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

Abstract

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.