Coloring mechanism and corrosion resistance of multi-order color hot-dip galvanized coatings

Abstract In this work, color coatings were prepared based on hot-dip galvanization employing a Zn-0.4Mn zinc alloy bath, and the surface color exhibited a three-order periodic cycle of colors (yellow, purple, blue, and green). The element Mn has a greater thermodynamic tendency towards oxidation than Zn in the air. As a result, the Mn atoms will be oxidized preferentially to form an oxide film of diverse colors on the surface. The morphology and composition of the oxide film were investigated by scanning electron microscope (SEM), x-ray diffractometry (XRD), and x-ray photoelectron spectroscopy (XPS). It has been shown that the diverse colors of the oxide film result from the different thicknesses of the oxide film caused by light destructive interference. The main components of the oxide film are MnO and MnO 2 . As the oxidation time increases, the oxide film grows and thickens. The thicker film showing the third-order color will crack, which may be caused by the internal stress of the film layer. The results of electrochemical impedance spectroscopy (EIS) showed that the impedance increased sequentially with the color progression, but decreased with the third-order color coating, and the charge transfer resistance reaches a maximum at the second-order in purple and blue, with both values exceeding 4000 Ω·cm 2 . Furthermore, neutral salt spray tests (NSS) demonstrated that the thickening of the oxide film enhances the corrosion resistance of the coatings.