Corrosion Resistance of Cr (III) Based Conversion Layer on Zinc Coatings in Comparison with a Traditional Cr (VI) Based Passivation Treatment

摘要

Industrial hot dip galvanized steels were submitted to a Cr(III)-basedpassivation treatment (containing phosphates) leading to Cr(VI)-free conversion layers. The characterization of the substrate and of the coatings was carried out before and after the passivation treatment by means of optical microscopy (OM) and scanning electron microscopy with energy X-ray dispersive spectrometry associated (SEM/EDS). In particular the zinc coating was preliminarily studied both superficially and in section in order to define its adequacy and its possible state of defectiveness. The corrosion behaviour was evaluated by means of electrochemical impedance spectroscopy (EIS) and salt spray test. A comparison between the protective effectiveness of the innovative Cr(III)-based passivation and the Cr(VI)-basedprocess usually employed by manufacturers was carried out. According to the exposed preliminary results, promising outcomes were obtained with the innovative Cr(III)-based passivation by optimizing the amount of chromium in the coating.