INVESTIGATION OF THE INHIBITIVE EFFECT OF N-PHOSPHONO-METHYL-GLYCINE ON THE CORROSION OF CARBON STEEL IN NEUTRAL SOLUTIONS BY ELECTROCHEMICAL TECHNIQUES

摘要

Steady-state current-voltage curves for various disk rotation rates were combined with electrochemical impedance measurements in order to investigate the corrosion inhibition of carbon steel by N-phosphono-methyl-glycine (NPMG) /Zn~(2+) mixtures with a concentration ratio of 1:1 between NPMG and zinc cations, in a model water. The efficiency of the NPMG/Zn~(2+) combination increases with concentration in the range 10-100 ppm, and it acts as a mixed type inhibitor, affecting both the anodic and cathodic reactions. The corrosion behavior in all the metal/environments investigated is characterized by the formation of a 3-D porous layer which tends to hinder the diffusion of oxygen. Such layer consists of corrosion products in the blank solution and of corrosion products and/or inhibitors in the presence of the NPMG/Zn~(2+) mixture. The impedance response of such layers depends on inhibitor concentration, exposure time and rotation rate of the disk. From electrochemical impedance measurements it was found that the oxygen reduction reaction is under mixed activation-diffusion control. It was shown by both steady-state and impedance measurements that there is a significant effect of rotation speed on the anodic reaction.