, which will accelerate the corrosion of high-temperature and high-pressure equipment. 4 Treatment and Countermeasures 4.1 Clearing the lining groove Before stopping the vehicle, ensure that the lining ring groove at the leakage point is unobstructed so that the leaked material can be discharged from the leak detection pipe along the groove without penetrating into the gap between the lining and the strength layer and corroding the strength layer. To unblock the lining groove, medium-pressure flushing water is generally used, which is added from one leak detection pipe and discharged from another leak detection pipe. If there are multiple leak detection pipes on the closed ring groove, the flushing water is best added in a dispersed manner and discharged from one leak detection pipe. The drain and leak detection pipe is best selected as the leak detection pipe where material leakage is found, because the location of this leak detection pipe is most likely to be close to the leakage crack. 4.2 Analysis and monitoring The degree of crack leakage must be analyzed and tested regularly, and the test sample should be the discharge liquid of flushing water. The flushing water discharged from the leak detection pipe is condensed by adding cooling water through the threaded coil. Samples are taken to analyze the contents of ammonia, CO2, urea, and iron ions in the solution. An analysis chart of the total amount of leakage and the trend of each individual amount is made to serve as a reference for selecting the timing of parking. After the stripper leaked in 2000, it was shut down twice. The leak was not found during the first stop, but the leak was found only after the vehicle continued to run for a period of time and was stopped again. The economic losses incurred were staggering, indicating that the timing of stopping the vehicle to deal with the leak is very important. According to several processing conditions, when the total discharge of materials is 600 mg/min, leak detection is possible. However, the specific shutdown processing time must be determined based on all aspects of the situation. If the material does not damage the strength layer, observation operation is feasible. 4.3 Stop and check for leaks When the time is right, you must stop and check for leaks. The principle of leak detection is basically the same. Fill one side of the lining with permeable air, apply coloring reagent on the other side, and determine the position according to the change of the coloring agent. At present, the leak detection method generally uses the ammonia leakage method and phenolphthalein reagent coloring. This is because ammonia has strong penetrating ability and is easy to obtain in large fertilizer plants. Before filling with ammonia, vacuum should be evacuated first. When the vacuum degree is around 49.0kPa, choose to add ammonia from the leak detection pipe at the leakage point of the material. When adding ammonia, care should be taken that the pressure is not too high. Generally, there is basically no problem in detecting leaks when the gauge pressure is below 78.4kPa. 4.4 After the location of the leakage point is determined for repair welding, the cracks at the leakage point should be polished. The grinding width and length should be based on the elimination of all cracks on the weld at that location. The grinding temperature should not be too high, and it is better to grind intermittently to avoid the increase of ferrite due to the increase in temperature. Argon arc welding is used for welding, and the welding material should be close to the lining material. There are three urea system equipments, and the welding material is 00Cr25Ni22Mo2 stainless steel. Coloring and flaw detection are performed after welding. 4.5 Routine inspections of major and medium maintenance high-temperature and high-pressure equipment are affected by operating conditions and highly corrosive media, and there are many potential hazards. Especially as the operation time increases, the hidden hazards are exposed at an accelerated rate. Therefore, when using large and medium repairs, a comprehensive inspection of the equipment must be carried out, including lining thickness measurement, weld inspection, corrosion weld treatment, tube head treatment, tube descaling, etc. General weld processing requirements: When the lining thickness is not less than 5mm, the weld grinding depth is not more than 2mm and no repair welding is required. ； When the lining thickness is less than 5mm, repair welding is required if the grinding depth is less than 2mm but greater than 1mm. Corrosion of high-temperature and high-pressure equipment in urea plants is common. The Fertilizer Plant of Jianfeng Chemical General Plant has accumulated valuable practical experience by dealing with corrosion and leakage of high-temperature and high-pressure equipment, especially in terms of treatment methods and timing of shutdowns. Although the synthesis tower has continuously leaked in the past two years, the losses caused by shutdown treatment have been greatly reduced. Since the corrosion mechanisms of high-temperature and high-pressure equipment are multifaceted, there is still a long way to go to solve the lining corrosion problem.

The urea synthesis tower linings in domestic urea production plants are made of various corrosion-resistant materials, including industrial pure titanium, 316L (MOD), X2CrNiMo25.22.2, and possibly other duplex stainless steels. However, 316L (MOD) and X2CrNiMo25.22.2 are mostly used as corrosion-resistant linings. After a certain period of use, most urine towers with 316L (MOD) corrosion-resistant lining will suffer from weld corrosion (welded with Thermanit19/15H electrode) and knife edge corrosion at the edge of the weld. The lining weld welded with BM310Mo-L electrode mainly produces knife-edge corrosion in the near seam area, while the weld is in a normal corrosion state. Some welds appear convex after corrosion, indicating that the corrosion resistance of the weld metal is better than that of the base material 316L (MOD). ? 1. Discussion on the causes of knife-edge corrosion. The main reason for the occurrence of knife-edge corrosion is the influence of welding heat. There is an area near the seam area of ​​the lining weld that is subject to sensitization (600~1000℃) temperature under the action of welding heat. Although 316L (MOD) is an ultra-low carbon stainless steel, its corrosion resistance will inevitably decrease in areas subject to sensitization temperatures. That is to say, the welded joint itself has inhomogeneity in structure and performance (including corrosion resistance), and the corrosion resistance of the sensitized (coarse grain) zone is the worst area in the entire welded joint, and it is also the area with the most serious corrosion. The size of this area depends on the size of the welding heat input. Of course, whether knife-edge corrosion occurs is directly related to the welding specification parameters, that is, the size of the heat input. When the heat input is too large, that is, the welding current is too large and the welding speed is too slow, it will inevitably cause the high temperature residence time in the near seam area to be too long, the precipitation of Cr23C6 chromium carbide to increase, and the grain size to grow, etc., all of which will lead to a decrease in corrosion resistance in this area. As the heat input increases, the width of this area also increases. In order to reduce the tendency of knife-edge corrosion, the heat input should be minimized, and it is best to control the welding heat input below 10kJ/cm. However, despite this, it is difficult to avoid knife-edge corrosion. In addition to process factors, the key issue should be the chemical composition of the lining base material itself, such as carbon content, chromium content, and the content of stable chemical elements that have a stronger affinity with carbon than chromium, etc. Based on the above understanding, we believe that although 316L (MOD) is ultra-low carbon steel, its Cr content is lower than that of X2CrNiMo25.22.2. Even if extra care is taken in the welding process, knife-edge corrosion will inevitably occur, especially in longitudinal welds. 2 Repair opinions on weld corrosion of urinary tower lining and knife edge corrosion in near seam area 2.1 Repair of welded cover plate ① Before welding the cover plate, the weld seam of the plain lining must be ground first to facilitate the close contact between the cover plate and the lining. Grind and remove corrosion from the connection between the lining and the cover to facilitate welding. ②The lining covered by the cover must be drilled so that the hole is connected to the original leak detection system to avoid forming a closed air chamber and at the same time serve as a leak detection channel for the weld connecting the cover and lining. ③It is best to use X2CrNiMo25.22.2 as the cover plate material, so that even if knife edge corrosion occurs in the near seam area in the future, it will only occur on one side of each weld. If the same 316L (MOD) as the lining is used, knife-edge corrosion may occur on both sides of each weld. ④The cover must have a certain width, and its width should not be less than 40mm. If it is too narrow and the distance between the two fillet welds is too close, it will inevitably affect the corrosion resistance of the cover plate. ⑤The cover plate must have a certain thickness. The thickness should not only consider the corrosion needs, that is, the corrosion resistance thickness of the fillet weld, but also consider whether the welding of the cover plate will affect the installation of the tray. The thickness of the cover plate should be slightly thicker than the thickness of the lining plate. 2.2 Post-weld treatment: Polish and eliminate the corrosion layer and knife-edge corrosion grooves, and take necessary repair welding measures to repair the lining weld, but the following issues must be paid attention to. ①Avoid overlapping the hot zone of the repair weld with that of the original weld. The width of the repair weld should be at least 2 to 3 mm wider on each side than the original weld, because the overheated coarse-grained zone is usually within 2 mm of the fusion line. ②When the welding seam is wide, multi-pass welding should be used. The swing amplitude of the electrode during each welding pass should be controlled within 2 times the diameter of the welding core. ③The groove after knife-edge corrosion grinding should be used as the final repair welding bead as much as possible. 2.3 Opinions on selecting the weld repair method for urinary tower lining ① After the above two repair methods are repaired, there is also the possibility of knife-edge corrosion occurring again in the near seam area. ②We believe that when the weld corrosion is serious and the width of the repair welding is greater than 20mm, the covering plate method can be considered for repair. ； When it is less than 20mm, grinding and repair welding can be considered to repair it. ③When only knife-edge corrosion occurs and the weld is still intact, there is no need to use a cover plate to repair it. We believe the exact method of repair should be determined on a case-by-case basis. The method of adding a welding cover plate is suitable for situations where the repair area is relatively large and the amount of welding is relatively large. For small repair areas, priority should be given to repair measures such as repair welding after grinding.

