Research on the Corrosion of Stainless Steel Heat Exchangers by Industrial Cooling Water and Countermeasures

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Research on the Corrosion of Stainless Steel Heat Exchangers by Industrial Cooling Water and Countermeasures Stainless steel heat exchangers are widely used in the production processes of the petrochemical and power industries. However, the rate of development and the damage caused by local corrosion of stainless steel pipes (mainly pitting corrosion and stress corrosion cracking) are also astonishing. This article briefly introduces the types of corrosion of stainless steel ； Regarding the operation of thermal power plants and the corrosion prevention of stainless steel tubes in unit condensers, corresponding chemical treatment measures and successful application examples are discussed. 1. Applications of stainless steel heat exchangers: Stainless steel is an alloy of iron, chromium, and nickel, which first appeared in the early 20th century. Chromium-nickel steels, particularly 18Cr–8Ni austenitic stainless steels, are widely used in the chemical industry due to their high stability in many chemical media and their resistance to corrosion by high-temperature gases. In the production of various organic products and polymers (such as urea, acetic acid, polypropylene, polyvinyl alcohol, etc.), most of the equipment is made from chromium-nickel alloy steels and austenitic stainless steels. Many of the tube heat exchangers, condensers, and jacketed reactors that come into contact with various industrial waters are manufactured from austenitic stainless steel (the main types being AISI 304, 304L, 316, 316L). In the power industry, the application of stainless steel is also becoming increasingly widespread. In power plants, stainless steel is mainly used to manufacture the cooling tubes of condensers. The condenser is one of the important auxiliary machines in a steam turbine generator set, and its performance has a direct impact on the operation of the set. Its main heat transfer component—the cooling tubes—is the most important part of the condenser, accounting for over half of its total cost. Therefore, the selection of material and type for the cooling tubes is key to the design of the condenser. As early as the early 1990s, China began to apply the theory of helical groove tube heat transfer to develop new types of condensers. Through repeated demonstrations and tests, the ideal cooling tube condenser was developed—a high-efficiency stainless steel spiral-welded tube condenser. The overall heat transfer coefficient of stainless steel spiral welded pipes is 25–30% higher than that of copper pipes. In the actual operation of several thermal power plants, with the vacuum level kept constant, the amount of water required for circulation is 20% less compared to when copper pipes are used ； When the circulating water volume remains unchanged, the vacuum level increases by more than 5%. Since stainless steel has higher strength and surface hardness than copper pipes, neither the high-speed steam and water droplets on the steam side nor the sediment, dirt, and inlet turbulence on the water side can cause erosion to the stainless steel pipes. Therefore, it is better suited to water with sediment and dirt such as that in rivers, as well as to circulating water heating systems with high exhaust temperatures. At present, thanks to the progress and development of science and technology, our country is able to produce stainless steel spiral welded pipes on its own. The quality of some of these pipes has reached levels comparable to those of advanced foreign products, and their prices are reasonable – they are about 10% cheaper than copper pipes. A condenser for a 300,000-kilowatt power unit can save 600,000 to 1,000,000 yuan in material costs. 2. Types of corrosion in stainless steel heat exchangers Although stainless steel exhibits a very low rate of general corrosion in various industrial waters (for example, in seawater with a flow rate of 0.3–0.6 m/s, the corrosion rate of 316 stainless steel is only 0.5 μm/year), in actual industrial production conditions, corrosion-related failures of stainless steel equipment, especially various industrial water coolers, occur quite frequently. In the more than a dozen large-scale fertilizer plants newly built in our country (with an annual production capacity of 300,000 tons of synthetic ammonia and 480,000 tons of urea), corrosion damage to the stainless steel water coolers in these plants began to occur after 1 to 2 years of operation. Dozens of such coolers have already been replaced, yet the damage continues to occur, resulting in significant economic losses. All of these corrosion damages are caused by localized corrosion (mainly pitting and stress corrosion cracking). The Japanese Society of Corrosion Engineers, in collaboration with the Japanese Stainless Steel Society