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Study on Microarc Oxidation Coatings and Composite Coatings on the Surface of Mg-Li Alloys

Author: LuYi
Tutor: JingXiaoYan
School: Harbin Engineering University
Course: Applied Chemistry
Keywords: Mg-Li alloys microarc oxidation composite coatings anti-corrosion properties
CLC: TG174.4
Type: Master's thesis
Year: 2009
Downloads: 62
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Abstract


As the lightest metallic structural materials, Mg-Li alloys have favorable application prospects in the fields of aerospace, spaceflight, automobile and electronics. However, the application of Mg-Li alloys has been mainly hindered by their poor corrosion resistance. In this thesis, microarc oxidation technique (MAO) was employed on the surface treatment of Mg-Li alloys to obtain in-situ ceramic coatings. On the basis of these, MAO/chemical conversion composite coatings were prepared. The microstructure, composition, properties and formation mechanism of MAO coatings and MAO/chemical conversion composite coatings were investigated by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, potentiodynamic polarization and electrochemical impedance spectroscopy, etc. measurements.The ceramic coatings were prepared in the silicate/molybdate and silicate/aluminate composite electrolyte. The microstructure, composition and anti-corrosion properties of two ceramic coatings were comparatively studied. In contrast to alloy substrate, the anti-corrosion properties of two coated alloys were both remarkably improved. Compared with the white ceramic coatings formed in the silicate/aluminate electrolyte, the dark brown ceramic coating formed in the silicate/molybdate electrolyte are more uniform and denser, the improvement of corrosion resistance may be ascribed to the formation of MgMoO4 and MoO3 and the improvement of compactness.The MAO process of Mg-Li alloys in silicate electrolyte with and without the addition of silica sol was scrutinized. Furthermore, the effect of silica sol on the thickness, micro-morphology, composition and corrosion resistance of ceramic coatings has been discussed. The results indicate that the microstructure of ceramic coatings become more uniform and denser, and the corrosion resistance of coatings containing thermodynamic stable SiO2 phase has been improved to some extent.The MAO/chemical conversion composite coatings were fabricated under the adjustment and control of component of conversion solution and technical parameters, the micro-morphology, composition and electrochemical performance of MAO and MAO/chemical conversion composite coatings were comparatively evaluated. The results reveal that numerous spherical-like microparticles with uniform size accumulate and disperse on the surface of composite coatings. Spherical-like microparticles composed of NaMgF3 and MoO3 can be deposited in the micro-pore and micro-crack, which can not only significantly fill the inherent structural defects of ceramic coatings, but also improve the compactness of coatings. As a result, the corrosion resistance of alloys is enhanced.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Corrosion control and protection > Metal surface protection technology
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