Analysis of Specific Causes of Corrosion in Low Temperature Storage Tanks and Application of Magnetic Particle Testing Technology for Regular Inspection

[1] Analysis of the specific causes of corrosion in liquefied gas storage tanks. If you encounter corrosion during the use of LNG storage tanks, it must be taken seriously because its corrosion can have a significant impact on your normal work if not well resolved. Therefore, we hope that everyone will take it seriously. So what are the reasons for the corrosion of liquefied gas storage tanks? Liquefied gas storage tanks do not require frequent cleaning, and sediment in the water can be discharged by regularly opening the drain valve at the bottom of the tank. But we will find that there is corrosion in the liquefied gas storage tank. What's going on? Let's take a look together below. ① Gap corrosion or oxygen accumulation cell corrosion is the corrosion that occurs when some precipitation or attachment occurs on the metal surface. ② A stone may clog the tube of the corroded copper alloy condenser. At this point, turbulence will immediately occur in the downstream direction of the stone. This will lead to corrosion or wear of the copper protective oxide film and expose unprotected copper alloy metal, resulting in further corrosion. ③ Pitting corrosion is usually caused by defects on the metal surface. Repeat the reaction at an appropriate rate to produce metal perforation phenomenon. ④ Electrochemical corrosion occurs when two or more different metals come into contact and connect in the presence of a conductive liquid electrolyte. [2] The application of magnetic particle testing in tank inspection is mainly due to the fact that the materials used in liquefied natural growth gas storage tanks are mostly carbon steel or low-alloy steel such as Q235 and 16MnR, which have the characteristics of low remanence and low coercivity. Therefore, the magnetic particle testing of LNG storage tanks can only be carried out by continuous method, that is, applying magnetic powder or magnetic suspension to the workpiece for magnetic particle testing while magnetizing with an external magnetic field. There are various magnetization methods for magnetic particle inspection, which are generally selected based on the characteristics of the tested workpiece. Common methods for circumferential magnetization include conduction method, center conductor method, bias core rod method, contact method, induced current method, and cable winding method. Methods for longitudinal magnetization include coil method, magnetic choke method, and magnetic iron method. Methods for multi-directional magnetization include cross magnetic choke method and cross coil method. Different methods have different characteristics. For example, the contact method uses two rod contacts to contact the surface of the workpiece, magnetizes it, and generates a distorted circumferential magnetic field on a flat workpiece. It is used to discover discontinuities parallel to the line connecting the two contacts. With a smaller magnetizing current, the required magnetic field strength can be deeply applied locally to the workpiece. It has good sensitivity and is easy to use. Although there are various methods for magnetic particle inspection, the inspection of LNG storage tanks mainly focuses on welds, including butt welds, fillet welds, etc. Fixed equipment cannot be used, and only portable equipment can be used for segmented inspection, which limits the selection of magnetic particle inspection methods for LNG storage tank inspection. Currently, the commonly used methods are as follows. (1) Magnetic choke method: This method is widely used, with simple equipment and easy operation. The movable joint magnetic choke can detect corner welds. In order to detect defects in various directions during use, at least two perpendicular inspections should be performed on the same part. The weld should be divided into several inspection sections, and there should be a certain overlap during the inspection operation. This method is inefficient and improper operation may result in missed detections. (2) Cross magnetic choke method: Currently, it is a widely used method in container inspection, which can generate a rotating magnetic field, has good inspection efficiency, sensitivity, simple operation, and can detect defects in all directions with one magnetization. It is suitable for detecting long butt welds, but not for diagonal welds. However, due to the need for a 380V power supply, there are some restrictions on its use inside containers in the petrochemical industry. (3) Contact method: It belongs to the unidirectional magnetization method, and the electrode spacing can be adjusted. The electrode spacing and current size can be determined according to the inspection site and sensitivity requirements, and can be flexibly adjusted for fillet welds. This method, like the magnetic choke method, requires two intersecting and perpendicular inspections of the same area. (4) Coil method: For the circumferential and corner weld seams of pipelines, the cable winding method can be used for inspection, which belongs to longitudinal magnetization and can detect longitudinal cracks in the weld seam and heat affected zone. In the actual inspection of LNG storage tanks, the author's unit mainly uses two methods: magnetic choke method and cross magnetic method. For the longitudinal and circumferential welds of container docking, these two methods are easy to operate, have good sensitivity and efficiency, and occupy an irreplaceable position; For the fillet weld of the connecting pipe, the cross magnetic method cannot be used for inspection. The movable joint magnetic choke can better solve the problem of detecting the fillet weld of the connecting pipe perpendicular to the container cylinder, but there are difficulties in determining the fillet weld of the connecting pipe at a certain angle. The contact method and coil method can better solve this problem. In fact, due to the complex stress conditions at the junction, corner welds are more prone to problems than butt welds. Therefore, how to solve the problem of corner weld inspection and how to introduce and apply the contact method and coil method well are the future work and efforts.