Inhibitory effects of nitrilotris(methylenephosphonic acid) on cathodic reactions of steels in saturated Ca(OH)2 solutions

Severe localized corrosion of buried steel pipes is likely to occur near the sites where pipes penetrate or contact concrete structures. This type of corrosion is caused by the more anodic potential of steel in concrete than in soil. Since the corrosion is induced by cathodic reactions on steel surfaces in concrete accompanied by anodic reactions (i.e., corrosion) on those in soil, cathodic inhibitors in concrete may be a means to prevent or lessen this problem. As the inhibitor, nitrilotris(methylenephosphonic acid) (NTMP) was examined for their potential to inhibit the cathodic reactions in a saturated Ca(OH)2 solution as a simulated concrete solution. Polarization measurements showed the cathodic reactions were inhibited by NTMP in accordance with the Langmuir adsorption isotherm. Additional polarization measurement for steels in a 0.05 mol/dm3 Na2SO4 solution to simulate soil solution and calculations indicated that the localized corrosion could be inhibited by NTMP in concrete. The results of surface analyses using Fourier transform infra-red spectroscopy and X-ray photoelectron spectroscopy suggested NTMP was probably adsorbed on the cathodic portions of the steels, mainly forming a polymer cross-linked one another by calcium ion through its three phosphono groups.