A brief discussion on the corrosion of coolers in refineries and countermeasures

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This document can serve as a reference for equipment managers or maintenance personnel in chemical processing plants. A Brief Discussion on the Corrosion of Coolers in Refineries and Countermeasures Abstract: An analysis is conducted on the corrosion and scaling issues affecting the cooling equipment in refineries. Under different environmental conditions, the “7910” coating is used for anti-corrosion treatment of the inner walls of carbon steel pipe bundles ； Adopt 5454 Al-Mg alloy tube bundles and “Ni-P” electroless plated tube bundles ； Chemical cleaning, online cleaning, and high-pressure water jetting techniques are employed for the tube bundles. It achieved very good results. Keywords: cooler, corrosion, causes of scaling, protective measures, scope of application. 1. General overview: A certain petrochemical refinery currently has over 300 shell-and-tube water coolers. Most of the materials are carbon steel, and the water used is recycled water. The cooler is one of the key equipment in production facilities; approximately 60% of the frequent failures and emergency repairs that occur on a daily basis are due to corrosion and leakage in the tube bundles of the cooling equipment. It has severely affected the safe, stable, and full-capacity operation of the production facilities. Additionally, when cooling water exchanges heat with a medium at a higher temperature (with water typically flowing through the tube side), it is prone to scaling, forming a layer of rust and scale. This increases the thermal resistance, significantly reducing the heat exchange efficiency and failing to meet production requirements. Therefore, the rational selection of materials for heat exchanger tube bundles and control methods to minimize corrosion is an issue that our scientific and technical personnel have always been concerned about. Since the 1980s, in response to the corrosion of heat exchange equipment, various countermeasures have been adopted under different corrosive environments. After years of efforts, good results and significant economic benefits have been achieved. 2 Causes of Scaling and Corrosion 2.1 Analysis of the causes of scaling and corrosion on the inner wall of the tube bundle In most coolers, water flows through the tube side, and this water contains calcium and magnesium ions as well as bicarbonates. When the cooling water flows over the metal surfaces involved in heat transfer, the following reactions occur: Mg2+ + HCO3- + H2O → MgCO3↓ + Mg(OH)2·3MgCO3 + CO2; Ca2+ + 2HCO3- → H2O + CO2 + CaCO3↓. When polyphosphates are added to the water as corrosion inhibitors, the following reaction takes place: 3Ca2+ + 2PO43- → Ca3(PO4)2↓. In addition, oxygen dissolved in the cooling water can also cause metal corrosion, leading to the formation of rust; the reaction is as follows: 2Fe + 2H2O + O2 → 2Fe(OH)2↓. As a result of these reactions, scale gradually forms on the heat transfer surfaces, along with the formation of rust. When the cooler is in operation, the heat exchange efficiency is severely reduced due to the effect of scale buildup. In some individual tubes, the heat exchange tubes became clogged within less than a year of use. Furthermore, the presence of scale can lead to under-scale corrosion on the inner wall of the tubes, reducing the service life of the tube bundle. The corrosion of metal surfaces by water is primarily electrochemical corrosion; in the corrosion cell, the cathodic reaction is mainly the reduction of oxygen, while the anodic reaction is the dissolution of iron. The corrosion reaction of carbon steel in water is as follows: Anodic reaction: 2Fe → 2Fe2+ + 4e-. Cathodic reaction: O2 + 2H2O + 4e- → 4OH-. Overall reaction: 2Fe + 2H2O + O2 → 2Fe(OH)2↓ During corrosion, iron is converted into iron hydroxide, which precipitates out of the solution. Since this ferrous compound is unstable in oxygenated water, it will further react with oxygen to form iron hydroxide. 2Fe(OH)2 + 2H2O + 1/2 O2 → 2Fe(OH)3↓; subsequently, iron hydroxide loses water to form rust. 2Fe(OH)3 → FeOOH ↓ + H2O. In other words, corrosion of metals beneath scale is accelerated due to the autocatalytic effect of their inherent electrochemical corrosion. 