1 Question raised Hunan Jinxin Chemical Co., Ltd. has an annual output of 260kt of urea and has two sets of new and old aqueous solution full cycle equipment with basically the same process flow. Among them, urea production equipment such as high-pressure ammonium methane pumps, liquid ammonia pumps and two-stage evaporation heaters (referred to as two-stage evaporation heaters), during the process production process, due to poor corrosion resistance and other reasons, the equipment has a short service life and frequent maintenance or replacement, seriously affecting the long-term economic operation of the system. The main situation is as follows. 1.1 High-pressure ammonium methane pump This equipment is the main equipment for urea production. The model is T15SV-160.68-136S3 triple horizontal injection plug pump. Its function is to pressurize the methylammonium liquid from 1.75MPa to 20MPa and send it to the urea synthesis tower. The original designed valve seat material is SVS33 (the sealing surface is plated with carbide), the valve core material is SCS16 (the sealing surface is cast material of SVS23), and the inlet and discharge valve spring is HASTELLOYC (hydrochloride-resistant nickel-based alloy). Due to the unreliability of the carbide embedded in the valve seat and valve core sealing surface, 0Cr18Ni12Mo2Ti was used as the valve seat and valve core material and the inlet and discharge valve spring materials, which solved the problem better. However, the one-way valve of the high-pressure ammonium methane pump is severely corroded and has a short service life. According to the maintenance files, the shortest one was only a few hours before pitting appeared on the surface (it was suspected that the wrong material was used at the time), the longest one was only 562 hours, and the average service life was only 178 hours. Every time the valve is inspected, it is found that the crystal lattice on the inner surface of the valve seat is exposed, the smoothness is poor, and the gap between the valve seat and the valve core is increased, usually 1.6 to 2.3mm, sometimes as high as 3.5mm (standard value 0.20 to 0.30mm). In addition, there is a common situation where the valve seat, valve core contact surface and spring seat are sunken. , the seal is not tight, the first-class pump does not work well, and the vibration of the pump body and mesh surface is intensified. Due to insufficient heat treatment hardness, the springs of the inlet and discharge valves are not as obvious as the corrosion of the valve seat and valve core. However, because they bear pulsating impact loads, fatigue corrosion is prone to occur, leading to fracture. The shortest time is 196h, and the longest time is about 1000h. 1.2 High-pressure liquid ammonia pump. The model of this pump is T18S-250.8-180SF (3). It has the same structure as the methane ammonium pump. The difference is that it pressurizes 1.75MPa liquid ammonia to 20MPa and sends it to the urea synthesis tower. The original design valve seat and valve core are made of SCS13 stellite alloy, and the inlet and discharge valve springs are SVS27. The high-pressure liquid ammonia pump is the same as the methylammonium pump. Due to poor inlay quality, 0Cr18Ni9 is used as the valve seat, valve core material and inlet and outlet valve spring material. Because liquid ammonia has almost no corrosion to single-phase materials (0Cr18Ni9) under normal circumstances, the corrosion of the inlet and outlet valves of the high-pressure liquid ammonia pump is not as obvious as that of the high-pressure methylammonium pump. However, due to unstable system production, the accumulation of liquid in the ammonia cooler can cause a small amount of ammonium carbonate to be carried in the liquid ammonia. In addition, the flow rate of liquid ammonia through the inlet and outlet valves is extremely high, resulting in erosion and corrosion. Therefore, the life of the inlet and outlet valves is only 2 to 3 times that of the high-pressure methane ammonium pump. At the same time, dents in the valve seat and valve core sealing surfaces and brittle breakage of the inlet and outlet valve springs also occur from time to time. 1.3 Secondary evaporation heater This equipment is the key equipment of the evaporation system. Its function is to further evaporate and purify 95% of the urine in a period to 99.7%. This equipment is a typical shell and tube heat exchanger, its technical characteristics are shown in Table 1. Its upper and lower tube plates are made of 0Cr18Ni12Mo2Ti, the tubes are made of 0Cr18Ni12Mo2Ti, the specification is φ25×2mm, and the shell side material is Q235-A, δ=6mm. http://www.nmtech.com.cn/jishuwang/upload/0602081023375155.jpg During production, due to severe erosion and corrosion from urea and steam, the average service life of each secondary steam heater is only about one year despite frequent maintenance. The main failure mode is failure caused by damage and leakage of the tube sheet, the welds between the tube sheet and the tubes, and the tubes. Every time I used condensate to check for leaks, I found that the weld area between the tubes and the upper tube plate was like a fountain. The surface of the weld was rough, with serious pit corrosion, pinholes, and cracks. The tubes were also significantly thinner. During the inspection, it was found that the tubes were silver-gray and the mouth of the tubes was knife-edge shaped. In addition to the short life of the heat exchanger, which seriously affects production, leakage causes steam to escape into the urine, which often causes the water content of urea to increase and become defective products. The annual direct economic loss is more than 30,000 yuan. Based on the above situation, it is of great significance to find out the causes of corrosion of equipment such as methylammonium pumps, liquid ammonia pumps and secondary steam heaters, and carry out targeted treatments to improve their service life, which is of great significance to the economic operation of the urea production system. 2 Analysis of the reasons for the short service life of the three types of equipment 2.1 Analysis of the high-pressure methane ammonium pump believes that there are many reasons for the short service life of the high-pressure methane ammonium pump inlet and outlet valves and the inlet and outlet valve springs, mainly including the following aspects. ①Improper material selection. 0Cr18Ni12Mo2Ti pipe has strong corrosion resistance in methane ammonium solution, but because the methane ammonium liquid flows very fast when passing through the one-way valve, it has strong erosion corrosion. Once corrosion forms on the surface of 0Cr18Ni12Mo2Ti, intergranular corrosion will occur, aggravating the corrosion. ②Stress corrosion. Because the inlet and discharge valves have been heat treated, residual stress exists after heat treatment, causing stress corrosion. The valve core and spring are subject to impact loads, so they are prone to corrosion fatigue damage. ③Improper heat treatment. The surface hardness of 0Cr18Ni12Mo2Ti nitrided treatment is Hv750~1000. The surface hardness is not enough, which is the main reason for the valve seat seal of the valve core to be dented. The brittle fracture of the inlet and outlet valve springs is also caused by improper handling. ④The design structure is unreasonable. 2.2 High-pressure liquid ammonia pump The cause of corrosion of this equipment is the same as that of the high-pressure ammonium methane pump. The only difference is that liquid ammonia is less corrosive to the valve core and valve seat material (0Cr18Ni9), mainly the erosion corrosion mentioned in the previous chapter. 2.3 Secondary steam heater The corrosion causes of this equipment can be summarized into the following factors. ①The material has poor corrosion resistance. Although 0Cr18Ni12Mo2Ti has strong corrosion resistance, its corrosion resistance temperature is below 190°C. Above this temperature, the corrosion rate increases exponentially. In addition, there are various local corrosions in 0Cr18Ni12Mo2Ti, especially the corrosion of welded joints is more serious. The original design capacity of our company's old urea device was 110kt/a. After technical transformation, the annual output of the device exceeded the design capacity. Due to long-term high-load operation and fast material flow rate, the outlet of the cracked pipe was eroded and corroded, and the wall thickness of the pipe mouth was thinned, forming a knife-edge shape. ②Crevice corrosion caused by the gap between the tube sheet and the tubes. ③Process operations and other reasons. 3 Apply titanium materials to improve equipment corrosion resistance. It can be known from relevant literature that titanium materials have excellent corrosion resistance under high temperature and pressure, which is an order of magnitude higher than 0Cr17Ni14Mo2Ti ultra-low carbon stainless steel. Their sensitivity to corrosion caused by oxygen and sulfide in urea and methylammonium solutions is lower than that of stainless steel. We used new titanium materials to transform the urea methylammonium pump, liquid ammonia pump, and secondary steam heater respectively. The transformation status and effects are described below. 