CORROSION INHIBITION BY 9-HYDROXY-ETHANE- 1,1-DIPHOSPHONIC ACID - AN ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDY

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.