Mechanism for protection of iron corrosion by an intrinsically electronic conducting polymer

The protection mechanism of an iron substrate, coated with intrinsically electronic conducting polymer (ICP) was devised on the basis of experimental data collected by various local measurement techniques around an artificial pinhole area; namely, potential, current, impedance spectra, and in situ Raman spectroscopy. The local potential and the current between ICP and the defect area allowed verification that the polymer coating is able to make passive a small area of bare iron. The local impedance spectra exhibited the characteristics of a passive iron electrode by capacitance behavior dominating the whole impedance spectrum. On the contrary, classical impedance spectra, defined by the overall potential over the overall current revealed merely the impedance spectra of ICP involving doping and undoping processes. The protection by ICP coating vanishes before exhaustion of the whole charge stored initially. Furthermore, with a polyaniline coating with an inner poly(diamino-naphthalene) layer, it was shown that even when active corrosion is taking place at the iron, the film remained in the oxidized state. Besides, the oxidation state near the defect area was higher compared to the overall oxidation state of the ICP coating, though the interface potential is the same on the whole surface area, including the defect area.