Corrosion analysis and countermeasures of ammonia stripping tower

Nickel and its alloys are widely used in corrosive media such as acids, bases, and salts in the chemical and chemical industry, covering a very broad range of applications. Nickel alloys are important industrial materials. In terms of corrosion resistance, they lie between stainless steel and rare materials such as tantalum. Nickel alloys are generally used in environments with corrosive inorganic acids and chlorides. In this corrosive environment, most stainless steels are prone to stress corrosion cracking, pitting corrosion, and crevice corrosion. Pure nickel is used in industry as a material resistant to alkali corrosion. Although zirconium shares the same corrosion resistance range as titanium alloys and corrosion-resistant nickel alloys, these alloys, known as \"active metals,\" possess special properties and cannot withstand the corrosion of certain ions, such as fluorides. Nickel alloys are suitable for resisting a wide variety of forms of corrosion; they exhibit high resistance to stress corrosion caused by chlorides, and can withstand the corrosion of corrosive reducing acids such as hydrochloric acid, hydrofluoric acid, and low-concentration sulfuric acid. Certain nickel alloys can also resist corrosion by strongly reducing acids as well as strongly oxidizing acids. Various nickel alloys exhibit strong corrosion resistance to alkalis. Some nickel alloys exhibit high resistance to local corrosion (pitting and crevice corrosion). All nickel alloys have excellent ductility, are easy to shape, and possess good weldability. Nickel itself can resist corrosion by certain chemical media, and it is metallurgically compatible with other metals such as chromium, copper, molybdenum, and tungsten, allowing the formation of various nickel-based alloys that thereby **improve its corrosion resistance. Chromium plays the same role in nickel alloys as it does in stainless steel; in the presence of oxygen, chromium accelerates the formation of a passivation layer that prevents corrosion. Adding iron to nickel-based alloys can also promote the formation of a passivation film to prevent corrosion. Copper, molybdenum, and tungsten can enhance the noble metal properties of nickel-based alloys under active corrosion conditions. Due to their large atomic size, molybdenum and tungsten, like niobium, are important strengthening agents for nickel-based alloys. In addition to industrially pure nickel, there are also nickel-based alloy systems. 3 binary alloy systems: Ni-Cr alloy, Ni-Cu alloy, and Ni-Mo alloy. Chromium promotes the formation of a passivation film, while copper is highly effective in resisting corrosion by seawater, alkaline water, and reducing acids, especially hydrofluoric acid. The most suitable binary nickel-based alloy system is the Ni-Cr-Mo system. Chromium and molybdenum can not only resist corrosion by oxidative and reducing acids, but they also exhibit excellent resistance to pitting corrosion, crevice corrosion, and stress corrosion caused by chlorides. The Ni-Cr-Fe and Ni-Fe-Cr alloy systems were designed to overcome the performance and cost disadvantages between Ni-Cr alloys and austenitic stainless steels. One of their advantages is that they enhance the resistance of stainless steel to stress corrosion cracking. This article analyzes and explains the characteristics of the chemical compositions of common nickel alloys, in order to guide the selection of appropriate nickel alloys for corrosion resistance against various chemicals. 1 Material Structure 1.1 Industrial pure nickel Many materials are sold under the guise of industrial pure nickel; most pure nickel has a nickel content of over 99%, with other trace elements added to control its specific properties. Many pure nickels are used in the electronics industry due to their excellent electromagnetic properties. Pure nickel possesses excellent corrosion resistance, primarily because it is resistant to caustic alkalis, and it maintains this resistance over a wide range of concentrations and temperatures of such alkalis ； Secondly, they can be easily combined to form various nickel-based alloys that are resistant to mildly corrosive media, making them suitable as materials for food processing equipment. 