Abstract: Less than two months after installation of the 65Mn steel spring washer of a fuel through pipe joint, a large area of rust occurred on its side. Through macroscopic observation, metallographic inspection, scanning electron microscope and energy spectrum analysis, hardness test and open circuit potential test, the reasons for corrosion of spring washer were analyzed. The results showed that the corrosion products on the side of spring washer was mainly Fe₂O₃. The main reasons for corrosion of spring washer were the friction between the flat washer and the lock nut during the assembly process, which caused the oxide film on its surface to fall off. In the soap liquid environment left by the air tightness check, the spring washer was corroded.