The corrosion inhibitor behavior of iron in saline solution by the action of magnesium carboxyphosphonate

Abstract

Abstract Herein, we report the synthesis, structural characterization and corrosion assay of a metal phosphonate – Mg(GLY)(H2O)2 obtained from a tridentate ligand N,N-bis-phosphonomethylglycine (GLY) and a magnesium salt (MgSO4·7H2O). The phosphonate was obtained by hydrothermal method at 80°C and also under ultrasounds conditions at 60°C. The FTIR, X-ray powder diffraction, elemental analysis and thermogravimetric analysis were performed in order to fully characterize the synthesized compounds and polarization experiments (CP) and electrochemical spectroscopy (EIS) to investigate the corrosion inhibition properties. The FTIR confirm the formation of magnesium phosphonate, and the X-ray diffraction showed the formation of a semi-crystalline compound. The elemental analysis confirmed the number of water molecules per formula unit of Mg(HO3PCH2)2N(H)CH2COO·2H2O. The presence of nitrogen atom and phosphonate groups in the metal phosphonate structure anticipated that the presence of the small quantity of Mg(GLY)(H2O)2 in saline solution will provide a positive effect on iron surface and act as a corrosion inhibitor. From the CP curves recorded in an aerated nitric saline solution, corrosion parameters (corrosion potential –Ecorr, corrosion density current – Jcorr, polarization resistance – Rp and corrosion rate – Rcorr) were extracted from Tafel plots. The decrease in Jcorr is associated with a shift in Ecorr to more negative values. These results suggest that metal phosphonate behaves as a mixed-type inhibitor, by reducing both the cathodic and anodic reactions. The optimum inhibitor concentration determined was 2 mM. At this concentration the corrosion rate decreases by 23% fold comparatively with iron in nitric acid solution without metal phosphonate. The EIS data in agreement with the polarization measurement resulted from polarization data.