Effects Of Anodizing On The Bonding Strength Of AZ91D Magnesium Alloy And Adhesion Strength Of Organic Coatings

Due to the unique combination of low density, high strength/weight ratio and good electromagnetic shielding, magnesium and its alloys are used in many industries such as communication, automotive and aerospace fields. However, surface treatments must be applied to increase corrosion resistance, owing to the high chemical reactivity of magnesium. Organic coating is an effective approach to improve the corrosion resistance of magnesium and its alloys. However, on magnesium alloys a loose corrosion product film is liable to form, which is detrimental to the bonding between the coating and substrate. Therefore, the surface of magnesium alloys must be properly pretreated before coating, otherwise the adhesion strength of organic coatings on magnesium alloys will be affected. The microstructure of the porous layer achieved by anodizing is favorable to enhance the adhesion strength of organic coatings, which makes anodizing an appropriate pretreatment before coating. This thesis is aimed at the effects of anodizing on the bonding strength of AZ91D magnesium alloy and adhesion strength of organic coatings. SEM,lap-shear strength of the bonding joints,neutral salt spray test were adopted to study the morphologies, bonding strength and the corrosion resistance of the oxidation film, and the optimum technological parameters were achieved. Pull-apart test were used to evaluate the effects of the optimum oxidation technology and microarc oxidation technology on the adhesion strength of epoxy primer varnish and Mg-rich epoxy primer.1 The effects of anodizing time,concentration of KOH,current density on the oxidation behavior,the microstructure of anodic oxidation films,and the lap-shear strength of bonding joints were studied. The optimum technological parameters were as follow:KOH100g/L+ Na2Si03100g/L+Na2B4O7100g/L+Na2CO350g/L+KF0.5g/L,i=10mA/cm2, t=6min.2 The microstructure of the anodic film achieved at the optimum oxidation technology and its influence on the Wetting ability of AZ91D magnesium and the adhesion strength of organic coating were studied. The porous structure of the anodic film is favorable to enhance the Wetting ability of AZ91D magnesium and the adhesion strength of organic coating. achieved, which is Na2Si0315g/L+ KOH20g/L+ KF3.2 g/L+ Na3C6H5O7·2H2O5g/L+C3H8O310mL/L,±i=10-50mA/cm2,±f=400-1000Hz±D=10%,t=8min. The microstructure of the microarc oxidation film achieved at the optimum technology and its influence on the Wetting ability of AZ91D magnesium and the adhesion strength of organic coating were studied. The porous structure of the microarc oxidation film is favorable to enhance the Wetting ability of AZ91D magnesium and the adhesion strength of organic coating.4 By comparison of the morphologies of anodic film and microarc oxidation film as well as their effects on the adhesion strength of organic coating, the effects of the microstructure such as the pore size,depth and the porosity on the adhesion strength of organic coating were studied.