Severe corrosion of the packing in the reabsorption tower of a catalytic cracking unit (case of intergranular metal corrosion)

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1. The reabsorption tower of a certain catalytic cracking unit is about 20 meters tall. The lean gas entering the tower contains small amounts of H2S, CO2, and H2O. It was put into operation in September 1997; during maintenance in 2000, severe corrosion was found at the upper part of the packing inside the tower, at temperatures of 35–45ºC. 2. The filler material is 1Cr18Ni9Ti. 3. The dry gas coming out of the reabsorption tower contains CO2, H2S, and H2O; the medium inside the tower is acidic, which provides the external conditions for intergranular corrosion. 4. It is possible that the filler remained in the sensitization temperature range for an extended period during processing, resulting in the precipitation of carbides at the grain boundaries. 5. Conclusion: The precipitation of carbides at the grain boundaries caused intergranular corrosion of the filler in the CO2-H2S-H2O acidic medium. 6. Recommendations • Use ultra-low carbon 304L stainless steel. • Switch to carbon steel packing but apply a layer of wear- and corrosion-resistant coating.