The effects of thioureido imidazoline and NaNO2on passivation and pitting corrosion of X70 steel inacidic NaCl solution were studied. The addition of thioureido imidazoline promoted passivation of thesteel in the NaCl-NaNO2solution, but the pitting potential decreased, which was attributed to the prefer-ential adsorption of residue thiourea monomers in the inhibitor. With longer immersion time, the pittingpotential in solution with thioureido imidazoline + NaNO2increased to the value with only NaNO2. Amulti-layer film formed with thiourea rich in inner and imidazoline rich in outer. The compoundedinhibition effect of thioureido imidazoline and NaNO2was discussed.