Abstract: The reflux pump in the reflux pipeline on the top of an atmospheric-vacuum tower C-102 was found abnormal. It was found that the pump impeller had been corroded and damaged after the disassembly inspection. To find out the failure causes of the impeller, the impeller was analyzed by macroscopic inspection, chemical composition analysis, metallography inspection, microscopic inspection, corrosion product analysis and the corrosive medium influence analysis. The results show that the root causes of the impeller failure were electrochemical corrosion and wet hydrogen sulfide stress corrosion. The corresponding suggestions and the countermeasures were put forward according to the reasons.