3.1 Transformation of the high-pressure ammonium methane pump In order to improve the corrosion resistance of the methane ammonium pump, according to the working characteristics of the methane ammonium pump, we use titanium materials to modify its one-way valve and spring. The material of the inlet and outlet valve seats and valve cores was changed to TC4, and the inlet and outlet valve springs were changed to TC3. At the same time, glow ion nitriding treatment technology is used, the treatment depth is 1.2~1.5mm, and then the residual stress is eliminated, so that the surface hardness is increased from Hv750~1000 of 0Cr18Ni12Mo2Ti to Hv1200~1300 of TC4. The titanium material transformation of the methylammonium pump has achieved gratifying results. It can be found from the maintenance records of the past three years: The service life of titanium valves has been increased by more than 5 times, with the longest reaching 106 days. ； The maximum service life of the inlet and discharge valve springs is 3899 hours, and the shortest service life is 1421 hours, which is more than 7 times longer than that of stainless steel springs. 3.2 The transformation of the high-pressure liquid ammonia pump is similar to the transformation of the methylammonium pump. The inlet and discharge valves of the high-pressure liquid ammonia pump also use TC4, and their springs use TC3. Since titanium material is relatively expensive, 3 to 4 times that of stainless steel, in order to reduce the cost of modification and improve economy, we changed the valve seats of the ammonium methane pump, liquid ammonia pump, and discharge valve to the shape shown in Figure 1. Since the modified valve seat is symmetrical, one valve seat can be used twice, doubling the service life of the valve body. The technical and economic indicators of the modified ammonium methane pump and liquid ammonia pump are very ideal, see Table 2. 3.3 Transformation of the secondary steam heater In order to inhibit the corrosion of the secondary steam heater, we have modified it with titanium as the main material.: The tube material is TA2, the upper and lower tube plates are made of TA2+16MnR explosive composite plates (the purpose of using titanium explosive composite plates is to reduce manufacturing costs), and the flux used to weld the tubes to the upper and lower steel plates is TA2M (GB3623-83). http://www.nmtech.com.cn/jishuwang/upload/0602081024285283.jpg Figure 1 Schematic diagram of the valve seat structure of the modified ammonium methane pump and liquid ammonia pump http://www.nmtech.com.cn/jishuwang/upload/0602081025008851.jpg Since titanium materials are sensitive to crevice corrosion in urine, that is, if the concentration of titanium ions and accumulated oxygen in the crevices is insufficient, it is easy to cause activation of titanium and intensify corrosion. In order to prevent the occurrence of this sensitive phenomenon, in the structural design of the heat exchanger, the titanium equipment of the tubes and tube plates is manufactured using a process of first expansion and then welding. The specific structural form is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/0602081025358747.jpg Figure 2 Schematic diagram of the tube plate and tube structure of the secondary steam heater. At the same time, in order to ensure that the manufacturing of titanium equipment meets the design technical requirements, a tender for equipment manufacturing technology was adopted. After the secondary steam heater modified with titanium material was put into use in the system for 2 years, during the overhaul inspection, the surface of the tube plate and tubes was smooth, with no obvious corrosion, that is, no obvious thinning, and no significant difference from the new heat exchanger. The equipment has been running for 6 years and is still in normal operation, indicating that the titanium material transformation is very successful. 4 Conclusion Titanium material is a new type of material with excellent corrosion resistance. The use of titanium materials to transform the urea methylammonium pump, liquid ammonia pump and secondary steam heater has greatly increased the service life of the equipment and achieved satisfactory results. The success of this transformation has important reference significance for the transformation of equipment in other anti-corrosion conditions of the urea system.

1 Introduction The saturated hot water tower is an important equipment in the ammonia synthesis system. The accident rate is relatively high. In the past, the hot water tower was made of 16MnR, and the wall thickness thinning phenomenon was very serious during use. Nowadays, it is often made of austenitic stainless steel materials, which solves the problem of too rapid wall thickness thinning. However, judging from the usage conditions, saturated hot water towers made of austenitic stainless steel materials also have varying degrees of corrosion phenomena. A saturated hot water tower put into use by a company in April 2004 consists of upper and lower parts. The upper part is a saturated tower. The medium is semi-water gas and water vapor. The working pressure is 2.01 MPa (design pressure 2.11MPa), the working temperature is 40~160℃ (design temperature 160℃), the diameter is 2000mm, and the wall thickness is 18mm. ； The lower part is the hot water tower, the medium is conversion gas and water vapor, the working pressure is 1.96MPa (design pressure 2.11MPa), the working temperature is 115~180℃ (design temperature 180℃), the diameter is 2200mm, and the wall thickness is 20mm. The materials of the upper and lower tower cylinders and heads are both 0Crl8Ni9, and are connected by a variable diameter section in the middle. The materials are also 0Crl8Ni9. In April 2005, the lower head of the hot water tower leaked. Penetration testing was carried out on the inner and outer surfaces of the annular joints and joints of the lower head. It was found that there were a large number of longitudinal cracks in the heat-affected zones on both sides of the joints on the inner surface, with an intermittent length of 1800mm, and some of them had been cracked (cracks were 100mm long). ； No defects were found in the circumferential seams. The hardness of the base metal, welds, and heat-affected zones at the cracked location was measured, and all were within the normal range. A simple test with a magnet showed no obvious magnetism in the material. I checked the operation records of the equipment and found no over-temperature phenomenon. The unit's process technicians also confirmed that the equipment has been operating under normal process parameters. After checking the quality certificate of the equipment, it was found that solution treatment was not performed after the head was formed. The design drawings also do not require solution treatment. 2 Cause Analysis Intergranular corrosion of austenitic stainless steel is the main reason for the formation of corrosion cracks and leakage. During the container manufacturing process, when hot processing, welding or heat treatment, the stainless steel material stays in the temperature range of 450 to 850°C, and chromium-rich carbide Cr23C6 precipitates at the grain boundary, thus forming a chromium-poor area at the grain boundary, affecting its corrosion resistance and causing intergranular corrosion tendencies. 0Crl8Ni9 is an austenitic stainless steel that does not contain stabilizing elements. After welding thermal cycle, intergranular corrosion sensitization zone will appear in the base metal near the welding joint. More importantly, during the hot press forming process, the austenitic stainless steel head has large thickness, large thermal hysteresis, large linear expansion coefficient, and a long time in the sensitization temperature range (450~850°C), resulting in intergranular chromium deficiency. If corresponding heat treatment measures are not taken, intergranular corrosion is prone to occur. Therefore, after manufacturing and processing 0Crl8Ni9 materials, it is best to undergo solid solution treatment (quick cooling at 1010~1150℃) to avoid intergranular corrosion. However, relevant regulations such as GB150-1998 and "Pressure Vessel Safety Technical Supervision Regulations" do not require such issues. Article 74 of the "Pressure Vessel Safety Technical Supervision Regulations": Austenitic stainless steel pressure vessels generally do not require heat treatment after welding ； GB150-1998 10.4.2.2 regulations: Unless otherwise specified in the drawing, cold formed austenitic stainless steel heads may not be heat treated. Therefore, it is compliant with the requirements that the manufacturer does not perform solution treatment, but this can easily cause intergranular corrosion. After on-site sampling for metallographic examination and analysis, it was found that carbides precipitated in the heat-affected zone, which was an important reason for intergranular corrosion caused by chromium deficiency. 