CORROSION INHIBITION BY 1-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 carbon steel corrosion as a function of inhibitor concentration. An op timal inhibitor concentration was found to be approximately 10(-4) mol dm(-3). A two-time constant equivalent circuit model including a cons tant phase element was consistent with experimental data in the freque ncy range studied. The system parameters obtained by a nonlinear fit p rocedure are discussed in terms of the model used. Inhibitor efficienc ies derived from impedance data are compared with those obtained from polarization and weight loss measurements. Results of these investigat ions show that at low concentrations, HEDP inhibits carbon steel corro sion by a precipitation mechanism forming insoluble iron complexes and repairing the porous oxide layer formed on the metal surface. Increas ing HEDP concentration beyond the optimal value decreases its inhibiti on efficiency due to the dissolution of the oxide layer.