I. Felhosi et al., Effects of bivalent cations on corrosion inhibition of steel by 1-hydroxyethane-1,1-diphosphonic acid, J ELCHEM SO, 146(3), 1999, pp. 961-969
The influence of calcium and zinc ions on the corrosion inhibition effect o
f 1-hydroxyethane-1,1-diphosphonic acid (HEDP) on carbon steel has been inv
estigated by electrochemical techniques, X-ray photoelectron spectroscopy,
and atomic force microscopy. Addition of calcium or zinc ions greatly incre
ased the inhibition efficiency of HEDP in a synergistic manner. The highest
inhibition effect was obtained for molar ratios of Ca/HEDP = 0.5 and Zn/HE
DP = 3 for 3 x 10(-4) mol dm(-3) HEDP. The corrosion inhibition mechanism i
n the presence of these additives proved to be different from that with HED
P alone and was related to the formation of different complex species betwe
en HEDP and cation additives. The frequency response of the studied system
was described by a transfer function containing two relaxation time constan
ts corresponding to the charge-transfer process and the porous oxide layer.
Higher zinc and phosphorous content in the inhibitor layer and a continuou
s change in the O-Fe and HO-Fe ratio were achieved by increasing zinc conce
ntration in the solution (in the range Zn/HEDP = 0.5-3) as supported by XPS
measurements. Changes in the chemical structure were established in the ox
ids-hydroxide layer developed spontaneously on the steel surface. The syner
gistic effect of inhibitor compounds appeared also in surface morphology, l
eading to the development of smooth, i.e., dense, protective layers as reve
aled by atomic force microscopy. (C) 1999 The Electrochemical Society. S001
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