and the Japanese Society of Chemical Engineers, analyzed 700 stainless steel shell-and-tube water coolers. As a result, 85 of them suffered from stress corrosion cracking (accounting for 12.1%); among these, 52.9% had been in use for 1 to 3 years, while only 9.4% had been in use for more than 10 years. This clearly illustrates the severity of corrosion-induced damage to stainless steel. Compared to the chemical industry, the use of stainless steel in the power industry has been short-lived and limited in scope. The serious problem of stainless steel corrosion and degradation in the chemical industry must draw our utmost attention. In the power industry, as a preventive measure, this article will briefly explain the types of corrosion of stainless steel, with a focus on the anti-corrosion strategies for it. The corrosion forms of stainless steel can be divided into general corrosion and local corrosion. In various industrial waters, stainless steel exhibits a very low rate of general corrosion; under ideal conditions, it corrodes only 1 centimeter per 1 million years. Therefore, the hazard of general corrosion is minimal. In practical applications, local corrosion of stainless steel (mainly pitting and stress corrosion cracking) can cause significant damage. This type of corrosion often starts and spreads in a certain area of the equipment, eventually leading to the corrosion and failure of the stainless steel equipment. 2.1 Pitting corrosion of stainless steel in industrial water Pitting corrosion is an extreme form of localized corrosion. After corrosion pits form on the metal surface, they spread deeper at a rate that is greater than or equal to their rate of spread laterally; as a result of this corrosion, pits or small holes are formed on the metal surface. Eroded spots are sometimes isolated from each other, while at other times they are quite close to one another, giving the appearance of a rough surface. The diameter of the pits can vary; however, in most cases they are relatively small, with some being only dozens of micrometers in size. It is often covered with corrosion products, making it difficult to detect. It is difficult to predict its corrosion rate based on laboratory experiments. Sometimes it takes a long time for pits to form—several months or even years. Once formed, it develops relatively quickly, often resulting in sudden corrosive damage (perforation). Therefore, pitting is a highly damaging and severe form of localized corrosion. Numerous studies have shown that pitting occurs beneath attachments or sediments. Once measures are taken to remove the attachments or deposits, the problem is avoided. 2.2 Stress corrosion cracking of stainless steel in industrial water Stress corrosion cracking of austenitic stainless steel in industrial water is induced by pitting corrosion; the influencing parameters for both phenomena are the same, with only the respective critical values required being different. For stress corrosion cracking that occurs in various water coolers, the effect of temperature is often more significant than that of Cl- concentration; therefore, attention should be paid to the combined effect of chloride ion concentration and temperature. Due to the differences between laboratory test results and the failure phenomena under actual production conditions, numerous investigations and statistical analyses have been conducted on the stress corrosion cracking conditions of stainless steel equipment, particularly heat exchangers, by examining failures in industrial-scale devices. An analysis by DuPont of the United States regarding the reasons for the scrapping of 685 pieces of stainless steel equipment made of 18Cr-10Ni type steel indicates that stress corrosion cracking and pitting corrosion account for 38% ； An analysis of the causes of damage to 954 pieces of such material equipment in Japan indicates that stress corrosion cracking and pitting corrosion account for 63% (with stress corrosion cracking making up 38% and pitting corrosion 25%). Nishino Tomoyoshi and Fujigami Sekiei reported their findings on the investigation into the failure of the welded parts made of austenitic stainless steel in chemical plants as early as 1990. 