Influence of zinc oxide on corrosion resistance of alumina based chemically bonded ceramic coatings

With thermal chemical reaction, the chemically bonded ceramic coatings are prepared on A3 steel. Influence of zinc oxide on corrosion resistance of the coating is investigated by corrosion polarization analysis. It is found that the chemically bonded ceramic coatings show the outstanding corrosion resistance. In addition, the coating with zinc oxide possesses a smaller reaction rate and corrosion current density than the coating without zinc oxide. Scanning electron microscope (SEM) analysis and X-ray diffraction (XRD) analysis are conducted to find the reason. Without adding zinc oxide, AlPO4 is the main binding phase connecting with the particles in the coating, and the micro structure of the particles can be seen clearly. However, with adding zinc oxide, a new binding phase Zn3(PO4)2 is found in the coating. Besides, the particles in the coating are fully covered with Zn3(PO4)2 and AlPO4, and the micro structure of the particles cannot be clearly seen. The particles have a better cover with the binding phase compared with the particles without zinc oxide. Therefore, adding zinc oxide makes the coating more compact that hinders access of H2O, O2, and Cl– to the substrate with the compact coatings, which improves the corrosion resistance of the chemically bonded ceramic coatings.