Study On Effect Of Inhibitors On Localized Corrosion Of Metal Using The Wire-Beam Electrode

Polarization curves, corrosion potentials and electrochemical impedance spectra inside and outside the crevice of mild steel, pure aluminium and stainless steel were measured in NaCl and inhibitor-containing NaCl solution using potentiodynamic polarization and the wire beam electrode technique. The kinetics of crevice corrosion and water-line corrosion of these metals and the effects of inhibitors on crevice corrosion and water-line corrosion were studied.The experimental results showed that the wire beam electrode could be used to study inhomogeneity of localized corrosion of metals. The distribution of corrosion potentials the wire beam electrode in NaCl solution took on annular character. Mild steel inside the crevice was anode and that outside the crevice was cathode in 5%NaCl solution. At early-immersed stage, the corrosion potentials outside the crevice were higher than those inside the crevice. With increase of immersion time, the corrosion potentials inside and outside the crevice shifted negatively, but those inside the crevice shifted more negatively. The distribution of corrosion potential became inhomogeneous gradually. It showed that metal corrosion inside the crevice developed step by step with the immersion time. Because of the addition of 5%NaNO₂ solution, the corrosion potentials inside and outside the crevice shifted towards positive, the difference of corrosion potentials was minished and corrosion rate of mild steel was reduced evidently. In 5%NaCl+5%NaNO₂ solution, compact oxide film having strong adhesion to the surface of mild steel was formed, and crevice corrosion of mild steel was restrained availably.Wire beam electrode of pure aluminium was immersed in NaCl solution, and the addition of Na₂SO₄ to NaCl solution minished the difference of corrosion potentials inside and outside the crevice, and corrosion rate of pure aluminium was reduced evidently. Na₂SO₄ was adsorption inhibitor of pure aluminium in neutral solution, SO₄^2- ions replaced aggressive Cl^- ions owing to their competitive adsorption on the surface of the wire beam electrode, and Cl^- concentration was decreased inside and outside the crevice. The time of breakdown of the passive film was extended, and the O₂ was consumed inside the crevice in SO₄^2--containing NaCl solution more slowly than in NaCl solution. So crevice corrosion was restrained availably.Crevice corrosion of stainless steel in NaCl solution and effects of NaOH on crevice corrosion were studied. On adding of NaOH to NaCl solution, OH^- ions replace...