Causes of corrosion in air cooler tube bundles and protective measures

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        1 Overview      As a type of large-scale heat exchange equipment, air coolers are widely used in the petrochemical industry. However, their tube bundles often operate under conditions of high temperature, high pressure, and high corrosivity; thus, their corrosion resistance is put to a severe test. This directly affects the safe and stable operation of the equipment. Therefore, it is necessary to analyze the common types of corrosion that occur in these tube bundles and propose protective measures to extend their service life.         2 Common types of corrosion in air cooler tube bundles (1) Pitting corrosion: In a medium containing dissolved oxygen and harmful anions (mainly Cl ions), the pressure-bearing materials of air cooler tube bundles show little or no corrosion on their surface after a certain period of time; however, pits or pockmarks appear at individual points or small areas on the surface. Over time, these corrosion pits deepen, forming small-hole-like corrosion defects. This is known as pitting corrosion. Since severe pitting can cause the equipment to develop holes, it is also referred to as hole corrosion. Important factors affecting the design and manufacture of air-cooled heat exchanger tubes include: a. The influence of heat treatment temperature: For stainless steel tube bundles, the main pressure-bearing components are generally not subjected to heat treatment after welding; however, austenitic stainless steels exhibit the best pitting resistance after solution treatment. For other stainless steel materials, heat treatments such as annealing or tempering at certain temperatures can result in the formation of precipitate phases, thereby increasing the tendency to pitting. b. As the surface finish of the tube bundle metal material improves, its pitting resistance increases; however, when cold working causes cold work hardening of the metal surface, its pitting resistance decreases. Therefore, for non-conventional materials under normal operating conditions, pitting corrosion tests are generally conducted for evaluation: these tests can be classified into three categories – chemical immersion method, electrochemical measurement method, and field test method. The parameters used to evaluate resistance to pitting corrosion include pitting depth, pitting density, corrosion rate, and corrosion area. And it combines the corrosion rate with the distribution, shape, size, density, depth, etc. of the erosion holes.         (2) Intergranular corrosion is a type of corrosion that occurs when tiny gaps (usually between 0.025 and 0.1 mm) are formed between metals or between metals and non-metals; the heat exchange medium gets trapped in these gaps, and when this medium contains harmful anions, it leads to corrosion. The result of such corrosion is a decrease in the material’s strength, an increase in local stress, and a reduction in the material’s load-bearing capacity. In air-cooled heat exchangers, gap corrosion usually occurs at the joints of the plates, at the connections of the flange sealing surfaces, at the locations where gaskets are used for sealing, or between the main tube and the lining tube; among these, gap corrosion is most likely to occur between the main tube and the lining tube. Therefore, in product design, the base tube and liner tube are often expanded together throughout their length to eliminate any gap between them. For air-cooled heat exchangers, a plug sealing structure is typically used, with sealing achieved through metal gaskets. The two sides of these gaskets come into contact with the plug and the tube sheet respectively; there is a certain gap at the microscopic level. The gaskets are exposed to corrosion by the heat exchange medium, and factors such as pressure, temperature, and vibration can cause gap corrosion at the sealing edges, which readily leads to failure of the gasket seal.         (3) Erosion-corrosion, including impingement erosion and wear-corrosion, is the damage to metal surfaces caused by high-speed relative motion between the metal surface and a fluid medium. The metal surface affected by erosion corrosion typically shows grooves, depressions, and teardrop-shaped patterns, with a smooth surface free of corrosion deposits. Compared to corrosion under other types of stress, the factors affecting erosion corrosion are more complex. In addition to the material’s chemical composition, microstructure, mechanical properties, surface roughness, and corrosion resistance, it is also greatly influenced by the temperature of the medium, pH value, dissolved oxygen levels, the size and hardness of solid particles, as well as the shape and structure of the components through which the fluid flows, as well as the flow velocity and pattern of the fluid. In air-cooled heat exchangers, the area most prone to erosion-corrosion is the weld joint between the base tube and the tube sheet. Since single- or double-layer welding is typically used for this joint, with a weld leg height ranging from 1.5 to 2 mm, prolonged exposure to flow erosion can cause the weld to thin down until it eventually leaks. Therefore, properly designing the heat exchanger’s structural configuration can effectively reduce the rate of erosion-corrosion. Commonly employed methods include using a return header to alter the fluid flow path, or installing flow-restricting strips inside the base tubes; these strips act as buffering elements that slow down the medium’s velocity and modify its flow pattern.         (4) Intergranular corrosion: A type of localized selective corrosion that occurs along the grain boundaries of metallic materials in certain corrosive media. Grain boundaries are the interfaces between different grains. Since the grains have different crystal orientations, the arrangement of atoms at the interfaces must gradually transition from one orientation to another. Therefore, grain boundaries are actually structural defects with incomplete “planar” characteristics. Conditions for intergranular corrosion: a. The metal or alloy contains impurities, or second phases precipitate along the grain boundaries. b. The difference in chemical composition between grain boundaries and within grains leads to the formation of a corrosion cell in a suitable medium; the grain boundaries act as anodes while the grains serve as cathodes, resulting in selective dissolution at the grain boundaries. c. A specific corrosive medium is present. In certain alloy-media systems, severe intergranular corrosion often occurs. For example, austenitic stainless steels may suffer from severe intergranular corrosion in specific corrosive environments, whether they are weakly oxidizing or strongly oxidizing.         Intergranular corrosion test methods and their evaluation methods: Currently, there are five basic methods listed in the standards: oxalic acid electrolytic etching method, boiling 65% nitric acid method, boiling sulfuric acid-ferric sulfate method, boiling sulfuric acid-copper sulfate method, and nitric acid-hydrofluoric acid method.         (5) Stress corrosion: The failure of a material caused by the combined effect of stress and a corrosive environment is known as stress corrosion. As a type of brittle failure mode, it poses significant hazards; whereas the failure of a material under the action of stress alone is classified as mechanical fracture or mechanical fatigue fracture ； When a material is corroded by the medium alone under corrosive conditions, it is considered environmental corrosion. Corrosive damage that results from the combined effect of stress factors and the corrosive environment, leading to fracture or cracking, is known as corrosion cracking. In air-cooled heat exchangers, leaks often occur due to corrosion cracking. The most common type of corrosion cracking is hydrogen sulfide stress corrosion cracking (SSCC), which refers to the brittle fracture of the pressure-bearing materials in the tube bundle under tensile stress in a sulfide-containing environment. Three conditions are typically required for stress corrosion cracking to occur: a. The metal has a tendency to suffer from corrosion cracking in that environmental medium ； b. Structures composed of pressure-resistant materials that are in contact with or exposed to selective corrosive media ； c. There is a tensile stress above a certain level.         3 Corrosion rate test The corrosion rate is an important indicator for evaluating the corrosion resistance of metals. During the corrosion process, metals undergo certain changes in their weight, microstructural composition, dimensions, surface condition, and mechanical properties. The rates of these macroscopic and microscopic changes can be used to indicate the extent of metal corrosion; hence, there are various methods for measuring the corrosion rate. For air cooler tube bundles, commonly used methods include the electrical resistance method, weight-loss method, and linear polarization method.         The resistance method is based on the principle that corrosion reduces the cross-sectional area of a metal sample, thereby increasing its resistance. By using this method, it is possible to continuously monitor the corrosion status of air cooler tubes during their operation; it allows for an accurate assessment of the corrosion rate at various stages of operation, as well as any changes in that rate. Moreover, this method can be applied to various types of media. The resistance method is fast, sensitive, and convenient, and can be used to monitor corrosion rates in conditions with high corrosion rates.         Linear polarization method is a highly suitable monitoring technique, featuring high sensitivity and rapid response to changes in corrosion conditions; it enables the determination of the instantaneous corrosion rate and allows for timely reflection of changes in the operating conditions of the tube bundle. However, this method relies on steady-state conditions to determine the corrosion rate, so it is applicable only to uniform or general corrosion, and not to localized corrosion.         The weight loss method is a commonly used technique for determining corrosion rates; it is simple to apply and yields reliable results. Typically, a coupon test is conducted; the material of the coupon is the same as that of the pressure-bearing components of the air-cooled heat exchanger. The shape and size of the coupon can be determined based on the test requirements and circumstances. Before the test, the test pieces should be surface-treated: they must first be sandblasted to remove rust and polished, then degreased using solvents such as acetone, rinsed thoroughly with distilled water, and dried at 50°C. The test piece is immersed in the corrosion solution, and its corrosion rate is determined by measuring its weight.         4 Corrosion Protection Measures 4.1 Proper Selection of Materials The correct selection of materials is one of the key factors in controlling corrosion, and the following principles should be followed: (1) The corrosion resistance of the materials must meet the requirements of the equipment’s operating environment; by considering factors such as the type of corrosive agent, its sensitivity, and the rate of corrosion, the operating environment should be analyzed in order to select appropriate materials ； (2) The physical properties, mechanical properties, and processability of the materials must meet the design and manufacturing requirements of the equipment, such as required strength, hardness, impact toughness, fatigue resistance, heat resistance, and weldability. For air-cooled tube bundles operating in environments containing hydrogen and sulfur, nickel-based alloys and duplex stainless steels are commonly used. Duplex stainless steel is a novel material that possesses numerous excellent properties such as superior corrosion resistance, high strength, and ease of manufacturing and processing. Its physical properties lie between those of austenitic stainless steels and ferritic stainless steels, being closer to those of ferritic stainless steels and carbon steel.         4.2 Rational structural design Product design must take corrosion prevention into account; based on the materials used and the surrounding environment, the operating stresses must be carefully calculated and determined in order to achieve a rational structural design for corrosion prevention. The following points should be noted: (1) The design should strive to be simple ； (2) Prevent water and dust accumulation ； (3) Try to avoid sharp corners, grooves, and gaps ； (4) Try to avoid connecting metals with large potential differences ； (5) Avoid the superposition of stress, assembly stress, and residual stress in the same direction.         4.3 Improvements in manufacturing processes Materials can also cause corrosion or create potential risks of corrosion during processing and assembly, such as: (1) Residual stresses are likely to be generated during machining ； (2) The heat treatment parameters should be selected carefully to avoid intergranular corrosion, stress corrosion, hydrogen embrittlement, etc. caused by improper heat treatment. Staying at the sensitization temperature range should be avoided as much as possible. For heat treatment processes that may generate significant residual stresses, measures should be taken to eliminate such stresses ； (3) The corrosion sensitivity of materials varies depending on the welding method. To reduce stress after welding, attention should also be paid to the design of the welding sequence in order to minimize deformation of the workpiece. Residues at the weld site after welding should be removed promptly to avoid localized corrosion ； (4) After welding and processing of the workpieces, degreasing and cleaning treatments should be carried out to prevent residual liquid from corroding the workpieces. Under specific operating conditions, the internal surfaces of air cooler tube bundles can be protected against corrosion by applying a nickel-phosphorus coating ； (5) During equipment assembly, excessive assembly stress should be avoided. During the design phase, accuracy should be improved and tolerances minimized; during assembly, reasonable assembly methods should be employed to prevent stress concentration.         5 Conclusion Only by gaining a thorough understanding of the types of equipment corrosion, implementing effective control measures, and taking preventive actions can we ensure the safe and stable operation of air cooler tube bundles and extend their service life. Therefore, research on the corrosion of materials and equipment is one of the important research topics in the industry.