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1 Corrosion: Corrosion is the most common type of defect that occurs in pressure vessels during their use, especially in chemical processing vessels. It is caused by chemical or electrochemical reactions between the metal and the medium it comes into contact with. Types of corrosion: The corrosion of containers can be uniform corrosion, pitting corrosion, intergranular corrosion, stress corrosion, and fatigue corrosion. Regardless of the type of corrosion, it can lead to the failure or destruction of the container in severe cases. Corrosion can occur on both the inner and outer surfaces of pressure vessels. The outer wall of the container is generally subject to atmospheric corrosion, and this form of corrosion is closely related to factors such as location and season. In dry areas or seasons, atmospheric corrosion is much less severe than in humid areas or rainy seasons. Corrosion on the outer wall of pressure vessels occurs mostly in areas that are frequently wet and prone to the accumulation of water or moisture. Corrosion tends to occur at the contact surface between the container and the bracket, as well as in the areas where the container touches the ground. Corrosion on the inner wall of the container is primarily caused by the working medium or the impurities it contains. Generally, for containers in which the working medium has a significant corrosive effect, anti-corrosion measures are taken during design, such as using corrosion-resistant materials, applying surface treatments or coatings, or lining the inner walls. Therefore, corrosion on the inner walls of these containers is often caused by the failure of anti-corrosion measures. Corrosion of the inner wall of the container can also be caused by disruptions in the normal operating conditions. For example, dry chlorine does not cause corrosion to steel containers; however, if chlorine contains moisture, or if the container filled with chlorine is not dried after a hydrostatic test, or if moisture gets inside for some other reason, then chlorine reacts with water to form hydrochloric acid or hypochlorous acid, which causes severe corrosion of the container’s inner wall. Structural factors can also cause or exacerbate corrosion; for example, containers with corrosive deposits, or discharge pipes that are positioned above the bottom surface of the container, allow corrosive deposits to accumulate over time at the bottom of the container, thereby leading to corrosion. In addition, welds and heat-affected zones, as well as the areas around rivets in riveted containers and their joint areas, are places where corrosion tends to occur easily. Since the corrosion of the outer wall of the container is generally uniform or localized corrosion, it can be detected by visual inspection. For containers with an outer wall coated in a paint protective layer, if the protective layer is intact and no other suspicious signs are detected, it is generally not necessary to remove the protective layer in order to check for corrosion of the metal wall. For containers with insulation layers or other covering layers outside, if the insulating material does not cause corrosion to the container wall material, or if the container shell has an anti-corrosion coating, the insulation layer can remain in place as long as it is intact. However, if leaks or other signs that may lead to corrosion are detected, part of the insulation layer at the suspected area should be removed for inspection. There may be various forms of corrosion on the inner walls of the container. Uniform corrosion and localized corrosion can also be detected through visual inspection. For intergranular corrosion and fracture corrosion (stress corrosion and fatigue corrosion), except that severe intergranular corrosion can be detected by hammer testing, it is generally difficult to identify through visual inspection; metallographic examination, chemical composition analysis, and hardness testing are commonly used. Generally, the lining must undergo a gas-tightness test, and any components that may interfere with this test should be removed. When uniform or localized corrosion is detected on the inner or outer wall of the container through visual inspection, the remaining thickness of the corroded area should be measured in order to determine the thickness of corrosion on the vessel wall and the corrosion rate. Treatment methods: The handling of corrosion defects depends on the specific usage conditions of the container. The general principles are as follows: (1) If defects such as intergranular corrosion or stress corrosion are detected on the inner wall, it is not advisable to continue using the container. If the corrosion is mild, use under the original operating conditions with modifications may be permitted depending on the specific circumstances. (2) When pitting corrosion is detected but it does not interfere with operational procedures (no cracks are present and the corrosion depth is less than half of the calculated wall thickness), the defect may be left untreated and the component may continue to be used. (3) For uniform corrosion and local corrosion, it is determined whether to allow continued use, reduce the inspection interval, operate at reduced pressure, or declare the material unusable, based on the principle that the remaining thickness should be no less than the calculated thickness. 2 Cracks: Cracks are the most dangerous type of defect in pressure vessels; they are responsible for causing brittle failure in these vessels, and they also contribute to the occurrence of fatigue cracking and corrosion cracking. Types of cracks: Cracks in pressure vessels can be roughly divided into two categories based on their formation process: cracks that arise during the raw material preparation or vessel manufacturing, and cracks that occur or spread during the vessel’s operation. The former includes rolling cracks in steel plates, drawing cracks in containers, welding cracks, and stress-relief heat treatment cracks ； The latter includes fatigue cracks and stress corrosion cracks. Rolling cracks in raw materials are linear defects that arise during rolling due to the accumulation of defects present in the metal material itself, such as porosity, shrinkage cavities, and non-metallic inclusions. This defect can be located inside the material as well as on its surface, without any specific direction or fixed location. Similar cracks are also often found in some drawn small