Effect of chloride ions on the corrosion behaviour of steel in 0.1Â M citrate

The corrosion behaviour of steel was studied in aerated near neutral citrate solutions without and with various concentrations of NaCl. The potentiodynamic anodic polarization curve in 0.1 M citrate solution exhibits four anodic peaks A1, A2, A3 and A4 prior to the oxygen evolution reaction. Addition of Cl− ions to the solution enhances the four peaks currents, specially A3, which is followed by pitting corrosion. The negative going scans of the cyclic voltammograms show two anodic reactivation peaks A5 and A6 and one cathodic plateau P1. A diffusion controlled process in the potential range of A1, A2 and P1 was detected by RDE experiments. The potentiostatic current time transients, at different concentrations of NaCl and applied potentials Ea > A3, were studied. The pit nucleation rate (ti−1) is found to increase with increasing the concentration of NaCl and the applied anodic potential. The impedance spectra exhibit four different behaviours depending on the potential range used. They were fitted with a single time constant circuit at Ea < −700 mV. However, at −700 mV < Ea < −480 mV, they were fitted with a circuit with two time constants. At Ea > −480 mV, the second semicircle is replaced by negative polarization resistance which is disappeared at Ea > −300 mV. The electrode impedance was found to decrease with the applied potential.