Preparation And Performance Of Microarc Oxidation Coatings And Composite Coatings On The Surface Of Mg-Li Alloys

As the lightest structural materials with high strength-to-weight ratio, superior mechanical characteristics, good magnetic screen ability and high energetic particle penetration resistanc, Mg-Li alloys have a wide application in aerospace, transportation, electronic and other industries. However, the poor corrosion resistance of Mg-Li alloys inhibits their practical application. In this thesis, microarc oxidation (MAO) technology was employed to fabricate MAO coatings on the surface of Mg-Li alloys, and MAO/sol-gel composite coatings were prepared by the combination of MAO and sol-gel technology. The microstructure, composition, anti-corrosion properties of the MAO coatings and MAO/sol-gel coatings were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and potentiodynamic polarization, etc. measurements.The MAO progress of Mg-Li alloys in an alkaline polyphosphate electrolyte without and with addition of titania sol has been studied. The effects of titania sol on the surface and cross-section microstructure, surface roughness, composition and anti-corrosion properties of MAO coatings were investigated. The results show that after titania sol was added to the alkaline polyphosphate electrolyte decorative blue ceramic coating could be obtained on the surface of Mg-Li alloy, the coating was composed of TiO2, and Ti2O3 besides MgO, the compactness and surface roughness of the MAO coating was improved and decreased, respectively, the anti-corrosion properties of the MAO coating was improved to some extent.MAO/sol-gel composite coatings were prepared on the surface of the Mg-Li alloy by the combination of MAO and sol-gel technology, the heat treatment technical parameters were confirmed by optimization experiment. The microstructure, composition and anti-corrosion properties of MAO coatings and MAO/sol-gel coatings were comparatively evaluated. The results indicate that after sol-gel treatment, micropores and microcracks on the surface of the MAO coating were covered by the sol-gel layer, and the compactness of the MAO coating was improved, the MAO coatings were composed of MgO and Mg2SiO4, and the sol-gel layer was composed of amorphous titanium organic-inorganic compounds. The anti-corrosion properties of the Mg-Li alloy were improved greatly by the MAO/sol-gel composite coating.The MAO process of LA141 Mg-Li alloys in an alkaline silicate electrolyte without and with addition of tungstate has been studied, the addition amount of tungstate was confirmed by reference to anti-corrosion properties of MAO coatings. The effect of MAO time on the surface morphology and phase composition of MAO coatings prepared in an alkaline silicate electrolyte with addition of tungstate was investigated. The morphology, composition, anti-corrosion properties and surface morphology after potentiodynamic polarization test of MAO coatings prepared in an alkaline silicate electrolyte without and with addition of tungstate were comparatively evaluated. The results show that as the MAO time prolong, the surface of the LA 141 Mg-Li alloys which were treated in alkaline silicate electrolyte with addition of tungstate was gradually covered by MAO coating, WO3 was appeared in the phase composition of the MAO coating except for MgO and Mg2SiO4. The MAO coating prepared in an alkaline silicate electrolyte was composed of MgO and Mg2Si04, a new phase of WO3 was detected in the MAO coating and the thickness of the MAO coating was increased after tungstate was added to the alkaline silicate electrolyte. The anti-corrosion properties of the MAO ceramic coating were improved with the addition of the tungstate. The corrosion type of the two kinds of MAO coatings in 3.5 wt.% NaCl solution was pitting corrosion.