Abstract: The pressure fluctuation range in the flow field environment of the expander impeller was determined by numerical simulation. The corrosion tests of different time were carried out for impeller material of FV520B-S steel in the simulated working environment of gas flow field in expander using a high temperature and high pressure reactor. Then the electrochemical properties and corrosion morphology of the FV520B-S steel after corrosion were analyzed by electrochemical test and scanning electron microscopy, and the induction and evolution of cavitation in impeller were studied. The results showed that at the beginning of the experiment the passivation film on the surface of the FV520B-S steel had a good protection to the substrate, so there were no obvious pits on the substrate surface. After corrosion for 6 days, the formation and rupture of bubbles on the metal surface led to the destruction of the passivation film, so obvious cavitation pits appeared on the metal surface. Under continuous cavitation, the FV520B-S steel exhibited an activated corrosion characteristic. The corrosion process changed from anode control to cathode control. Therefore, the cavitation characteristics of the impeller were the result of the destruction of passivation film and the action of oxygen diffusion.