Abstract: An abnormal failure occurred in the outlet sour gas pipeline of a sulfur recovery unit in a certain plant. In order to find the root cause of the failure of the pipeline, a series of inspections and analyses were carried out on the failed samples,such as material chemical composition analysis, mechanical property analysis, macro and micro observation of corrosion holes, energy spectrum analysis and metallographic structure analysis, and blue dots were performed on the inner wall of the pipeline. The results show that the chemical composition, mechanical properties and metallographic structure of the pipeline meet the requirements of relevant standards. Desalted water is deposited on the bottom of the pipeline due to wrong edges and the failure to purge the inner wall of the pipeline after the water pressure test, which makes the chloride ions concentrate and accumulate there, and the overall passivation effect of the inner wall is not good and there is more polishing traces, and eventually lead to chloride ion corrosion perforation. Chloride ion corrosion is the root cause of the failure; the accumulation of desalinated water is caused by wrong edges, the failure to purge the inner wall after the hydraulic test and the poor passivation effect of the inner wall are important reasons that lead to the concentration and enrichment of chloride ions and their destruction. At the end of the thesis, the solutions are put forward, which have a certain guiding effect on the safe operation of such devices.