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There are many equipment that use stainless steel as heat exchangers in chemical plants. Heat exchangers using stainless steel have relatively high requirements for Cl- content in circulating water (for example, when using ZH-371-ZD polyphosphate composite formula as a corrosion inhibitor, the requirement is not higher than 600 mg/L, and in neutral media without corrosion inhibitors, the requirement is not higher than 300 mg/L). Due to the influence of the Cl- content of the primary water replenishment, the evaporation of the circulating water and the chlorine bactericide, the Cl- content in the circulating water of most stainless steel heat exchangers is slightly high, which will cause corrosion of the stainless steel heat exchanger. Without appropriate measures and countermeasures, once the stainless steel exchanger leaks during use, it will affect production, not only increase equipment investment and product manufacturing costs, but also affect the long-term safe operation of the entire system in severe cases. Therefore, it is very important to study the causes of corrosion of stainless steel heat exchangers and find solutions to the corrosion. The following statements are made on this point. 1 Corrosion types of stainless steel heat exchangers Generally, stainless steel has a very low overall corrosion rate in industrial water. For example, in seawater with a flow rate of 0.3~0.6m/s, the corrosion rate of stainless steel is only 0.0005 mm/a. "Design Code for Industrial Circulating Cooling Water Treatment" (GB50050-95) stipulates: The corrosion rate of the stainless steel heat exchanger tube wall should be less than 0.005 mm/a. However, under actual industrial production conditions, stainless steel equipment, especially various industrial water coolers, have many corrosion and damage accidents, causing great economic losses to enterprises. The reason why these stainless steel equipment corrode faster is that: Although stainless steel has a very low overall corrosion rate, its local corrosion (mainly crevice corrosion, pitting corrosion and stress corrosion cracking) can cause huge damage. Taking Japan's Mitsubishi Chemical Machinery Company's survey of 166 damage accidents in ten years as an example, comprehensive corrosion accounted for 8.5%, high-temperature oxidation corrosion accounted for 4.9%, and the rest were localized corrosion. This kind of corrosion often occurs and expands somewhere in the equipment, eventually leading to corrosion and scrapping of stainless steel equipment. Therefore, preventing local corrosion of metal is very important to extend the service life of stainless steel equipment. 1.1 Crevice corrosion There are some gaps on the metal surface, and the solution in these gaps causes corrosion of the metal in the gaps due to the difficulty in migration of corrosion-related substances, which is called crevice corrosion. In the various gaps formed by stainless steel, oxygen in the solution can only enter through diffusion. The inner surface of the stainless steel gap will quickly consume the dissolved oxygen in the solution in the gap. When the dissolved oxygen drops to zero, the passive film on the surface of the stainless steel in the gap begins to undergo reductive dissolution, resulting in crevice corrosion. 1.2 Pitting corrosion Pitting corrosion, also known as pitting corrosion or pitting corrosion, is a local form of corrosion that produces small holes on the metal surface. The diameter of this hole can be large or small, but in most cases it is relatively small. Some pits exist in isolation ； Some pits are compacted together, like a rough surface. Pitting corrosion is a unique form of metal dissolution. When pitting corrosion occurs, iron dissolves in the pits to generate Fe2+. At the same time, excess positive charges are generated in the pits, causing Cl- to migrate into the pits to maintain the electrical neutrality of the solution in the pits. Therefore, there will be a high concentration of FeCl2 in the corrosion holes. Hydrolysis of FeCl2 will produce high concentrations of H+ and Cl-: FeCl2＋2H2O→Fe(OH)2↓＋2H＋＋2 Cl- Pitting corrosion is one of the most destructive and potentially dangerous forms of corrosion. It perforates and destroys equipment, so it is particularly harmful. It is a local but severe form of corrosion. Equipment with severe pitting corrosion will suddenly experience perforations and leaks, which can easily lead to accidents. It is often difficult to detect and detect pitting corrosion, which is usually minimal and often covered by corrosion products or deposits. 1.3 Stress corrosion stress corrosion cracking refers to the cracking of metal or alloy caused by the combined action of tensile stress and specific corrosive media. Its characteristic is that most of the surface is not damaged, and only some fine cracks penetrate the interior of the metal or alloy. Stress corrosion cracking can occur within commonly used design stress ranges and has serious consequences. Important factors that cause stress corrosion cracking include temperature, solution composition, metal or alloy composition, stress and metal structure. 2. Measures to prevent local corrosion damage of stainless steel in cooling water. Based on years of experience in the use and maintenance of stainless steel heat exchangers, we have summarized a set of methods to prevent stainless steel corrosion. 2.1 Reduce the Cl- concentration or take local protective measures. 2.1.1 Replacement. Each time the chlorine fungicide has exhausted its effectiveness, the circulating water system should be replaced on a large scale. 