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According to literature, under normal circumstances, MDEA decarboxylation systems do not experience destructive corrosion problems; therefore, when adopting this technology, many companies choose carbon steel for the materials of the relevant equipment and fillers. However, corrosion problems in the MDEA decarbonization systems of many enterprises are quite common. This paper analyzes the corrosion problems that occur during the operation of MDEA decarburization units and proposes effective preventive and control measures. 1 Analysis of the causes of corrosion in MDEA decarburization equipment: From 1995 when our company adopted the MDEA decarburization process until the end of 2001, the operation was generally stable, which also brought certain economic benefits to the enterprise. However, after the production capacity of the unit increased gradually in 2002, a series of destructive corrosion phenomena occurred in the system, severely affecting the stable operation of the ammonia synthesis system. 1.1 Valve corrosion: The frequency of valve corrosion accounts for 16% of the total corrosion incidents in the installation, and the resulting effects of this corrosion are external leaks and internal leaks. External leakage is relatively easy to detect; once corrosion-induced leakage occurs, it can be identified promptly and temporary clamping measures can be taken. However, after an external leak occurs, the corrosion area around the leak point expands within a short time, and the clamping band can only hold for 10 to 15 days. External leakage corrosion generally occurs on the valve of the rich liquid outlet pipe in the absorption tower; upon inspection of valves with such leakage, corrosion is usually found in the valve body section behind the valve core. Internal leakage is generally difficult to detect; it is usually only discovered during shutdown for maintenance, when the valve is taken apart for inspection, especially when the level control is not sensitive or even turning the valve on and off has no effect. Internal leakage corrosion usually occurs at the bypass valve of the liquid level control valve at the flash tank outlet. The corrosion of the valve on the rich liquid outlet pipe of the absorption tower is mainly caused by erosive corrosion resulting from flow pattern changes at the valve’s flow-through area due to excessively high solution flow rates. The corrosion of the bypass valve of the liquid level control valve at the flash tank outlet is caused by CO2 cavitation resulting from the depressurization of the rich liquid. 1.2 Pipe corrosion: The frequency of pipe corrosion accounts for 42% of the total corrosion incidents in the facility, with pipe corrosion occurring after the self-regulating valve of the solution at the flash tank outlet accounting for 90% of such cases. To extend the service life of pipelines, thick-walled pipes were used as a substitute for thin-walled pipes, but the results were not satisfactory. According to statistics, the maximum service life of this section of pipeline is 4 months, while the minimum is only 26 days. Based on the corrosion surface of the pipeline, the corrosion exhibits a porous and loose structure, which is a typical case of cavitation; this phenomenon is particularly evident in the area 1.2 meters behind the level control valve. This is because the pressure inside the flash tank is 0.45 MPa, while the pressure behind the liquid level control valve is 0.18 MPa. When the solution’s pressure is reduced from 0.45 MPa to 0.18 MPa, CO2 in the liquid phase becomes supersaturated and is released from the liquid phase, resulting in cavitation in the pipe section behind the liquid level control valve. 1.3 Corrosion of tower equipment: The frequency of corrosion on the walls of atmospheric pressure desorption towers accounts for 2% of the total corrosion incidents in such installations; the phenomenon involves solution seeping out from the tower walls. After analyzing the corrosion site and the internal structure of the equipment, it was determined that a failure in the liquid distributor of the atmospheric pressure desorption tower was to blame, causing the solution after flash evaporation to be sprayed directly against the tower wall without being properly distributed by the liquid distributor. Taking advantage of the maintenance opportunity, the liquid distributor of the atmospheric pressure desorption tower was inspected, confirming the analysis results: in one instance, 8 branch pipes of the liquid distributor at the top of the atmospheric pressure desorption tower fell onto the tray below it, while the main pipe was almost completely corroded due to cavitation; in another instance, the gasket used for the flange connection of the liquid distributor was damaged. During operation, no leaks were observed in the walls of the absorption tower. However, during a major system overhaul when the packing inside the absorption tower was replaced, corrosion was found on the wall of the lower section of the lean liquid section; the walls of the upper section of the lean liquid section and the semi-lean liquid section were essentially intact. The corrosion area in the corroded zone is relatively large (accounting for about 60% of the area of the lower section of the cylinder in the lean liquid section); pitting is evident and is distributed fairly evenly. Based on the signs of corrosion, the chemical corrosivity of the solution itself can be ruled out; therefore, the focus is shifted to the packing. During the last major overhaul, due to an insufficient amount of structured packing in the lean liquid section of the absorption tower, some small-diameter structured packing was used to fill the upper layer of the lower part of this section, with stepped ring packing used to fill the surrounding area. The mixing of structured packing and random packing affected the distribution of the solution, preventing the annular gaps in the tower wall beneath the layer of small-diameter structured packing from being wetted by the solution. When the semi-degassed gas, containing saturated water vapor and CO2, rose to these unwetted gaps from the semi-depleted liquid section, the gas cooled down, causing the saturated water vapor to condense; the presence of CO2 gas led to acid corrosion. By comparing the packing height of the small-diameter structured packing with the area where corrosion occurs, it was confirmed that the corrosion zone lies within 800 mm below the layer of small-diameter structured packing, thus validating the analysis results. 