Gaps in the equipment can lead to severe corrosion and damage to the equipment

Thread Content

The bottom reboiler of the regeneration tower in a plant’s light oil hydrogen production unit is a U-tube heat exchanger. The tube side operates at a low temperature of 167℃, while the shell side uses a Benfield solution at 117℃. 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 pipe surface is covered with interconnected deep pits, as well as numerous sharp and deep erosion holes. The corresponding areas on the tube sheet also show continuous deep pits, with a greater depth than those on the tube surface. 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 region, material migration is difficult, which leads to intensified corrosion conditions within that region 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 conditions 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.