Preparation And Performance Of Phytic Acid Conversion Coatings For AZ91D Magnesium Alloys

Magnesium alloys are extensively used in automobile，electron and aerospace owning totheir high specific strength and stiffness，excellent shock resistance and high thermalconductivity. However, magnesium alloys have high chemical activity, and its naturaloxidation coating have poor corrosion resistant, which largely limited its extensive use.Among the various corrosion protection, chemical conversion treatment is one of the mosteffective and economic methods. Traditonally, conversion coatings for magnesium alloys arebased on hexavalent chrominm(Cr6+) compounds, which are toxic and caustic for human andenvironment. Due to the imminence of associated restrictions, there is a great need for thedevelopment of environment-friendly treatment methods.A new innoxious chemical protective coating for magnesium alloys，phytic acid convers-ion coating，was prepared. Calcium nitrate、ammonium metavanadate and sodium tartratewere chosen as the additives. The formula and the coating preparation process were optimim-ed through orthogonal experiments and single factor experiments. The morphology, composi-tion and functional group were carried out by means of scanning electronic microscopy(SEM),Energy Diffraction Spectrum(EDS)，Energy dispersive analysis of x-ray（EDAX），Fourier transform infrared spectroscopy（FTIR）and Electrochemical workstation，respectively.The coating formation mechanism was obtained. The influence of phytic acid conversioncoating as a middle layer on the polyurethane coating on magnesium alloys was investigatedthrough salt spray test，adhesion test and hardness test.1）Formula of phytic acid conversion solution and process of chemical conversion wereoptimized through orthogonal test and single factor experiments respectively.The best conditi-on were:pure phytic acid of15g/l，calcium nitrate of1g/l，ammonium metavanadate of0.5g/l，sodium tartrate0.5g/l，pH of4，temperature of70℃and time of35min.The loss ratio ofcoating obtained in the optimum process was8%of that in pure phytic acid,and50%of thatin chromic acid,which illustrated that corrosion resistance was improved a lot.2）Potential-time curve of coating-forming process of Mg alloy was adopted to monitorpotential on the surface of coating.Conversion coating composition in different forming timewas analysed.From the results of both the above experiment,phytic acid conversion on Mg alloy could be divided into two process those were dissolution of the substrate and depositionof the insolubles.3）The main ingredients of coating were phytic acid salt of metal，as well as metal oxideor hydrogen oxidation,which was detected through EDS and FTIR.The formation mechanismof the coating was discussed.When the coating begun to form,it mainly contained compoundsof Mg, which consisted of magnesium salt of phytic acid, Mg(OH)2and MgO; As the proces-sing time went on, the concent of the metal salt of phytic acid became bigger, and Al begun toappear in the coating, and increased as the conversion time extented.4）Corrosion resistance of the combined film with the polyurethane on the surface ofphytic acid conversion coating was measured by salt spray test.The result showed that saltspray test time was more than1200hours，which implied that the corrosion resistant perfor-mance was greatly improved.Formation and anticorrosion mechanisms of phytic acid conversion coating was studiedfor one kind of magnesium alloy(AZ91D)theoretically,and it will widen the application ofphytic acid surface treatment technique for magnesium alloys practically.