Characterization and corrosion behavior of ceramic coating on magnesium by micro-arc oxidation

In this study, the commercial pure magnesium was coated in different aqueous solutions of Na2SiO3 and Na3PO4 by the micro-arc oxidation method (MAO). Coating thickness, phase composition, surface and cross sectional morphology and corrosion resistance of coatings were analyzed by eddy current method, X-ray diffraction (XRD), scanning electron microscope (SEM) and tafel extrapolation method, respectively. The average thickness of the coatings ranged from 52 to 74 mu m for sodium silicate solution and from 64 to 88 mu m for sodium phosphate solution. The dominant phases on the coatings were detected as spinal Mg2SiO4 (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg-3(PO4)(2) (Farringtonite) and MgO (Periclase) for sodium phosphate solution. SEM images reveal that the coating is composed of two layers as of a porous outer layer and a dense inner layer. The corrosion results show the coating consisting Mg2SiO4 is more resistant to corrosion than that containing Mg-3(PO4)(2). (C) 2011 Elsevier B.V. All rights reserved.