Preparation Of Mo-ce Based Conversion Coating On AZ91Magnesium Alloy And Its Microstructure Andanti-corrosion Properties

As the lightest metallic structural materials, magnesium alloy with high specific strengthhave been widely used in in aerospace, automobile, electronics and other filds. However,wider application is limited since the magnesium alloys are susceptible to corrosion in air,due to low electrode potential. Surface protection of magnesium alloys and their products isvery important to determine their qualities and service life. The present study aimed todevelop a chrome-free and phosphate-free conversion coating technology for magnesiumalloy. The commercial magnesium alloy of AZ91widely used in industries was chosed as theexperimental material. Anovel Mo-Ce-based composite conversion coatings technology weredeveloped. Microstructure and corrosion resistance of coatings were analysed by spot test,electrochemical test, SEM, EDS, XRD, XPS, FT-IR and other testing methods.Cerium nitrate and sodium molybdate were determined as the main components of basicchemical conversion system. The processing parameters were optimized by orthogonalexperiments and single factor experiments. The optimal parameters were as follows:Ce（NO₃）₃of8g/L, Na₂MoO₄of4g/L, deposition time of10mins and the pH value of2.5forthe conversion solutions. Thereafter, sodium citrate was identified as the appropriate additivefor the optimal chemical conversion system by screening tests. The optimal processingparameters for chemical conversion system containing sodium citrate were as follows:Ce（NO₃）₃of9g/L, Na₂MoO₄of3g/L, C₆H₅Na₃O₇of2-3g/L, solution pH value of2.5-2.8and the processing time of8to10mins.The conversion coating prepared in the basic conversion system was homogeneous withmicro-cracks and remarkable white granular particles distributed in the film. Mo-Ce-basedcoating was amorphous structure and mainly composed by metal oxides and hydroxides ofMo, Ce and Mg elements. During the forming of coating, local pH value increased resultingin the deposition of metal ions due to electrochemical unevenness of substrate. After sodiumcitrate was added, the conversion coating became more uniform and dense. No obvious whiteparticles existed in the coating. The elements, phase compositions and structuralcharacteristics of the coating were not affected significantly by citrate. Contents of elementsMo and Ce in the coating were decreased while element C increased significantly. Citrate participated in the formation of coatings and the conversion coating mainly constituted bymetal oxide and complex of metal and citrate.Tafel polarization and impedance analysis demonstrated that the corrosion currentdensity of Mo-Ce based coating decreased about2orders of magnitude from9.632e-005A/cm²of magnesium subatrate to1.302e-006A/cm². The polarization resistance of coating（Rp） increased remarkably from10220Ω·cm²to1607Ω·cm²Rpof substrate. Experimentaldata indicated that the Mo-Ce based coating on AZ91magnesium coating can effectivelyinhibit the corrosion process so as to improve the corrosion resistance of magnesium alloys.