Pore Structure Control Of Oxide Films On Aluminum And Magnesium Alloys And Its Relationship With Film Properties

Anodizing and microarc oxidation of aluminum alloys and magnesium alloys are widely applied in industry fields to further improve the mechanical and corrosion resistant properties of the alloys.Previous researches mainly focused on the oxidation technology and the properties of the films on aluminum alloys and magnesium alloys.However,there are few reports on the relationship between the film properties and the pore structures.Moreover,the quantitative characterization of the pore structure is also seldom studied.Thus,the relationship between the pore structure and the film composition and film properties is a valuable research issue.In this thesis,the anodic films and the microarc oxidation films with the controllable pore structures on 2024 aluminum alloy and AZ91D magnesium alloy were prepared and the relationship between the film property and the pore structure was studied.The anodic films on aluminium alloy were prepared in sulfuric acid solution with different concentrations.The variations of sulfuric acid concentration could affect the morphology and the pore structure of anodic film greatly.As the sulfuric acid concentration increases,the film porosity increases and the regularity of pore structure decreases.The microhardness decreases and bonding strength increases with the increased film porosity.The different sulfuric acid concentrations result in the variations of pore parameters,and the variations of pore parameters would affect the thermal cracking behavior of the anodic films.Not only the increased interpore distance and pore circularity but also the decreased porosity and number of partial dissolved pores could result in the decreased thermal cracking tendency of the films.The crack density changes faster than the crack width.The barrier layer resistance of the films decreases with the increased film porosity.After immersion in 3.5%NaCl solution for 4800 hours,the barrier layer resistance of the films with porosity of 24.3%and 28.2%decreased slowly,but the barrier layer resistance of the films with porosity higher than 30%decreased much faster.From the view of corrosion protection,the porosity of anodic films should be controlled below 30%.The microarc oxidation films on aluminium alloy were prepared in electrolyte modified by adding oxide and SiC nanoparticles.The oxide and SiC nanoparticles are embedded and melted into the microarc oxidation films.Then the nanoparticles may fill the pores,cracks and defects of the films,and result in an increase of film thickness.After long-term immersion for 1440 hours,the resistance of the inner compactness layer decreases slightly and the capacitance of the inner compactness layer increases slowly with the addition of Ti02,CeO2 and SiC nanoparticles into the electrolytes.The resistance of the outer loose layer shows similar variations to the resistance of the inner compactness layer.The corrosion resistance of the microarc oxidation films during the long-term immersion mainly depends on the good shielding property of the inner compactness layer.The addition of the nanoparticles could result in increased film porosity while the corrosion resistance of the films was improved.When the heating temperature was lower than 600?,all the microarc oxidation films did not crack.This implies that the nanoparticles would not affect the heat resistance of the films,which could be attributed to the uniform distribution of the fine Al2O3 crystalline phase as the main component in the films.As the film porosity increases,the bonding strength of the films increases but the film microhardness decreases.Film porosity is the main factor determining the microhardness and the bonding strength of the films.Chemical composition could also affect the film microhardness to some extent.Particularly,the addition of SiC nanoparticles could increase the film microhardness greatly.The microarc oxidation films were prepared on AZ91D magnesium alloy under different current frequencies.As the current frequency increases,the relative content of Mg element and the relative content of MgSiO3 phase increase to some degree.The higher current frequency results in the lower film thickness.Film porosity could affect the microhardness and bonding strength obviously.For the same porosity,the higher pore diameter increases the bonding strength,but decreases the film microhardness.As the current frequency increases,the interpore distance and the pore circularity increase while the pore diameter and the film porosity decrease.Not only the increased interpore distance and pore circularity but also the decreased pore diameter and film porosity could decrease the thermal cracking tendency of the films.Crack density is a better parameter than crack width for evaluating the heat resistance of the films.The decreased film porosity could result in increased film impedance.The lower porosity and the better continuity of the films improve the anti-corrosion ability of the films.Addition of the oxide and SiC nanoparticles into the electrolytes also enhanced the properties of anodic films on magnesium alloy.The oxide and SiC nanoparticles are adsorbed and embedded in the anodic films as a whole,which increases the film thickness to some degree.The corrosion resistance of anodic films at the initial immersion stage is similar.After 1200 hours immersion,the barrier layer resistance of anodic films prepared by adding SiC,TiO2,ZrO2 and CeO2 nanoparticles to the electrolytes decreases,but is still higher than 105 ?·cm2.And the barrier layer capacitance increases very slowly.The addition of the nanoparticles results in the decreased film porosity,which increases the film microhardness but decreases the bonding strength.Then the increased interpore distance and the decreased film porosity would decrease the thermal cracking tendency of the anodic films.Particularly,the anodic film prepared with CeO2 nanoparticles did not crack after heating.The above results show that the effects of film porosity and pore parameters on the mechanical properties and the corrosion resistance of oxide films on aluminum alloy and magnesium alloy show similar rules.The film properties are mainly determined by the pore structure.Based on the relationship between the film properties and the pore structure,the oxide films with the better properties could be obtained by designing and controlling of the pore structure of the films.