Molybdate conversion coatings on AZ31 magnesium alloy

A brown-yellow conversion coating was obtained on the surface of AZ31 magnesium alloy in molybdate solution.The morphology,composition and corrosion resistance of the coating were studied by scanning electron microscopy,X-ray photoelectron spectroscopy,and potentiodynamic polarization analysis,respectively.Potentiodynamic polarization analysis was used to study the corrosion resistance of the coating.The results show that the conversion coatings consist of spherical particles and the thickness of the coating is about 12 μm.The exterior conversion coating consists of MoO3.However,the inner conversion coating mainly consists of MoO2 and MoO(OH)2 as well as a little amount of MoO3.An obvious passivation occurs on the molybdate conversion coating during anodic polarization.The corrosion potential shifts 683 mV positively,and the corrosion current density decreases two orders of magnitude.The corrosion resistance of AZ31 alloy is remarkably improved by the molybdate conversion coating.The mechanism of the coating was discussed briefly.