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1 Overview of the CO2 compressor: In the mid-to-late 1980s, the Ningxia Petrochemical Branch of China National Petroleum Corporation introduced the first large-scale fertilizer production facility using residue oil as raw material from countries such as Japan and West Germany; this facility had an annual production capacity of 300,000 tons of synthetic ammonia and 520,000 tons of urea. In September 1999, the second set of large-scale fertilizer production facilities was put into operation officially. This facility uses natural gas from the Shaanxi, Gansu, and Ningxia gas fields as raw material, producing large-grain urea. Its designed capacity is to produce approximately 273,000 tons of synthetic ammonia and 520,000 tons of urea per year. Some of the key equipment for the urea fertilizer production plant was purchased from Mexico’s state-owned fertilizer company as idle equipment, while the remaining equipment was sourced domestically. The production process uses the ammonia stripping method; the original designed capacity was 1,500 t/d, and the capacity after restoration of the design is 1,740 t/d. The CO2 compression unit in the urea plant was manufactured by the Italian company NUOVOPIGNONE in 1988 for the original plant, but it has remained unused ever since. The compressor is driven by a steam turbine. The model and main design parameters of the unit are shown in Table 1. The compressor process is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0605181549247269.jpg Materials of the main components of the unit: the low-pressure cylinder is made of cast steel ASTM105/11 (equivalent to 35# steel); the main shaft is made of 40NiCrMo7; the impeller is forged and welded from X15Cr13, which has good corrosion resistance. Rated speed is 6,929 rpm. The partition is forged steel ASTM105/11. 2 Existing problems: (1) In the first half of 2000, the CO2 compressor encountered the following issues: water entered the oil tank of the unit, with a maximum water content of 3%; liquid was present in the exhaust vent of the low-pressure cylinder’s shaft seal; the vibration level of the low-pressure cylinder kept increasing, reaching up to 90μm at its peak; the gas production capacity of the unit declined day by day, accompanied by surge phenomena. On July 26, 2000, maintenance was carried out due to the inability to eliminate unit surge. Inspection revealed corrosion on the inner cylinder surface at the inlet of Stage 1 and Stage 2 of the low-pressure cylinder. The first-stage impellers in each stage were partially blocked to varying degrees, especially the impeller of Stage 4 in the high-pressure cylinder, which was almost completely blocked. Chemical analysis showed that the blockage was mainly composed of iron oxide. (2) During the major overhaul of the low-pressure cylinder of the CO2 compressor on July 3, 2001, corrosion covering an area of about 1 m2 was found on the inner cylinder surface at the inlet of Stage 1 and the inlet of Stage 2, with a corrosion depth of 7–13 mm. In some areas, there are corrosion pits with a diameter of about 10 mm and a depth of 15–20 mm. The original design for the shaft seal area of the two-stage cylinder block included three positioning shoulders used to fix the shaft seal. Upon disassembly, it was found that the two innermost shoulders had been largely corroded, with only small, discontinuous protrusions remaining. The exhaust passage between the shaft seal sleeve and the cylinder block is almost completely blocked by rust deposits, and scaling has occurred in the flow channels of the first-stage impellers in sections 1 and 2 of the low-pressure cylinder. 3 Analysis of corrosion causes 3.1 Operating conditions of the CO2 compressor The CO2 entering the first stage of the CO2 compressor comes from the decarboxylation unit of the synthesis plant; it contains saturated decarboxylation liquid (MDEA) as well as water vapor. MDEA is a colorless liquid that is miscible with water, and under certain conditions it has a strong capacity to absorb acidic gases such as CO2. Furthermore, MDEA has a low heat of dissolution for acidic gases, the bond energy of the resulting reactants is weak, allowing for easy regeneration; therefore, MDEA solutions are used in decarboxylation systems. Before entering the compressor, CO2 is cooled and condensed by the circulating water cooler 110 C1/C2. A portion of the liquid is removed in separator 103F, and the remaining vapor along with liquid droplets are carried within the CO2 gas, which then enters the inlet of the first stage of the compressor along with the CO2 gas at a temperature of 40°C. At full load, the CO2 flow rate is 24,700 m3 (standard) per hour; actual measurements show that the amount of liquid separated by the separator per hour is 5 m3 per hour. Taking advantage of the parking opportunity, the inter-stage coolers were inspected to ensure there were no leaks, which indicates that all the water comes from the decarburization section of the synthesis system. To reduce the amount of liquid carried by the CO2 stream, during a major maintenance campaign in 2000, an ammonia cooler 110–C3 and a pipeline separator were added between 103F and the compressor. As a result, the temperature of the CO2 dropped to 20–25°C. Actual measurements showed that 3.7 m3 of liquid was separated per hour; 110–C3 removed most of the liquid phase, which reduced the levels of MDEA and water vapor in the gas phase. However, the problem of liquid being carried along with the CO2 still persisted. 