Study on the microstructure and corrosion resistance of ZrO2-containing ceramic coatings formed on magnesium alloy by plasma electrolytic oxidation

ZrO2-containing ceramic coatings were prepared on AZ91D magnesium alloy by plasma electrolytic oxidation (PEO) technique in three different zirconate electrolytes. The morphology and phase composition of the ceramic coatings were characterized by environmental scanning electron microscopy (ESEM) and X-ray diffractometer (XRD). The corrosion properties of the coatings were examined by immersion test and electrochemical corrosion test in 3.5 wt.% NaCl solution at 20 degrees C and 60 degrees C. The results show that the corrosion resistance of the coatings is related to their thickness, microstructure and phase composition. The better results are obtained for the coating formed in K2ZrF6-Na2SiO3-KOH electrolyte because it has a uniform surface, a compact inner layer, the maximum thickness and more corrosion resistant phases relative to those formed in Zr(NO3)(4)-KOH and ZrOCl2-KOH electrolytes. Compared to the bare AZ91D magnesium alloy, the corrosion resistance of the same coating in the NaCl solution at 60 degrees C is higher than that in the NaCl solution at 20 degrees C, which might be attributed to ZrO2 in the coating being propitious to improve the corrosion resistance of the specimens, especially the relatively high temperature corrosion resistance of those. (C) 2008 Elsevier B.V. All rights reserved.