3 Preventive measures for intergranular corrosion 3.1 Material selection 3.1.1 Reduce the carbon content of steel Intergranular corrosion is related to the precipitation of carbides. Reducing the carbon content in steel is an effective way to prevent intergranular corrosion of austenitic stainless steel. In order to prevent intergranular corrosion of stainless steel, especially to prevent the risk of knife edge corrosion in stainless steel welds, ultra-low carbon stainless steel can be used. Choosing ultra-low carbon austenitic stainless steel with a carbon content of ≤0.03% can basically eliminate intergranular corrosion. 3.1.2 Adding alloying elements Austenitic stainless steel can also reduce the occurrence of intergranular corrosion by reasonably adding alloying elements. like: Adding titanium, niobium or tantalum, etc., these elements have a strong affinity with carbon. When the content is high enough, they can prevent the deposition of chromium carbide and prevent local depletion of chromium from causing intergranular corrosion. If molybdenum is added, a partially dispersed α phase can be produced under certain conditions to become a multi-phase steel, which can improve its ability to resist intergranular corrosion. Practice has shown that to eliminate intergranular corrosion, some trace elements, such as boron, can be added, which can change the properties of the grain boundaries so that Cr23C6 or σ phase cannot precipitate between the grains. 3.2 Material processing 3.2.1 Solution treatment When austenitic steel is heated to 450~850℃ and cooled slowly, chromium carbide will precipitate and cause intergranular corrosion. In order to restore its ability to resist intergranular corrosion, solid solution treatment is used to redissolve the chromium carbide deposited on the grain boundaries into the austenite structure. Solid solution treatment is generally heated to 1010~1150℃, kept for a period of time, and then rapidly cooled so that carbides cannot precipitate, thereby achieving the purpose of maintaining the single-phase austenite structure at high temperatures and improving the ability to resist intergranular corrosion. 3.2.2 Stabilization treatment Stabilization treatment means that austenitic stainless steel containing titanium and niobium is solution treated and then maintained at 850-900°C (slightly less than the temperature at which chromium carbide is re-dissolved) for a sufficient time (1-4 hours), and then air-cooled to ensure the formation of stable TiC or NbC, thereby reducing the tendency of intergranular corrosion. Moreover, due to the removal of titanium from austenite, its stability is increased, reducing the possibility of phase transformation of austenite, and preventing future intergranular corrosion under long-term heating. 3.3 Develop appropriate welding processes (1) Perform stabilization annealing treatment on weld joints. (2) Use ultra-low carbon or niobium-containing welding rods. (3) Use two-way welding rods, that is, increase the chromium content of the austenitic stainless steel weld metal to 21% or slightly higher, or add other ferrite-forming elements. (4) When arranging the welding sequence, put the side in contact with the medium last for welding. (5) Reduce the time the heat-affected zone is in the sensitization temperature range, such as using a small welding current and a large welding speed to avoid overheating of the welding joint and prevent carbonization of the weld. Cool quickly after welding, and wait for each layer to cool completely before welding the next layer. 4. To deal with the existing saturated hot water tower, there are too many cracks and they are widely distributed. The most suitable solution is to replace the lower head of the hot water tower as a whole. However, this takes too long and is not allowed in production. Therefore, the only way is to patch the cracks with stainless steel plates to cover the cracks, and perform penetrant inspection on the fillet welds after the patching is completed. During the annual overhaul, the company decided to completely replace the lower head of the hot water tower. First of all, it is required that the lower head provided by the manufacturer must undergo solution treatment after being formed. Weld the supporting parts on the hot water tower, and then replace the lower head of the hot water tower as a whole. Welding of the lower head should be carried out strictly in accordance with the welding process formulated before welding to avoid overheating of the welded joint and prevent carbonization of the weld. Cool quickly after welding, and wait until each layer is completely cooled before welding the next layer to avoid intergranular corrosion. After welding, 100% radiographic flaw detection was carried out on the head butt welds, and all passed the second level. A 100% penetrant inspection was conducted on the head butt welds and head seams, and no defects were found. The hardness measurement data are also within the normal range. While replacing the lower head, other parts of the saturated hot water tower were inspected, and radiographic and penetrant spot checks were conducted, but no defects were found. Finally, a pressure test was conducted on the entire saturated hot water tower, and the results were qualified. 5 Conclusion After the lower head of the hot water tower was replaced, the equipment has been running normally. It is recommended that the lower head welds and other welds of the hot water tower should be randomly inspected every year during the annual overhaul to ensure the safe operation of the saturated hot water tower.

1 Overview In the urea production process, in order to process the urea aqueous solution with a temperature of 90 to 120°C and a concentration of about 68% to 75% (mass) after synthesis, heating under reduced pressure, and separation of unreacted materials into solid granular urea, a two-stage vacuum steaming process is generally used to evaporate most of the water in the aqueous solution to obtain a urea melt with a concentration of 98% to 99.8%, which is sent to a granulation tower for granulation. The process is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/060823911003819.jpg The main task of the evaporation process is to evaporate the water in the urea aqueous solution through vacuum to obtain a urea melt with a water content of less than 0.5% (wet basis), a biuret content of less than 1% (wet basis), and other impurity content less than the specified indicators and send it to the granulation tower for granulation to obtain qualified solid granular urea. In order to obtain granular urea that meets the conditions, the analysis and selection of process conditions are generally carried out using the urea-water equilibrium phase diagram. Urea aqueous solution has the characteristics of double boiling points. In order to prevent the operating curve from entering the area between the first and second boiling points, part of the solid urea will precipitate out of the urea solution, block the equipment and pipelines, and cause serious wear and tear on the equipment and pipelines, making the evaporation process unable to proceed normally. Generally, the process control conditions for evaporation are:: flash tank: Pressure 32kPa (absolute), temperature 100℃ ； A period of evaporation: Pressure 26.6~31kPa (absolute), temperature 130℃ ； Ur≈95% two-stage evaporation: Pressure 3.3~9kPa (absolute), temperature 136~142℃ ； Ur≈99.7%. During the operation of the evaporation equipment, the cyclone separator, especially the pipeline between the outlet of the second-stage evaporation heater and the inlet of the second-stage cyclone separator, is prone to corrosion and leakage. Air is sucked into the system, making it difficult to control the second-stage vacuum degree within the specification, affecting the quality of the finished product. In severe cases, the separator may become unstable and scrapped. We analyzed the corrosion problem of the cyclone separator and the process is as follows. 2 Causes of cyclone separator corrosion 2.1 Parts of the cyclone separator that are corroded The parts of the cyclone separator that are most susceptible to corrosion are in order:: See diagram 2 for the upper tubes of the rising film heater, the heater outlet elbow, the rectangular reducer tube, the lower liquid pipe of the separator, etc. http://www.nmtech.com.cn/jishuwang/upload/060823914053657.jpg 2.2 The process media considered in the material selection and processing of the cyclone separator are mainly Ur(L), H2O, Bi, Am, NH3, CO2, etc. In terms of equipment material selection, 304L stainless steel is generally used based on the operating temperature and operating pressure. Argon arc welding is used for processing and welding to reduce the input of linear energy and reduce the sensitization of the welded joints. ； Use 25-22-2 welding materials to obtain ideal corrosion resistance. 