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

摘要

Acciai zincati a caldo sono stati sottoposti a trattamenti di passivazione con prodotti industriali a base di Cr(Ⅲ) e le proprietà degli strati sono state confrontate con quelle degli strati di passivazione ottenuti con prodotti tradizionali a base Cr(Ⅵ), trattamento con acido cromico. La caratterizzazione del substrato e degli strati protettivi è stata effettuata prima e dopo il trattamento di passivazione mediante microscopio ottico (OM) e microscopio elettronico associato a microsonda a dispersione di energia (SEM/EDS). In particolare lo strato di zinco è stato osservato sulla superficie ed in sezione per evidenziarne la possibile difettività.%Industrial hot dip galvanized steels were submitted to a Cr(Ⅲ)-based passivation 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)-based process 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.