Preparation And Characterization Of The Corrosion-resistant Composite Coatings On AZ31Magnesium Alloy

Magnesium alloy is a biodegradable metallic biomaterial with great potentialapplications. However, the rapid degradation of magnesium alloy in human body environmenthas prevented their clinical applications. In this paper, the AZ31Mg alloy was surfacemodified by micro-arc oxidation. Ca-P and MgO/Ca-P coatings were prepared on the surfaceof Mg alloy by sol-gel method. PLA and MgO/PLA coatings were fabricated on the surface ofMg alloy by dip coating. The surface morphologies, compositions and phase structure of thecoatings were respectively investigated by SEM, EDS and XRD. The adhesion strength of thecoatings was measured by a pull-off method. The effects of surface modifications on thecorrosion resistance of the AZ31Mg alloy in SBF were evaluated by immersion test,potentiodynamic polarization and electrochemical impedance spectroscopy tests.A layer of uniformly distributed Ca-P compound could be prepared on the AZ31and themicro-arc oxidized AZ31magnesium alloy by a sol-gel technology in the solution of Ca（NO3）2·4H2O, P2O5and C2H5OH. The sol-gel coating could be transformed to Ca2P2O7,mixture of Ca2P2O7, Ca3（PO4）2and Ca10（PO4）6（OH）2, and Ca10（PO4）6（OH）2, bya heat treatment respectively at300℃,400℃and500℃. The bonding strength between theCa-P coating and the Mg substrate is no less than22MPa with sintering temperatures from300℃to500℃. The bonding strength between the coating and the substrate is improved,and the area portion of the micro cracks inside the coatings increase, with increasing sinteringtemperatures. The bonding strength of the MgO/Ca-P composite coatings to the substrate is noless than33MPa. The MgO/Ca-P composite coatings have a higher bonding strength and asmaller amount of the micro cracks than that of the only Ca-P coating at the same sinteringtemperature. It could be attributed to that the MgO inner layer could reduce the difference ofexpansion coefficient between the Ca-P coating and the Mg substrate, and decrease theresidual stress inside the Ca-P coating during sintering process. The Ca-P and MgO/Ca-Pcomposite coatings prepared by sol-gel could greatly improve the corrosion resistances of theMg substrate. The functions of the Ca-P layer decrease with increasing sintering temperatures.There are more improvements of corrosion resistance for the MgO/Ca-P composite coatingsthan that for the only Ca-P coating. It could be attributed to the good corrosion resistance ofMgO, the close expansion coefficient of MgO and Ca-P compound, and the high bondingstrength of the composite coatings.It shows that a uniformly distributed coating could be prepared on surface of the Mgsubstrate and MgO layer by a dip-coating method in the trichloromethane with the addition ofPLA. The bonding strength of the PLA and the MgO/PLA were no less than35MPa and40MPa, respectively. The porous structure of MgO coating could strengthen the adhesion ofPLA coating via mechanical interlocking. The MAO coating sealed by the PLA could significantly improve the corrosion resistance of the AZ31alloy, and the corrosion resistancesealed in the10%PLA is higher than that for the5%PLA. The contribution of MgO/PLAcomposite coatings to the corrosion resistance of AZ31alloy is more than that of the onlyPLA coating.The MgO/PLA composite coatings could provide higher corrosion resistance than theMgO/Ca-P composite coatings for Mg substrate.