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.