An evaluation of the corrosion resistance and adhesion properties of an epoxy-nanocomposite on a hot-dip galvanized steel (HDG) treated by different kinds of conversion coatings

Hot-dip galvanized steel (HDG) samples were chemically treated by Cr(III), Cr(VI), Cr(III)-Co(II) and Cr(III)-Ni(II) conversion coatings. Epoxy nanocomposites were prepared using 2 wt.%, 3.5 wt.%, 5 wt.% and 6.5 wt.% nano-ZnO. Electrochemical impedance spectroscopy (EIS), pull-off adhesion tester and scanning electron microscope (SEM) were utilized to evaluate epoxy coatings properties on the surface of pre-treated HDG samples. Results showed that addition of nano-ZnO particles (specially 3.5 wt.%) can significantly improve the corrosion resistance of the epoxy coating on HDG. Decrease of contact angle (φ) and increase of surface roughness (Ra) of the pre-treated HDG samples were obtained. Decrease of the value of φ was more pronounced when the Cr(III) pre-treated samples were post-treated by Cr(III)-Co(II) and Cr(III)-Ni(II) conversion coatings (CCs). The dry and wet adhesion strengths of the epoxy coating to HDG were significantly increased after the surface treatment of the samples. Increase of the adhesion strength and decrease of the adhesion loss were more pronounced on Cr(III)-Co(II) and Cr(III)-Ni(II) post-treated samples. The corrosion resistance of epoxy coating was also increased on the surface of pre-treated HDG samples. Increase of the corrosion resistance of the Cr(III) pre-treated HDG samples was more pronounced on the samples which were post-treated by Co(II) and Ni(II).