3. Ways to prevent localized corrosion and damage of stainless steel in water. Although the mechanism of corrosion has not yet been fully understood, and no boundary conditions that can completely prevent or eliminate corrosion have been established. In actual production, this situation is often observed: two pieces of equipment operating under roughly the same working conditions can have vastly different lifespans ； Furthermore, the same stainless steel suffered stress corrosion cracking in cooling water with a low chloride concentration (only 10–20 mg/L), yet it could be used safely for a long time in seawater with a high chloride concentration. Statistical analysis of the operation conditions of a large number of industrial devices, along with numerous in-depth laboratory studies, have enabled us to identify the main factors affecting pitting and stress corrosion cracking in stainless steel. By establishing certain statistical patterns, it is undoubtedly possible to extend the service life of stainless steel equipment. Since the failure of stainless steel equipment is entirely caused by localized corrosion, primarily pitting and stress corrosion cracking, finding reasonable, economical, and effective anti-corrosion measures for industrial water has always been a subject of careful research in various countries. Although it has not yet reached a perfected stage, various effective methods have been found that can be applied according to the needs and conditions of actual production. 3.1 Selection of alloy materials resistant to localized corrosion failure. It has long been believed that nickel-chromium austenitic steels have a tendency to suffer from stress corrosion cracking, while only pure ferritic high-chromium stainless steels lack such a tendency. In fact, austenitic stainless steels with high nickel content (35–40%) are also immune to stress corrosion cracking; only 18%Cr steels containing 8–10% nickel are sensitive to it. Now, researchers have used focused ion beam secondary ion mass spectrometry to confirm that improvements in material processing techniques can also reduce the corrosion of lower-grade stainless steels. 3.2 An annealing treatment is used to eliminate stress. To completely remove stress, annealing should be carried out at temperatures above 850°C; in practical applications, this is often not possible due to the large size of the equipment or the risk of deformation. To prevent intergranular stress corrosion cracking in stainless steel, low-temperature annealing at 550–600°C is typically carried out, which is also effective in relieving stresses. Furthermore, when manufacturing equipment, the following should also be taken into account: (1) Media that may cause stress corrosion cracking must not be used during polishing ； (2) After pickling, the residual liquid should be thoroughly removed, and a passivation process should be applied ； (3) When welding thin-walled tubes, uneven deformation caused by misalignment should be eliminated. 3.3 Other protection methods: Many anti-corrosion measures can be effective under certain conditions; for example, coatings are effective as long as flaky peeling can be avoided. Here are a few more protection methods: (1) Cathodic protection. At a potential of –0.8V (relative to the Ag/AgCl electrode), crevice corrosion in CrNi steel in contact with carbon steel can be suppressed, and this method is particularly effective for 304 and 316 stainless steels. Cathodic protection can be achieved using aluminum sacrificial anodes in calm seawater. (2) Preventing stress corrosion cracking with corrosion inhibitors: Stress corrosion cracking in austenitic stainless steels occurs in the activation potential region, and adsorptive corrosion inhibitors must be used to cover this area. The main components of adsorptive corrosion inhibitors are organic heterocyclic compounds and derivatives based on organic amines; it is the organic amine derivatives that exhibit an activation potential region in the cathodic area on the metal surface ； The unshared electron pairs in the hydrophilic group undergo chemical adsorption through coordination bonding with the d-orbitals of metal elements ； The unsaturated bonds in the corrosion inhibitor molecules also form conjugation on the metal surface through Л bonds, thereby enhancing chemical adsorption. Thus, a dense adsorption film is formed on the metal surface, suppressing the metal corrosion process. (3) Take measures to reduce the surface temperature of the heat transfer surfaces in contact with the cooling water. By lowering the surface temperature below the critical value, it is possible to **reduce the likelihood of stress corrosion cracking, and this method is more effective than any other approach. To reduce the surface temperature, high-flow-rate cold water should be used for cooling as much as possible. (4) Removing deposits from the surface of stainless steel pipes ① For the operating units in thermal power plants, the deposits on the water-side surface of the condenser’s stainless steel pipes are rust and scale. Any existing rust or scale should be removed using nitric acid; when carrying out chemical cleaning, it is necessary to prevent the introduction of chloride ions. As long as the surface of the stainless steel is kept clean, good protection can be achieved. The chemical cleaning system consists of: cleaning tank → cleaning pump → temporary inlet pipe → condenser → temporary return pipe → cleaning tank. During operation, plants should determine the operational parameters (such as turbidity, hardness, and flow rate) through testing, based on the specific water quality and the