AN ELECTROCHEMICAL STUDY OF THE CORROSION OF POLYMER COATED IRON

In this study unconventional electrochemical techniques are introduced, which allow analysis of the corrosion of polymer-coated metal surfaces. Due to the high ohmic resistance of polymer films conventional electrochemical techniques like impedance spectroscopy frequently provide no information about the corrosion reaction at the phase boundary metal/organic coating. Therefore, in this study a second reference electrode positioned below the polymer film is used, which allows to measure or control the electrode potential of the interface metal/coating directly. Using this reference electrode dc- and ac-measurements were performed. The diffusion coefficient of O2 in an adherent coating is determined using well known transport equations. Furthermore, the corrosion kinetics of iron below the coating is elucidated. Whereas a Tafel-slope of the iron dissolution of about 60 mV/decade is observed, the rate constant has to be much lower than for uncoated metals. Combining the results of dc- and ac-measurements, the corrosion rate of coated iron in O2-saturated electrolyte is calculated, which is mainly controlled by the transport of oxygen through the coating.