Inhibition effects of N-phosphono-methyl-glycine/Zn2+ mixtures on corrosion of steel in neutral chloride solutions

Electrochemical and surface analytical techniques were combined in order to investigate the corrosion inhibition of a carbon steel rotating disc electrode (rde) by N-phosphono-methyl-glycine (NPMG)/Zn2+ mixtures with a concentration ratio of 1:1 between NPMG and zinc cations, in chloride solutions. From polarization curves it was shown that the NPMG/Zn2+ mixture retarded both the anodic and cathodic partial reactions of the corrosion process. Inhibition efficiency, estimated from electrochemical impedance measurements, increased from approximately 85 to 95% when inhibitor concentration increased from 10 to 100 mg/l. Analysis of the composition with XPS suggested that the films consisted mainly of hydrous ferric oxides (Fe(OH)3 and FeOOH), with small amounts of Fe–NPMG complex, ZnO and corrosion products. AES depth profiling showed that the most protective surface film (that formed in the solution containing 100 mg/l of inhibitor) was thicker compared to that formed in the solution with low inhibitor concentration.