corrosion and scale inhibition agents used, in order to prevent the formation of deposits on the surfaces. ② For the capital construction units of thermal power plants, the deposits on the inner and outer surfaces of the new stainless steel pipes are a harmful film with a complex composition, chemical polishing residues, and pollution deposits. Among them, the harmful films and chemical polishing materials are generated during the processing of stainless steel pipes ； Polluting deposits include soil, sand, gravel, cement and other silicon-containing substances that contaminate and adhere to stainless steel pipes during storage and installation (this problem is more severe in the northwestern regions due to strong winds and sandstorms). At the same time, the new pipes generate residual stresses during production, transportation, and installation. Since pitting occurs beneath deposits or accumulations, and stress corrosion cracking of stainless steel is induced by pitting, it is essential to thoroughly remove any harmful films and deposits from the surface of stainless steel pipes in order to eliminate the key factors that lead to localized corrosion in stainless steel (primarily pitting and stress corrosion cracking). Anti-corrosion theories and experiences from both domestic and international sources show that the excellent corrosion resistance of stainless steel depends on the passive oxide film present on its surface, and a clean surface along with a uniform structure are prerequisites for the formation of a uniform and dense passivation film. It was by learning from past lessons that the chemical processing system used a cleaning agent with strong penetration, stripping, cleaning, and corrosion-inhibiting capabilities to clean its stainless steel heat exchangers before putting them into operation, which helped to **reduce** corrosion incidents. According to available information, this product is manufactured using American technology and is primarily used for cleaning and passivating stainless steel surfaces. It does not cause corrosion; cleaning and passivation are accomplished in one step, with a fast reaction rate and thorough cleaning. This product is widely used for the surface treatment of stainless steel equipment, pressure vessels, and engineering components in industries such as chemicals, power generation, food processing, printing and dyeing, and aviation. Based on the cleaning tests and quality analyses we have conducted, it has been proven that such agents can indeed completely remove contaminants and harmful films from the surface of stainless steel pipes, do not corrode the pipes, and provide passivation during the cleaning process. The quality of the pharmaceuticals also meets the requirements of the revised standard Q/SDP008-2002. Tests conducted by the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, have shown that the surface protective layer of the stainless steel tube is dense, has a uniform chemical composition, and exhibits strong corrosion resistance. Therefore, specialized cleaning agents for stainless steel should be used in the chemical cleaning of the inner and outer surfaces of the stainless steel tubes in the condensers of utility-scale units in thermal power plants. Due to the close arrangement of the stainless-steel tubes in the condenser, manual cleaning is not possible; only chemical circulation cleaning can be performed. For the steam-side surface of the stainless steel tubes in the condenser, the cleaning agent should be CA-1s. The chemical cleaning system should be: bottom of the condenser → condensate pump → upper part of the condenser. Chemical dosing is carried out through the access hatch at the top of the condenser; the condensate pump is started to enable a closed-loop circulation, thereby enhancing the cleaning effect. During the cleaning process, the temperature must be monitored, and it is absolutely not allowed to exceed the operating temperature of the condenser. For the water-side surface of the stainless steel tubes in the condenser, the cleaning agent should be CA-1s. The chemical cleaning system consists of: circulating cooling water tower → circulating water pump → condenser itself → circulating water return pipe → circulating cooling water tower. 4. Conclusion Stainless steel is resistant to corrosion; however, once local corrosion sites form, they can develop rapidly (with a corrosion rate of up to 10,000 mm/year, or 1.15 mm/hour), often leading to sudden corrosion damage (piercing or cracking) with catastrophic consequences. Therefore, it should be given high priority in terms of awareness and in work. 　　Although corrosion occurs only during the operation phase of equipment, from the perspective of \"Comprehensive Equipment Engineering,\" its causes lie in stages such as research, design, manufacturing, installation, commissioning, and maintenance. The solutions must also be implemented at each stage.