Corrosion inhibition by strainless complexes

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 Ifs 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. (C) 2001 Elsevier Science Ltd. All rights reserved.