Abstract Polarization and semiconductor behavior of alkyd coating coated on carbon steel electrode in 5% NaCl solution during its degradation was investigated by potential-capacitance measurement, Mott-Schottky analysis and electrochemical impedance spectroscopy in this paper. A MIS(Metal-Insulator-Semiconductor) structure generated when the electrode immersing in the electrolyte for 2 hours, and carrier density of the n type coating semiconductor was 4.99×109cm-3. The corrosion was controlled by diffusion of aggressive ions and water in the coating at this stage. Dipole polarization happened under voltage in the coating which was immersed for 1 and 2days, and the electric field caused by the dipole polarization baffled charge carrier’s transfer. Because of dipole relaxation, the differential capacitance decreased with rising absolute value of voltage, frequency dependence of potential-capacitance behavior also occurring; after immersing for 7 days, space-charge polarization dominated and carbon steel/coating transformed into a metal-semiconductor contact. With the extending immersion time, carrier concentration of coating increased, and flat band potential moved positively, so the work function of it decreased gradually, which resulted in weakening binding ability of coating to electrons. Potential barrier on the interface of metal and semiconductor heightened with rising voltage, also a barrier layer initiated on the interface. Carrier transport on the electrode was controlled by pore resistance of the coating, space charge layer and kinetics of corrosion reaction of substrate.

Key words: Potential-capacitance measurement, Mott-Schottky analysis, Semiconductor, Corrosion, Polarization