1.2 Nickel-copper alloys (Ni-Cu) Nickel and copper are two elements that are adjacent to each other in the periodic table; they have identical atomic structures, both belonging to the face-centered cubic crystal structure. In their solid state, these two elements exhibit good metallurgical compatibility under various temperature conditions. There are several important Ni-Cu and Cu-Ni alloys, and this article focuses on those Ni-Cu alloys with a copper content of 30% to 45%, which are related to Monel alloys. The Cu-Ni alloy with a nickel content of about 30% has achieved commercial success in terms of its corrosion resistance. In fact, these Cu-Ni alloys are resistant to corrosion by aqueous solutions, and in particular, they exhibit excellent resistance to hydrofluoric acid corrosion. An important characteristic of nickel-copper Monel alloys is their resistance to corrosion by seawater and alkaline water. In fact, these alloys are also resistant to corrosion caused by biological wastewater, exhibit strong resistance to hydrofluoric acid, as well as resistance to corrosion by other non-oxidizing acids. They can also withstand cavitation, which makes them suitable as materials for propeller blades and pumps that are exposed to flowing water. The mass fractions of the nickel alloy components resistant to aqueous solution corrosion are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload1/0807301457049355.jpg 1.3 Nickel-molybdenum alloys (Ni-Mo): Nickel-molybdenum alloys, or Hastelloy B alloys, have been used in the chemical industry for a long time. Their advantage is their strong corrosion resistance to pure hydrochloric acid and sulfuric acid, as well as a wide range of concentrations and temperatures for which they remain resistant to corrosion. These alloys can also resist corrosion in pure hydrofluoric acid, as well as in solutions of hydrobromic acid, food-grade phosphoric acid, acidic chlorides, and other non-oxidizing halide solutions. The biggest limitation of nickel-molybdenum alloys is their susceptibility to corrosion by oxidizing acids, such as nitric acid or various oxidizing acids containing oxygen, hydrogen peroxide, chlorine, bromine, iron ions, and copper ions. 1.4 Nickel-chromium alloys (Ni-Cr): Experiments have shown that adding chromium to nickel not only improves the alloy’s resistance to oxidizing acids but also paves the way for the development of high-temperature alloys resistant to oxidative corrosion. The most commonly used corrosion-resistant Ni-Cr alloy forgings are the Inconel 600 series alloys, which are characterized by their excellent resistance to NaOH and their good resistance to stress corrosion cracking, outperforming many stainless steel materials in these aspects. 1.5 Nickel-chromium-molybdenum alloy (Ni-Cr-Mo) The nickel-chromium-molybdenum alloy is characterized by its versatility. Chromium and molybdenum improve the properties of nickel-based alloys; chromium acts as a passivator in oxidizing acids, in the same way as it does in stainless steel. Molybdenum can resist corrosion by reducing acids, especially hydrochloric acid. Nickel-chromium-molybdenum alloys have achieved great success in resisting corrosion by hydrochloric acid and chlorides in the industrial sector. In the chemical and processing industries, hydrochloric acid and chlorides cause severe damage to stainless steel. Nickel-chromium-molybdenum alloys are particularly resistant to pitting corrosion and crevice corrosion, as well as to stress corrosion cracking; they can also withstand latent and unpredictable forms of corrosion caused by chlorides. In heat exchangers, not only the corrosion of the process steam must be considered, but also the corrosion of the cooling medium. For example, cooling water often contains chlorides due to biological contamination, and heat exchangers located near the coast commonly use seawater (which can be considered the most common chloride solution) as a coolant. Nickel-chromium-molybdenum alloys can resist corrosion by hydrochloric acid and its related salts, as well as by compounds of bromine and fluorine. In fact, nickel-chromium-molybdenum alloys are among the metal materials that are resistant to heat and corrosion in hydrofluoric acid environments with high humidity. Another property of nickel-chromium-molybdenum alloys is their resistance to corrosion by sulfuric and nitric acids, as well as moderate resistance to corrosion by caustic soda and caustic potash. 1.6 Nickel-chromium-iron alloys (Ni-Cr-Fe) The emergence of nickel-chromium-iron alloys addressed the performance shortcomings between high-molybdenum stainless steels and nickel-chromium-molybdenum alloys. Therefore, these alloys exhibit good corrosion resistance against chloride-induced forms of corrosion such as pitting corrosion, crevice corrosion, and stress corrosion cracking, and also possess a certain degree of corrosion resistance to hydrohalic acids, particularly hydrochloric acid. Some nickel-chromium-iron alloys contain a certain amount of copper, which can improve their corrosion resistance in environments involving sulfuric acid and hydrofluoric acid. These alloys evolved from Hastelloy G, which in turn originated from the nickel-chromium-iron alloys with varying copper contents mentioned above, as well as from Hastelloy F alloys. 1.7 Nickel-iron-chromium alloys (Ni-Fe-Cr): When the iron content in these alloys reaches 30%, their composition is similar to that of high-nickel austenitic stainless steels. The negative impact is that such a high iron content reduces the compatibility of the nickel alloy with major metal elements such as molybdenum, thereby decreasing its resistance to crack corrosion; moreover, a high iron content increases the cost of the alloy.

