Electrochemical and surface analytical studies of carbon steel protected from corrosion in a low-chloride environment containing a phosphonate-based inhibitor

Electrochemical and surface analytical techniques have been used to study carbon steel protected from corrosion by use of a new ternary inhibitor formulation containing nitrilotris(methylenephosphonic acid) (NTMP), zinc ions, and nicotinic acid (NA). Potentiodynamic polarization studies indicate that the ternary inhibitor acts as a mixed-type inhibitor. Electrochemical impedance studies imply formation of a protective film at the metal–solution interface. Surface analysis by X-ray photoelectron spectroscopy (XPS) showed that the protective surface film contains Fe, Zn, P, N, C, and O. On the basis of shifts in the binding energies of these elements, it is inferred that the protective film comprises a multiligand complex, Fe(III), Zn(II)–NTMP–NA, Zn(OH)2, and smaller quantities of oxides and/or hydroxides of iron. Analysis of the protective film by Fourier-transform infrared spectroscopy also supports this interpretation of the XPS results. Surface morphology and topography were studied by scanning electron microscopy and atomic force microscopy, respectively. On the basis of the results from all these studies, a plausible mechanism for inhibition of corrosion by the formulation is proposed.