Corrosion inhibition of steel in neutral chloride solutions by mixtures of N-phosphono-methyl-glycine with zinc ions

Steady state current-voltage curves were combined with electrochemical impedance measurements in order to investigate the corrosion inhibition of a carbon steel rotating disc electrode by N-phosphono-methyl-glycine (NPMG)/Zn2+ mixtures with a concentration ratio of 1:1 between NPMG and zinc cations, in chloride solutions. The NPMG/Zn2+ mixture retarded both the anodic and cathodic partial reactions. Its efficiency was better for a concentration of 40 ppm than with concentrations of either 10 or 100 ppm. At the corrosion potential, electrochemical impedance diagrams exhibited a high frequency loop, which was ascribed to the charge transfer process, and a low-frequency loop that was related to diffusion across the protective film. Analysis of impedance spectra in terms of an equivalent circuit model showed that the corrosion behaviour of the metal-layer-solution system was strongly dependent on the physicochemical properties of the corrosion products-corrosion inhibitor porous layer.