Guizhou Chitianhua Co., Ltd.’s new urea production plant utilizes the Snam ammonia stripping process from Italy; the 4 high-pressure units in this plant (urea synthesis tower, stripping tower, high-pressure ammonium methylate condenser, and high-pressure ammonium methylate separator) were all manufactured by the Italian company FBM HUDSON. The unit was put into operation in August 1996. After 10 years of operation, during the major overhaul in November 2006, a comprehensive corrosion inspection was carried out on the 4 high-pressure units. Based on the results of this inspection as well as those from previous years, and taking into account the structural characteristics of the units, an analysis of the corrosion conditions was conducted. 1. Urea synthesis tower: In 2005, an explosion occurred in a urea synthesis tower at a factory in China, resulting in 4 deaths. The General Administration of Quality Supervision, Inspection and Quarantine issued a notice titled \"On Further Strengthening the Inspection Work Related to the Production and Use of Urea Synthesis Towers.\" Guizhou Chitianhua Co., Ltd. fully recognizes the importance of regular equipment inspections, conducting such inspections 2 years in advance and strengthening online leak detection efforts. 1.1 Structural features: Unlike many nitrogen fertilizer production plants in China, which feature multi-layered structures, this synthesis tower has a single-layer outer shell. Its welds are easy to inspect using ultrasonic testing, which helps to determine the equipment’s capacity to withstand pressure safely. During this major overhaul, the entire external insulation of the equipment was removed to expose all welds, which were then subjected to 100% MT and UT inspections; no abnormalities were found. 1.2 Liner corrosion condition: The corrosion-induced thinning of the liner base material and the corrosion in the heat-affected zone of the welds are more severe in the upper and middle sections compared to the lower section. Significant corrosion grooves are present in the heat-affected zone of the lining weld, with a depth of <1.0 mm ； Selective edge corrosion is relatively severe at the lining ring and the longitudinal weld fusion lines; grinding, repair welding was carried out on the selective edge corrosion at the lining ring weld fusion line where the topmost cylinder body is connected to the upper shell. In recent years, leaks in the linings of synthesis towers in various plants have basically occurred in the weld areas; therefore, corrosion inspection of the welds and their heat-affected zones should be regarded as the most important inspection points for synthesis towers. Table 1 shows a comparison of the lining wall thicknesses in recent years. For the cylinder section, the corrosion rate from 2002 to 2006 was 0.133 mm/year, indicating an acceleration in corrosion that warrants attention; it is necessary to ensure annual inspections of the lining. Leaks have occurred in the urea synthesis towers of several urea plants using the ammonia stripping process in the surrounding area. The lining thickness of one Snam urea synthesis tower using the ammonia stripping process, which also has a nominal wall thickness of 5 mm and has been in operation for 11 years, is shown in Table 2. This suggests that urea synthesis towers with similar structures using the ammonia stripping process are subject to severe internal corrosion. http://www.nmtech.com.cn/jishuwang/upload1/0805261003349818.jpg Corrosion in the lining is inevitable, but what thickness is safe for the lining wall? According to relevant calculation formulas, taking into account the condition of no instability, the minimum thickness calculated is 3.07 mm ； Considering the need to prevent warping of the lining due to differences in thermal expansion between the lining and the shell, the minimum thickness was calculated to be 3.276 mm. Therefore, the minimum wall thickness that must be ensured is 3.276 mm. 1.3 Weld defect in the old supports: The old supports of the original tower tray were of a gate-shaped design; during initial manufacturing, no bevel was created at the junction with the lining, which resulted in incomplete welding in the middle of the welds. Once pores form on the surface of these welds due to corrosion, the hollow areas become prone to oxygen-deficient corrosion. During the renovation of the bottom 4 layers of trays, a single-sided lug structure was used; the old supports were discarded and not utilized, and they were thoroughly polished during this major repair. It is planned to replace the remaining old I-shaped supports on the upper part with a single-sided lug structure during the next major overhaul. 