Effects of manganese addition on microstructures and corrosion behavior of hot-dip zinc coatings of hot-rolled steels

In the present work, the Zn coatings of hot-rolled steel sheets with different Mn additions were prepared by hot dipping process. The effects of Mn addition on the microstructures and corrosion resistances of the hot-dip Zn coatings were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffractometry (XRD). The Mn distribution in δ-layer is lower than that of ζ-layer. Mn addition could promote the growth of δ-layer of the coating and inhibit the growth of the columnar ζ phase which morphology was changed to discontinuous phase. It is also found that the surface color of Zn-Mn coating varies with the dipping temperatures. Such phenomenon is supposed to be attributed to the optical interference effect of the Mn oxide film formed on the coating surface. For the corrosion behavior, the as-dipped and ζ layer-exposed Zn coatings were subjected to both Tafel polarization measurements and electrochemical impedance spectroscopy (EIS) tests in 3.5% NaCl solution. The results indicate that Mn addition enhances the corrosion resistance of the as-dipped coatings. The dense corrosion products formed on the top η layer of the Zn-Mn coatings after Tafel polarization corrosion are assumed to inhibit the further corrosion of the coating. However, minor effects of Mn addition to the anti-corrosion performance of ζ layer were noticed.