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Research Of Rare Earth-phytic Acid Composite Conversion Coatings On Magnesium Alloys

Posted on:2013-01-15Degree:MasterType:Thesis
Country:ChinaCandidate:X Q LiuFull Text:PDF
GTID:2251330392968341Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
In this work, the AZ31samples were firstly treated in rare earth solution, thenthey were treated with phytic acid solution, and composite conversion coatings wereprepared successfully on magnesium alloys. The effect of six process parameters,(concentration of Ce(NO3)3, concentration of phytic acid, pH values of phytic acid,immersion time in rare earth salts and phytic acid solution, temperature) on themorphologies, structure and corrosion resisitance of conversion coatings werestudied syetemtically. Scanning electron microscope (SEM) was operated to observethe surface morphologies of the conversion coatings. The composition, content andchemical state of elements were analyzed by Energy dispersive X-ray spectroscopy(EDS) and X-ray photoelectron spectroscopy (XPS). The anticorrosion performanceof coatings was measured with the potentiostat and electrochemical impedencespectroscopy (EIS).Surface morphologies of composite coatings varied with process parameters,proper increase of concentration of Ce(NO3)3and phytic acid, time and temperatureand decrease of pH values could make the reaction more fully, and homogeneousand compact coatings with microcracks were easily obtained. When the changes ofthese parameters were out of the range, the compactness of coatings decreased, themocrocracks expanded and in some areas several pieces of films fell off. When thecoatings were compact, the content of O, P and Ce was higher, and when less density,the content of Mg increased, that of other elements was declined.According to EDS and XPS, the composite conversion coating consisted of C,Mg, O, P and Ce. The peaks of C1s corresponded to contaminations and phytic acid.The peaks of O1s belonged to CeO2, P=O, Mg(OH)2and C-O-P in the phytic acid,respectively. Mg element was mainly in the form of Mg(OH)2, the content of MgOand Mg3(PO4)2was very low; the peaks of P2p pertained to PO43-and HPO42-.The results of polarization curves and EIS demonstrated that, the corrosionresistance of conversion coatings was controlled by the difference of processparameters and complied with the parabolic rule. When extending the concentrationof treatment solutions, temperature, pH values and time, the anticorrosion ofcoatings increased in initial stage and then declined. the corrosion potential of blanksample was-1510mV, and that of sample prepared in optimized conditions raisedabout192mV, reaching-1318mV, the pitting potential reached to-1357mV, and thecurrent density decreased mort than two orders. The electric resistance of conversionfilm and polarization resistance was4780cm2and1816cm2, increasing two orders. Above all, the corrosion resistance of matrix was extremely enhanced bychemical conversion treatment.
Keywords/Search Tags:AZ31magnesium alloy, composite conversion coating, morphology andstructure, corrosion resistance
PDF Full Text Request
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