1.4 Other internal components: The material of the downcomer is CrNiMo 25-22-2; its surface is as smooth as new, appearing silver-white, with no signs of corrosion, and even the steel stamp on the surface is clearly visible. 1.5 Leak detection pipes: During initial installation, all leak detection pipes extend vertically from the equipment’s insulation by about 10 cm, distributed on both sides of the entire cylinder. In the event of a leak at the welds, it is difficult to detect such leaks. To ensure the safe operation of the equipment, taking advantage of the regular inspection period, all insulation from the equipment was removed, and all leak detection tubes for the welds were gathered together on 3 different floors. Later, based on the experience of related manufacturers, nitrogen was introduced into the leak detection tube, and a liquid collection tank was installed; the integrity of the leak detection tube was confirmed by checking for the presence of bubbles. 2 Stripping Tower 2.1 Heat Exchange Tubes The heat exchange tubes in the stripping tower are zirconium-lined composite tubes; the outer layer is made of 25-22-2 stainless steel, while the inner layer is made of zirconium. The nominal thickness of the zirconium lining is 0.7 mm (the actual thickness is slightly greater). As long as the zirconium lining does not corrode, the outer layer will not thin out. Therefore, calibration is carried out based on the thickness of the zirconium lining, and the data obtained from on-site inspections also show the thickness of this lining. The average thickness of the zirconium lining on the tubes was measured to be 0.76 mm. Based on the inspection results, no signs of corrosion-induced thinning could be observed; in a few tubes, slight thinning could be seen at the upper end of the tubes, but it was less than 0.05 mm, well within the accuracy range of the testing equipment. Compared with the test report from 2000, there is no significant corrosion-induced thinning of the zirconium lining on the tube bundles. The classification statistics of tube wall thickness are shown in Table 3. http://www.nmtech.com.cn/jishuwang/upload1/0805261004119163.jpg Ammonia stripping plants in China that use titanium heat exchange tubes suffer from severe erosion; most of them have switched to other types of tubes. Moreover, heat exchange tubes made of 2RE69 in carbon dioxide stripping plants also experience a certain degree of corrosion. In comparison, zirconium-lined composite tubes have a more rational design. 2.2 Liquid Distribution Tubes The liquid distribution tubes used in the stripping tower are of the inserted type, as shown in Figure 1. A portion of the tube end is inserted into the heat exchange tubes, while another portion of its extreme end is inserted into the zirconium-lined area. Compared to the extrapolation method used in the older carbon dioxide stripping process (Figure 2), it reduces erosion and corrosion at the top of the tube bundle. This structure sacrifices the scouring and corrosion-thinned condition of the tube bundle section due to the insertion of distribution tubes, but it protects the tube end portion, which is unlined with zirconium and represents the weak point of the entire tube bundle. The pipe ends of some liquid distribution pipes are severely corroded and thinned, and have become deformed, as shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload1/0805261004597345.jpg 2.3 Welded layers on the upper and lower covers: The inner surfaces of the upper and lower covers are covered with welded layers. The outer part of these welded layers has corroded, turned black, and become porous; in addition, the ferrite content is above the specified limit (≤1.5%). The minimum thickness of these welded layers is 2.5 mm. These layers need to be polished and repaired through welding. It is likely that this is due to defects from the original manufacturing process, and they should be carefully inspected during the next major maintenance session. 