When welding pipes in wet flue gas desulfurization and decarbonization systems, attention should be paid to the selection of welding electrodes (case study)

Thread Content

When welding pipes in wet desulfurization and decarbonization units, attention should be paid to the selection of welding electrodes (case study). Problem: Pitting corrosion is relatively severe in the lean liquid, semi-lean liquid, and rich liquid pipelines of wet desulfurization and decarbonization units in the petrochemical industry, and all punctures in these three types of pipelines occur at the welds. The medium in the pipeline is an aqueous solution of 10%–15% monoethanolamine (which was later replaced by an aqueous solution of diisopropanolamine, and further later by an aqueous solution of methyldiethanolamine). It contains 2–7 kg/m of hydrogen sulfide and 2–30 kg/m of CO. The temperature of the rich liquid is 80–85°C, while the temperatures of the lean liquid and semi-lean liquid are 115–120°C. The type of corrosion at the welds in the pipeline is pitting corrosion. Analysis: The base material is A20 steel, while the welding rods used to be “Ket 42” (an acidic welding rod); later on, “Ao 302” (an alkaline welding rod) was used. The new pipe was not annealed at the welds, and to date, no perforation corrosion has occurred at those welds after many years. After replacing the welding electrode, there was little change in the flow rate of the rich and lean fluids, thereby significantly extending the startup time. Welding rod O302 meets international standard TA.2-2; it belongs to the titanium-calcium type of flux coating. The welding current can be either AC or DC, with the main components being Cr 22%–26% and Ni 11%–24%. The mechanism is as follows: first, the Ao302 electrode can resist sulfur corrosion, whereas the Jie42 electrode may not be resistant to sulfur ； Secondly, the Ao 302 electrode has good intermiscibility with the base metal A steel, resulting in a low tendency for corrosion; whereas the Jie 42 electrode has poor intermiscibility with the A320 base metal, leading to a higher tendency for galvanic corrosion or corrosion due to potential differences ； Thirdly, the internal stress generated after the interdiffusion of the Ao 302 welding rod and pipeline A20 steel is very low, resulting in a low tendency for stress corrosion ； Fourth, the plasticity of Ao 302 is better than that of Jie 42. The above experience can serve as a reference for units in the petrochemical industry that have wet desulfurization and decarbonization facilities.