2.3 The working medium and process conditions of each part of the cyclone separator are determined by material balance and selection of process conditions. The working medium and process conditions of each part of the cyclone separator are as follows in Table 1. http://www.nmtech.com.cn/jishuwang/upload/060823916387191.jpg 2.4 Main causes of corrosion to cyclone separators 2.4.1 A form of damage caused by abrasive metal surfaces being simultaneously abraded and corroded by fluid media. In the upper part of the heater tube, the velocity of the fluid medium in the tube is very high, close to turbulent flow, and turbulent corrosion occurs. Within this distance, the corrosion rate of the tube is much greater than that of other parts. Since the second-stage evaporation heater uses a rapid rising film heater, among various corrosion factors, abrasion should be an important factor causing equipment corrosion. Based on the material balance, we conducted a simple analysis on the change in the speed of the material flowing through the tubes caused by the change in the material in the heater. The results are as follows: a. The material form changes in the tube at the bottom of the heater. The temperature is low and the urea solution appears in a liquid state.: slower ； In the middle and lower part of the tube, the temperature is higher, and the solution part bubbles and boils faster. ； In the middle and upper part of the tube, the fluid is in a constant-temperature boiling state. The high-speed flowing air column in the center of the tube drives the liquid film on the inner wall of the tube to rise rapidly. The fluid flows very fast. b. Calculated based on Ur150t per shift, when the urea aqueous solution flows through the two-stage evaporation heater tube, the speed of each part of the tube under different control pressures is as follows (tube specification φ25×2000, 85 tubes in total): 2.4.2 Erosion corrosion: The mechanical erosion caused by high-speed liquid on elbows, tees, etc. will destroy the passivation film on the surface of stainless steel or titanium and hinder its passivation, so the corrosion rate increases significantly. It can be seen from the above table that within the normal operating pressure range, the flow rate of the fluid is particularly large, especially for the upper part of the tubes, the effect of erosion and corrosion is more obvious. Since the rising film heater is a tubular heater, the heater can be reverse-installed in a planned manner according to the usage and cycle of the heater to effectively extend the service life of the heater. 2.4.3 Corrosion caused by process control factors mainly means that changes in temperature and pressure cause the operating curve to enter the K1-K2 range of the Ur-H2O equilibrium phase diagram, causing urea crystals to precipitate in the solution, and the high-speed fluid entrains solid urea to cause strong erosion and corrosion of the equipment. http://www.nmtech.com.cn/jishuwang/upload/060823918044922.jpg There are many factors that can change the temperature process indicators of the evaporation system during production. However, the main factors that have a greater impact on the second-stage evaporation are large changes in production load and pressure changes in low-pressure steam. Both of these factors will cause a sudden drop in the temperature of the second section. When the temperature loop of the automatic control system adopts simple control, the temperature fluctuations are more obvious. 2.5 Treatment process for abnormal evaporation situations When the liquid phase outlet temperature of the cyclone separator suddenly drops, the process medium system enters the supersaturated zone and crystallizes. In order to avoid further deterioration of the working conditions, according to the operating procedures, the operating pressure is generally increased for a short time, that is, the vacuum degree of the system is reduced to promptly exit the system from the supersaturated zone. It can be seen from the equilibrium phase diagram that if the pressure is increased in a short time, since the operating point moves to the right (temperature decreases) and upward (pressure increases), the water content in the system increases and the urea content decreases, which will cause the quality of the product to decrease. There is always a certain time lag in this manual operation process. Before the system breaks the vacuum operation, the operating point will enter the supersaturated zone along the isobaric line, causing part of the solid urea to precipitate out of the solution. Due to timely adjustment and control, the system exits the supersaturated zone in a short time, slowly adjusts the pressure and temperature, and the system returns to normal process status. During this adjustment process, partially crystallized urea, driven by high-speed fluid, will cause severe abrasion and erosion corrosion to the secondary steam heater, rectangular pipe and cyclone separator lower liquid pipe. The operation process of breaking the vacuum and restoring normal control in the urea-water (pressure-temperature-concentration) equilibrium phase diagram: http://www.nmtech.com.cn/jishuwang/upload/060823919348127.jpg 3 Main optimization measures taken 3.1 Equipment selection and welding process control 3.1.1 Equipment selection considerations Although 304L stainless steel is relatively corrosion-resistant in the urea production system, because the operating temperature of the secondary steam heater is 140±2°C, and the flow rate of the fluid in the upper tube of the heater is basically at a high subsonic speed, the system has very severe abrasion and erosion corrosion of 304L stainless steel, so it is not suitable to use 304L stainless steel as the heater manufacturing material. After comprehensively considering various factors, we entrusted Shandong Linyi Chemical Machinery Factory to produce the Ti-series heater, which has been put into use for more than 2 years without corrosion or leakage in the heater. 3.1.2 Welding process control requires equipment manufacturing and installation units to use welding materials in strict accordance with the requirements, and use argon arc welding technology for welding and repair welding of stainless steel to reduce the input of line energy and reduce the sensitization of welded joints. 3.2 Operation process control is mainly based on the double boiling point characteristics of urea solution and the process characteristics of two-stage vacuum evaporation, and has been optimized from both automatic control and operation aspects. 3.2.1 Automatic control system a. Stabilize and adjust the low-pressure steam pressure used in the evaporation system to reduce the impact of large fluctuations in steam pressure on the evaporation production system. b. Adjust the evaporation temperature control parameters of the urea DCS system to improve the sensitivity of the evaporation temperature control system and ensure the relative stability of the second-stage evaporation temperature. 3.2.2 Strengthen theoretical and practical training for operation controllers. Operation controllers are required to master the production principles of the urea evaporation process and understand the basic knowledge of phase diagrams. The "Urea-Water Equilibrium Phase Diagram" can be used to guide production operations, especially the process of the system entering and exiting the supersaturated zone after changing the temperature and pressure in normal production. During production control, attention should be paid to changes in liquid level and temperature in the two-stage tower of the circulation system, and the operation of the first-stage evaporation should be adjusted in a timely manner to avoid the front system having a greater impact on the second-stage evaporation. The purpose is to improve the consciousness of controllers to strictly implement process operating procedures, minimize and avoid the system entering the supersaturated zone, so as to reduce the abrasion of the system during this process, while ensuring the stability and uniformity of product quality. 4 Optimization effect of cyclone separator corrosion control http://www.nmtech.com.cn/jishuwang/upload/060823920279634.jpg 5 Conclusion The corrosion resistance of chemical production equipment has always been the primary factor in equipment selection, especially in urea production. During the use of equipment, it is necessary to promptly find out the causes of equipment corrosion and solve them according to changes in usage conditions. This is an important guarantee measure for safe, stable, high-yield, high-quality, and low-consumption urea production.