2.4 Introduction of passivation air to the bottom tube bank: In the original design, considering that the temperature at the bottom of the stripping tower was as high as 192–209 °C, resulting in harsh operating conditions, a passivation air compressor was installed to supply air to the bottom of the stripping tower in order to facilitate the formation of a passivation film. Many ammonia stripping plants in the country do not have this equipment, or it is removed after operating for a period of time. The passivation air compressor in the urea production unit of Guizhou Chitianhua Co., Ltd. was shut down for about 3 months due to a lack of imported spare parts; no abnormalities were detected in nickel content analyses during that period, and there was also no abnormal corrosion in the pipe trays at the bottom of the equipment during the major repair. Currently, there is a lack of detailed comparative data regarding whether the introduction of air serves to prevent corrosion in this equipment. 3 High-pressure ammonium methanate condenser 3.1 The horizontal design prevents stress corrosion cracking of the tubes. Like in other ammonia-stripped urea production units, the high-pressure ammonium methanate condenser is installed in a horizontal position, which avoids the tubes from being in areas where there is a alternation between wet and dry conditions. This prevents the formation of a Cl‑rich environment, thereby reducing the risk of stress corrosion cracking (SCC) in the tubes – an advantage over the older carbon dioxide stripping method. During the major overhaul, the first ammonia leakage check since the equipment was put into operation was conducted, and no abnormalities were found. 3.2 Tube corrosion status: The tubes are made of 25-22-2 stainless steel, with an average wall thickness of 2.13 mm; the average wall thickness in 2002 was also 2.13 mm. The wall thicknesses of the tubes are mainly concentrated between 2.10 mm and 2.15 mm. Compared to the inspection report from 2002, there is no significant thinning due to corrosion, so the condition is excellent. Each tube has virtually no noticeable thinning area; only a slight thinning can be observed near the tube opening, but it is less than 0.05 mm, well within the detection accuracy of the instrument. 4 High-pressure Ammonium Methane Separator: The high-pressure ammonium methane separator is the simplest in structure among several high-pressure devices; it has no other internal components aside from the liquid inlet baffle. The thickness of the liner in this thickness gauge cylinder ranges from 5.4 to 6.0 mm, with an average thickness of 5.627 mm. In 2000, the thickness of the liner in the cylinder was between 5.7 and 6.1 mm, with an average thickness of 5.761 mm. It can be seen that corrosion is not significant, and the equipment is currently in a state of normal, uniform corrosion. The morphology of the local corrosion is characterized by numerous arc burn marks on the surface of the lining; 9 of these arc burn marks showed corrosion-induced blackening and porosity, with an elevated ferrite content. One of these areas was corroded into a pit, with a depth of approximately 2.0 mm. These minor manufacturing defects were all repaired through grinding and welding. 5 Conclusion: The 4 high-pressure units are the core equipment of the plant. Based on the current conditions of corrosion and operation of these devices, the focus of corrosion monitoring should be placed on the urea synthesis tower and the stripping tower. The interval between corrosion inspections should be shortened, with inspections being carried out during each major maintenance session in order to keep an eye on the condition of these devices. At the same time, it is necessary to strengthen exchanges regarding the corrosion of high-pressure equipment in other urea plants, especially those using ammonia stripping, to gather relevant information from various sources and carry out proper equipment management in order to ensure the stable, continuous, and efficient operation of the facilities.

