Impedance spectra of carbon steel in the presence of 1-hydroxy-ethane-1,1-diphosphonic acid (
hedp corrosion inhibitor) has been investigated in neutral solutions. The aim of the measurements
was to study the effect of HEDP on the carbon steel corrosion as a
function of inhibitor concentration. An optimal 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 nonlinear fit procedure are discussed in
terms of the model used. Inhibitor efficiencies derived from impedance
data are compared with those obtained from polarization and weight loss
measurements. Results of these investigations show that at low
concentrations, HEDP inhibits carbon steel corrosion by a precipitation
mechanism forming insoluble iron complexes and repairing the porous
oxide layer formed on the metal surface. Increasing HEDP concentration
beyond the optimal value decreases its inhibition efficiency due to the
dissolution of the oxide layer.
