Conservation state of cast iron metalworks in European street furniture

This work evaluates the state of conservation of three artefacts of historical street furniture in cast iron, dating back to the second half of the nineteenth century and coming from foundries located in Italy, France and Great Britain. Form, distribution and size of graphite in cast irons, and the constituents of microstructures were evaluated by optical microscopy, whereas the alloy composition was determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). The same analytical technique, in association with Diffuse Reflectance Infrared Fourier Transform (DRIFT) and micro-Raman spectroscopies, was employed to characterize corrosion attack morphology, patina stratification and thickness. The protectiveness of the corrosion layers of the artefacts was evaluated by Electrochemical Impedance Spectroscopy (EIS). The results showed that the microstructure and alloy composition were similar to those frequently encountered in complex shape castings with variable wall thickness. On the corroded surfaces, the graphitization of the cast irons was detected. This phenomenon was accompanied by the deposition of corrosion layers of variable thickness, containing iron oxides, oxy-hydroxides, chlorides and silicon oxides. The corrosion layers of most artefacts showed a poor protectiveness, particularly when they included chlorides, detected in association with akaganeite among corrosion products.