Our company's ammonia recovery system was originally designed to produce ammonia water by atmospheric pressure absorption. The ammonia absorption tower is a packed tower. The ammonia water produced is sent to the ammonia water station, mainly for farmers. During the off-season, a large amount of ammonia water is discharged, which not only pollutes the environment but also causes waste. In 1994, ammonia recovery was transformed. The ammonia absorption part was changed to pressurized absorption, the ammonia absorption tower was changed to an isothermal plate bubble tower, and an ammonia evaporation system was added. The produced liquid ammonia was sent to urea, and the tail gas was sent to the combustion gas pipe network, completely eradicating pollution. Although the ammonia recovery transformation has achieved good economic and social benefits, the subsequent corrosion problem of the ammonia evaporation tower has become the main factor affecting the stable production of ammonia recovery. 1 Technical parameters and corrosion conditions of ammonia recovery process 1.1 Ammonia recovery process After the tank gas from the liquid ammonia storage tank is mixed with the purge gas from synthesis, it enters from the lower part of the ammonia absorption tower and comes into contact with the dilute ammonia water sprayed from the top of the tower in countercurrent. The ammonia in the gas is absorbed, and the tail gas is sent to the post-process through the gas-liquid separator. The ammonia in the gas is absorbed by the dilute ammonia water to generate 10% to 12% concentrated ammonia water with a temperature of 50 to 55°C. It enters the ammonia water heat exchanger by virtue of the pressure difference and is heated to 150 to 160°C. It enters the tower from the middle and upper part of the ammonia evaporation tower. In the ammonia evaporation tower, the ammonia in the concentrated ammonia water is evaporated, escapes from the top of the tower to the ammonia condenser, and is condensed into liquid ammonia by fresh water at 14°C. Part of it flows back to the upper part of the ammonia evaporation tower, and the rest enters the liquid ammonia storage tank as a product. The dilute ammonia water after evaporating ammonia is sent to the ammonia water heat exchanger to exchange heat with the concentrated ammonia water from the ammonia absorption tower. The dilute ammonia water is cooled to 60-80°C, and sent to the ammonia water cooler for further cooling to 35°C. After being pressurized by the ammonia water circulation pump, it enters the ammonia absorption tower and is circulated and absorbed in the system. The heat required in the ammonia distillation tower is provided by the steam flowing into the reboiler. To ensure that the liquid level is normal, steam condensate needs to be replenished into the system regularly. The ammonia recovery process is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0512291411204922.jpg 1.2 Main technical parameters of ammonia evaporation tower (1) Design parameters: diameter ￠600m, H＝15 640 mm ； There are 23 float valve trays inside, and there are 10 float valves on each tray. ； Tray spacing 400mm ； Working temperature 200～180℃ ； Working pressure 1.3MPa. (2)Material simplified 16MnR ； Tray Q235A-F, ； Fixed distance pipe 10# carbon steel ； Float valve 1Crl8Ni9Ti. (3) The operating pressure of each part of the medium and process indicators of the ammonia distillation tower is 1.3~1.4MPa ； Ammonia steaming tower kettle temperature is 180°C, medium, 0.5% ~ 1% ammonia water ； The feeding temperature of the ammonia steaming tower is 150~160℃, and the medium is 10%~12% ammonia water. ； The temperature at the top of the ammonia distillation tower is 55-65°C, and the medium is 99.5% liquid ammonia. 1.3 Corrosion situation of ammonia distillation tower After the ammonia recovery system was put into operation in October 1994, the operation has been good. During the overhaul in May 1997, it was discovered that the ammonia evaporation tower was severely corroded.: The thinnest part of the 8mm-thick tower is only 3.6 mm ； Most of the float valves on the tray have fallen off due to corrosion, and the welds on the tray have penetrated in many places. ； The 3mm arc-shaped downcomers on the 12th, 13th and 14th floors were penetrated due to severe corrosion, and there were holes about 15mm long. For this reason, Section 4, 6 trays, all distance tubes and all float valves were replaced. In February 1998, it was discovered that there was a leak in the 5th section of the ammonia distillation tower (at the thermometer) and the overall corrosion was serious. In just 9 months, the wall thickness of the cylinder was reduced from 7.8mm to 2.4mm, and the corrosion rate reached 7.2mm/a. Judging from the distribution of corrosion, the feed area of ​​the ammonia evaporation tower is seriously corroded, there is also a certain degree of corrosion in the lower part, and the top is lighter. 2 Analysis of corrosion causes According to the form of corrosion, corrosion can be divided into uniform corrosion and local corrosion. The former occurs relatively uniformly on the entire surface, while the latter only occurs locally. According to the principle of corrosion, it can be divided into chemical corrosion and electrochemical corrosion. The degree of corrosion is expressed by the corrosion rate. The judgment criteria are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0512291413469820.jpg According to information, about 60% of the damage to chemical equipment is caused by corrosion, 30% of the corrosion damage is uniform corrosion, and 70% is dangerous local corrosion, of which stress corrosion cracking is the most common. From the analysis of corrosion data, 16MnR has excellent corrosion resistance for both ammonia and liquid ammonia, with a corrosion rate below 0.1 mm/a. For wet ammonia and ammonia solution, the corrosion rate is not high, about below 1mm/a. The causes of corrosion in ammonia evaporation towers are analyzed as follows. 2.1 Chemical corrosion is analyzed from the aspects of process and medium. The temperatures at the top of the tower, the feed point, and the lower part of the tower are different, and the concentration of the medium is also different. The ammonia concentration at the top is high (99.5%) and the temperature is low (34~40℃) ； The ammonia concentration at the feed point is high (10% ~ 12%) and the temperature is also high (150 ~ 160℃) ； The ammonia concentration in the lower part is low (<1%) and the temperature is high (180°C). Since the medium in the tower is alkaline, the O2 in the alkaline solution has a great impact on steel corrosion. In addition, Cl- and other impurities also have a great impact on steel corrosion. The O2 and Cl- in the ammonia recovery system come from the chemical soft water added during startup and the condensate added at ordinary times. The chemical soft water used is deoxygenated soft water, in which dissolved O2 is <15 μg/L and Cl- is 6.0 mg/L. ； The dissolved O2 in the added condensate is <15 μg/L, and C1- is 3.0 mg/L. As far as Cl- is concerned, according to production needs, the more water is lost by evaporation, the more condensate must be replenished (the replenishment frequency is 1 time/d). However, the Cl- taken away with the evaporation of water is very small, and most of it remains in the circulating fluid, causing more and more condensate to accumulate in the middle of the circulation system. According to the analysis of ammonia water by the Quality Inspection Center, sometimes the Cl- concentration is as high as 300 mg/L or more. According to the data, stress corrosion cracking may occur in the stressed parts (such as near the weld) in solutions containing Cl-, OH-, etc., which are sensitive to stress corrosion. The sensitized areas on both sides of the weld are also prone to intergranular corrosion. Carbon steel corrodes very lightly in dilute ammonia water, but corrodes faster in hot, concentrated ammonia water. Trace amounts of Cl- and O2 have a great impact on equipment corrosion. A few × 10-6 C1- can cause stress cracking of 18-8 stainless steel. Austenitic stainless steel is very susceptible to stress corrosion cracking in the high-temperature environment of aqueous solutions containing chloride. The pitting corrosion tendency of type 18-8 stainless steel in water containing Cl- is greater than that of general aqueous solutions. 2.2 Electrochemical corrosion Ammonia is alkaline, and electrochemical corrosion occurs in materials in alkaline solutions. Before starting the ammonia recovery, chemical soft water needs to be added to the system to establish the liquid level. During the production process, in order to make up for the water loss, steam condensate needs to be intermittently added to the system. When there is a trace amount of oxygen in the soft water, an electrochemical reaction occurs. The reaction formula is as follows: Fe—→Fe+2eO2+2H2O+4e—→4OH— According to the data, the corrosion rate can be increased under conditions such as stirring, ventilation, and increasing temperature. During the normal production process of the ammonia evaporation tower, the ammonia water is in a boiling state, and the maximum temperature at the bottom is 180°C. The corrosion is quite serious. This kind of corrosion is manifested as total corrosion, and the corrosion is lighter at low temperatures than at higher temperatures. 2.3 Erosion corrosion Looking at the entire corrosion situation, the corrosion at the feed is the most serious. As the corrosion data shows, high flow rates can increase the corrosion rate several times. The feeding temperature here is above 130°C, and the ammonia concentration is 10% to 12%. At high temperatures, the corrosion caused by the erosion of concentrated ammonia is the most serious. The temperature distribution of the ammonia evaporation tower gradually increases from the top to the bottom. The top is below 40°C and the bottom reaches 180°C. The ammonia concentration tower is 99.5% at the top, 10% to 12% at the middle feeding point, and 1% at the bottom. The temperature at the bottom of the ammonia evaporation tower is high, but the ammonia concentration is low ； The concentration of ammonia at the top is high, but the temperature is low, while the corrosion of pure liquid ammonia is lighter ； The concentration of ammonia water at the feed point is high and the temperature is also high, so the corrosion is the most serious. Corrosion in other locations is relatively light and manifests as localized corrosion. From the above analysis, it can be seen that the corrosion of the ammonia evaporation tower made of 16MnR and its stainless steel float valve, tray, etc. is the result of chemical corrosion, electrochemical corrosion, erosion corrosion, intergranular corrosion, pitting corrosion and other corrosion. The main factors causing corrosion in ammonia evaporation towers are Cl-, dissolved O2, etc. in ammonia alkaline solution. Top corrosion with NH3 as the main form is lighter ； The concentration is low, but the middle part with higher temperature and concentration is seriously corroded. ； The lower part, where the temperature is higher and the concentration is lower, is second. 3 Protective measures and effects 3.1 Removal of Cl- Adding a shallow desalination device can reduce the amount of Cl- in soft water by 1/2. In view of the accumulation of Cl- in the system due to the replenishment of condensate, the improvement method is to strengthen the analysis and regularly conduct large-scale discharge and replenishment of the ammonia recovery system to control Cl- below 10mg/L. 3.2 The soft water used to control the oxygen content is deoxygenated soft water sent by power, which has undergone vacuum deoxygenation and atmospheric deoxygenation. The improvement method is to improve the O2 removal efficiency of the vacuum O2 removal device and control the O2 content below 10 μg/L, that is, to reach level 10-9. 3.3 Material replacement According to existing conditions, the C1- content is generally above 1×10-6. To prevent corrosion, special materials must be used. In May 1999, the material of the ammonia evaporation tower cylinder was replaced with 1Crl8Ni9Ti, and the materials of the ammonia evaporation tower trays, spacer tubes, etc. were all replaced with 1Crl8Ni9Ti. After taking the above measures, the corrosion problem of the ammonia distillation tower was fundamentally eradicated. In May 2003, the ammonia distillation tower was overhauled and the ammonia distillation tower was opened for inspection. The float valve and tray were all intact, and the wall thickness of the simplified body had not been reduced.