2.2 Causes of corrosion of the oil phase on the outer wall of the tube bundle Corrosion of the outer wall of the tube bundle in heat exchangers is a common problem in the operation of petroleum refining plants; particularly, corrosion in the low-temperature areas at the tops of towers, as well as in the cooling systems used in units for primary and secondary processing such as atmospheric and vacuum distillation, catalytic cracking, and delayed coking, is quite severe. Since most of the media in the shell side of coolers are oil and steam, with operating temperatures ranging from 100 to 160°C, both the corrosion morphology and metallographic analysis indicate that the corrosion is electrochemical in nature. Although the corrosive media and operating conditions vary, resulting in some differences in corrosion characteristics, the fundamental corrosion mechanisms remain essentially the same. Generally, corrosion is mild in the gas phase region, more severe in the liquid phase region, and most severe at the gas-liquid phase transition zone. The corrosion pattern consists of both general corrosion and local corrosion, with pitting and perforation being particularly prominent. The maximum rate of local corrosion can exceed 6 millimeters per year, while the average rate ranges from 1.2 to 5 millimeters per year. Due to varying degrees of presence of HCl, H2S, HCN, NH3, and H2O in the oil phase system, these substances volatilize along with the light components. When in gaseous form, their corrosive effect is generally minimal. However, after condensation and heat exchange, when the temperature drops by more than 100°C and liquid water appears in the condensation area, corrosion caused by the HCl-H2S-H2O and HCN-NH3-H2O systems occurs within the cooler shell. The severe corrosion and damage to the primary processing equipment are caused by a cyclic corrosion process driven by the interaction between HCl and H2S. The relevant reactions are as follows: Fe + 2HCl → FeCl2 + H2↑; FeCl2 + H2S → FeS↓ + 2HCl; Fe + H2S → FeS + H2↑; FeS + 2HCl → FeCl2 + H2S. The reasons for corrosion in the condensation system of the secondary processing equipment differ from those in the primary processing equipment, but the degree of corrosion at the gas-liquid phase transition areas in the condensation zone remains the same. Judging from the appearance of the corroded tube bundle, the spaces between the tubes are blocked by loose corrosion products and debris, and pits appear on the metal surface. 3 Methods to address tube bundle corrosion: Generally, it is not advisable to use tube bundles made of highly corrosion-resistant alloys for heat exchange equipment. Because, first, the cost is high, and second, the heat transfer efficiency is poor. Over the past decade or so, our factory has adopted different anti-corrosion methods tailored to various corrosion-resistant environments, achieving very good results. The situation is as follows: 3.1 Addressing corrosion and scaling on the inner wall of the tube bundle 3.1.1 To deal with corrosion on the inner wall of the tube bundle, 7910 coating is used for anti-corrosion protection. The main component of 7910 coating is a synthesis of epoxy and amino resins; this material is of the thermosetting type and is composed of polymers. It has excellent resistance to weak acids, strong alkalis, and oxidizing agents in water. The anti-corrosion-treated tube bundle can be used at a shell-side inlet temperature of less than 160°C, effectively solving the problem of corrosion on the metal surface inside the tube bundle. A total of about 250 units have been anti-corrosion treated, resulting in savings of approximately 5 million yuan in manufacturing costs. 3.1.2 Addressing tube bundle scaling: Due to the varying operating conditions of production units, when water exchanges heat with media at higher temperatures, a considerable amount of scale forms on the inner walls of the tube bundles, which reduces the efficiency of the heat exchange equipment and affects its heat exchange performance. Based on the actual circumstances, the following measures were taken: (1) Application of high-pressure water jet technology – While maintaining the production equipment, high-pressure water jet technology was used to clean the rust layer from the inner walls of the tube bundles. This technology is suitable for equipment under mechanical cleaning conditions. It features strong cleaning ability and a wide range of applications. When the nozzle pressure is 70 MPa or higher, water is ejected from a specialized nozzle at supersonic speed toward the object to be cleaned, removing various types of scale as if with a sharp blade. Its cleaning efficiency is also beyond comparison