3.2 Causes of corrosion in the low-pressure cylinder (1) Electrochemical corrosion: Despite certain measures being taken, MDEA and water are still present in the CO2 that enters the compressor. Especially after the CO2 undergoes compression and inter-stage cooling, liquid forms within the CO2 as it enters the second stage; part of this liquid is separated out in the exit separator of the first stage, while the remaining liquid is carried by the CO2 stream to the inlet of the second stage of the compressor. Due to the pre-rotation effect, the liquid droplets are propelled toward the cylinder wall by centrifugal force and accumulate there continuously. Compared to the droplets, the amount of CO2 is sufficient to cause water to become saturated with carbonic acid; the measured pH value is 4.3. Under such conditions, low-pressure cylinders made of low-carbon cast steel suffer from electrochemical corrosion. Observations show that the corrosion of the cylinder block is much more severe than that of the partition, mainly because the cylinder block is made of cast material, which contains numerous metallurgical defects. The unevenness of the material structure and the large size of the grain structure create numerous galvanic cell formations in an acidic environment, leading to faster corrosion ； The partition is a forged piece with a uniform distribution of material components and fine metallographic grains; as a result, the number of galvanic cells formed and the electrode area are small, leading to a relatively low corrosion rate. (2) Erosive corrosion: As CO2 flows through the pipelines and is cooled by air, especially in winter when the temperature is around -20°C, a liquid phase forms due to the long length of the transmission pipelines. This leads to corrosion of the carbon steel pipes. These CO2 gases, containing liquid and corrosive substances (rust), enter the compressor at a speed of 15–25 m/s, eroding the cylinder walls and blocking the flow channels in the impellers, thereby causing surging and vibration in the compressor. Since this cylinder block was originally designed for the low-temperature methanol stripping carbon removal process, CO2 should be in a dry gas state without any liquid present; therefore, corrosion is inevitable under the current operating conditions. How to reduce the corrosion rate and restore the original alignment reference of the shaft seal sleeve became the primary issue during the major maintenance in 2002. 4 Anti-corrosion treatment of the inner surface of the cylinder 4.1 Supersonic arc spraying technology Supersonic arc spraying is a process method within thermal spraying technologies; it is an important branch of surface engineering and represents a new technique for strengthening and modifying material surfaces. It uses two coated metal wires as consumable electrodes; the arc generated at their ends serves as a heat source to melt the metal. The molten droplets are atomized by compressed air, and the spray gun propels these droplets at supersonic speeds onto the surface of the equipment or components, allowing the molecules of the coating material to integrate into the substrate being coated, thereby forming a coating on its surface. Compared to flame spraying, arc spraying is characterized by high thermal efficiency, with heat utilization reaching 60% to 70% ； It has high productivity, with a spraying rate that is more than 3 times that of flame spraying ； Low cost, over 30% lower than flame spraying ； The coating has high adhesion strength, exceeding that of flame spraying by over 50% ； Pseudo-alloy coatings can be easily prepared, thereby enabling the acquisition of various desirable properties such as wear resistance, corrosion resistance, and resistance to high-temperature oxidation on the surface of the equipment or component substrate to be coated. Since spray coating technology does not require heating of the substrate and operates at high speeds, resulting in a very thin coating layer, it does not cause a significant increase in the temperature of the equipment or components. It overcomes the substrate deformation that occurs in spray welding and surface cladding, making it highly suitable for the surface treatment of critical equipment or components with strict requirements regarding shape and dimensions. 4.2 Repair Procedures (1) Material Selection: Given that the main purpose of spraying is to prevent acid corrosion and erosion, and taking into account the material used in the original cylinder body, 7CR13 high-carbon martensitic stainless steel is used as the material for spraying. Technical control: The surface temperature of the workpiece is less than 200℃ ； Mechanical properties: Surface bonding strength 50 N/mm2, surface porosity