The process media in urea production, such as ammonium carbamate and urea aqueous solution containing carboxamide, are highly corrosive media, and the high temperature and high pressure characteristics of urea production further aggravate the corrosion of urea equipment materials. There are six main types of co-corrosion: uniform corrosion, pitting and crevice corrosion, intergranular corrosion, selective corrosion, stress corrosion, and fatigue corrosion. There are five main factors that affect corrosion: ammonia-to-carbon ratio, water-to-carbon ratio, inert gas influence, oxygen content influence, and sulfur content influence. The author discusses them respectively as follows. 1 Corrosion Characteristics and Types 1.1 Uniform corrosion Uniform corrosion causes the metal surface in contact between the equipment and the medium to lose its luster and become rough, especially the welds, weld fusion lines and welding heat-affected zones. The main factor affecting the uniform corrosion of materials is the state of the passivation film on the surface of stainless steel. The corrosion rate of stainless steel in the activated state differs by several orders of magnitude from that in the passivated state. The relationship between oxygen activation and passivation of metals is shown in Figure 1. In a good passivation film state, the corrosion rate of stainless steel materials used in urea production is 0.01 to 0.1 mm/a. Uniform corrosion is less likely to cause destructive accidents because the corrosion rate can be correctly selected in the design. http://www.nmtech.com.cn/jishuwang/upload1/080811914599346.jpg For the selection of stainless steel materials in urea production, the corrosion rate should be calculated separately according to the corrosion status of different parts. The corrosion rates of stainless steel materials commonly used in urea plants are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload1/080811915209887.jpg 1.2 Pitting corrosion and crevice corrosion Pitting corrosion, also called pit corrosion, is a common localized corrosion of stainless steel. Pitting corrosion can be large or small. Generally speaking, pitting corrosion only concentrates on certain specific points and forms corrosion holes at these points. The depth is much larger than its diameter, while most other surfaces remain passive. The form of corrosion that selectively proceeds under metal surface attachments or in the gap between metal and another object is called crevice corrosion. These two types of corrosion are caused by the so-called active-passive battery, and corrosion occurs on the same metal surface. Since the active state and the passive state coexist on the same metal surface, it indicates that the medium flow conditions in the active zone and the passive zone are different, that is, different forms of movement of substances, which promotes the formation and growth process of this type of corrosion. This is why crevice corrosion often occurs in dead corners of equipment and stagnant areas of medium flow. In these parts with special geometric shapes, the flow or diffusion of the medium is restricted, causing the chemical composition of the highly corrosive solution in the blocked dead-end gap cavity to be greatly different from the composition of the overall solution, resulting in a decrease in the electrode potential in the cavity. The metal surface of the gap cavity is anode, and the entire outer surface is cathode, forming local corrosion of the metal surface. This kind of corrosion behavior often interacts with alternating stress to cause corrosion holes to rapidly initiate and expand. For example, the gap between the distributor riser pipe and the fixed plate in the tube box of a CO2 stripping tower in a certain factory was too small, causing the urea melt to flow poorly and causing crevice corrosion. An example of crevice corrosion is shown in Figure 2. In Figure 2, crevice corrosion occurs when a<0.3mm, but does not occur when a>1 mm. http://www.nmtech.com.cn/jishuwang/upload1/080811915436162.jpg 13 Intergranular Corrosion Intergranular corrosion of urea steel is medium erosion along the grain boundaries to form cracks. There may not be any significant macroscopic changes on the surface, but it can cause a sharp reduction in the mechanical strength of the equipment. The cause of this kind of corrosion is the corrosion of the dechromized part of the substrate due to the deposition of carbides in the underlying metal, that is, the formation or precipitation of chromium carbides without grain boundaries. Since the diffusion rate of chromium is lower than that of carbon, when carbides precipitate at the grain boundary, carbon will be quickly replenished, but chromium will not have time to diffuse to the grain boundary, thus causing a chromium-depleted area near the grain boundary. When the chromium content is reduced to less than 2%, it reaches below the limit content required for passivation, and strong corrosion occurs. Intergranular corrosion mainly occurs in heat-affected zones or stainless steel parts that have been heated without solution treatment. Figure 3 shows the heat-affected zone in urea equipment. Under the action of urea-methylammonium solution, intergranular corrosion occurs. If the underlying metal in the urea plant is suitable (proper carbon content, stress relief), this type of corrosion behavior will not occur in the above media. The intergranular corrosion metallographic structure is shown in Figure 3. http://www.nmtech.com.cn/jishuwang/upload1/080811916064570.jpg 1.4 Selective corrosion In the urea-methane ammonium medium, due to the imbalance of the metal structure, ferrite appears when subjected to high temperatures. During the heating process of 600 to 900°C (welding is performed nearby), it is easy to transform into α phase. Selective corrosion of the tissue will occur if the above-mentioned intermetallic compounds occur or appear in nearby areas where the intermetallic compounds precipitate and affect the structure. The application of multi-phase steel in the medium can produce both selective corrosion of the ferrite phase and selective corrosion of the α phase. Generally, the ferrite content is often stipulated within a very low limit in the specifications of steel for urea. However, ferrite itself is not harmful, but its selectivity, which is most easily converted into α phase, is harmful. In fact, in most methods of producing urea, urea synthesis is carried out in the presence of oxygen in the reactants. Oxygen plays a passivating effect on stainless steel during the operation, causing the redox potential of the medium to move to a higher level beyond the activation zone. However, when the oxygen content is sufficient, the medium has a strong ability to selectively corrode the ferrite phase. In the absence of oxygen, the medium has a weak ability to selectively corrode the ferrite phase. 1.5 Stress corrosion Stress corrosion is a localized corrosion phenomenon caused by the joint action of stress and corrosion and the special cooperation of metal and environmental systems. Transmission equipment in contact with urea-methane ammonium liquid, such as high-pressure methane ammonium pumps, etc., have such corrosion behavior. At the same time, for reaction equipment in urea production, it can be considered that the metal surface under pressure under the action of pressure produces alternating stress when the pressure is not constant. If we start from the protective film destruction theory, this film causes damage under the action of stress, and as a result, a fresh surface is exposed. This fresh surface is placed in the medium and becomes an anode. It and other metal surfaces with protective films become cathodes form a corrosion battery with a small area anode and a large area cathode. On the other hand, the fresh metal surface placed in the corrosive medium will automatically form a film, making the damaged film capable of repairing. Transgranular stress corrosion occurs when dislocations move along the slip surface under the action of stress and merge at the surface to form a slip ladder, causing permanent deformation of the metal. If the passivation protective film on the surface cannot deform correspondingly with the formation of this step, that is, the passivation protective film will be destroyed. The greater the slip, the greater the potential for failure. When the protective film on the surface of stainless steel is damaged due to slippage, part of the surface is re-passivated. In the dead corner, due to the lack of necessary conditions for re-passivation, the tiny area there remains activated, resulting in a stress corrosion zone formed by concentrated corrosion. For example, at the connection bolts between the hook bolts and the tower plate of the urea synthesis tower, the bolts that have been subjected to long-term tensile stress have experienced a necking phenomenon due to corrosion, resulting in corrosion of the threads. This type of corrosion is extremely destructive, that is, there are tiny pits and fine cracks in the corrosion-sensitive parts of stainless steel, and the cracks propagate very quickly and can cause serious damage in a short period of time. Conditions that produce stress corrosion: ①Stainless steel must be subjected to specific corrosive media, such as austenitic stainless steel used in solutions containing chloride ； ②Tensile stress must exist, including applied stress, phase transformation stress and residual stress ； ③Stainless steel must have a susceptibility to stress corrosion cracking, which is related to the metal structure and chemical composition. Stress corrosion cracking also occurs in a certain potential range, that is, in the potential range where the passivation film is incomplete. To sum up, stress corrosion is a unified concept in which stress and corrosion work together and influence each other. For smooth metal surfaces without defects, stress corrosion damage goes through 4 stages.: ①Pitting corrosion begins to occur at impurity points and corrosion pits are gradually formed. ； ②Under the action of stress, microcracks form at the bottom of the corrosion pit ； ③Microcracks expand to macrocracks larger than 0.05 mm ； ④The crack reaches a critical state and destroys rapidly. The most important thing to inhibit this kind of corrosion is to ensure that the equipment is defect-free and to eliminate the residual stress caused by various processing methods. 