In certain operating environments, the gaps in the device’s structure are subject to severe corrosion

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The last edit to this post was made by Wang Wei2 on 2023-12-30 at 10:06. In certain operating environments, the gaps in the device structure can suffer severe corrosion; the bottom reboiler of the light oil hydrogenation unit at a certain petrochemical company is a U-tube heat exchanger. The tube side operates at a low temperature of 167°C, while the shell side uses a Benfield solution at 117°C. The tubes of the heat exchanger are made of 1Cr18Ni9Ti stainless steel, and the tube sheet is made of 16Mn steel. A leak was discovered after two years and four months of use. The reason is corrosion occurring in the gap at the junction of the shell-side tubes and the tube sheet. The tube surface is covered with interconnected deep pits, as well as numerous sharp and deep erosion holes. The corresponding area on the tube sheet also shows a continuous deep pit, with a greater depth than that of the erosion pits on the tube surface. Analysis: Gaps in the equipment’s structure are often subject to severe corrosion. Crevice corrosion that occurs within slits is characterized by rapid progression and concentrated damage, posing a severe threat to equipment, especially those made of corrosion-resistant alloys that are prone to passivation. This should be given full consideration during the design process. The rapid development of crevice corrosion can be explained by the closed-cell model. Due to the geometric conditions of blockage in the gap area, material migration is difficult, which leads to intensified corrosion conditions within that area and the occurrence of a corrosion process with autocatalytic characteristics. In this case, the outer surfaces of both the 16Mn steel pipe plates and the 1Cr18NI9Ti stainless steel pipes were immersed in the Benfield solution. The main components of the Benfield solution are K2CO3 and KHCO3; it is a high-temperature alkaline solution to which V2O5 is added as a corrosion inhibitor. V+5 is a passivator that can passivate 16Mn steel to maintain a very low corrosion rate. A basic requirement for using a passivator is that its concentration must exceed the critical passivation concentration in order to passivate the metal. If the concentration is too low, it not only fails to passivate the metal but also promotes its corrosion or causes localized corrosion. This is exactly the situation in the gap area between the tube sheet and the tubes. Due to the geometric constraints of the blockage, the depletion of V+5 ions cannot be replenished, preventing the V+5 ion concentration inside the gap from reaching the critical passivation level, which results in severe corrosion of the 16Mn steel pipe plates.