| As the lightest structural metal materials,magnesium alloys endow with a series of advantages such as high specific strength,high specific stiffness and good electromagnetic shielding performance,which make them widely of using in the fields of automobile,electronic manufacturing and communications industry,but the poor corrosion resistance of magnesium alloy limited its application development.However,when the magnesium materials adopt Dacromet technology,as the magnesium alloy has high chemical activity,the acid materials in Dacromet coating liquid will react firstly with the magnesium alloy to produce loose corrosion materials,and a compact barrier layer cannot be obtained during the Dacromet film forming process.On the basis of optimization for the composition and preparation parameter of the Dacromet coating,the microstructure and corrosion resistance of Dacromet coating with different content of boric acid for AZ31 magnesium alloy sheet were investigated in the paper by the electrochemical tests,X-ray Diffraction?XRD?,scanning electron microscope?SEM?,and the Dacromet-Silane composite coating was prepared for further enhancing the corrosion resistance.The main conclusions were as follows:Optimization composition of Dacromet coating on magnesium alloy was metal powder 300 g/L,chromic anhydride 80 g/L,ethylene glycol 180 ml/L.Thickener was hydroxyethyl cellulose,p H regulator was Zn O;defoaming agent was isooctyl alcohol.The best curing process was 80?holding10 min,and then 300?keeping 30 min.The Dacromet coating for magnesium alloy was consisted of Zn,Mg,Mg Cr O4 and oxides Zn O,Mg O,Cr2O3,Cr O3.The thickness of coating was about 10?m.There were B2O3 appears in the coating with the addition of boric acid.The content of Mg Cr O4,Mg O decreased while the content of Cr2O3,Cr O3 increased with the increase addition of boric acid,and the limit values were reached in the coating with the addition of 2%boric acid,And the film compactness was the best.The corrosion potential of coating reduced and the corrosion current decreased first and then increased with content of Boric acid increased.The limit values were reached with the addition of 2%boric acid.The Rct value increased first and then decreased with content of Boric acid increased,the limit values were reached with the addition of 2%boric acid,the corrosion resistance was best.There were corrosion products on the coating with the addition of 2%boric acid in salt fog corrosion about for 75h.Dacromet-Silane composite coating was prepared by recoat the silane coating on the basis of Dacromet coating of magnesium alloy.The new phase of Si O2 and Zn2Si O4 is produced in composite coating.The composite coating is compact with a thickness of about 20?m.Compared with that of Dacromet coating,linear polarization resistance Rp value of the composite coating was increased from208.1?·cm2 to 770.3?·cm2,the corrosion current Icorr value was reduced from 5.068×10-5A·cm-2 to2.716×10-6A·cm-2,and the charge transfer resistance Rct value was increased from 107.5?·cm2 to217.2?·cm2.Compared with the Dacromet coating with boric acid replace part of chromic acid,salt fog resistance of composite coating was significantly enhanced. |
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