to manual cleaning. The application of this technology in heat exchange equipment to remove scale, rust, and some coke deposits from metal surfaces is an effective method. (2) Chemical cleaning technology is employed; the tube side of heat exchangers is mostly filled with circulating water. When exchanging heat with a medium at a higher temperature, scaling is likely to form on the inner wall of the tube, creating a scale layer that reduces the cooling efficiency. Some coolers cannot be stopped once they’re in operation; forcibly shutting them down would result in significant losses. We used chemical cleaning to avoid shutting down the equipment. The situation is as follows: most cooling water contains calcium and magnesium ions as well as bicarbonates. When cooling water flows over the metal surface, carbonates are formed. Additionally, oxygen dissolved in the cooling water can also cause metal corrosion, leading to rust formation. The heat exchange efficiency decreases due to the formation of rust scale. In severe cases, it is necessary to spray cooling water outside the housing; severe scaling can block the pipes, rendering the heat exchange ineffective. Through experimental screening, nitric acid has been identified as a suitable solvent for removing scale from pickling water. This is because nitric acid reacts with the scale (calcium carbonate, magnesium carbonate) as follows: 2HNO3 + CaCO3 → Ca(NO3)2 + CO2 ↑ + H2O; 2HNO3 + MgCO3 → Mg(NO3)2 + CO2 ↑ + H2O. It offers advantages such as rapid scale dissolution, high solubility of the resulting nitrates in water, and ease of use. Since nitric acid solution itself has a strong corrosive effect on metals, a certain amount of corrosion inhibitors and other additives must be added during the pickling process to protect the metal surface. Cleaning is carried out to meet production requirements. This method addresses the requirement that some cold exchange equipment cannot be stopped for cleaning. It saves labor and resources, reduces time, ensures high cleaning quality, and increases work efficiency by more than ten times. (3) Employ online cleaning technology to prevent scaling in the tube bundles of heat exchange equipment during operation, or to remove scale from already scaled tube bundles. The use of manual machinery, high-pressure water jet technology, and chemical cleaning techniques has all yielded good results. However, the aforementioned methods are only passive measures and do not solve the problem at its root. To address this issue, the “online automatic cleaning technology for heat exchangers” is employed. This method involves inserting inserts of a specific shape into each individual tube within the tube bundle; the flow of water through these tubes helps prevent the formation of rust and scale, yielding very good results. Online automatic cleaning technology (also known as spring-based automatic online cleaning) is a mechanical method. Its core is a combination of a helical spring and solid elements forming a simple mechanical system, which is installed inside the heat exchanger tubes. Under the action of the fluid, continuous radial and axial vibrations are generated, disturbing the laminar layer at the inner wall of the pipe and increasing the degree of turbulence. This effectively prevents the accumulation of dirt, thereby reducing the thermal resistance within the pipe and enhancing its heating efficiency. Additionally, the vibration of the helical spring and its repeated friction against the pipe wall also help to remove dirt. It can be seen that the greatest advantages of online automatic cleaning technology are scale prevention and removal, as well as enhanced heat transfer. From the perspective of removing scale, compared to chemical cleaning and other mechanical methods, this approach does not require any external power equipment, nor does it necessitate shutdowns or pauses in production; it can be applied during normal operation, enabling the equipment to remain in good working condition for a long time and maintain an optimal heat transfer state. An experiment on the application of \"online automatic cleaning technology for cooling equipment\" was conducted on a floating-head cooler with a diameter of φ500 in the Constant Pressure-Reduced Pressure Unit’s Constant Third Line. This device tends to develop scale when in use, with a thickness of around 2 millimeters. This reduces the heat exchange efficiency; in severe cases, some tubes become completely blocked within a year. During the