Why is CS still used at the bottom of the regeneration tank while 304 is used for the side walls?
Thread Content
The bottom of the regeneration tank is made of carbon steel (CS) while the side walls are made of 304 stainless steel; this is mainly because the operating conditions and stress levels to which the bottom and side walls are exposed differ, requiring a tailored selection of materials. The environment at the bottom of the tank is relatively stable: the bottom of the regeneration tank is usually immersed in an alkaline solution, with minimal temperature fluctuations, and no significant evaporation or concentration occurs. As long as the temperature of the alkaline solution is kept below 46°C and post-weld heat treatment is carried out to relieve stress, carbon steel can meet the corrosion resistance requirements without the need to upgrade the material. There is a risk of evaporation and concentration on the side walls: the areas around the liquid surface on these side walls are prone to the evaporation and concentration of the alkaline solution due to temperature fluctuations or localized heating, resulting in a harsh environment with high concentrations and high temperatures; under such conditions, carbon steel is highly susceptible to alkali embrittlement cracking. 304 stainless steel exhibits better corrosion resistance under these conditions, enabling it to effectively resist stress corrosion. Structural and cost considerations: The large bottom area of the tank means that using 304 stainless steel throughout would significantly increase costs. While ensuring safety, the composite structure of “bottom CS + side walls 304” is adopted to control costs and guarantee the safety of critical areas. Therefore, this differentiated material selection is a comprehensive decision based on corrosion mechanisms, temperature distribution, and cost-effectiveness.
"The analysis of this material selection is very professional! Indeed, the corrosion conditions at the bottom and sides of the tank differ significantly. Here are two additional points of experience for reference: In our projects, we have found that alkaline concentration tends to occur most frequently in areas where the liquid level fluctuates; the passivation layer of 304 stainless steel remains more stable in such areas. It is also important to pay attention to the heat treatment processes in the welding transition zones, as we have encountered problems resulting from insufficient stress relief during welding of dissimilar steels. Additionally, it is recommended to conduct regular inspections using endoscopes to check for changes in wall thickness near the liquid level line, as this area is prone to being overlooked"