Effects of bivalent cations on corrosion inhibition of steel by 1-hydroxyethane-1,1-diphosphonic acid

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 3-4651(98)02-071-0. All rights reserved.