Leakage of 316Ti SS pipeline transporting 98% H₂SO₄ due to CUI and changed microstructure from welding

With the properties of good mechanical strength, excellent resistance to uniform corrosion and superior weldability, austenitic stainless steels are widely used in petrochemical industries. Among them, 316Ti austenitic stainless steel is a titanium stabilized version of 316 molybdenum-bearing steel. The addition of titanium can greatly increase the resistance to sensitization and significantly reduces the susceptibility to the formation of chromium carbides. So, 316Ti stainless steel is more resistant to general corrosion and pitting/crevice corrosion as well as intergranular corrosion, it is even more resistant to sensitization as compared to low carbon 316L stainless steel. Based on these advantages, 316Ti stainless steel pipelines are chosen to transport 98% H₂SO₄ in one factory. The pipelines for transporting concentrated H₂SO₄ were manufactured by welding segments of 316Ti stainless steel pipes. The external surface of the pipelines was insulated to maintain heat and prevent crystallization as well as for safety and health reasons. After being in use for about two years, leakage occurred on one pipe and concentrated H₂SO₄ was seeping from the pits on the pipeline. The investigation about leakage was carried out. The results show that leakage was caused by corrosion under insulation (CUI) and the pits formed by CUI are just located on the welding arc contact zone (WACZ) of the base metal caused by the welder's faulty operation during welding the joints. The external surface metal of the pipe contacted by the welding arc was partially re-melted by arc heat, which solidified in air and finally formed into another new microstructure that is completely different from that of 316Ti stainless steel. The microstructure in re-melting and solidification zone of the pipe is similar to that of cast metal and didn't experience any heat treatments. This has less resistance than austenite in 316Ti stainless steel. As a result of the continuous rainy days in the summer, the insulation on the external surface of the pipe was dampened or wetted, which made moisture penetrate and accumulate between the insulation and the pipe, finally resulting in CUI. At the same time, the accumulation and concentration of chloride from the insulation as well as the relatively high temperature in summer, greatly increased the corrosion rate of metal under insulation. Therefore, the presence of the less resistant microstructure in the re-melting and solidification zone, together with the dampened or the wetted insulation, as well as existence of chloride, made the external surface metal of the pipe under the insulation suffer from corrosion, finally resulting in the occurrence of the leakage. The concentrated H₂SO₄ which was seeping from the pipeline was easily diluted by the moisture or water trapped in insulation. The diluted H₂SO₄ is a strong corrosive medium, which in turn made the metal around the pits heavily corroded.