Nano Al2O3/phosphate Composite Conversion Coating Formed on Magnesium Alloy for Enhancing Corrosion Resistance

Abstract

Novel nano Al2O3/phosphate composite conversion coatings with no visible defects were formed on AZ91D magnesium alloy in a phosphate solution containing Al2O3 nanoparticles by applying a magnetic field. The formation of these coatings was investigated using the synchrotron radiation real-time imaging technique. The coatings’ surface morphology, chemical composition and structure were examined by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). Scanning vibrating electrode (SVET) and polarization curves were employed to evaluate their corrosion resistance in NaCl solution. The results show that the superposition of a magnetic field during the coating formation process can promote the generation of small hydrogen bubbles and accelerate their desorption, which favors the embedding of Al2O3 nanoparticles in the amorphous matrix. In the absence of a magnetic field, hydrogen bubble evolution could be one of the main reasons for the development of microdefects in phosphate conversion coatings. This defective phosphate conversion coating is amorphous and contains no Al2O3 nanoparticles, although the phosphate solution contains Al2O3 nanoparticles. The results demonstrate that the nano Al2O3/phosphate composite conversion coating obtained using a magnetic field exhibits better corrosion resistance than defective phosphate conversion coatings obtained without a magnetic field.