Introduction of a methylene group in the carbon chain of carboxymethyl phos
phonic acid (CMPA) results in 2-carboxyethyl phosphonic acid (2-CEPA), The
inhibition efficiency (IE) of these two phosphonic acids, in the absence an
d presence of Zn2+, in controlling corrosion of carbon steel immersed in a
neutral aqueous environment, containing 60 ppm Cl-, has been evaluated by w
eight loss method. The Ifs are measured as a function of immersion period,
and compared. The protective film has been analysed by X-ray diffraction, F
ourier 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 com
plex, which is more relaxed, strainless and more stable than Fe2+-CMPA comp
lex. The protective film is found to consist of Fe2+-phosphonate complex an
d Zn(OH)(2); it is found to; be luminescent. (C) 2001 Elsevier Science Ltd.
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