Corrosion inhibition by strainless complexes

Abstract Introduction of a methylene group in the carbon chain of carboxymethyl phosphonic acid (CMPA) results in 2-carboxyethyl phosphonic acid (2-CEPA). The inhibition efficiency (IE) of these two phosphonic acids, in the absence and presence of Zn2+, in controlling corrosion of carbon steel immersed in a neutral aqueous environment, containing 60 ppm Cl−, has been evaluated by weight loss method. The IEs are measured as a function of immersion period, and compared. The protective film has been analysed by X-ray diffraction, Fourier transform infrared and luminescence spectra. A suitable mechanism of corrosion inhibition is proposed based on the results of weight loss study and various surface analysis techniques. The 2-CEPA–Zn2+ is found to offer better IE than the CMPA–Zn2+ system. This is due to the facts: (i) 2-CEPA keeps Zn2+ in a more solubilised form, (ii) larger size of Fe2+–2-CEPA complex, which is more relaxed, strainless and more stable than Fe2+–CMPA complex. The protective film is found to consist of Fe2+–phosphonate complex and Zn(OH)2; it is found to be luminescent.