The correlation between depassivation and repassivation processes,which is significant in erosion-corrosion,was quantitatively investigated by single particle impingement tests at various flow velocities and impact angles.The results show that both repassivation and depassivation processes are associated with the kinetic energy of solid particle,and demonstrate that the repassivation is retarded by depas-sivation.This phenomenon probably results from the depassivation-induced microstructure evolution.On this basis,the dependence of critical flow velocity (CFV) for erosion-corrosion on the solid particle concentration and diameter is further theoretically predicted and experimentally verified.Accordingly,the crucial role of depassivation-repassivation in CFV phenomenon is further highlighted.