The effects of thioureido imidazoline and NaNO2 on passivation and pitting corrosion of X70 steel in acidic NaCl solution

The effects of thioureido imidazoline and NaNO2 on passivation and pitting corrosion of X70 steel in acidic NaCl solution were studied. The addition of thioureido imidazoline promoted passivation of the steel in the NaCl-NaNO2 solution, but the pitting potential decreased, which was attributed to the preferential adsorption of residue thiourea monomers in the inhibitor. With longer immersion time, the pitting potential in solution with thioureido imidazoline + NaNO2 increased to the value with only NaNO2. A multi-layer film formed with thiourea rich in inner and imidazoline rich in outer. The compounded inhibition effect of thioureido imidazoline and NaNO2 was discussed.