nearly 28 months of testing, no leaks occurred in this equipment, meeting the production requirements. During the equipment maintenance in September 1995, when the device was opened, it was found that the inner walls of the tubes in the tube bundle were free of any scale, remaining as smooth as before. All 116 springs installed are in good condition and will continue to be used. Through a benefit analysis, this equipment can generate direct economic benefits of 17,800 yuan per cycle (840 days). Or, in other words, by applying this technology, production needs can be met, thus preventing unnecessary losses. According to relevant data, without replacing the original heat exchange equipment, it is possible to increase the heat transfer efficiency by more than 30%. 3.2 Addressing the issues of corrosion on the outer wall of the tube bundle and rust deposits: Generally, light-grade oils are used as the fluid flowing through the shell of heat exchangers; however, due to the harmful impurities present in these oils, corrosion on the outer wall of the tube bundle becomes quite severe. From the corroded surface, the spaces between the tubes are clogged with loose corrosion products and dirt, and pitting is observed on the metal surface. Meanwhile, corrosion products increase fluid and thermal resistance, thereby reducing the heat transfer efficiency of the equipment. 3.2.1 Use of 5454AI-Mg alloy tube bundles: Since 1987, there has been no effective method to address the corrosion issue of the outer wall of tube bundles. If stainless steel is used for the tube bundle, it can indeed solve the problem of corrosion of the tube bundle, but it is expensive. Furthermore, it is inappropriate from the perspective of the pipe’s thermal conductivity. (1) Rationale for using the 5454 tube bundle: Based on a thorough examination of the 5454 Al-Mg alloy tube bundle (hereinafter referred to as the 5454 tube bundle), application tests were conducted on this material, yielding very good results. The reason why the 5454 tube bundle is resistant to oil and steam corrosion is that, when used within certain conditions, its corrosion resistance is similar to that of stainless steel. This is because the electrochemical standard potential of pure aluminum is very negative, at around -1.66 volts. Since the 5454 tube bundle readily combines with oxygen to form a stable Al2O3 passivation film, its potential rises rapidly to -0.5 volts, reducing the potential difference across the entire corrosion cell. Furthermore, this alloy tube bundle is also particularly good at resisting cooling water corrosion. The corrosion of the inner wall of the carbon steel cooler tube bundle by water is an electrochemical process, namely oxygen depolarization corrosion caused by dissolved oxygen in water. This tube bundle contains no iron elements and is not affected by the corrosion mechanism of cooling water on carbon steel. On the contrary, it undergoes a hydration reaction in cooling water with a pH value of 4.8–8.6: 2Al + 6H2O → Al2O3·3H2O + 6H+ + 6e-. This reaction results in the formation of a dense, chemically stable layer of hydrated oxide (Al2O3·3H2O) on the surface in contact with the cooling water. Should this layer be damaged, it can regenerate rapidly, keeping the metal surface in a passivated state and thereby enhancing its corrosion resistance. (2) Benefits of using a 5454 tube bundle: Carbon steel water coolers tend to develop a layer of fouling on the inner surface of their tubes during use. Although anti-corrosion measures are in place, some degree of fouling still occurs. The outer wall of the tube bundle has a thick layer of rust and oil residue due to corrosion, which reduces the cooling efficiency. A 5454 tube bundle is used; there is very little fouling on the inner wall of the tubes, and no rust layer on the outer wall, so the thermal resistance caused by these fouling layers can be ignored. In terms of thermal conductivity, the thermal conductivity of the 5454 tube bundle is 3.09–2.56 times that of 10# steel. Calculations show that using a 5454 tube bundle with a diameter of φ800 can save 48,660 yuan per year compared to carbon steel tube bundles of the same type. In other words, under the same process conditions, the heat exchange area of 5454 tube bundles can be smaller than that of carbon steel tube bundles, which is of great significance. The service life of this tube bundle is 5 to 8 times that of a carbon steel tube bundle. It is a material with excellent comprehensive properties that solves the corrosion problem in refinery coolers. 