The corrosion-resistant materials used for lining the urea synthesis towers in domestic urea producers are diverse; examples include industrial pure titanium, 316L(MOD), X225Cr22Ni2Mo, and other duplex stainless steels. However, 316L(MOD) and X225Cr22Ni2Mo are most commonly used as corrosion-resistant linings. Urea synthesis towers lined with 316L(MOD) for corrosion resistance develop weld corrosion (welded using Thermanit 19/15H electrodes) and corrosion at the edges of the welds after being in use for a certain period of time. The lining welds welded with BM310Mo-L electrodes exhibited knife-edge corrosion mainly in the near-weld area, while the welds themselves were in a normal state of corrosion. In some cases, the corroded welds appear protruding, indicating that the corrosion resistance of the weld metal is better than that of the base material 316L(MOD). Discussion on the causes of knife-edge corrosion: The main cause of knife-edge corrosion is the effect of welding heat; in the area near the weld seam of the lining, there are regions that are exposed to sensitization temperatures (600–1000 °C) as a result of this welding heat. Although 316L(MOD) is an ultra-low carbon stainless steel, its corrosion resistance in the sensitization temperature range also inevitably decreases. In other words, the welded joint itself exhibits unevenities in its microstructure and properties, including corrosion resistance. The sensitized (coarse-grained) area has the worst corrosion resistance of the entire welded joint and is also the most susceptible to corrosion; the size of this area depends on the amount of heat input during welding. Notch corrosion is directly related to the welding specification parameters, namely the amount of heat input. When the heat input is too high (i.e., the welding current is too large) and the welding speed is too slow, it inevitably leads to an excessive duration of high temperatures in the area near the weld. This results in an increased amount of Cr23C6 chromium carbide being precipitated, as well as grain growth, all of which contribute to a decrease in the corrosion resistance of that area. As the heat input increases, the width of this region also increases. To reduce the tendency for notch-like corrosion, the heat input should be minimized as much as possible; it is advisable to keep the welding heat input below 10 kJ/cm. Apart from process factors, the key issue is the chemical composition of the lining base material itself – such as carbon content, chromium content, and the amount of stable chemical elements that have a stronger affinity for carbon than chromium – as these determine the material’s resistance to notch corrosion. Based on the above understanding, the author believes that although 316L(M01) is an ultra-low carbon stainless steel, its Cr content is lower than that of X225Cr22Ni2Mo; even with great care in welding procedures, knife-edge corrosion still tends to occur in the welds, especially in the longitudinal welds. 2 Repair methods 2.1 Repair by welding a cover plate (1) Before welding the cover plate, it is necessary to grind flat the welds of the lining in order to ensure a good fit between the cover plate and the lining. Grind away the corrosion at the junction of the lining and the cover plate to facilitate welding. (2) The lining under the cover plate must be drilled to create holes that communicate with the original leak detection system, in order to prevent the formation of a sealed air chamber; these holes also serve as leak detection channels for the welds connecting the cover plate to the lining. (3) The cover plate material should preferably be X225Cr22Ni2Mo, so that even if knife-edge corrosion occurs in the near-weld area in the future, it will occur only on one side of each weld. If 316L (MOD), the same as the lining material, is used, knife-edge corrosion may occur on both sides of each weld. (4) The cover plate must have a certain width, which should be greater than 40 mm. If it is less than 40 mm, the distance between the two fillet welds is too small, which will inevitably affect the corrosion resistance of the cover plate. (5) The cover plate must have a certain thickness; this thickness should take into account the requirements related to corrosion, that is, the corrosion resistance of the fillet welds, as well as any impact that adding a cover plate might have on the installation of the tray. It is advisable for the thickness of the cover plate to be slightly greater than that of the lining plate. 2.2 Precautions: Precautions for grinding away the corrosion layer and groove-shaped corrosion, as well as for taking welding repairs to fix the lining welds. (1) It should be avoided that the heat affected zone of the repair weld overlaps with that of the original weld; the width of the repair weld on each side should be 2–3 mm wider than that of the original weld, as the overheated coarse-grained area is usually within 2 mm of the fusion line. (2) When the weld seam is wide, multiple passes of welding are advisable; during each pass, the swing amplitude of the welding electrode should be kept within 2 times the diameter of the electrode core. (3) The grooves resulting from blade-like corrosion grinding should, as much as possible, serve as the weld passes for the final repair welding. 2.3 Recommendations for selecting a repair method The specific repair method to be used should be determined based on the particular circumstances. The method of adding welding plates is suitable for situations where the area to be repaired is large and the amount of welding required is significant ； For areas that require repair and are relatively small, the repair method of grinding followed by welding should be given priority. (1) After repair using the above two methods, there is still a possibility of reoccurrence of knife-edge corrosion in the area near the weld. (2) When the weld corrosion is severe and the width that requires rewelding is greater than 20 mm, the method of using cover plates can be considered for repair ； When it is less than 20 mm, grinding and patch welding can be considered as a method for repair. (3) When only kerf corrosion occurs while the weld remains intact, it is not necessary to use a cover plate for repair.