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During the service time of 316 stainless steel elbow of sewage stripper, obvious local thinning and leakage occurred on both sides and the inner ridge of the elbow bend. The causes of corrosion were analyzed by means of macroscopic examination, computational fluid dynamics (CFD) simulation, and electron energy spectrum analysis on substrate and corrosion sediment. Results showed that the main reasons for the local thinning at the bend were the uneven distribution of flow pattern in the later stage caused by the drainage separation of the thermocouple, and the erosion corrosion caused by the excessive local flow velocity due to the sudden change of the flow direction at the bend. Based on the above causes of corrosion, corresponding protective recommendations were put forward.