Effects Of Organic Corrosion Inhibiotor On Pitting X70 Steel In Sour Chloride Solution

X70 steel pipeline is widely used in oil gas gathering, and its service life is always in the acidic environment with the aggressive ions, such as Cl-. Pitting corrosion occurs frequently on X70 steel, this had caused serious damage to property. Inhibitors can effectively reduce the corrosion rate of rebars and prolong the service life. But there was very few study about controlling pitting corrosion by using inhibitors. So the research of organic corrosion inhibitors in sour chloride solution has important significance to study on pitting corrosion of X70 steel.Potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), M-S curve, scanning electron microscopy with energy dispersive spectrometer (SEM-EDS) and X-ray photoelectron spectroscopy (XPS) were applied to study the corrosion behaviors and inhibition mechanisms of pitting corrosion for X70 steel in the solution of 0.2 mol/L NaCl+0.05 mol/L NaNO2 with inhibitors. The additional inhibitors were thiourea and thiourea-based imidazoline (TAI), through changes the concentration and soaking time to discuss the mechanism of corrosion inhibitors. Through the relevant electrochemical parameters of the pitting corrosion potential Eb and corrosion current density icorr to discuss the effects of two kinds of corrosion inhibitors on X70 steel pitting corrosion. What’s more, the M-S, EIS, XPS and other testing methods, analysis the influence of the film on X70 steel surface with the corrosion inhibitor at different concentrations and different soaking time, such as the components and contents of the film. The corrosion inhibition mechanism of pitting corrosion was interpreted. The main conclusions are as followed:(1)The thiourea inhibitor has good inhibition effect on uniform of X70 steel with right concentration. With 90 ppm thiourea in the solution, the inhibition rate can reach 92%. With the added to consumption, the inhibition effect decreased gradually. The X70 steel in the solution with 90 ppm thiourea after 24 h immersion, the inhibition rate increased to 98%, but two cases of thiourea inhibitor will reduce the stable pitting corrosion potential, promote the occurrence of pitting corrosion, may be the film on the steel with thiourea is imperfect.(2)The TAI inhibitor has good inhibition effect on uniform of X70 steel with right concentration. Imidazoline add from 40 ppm to 110 ppm, could promote the uniform corrosion of the steel, adding up to 160 ppm, start to appear the inhibition effect, the inhibition rate reached 99%, icorr decreased about 4 orders of magnitude, adding up to 240 ppm, the inhibition effect weakened. X70 steel with 160ppm TAI, the Fe2+ almost completely transformed to Fe3+, this film covers on the surface can suppress the uniform corrosion, but the pitting corrosion potential decreased with the increase of TAI gradually.(3) The time that X70 steel immersed in the solution with TAI will change the effect. Extend the steel in solution with 160 ppm TAI soaking time to 24 h, it has good inhibition effect on uniform corrosion, and pitting corrosion potential increased significantly, the film covers on the surface with promoting the passivation of the surface what suppresses the pitting corrosion.(4)Through the XPS surface testing methods, in-depth discuss the surface film layer after 24 h immersion, found the imidazoline molecule mainly above the film, with the gradually rising film stripping, thiourea molecules content increased. Because the thiourea molecules has relatively small molecular weight, preferentially adsorbed on the substrate surface, promoting substrate pitting corrosion because the film is loose. While the increase of immersion time, imidazoline molecule coated in the thiourea molecule, forming a dense oxide film with the formation of Fe-N coordination bond, the imidazoline formed a layer of hydrophobic membrane, effectively improve the suppressing occurrence of uniform corrosion and pitting.