high-pressure vessels. Welding cracks mainly occur during the container manufacturing process, either due to lax quality control by the container manufacturers or because minor existing defects go unnoticed and develop over time during use. Stress-relief heat treatment cracks are branch-like intergranular cracks that arise during stress-relief heat treatment after welding, and they can also expand over time during use. Fatigue cracks occur due to poor structural design of the container or defects in the material, which result in excessive local stress; these cracks appear after the container is subjected to repeated cycles of pressurization and depressurization. Such cracks can be found in pressure vessels that are frequently started up and shut down. Corrosion cracks are gradually formed as the corrosive medium corrodes the material under certain operating conditions; such cracks are often related to stress. This is because stress and corrosion reinforce each other: corrosion creates notches on the material’s surface, leading to stress concentration or weakening of the intergranular bonds in the metal, while stress accelerates the progression of corrosion, causing the surface notches to deepen. Although cracks in pressure vessels can occur in various areas on their inner and outer surfaces, the places where cracks are most likely to appear are typically the welds and the weld heat-affected zones, as well as areas with excessively high local stress. Treatment methods: Crack inspection can be carried out through visual inspection and non-destructive testing. Generally, signs of cracks are detected or initially identified through visual inspection, and then confirmed further through non-destructive testing. Non-destructive testing methods, whether it is liquid penetrant testing, fluorescent testing, or magnetic testing, are highly effective for detecting surface cracks, and the appropriate method can be chosen depending on the specific circumstances. When cracks are detected in a pressure vessel, it is first necessary to analyze the causes of these cracks based on their location, quantity, size, distribution, as well as the operating conditions of the vessel. If needed, metallographic testing can be conducted to determine whether the cracks are due to defects in the raw materials, remnants from the vessel’s manufacturing process, or resulting from use over time. Then, the method for dealing with the defect or the container with the defect is determined based on the severity of the defect and the specific conditions of the container. The micro-cracks left by rolling or drawing the container from the material are generally shallow and can be removed using a hand file or grinding wheel. Weld cracks should be removed as soon as they are detected during inspection. Components that develop cracks due to poor structure and excessive local stress are generally not suitable for continued use. Containers with corrosion cracks should not be used either after the cracks have been removed or welded up. Under special circumstances, when cracks remaining from container manufacturing or raw materials are indeed difficult to eliminate, and after inspection and assessment by a qualified pressure vessel defect evaluation agency, as well as through analysis and calculations based on fracture mechanics, it is confirmed that the cracks will not propagate and that there is sufficient safety margin, the container can continue to be used under reliable monitoring measures; however, the inspection interval must be shortened, and close surveillance of the development of the cracks is required. 3 Deformation: Deformation refers to a change in the geometric shape of a container, either overall or in certain parts, after it has been in use. Such defects are generally rare in pressure vessels. Types of deformation: The deformation of containers generally manifests in various forms such as local dents, bulges, overall flattening, and overall swelling. Local depression is a surface indentation that occurs in a particular area of a container shell or head as a result of external impact or compression. This type of deformation generally only occurs in small containers with thin wall thicknesses; it does not cause any change in the wall thickness of the container, but rather results in a loss of the original geometric shape of that specific area of the surface. A bulge is an outward protrusion that occurs when the pressure-bearing surface of a certain part of a container experiences severe corrosion, resulting in a significant reduction in wall thickness, and thus under the effect of internal pressure. In some cases, excessively high local temperatures in the container can also lead to a decrease in the mechanical properties of the material, resulting in bulging; such deformation further reduces the wall thickness in that area of the container. The overall flattening occurs because the wall thickness of the shell, under external pressure, is too thin, resulting in a loss of stability and the loss of its original shape; this deformation happens only in those parts of the container that are subjected to external pressure, such as the inner cylinder of a jacketed container. Overall expansion deformation occurs when the wall thickness of the container is too thin or when it is operated under overpressure, causing yield deformation in the entire container or in certain cross-sections. This kind of deformation generally occurs slowly and can only be detected under special monitoring. Treatment method: Deformation can generally be checked by visual inspection, and less severe deformations can be detected using measuring tools. Containers with deformation defects generally should not be used any longer, except for those with minor local dents. This is because in containers that have undergone plastic deformation, the wall thickness always decreases to some extent, and the deformed material also loses its toughness due to strain hardening; in addition, its corrosion resistance is reduced as well. For minor bulging deformations, if the affected area is not large and it does not affect other parts of the container, patching can be considered provided that the weldability of the container material is good. The portion where bulging is about to occur should be cut out, and then a plate of the same shape and material should be welded in its place. After welding, the weld seam must be technically inspected in accordance with the original technical requirements of the vessel.