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During the urea production process, under high temperature and high pressure environments, liquid ammonia, CO2, urine, ammonia, condensate and steam will corrode the equipment, but the main corrosive effect on the equipment is the corrosion of the urea carbamate liquid on the equipment. The urea synthesis tower (urine tower) is a key equipment in urea production. The urea synthesis medium has good electrical conductivity, and the corrosion of metal is an electrochemical corrosion process. The corrosion-resistant materials such as stainless steel, titanium, and zirconium currently used in urea equipment are all passivation alloys, and their electrochemical corrosion behavior is similar to that in inorganic acids. The corrosion-resistant materials selected for the methane-ammonium preheater and methane-ammonium separator can generally meet the corrosion resistance requirements. Corrosion in urea production equipment includes various electrochemical corrosion types such as general corrosion, local corrosion pitting corrosion, galvanic corrosion, crevice corrosion, selective corrosion, stress corrosion, corrosion fatigue, erosion corrosion and cavitation corrosion. During the urea production process, the equipment that comes into contact with the urea carbamate solution under high temperature and high pressure is shown in Table 5-18. Picture: The urea methane solution, an intermediate product in the urea production process, is highly corrosive under high temperature and pressure. Without oxygen, stainless steel is not resistant to corrosion by the methane solution. There are several explanations for the corrosion mechanism of ureacarbamate solution. ①Corrosion of Carbamate Ammonium carbamate dissociates in water to form carbamate (COONH2-) which is reducing and can prevent the formation of oxide film on the metal surface and cause activation corrosion of metal. ②Corrosion of urea methylammonium solution is due to the fact that urea will produce the allotrope cyanate under high temperature and high pressure conditions. For example, 5% of urea can be converted into ammonium cyanate at 100°C. Oxycyanate has strong reducing properties, making it difficult for passivated metals to form a passivation film and cause activation corrosion. ③Corrosion caused by ammonia complexation Someone studied the corrosion of S31603 stainless steel in the gas phase medium of the urine tower. The oxides on the stainless steel surface are complexed by ammonia and dissolved into the ammonia liquid, and at the same time the metal surface is dissociated by water. This forms a cycle of complexation, dissolution, and dissociation, causing metal corrosion. During the process of complexation and dissolution of stainless steel by ammonia, the dissolution order of the surface oxide film is: nickel oxide - chromium oxide - iron oxide - molybdenum oxide (molybdenum oxide is not complexed at all). ④Formation of carbonyl substances Some people believe that the corrosion of stainless steel in urea-methylammonium solution is due to the carbonylation reaction between the metal and the medium, resulting in the formation of metal carbonyl substances. Nickel in nickel and stainless steel easily generates Ni(CO)4, so nickel is the least corrosion-resistant. ⑤ The role of HCO3- According to the test results of the influence of NH4+ concentration and HCO3- concentration on the cathode current density of S31603 stainless steel, it was found that the cathode current only increases with the increase of HCO3- concentration. People also use experimental devices that simulate actual production to conduct electrochemical tests. It was found that the oxygen added to the solution plays an important role in reducing the corrosion rate of stainless steel. Oxygen increases the corrosion potential of stainless steel, but only when the oxygen content is higher than a certain critical value, it can protect stainless steel. There are certain differences in the corrosion characteristics of different urea production processes, so the oxygen content (critical value) for passivation is also different; too high oxygen content will have an adverse impact on the production process. The corrosion and protection of the urine tower is a relatively complex issue. In addition to being closely related to the responsibility and operating level of the operators and managers, it is also related to the materials, design structure, manufacturing quality, installation and maintenance of the synthesis tower itself. Only by strictly following the operating procedures, implementing process indicators, and regularly testing and maintaining the urine tower can the safety, stability, and long-term operation of the urine tower be ensured. In the urea plant, although ammonia and urine are highly corrosive to carbon steel, the equipment in contact with them is now made of stainless steel or non-metallic materials, so corrosion problems in these equipment are rare.