Corrosion analysis and prevention measures for liquid ammonia storage tanks

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Source: Hubei Work Safety Information Network. Abstract: Several factors that may cause corrosion in liquid ammonia storage tanks are analyzed, and corresponding protective measures are proposed. Ammonia is an important chemical product and industrial raw material, widely used in industries such as oil refining, chemicals, agriculture, pharmaceuticals, and refrigeration. For ease of storage and transportation, the ammonia produced by synthetic ammonia plants is usually converted from gaseous ammonia into liquid ammonia through pressurization or cooling. Liquid ammonia tanks, as special types of pressure vessels, are also widely used in these industries. 　　Years of experience in the use and inspection of liquid ammonia storage tanks have shown that such tanks rarely suffer from strength failures; most failures are caused by corrosion cracks. Based on years of practice, this paper analyzes several factors that may cause corrosion in liquid ammonia storage tanks, and proposes corresponding protective measures to prevent such corrosion from occurring. 1 Corrosion characteristics of liquid ammonia storage tanks  Inspections of various types of liquid ammonia storage tanks have revealed that corrosion cracks are relatively severe in the weld areas on the inner surfaces of these tanks; most of these cracks occur in the circumferential welds. The fracture surfaces of these cracks show no plastic deformation, indicating typical characteristics of brittle cracks. Most of the cracks are shallow and long surface cracks with distinct branching; the main crack lines are perpendicular to the weld direction. The cracks are more severe, especially at the start and end points of manual arc welding, at T-joints, and at the intersections of the head ring welds with the cylinder’s longitudinal welds. Magnetic particle inspection revealed that the weld cracks were dendritic in shape; the main cracks were mostly linear, with short branches, sharp ends, and slightly wider bases. 2 Corrosion analysis of liquid ammonia storage tanks The liquid ammonia contained in these tanks is in a liquefied state, obtained by applying pressure or cooling; its operating pressure corresponds to the saturated vapor pressure at atmospheric temperature. Operating temperature and operating pressure fluctuate with climate changes. The \"Regulations on Safety Supervision of Pressure Vessels\" stipulate that for storage tanks at normal temperature that contain low-pressure liquefied gases and have no insulation or cooling system, the design temperature is set at 50°C, while the maximum operating pressure is determined to be the saturated vapor pressure of the medium contained within at 50°C. In the Guangdong region, the maximum room temperature in summer generally does not exceed 40°C. At 40°C, the saturated vapor pressure of ammonia is 1155 M Pa, while the typical operating pressure ranges from 0.18 to 1.12 M Pa; therefore, storage tanks are not likely to suffer structural damage due to overload. Due to the very high expansion coefficient of liquefied gas, which is dozens of times that of water, if the tank is filled with liquid, the pressure inside the tank is no longer the vapor pressure but rather the expansion pressure of the liquid. The operating pressure of storage tanks is directly affected by temperature; for every 1°C increase in temperature, the pressure in liquid ammonia storage tanks can rise by 11,316–11,875 M Pa. A rise of just 3–5°C in temperature can cause the tank to explode due to severe overloading. Therefore, the \"Regulations on Safety Inspection of Pressure Vessels\" specify the filling coefficient for storage tanks at different liquid filling temperatures, in order to ensure that there is sufficient gas space inside the tanks. If the storage tank is not properly evacuated before being put into use, air can easily get mixed in. During filling, discharging, and maintenance, liquid ammonia can also be contaminated by air. Pulling stresses caused by operating pressure and residual welding stresses exist at the welds of the storage tanks. Under tensile stress, carbon steel is prone to stress corrosion failure in an air-contaminated liquid ammonia environment. 　　O 2, CO 2, and N 2 in the air all contribute to the corrosion of liquid ammonia on the tank wall material. Whether in the gas phase or liquid phase, ammonia, O2, and N2, together with carbon steel or low-alloy steel, create a stress corrosion environment that leads to stress corrosion cracking (SCC). The mechanism of corrosion is as follows: In liquid ammonia containing O2, O2 adsorbs on the steel surface to form an oxygen film, which keeps the corrosion potential at a positive value. When the material is subjected to tensile stress and deformed, this film is broken, and the exposed fresh surface (slip planes) together with the metal surface covered by the oxygen film form microcells, leading to rapid dissolution. In the absence of other impurities, O2 can re-form a film on exposed metal surfaces, thereby suppressing the occurrence of stress corrosion. However, when N2 is also present in liquid ammonia, the \"competitive adsorption\" between N2 and O2 at