Study of the conservation state of European street furniture in painted cast irons

This work evaluates the state of conservation of four painted and partially corroded post-mounted luminaries in cast irons, dating back to the nineteenth and the twentieth centuries and coming from foundries located in Italy and France. The form, distribution and size of graphite in the cast irons were evaluated by optical microscopy, in accordance with the standard EN ISO 945-1:2008. The same technique was also used to determine the constituents of the microstructures. The corrosion attack morphology, the patina stratification and the main corrosion products were characterized by scanning electron microscopy, equipped with variable pressure technology (VPSEM) and energy dispersive spectroscopy (EDS), diffuse reflectance FT-IR (DRIFT) spectroscopy and X-ray diffractometry (XRD). The protectiveness of the ancient paints was evaluated by electrochemical impedance spectroscopy (EIS). In order to check the reliability of the technique, EIS measurements were also performed on plates in modern gray cast iron with the current standard painting cycle of Neri S.p.A. (Longiano, FC, Italy), during neutral salt spray testing for 83 days (corresponding to 2000 h). The metallographic analyses showed that the microstructures were consistent with the procedures employed for manufacturing complex shape casting with variable wall thickness. On the corroded surfaces the graphitization of the cast irons was detected. This phenomenon was accompanied by the deposition of thick corrosion layers, containing lepidocrocite, goethite and impurities from the corrosive environment. No historical paints were recognized, suggesting the removal of the deteriorated paint layers, before the application of modern paint systems. Finally, the electrochemical characterization established the poor protectiveness offered by the paints on the post-mounted luminaries.