Abstract: Effect of chloride ion on corrosion behavior of J55 steel under gas-liquid biphase fluid condition was detected using potentiodynamic polarization curve,electrochemical impedance spectroscopy (EIS) and microstructure analysis.The results show that when J55 steel was corroded in 35 g·L-1 NaCl solution,the corrosion rate minished first and increased after 24 h,then kept invariable after 48 h.When J55 steel was corroded in the solution with different concentrations,there was a critical potential of -0.4 V.Below the critical potential the corrosion rate increased with the concentration increasing when the concentration of NaCl solidion was lover than 35 g·L-1,while minished when the concentration of NaCl solidion was higher than 35 g·L-1.Above the critical potential the corrosion rate increased with the increase of the chloride ion concentration.There was a maximum of the corrosion rate corresponding to the critical Cl- concentration when J55 steel was corroded in the solution containing chloride ion.And the critical concentration increased with the potential increasing.