1.4 Corrosion of pumping equipment: The frequency of corrosion in pumping equipment accounts for 38% of the total corrosion incidents in installations. Under the existing load, the pump configuration is 2 and a half lean liquid pumps and 1 lean liquid pump ； After the unit’s load is increased, it is matched with 3.5 lean liquid pumps and 1 lean liquid pump. However, after the unit's load was increased, corrosion of the pump shaft and impeller occurred repeatedly in the semi-poor liquid pump, with the shortest service life of the impeller being only 15 days. Measurements showed that the pump inlet pressure was only 18 kPa, which is lower than the sum of the pressure of the regenerated gas (28 kPa) and the 32 kPa pressure generated by the liquid level in the atmospheric desorption tower. At the original load, the pressure at the pump inlet can reach 52 kPa. Therefore, in the semi-poor liquid that has reached gas-liquid equilibrium in the atmospheric pressure desorption tower, the release of liquid-phase CO2 occurs when the pump inlet pressure is lower than the regeneration pressure, and this is a factor contributing to the corrosion of the semi-poor liquid pump. The function of the turbopump is to recover the energy of the medium-pressure solution, and its operating rate is directly related to the energy consumption of the plant. In the early stages of using a turbopump, it was believed that as long as the pressure difference between the inlet and outlet of the turbopump was met, its service life would be ensured. However, multiple analyses of turbopump failures have shown that the service life of turbopumps is closely related to the control of solution components. The sign of corrosion in the turbine pump was a reduction in the recovered energy by nearly 30%. Upon disassembly, it was found that both the impeller and the central shaft of the turbine pump were corroded to varying degrees; the outer edge of the entire impeller had essentially disappeared, with the remaining part having an abnormally sparse and porous structure, which is a typical symptom of cavitation. In the initial stage of using MDEA for decarboxylation, the total alkalinity of the solution was strictly controlled at (520±20) g/L. After years of operation, it was found that too high a total alkalinity of the solution increased energy consumption during operation; therefore, the total alkalinity of the solution was adjusted to (460±20) g/L, resulting in a reduction of ammonia steam consumption per ton by nearly 60 kg. Therefore, it is believed that as long as the purity of the gas can be ensured, the lower the total alkalinity of the solution, the better; hence, the total alkalinity level of the solution is maintained at (400±20) g/L over the long term. Statistics show that corrosion of turbine pumps generally occurs during the period when the total alkalinity of the solution is maintained at (400±20) g/L. Upon analysis, it was found that after the total alkalinity decreased, although the absorption capacity of the solution remained unchanged, its saturation level increased. Once a reduced-pressure condition occurred, CO2 in the liquid phase was immediately released into the gas phase, leading to corrosion of the turbine pump. 1.5 Packing corrosion: Packing corrosion mainly occurs in the stripping regeneration tower. Most of the packed material discharged is damaged; the degree of damage is particularly high in the lower section of the packing. Corrosion occurs throughout the entire packing layer, manifesting as both uniform thinning and erosion-like patterns. This is mainly due to the high CO2 concentration, high humidity, and high temperature (>100°C) in the lower section of the stripping regeneration tower; acidic corrosion tends to occur under such conditions. 2 Corrosion prevention measures: (1) The designed flow rate for the rich liquid pipeline at the outlet of the absorption tower is 1.0 m/s, whereas it has currently reached 3.2 m/s. When the flow velocity in the rich liquid pipeline exceeds 2.0 m/s, vortices are formed at the bottom of the absorption tower, carrying the shift gas into the rich liquid pipeline ； At the same time, the high flow rate increases the flow resistance, resulting in excessive pressure loss in the rich liquid; as a consequence, the CO2 in the rich liquid is released prematurely, leading to corrosion of the pipes and valves. To address the issues of excessive flow velocity in the rich liquid stream and easy corrosion at the flow-through parts of the valve body, the pipe diameter was enlarged and the valve was replaced with stainless steel. (2) The corrosion of the bypass valve of the liquid level control valve at the flash tank outlet is due to improper material selection and operational adjustments; therefore, operators are required to use the main line for adjustment as much as possible when controlling the liquid level in the flash tank. However, since the pressure difference between the flash pressure and the normal overhead pressure is too small, the solution encounters high resistance as it passes through the pneumatic control valve, making this approach impractical. While retaining the original pneumatic control valve, the material of the bypass control valve was replaced with stainless steel. (3) Pipe corrosion occurs after the self-regulating valve at the outlet of the flash tank. Due to the special location of this area, a large amount of CO2 gas is released as the solution passes through the self-regulating valve, resulting in the area remaining in an