Abstract: Causes of elbow perforation of 20 steel pipeline for crude oil in an oil field were analyzed by microscopic/macroscopic morphology analysis, chemical composition analysis, metallographic analysis, corrosion product analysis, water quality analysis, and force simulation, and so on. The results show that the ceramic coating on inner wall surface of the elbow had some quality defects such as pores and bubbling. The thickness at relatively large curvature area of the outer arc was the smallest, forming a low-lying area. The water in the crude oil was easy to produce CaCO₃ scales, which deposited in the low-lying area and the corrosion under scales occured. Under the synergistic action of CO₂, H₂S, sulfate-reducing bacteria and Cl^- in the crude oil medium inside the elbow, corrosion perforation was formed in the area with large curvature of outer arc where the fluid impact force was the largest and the coating damage was the most serious.