Electrochemical study on the corrosion of Cr III-based conversion layers on zinc coatings

Zinc coatings submitted to a Cr III-based passivation treatment, with and without a sealing treatment, were studied. For comparison, similar coatings with a Cr VI-based passivation treatment were also examined. The corrosion behaviour was studied by polarization resistance (Rp) measurements, anodic polarization and mainly by electrochemical impedance spectroscopy (EIS) in aerated 0.85 M NaCl solution. Salt spray tests and morphological observations on the coating surface were also performed. The results indicate that the Cr III passivated coatings with sealing treatments have a corrosion resistance comparable with that of coatings with a Cr VI passivation treatment without a sealant; therefore, they could be a less polluting alternative to the traditional chromated coatings.