Abstract: The corrosion behaviors of 20Cr9Ni5Co14 ultra high strength steel (USS) were studied by the electrochemical polarization methods (potentiodynamic polarization， linear polarization and cyclic polarization technologies) and alternating current impedance spectrum methods in different corrosion conditions， and after polarization the corrosion morphologies were characterized by Scanning Electron Microscopy(SEM). The results showed that the passivation phenomenon of 20Cr9Ni5Co14 steels appeared in 3.5wt.% NaCl solution while disappeared with the increase of NaCl concentration. The corrosion current also reduced from 8.223×10^-7A/cm² to 1.129×10^-7A/cm² with the increase of NaCl concentration. The passivation and transpassivation potentials of 20Cr9Ni5Co14 steels increased with the increase of hydrogen ion concentrations. When the pH value was higher than 3， the corrosion product film had excellent corrosion resistancewhich led the electrochemical polarization step as a dominated step， while the pH value decreased to 2， the corrosion products dissolution rate was fast， the corrosion rate of the interface of the metal surface and the solution was also fast， thus the polarization caused by concentration differences became the dominated step. Corrosion morphology studies showed that the corrosion of 20Cr9Ni5Co14 steels in the polarization process oriented from pitting corrosion， which leads to the reduction of corrosion resistance.

Key words: electrochemical polarization, electrochemical impedance spectroscopy, 20Cr9Ni5Co14, stainless steel