Investigation On Growth Mechanism And Corrosion Resistance Of Plasma Electrolytic Oxidation Coatings On Commercial Pure Aluminium

Aluminum and its alloys are widely used in automobile,aerospace,biomedicine and military industries due to their low density,high specific strength and good casting and molding capabilities.It is found that the plasma electrolytic oxidation ceramic coatings formed on the surfaces of aluminum and its alloys can basically meet the requirements of their corrosion resistance in the corrosive environment after being modified by plasma electrolytic oxidation?PEO?technology.However,due to the short history of the preparation of Al-based PEO coatings by PEO technology,the complicated basic process and the influence of multiple experimental conditions during coating growth,there exists dispute among scholars on knowing and understanding the microstructure evolution and growth mechanism of Al-based PEO coatings,which further restricts the development speed of PEO technology in theory and the optimization and control of corrosion resistance.Therefore,the exploration of the law of microstructure evolution and the growth mechanism of PEO ceramic coatings during PEO will provide strong theoretical and technical support for theoretical development of PEO technology and the optimization and control of corrosion resistance of the obtained Al-based coatings.AA1060 commercially pure aluminium and the alkaline silicate electrolyte are used as substrate material and the basic electrolyte in the thesis,respectively.The technological conditions,including time,power mode,electrolyte composition and PEO processing sequences,are used.Both the observation techniques,including scanning electron microscope?SEM?,X-ray diffraction?XRD?and laser scanning confocal microscope?LSCM?technology,and the corrosion resistance testing techniques,including electrochemical impedance spectra?EIS?and potentiodynamic polarization curves,are used in the thesis.The thesis focuses on the phase and microstructure evolution of the Al-based PEO coatings,which is combined with the observation analysis of the PEO ceramic coating obtained by electrochemical detachment method.And on this basis,the comprehensive analysis of PEO coating morphologies is made,and the growth mechanism of the Al-based PEO coatings is established.Further,the relationship between the corrosion resistance and the phase and microstructure of the obtained PEO coatings are analyzed,and the corrosion resistance of PEO coatings prepared by different techniques are compared,so as to achieve the optimization and control of the corrosion resistance of Al-based PEO coatings.The phase and microstructure evolution of pure Al-based PEO coatings prepared by different treatment techniques are studied.The results show that all the PEO coatings prepared by different techniques contain?-Al₂O₃.The PEO coatings,which are treated for relatively long processing times?10-30 min;594 V-30 m?under the galvanostatic and potentiostatic modes in E-Si electrolyte,contain?-Al₂O₃ and mullite phase.Under one-and two-step treatment regimes,?-Al₂O₃ and elemental tungsten phase appears in P2 and OS2-TS2 coatings related to be prepared in E-SiW electrolyte.Based on the law of microstructure evolution of PEO coatings,the growth model of pure Al-based PEO coatings is established by comprehensively analysing the morphologies of pure Al-based PEO coating prepared by different treatment techniques.The results show that the comprehensive analysis of the morphologies can display the one-to-one correspondence of the characteristic structures and their corresponding sizes among the surface,fracture cross-section and the inner surface morphology of the coatings.During PEO treatment processed under potentiostatic mode,the submicron hemispheres at the interface between the barrier-type anodic film and pure aluminum foil substrate results from the difference of oxidation rate between the?-AlFeSi second particle and the metal substrate in the anodic oxidation stage,and the average diameters of the submicron hemispheres increase with the voltage rising?from⁰.27?m to⁰.50?m?;while the micro-hemisphers are the direct products from plasma discharge acted on the coating/substrate interface in the sparking discharge stage,and their sizes remain basically constant with prolonging of treatment time.The evolution of pure Al-based PEO coatings is as follows:it originates from the one-layer barrier-type anodic film during the anodic stage,and transforms into the partially one-layer PEO coating during the initial period of sparking discharge stage,further into partially two-layer PEO coating in stage II,and eventually into completely two-layer PEO coating in later period of stage II and whole period of stage III and IV.Further,the corrosion resistance of pure Al-based PEO coatings prepared by different treatment techniques is studied.The results show that when pure aluminum foil is subjected to PEO treatment in the E-Si electrolyte,the PEO coating possessing superior corrosion resistance can be easily obtained using the galvanostatic mode compared with using the potentiostatic mode,and the R_p value of OS1 specimen is³.93,⁴.10 times as many as that of 594 V-8 m and 594 V-30 m,respectively.Among all the pure Al foil-based PEO specimens,OS2 specimen prepared in E-SiW electrolyte shows the best corrosion resistance,and its R_p reaches to a value of¹.27×10⁸?·cm².