3.2.2 The tube bundles of the heat exchangers are coated with a chemical “Ni-P” coating. Since in some coolers the medium temperature is relatively high (t > 160°C, pressure P > 1 MPa), the use of 5454 tube bundles is not suitable; however, anti-corrosion measures are still necessary in such cases. Since 1994, the tube bundles of the cooling equipment in some units have been coated with a Ni-P electroless coating, yielding excellent results. Since the Ni-P coating is a metallic layer with an amorphous structure, it lacks crystal defects such as grain boundaries and dislocations; it has a uniform structure that makes point corrosion unlikely, thereby granting it high corrosion resistance. In some media, the Ni-P coating performs better than titanium alloys; it does not exhibit tendencies toward pitting, intergranular corrosion, stress corrosion, or localized corrosion. Coating low-carbon steel with a chemically deposited Ni-P alloy coating can replace some stainless steels, thereby **reducing costs**. At the same time, the Ni-P coating features good uniformity, strong adhesion, high hardness, and excellent wear resistance. Core elements with Ni-P plating on the surface of carbon steel tube bundles have been used in the heat exchange equipment of atmospheric and vacuum distillation, catalytic cracking, gas fractionation, and furfural plants, and excellent results have been achieved after several years of use. For example: the 2 steam generators in the furfural plant (model FL800-180-16-2), with the tube side containing furfural vapor and the shell side containing water vapor. Due to the strong corrosive effect of high-pressure aldehyde vapor on the equipment, corrosion in this area is very severe; 2 cores have to be replaced during annual maintenance. To address the aforementioned issues, and in conjunction with equipment maintenance, a Ni-P layer was applied to the inner and outer walls of the carbon steel tube bundle in this area. After more than 4 years of use, there has been no leakage due to corrosion. Therefore, after undergoing Ni-P chemical treatment, the corrosion resistance of carbon steel tube bundles increases by more than 4 times compared to before, while the cost of coating is 70-80% of the manufacturing cost of carbon steel tube bundles. This coating has anti-fouling properties, which prevent the tube bundle from scaling. It is excellent in resisting water and oil vapor corrosion. Especially for coating corrosion protection and in cases where 5454 tube bundles cannot be used, this method is a good supplement. Furthermore, the Ni-P layer is a metal coating whose thermal conductivity is similar to that of steel, so it does not reduce heat transfer efficiency. This anti-corrosion method is highly valuable for use in heat exchange. 3.3 Use of chemical cleaning techniques: As mentioned earlier, using nitric acid to clean the scale on the inner walls of the tube bundles in heat exchange equipment has yielded very good results. However, using nitric acid to remove the rust layer from the outer wall of the tube bundle does not yield satisfactory results. Because the outer surface of the tubes without anti-corrosion measures is covered with many corrosion products, this not only affects heat transfer efficiency but also accelerates the corrosion of the metal surface. Although high-pressure water jet technology is used to extract the core during maintenance, the results are not satisfactory. To address this issue, X-ray diffraction analysis was conducted on the corrosion products on the outer surface of the tubes; the main components were FeS2, Fe3O4, and Fe2O3. Comparative tests show that using a hydrochloric acid solution with an appropriate corrosion inhibitor yields the best results, while also being the most cost-effective approach. An aqueous hydrochloric acid solution has a dissolving effect on iron oxides; the mechanism is as follows: FeO + 2HCl → FeCl2 + 2H2O; Fe2O3 + 6HCl → FeCl3 + 3H2O; Fe3O4 + 8HCl → FeCl2 + 2FeCl3 + 4H2O; Fe + HCl → FeCl2 + H2↑. Hydrochloric acid is an inorganic acid commonly used for the chemical cleaning of metal equipment. It exhibits a high dissolution rate and capacity for various iron oxides; the resulting salts are readily soluble, the process is simple and safe, and the equipment remains in excellent condition after cleaning. Chemical cleaning was carried out on the outer wall of the cooler tube bundle that was severely corroded. The main methods are as follows: equipment