Determination of the long term corrosion behavior of coated steel with A.C. impedance measurements

A previous study of the a.c. impedance of thin polymer films on aluminum alloys, steel, and phosphated steel, determined over a wide frequency range, has resulted in a general model for the electrochemical impedance across the coated surfaces. According to this model, penetration of the coating by the electrolyte results in an impedance behavior which is typical of a dielectric film short-circuited by conducting electrolytic paths perpendicular and parallel to the polymer/metal interface. Comparison of the time dependence of the impedance of coated steel substrates with the impedance across a free film has demonstrated that the corrosion of the substrate enhances the development of parallel paths of ionic conduction in the coating. A.c. impedance measurements carried out on polybutadiene-coated steels pretreated by different procedures determine quantitatively the resistance R_p of the conducting paths. The time dependence of R_p for the coated substrates has been determined for exposure periods of up to three weeks. Visual observation of the corroded and delaminated areas according to ASTM D610 has been carried out simultaneously, and correlations have been established with R_p. The results are discussed in terms of the possibility to predict the corrosion behavior of a coated metal, based on the measurement of R_p.