E. Kalman et al., CORROSION INHIBITION BY 9-HYDROXY-ETHANE- 1,1-DIPHOSPHONIC ACID - AN ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDY, Magyar kemiai folyoirat, 102(4), 1996, pp. 211-216
Impedance spectra of carbon steel in the presence of 1-hydroxy-ethane-
1,1-diphosphonic acid (HEDP) has been investigated in neutral solution
s. The aim of the measurements was to study the effect of HEDP on the
corrosion of carbon steel as a function of inhibitor concentration. Th
e optimum inhibitor concentration was found to be approximately 10(-4)
mol . dm(-3). A two-time constant equivalent circuit model including
a constant phase element was consistent with experimental data in the
frequency range studied. The system parameters obtained by a non-linea
r fit procedure are discussed in terms of the model used. Inhibitor ef
ficiencies derived from impedance data are compared with those obtaine
d from polarization and weight loss measurements. Results of these inv
estigations show that at low concentrations, HEDP inhibits carbon stee
l corrosion by a precipitation mechanism forming insoluble iron comple
xes and repairing the porous oxide layer formed on the metal surface.
Increasing HEDP concentration beyond the optimum value decreases its i
nhibition efficiency due to the dissolution of the oxide layer.