pretreatment----oil and fat removal----acid washing----rinse----passivation. Chemical cleaning of the rust layer has yielded excellent results. For example, in July 1990, chemical cleaning was carried out on a total of 5 condensers, namely the first and second stage top condensers of Unit 2 for vacuum distillation. After cleaning, through on-site testing and calculations, the 5 condensers can generate direct economic benefits of nearly 200,000 yuan as a result of this cleaning process. The service life of the tube bundle is extended through cleaning, as this allows the onset stage of under-scale corrosion to be delayed. Our plant has a large number of coolers suitable for chemical cleaning to remove rust; this cleaning process improves the heat transfer efficiency of the coolers in use, thereby avoiding unnecessary losses. 4 Conclusion: Ten years of corrosion and protection management for cold exchange equipment have yielded excellent results. However, for heat exchange equipment, although water is used as the cooling medium to exchange heat with oil in order to achieve heat transfer. However, due to different operating conditions, the forms of corrosion that occur vary, and therefore the protective measures employed also differ. For example, a protective measure may work well under certain conditions, but it may not be effective under other conditions. Therefore, the protective measures adopted can achieve optimal effectiveness within a certain range. The application scope of the above protection methods in different environments is as follows: 4.1 Using 7910 coating to prevent corrosion on the inner wall of pipes – this method is suitable for lubrication systems, as the cooling equipment in such systems typically uses lubricant as its medium, resulting in little or no corrosion on the outer surface of the pipe bundles. At the same time, there were no rust products on the metal surface of the outer wall of the tube bundle. Therefore, the 7910 coating is very effective in preventing corrosion on the inner walls of tubes, but this material should be used at temperatures below 160°C. 4.2 Use of 5454 tube bundles: These tube bundles can be employed in the cooling heat exchange equipment of fuel oil systems, such as condensers and coolers at the top of towers, as both the inner and outer walls of these tube bundles are subject to corrosion, resulting in significant surface rust. The operating conditions are: the pressure should be less than 1 MPa and the temperature less than 180°C, which will yield excellent results. 4.3 With a Ni-P electroless coating, this tube bundle can be used in the applications where 7910 coated anti-corrosion tube bundles and 5454 tube bundles are employed, and it can also be used in conditions with high temperatures (t>160–220°C, P>1 MPa). However, in practical applications, it is essential to pay close attention to the quality of Ni-P coated tube bundles. Otherwise, the goal will not be achieved. 4.4 Use of chemical cleaning technology: This technology should be applied appropriately when it is necessary to keep the production equipment running without interruption. It is possible to restore the equipment to normal operation in the shortest time without removing it, thereby improving its heat exchange efficiency. 4.5 High-pressure jet technology: This method can be used in conjunction with equipment maintenance, applied while disassembling the equipment. It provides excellent cleaning results, especially for the inner walls of tube bundles; it is cost-effective and efficient. 4.6 Adoption of online cleaning technology: Using this technology is a proactive approach to prevention, and when applied properly, it can yield significant economic benefits. When in use, pay attention to the quality of the circulating water and ensure that there are no solid debris such as stones in it. The presence of these elements affects the proper functioning of the spring system. Therefore, based on the corrosion and scaling conditions of the heat exchangers, the approach of „first diagnosing the problem, then determining the solution, and finally addressing it“ should be adopted. In other words, by adopting different protection methods based on the usage and corrosion conditions of the heat exchangers, the best economic benefits can be achieved. To determine the appropriate methods, we need to continuously conduct research and experiments in practical work, and keep summarizing experience in order to minimize the corrosion losses of heat exchange equipment.