the slip planes prevents the repassivation of some of the exposed slip planes, thus increasing the sensitivity of steel to stress corrosion cracking. For the conditions related to stress corrosion mentioned above, stress corrosion cannot occur if even one of them is missing. CO2 in the air, on the other hand, causes general corrosion. The corrosion mechanism is as follows: Cathodic reaction: O2 + 2NH4+ + 4e– → OH– + 2NH3. Anodic reaction: 2Fe → 2Fe2+ + 4e–. The overall reaction is: O2 + 2NH4+ + 2Fe → 2Fe2+ + 2OH– + 2NH3. In the presence of CO2, ammonium carbonate is formed: 2NH3 + CO2 → NH4CO3. NH4CO3 → NH4+ + HCO3–. The compound NH4CO3NH2 produced in this reaction has a strong corrosive effect on carbon steel; it causes the passivation layer on the surface of the steel to crack at the slip steps, and corrosion cracks of the anodic type develop along these areas. Therefore, these two types of corrosion reinforce each other, exacerbating the corrosive damage to the material. This stress corrosion is related to the strength of the base material; the higher the strength, the greater the susceptibility to corrosion. The storage temperature also has an impact on stress corrosion; in storage tanks that operate at normal temperatures above 0°C, stress corrosion is likely to occur. 3 Protective Measures and Safety Assessment If improper methods are used at every stage of liquid ammonia storage tank design, manufacturing, and operation, it can lead to hidden risks of stress corrosion. Intentional corrosion protection requires the adoption of reliable measures at every stage of the entire quality assurance system, from design and manufacturing to the usage phase. 　　(1) Material selection. Practice has shown that the higher the material strength, the greater the likelihood of stress corrosion. However, the minimum strength limit at which stress corrosion does not occur is related to factors such as impurity content and properties, stress level, and operating speed. To prevent stress corrosion, steel with lower strength should be selected as much as possible, taking into account factors such as operating pressure, residual stress, as well as safety and cost-effectiveness. 　　(2) Adopt a reasonable structure and welding process. Structurally, stress concentration caused by excessive and overly concentrated welds, asymmetric welds, intersecting welds, and unreasonable welding sequences should be avoided. During manufacturing, forceful welding should be avoided to prevent defects such as undercutting and misalignment, ensuring that the surface in contact with the medium is as smooth as possible. After manufacturing, annealing treatment should be carried out to remove the residual thermal stress resulting from welding. Proper post-weld heat treatment can **reduce residual stresses during the manufacturing process, as well as lower the peak hardness in the weld heat-affected zone.** 　　(3) For new storage tanks before they are put into use, the air inside them must be completely removed; during processes such as filling, discharging, and maintenance, certain measures should be taken to prevent any air from entering. Ammonia vapor should be continuously condensed in large storage tanks, and since most of the non-condensable gases are air, they should be vented. For smaller devices, the air inside the tank is removed by pumping or evaporation. In short, eliminating air pollution inside the storage tank can effectively prevent stress corrosion. 　　(4) Newly put into use storage tanks shall undergo internal and external inspections as specified, as well as periodic regular inspections. Areas prone to corrosion, such as the liquid-gas interface, the arc start point, and T-joints, should be inspected carefully; 100% magnetic particle or ultrasonic testing should be carried out on all welds below the liquid level, and if conditions permit, 100% magnetic particle testing should be performed on all welds. The detected cracks should be evaluated. Since the stress corrosion threshold fracture toughness J ISCC is only about 1/100 of the material’s normal fracture toughness D0105, it is necessary to evaluate the safety level of the cracks using fracture mechanics criteria, and to provide recommendations for treatment as well as the timing for the next inspection. For shallow cracks with a wall thickness of no more than 1ö4 and a depth of less than 4 mm, they can be mechanically removed by grinding, but the grinding process must be carefully controlled; for deeper cracks, grinding is performed first followed by patch welding. Preheating should be carried out before patch welding to prevent welding hardening; low-hydrogen electrodes are recommended for welding, and flaw detection inspections should be performed after welding, along with stress-relief treatment. 　　(5) Regularly monitor the concentration of liquid ammonia and its moisture content; if the moisture level falls below the critical value, water should be added promptly to keep the moisture content within the range of 0.12% to 1%. In addition, other inhibitors can also be added; for example, 100 L of refrigeration oil, 5 L of rapeseed oil, or 10–50 L of silicone oil can be used as stress corrosion inhibitors, all of which can effectively prevent stress corrosion caused by liquid ammonia