Investigation of inhibitive effects of phosphonic acids on corrosion of iron in 3% sodium chloride solution

Abstract

Thiomorpholin-4-ylmethyl-phosphonic acid (TMPA) and morpholin-4-methyl-phosphonic acid (MPA) have been synthesised using a microwave technique. The crude product obtained was purified and characterised by nuclear magnetic resonance spectroscopy (¹H NMR,¹³C NMR,³¹P NMR) and infrared spectroscopy. The inhibition efficiencies of the synthesised compounds have been studied by Tafel polarisation curve determinations and weight loss techniques. The comparison of the results with those reported previously for piperidin-1-yl-phosphonic acid showed that TMPA and MPA were better inhibitors. The addition of these compounds to aggressive media was accompanied by a decrease of the corrosion current density and a corresponding reduction of the corrosion rate. The inhibition efficiency increased with increasing inhibitor concentration. It reached 99·6 and 96·3% at a concentration of 5 × 10⁻³ M for TMPA and MPA respectively. The adsorption of the inhibitors was found to follow the Langmuir's isotherm. However, the determination of the film chemical nature required the use of Fourier transform infrared spectroscopy.