1.6 Fatigue corrosion Fatigue corrosion is caused by the simultaneous action of corrosive media and alternating or pulsating tensile stress. The alternating stress causes small cracks to form at the stress concentration points, and the cracks expand to form cracks under the action of corrosion, forming intergranular or transgranular cracks. For example, the domestic methane ammonium pump cylinder will almost crack under the action of fatigue corrosion. Fatigue corrosion is related to the maximum stress and the maximum amplitude caused by the stress. After the transmission parts are corroded, the ferrite phase of stainless steel will all decompose into α phase, and carbides will precipitate at the grain boundaries. Measures to prevent fatigue corrosion are to select appropriate materials, modify structures and relieve stress. 2 Main influencing factors of corrosion There are many factors that influence corrosion in the urea production process, such as the selection of base metal and lining materials, the corrosion resistance of welded metal materials, processing and manufacturing processes, heat treatment and processing quality, etc. However, for urea devices that have been produced, more attention should be paid to the factors that can cause corrosion behavior during the production process. 2.1 Media factors 2.1.1 Ammonia-to-carbon ratio The ammonia-to-carbon ratio not only involves the conversion rate of carbon dioxide, but also affects the production capacity of the reaction equipment, the heat balance of the reaction process and the corrosion state of the equipment. During the production process, the molecular ratio of ammonia and carbon dioxide synthesis reaction is 2:1. In order to prevent side reactions and reduce equipment corrosion, excess ammonia is often used to make the ammonia-to-carbon ratio equal to 3, which can neutralize the acidity of the methylammonium solution. If the ammonia-to-carbon ratio is less than 2, cyanate radicals may be produced and cause corrosion, so the ammonia-to-carbon ratio must be strictly controlled. 2.1.2 Water-to-carbon ratio Reasonable control of the water-to-carbon ratio in urea production can not only lower the melting point of the ammonium carbamate solution to facilitate urea production, but also eliminate the clogging of carbamate crystals. If the water-to-carbon ratio is too high, the decomposition of ammonium hydroxide will produce hydroxide radicals, which will corrode the equipment. The water-to-carbon ratio of the CO2 stripping urea unit is 0.37, and the water-to-carbon ratio of the aqueous solution full-circulation urea unit is 0.67, which is more reasonable. 2.1.3 Oxygen content The oxygen content in the urea synthesis medium seriously affects the corrosion resistance of the equipment. Stainless steel equipment adopts oxygenation method to maintain the formation and existence of metal passivation film, which can act as a slow-release agent. The passivation film completely isolates the metal surface from the corrosive medium, but allows electrons to pass through. Therefore, it has no hindrance to the cathode process. It only prevents oxygen ions from entering the solution and hinders the anodic process. Metal passivation is because the anode process is blocked and the metal no longer enters the dielectric solution, thus being protected. If the passivation film is damaged, the metal is in an active state and the corrosion rate can increase sharply by 1,500 times. For the CO2 stripping urea device, generally adding about 0.67% of the total amount of CO2 oxygen is enough to maintain the oxide film in a passivation state. The amount of oxygen added to different materials also affects corrosion. The 316 L type material should keep the critical oxygen content in its passivation medium above 10×10-6. The saturated solubility of oxygen in the liquid phase of the medium is very low (<1×10-6), and the critical oxygen content of stainless steel to maintain passivation is higher than this value, that is, it is in a supersaturated dissolved state in the medium. Therefore, when the liquid phase does not flow, the supersaturated oxygen will escape to the gas phase, causing the oxygen content to drop below the critical value. Although some oxygen still exists, the equipment has begun to corrode in the activated state. 2.1.4 Sulfur content The presence of sulfide in CO2 raw gas will form HS- state in water, which can destroy the generated oxide film or prevent its regeneration, thereby promoting corrosion and forming corrosion pits. For example, the corrosion rate of 0Cr17Ni16Mo3Ti stainless steel material in a sulfide-free medium is 2.08 g/(m2·h). If a mass fraction of 0.1% sulfide is added to the solution, the corrosion rate is 8 g/(m2·h). Therefore, the sulfide content in CO2 gas must be strictly controlled to less than 15 mg/m2. In addition, inert gas reduces oxygen and ammonia in the liquid phase and increases the corrosion rate of equipment. 2.2 Production and operation factors 2.2.1 Frequent startup and shutdown and too long shutdown and pressure holding time Frequent startup and shutdown, frequent shutdown and tower sealing times, and long time, which poses a great threat to the corrosion of the lining material. During the shutdown and pressure-maintaining state, the oxygen medium in the system does not flow. As the tower sealing time increases, the system pressure continues to decrease, and the oxygen dissolved in the medium is continuously released, causing the oxide film on the metal surface to continue to dissolve until it is destroyed, and the equipment is in an activated corrosion state. Practice has proved that the longer the tower is closed, the more serious the corrosion will be. For example, the synthesis tower of a factory was shut down to maintain pressure for 53 hours, and the finished products produced nearly 2 hours after restarting were all black and red. The author recommends that you try not to stop and maintain pressure, and even if you maintain pressure, it should not exceed 12 hours. In addition, during the tower sealing and pressure-maintaining stage, the oxygen content is reduced due to too low medium pressure, which can also cause equipment corrosion. Generally, the holding pressure shall not be less than 150 kg/cm2. 2.2.2 Low oxygen start-up and oxygen-cut production. When low oxygen or oxygen-cut production occurs during startup or production, the corrosion rate of materials increases very quickly. Occasional hypoxia or oxygen outage will also cause the annual corrosion rate of the lining material to exceed the target. For example, the distribution plate of a synthesis tower in a certain factory has been corroded by nearly 1 mm in less than 3 years, which is much larger than the 0.05 mm/a in foreign countries. 2.2.3 Temperature Overtemperature during operation has a great impact on the corrosion resistance of materials. For 316L material, the corrosion rate increases linearly with temperature rise between 165 and 210°C in urea medium. This is because the electrochemical corrosion rate increases as the temperature increases. As the temperature increases, the electrode process is strengthened and the diffusion rate increases. The resistance of urea-methane ammonium solution decreases, the oxygen content in it decreases, and the free constant of water increases, which increases the acidity of the solution. All of these are changes in the direction of enhancing the work of corrosion batteries. For this reason, the process indicators must be strictly adhered to, so that the outlet temperature of the synthesis tower is 188~190℃ and must not exceed 195℃. The relationship between temperature and corrosion rate is shown in Figure 4. http://www.nmtech.com.cn/jishuwang/upload1/080811916441209.jpg 2.3 Water quality factors The mechanism of corrosion behavior of urea equipment is basically electrochemical corrosion. The high chloride ion content of water in production is more likely to cause corrosion, especially stress corrosion of stainless steel equipment. 3 Conclusion (1) Types and influencing factors of corrosion in urea production units. In addition to the above, the insulation quality of the equipment also has a great impact on corrosion. Poor insulation will aggravate the corrosion of the lining of high-pressure equipment. The gas-phase contact parts of medium and high-pressure equipment often cause local corrosion due to poor insulation. (2) Large-scale urea plants mainly use carbon dioxide stripping and ammonia stripping to produce urea. The most basic purpose of process improvement is to prevent corrosion in the high-pressure system. Although different materials are used in high-voltage equipment, such as titanium, pickaxe, etc., corrosion phenomena still occur. Therefore, the corrosion mechanisms of various materials and welding heat-affected zones during the urea production process need to be studied in depth.