erosion-prone condition; thus, cavitation is inevitable. Therefore, high-grade stainless steel is used for this section of pipe to extend its service life. (4) The corrosion in the atmospheric pressure desorption tower is entirely caused by the loss of normal distribution of the solution under reduced pressure; resolving the issue of solution distribution will suffice. The main reason is the poor cavitation resistance of the material used for the solution distributor in the atmospheric pressure desorption tower; another reason is that the gaskets used for sealing the distributor’s branch pipes do not meet the requirements of the operating conditions. As a result, the carbon steel solution distributor in the original atmospheric pressure desorption tower was replaced with one made of stainless steel, and the asbestos gaskets used for flange sealing were replaced with metal wound gaskets. (5) Regarding the corrosion of the absorber tower wall, it can be attributed to technical issues related to the packing installation. After realizing that mixing fillers of different types and specifications caused issues with solution distribution, mixing fillers of various types and specifications within the same section was eliminated. (6) The corrosion of the semi-poor liquid pump is caused by the pipe resistance loss from the atmospheric pressure desorption tower to the pump inlet, and it can be resolved by appropriately raising the liquid level in the atmospheric pressure desorption tower. However, raising the liquid level in the atmospheric pressure desorption tower is only a passive approach, and it affects operational flexibility. The only way to completely resolve this issue is to reduce the pipe resistance, that is, by replacing the inlet main pipe of the semi-poor liquid pump; for this purpose, the inlet main pipe of the semi-poor liquid pump was changed from DN300 to DN400. (7) When the turbine pump is operating, the entire system is in a liquid-rich depressurized zone; the release of CO2 from the liquid-rich phase is closely related to the pressure resulting from the decompression by the turbine pump and to the total alkalinity of the solution. The corrosion problem of turbine pumps can be addressed by increasing the flashing pressure or raising the total alkalinity of the solution. However, an increase in the flash pressure affects the desorption of N2 and H2 as well as other inert gases to a certain extent, which in turn impacts the CO2 concentration in the regenerated gas; therefore, the flash pressure should only be increased appropriately within the range permitted by the design. By calculating the equilibrium partial pressure of CO2 in the rich liquid and comparing it with the actual operating conditions, it is necessary to raise the total alkalinity of the solution from 380 g/L to over 450 g/L; operators must strictly adhere to this requirement. (8) The corrosion of the packing in the air lift regeneration tower is of chemical acidic nature; therefore, acid-resistant packing is sought for replacement. Acid-resistant fillers mainly include plastics and stainless steel. Through operational condition analysis and cost analysis, the use of reinforced polypropylene packing was selected. 3 Control Effect: Through preventive measures taken at different times for various corrosion sites, the MDEA decarboxylation unit, which had been plagued by corrosion issues for nearly 4 years, finally achieved stable operation, and the control efforts yielded certain results. Since the valve in the rich liquid line of the absorption tower was replaced with a fully stainless-steel valve and the diameter of the rich liquid line was increased, there have been no issues with valve corrosion over the past 3 years ； The use of stainless steel pipes and valves in areas prone to cavitation has effectively extended the service life of the outlet pipe downstream of the flash tank level control valve as well as the auxiliary outlet valve of the flash tank. Since their installation, nearly 10 months have passed without any signs of corrosion ； After the issue of abnormal liquid distribution in the liquid distributor of the atmospheric pressure desorption tower was resolved, the corrosion problem on the walls of this tower was completely eliminated ； Strict control is exercised over the quality of packing installation in the absorption tower to prevent uneven gas-liquid flow. Regular on-line inspections of the tower wall thickness show no signs of thinning; the internal condition will need to be further verified during the next maintenance session ； After increasing the inlet diameter of the semi-poor liquid pump, tests conducted under high-flow conditions showed that the inlet pressure remained above 50 kPa, and the service life of the semi-poor liquid pump increased to over 20 months ； The total alkalinity level of the solution was strictly controlled; upon inspection 8 months after the new turbine pump was put into use, no further signs of corrosion were found ； After replacing the carbon steel stepped rings in the air lift regeneration tower with enhanced polypropylene packing, no corrosion issues were observed after 18 months of operation, thereby completely resolving the problem of packing corrosion in that tower. 4 Conclusion The corrosion issues of the MDEA decarbonization units analyzed above (except for corrosion on the absorber tower walls) are not isolated phenomena; many companies have encountered similar problems. The corrosion problem in the MDEA decarboxylation unit has caused the company direct losses of nearly 1.5 million yuan per year; taking into account the impact of corrosion products contaminating the solution, the total losses have reached 5 million yuan, thereby increasing the operating costs of the unit. However, by analyzing the mechanisms behind corrosion and properly understanding the causes of decarburization corrosion in MDEA systems, as well as by implementing appropriate preventive measures, practice has shown that the corrosion problems in MDEA decarburization units can be effectively controlled.