2.1.2 Cathodic protection uses aluminum sacrificial anodes for cathodic protection, that is, aluminum plates, aluminum rods and other aluminum products are placed inside the heat exchanger on the water pipe side and at the elbow of the heat exchanger. Soak in acid before placing to remove the oxide film on the surface of the aluminum product. This is a solution considered based on the electrochemical corrosion mechanism of metals. Our (branch) company’s diuretic circulating water, triuric circulating water, 5000 m3/h circulating water (for the second and fourth stage coolers of the compressor of the third plant), compressed circulating water of the first plant (for the first and second stage coolers of the 1# and 2# compressors of the first plant, and synthetic cold drainage), methanol circulating water (for the hindered amine regeneration gas cooler, final cooler and pressure The Cl- content is relatively high, which is 1188, 674, 638, 496, 549, and 660 respectively. mg/L (data is the average measurement in April 2006). Practice has shown that after taking protective measures to place aluminum products, the service life of stainless steel heat exchangers can be increased by about 40%. 2.1.3 Eliminate potential difference It only took two years for the reflux condenser of the atmospheric tower in the alcohol ether section of the methanol plant to leak. The inspection found that the leakage point was concentrated at the weld between the tube plate and the tubes. The tubes of this heat exchanger are made of stainless steel corrugated tubes. The tube sheets are made of carbon steel. Stainless steel surfacing is used between the tube sheets and the tubes. There is a potential difference between stainless steel and carbon steel, which accelerates the corrosion of the heat exchanger. At present, our company's methanol branch is gradually replacing this type of heat exchanger with stainless steel. 2.1.4 Adding corrosion inhibitors Currently, our factory uses HLN-03 scale inhibitor (organic phosphonate) and ZH-371-ZD general corrosion and scale inhibitor (polyphosphate compound), which have good effects. Before 2006, part of our plant's desalted water system used electrodialysis. After desalted water was produced, the system discharged polar water (because the primary water passed through a sodium ion exchanger before electrodialysis, the total hardness of Ca2+ and Mg2+ in the polar water was low, generally less than 5 mg/L, and the Cl- concentration in the polar water after electrodialysis desalination was as high as 300 mg/L). In order to save water resources at that time, polar water was once used as circulating water supply water. ； Because our plant's circulating water system uses a polyphosphate formula, and the divalent metal cations required for polyphosphate complexation do not meet the requirements, it is difficult to form a corrosion inhibitor film. At the same time, because the Cl- content in the supply water is high, the decarbonization circulating water system of the second plant that uses polar water as the supply water leaked twice in the CO2 gas cooler and lean liquid cooler of the second plant in 2005, causing two shutdowns. After finding the crux of the problem, the extreme water was used for another purpose and primary water was used as the supply water for the circulating water. The stainless steel heat exchanger of this system has not leaked so far. 2.2 Stress relief Stress can come from various sources, such as applied stress, residual stress, welding stress and stress generated by corrosion products. In order to completely eliminate stress, the following should be noted. (1) After chemical cleaning of the heat exchanger, all residual liquid should be removed and passivated. When the equipment is left unused for a long time, the water should be drained completely to prevent water accumulation. (2) Prevent the water cooler from being used in alternating wet and dry conditions. That is, after switching the compressor, do not drain the cooling water. If it is not used for several days, allow the cooler to drain part of the old water and replenish it with new water to prevent the hydrolysis or degradation of polyphosphate from depositing calcium phosphate scale on the surface of the heat exchanger. (3) Our factory joined forces with Shanxi Fengxi Yongji Chemical Machinery Factory in 2005 and began to manufacture its own equipment. After the equipment is manufactured, it is generally left for 15 to 20 days until the residual stress is eliminated by itself before reuse. The effect is very good. 3 Measures for process management 3.1 Optimize process conditions and reduce the occurrence of scaling. Take the No. 1 Synthetic Ammonia Branch of our company as an example. Due to the dense tube bundles and the high temperature of the gas in the pipes (110°C at the inlet and 48°C at the outlet), the synthetic radiator of the No.1 Synthetic Ammonia Branch of our company suffers from serious scaling and poor heat exchange effect. Chemical cleaning is required at least once a year. ； However, the cold row scaling in each section of compressors No.1, No.2, and No.3 of a factory using the same water quality was very light. Considering system stability and protection equipment, we have strengthened production management. At present, in the synthetic ammonia plants No. 1, No. 2 and No. 3, the excess high flash gas (main component is CO2 gas) generated in the production of synthetic ammonia is passed into the cold radiator receiving pool to reduce the pH value of the circulating water. The scaling phenomenon of the synthetic cold radiator has been significantly improved. 3.2 Eliminate sediments on the surface of stainless steel pipes in a timely manner. Since pitting corrosion mostly occurs under attachments or sediments, for heat exchangers that need to be cleaned (with severe scaling and a certain amount of rust), nitric acid cleaning is used in the process.